A Clinical Guide to Crowns

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BRITISH DENTAL JOURNAL VOLUME 192 NO. 3 FEBRUARY 9 2002 143

Crowns and other extra-coronal restorations:Introduction R. W. Wassell1 J. G. Steele2 and A. W. G.Walls3

1*Senior Lecturer in Restorative Dentistry, 2Senior Lecturer in RestorativeDentistry, 3Professor of Restorative Dentistry, Department of RestorativeDentistry, The Dental School, Framlington Place, Newcastle upon Tyne NE2 4BW*Correspondence to: R. W. WassellE-mail: [email protected]

Refereed Paper© British Dental Journal 2002; 192: 143

● An introduction to the series on Crowns and other Extra-coronal Restorations

I N B R I E F

This series of articles is aimed at anybody who places crowns and otherextra-coronal restorations (ie veneers and adhesive onlays or ‘shims’)on individual teeth. We hope that everyone from experienced practi-tioners to undergraduate students may find something of value. The aimof this series is to give guidance, based on available scientific evidencewhere possible, towards the provision of high quality restorations.

Treatment planning issues and materials choices feature at least asstrongly as technique description.

We have concentrated on single tooth restorations, but all of theprinciples described also apply to more complex multiple restorations,including fixed bridges. However, we have not specifically coveredreplacement of missing teeth with bridges or implants. Replacement ofteeth involves consideration of a range of additional issues and treat-ment planning decisions, whilst an entirely different set of technicalrules are required for the consideration of implants. These will be leftfor other authors to address.

Where possible, we refer to published scientific evidence. Admitted-ly, randomly controlled clinical trials and systematic reviews are muchscarcer in dentistry than in medicine. Consequently, much evidence isbased on the experience of clinicians, or on laboratory or theoreticalconsiderations. This does not necessarily devalue existing practices,but it does make it more difficult to make objective choices about treat-ment planning, materials or techniques. We have therefore tried to behonest when our advice stems solely from experience or theory.

The series consists of 13 individual articles, each of which should beable to stand alone. Each article will start with the specific learningobjectives we hope to meet. In a series like this it is impossible toexplore every technique or material ever described, this would makefor very heavy reading and very dull writing. We hope though thatthese articles should enable the reader to evaluate his or her own prac-tices against a set of fundamental principles.

CONTENT OF THE SERIESA series of 13 Articles coveringthe following:1. Changing patterns and the

need for quality2. Materials considerations3. Pre-operative assessment4. Endodontic considerations5. Jaw registration and

articulator selection6. Aesthetic control 7. Cores for teeth with vital

pulps8. Preparations for full veneer

crowns9. Provisional restorations 10. Impression materials and

technique11. Try-in and cementation of

crowns 12. Porcelain veneers13. Resin bonded metal

restorations

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144 BRITISH DENTAL JOURNAL VOLUME 192 NO. 3 FEBRUARY 9 2002

Changing patterns and the need for qualityJ. G. Steele1 R. W. Wassell2 and A. W. G. Walls3

This series of articles is aimed at anybody who places crowns and other extra-coronal restorations (ie veneers and shims) onindividual teeth. We hope that everyone from experienced practitioners to undergraduate students may find something ofvalue. Whoever reads them, we would ask to do so with an open mind. We have tried not to be dogmatic, and the techniquesand materials described are not the only ones available, but are the ones which accord with the principles we describe.

1*Senior Lecturer in Restorative Dentistry,2Senior Lecturer in Restorative Dentistry,3Professor of Restorative Dentistry,Department of Restorative Dentistry, The Dental School, Framlington Place,Newcastle upon Tyne NE2 4BW*Correspondence to: J. G. SteeleE-mail: [email protected]

Refereed Paper© British Dental Journal 2002; 192:144–148

● Crown provision has seen an enormous increase over the past three decades. In the UKaround 1 million teeth are fitted with crowns every year, many of these requiring complexadditional treatment prior to crown placement

● Around 15% of NHS dental spending annually is on crown provision and maintenance, so thetotal cost is very large

● The pressures on dentists to produce high quality restorations is probably increasing and willnot diminish

● There are few data on the quality or longevity of restorations placed ● Technological advances have changed the way we are able to practice over recent decades,

and further developments will open up new possibilities

I N B R I E F

Our aim in these articles is, by working from asound theoretical base, to try to give the reader thebackground to pick the best treatment optionsfrom the wide, and continuously changing, rangeavailable. Technical issues are important and willbe addressed, but there is a need to recognise thatthe provision of extra-coronal restorations is morethan just cutting a shape which is free of under-cuts, on to which something can be cemented.Cutting a preparation and cementing a restorationare relatively easy. Cutting a preparation andcementing a restoration which will last for manyyears without any further damage to the dental tis-sues is a different matter. Treatment planningissues and materials choices will occupy a greaterpart of this series of articles than the technicalitiesof tooth preparation.

We have concentrated on single toothrestorations, but all of the principles describedalso apply to more complex multiple restora-tions, including fixed bridges. However, we havenot specifically covered replacement of missingteeth with bridges or implants. Replacement ofteeth involves consideration of a range of addi-tional issues and treatment planning decisions,whilst an entirely different set of technical rulesare required for the consideration of implants.These will be left for future authors to address.

In an ideal world we would have been able todraw on the conclusions of full systematicreviews of the literature, based on randomisedcontrolled clinical trials. Such an approach iswidely used in many areas of medicine andallows objective assessment of the availabletechniques, drugs or materials. Few such reviewsare available in this area of dentistry though,probably because the raw material from whichthey are derived, namely randomised controlledclinical trials, are themselves so sparse. Where

possible, we have tried to support what we writewith appropriate previously published scientificevidence, but good quality evidence is scarceand much of what has been written in the past isbased on the experience of clinicians, or on the-oretical considerations. This does not necessarilydevalue existing practices, but it does make itmore difficult to make objective choices abouttreatment planning, materials or techniques. Wehave tried to identify where there is and where

1

Fig. 1a and 1b. The provision of good quality crowns oralternative adhesive restorations can result in atremendous improvement in oral health for the patient aswell as being a fulfilling experience for the dentist.Despite being a relatively minor procedure, this patient’saesthetics and confidence were improved enormously bythe provision of these anterior dentine bonded crowns

CROWNS AND EXTRA-CORONALRESTORATIONS:

1. Changing patterns andthe need for quality

2. Materials considerations3. Pre-operative

assessment4. Endodontic

considerations5. Jaw registration and

articulator selection6. Aesthetic control 7. Cores for teeth with

vital pulps8. Preparations for full

veneer crowns9. Provisional restorations 10. Impression materials and

technique11. Try-in and cementation

of crowns 12. Porcelain veneers13. Resin bonded metal

restorations

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Fig. 2 A series of radiographs taken over a five year period showing how the provision of crowns can be detrimental to oral health where the treatment is illthought out and, in this case, poorly executed. This is a graphic illustration of ‘the need for quality’

Fig. 2a Initial presentation revealedcrowns on lower anterior teeth withearly root caries on several teeth

Fig. 2b Treatment is planned andstarted but the caries has progressedand periapical pathology quicklybecomes apparent. Root treatment isstarted on affected teeth, withoutdealing with the root caries and its causes

Fig. 2c Root treatment continuesapace but with little attention tocoronal seal

Fig. 2f A successful outcome, but alittle late in the day

Fig. 2d A decision is made to progress to overdentures Fig. 2e Periodontal attachment loss and periapical pathologyrender overdenture abutments useless

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there is not strong evidence to support what wewrite, but inevitably we have often had to writefrom experience or theory.

The series consists of 13 individual articles,each of which should be able to stand alone. Foreach of these we will set, at the start of the paper,the specific learning objectives which we hopeto meet. In a series like this it is impossible toexplore every technique or material everdescribed, this would make for very heavy read-ing and very dull writing. We hope though thatthese articles should enable the reader to evalu-ate his or her own practices against a set of fun-damental principles.

The rest of this article will present some back-ground data which illustrate the health benefits,the health problems, the scale and the cost ofcrown provision, and the potential for techno-logical change to help us to improve oral health.

BACKGROUND: THE NEED FOR QUALITY

Crowns and oral healthAs we progress in the new millennium, a richblend of improving technology, better oralhealth, a strengthening scientific base and thetimeless ability to employ fine manual skills andartistry are making it increasingly possible, andenjoyable, for the dentist to deliver good qualityrestorations. Moreover, it is also increasinglypossible to do this with the minimum of damageto the dental tissues.

The health benefits of providing high qualityrestorations are essential, substantial and longlasting. Current concepts of health include posi-tive aspects of quality of life; self esteem and theability to undertake daily activities, such as eat-ing, speaking and socialising, comfortably andwithout embarrassment. It is here that restora-tive dental care has a major impact on health.Even in a simple case, such as that in Figure 1a,where an incisal tip has fractured and the com-posite repair is functional but dull, lifeless andunsightly in the eyes of the patient, the pre-treatment restoration can be a cause of embar-rassment. Well executed restorations could (anddid, see Fig. 1b) make a substantial positiveimpact on this individual’s well being. In a den-tition extensively broken down by years of dis-ease and wear the functional and aesthetic dis-ability can be much greater and the benefit moredramatic. It is true that poorly executed treat-ment may have the opposite effect. Figure 2shows a series of radiographs of a patient wherecrowns have been poorly conceived, ill-plannedand carelessly executed. The case is as graphican illustration of the need for both good plan-ning and skilful execution as it is possible tofind, and we will be using this case to illustrate anumber of specific points in a later article.

Historical trendsThe number of crowns placed by dentists inEurope and North America is vast and in manycountries the volume has been increasing year onyear. In England and Wales, where data are avail-able from the National Health Service (NHS), thenumber of treatments showed a rapid increaseover the past 30–40 years with the cost peakingat nearly £180M in 1991/1992 (Fig. 3).1–5 Itreduced significantly in the early 1990s whensome dentists moved out of NHS practice fol-lowing contractual changes, and then verymarkedly in 1997 when porcelain fused to metalrestorations on molar teeth were removed fromthe list of permitted items. Despite the reduction,the sums spent are still vast: around £142M in2000-2001 for crowns alone, but £6M forveneers, £11.5M for large onlays (having under-gone a very sharp rise since 1997) and £33.5M

0

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20012000199919981997199619921982197219630

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Percentage of total dental budget (£M)

Fig. 3 The cost of overall crown provision under the National Health Service for adults aged over 18years, 1963 to 1995/96 (figures for cost not taking into account annual inflation) (Note: The scaleon the x-axis is non-linear)

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Fig. 4 The total number of crowns (in thousands) recemented under the National Health Service foradults aged over 18 years, 1963–1995/96 (Note: The scale on the x-axis is non-linear)

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for bridges. Around a third of the dentate adultpopulation have at least one artificial crown,and amongst adults now in their 50s and 60s,half have a crown. Those with crowns havearound three per head on average.6 These figuresare derived from the 1998 UK Adult DentalHealth Survey and show a marked increase fromthe situation a decade earlier. Data from othercountries are less easily available, but what datathere are suggest that trends in Western Europeand North America are broadly similar.

The dental and demographic trends in thepopulation will ensure that the need for full cov-erage and other extra-coronal restorations willnot diminish in the foreseeable future. In addi-tion to the need to maintain and replace existingcrowns, there is a tidal wave of heavily restoredteeth in the middle-aged group in many westerncountries which resulted from treatment ofhigher levels of caries in the 1950s, 60s and 70s.6

These heavily restored teeth are now getting old.As the restorations in them age they will need tobe replaced, and as they are replaced they will getlarger. The number of cases where there has beenextensive loss of tooth tissue, and therefore wherefull coverage restorations may be indicated, looksunlikely to diminish. It may even increase. Fur-thermore, the need for such restorations willtend to be concentrated towards the older end ofthe age spectrum, providing an additional chal-lenge for the clinician. At the same time increas-ingly litiginous consumers will demand highquality restorations, and may be prepared torecover their costs through the courts if theirtreatment is ill-conceived or poorly executed.The pressures on practising dentists to producehigh quality restorative dentistry has never beenhigher, and will not reduce.

Quality issuesDespite the vast number of crowns placed andthe need for high quality, we know very littleindeed about the performance of these restora-tions. Even simple data on the longevity ofcrowns, and the rate of loss or replacement arelimited. Research findings are often difficult tointerpret because of variability in the selectioncriteria and treatment techniques used. Certainly,the number of crowns which are dislodged andrequire to be recemented is large in the UK; wellover half a million crowns are recemented everyyear under the NHS. This figure rose rapidly inthe 1980s and 90s reflecting the increase inoverall crown provision(Fig. 4).1–5 But, crownswhich fall out are only one manifestation of fail-ure. Others include those which become carious,periodontally compromised or in which thepulps become inflamed or necrotic as a result ofthe treatment provided. Evidence for the long-term effects of crown preparation on vital teethis very sparse indeed. There are estimates of thedamage to pulp vitality which occurs followingtooth reduction for crowns and bridges. Thisindicates that anything from 1–15% of teeth losevitality after preparation, with two of the threestudies cited here approaching the higher end of

0.0

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Porcelain Jacket Crown

Porcelain fused to metal crown

Full/partial veneer metal crown (usually gold)

2001200019991998199719961992198219721963

Number of teethtreated (millions)

Fig. 5 The total number of crowns of different types (all metal, porcelain fused to metal and allporcelain) provided under the National Health Service for adults aged over 18 years,1963–1995/96 (Note: The scale on the x-axis is non-linear)

Fig. 6a and 6b Adhesive dentistry has much to commend it.Contrast the destructive preparation for conventionalporcelain fused to metal restoration (4a) with the muchmore conservative preparation for a porcelain veneer (4b)

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the range.7–9 For various reasons these estimatesmay not be representative of typical adult popu-lations, but they do indicate that irreversiblepulp damage is likely to be a substantial problem. There are very few useful quantitativedata on other forms of failure. In these articles we have tried to use the limited evidence avail-able to identify and then to address the reasonsfor failure.

Technological solutionsTechnological advances over the past 30 yearshave offered many potential solutions to some ofthese problems, and have in turn played their partin changing treatment patterns as well. The tradi-tional boundaries between routine and advancedrestorative work have become blurred as newtechniques and materials have proliferated, somuch so that we found it difficult to decide on atitle for this series of articles. All metal and allporcelain restorations have largely given way toporcelain fused to metal restorations (Fig. 5 showsdata for the UK National Health Service),1–5 butchanges in porcelain technology are starting toprovide improved all-porcelain alternativesagain, though it may be some time before theiruse becomes widespread. Looking beyond this, itis possible to envisage a greater place for compos-ite resins as an alternative to porcelains in futureyears. However, it is the rapid development ofadhesive technology, particularly where bondingto dentine is concerned, that has provided thegreatest potential for change and the most excit-ing possibilities. Above all, preparations can bemade much less destructive and are often quickerto cut (Figs 6a and 6b). For all their attractionsthough, adhesive restorations create their ownproblems. Provisional restorations are difficult tomake, and the final restoration is often awkwardto adjust and to cement. They are highly tech-nique sensitive, requiring additional skills andcare if they are to be successful.

In this series we will attempt to outline a setof principles for the provision both of conven-tional and adhesive restorations (Figs 7a and 7b)with the aim of improving oral health.

Fig. 7a and 7b With the spectrum of restorations nowavailable, some cases are best treated with a combinationof conventional and adhesive approaches

1 Dental Practice Board. Digest ofStatistics 1995-2001. Part 1-Detailed analysis of GDS treatmentoutcomes. DPB, Eastbourne. Data for1996-2001 available at:http://www.bdta-dentistry.org.uk/dentaldata/index.html

2 Dental Practice Board. Digest ofStatistics 1991-92. Part 1-Detailedanalysis of GDS treatmentoutcomes. DPB, Eastbourne.

3 Dental Estimates Board. Statistics1963. DEB, Eastbourne.

4 Dental Estimates Board. AnnualReport 1972. DEB, Eastbourne.

5 Dental Estimates Board. AnnualReport 1982. DEB, Eastbourne.

6 Kelly M, Steele J, Nuttall N, BradnockG, Morris J, Nunn J, Pine C, Pitts N,Treasure E, White D. Adult DentalHealth Survey: Oral Health in theUnited Kingdom in 1998. (2000)London: TSO.

7 Felton D, Madison S, Kanoy E, KantorM, Maryniuk G. Long term effects ofcrown preparation on pulp vitality. JDent Res 1989; 68 (specialissue):1009. Abstract 1139.

8 Bergenholtz G, Nyman S. Endodonticcomplications following periodontaland prosthetic treatment of patientswith advanced periodontal disease. J Periodontol 1984; 55: 63-68.

9 Reuter J E, Brose M O. Failures in fullcrown retained dental bridges. Br Dent J 1984; 157: 61-63.

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Crowns and extra-coronal restorations:Materials selectionR. W. Wassell1 A. W. G. Walls2 and J. G. Steele3

Materials selection is the second in the series on crowns and other extra-coronal restorations. Some of us are less thaninspired by dental materials science. Nevertheless, many of the things that concern us clinically with crowns and theiralternatives are based on material properties. We worry about the strength of the restoration, how well it fits and itsaesthetics. We also worry about wear, occlusal control and biocompatibility. Not least of our concerns are dental laboratorycharges, which inevitably have to be passed on to the patient.

1*Senior Lecturer in Restorative Dentistry,2Professor of Restorative Dentistry,3Senior Lecturer in Restorative Dentistry,Department of Restorative Dentistry, TheDental School, Framlington Place,Newcastle upon Tyne NE2 4BW*Correspondence to: R. W. WassellE-mail: [email protected]

Refereed Paper© British Dental Journal 2002; 192:199-211

● Familiarisation with the broad spectrum of contemporary materials allowing better-informeddecisions to be made

● Discovering the factors that influence choice of alloys for indirect metallic restorations● Knowing the pitfalls of selecting a cheap alloy for cast post and core construction● Familiarisation with the different types of all ceramic crown and which ones can be used

posteriorly● An awareness of the many different types of metal copings for ceramo-metal restorations● Consideration of the use of composite crowns for specific clinical situations

I N B R I E F

An understanding of the materials available,their application and limitations will not onlyhelp with selection but will allow more effectivecommunication with patients and laboratories.Promotion of such understanding allowing bet-ter-informed decisions to be made is in essencethe aim of this article.

There are now more materials availablethan ever before for indirect restorations.These fall under the broad headings of allmetal, ceramics, metal-ceramics and resincomposites. Cements and cementation, eitherconventional or adhesive will be considered inlater articles. Manufacturers’ details areshown at the end of the article.

ALL METAL RESTORATIONSAll metal extra-coronal restorations includecrowns, onlays and shims (onlays relyingsoley on adhesive retention). Cast posts andcores can cause particular difficulties and areconsidered separately at the end of this sec-tion. In the UK as in other countries we areaware that economic pressures are driving the adoption of less expensive alloys. Thesealloys contain either low concentrations ofnoble metals or are comprised entirely of basemetals.

In this section we consider the various typesof alloys and the factors which influence theirselection including:• Cost• Castability and handling• Physical properties• Resin bonding• Corrosion and tarnish• Biocompatibility

Gold and palladium alloysIn the 1950s making an alloy selection for a castmetal restoration would simply have entailedchoosing an ADA specification high gold alloy,all of which had a greater than 75% gold andplatinum content. The soft type I alloy containedmost noble metals (83%) with the harder alloys(types II, III and IV) containing an increasingamount of silver and copper. Type I alloy wasused for small inlays, type II for larger inlays,type III for crowns and bridges, and type IV forpartial denture frameworks.

In the early 1970s the gold price increasedfour fold resulting in the introduction of lessexpensive alloys of lower Karat.1 Palladium,which had been used since the 1930s to producea cheaper white gold (Au 30%, Pa 10–35%, Ag35-60%, Cu 6–25%), became the noble con-stituent of many of the new precious alloys.These alloys include:• Silver-palladium • Palladium-silver-gold• Gold-silver-copper-palladium (with a gold

content greater than 40%)• Palladium-copper• Palladium-tin

Palladium has a strong whitening effectwhich means that most of these alloys will havea silvery appearance unless the gold content isgreater than 40% and the palladium less than6%.2 Unfortunately, both palladium and silverabsorb oxygen when molten, which can resultin porous castings especially if casting buttonsare reused. Also, reducing the gold contentlowers the specific gravity (density), which maymake casting less reliable than with high goldalloys. This is because less dense alloys have

2

CROWNS AND EXTRA-CORONALRESTORATIONS:

1. Changing patterns andthe need for quality

2. Materials considerations3. Pre-operative








restorations

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lower kinetic energy during casting which inturn reduces the penetration of molten metalinto the mould. Nevertheless, some authoritiesargue that with ideal conditions almost allcommercial alloys result in acceptable cast-ings,3 but experience indicates that techniciansmay find such conditions difficult to achieve.

Tarnishing of some alloys is noticeable incertain patients, particularly around the mar-gins of their restorations. This observation isborn out by a five-year clinical study compar-ing two palladium silver alloys to a type III goldalloy.4 In the UK the National Health Servicehas encouraged the use of alloys containingonly 30% gold. The potential for corrosionproblems to result from this change is currentlyunknown.

Allergies to gold, palladium and platinumare extremely rare.5 In-vitro studies6 show thathigh gold alloys have excellent corrosionresistance, which implies that few metal ionsare eluted from restorations. Metal ions areeluted more easily from alloys of low noblemetal content, including those of copper andsilver. Copper ions have been implicated in producing lichenoid reactions.7 However,lichenoid reactions to metal ions from crownsare not as well described as for amalgam wheremany lichen planus series show up to a third ofpatients to be sensitive to mercury salts.8 Thisraises the possibility that some cases of lichenplanus adjacent to crowns may be linked to theunderlying amalgam core. Nevertheless, withcurrent trends to use more easily corrodiblecasting alloys we should be alert to the possi-bility of more lichen reactions in the future.

Base metal alloysBase metal alloys used to make indirect restora-tions include: • Nickel-chromium• Nickel-chromium-beryllium• Titanium• ‘Progold’

Nickel-chromium alloysThe most commonly used base alloys are nickel-chromium and nickel-chromium-beryllium.Beryllium is added to improve the alloy’s physi-cal properties; it is used as a hardener, grainstructure refiner and to reduce the alloy’s fusiontemperature.9 As a result of health concerns (seebelow) some alloys contain molybdenuminstead of beryllium.10 All of these alloys have ahigher modulus of elasticity than noble alloys.This means that they are more rigid which ishelpful in preventing flexion of long spanbridges. Rigidity in thin section is necessary foradhesive bridge frameworks and for adhesivelyretained shims used to restore the palatal sur-faces of worn incisors. It is also possible withselected resin adhesives to obtain high bondstrengths to the surface of the sandblasted alloy.For instance Nery et al.11 reported a bondstrength of 22 Mpa for a nickel chromium alloybonded to dentine with All Bond 2 (Bisco Inc,

Itasca, USA). A worrying trend, however, is forsome laboratories to substitute a precious metalinstead of nickel chromium without telling thedentist. Clearly, this could reduce the expectedbond strength.10

Leaving aside health concerns for a moment,nickel chromium alloys are not without prob-lems. Casting conditions need to be carefullycontrolled to obtain well fitting restorationsand any technician will tell you that thesealloys are hard to adjust and difficult to finish.This in turn can be a nuisance clinically whenfitting restorations.

Experience of use differs markedly fromcountry to country. In the USA these alloys havehad a relatively good press12,13 and are used forcasting crowns and bridges by the majority oflaboratories.9 In the UK these alloys account foronly 1% of the crowns provided on the NationalHealth Service. In Sweden, the National Board ofHealth and Welfare has warned against the use ofalloys containing more than 1% nickel,9 effec-tively precluding nickel-chromium alloys as theyall contain between 62 and 82% nickel.14

The health problems are worth exploring inmore detail. Many laboratories use non-beryl-lium containing nickel chromium alloys, asberyllium grindings and casting fumes areextremely hazardous unless controlled by anadequate exhaust and filtration system. Acuteproblems include conjunctivitis, dermatitisand bronchitis. Chronic beryllium disease maynot express itself for several years after expo-sure.9 Similarly, nickel can cause techniciansdermatological and lung problems with high levels of nickel or nickel compoundsbeing carcinogenic. Tumours include rhab-domyosarcoma, nasopharyngeal and lungcarcinomas. Again an effective exhaust sys-tem is needed to keep concentrations belowrecommended levels (Table 1). These levelsvary from country to country, but if you arefitting nickel chromium restorations youshould ensure adequate aspiration duringadjustments.

Nickel is well known to cause contact der-matitis. Current estimates show that between 10–20% of women and 1–2% of men are sensi-tised to the metal, possibly as a result of wearingnickel containing jewellery.15 Surprisingly, therehave been few reports of patients reactingadversely to nickel containing dental restora-tions and little evidence that nickel adsorptionintra-orally exacerbates existing dermatitis.Nevertheless, there are authorities that counseldentists against using nickel in those patientsknown to be sensitive to the metal.5,14

Table 1 Nickel and beryllium are toxic. In the UKmaximum permitted air levels (in µg.m-3) of nickel foran eight-hour shift have recently been reduced fivefold78 but are still over six times higher than in the USA9

UK USA

Beryllium 2 2

Nickel 100 15

GOLD AND PALLADIUMALLOYS KEY POINTS:• Porous castings can

result from reused casting buttons

• Some alloys — especiallythose of low noblemetal content — areprone to corrosion

• The potential for cheap alloys to causelichenoid reactions is currently not knownbut needs to be kept inmind

BASE METAL ALLOYS KEY POINTS:• Ni-Cr alloys are

popular in the USA butare effectively bannedin Sweden

• A good bond strengthcan be achievedbetween certain Ni-Cr alloys and resin luting agents

• Ni-Cr alloys are stifferthan most noble alloysand can therefore beused in slightly thinnersection

• Technicians and dentists must avoidinhaling Ni-Cr casting fumes or grind-ing dust especially ifthe alloy containsberyllium

• In patients sensitive tonickel it is best to avoidusing Ni-Cr alloys

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TitaniumTitanium and its alloys are well known forbiocompatibility. Restorations can be either castor electro-formed. Casting requires hightemperatures (1650°C) and a special magnesiuminvestment.16 Titanium oxidises easily so anargon arc is used to melt the metal and castingperformed under vacuum. In 1985, Ida et al.17

reported that the fit of cast titanium crowns wasintermediate between those made from a highnoble alloy and nickel chromium. Electro-forming was introduced in 198918 and involvesthe milling of a titanium blank by sparkerosion. Two year clinical follow-up of electro-formed copings veneered with composite haveshown encouraging results.19

‘Progold’Do not be misled by alloys such as ‘Progold’. Likebrass they consist largely of copper and zinc,and tarnish easily.

Cast post and coresTo avoid the post bending or breaking, thealloy chosen should have relatively high hard-ness, proportional limit and ultimate tensilestrength. Occasionally, cost considerationsdrive the selection of the cheapest alloy, how-ever corrosion and problems with castabilityshould always be born in mind; porosity with-in a cast post can often result in post fracturewith unfortunate consequences and thin postsare more likely to suffer critical porosity thanthicker ones.

It is important to note that many gold alloysused for post and cores can be either soft or harddepending on heat treatment. A soft post willbend (Fig.1) and break more easily underocclusal load than a hard one. Laboratories mayunwittingly produce soft posts simply byquenching the hot casting ring to allow theinvestment to be easily removed. If this is donethe casting must be heat-treated according tomanufacturer’s specifications. The alloy used atour dental hospital (45% Ag, 28.8% Pd, 18.8%Cu, 5% Au, 0.6% Pt) is heat treated at 400°C for20 minutes. In practice our technicians avoid theneed for a separate heat treatment by allowing

the casting ring to bench cool slowly to roomtemperature.

Some of the alloys used for all metal restora-tions can also be used for metal-ceramic restora-tions. However, we will first consider all ceramicrestorations.

ALL CERAMIC RESTORATIONSDental ceramics, also termed porcelains, havea composite structure consisting of a crys-talline phase or phases within a glassy matrix.Dental ceramics consist of oxides, largely ofmetals and silica, and are essentially inertmaterials, but we should be aware that theycan be attacked by acidulated phosphate fluo-ride (APF) gel. Clinical experience with allceramic restorations breaking under occlusalload confirms that these materials are general-ly susceptible to stress corrosion and slowcrack growth.20

In 1965 McClean introduced aluminousporcelain.21 Until then only feldspathic porce-lain had been available to make PJCs whichalthough aesthetic was extremely brittle. Manyof us would now consider aluminous porcelainas the standard material for PJCs. Nevertheless,despite it being a significant advance and high-ly aesthetic, aluminous PJCs are still prone tofracture and cannot be recommended as reliablemeans of restoring posterior teeth.

There are now a bewildering number ofporcelain systems on the market and researchis continuing to develop materials which are strong, aesthetic and suitable for multipleapplications, including crowns, bridges,inlays and onlays. Some materials rely on theproduction of an opaque, heavily reinforcedcore over which weaker but more aestheticlayers of porcelain are built eg In-Ceram and AllCeram. Other materials have thestrengthening agent dispersed throughout thematerial e.g. the glass ceramic, Dicor. Detailsof the strengthening mechanisms are welldescribed elsewhere22 but all of them rely onhaving a crystalline phase dispersed within aglassy matrix.

Traditionally, porcelains are classified ashigh fusing, medium fusing or low fusing butthis is not terribly helpful in understanding theincreasing number of materials available. Alu-minous porcelain PJCs are made simply by sin-tering (see later) but there are five other meth-ods of making indirect porcelain restorationswhich together form a useful means of classifi-cation:

• Sintered Porcelains• Glass Infused Ceramics• Cast Glass Ceramics• Hot Pressed, Injection Moulded Ceramics• Machined Glass Ceramics• Machined Densely Sintered Ceramics

We will first consider some of the materialswithin this classification and then look at themajor clinical factors that influence materialchoice.

Fig. 1 A bent cast post has resulted in failure (note thefailed composite cementation). Proper alloy selectionand heat treatment will produce a more rigid casting,capable of resisting distortion from occlusal forces.

TITANIUM KEY POINTS:• Specialised casting

requires an argon arcunder vacuum

• Restorations can also be‘electroformed’ usingspark erosion

CAST POST AND CORESKEY POINTS:• Avoid cheap alloys

which may corrode orcast poorly

• The alloy needs to havea high modulus of elasticity and high yield strength

• Proper heat treatmentensures a stiff postwhich is less likely tobend

PRACTICE


Methods of making indirect porcelainrestorations

Sintered porcelainsSintered porcelains are built up from an aque-ous slurry of porcelain particles condensedonto a platinum foil matrix or a refractory die.Sintering occurs at a temperature above thesoftening point of porcelain whereby the glassymatrix partially melts and the powder particlescoalesce. There is volume shrinkage of30–40%. Porosity can be reduced from 5.6 to0.56% by vacuum firing.23 Sintering is themost commonly used technique of makingPJCs and veneer restorations with a number ofdifferent materials available:• Aluminous porcelain eg Vitadur-N, Hi-Ceram• Feldspathic porcelain reinforced with Zirconia

fibres eg Mirage II• Feldspathic porcelain reinforced with leucite

eg Optec HSP

Glass infused ceramicsIn-Ceram is a glass infused ceramic used forcrowns (Fig. 2). It consists of a core containing90% alumina, which is built up on a refractorydie. During firing, at 1150°C for four hours, thedie shrinks so that it can be withdrawn from thecore. This process (which is also used to manu-facture ceramic lavatory pans) is called ‘slip-casting’. At this stage the core is a weak, porousstructure consisting of partially sintered alumi-na particles. Strength is conferred by painting aslurry of lanthanum containing glass onto theoutside of the core and refiring it. During refir-ing the molten glass is drawn into the porousstructure thus eliminating voids and creating aglass-ceramic composite. The excess glass isground away and porcelain with a matchedcoefficient of thermal expansion is built onto thesurface. The manufacturers recommend thatrestorations be cemented with conventional acidbase cements.

In-Ceram Spinnel is a similar type of materialbut uses the less hard magnesium spinnel(MgAl2O4) instead of alumina. The material isspecifically designed for inlays and onlays. Mostrecently introduced is In-Ceram Zirconia, whichhas a very high flexural strength.

A novel material called Techceram has been

introduced in the UK, which also relies on hav-ing a glass infused core. The core is built of small‘splats’ of alumina sprayed from a plasma gun ata rotating refractory die. Again, after glass infu-sion, the restoration is formed conventionally onthe core with a matched sintered porcelain. Thecompany claim a flexural strength as high as forIn-Ceram.

Cast glass ceramicsGlass ceramics are polycrystalline solids pre-pared by the controlled crystallisation of glasses.The best known of these systems, Dicor, is basedon mica crystals although there is another, Cera-pearl, based on hydroxyapatite and experimen-tal lithia-based materials.24 A Dicor restorationis made by investing a wax pattern and casting.Heating the reinvested crown for six hours at1070°C carries out controlled crystallisation,termed ‘ceraming’. This causes the Mica to form astrong ‘house of cards’ structure, which makesfracture propagation equally difficult in alldirections. However, a reaction between themica and the surrounding investment may resultin a weakened surface layer, which reduces sig-nificantly the overall strength of the material.25

Characterisation of the crown is achieved by sur-face glaze. Prior to glazing the material has theappearance of frosted glass.

To overcome the limitations of surface glaze,which are considered in more detail later, a tech-nique of laminating a Dicor coping with felds-pathic porcelain has been developed with theintriguing name ‘Willi’s Glass’.26 Providing thecorrect porcelain (Dicor Plus) is used for lamina-tion, the strength of the restoration should notbe adversely affected.27

Hot pressed, injection moulded ceramicsIPS Empress is a leucite containing porcelain. Aswith cast glass ceramics the restoration is firstwaxed-up and invested, however the ingot,which is made of sintered ceramic, is not moltenbut softened before being pressed into a mouldunder pressure at 1150°C (Fig. 3). The pressure ismaintained for 20 minutes during which timethe tetragonal leucite crystals are dispersedthroughout the restoration giving a 40% con-centration by volume. The shade of the ingotprovides the basic shade, which can be modifiedby either glazing or veneer porcelains. To ensurecompatibility with veneer porcelains the ingotshave a lower coefficient of thermal expansion(14.9 x 10-6/°C) than those for the glazed materi-al (18 x 10-6/°C). The application of veneerporcelains may require multiple firings that canenhance the strength of the material.28

Another material, less well known in the UKis Alceram, which is based on magnesium spin-nel. It is important to emphasise that this is aquite different material to the much strongerProcera AllCeram which is mentioned later.

Machined glass ceramicsThere are a number of milling systems availablefor milling ceramic blanks,29 which may be con-

Fig. 2 In-Ceram glass infused alumina cores prior toporcelain application (Courtesy of Vita)

ALL CERAMIC RESTORATIONS KEY POINTS:• Ceramics are

considered inert but can be attacked by APF gel

• Ceramics are strength-ened by the dispersionof a crystalline phasethrough a glassy matrix

• Ceramics can be classified according tofusion temperature andmode of manufacture

• Ceramic systems cannot be evaluated onstrength data alone

• In-Ceram and ProceraAllceram are suitablefor posterior crownsbased on long-termclinical evaluation

• Resin bonding of In-Ceram requires specific silanisationtechniques

PRACTICE


trolled either by computer aided design/comput-er aided manufacture (CAD/CAM) or mechani-cally. The best-known CAD/CAM system in theUK is Cerec and the best-known manual systemis Celay. Both of these machines were introducedfor inlays and onlays, but the original Cerec 1software gave questionable marginal fit.30–32

The accuracy of fit is better with Cerec 233 andmay be improved still further with the recentintroduction of Cerec 3.

The ceramic blanks are manufactured tohigher levels of strength than can be achievedmanually in the dental laboratory. Materialsinclude Dicor MGC, Vita Mark II and Vita Celay;the latter two contain sanidine (KAlSi3O8) as thecrystalline phase. Unfortunately, sanidine makesthe ceramic very opaque.22 A newer material,Corning MGC-F is a tougher material with ahigher flexural strength.34

Despite the relatively high strength of theceramic blank, machining may cause weakeningthrough the introduction of surface flaws.35

Machined densely sintered ceramicsProcera AllCeram is a densely sintered, highpurity alumina core on which low fusing porce-lain is built.36,37 AllCeram cores have some ofthe highest measurements of flexural strengthyet recorded for a commercially available sys-tem. Despite this, even stronger materials basedon zirconia are being developed.

Interestingly, the cores are made centrallyin Sweden or New Jersey, USA. Local dentallaboratories scan the dies (Fig. 4) and theinformation is sent to one of the two centresvia the Internet enabling oversized dies to bemade. The dies are oversized to compensatefor the shrinkage of alumina during sintering.A mixture of alumina and binder is pressedonto the dies under pressure and, before sin-tering, machined to the required thickness(0.25, 0.4 or 0.6 mm). In this way any smalldefects caused by machining are eliminated

during sintering. The thicker cores are recom-mended for posterior teeth and the thinnercores for veneers. The 0.4 mm cores are usedfor aesthetically critical crowns on anteriorteeth and first premolars.

Choice of ceramic systemWhen faced with the task of evaluating a new oralternative ceramic system you should have anumber of questions in mind — as consideredunder the six headings below.

Should I choose the strongest ceramic?The flexural strength of ceramics is often quoted(see Table 2 for moduli of rupture), but it is worthpointing out the limitations of such data (seelater) before automatically choosing whatappears to be the strongest system on the list.

• Fracture toughness and fatigue are alsoimportant

• Comparisons between tests may be unreliable 38,39

• An In-Ceram core, although strong in itself, isweakened by increasing the thickness of over-lying ceramic40

• Resin cements are an important adjunct to thestrength of the finished restoration. This con-cept has been shown with veneers.41 A poorbond of resin to tooth or restoration will notenhance the strength of the cemented restora-tion. Some ceramic cores are smooth internal-ly and resistant to etching with hydrofluoricacid (eg In-Ceram), which prevents effectiveresin bonding unless specific silanisationtechniques are used .42,43

• Some ceramic veneers omit the strengtheningcore and rely mainly on the resin bond toenhance the strength of the comparativelyweak veneering ceramic.

How much tooth reduction is needed?Most systems specify a similar amount oftooth reduction as for a conventional alumi-

Fig. 3 The press ceramic furnace used for the constructionof injection moulded IPS Empress crowns (Courtesy ofIvoclar).

Fig. 4 A die is scanned for a Procera AllCeram coping(outline of scan is in the background), which is madecentrally in either Sweden or the USA (Courtesy of Nobel Biocare)

PRACTICE


nous PJC; that is a 1 mm shoulder and 1.5 to2 mm incisal reduction for anterior teeth and2 mm occlusal reduction for posterior teeth. A 1.2 mm axial reduction is specified for In-Ceram but a much heavier axial preparationis needed (1.5 mm) when Dicor crowns are pre-scribed for posterior teeth. Many dentists willfind this unacceptably destructive of tooth tis-sue or they simply will not cut it and thenwonder why the crown fractures.

Do all ceramic restorations fit well?The marginal fit of indirect restorations is formany dentists an emotive but inconclusivesubject. There is currently no consensus as towhat constitutes an unacceptable marginalopening 44 although clinical experience andempirical data suggest 100µm45 especiallywhere the margin can only be probed.46 Norelationship has been found between size ofmarginal opening and microleakage,47 howev-er an open margin will predispose to loss ofcement lute, which has implications for sec-ondary caries and periodontal disease.48

Most marginal fit data is from in-vitro studieswhere variables are carefully controlled butunfortunately comparisons of crown types arelimited and studies cannot be compared becauseof differences in methodology. Also, measure-ments can be made difficult by rounding ofshoulder ceramic margins. These in-vitro studiesgenerally show mean marginal openings in the20–80 mm range and while statistically signifi-cant differences can be shown between systems49

the clinical relevance is unclear. However, asalready mentioned1 some of the machinedceramic restorations have much greater discrep-ancies. Those would certainly be problematic if

the restorations were cemented with conven-tional cements.

A common perception is that the most reli-able margins are achieved with metal ratherthan with ceramic. Whilst this contention is sup-ported by some studies49 others do not supportit.50 Clearly technical skill will play a large partin achieving satisfactory results with whateversystem is chosen. Comparing ceramo-metalcrowns to metal crowns with all ceramic crownsthe former generally have better resistance tomarginal distortion from repeated firings thanthe latter, which has implications for some allceramic systems where the ceramic is appliedincrementally.51 In-Ceram, however, has goodresistance to multiple firings.52

Will the system be aesthetic?Although all ceramic restorations have thepotential to provide the best aesthetics, aesthet-ics are material and skill dependent. Obtaining agood result is not simply a matter of colourmatching. Translucency, which is determined bythe match in reflective indices of the crystallinephase and surrounding matrix, is also important.Better control of colour can be achieved bybuilding it into the substance of the restorationrather than merely relying on surface glazewhich may wear thin with time or be removedwith adjustments. Some of the systems describedabove are coloured entirely by surface glazing.

Will the restoration abrade the opposingtooth?Ceramic has a reputation for causing wear toopposing tooth structure. This is born out by in-vitro studies,53 and certainly there is a potentialfor greater wear if ceramic is left rough afterocclusal adjustment.54 It is currently not clear ifone type of ceramic is markedly less abrasivethan the others, although the mica-glass, Dicor,the machinable glass Vita Mark II and the lowfusing ceramics (eg Procera and Duceratin) haveshown a trend in this direction.55,56,57 The situa-tion is complex however; chemicals in food-stuffs (eg cola drink) may affect some low fusingceramics resulting in them becoming more abra-sive and less resistant to wear.58 Further researchis needed before dentists can confidently pre-scribe ceramic occlusal surfaces with similarabrasivity to gold.

Is the system supported by meaningful clinicalstudies?The problem with clinical studies is that theytake time. Longevity data at one or two years isof limited use unless it is showing a high failurerate. Also clinical success with anterior crownscan not be extrapolated posteriorly; a 3-yeartrial of Dicor molar crowns resulted in a failurerate of 64%. With the brittle characteristics of allceramic restorations, a trial of 5 years is prefer-able to be confident of future performance.Good data is available for In-Ceram59,60 and forProcera AllCeram61 showing fracture levels ofless than 1% per year.

Table 2 Comparative flexural strength data for dental ceramics classified according to techniqueof restoration production

Sintered Porcelains Crystalline Phase Example Flexural Strength (MPa) Reference

Aluminous (Pt foil) Vitadur-N 123 79

Aluminous (refractory die) Hi-Ceram 139 79

Leucite reinforced Optec HSP 104 79

Zirconia based Mirage II 70 79

Glass Infused CeramicsAlumina based In-Ceram 446 80

Magnesium spinnel In-Ceram Spinnel 378 80

Zirconia In-Ceram 604 80

Zirconia

Cast Glass CeramicsMica based Dicor 125 79

Hot Pressed Injection Moulded CeramicsLeucite based IPS Empress 97 80

160-180 28

Zirconia and Alumina Alceram 162 81

Machined Glass CeramicsMica based Dicor MGC 229 80

Sanidine Vita Mark II 122 80

Machined Densely Sintered CeramicsAlumina Procera AllCeram 687 36

PRACTICE


Are all ceramic bridges possible?It is possible to make small anterior bridges withmost of these systems, but with the exception ofVita’s In-Ceram few manufacturers actively pro-mote this because of the risk of fracture, espe-cially at the connectors. The Procera AllCeramspecifies a minimum connector height of 3 mmand a maximum span of 11 mm. Clinical studiesare underway but long-term results are not yetavailable.

Cost?The cost of high strength ceramic restorationssuch as In-Ceram, AllCeram and Empress willtake into account a laboratory’s investment innew equipment and training as well the timetaken to make a restoration. In the UK, highstrength ceramic crowns are up to £40 to £60more expensive than an aluminous PJC. Thesematerials can be used on the National HealthService but not without prior approval. Ifapproved, a discretionary fee is awarded whichin most cases will not cover the laboratory bill.

METAL-CERAMIC RESTORATIONSStress concentrations within PJCs often lead tocracks propagating outwards from the fit surfaceof the restoration. A comparatively tough metalcoping effectively bonded to the ceramic willhelp stop cracks developing in this way. The firstmetal copings were cast but other methods ofcoping construction, including foil and metalcomposite copings, have since been developed.

Cast copingsPorcelain fused to metal (metal-ceramic) tech-nology was first described in 195662 and patent-ed in 1962.63 Alloys were produced with meltingpoints sufficiently high to resist the firing ofporcelain. The first alloys had a high noble metalcontent of around 98% with iron, indium and tinused for hardening, and to create a superficialoxide layer to which the ceramic could be bond-ed. The ceramic had to be specially formulated tohave a high coefficient of thermal contraction toprevent unwanted stresses being built upbetween it and the coping on cooling after fir-ing. This was achieved with a ceramic contain-ing 15–25 vol% leucite as its crystalline phase.

With such a high gold content the originalalloys were extremely expensive, resulting inmany laboratories preferring high palladiumlow gold alloys – although paradoxically, palla-dium prices have recently been so high that thehigh gold alloys are sometimes the more afford-able alternative! High palladium alloys have theadvantage of having a high modulus of elastici-ty and are therefore more rigid allowing slightlythinner copings to be made. This rigidity is par-ticularly useful in bridgework where flexion ofthe pontics under load can result in fracture ofthe overlying porcelain. The alloy used by ourdental hospital contains 78.5% Pa, 6.9% Cu,5.5% Ga, 4.5% In, 2% Sn, 2% Au. Other alloysalso contain either gallium or indium or both topromote chemical bonding to the porcelain.10

Some of the alloys already considered under‘all metal restorations’ can be used for metal-ceramic copings. These include nickel chromi-um, nickel chromium beryllium, silver palladi-um alloys and titanium.

Control of the oxide layer thickness is impor-tant to avoid problems of porcelain debonding.Some silver palladium alloys can cause a green-ish hue to appear due to diffusion of silver com-pounds into the porcelain. Titanium oxidiseseasily and a thick nonadherent oxide layer canform under regular feldspathic porcelains. Thuslow fusing porcelains (eg Procera or Duceratin)are used to avoid problems of the oxide layercompromising strength.64 Porcelain bondstrengths to titanium are in any case not as highas with other alloys which may explain the high-er risk of metal-ceramic failure seen in a 6-yearclinical follow-up.65

Low fusing ‘hydrothermal’ porcelains (Duc-eram, Duceragold) can also be fused to type IVgold in what is known charmingly as the ‘Gold-en Gate System’. Once again control of theoxide layer is crucial for achieving bondingand good aesthetics.66 Aesthetics are improvedby the underlying gold shining through theporcelain. The major perceived advantage ofthis system is the potential to limit the numberof different alloys used in a patient’s mouth.

Cast copings are the most commonly usedmethod of strengthening porcelain and haveserved us well, but consistently good aestheticsare difficult to achieve because the metal has tobe covered by an opaque layer which in turnlimits the thickness of an adequate overlyinglayer of porcelain. However, proper tooth reduc-tion and excellent technical support will largelyovercome this problem.

One of the main advantages of cast copings isthat the coping can be waxed to create a metalocclusal surface — a facility that is either absentor more difficult to achieve in the following twosystems.

Foil copingsIn 1976 McClean 67 reported a technique of fus-ing platinum foil to the fit surface of an alumi-nous PJC. The foil was made adhesive to theporcelain by electroplating with tin and subse-quent oxidisation. The crown was made usingtwo layers of foil with the first layer beingremoved after firing.68 Any improvements incompressive strength are controversial withsome reports showing a positive effect and oth-ers negative.69 Such differences are explained byvariations in test methodology. Other foils havebeen tried with aluminous porcelain includingpalladium70 and gold coated platinum.71,72 Morerecently a gold foil reinforced crown has beenintroduced.73 The foil of this ‘Sunrise Crown’ is50 µm thick and contains gold, platinum and anoxidising element designed to facilitate porce-lain bonding. Unlike platinum foil, the goldalloy has too high a coefficient of thermalexpansion to be used with aluminous porcelain.Standard metal-ceramic porcelains are used

METAL-CERAMIC RESTORATIONS KEY POINTS:• High palladium low

gold alloys have significant advantagesover high gold alloyswhere rigidity is needed

• Oxides of gallium, indium and tin are usedto promote adhesion ofalloy to ceramic

• Too thick an oxide layercan result in ceramicdebonding or discolouration

• A metal occlusal surface can be incorpo-rated in a cast copingbut not in foil copings

• Foil copings are notstrong enough for posterior restorations

• Metal composite copings (Captek) are apromising alternativeto cast copings wherethe occlusion is to bebuilt in porcelain

• Electroformed copingsare an interesting butunproven technology

• Surface treatment ofthe ceramic by ionexchange gives onlylimited strengthening

PRACTICE


instead. These crowns can fit well74 but meas-urements of compressive strength are unimpres-sive,73 and, in the absence of clinical trials to thecontrary, do not support their use for restoringposterior teeth.

Metal composite copingsThe Captek system was developed over adecade ago for producing composite metalframeworks for metal-ceramic crowns,bridges, inlays and onlays (Fig. 5). The tech-nique, which involves the fabrication of ametal composite (Fig. 6) coping (88% Au, 4%Pt, 4% Pa) is comparatively simple but doesrequire some technician training.75 A metalimpregnated wax sheet is adapted over arefractory die, sintered in a porcelain furnace,and the resulting granular structure infusedwith molten gold. The coping thickness is250 µm for anterior teeth and 350 µm for pos-terior teeth. The margins can be thinned downto 50 µm to give a gold micro-margin or aporcelain butt fit produced. Conventionalmetal-ceramic porcelains are applied to thegold coloured coping. As with the Golden GateSystem and Sunrise Crown there is some reflec-tion from the gold coping, which can enhanceaesthetics. Unlike conventional cast metalceramic restorations, tooth preparation can be reduced significantly, it being possible to make crown margins 0.3 mm wide. However,

our experience with the material shows that itcan be difficult to obtain a good crown marginunless an obvious chamfer or shoulder is present.

The manufacturer’s literature quotes a studyfrom the University of Boston, which comparesrestorations, made from Captek with thosemade from a high quality palladium-contain-ing alloy. The sheer bond strength of porcelainto Captek was almost twice as high whilstcrowns of both materials loaded at the incisaledge showed similar load bearing characteris-tics. Marginal fit was also reported to be excel-lent and significantly better than conventional

Fig. 5 A Captek coping for a ceramo-metal restorationof a central incisor. The buccal margin will be thinnedto a micro-margin prior to porcelain application.Captek is unusual in that it resists firing distortionresulting from differential thermal contraction ofmetal and porcelain.

Fig. 6 Schematic diagram showing how a Captek core is laid down as gold alloy impregnated waxsheets: a) the granular appearance of Captek W after the first wax layer has been burnt off; b) application of the second wax layer (Captek G); c) perfusion of the Captek W by the Captek Gduring the second firing; and d) the resulting composite metal structure after firing (Courtesy ofSchottlander).

a

c

b

d

PRACTICE


metal ceramic crowns. Long-term studies areneeded to confirm anecdotal evidence of goodclinical performance. Occlusal contacts areusually formed in ceramic, as metal contactsare difficult to build.

Electroformed copingsThe GES Gold Electroforming System uses anionic solution to electroplate a 0.3mm thick-ness of pure gold directly onto the die. The dieis coated with a metallic silver varnish to ren-der it conductive. The manufacturers claim asimilar strength characteristic to type III goldbut the system is not supported by any clinicalstudies.

Strengthening of feldspathic metal-ceramicporcelainsFeldspathic porcelains are inherently weak intension and strong in compression. Hence theyrely on bonding to metal and coping design todissipate tensile stresses. Another approach is togenerate compressive stresses either internallyor in the immediate subsurface layer. A recentinnovation is the use of ion exchange wheresmaller diameter sodium ions in the surface ofthe porcelain are replaced by larger diameterpotassium ions. This exchange has the potentialto strengthen by subsurface compression, how-ever any strengthening effect is lost if therestoration is subsequently self-glazed or fin-ished. Surprisingly, the ion exchange techniquedoes not give a significantly greater increase inflexural strength than simple overglazing.76

Where overglazing is not practical, fine polish-ing with diamond pastes helps remove surfaceflaws and gives a modest increase in strengthover self-glazing.

RESIN COMPOSITESThe use of resin composites for indirect inlaysand onlays is well known.77 Recently, manu-facturers have introduced highly filled com-posite materials for making indirect crownsand bridges. Clearly the major advantage is inreduced laboratory costs, but it should also beborn in mind that modern composites inducemuch less wear against opposing teeth thanporcelain. It is too early to say whether thesematerials will perform well in the long-term,however they at very least offer a good solu-tion where restorations are not expected tolast for extended periods eg young patients.Materials falling into this category includeArtglass, and Targis and Vectris. The Vectismaterial is interesting in that it uses a silanat-ed glass fibre mat reinforcement for crownsand glass fibre strands to give strength tobridge spans.

Finally, several systems have been developedfor bonding composite to metal substructures.These include the Silicoater and Kevloc tech-niques which both require specialised laboratoryequipment. Restorations of this type are morepopular in continental Europe than in the UKwhere metal-ceramics predominate.

CONCLUSIONWith more and more materials being introducedit is important that dentists understand the vari-ety available and the factors which will con-tribute to the success or failure of their restora-tions. In the final analysis established orpromising materials should be subjected to ran-domised clinical trial. Several manufacturers areadopting this approach but more need to do so.While there is considerable clinical research dataon intra-coronal restorations much more isneeded on those placed extra-coronally.

1. O’Brien W. Evolution of dental casting. In: Valega T, editor.Alternatives to gold alloys in dentistry, Conf. Proc. DHEW Pub.No. [NIH]77-1227. Washington DC: U.S. GovernmentPrinting Office, 1977.

2. Craig G, O’Brien W, Powers J. Dental Materials properties andmanipulation. pp.236-250. St Louis: Mosby, 1983.

3. Cohen S, Kakar A, Vaidyanathan T, Viswanadhan T. Castabilityoptimization of palladium based alloys. J Prosthet Dent 1996;76: 125-131.

4. Marzouk M, Saleh L, Emanuel R, Malone W. Clinical behaviourof silver-palladium alloy castings: a five year comparativeclinical study. J Prosthet Dent 1991; 65: 19-26.

5. Kansu G, Aydin A. Evaluation of the biocompatibility ofvarious dental alloys: part II - allergenical potentials. Eur JProsthodont Rest Dent 1996; 4: 155-161.

6. Mulders C, Darwish M, Holze R. Corrosion behaviour ofdental alloys - an in-vitro study. J Oral Rehabil 1996; 23:825-831.

7. Frykholm K, Frithiof L, Fernström A, Moberger G, Blohm S,Björn E. Allergy to copper derived from dental alloys as apossible cause of oral lesions of lichen planus. ActaDermato-Venereologica 1969; 49: 268-281.

8. Smart E, Macleod R, Lawrence C. Resolution of lichen planusfollowing removal of amalgam restorations in patients withproven allergy to mercury salts: a pilot study. Br Dent J 1995;178: 108-112.

9. Pierce L, Goodkind R. A status report of the possible risks ofbase metal alloys and their components. J Prosthet Dent1989; 62: 234-237.

10. Northeast S E, van Noort R, Johnson A, Winstanley R B, WhiteGE. Metal-ceramic bridges from commercial dentallaboratories: alloy composition, cost and quality of fit. BrDent J 1992; 172: 198-204.

11. Nery S, McCabe J F, Wassell R W. A comparative study ofthree dental adhesives. J Dent 1994; 23: 55-61.

12. Weiss P. New design parameters: utilising the properties ofNi-Cr super alloys. Dent Clin N Am 1977; 21: 749.

13. Kelly J, Rose T. Nonprecious alloys for use in fixedprosthodontics: a literature review. J Prosthet Dent 1983; 49:363-370.

14. Equipment. CoDMIa. Report on base metal alloys for crownand bridge applications: benefits and risks. J Am Dent Assoc1985; 111: 479-483.

15. Wiltshire W, Ferriera M, Ligthelm A J. Allergies to dentalmaterials. Quintessence Int 1996; 27: 513-520.

16. Ida K, Togaya T, Tsutsumi S, Takeuchi M. Effect of magnesiainvestments in the dental casting of pure titanium ortitanium alloys. Dent Mater J 1982; 1: 8-21.

17. Ida K, Tani Y, Tsutsumi S. Clinical application of pure Ti-crowns. Dent Mater J 1985; 4: 191-195.

18. Andersson M, Bergman B, Bessing C, Ericson G, Lundquist P,Nilson H. Clinical results with titanium crowns fabricatedwith machine duplication and spark erosion. Acta OdontolScand 1989; 47: 279-286.

19. Bergman B, Bessing C, Ericson G, Lundquist P, Nilson H,Andersson M. A two year follow up study of titanium crowns.Acta Odontol Scand 1990; 48: 113-117.

20. Michalske T, Freiman S. A molecular interpretation of stresscorrosion in silica. Nature 1982; 295: 511-512.

21. McClean J, Hughes T. The reinforcement of dental porcelainwith ceramic oxides. Br Dent J 1965; 119: 251-267.

22. Denry I. Recent advances in ceramics for dentistry. Crit RevOral Biol Med 1996; 7: 134-143.

23. Jones D, Wilson H J. Some properties of dental ceramics. JOral Rehabil 1975; 2: 379-396.

24. Anusavice K, Zang N, Moorhead J. Influence of colorants oncrystallization and mechanical properties of lithia-basedglass ceramics. Dent Mater 1994; 10: 141-146.

25. Denry I, Rosensteil S. Flexural strength and fracture

RESIN COMPOSITESKEY POINT:• Improvements in

technology (includingfibre filler) look promising but requirelong-term evaluation

PRACTICE


toughness of Dicor glass-ceramic after embedmentmodification. J Dent Res 1993; 72: 572-576.

26. Geller W K, Kwiatkowski S J. The Willi’s glass crown: a newsolution in the dark and shadowed zone of aestheticporcelain restorations. Quintessence Dent Tech 1987; 11:233-242.

27. Al-Shehri S, Mohammed H, Wilson C. Influence of laminationon the flexural strength of a dental castable glass-ceramic. JProsthet Dent 1996; 76: 23-28.

28. Dong J, Luthy H, Wohlwend A, Sharer P. Heat-pressedceramics: technology and strength. Int J Prosthodont 1992;5: 9-16.

29. Qualtrough A, Piddock V. Dental CAD/CAM: A millstone or amilestone? Dent Update 1995; 22: 200-204.

30. Mormann W, Krejci K. Computer designed inlays after 5 yearsin situ: clinical performance and scanning electronmicroscopic evaluation. Quintessence Int 1992; 23: 109-115.

31. Sjögren S, Bergman M, Molin M, Bessing C A. A clinicalexamination of ceramic (Cerac) Inlays. Acta Odontol Scand1992; 50: 171-178.

32. Isenberg B P, Essig M E, Leinfelder K F. Three year clinicalevaluation of CAD/CAM restorations. J Aesthet Dent 1992; 4:173-176.

33. Inokoshi S, B. VM, Willems G, Lambrechts P, Braem M,Vanherles G. Marginal accuracy of CAD/CAM inlays madewith the original and the updated software. J Dent 1992; 20:171-177.

34. Thompson J, Bayne S, Heymann H. Mechanical properties ofnew mica-based machinable glass ceramic of CAD/CAMrestorations. J Prosthet Dent 1996; 76: 619-623.

35. Tinschert J, Zwez D, Marx R, Anusavice K J. Structuralreliability of alumina-, feldspar-, leucite-, mica- andzirconia-based ceramics. J Dent 2000; 28: 529-535.

36. Wagner W, Chu T. Biaxial flexural strength and indentationtoughness of three new dental core ceramics. J Prosthet Dent1996; 76: 140-141.

37. Brunton P A, Smith P, McCord J F, Wilson N H F. Procera all-ceramic crowns: a new approach to an old problem. Br Dent J1999; 186: 430-434.

38. ISO. International standard 6872-1984. Dental Ceramic. 1sted.pp.1-14. Geneva, Switzerland: International Organisationfor Standardization, 1984.

39. Ban S, Anusavice K J. Influence of test method on failurestress of brittle dental materials. J Dent Res 1990; 69:1791-1799.

40. McClean J, Jeansonne E, Chiche G, Pinault A. All ceramiccrowns and foil crowns. In: Chiche G, Pinault A, editors.Esthetics of anterior fixed prosthodontics. pp.97-113.Chicago: Quintessence Publishing Co., Inc., 1994.

41. Brandson S J, King P A. The impact fracture resistance ofrestored endodontically treated anterior teeth. J Dent Res1992; 72: 1141.

42. Madani M, Chu F C, McDonald A V, Smales R J. Effects ofsurface treatments on shear bond strengths between a resincement and an alumina core. J Prosthet Dent 2000; 83: 644-647.

43. Blixt M, Adamczak E, Linden L A, Oden A, Arvidson K. Bondingto densely sintered alumina surfaces: effect of sandblastingand silica coating on shear bond strength of luting cements.Int J Prosthodont 2000; 13: 221-226.

44. Karlsson S. The fit of Procera titanium crowns. Acta OdontolScand 1993; 51: 129-134.

45. McClean J W, von Fraunhofer J A. The estimation of cementfilm thickness by an in vivo technique. Br Dent J 1971; 131:107-111.

46. Christensen G J. Marginal fit of gold inlay castings. J ProsthetDent 1966; 16: 297-305.

47. White S N, Kipnis V. Influence of marginal opening onmicroleakage of cemented artificial crowns. J Prosthet Dent1994; 71: 257-264.

48. Björn AL, Björn H, Grkovik B. Marginal fit of restorations andits relation to periodontal bone level. Odont Rev 1970; 21:337-346.

49. 147/242. PoCN, Morris H F. Department of Veterans AffairsCooperative Studies Project No. 242. Quantitative andqualitative evaluation of cast ceramic, porcelain-shoulder,and cast metal full crown margins. J Prosthet Dent 1992; 67:198-203.

50. Holmes R J, Sulik W D, Holland G A, Bayne S C. Marginal fit ofcastable ceramic crowns. J Prosthet Dent 1992; 67: 594-599.

51. Castellani D, Baccetti T, Clauser C, Bernadini U D. Thermaldistortion of different materials in crown construction. JProsthet Dent 1994; 72: 360-366.

52. Shearer B, Gough M B, Setchell D J. Influence of marginalconfiguration and porcelain addition on the fit of In-Ceram

crowns. Biomater 1996; 17: 1891-1895.53. Ratledge D, Smith B, Wilson R. The effect of restorative

materials on the wear of human enamel. J Prosthet Dent1994; 72: 194-203.

54. Monasky G, Taylor D. Studies on the wear of porcelain,enamel and gold. J Prosthet Dent 1971; 25: 299-306.

55. Seghi R, Rosensteil S, Bauer P. Abrasion of human enamel bydifferent dental ceramics in vitro. J Dent Res 1991; 70: 221-225.

56. Krejci I, Lutz F, Reimer M. Wear of CAD/CAM ceramic inlays:restorations, opposing cusps and luting cements.Quintessence Int 1994; 25: 199-207.

57. Hacker C H, Wagner W C, Razoog M E. An in-vitroinvestigation of the wear of enamel on porcelain and gold insaliva. J Prosthet Dent 1996; 75: 14-17.

58. Al-Hiyasatt A S. An investigation of the wear of humanenamel and dental ceramics. [PhD]. University of Glasgow,1997.

59. Probster L. Four year clinical study of glass infiltrated,sintered alumina crowns. J Oral Rehabil 1996; 23: 147-151.

60. McLaren E A, White S N. Survival of In-Ceram crowns in aprivate practice: a prospective clinical trial. J Prosthet Dent2000; 83: 216-22.

61. Oden A, Andersson M, Krystek-Ondracek I, Magnusson D.Five-year clinical evaluation of Procera AllCeram crowns. JProsthet Dent 1998; 80: 450-6.

62. Brecker S C. Porcelain baked to gold: a new medium inprosthodontics. J Prosthet Dent 1956; 6: 801-810.

63. Weinstein M, Weinstein A. Ney Company, assignee. FusedPorcelain-to-metal teeth. US Patent 3,052,982, September11. 1962.

64. Pang I, Gilbert J, Chai J, Lautenschlager E. Bondingcharacteristics of low-fusing porcelain bonded to puretitanium and palladium-copper alloy. J Prosthet Dent 1995;73: 17-25.

65. Walter M, Reppel P D, Boning K, Freesmeyer W B. Six-yearfollow-up of titanium and high-gold porcelain-fused-to-metal fixed partial dentures. J Oral Rehabil 1999; 26: 91-96.

66. Mattmuller A, Wassmann J, Biffar R. Hydrothermal ceramicfor porcelain-fused- to-metal crowns: an initial experiencereport from clinical practice. Quintessence Int 1996; 27:521-526.

67. McClean J, Sced I. The bonded alumina crown. 1. The bondingof platinum to aluminous dental porcelain using tin oxidecoatings. Aust Dent J 1976; 21: 119-127.

68. McClean J, Kedge M, Hubbard J. The bonded alumina crown.2. Construction using the twin foil technique. Aust Dent J1976; 21: 262-263.

69. Philp G, Brukl C. Compressive strengths of conventional, twinfoil, and all-ceramic crowns. J Prosthet Dent 1984; 52:215-220.

70. Piddock V, Marquis P, Wilson H. Comparison of the strengthsof aluminous porcelain fired onto platinum and palladiumfoils. J Oral Rehabil 1991; 13: 31-37.

71. Southan D. Defects in porcelain at the porcelain-to-metalinterface. In: Yamada H, Grenoble P, editors. Dental porcelain:the state of the art -1977. pp.48-49. Los Angeles: Universityof Southern California, 1977.

72. Hopkins K. A method of strengthening aluminous porcelainjacket crowns. Br Dent J 1981; 151: 225-227.

73. Hummert T, Barghi N, Berry T. Effect of fitting adjustments oncompressive strength of a new foil crown system. J ProsthetDent 1991; 66: 177-180.

74. Hummert T, Barghi N, Berry T. Postcementation marginal fitof a new ceramic foil crown system. J Prosthet Dent 1992;68: 766-770.

75. Shoher I, Whiteman A. Captek - A new capillary castingtechnology for ceramometal restorations. Quintessence DentTech 1995; 18: 9-20.

76. Giordano R A, Campbell S, Pober R. Flexural strength offeldspathic porcelain treated with ion exchange, overglaze,and polishing. J Prosthet Dent 1994; 71: 468-472.

77. Burke F, Watts D, Wilson N, Wilson M. Current status andrationale for composite inlays and onlays. Br Dent J 1991;170: 269-273.

78. EH40/97. Occupational exposure limits 1997 for use with TheControl of Substances Hazardous to Health Regulations1994. pp.14-17. Sudbury: HSE Books, 1997.

79. Seghi R, Daher T, Caputo A. Relative flexural strength ofdental restorative ceramics. Dent Mater 1990; 6: 181-184.

80. Seghi R, Sorensen J. Relative flexural strength of six newceramic materials. Int J Prosthodont 1995; 8: 239-246.

81. Wohlwend A, Strub J, Scharer P. Metal ceramic and allporcelain restorations: current considerations. Int JProsthodont 1989; 2: 13-26.

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List of products mentioned in the text

Alceram Innotek Dental Corp., Lakewood, USA

Artglass Heraeus Kulzer, Wehrheim, Germany

Captek Schottlander, Letchworth, UK

Cerapearl Kyocera, San Diego, USA

Corning MGC-F Corning Inc., New York, USA

Dicor Corning Inc., New York, USA

Dicor MGC Corning Inc., New York, USA

Dicor Plus Corning Inc., New York, USA

Duceratin Degussa AG, Frankfurt, Germany

Duceram-LFC Degussa AG, Frankfurt, Germany

Duceragold Degussa AG, Frankfurt, Germany

GES Gold Gramm Technology, 75233 Tiefenbronn, Germany

Hi-Ceram Vita Zahnfabrik, D-79704 Bad Säckingen, Germany

In-Ceram Vita Zahnfabrik, D-79704 Bad Säckingen, Germany

In-Ceram Spinnel Vita Zahnfabrik, D-79704 Bad Säckingen, Germany

In-Ceram Zirconia Vita Zahnfabrik, D-79704 Bad Säckingen, Germany

IPS Empress Ivoclar-Vivadent, Schaan, Liechtenstein

Kevlock Heraeus Kulzer GmbH, Wehrheim/Ts, Germany

Mirage II Myron International, Kansas City, USA

Optec HSP Jeneric/Pentron Inc.,Wallingford, USA

Procera AllCeram Nobel Biocare, Göteburg, Sweden

Procera Porcelain Nobel Biocare, Göteburg, Sweden

Silicoater Heraeus Kulzer GmbH, Wehrheim/Ts, Germany

Sunrise Foil Crown Tanaka Dental Products, Skokie, USA

Targis/Vectris Ivoclar-Vivadent, Schaan, Liechtenstein

Techceram Techceram Ltd., Shipley, UK

Vitadur-N Vita Zahnfabrik, D-79704 Bad Säckingen, Germany

Vita Mark II Vita Zahnfabrik, D-79704 Bad Säckingen, Germany

BRITISH DENTAL JOURNAL VOLUME 192 NO. 5 MARCH 9 2002 257

Crowns and extra-coronal restorations:Considerations when planning treatment D. J. Jacobs1 J. G. Steele2 and R. W. Wassell3

Considerations when planning treatment is the third in the series of crowns and other extra-coronal restorations. Articles orchapters on treatment planning in restorative dentistry can make pretty dry reading, often built around a list of factors thatmight influence your decision-making. In truth though, planning and placing crowns or other extra-coronal restorationscannot be distilled into a series of lists. The decision-making involved requires experience, subtle understanding and a flexibleapproach, none of which come easily.

1*Consultant in Restorative Dentistry,Newcastle Dental Hospital, RichardsonRoad, Newcastle upon Tyne NE2 4AZ2Senior Lecturer in Restorative Dentistry,3Senior Lecturer in Restorative Dentistry,Department of Restorative Dentistry, The Dental School, Framlington Place,Newcastle upon Tyne NE2 4BW*Correspondence to: D. J. JacobsE-mail: [email protected]


● A long-term view of crown provision is essential (ten years or more)● Meeting the patient’s expectations is a critical measure of success● The patient’s ability to tolerate treatment and maintain the restorations is a key factor

when planning treatment● One's own ability to provide high quality crowns requires honest reflection in every case● The damage to tooth tissue when providing crowns is considerable and should be weighed

against the benefits● Control of the environment to minimise disease and damage is a fundamental part of the treatment plan● The biological and biomechanical safety of the restorations is taken as a prerequisite

I N B R I E F

No crowns can really be regarded as perma-nent. If we lived for long enough, wear andtear, disease and the realities of intra-oral exis-tence mean that even the most carefully con-structed and cemented crown would probablyeventually fail. We should though expect to getmany useful years from our crowns, andshould plan to have a situation we can recover,if and when they eventually fail. This articleaims to address the issues of planning, both byplanning to avoid failure and also by planningto cope with failure. It will address both thetreatment plan itself and the planned deliveryof treatment (these are subtly different things),and it aims to set the scene for the moredetailed and specific analysis of treatmentplanning and delivery issues within the rest ofthis series of articles.

Learning is most effective when you learnfrom your mistakes, but this can be a painfulprocess, both for patient and operator. Here wewill use examples where mistakes have beenmade (some quite close to home) to illustratethe points we are trying to make. Take a fewmoments to look at the illustrations of the twocases shown, and read the text in the boxes.

Many of the cases that fail miserably sufferfrom decisions made right at the beginning. In other cases the decisions are sound but theexecution is the problem. These two caseswere abject failures at several points and illus-trate, perhaps in a rather extreme way, some ofthe fundamentals of planning. One was donein a dental hospital, the other in a practice. Bad planning and bad execution were contributorsin both. We will refer to these two cases as wego through.

THE LONG-TERM STRATEGY: WHAT WILL THISDENTITION BE LIKE IN 10 YEARS?A long-term strategy is different from a treat-ment plan. It is temptingly easy to focus immedi-ately at the level of the tooth, discussing itscrown height, pulp vitality or its role in theocclusion. Whilst these detailed ‘close focus’issues are all critically important if a crown is tosurvive, they are not as important as making adecision about treating the tooth in the contextof the rest of the dentition, and about managingthe rest of the dentition in the context of theindividual.

Look at Case 1 (see box overleaf). This failedfor many reasons, including poor technical out-comes and perhaps insufficient attention tohygiene and diet. But as you look at the firstradiograph, ask yourself this question: ‘if thispatient attended my surgery now, what condi-tion would I expect this dentition to be in, in10 years time?’ The answer might be ‘edentu-lous’, it may be that there will be a couple ofremaining teeth and an overdenture, but theanswer probably is not ‘the same as it is now’.Given the widespread caries and the lack ofremaining sound tooth tissue, tooth loss herewas always very likely. Had this been acknowl-edged right at the beginning, rather than tryingto save and re-crown every tooth (and on a ‘closefocus’ tooth-by-tooth basis this seemed quitepossible no doubt), then a reasonable end stagewould have been reached more quickly, moreefficiently and less traumatically. Who knows,implants may not have been necessary if thetransition to edentulousness had been planned,or a couple of overdenture abutments had beenidentified at the start. If you ask yourself: ‘What

3

CROWNS AND EXTRA-CORONALRESTORATIONS:1. Changing patterns and

the need for quality2. Materials considerations3. Pre-operative








restorations

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258 BRITISH DENTAL JOURNAL VOLUME 192 NO. 5 MARCH 9 2002

CASE 1In 1989 this patient had all of herlower teeth crowned.

Two years later there was evidenceof caries around the margins of sev-eral of them (Fig. 1).

A further decision was made to roottreat all of the teeth, initially with aview to restoring them with crownsand this treatment was started a shorttime later, initially leaving the crownsin place (Fig. 2).

The root treatments were under-taken, but within a year they beganto fail because, among many otherreasons, it was proving very difficult toensure a coronal seal (Fig. 3), in fact itis doubtful whether this biological pre-requisite to successful endodonticshad been considered at all.

As no progress was being made(things were actually getting worse),the decision was made to revert to anoverdenture (Fig. 4).

Within another year even the over-denture abutments became mobileand infected (Fig. 5) and in the endthey too were removed, leaving thepatient with a denture which shecould not wear.

The end result, a further 2 yearsdown the line, was the placement offour implants and a very successfullower implant retained fixed pros-thesis (Fig. 6).

The whole case cost several thou-sand pounds to manage, much ofwhich was used to provide treatmentwhich soon failed.

Fig. 4

Fig. 5

Fig. 6

Fig. 1

Fig. 2

Fig. 3

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will this be like in 10 years, or 20?’ and cananswer, a sensible strategy for how you want toget there can be developed.

Essentially this is gambling, a game of odds.You cannot possibly hope to know what thingswill be like in 10 years, can you? Perhaps not,but as a professional you are better placed toassess the ‘form’ of the dentition than anyoneelse. Often, it is unwise to develop your finalstrategy straight away, before you have seen theresponse to basic preventive measures such ashygiene and dietary management. Increasinglywe find that the ‘one off’ treatment plan is actu-ally inappropriate, as it was in the case inFigure 7 where isolated coronal restorations andbridgework have been provided in an environ-ment of generalised toothwear. This clearly war-ranted fuller investigation and an overall man-agement strategy. Taking your time andplanning in stages, will often improve yourchances of winning this game of odds.

With a strategy in place, it is now time to asksome more specific questions, starting withsome issues relating to the patient as a wholebefore finally moving on to technicalities asso-ciated with the patient’s mouth and teeth.

CASE SELECTION: THE PATIENT

Can I meet the expectations of the patient?The first fundamental issue that you need toresolve with the patient is whether your expecta-tions and theirs converge. It is all very well beingable to sell something to your patient, but youhave to be able to deliver and, in the long term,‘expectation modification’ can be a key skill. Theissue of expectations most often arises in termsof aesthetics (see Part 6 in this series) thoughthere are issues in other areas too. We know thatpatients’ expectations for tooth retention areincreasing all the time,1 and this applies as muchto treatment decisions on individual teeth as itdoes to the dentition as a whole. Where you real-ly feel that expectations cannot be met or modi-fied it is better to make an appropriate referral atthe beginning than risk later retribution.

Will the patient be able to tolerate thetreatment and then maintain the restorations?Planning some restorations based on the techni-calities of pulp state, angulation, crown heightand so on is all very well, but if the patient can-not lie flat for more than 20 minutes, or cannotopen their mouth beyond 2 centimetres, actuallyproviding treatment may cause you some prob-lems. It is surprisingly easy to forget to thinkthese things through before lifting a handpiece. Physical limitations, such as neuromuscular orskeletal disorders, may also prevent patientsfrom maintaining their restorations, no matterhow devoted they are to good hygiene. Further-more, it is not simply the state of hygiene youencounter when you see the patient for the firsttime that matters, it is also how you see thembeing able to cope in 5 or 10 years. This may notbe an issue for young or middle-aged patients,

but for an increasingly dentate and demandinggroup of older adults it is highly relevant. Thetreatment planning strategy of ‘shortened den-tal arch’ (SDA) is built around the differences inthe ease with which different types of tooth canbe maintained and cleaned. Anterior teeth areeasier for the dentist to access for treatment andlong-term maintenance and for the patient toclean. Molars on the other hand are difficult.They have multiple roots with furcationsbetween them, as well as fine and curved rootcanals. Their inaccessible location means that, ifthere are real problems cleaning, these are theteeth most likely to suffer. SDA gives priority tothe anterior and premolar teeth to ensure thatlimited resources are targeted to the teeth whichhave the best chance of long-term survival, andwhich will provide adequate mechanical andaesthetic function.

My own skills: Can I do this to a high enoughstandard?This can be a difficult question to face up to butit is a question that should be answered honestly.Tackling a complex case requiring multiplecoronal restorations with an excess of bravadoand a lack of insight and understanding of thepotential pitfalls can lead to disaster. Goodrestorative work is more than being able to cutsome shapes and stick on crowns. Dentistry isdifficult. The knowledge and experience, not tomention the technical skills that are required tomake the complex judgements necessary to planand then carry out treatment take a long time toacquire. Other factors, which enter the equationand may affect the outcome, include the avail-ability of adequate treatment facilities, materialsand appropriate technical support. Ultimatelythe decision is yours, but there is usually morethan one treatment option and simpler alterna-tives may be more successful in the long-term.

Can I justify the damage I am going to do tothe patient’s teeth? Look at Case 2 again (see box overleaf). This is acase where a decision was made to place crowns onperfectly sound teeth. Admittedly, the clinical workwas poor, but the resultant damage was almost ter-minal for the teeth. This was a planning failureevery bit as much as a clinical failure. It is easy tobe seduced by the technical possibilities and to forget the biological realities when creating a treat-ment plan. In a recent review of patients visiting aUK dental hospital, 19% of all crowned teethwithout root fillings had evidence of periapical

Fig. 7 ‘isolated coronal restorationsand bridgework with generalisedtoothwear’

PRACTICE


CASE 2This 18-year-old female patientattended Newcastle DentalHospital requesting treatment toimprove the appearance of herupper anterior teeth which werechipped as a result of trauma withUL1 (21) having been root filledand discoloured (Fig. 8). She wasplaced on a waiting list for con-servative management involvingthe provision of a labial porcelainveneer to UL1 (21) and incisalcomposite restorations to UR1 (11),UR2 (12) and UL2 (22).

Inevitable delays with treatmentat the Dental Hospital led to thepatient seeking treatment else-where. She did, however, return tothe Dental Hospital some 5 yearslater with PJCs of poor quality on allthe upper anterior teeth and irre-versible pulpitis in UR2 (12) andUL2 (22) (Fig. 9). A further treat-ment plan was formulated involvingendodontics to UR2 (12) andUL2 (22) followed by replacementcrowns for the upper anterior teeth.Once again, Dental Hospital waitinglists resulted in the patient obtain-ing treatment elsewhere. A further8 years later, she was referred backto the Dental Hospital by her latestdentist who was suitably horrifiedby what he found! The results of 13years of treatment were six poorcrowns with carious margins,unrootfilled or inadequately root-filled teeth, short or perforatingposts and several teeth of verydoubtful prognosis (Fig. 10). In sum-mary, an unnecessarily mutilateddentition.

Fortunately, remedial treat-ment from her own dentist waspossible in this case and the resultis much better than could havebeen hoped for initially (Fig. 11).This is also a very good illustrationof what can be achieved in theGeneral Dental Services underideal circumstances although it isimportant to note that the treat-ment required a further 17 visitsover a 9-month period, includingtwo surgical procedures, and thelongevity of the restorationsremains unpredictable.

Fig. 8

Fig. 9

Fig. 10

Fig. 11

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pathology.2 For a number of technical reasons it isvery difficult to get an accurate indication of justhow often teeth die as a result of crown prepara-tion, but this finding and other published evidencesuggests it is probably a fairly common occurrence,unless the technical quality of the work is of a veryhigh standard.3,4 Furthermore, the tooth itself isoften weakened by preparation, and fracture of thetooth at gum level is not uncommon. Crown prepa-ration on virgin teeth is not something to be under-taken lightly.

What is the main lesson to be learnt fromCase 2? We have already stated in the introduc-tion that all crowns may eventually fail but thepoor standard of clinical work in this instancehas certainly resulted in unacceptable early anddamaging failure. The message is clear that ifthere are simpler, less invasive, but effectivemeans of achieving the desired result, as therewere in this case, these should always be consid-ered before embarking upon more complextreatments which may actually accelerate theloss of the dentition.

CASE SELECTION: THE MOUTH AND TEETH

The oral environment: Can I, and the patient, control the environment to minimise the risk of disease or damage in the long term?It is tempting sometimes to want to crack on anddo the difficult, skilled and lucrative technicalthings, to see the final result, and to witness thetransformed smile. The new crowns though areonly as good as their environment. Sometimes ittakes a lot of time to sort out the environment,for little immediate reward. At other times youjust have to work within a difficult environment;for example it may be rather difficult to stop abruxist from grinding and you may need to planaround this. Usually you have to accept less thanperfection, often much less, but plaque control,caries risk and the occlusion are at the heart ofsuccessful crown work. All can be managed tosome degree. If you should be in any doubtabout the importance of managing the environ-ment, look again at Case 1 where neither plaquecontrol, nor caries management were properlydealt with. The result was time consuming andexpensive failure.

Is the plaque control good enough?Little needs to be said about plaque. We knowthat it is a fundamental factor in both caries andperiodontal disease. In simple terms, plaquearound restorations is likely to shorten their life.Tooth cleaning is a surprisingly complexprocess, and sometimes we forget this or eventrivialise it. Helping people to find the skills tokeep their own teeth clean is probably the leasttrivial thing dentists do.

It is truly remarkable that any form of opera-tive dentistry should be considered in an envi-ronment like this (Fig. 12), let alone the provisionof crowns that have been carefully contoured tofollow the aberrant gingival margins! The pres-

ence of marginal deficiencies in an environmentwhere plaque control is inadequate can result inrapid caries progression and potential tooth loss(Fig. 13). Contrast this situation with Figure 14which illustrates poor open crown margins butwhere caries is minimal. Again, this quality oftreatment cannot be condoned but the savinggrace has been the patient’s maintenance of ahigh standard of plaque control.

It is the dentist’s responsibility to provide thehighest standard of technical work possible, butalso to communicate to the patient the impor-tance of good hygiene and maintenance. Thepatient’s responsibility is to follow that adviceon the understanding that they may directlyinfluence the longevity of their dental restora-tions. The fact that they may have made a con-siderable investment in terms of time andexpense for their dental treatment represents apowerful incentive to comply with future pre-ventive measures.

Has the risk of caries in the future beenaddressed? Caries risk is directly related to both plaque anddiet, and also (in the case of root caries) to thepresence of a partial denture.5–7 The oral healthprofile of most western populations is such thatour crown patients of the next few decades will,

Fig. 12 ’poor oral hygiene andcrowns’

Fig. 13 ‘deficient margins andrecurrent caries’

Fig. 14 ‘deficient margins with goodplaque control’

PRACTICE


for the most part, be a steadily ageing band.1

Caries management is an issue at all ages, but inthis cohort root caries is likely to be an increas-ing threat. This is a wretched disease to dealwith. The best crown in the world, with an unde-tectable margin, perfect occlusion and stunningaesthetics will still fail if the risk of root caries isnot addressed. In this case (Fig. 15), the risk ofcaries around the upper anterior restorations hasclearly not been properly addressed with rootcaries and progressive periodontal destructionresulting. Treatment has involved the mechani-cal repair of caries at the crown margins (and insome areas the repair of the repairs) with little orno emphasis on prevention. The solution formanaging root caries is biological not mechani-cal. It requires simple communication abouthygiene and attention to some ‘little things’ inthe diet, such as the odd teaspoon of sugar inregular cups of tea or coffee, biscuit nibbling anda range of other apparently innocuous activitieswhich patients may not associate with a problem.If you are in any doubt as to how important thisis, just scan Case 1 again. The dentist has at his orher disposal a full armamentarium of approaches,including hygiene and dietary advice and theappropriate use of fluorides and varnishes.Caries, particularly root caries, is preventable. Ifyour crowns fail because of caries it is partly afailure of your own management.

Has the risk of damage from the occlusionbeen minimised?A few basic checks and a little thought and careshould eliminate the risk of problems in the largemajority of cases. It is important that you knowwhere the contacts are on the tooth you are totreat, especially those that are involved in guid-ing the jaw movement in lateral and protrusiveexcursions. It is also important to know whichother teeth are involved in guidance, andwhether the tooth you are about to prepare is adeflective contact or interference to guidance.These may sound complex but are readilychecked and there is rarely a problem. Tech-

niques for occlusal examination and adjustmentare dealt with in Part 5.

Figure 16 shows a de-bonded anterior postcrown. Many kilos of zinc phosphate must be used and many unnecessary hours spent re-cementing such restorations. Often the blameis put on a short or non-retentive post (whichthis tooth shows), but in our experience a largeproportion dislodge because inadequate care istaken to ensure that it is not providing all of theanterior guidance. Look at the profile of thepalatal surface of this crown, it is bulky and con-vex and it is difficult to believe that this com-plied with the natural pattern of guidance pro-vided by the adjacent natural teeth. Althoughthe post is not long, the contour of the guidancesurface must have been a contributory factor.Any anterior tooth is likely to provide some ele-ment of protrusive guidance and you can andshould spend a few moments thinking abouthow to manage this right at the outset. Againdetails are given in Part 5.

The nature of the opposing tooth contacts isalso important in terms of the material used forthe occlusal/palatal surfaces of coronal restora-tions. In this case (Figs 17,18) there is already anongoing toothwear problem caused by erosionand the provision of upper anterior crowns withporcelain palatal surfaces, which have beenadjusted to make them even more abrasive, isnot a good example of planning and the poten-tial for further wear is self-evident.

The tooth: Are the foundations biologically andbiomechanically sound?The periodontal tissuesA tooth with any amount of attachment loss canbe crowned, but where the disease is advancedand uncontrolled it is a hopeless investment. Themajor issues though do not surround the risk to

Fig. 15 ‘root caries and attemptedrepairs’

Fig. 16 ‘de-bonded anterior postcrown’

Figure 17,18 ’worn dentition with porcelain abrasion’

PRACTICE


your crown from periodontal disease but the riskto the periodontal tissues from your crown. Poorcrown margins can certainly result in gingivalproblems (Fig. 19), as can an incorrect emergenceprofile. All crown margins should, ideally, beplaced supragingivally to avoid problems relatedto gingival inflammation. Where a subgingivalmargin is indicated, it is essential that the marginbe placed within the limits of the sulcus and thatthe biologic width is not encroached upon (Fig. 20). The biologic width is a band of approxi-mately 2 mm of supracrestal connective tissueattachment and junctional epithelium aroundevery tooth. If a restoration encroaches upon oreliminates this 2 mm band of attachment, aninflammatory response occurs and attachmentloss, apical migration and pocket formation mayresult. Contrast the healthy gingivae associatedwith the supragingival crown margin on toothUR1 (11) in Fig. 21 with the subgingival margin ontooth UL1 (21). Marginal position is something tobe planned in advance, and as a rule of thumb it iswise to minimise encroachment into the sulcus.

Apart from encouraging periodontal prob-lems, the subgingival placement of margins canalso make accurate impression recording difficultor impossible. In Figure 22 subgingival marginplacement has led to gingival inflammation,either as a result of biologic width encroachmentor poor marginal fit resulting from obvious diffi-culties with impressions, as in Figure 23. Poorlycontoured temporaries can also result in prob-lems with impressions because of poor gingivalcondition (Fig. 24). The replacement of suchrestorations with well fitting and contoured pro-visional restorations may need to be a plannedfirst step prior to definitive treatment.

Is the endodontic state stable enough to allow acrown to be planned?Managing the endodontic state can be a particu-larly difficult problem, in fact we have devotedan entire article to the subject (see Part 4). Generally speaking though, for vital teeth theissues are quite straightforward. The pulp is a liv-ing tissue and everyone’s life is easier where itremains a living tissue. Pulp protection is funda-mental to crown provision. Non-vital or pulpallyinvolved teeth that are not already filled alsousually make for a relatively straightforwardplanning decision, as the infection clearly needsto be managed. There are very few, if any, cir-cumstances where a crown should be placed overa tooth with an infected pulp where no attemptat endodontic treatment has been made.

The greatest clinical quandary arises withteeth that have already been root treated, butwithout a long-term resolution of the pathology,or where the technical quality of the root fillingis dubious. Data from the UK suggest that mostroot treated teeth would actually fall into the latter category.2,8,9 Endodontics can be difficultenough at first attempt, but a technically satis-factory result is even more difficult where anunsatisfactory attempt has already been made.Re-treatment decisions have to be made on a

Fig. 19 ‘gingival inflammationrelated to anterior crown margins’

Fig. 21 ‘biologic widthencroachment’

Biologic width

Fig. 20 ‘biologic width’

Fig. 22 ’subgingival margins andgingival inflammation’

Fig. 23 ‘poorly fitting anteriorcrowns resulting from inaccurateimpressions’

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tooth-by-tooth basis and will be influenced byyour own skill as an endodontist, or the access toa specialist in this demanding discipline. Theseare the foundations though, and you wouldthink twice before replacing a roof where there isdry rot in the rafters.

Part 4 of the series covers these issues in somedetail, including ways of restoring the core ofthe tooth using the root canal for retention.

Is the tooth itself strong enough to receive andretain a crown?A number of basic factors should be consideredbefore preparing any tooth for a crown. Theextent of an existing restoration will influencethe strength of the remaining tooth tissue andadequate retention of an extensive restoration isimportant to prevent complete coronal break-down, even with a crown in place. The patientwho enters the surgery with a crown containingan inadequate core or fractured tooth (usuallyneatly wrapped in tissue paper) is unfortunatelya common scenario.

A properly constructed core with appropriateretentive features on a sound, disease free toothcan usually prevent this situation. Previousendodontic treatment will also affect thestrength of the remaining tooth tissue, particu-larly where access cavities and coronal destruc-tion are extensive. Parts 4 and 7 in this seriescover the issues of core build up in detail, butdecisions about the need for a core are an inte-gral part of the planning process.

Will the preparation be retentive enough?Adequate clinical crown height is a critical fac-tor for the retention of full coronal restorations.Unless you plan to increase the occlusal verticaldimension, teeth with short clinical crowns willbe even shorter and potentially very unretentivefollowing crown preparation. This is particularlyrelevant for the restoration of worn teeth andalthough recent improvements in adhesive den-tistry have increased the options for enhancingretention, mechanical retention by appropriatetooth preparation remains essential for pre-dictable success and longevity of coronalrestorations. Specific design features to optimiseretention as well as crown lengthening will becovered in Part 8 in this series.

Is there enough space for my restoration?Tooth wear not only results in problems ofretention but also in providing adequate

occlusal space for your restoration. As we havejust discussed increasing the occlusal verticaldimension can obviate the need for occlusalreduction on the preparation and can provideadditional length that will improve retention,and often allows for improved aesthetics as well.Clearly, such an approach will involve extensiverestorations and could involve the whole of oneor both arches. However other strategies areavailable to make space.

The easiest way to provide sufficient clear-ance is to reduce the height of the opposingtooth, but whilst simple, this is often not advis-able. Another approach, which is conservative oftooth tissue, is to make a local temporaryincrease in vertical dimension to promote axialorthodontic tooth movements (eg a Dahl appli-ance, or an occlusal composite build-up). Thepatient needs to be informed that the requiredtooth movements can take several months andyou have to be careful to ensure that the built-upteeth are being loaded axially. This approach isbeing used increasingly on supra-occludingrestorations by at least one centre.10

Occasionally space can be created anteriorlyby adjusting the posterior teeth to eliminatedeflective contacts, which repositions themandible distally.

None of the above options should be under-taken lightly, but a detailed consideration fallsoutside the remit of this series. All of these tech-niques are well described in a recent article byDyer et al. (2001), which provides a useful intro-duction to contemporary methods of managingspace and problems of reduced crown height.11

What next? The staged treatment planAlthough a clear vision of the finished treatmentis indispensable at the outset we sometimes findthat the initial plan is inappropriate and changeshave to be made. This is because one or other ofthe questions listed above are answered in a dif-ferent way from the way you expected. Perhapshygiene continues to be a problem, or perhaps itimproves unexpectedly, maybe the core breaksdown or you find a root crack during endodon-tics. It pays to keep flexible, and to review theplan at key stages as you progress. Failure to dothis can leave you committed to a course ofaction which may become increasingly difficultto sustain and which may fail much sooner thananticipated.

It is always worth discussing the alterna-tive treatment options with your patientbefore starting work. In this way a change ofplan can be made at an early stage rather thancarrying on regardless and subsequently hav-ing to explain an expensive failure later.Anticipating problems is one of the key skillsof the experienced restorative dentist andcontingency planning is an essential elementof any complex treatment plan.

ConclusionIn the course of this article we have posed sevenkey questions to ask yourself before you finally

Fig. 24 ‘poor temporisation’

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plan treatment and pick up a handpiece. Suc-cess with crowns and other extra-coronalrestorations depends on many interacting fac-tors; technical issues related to tooth prepara-tion, the relationship with the pulp and peri-odontal tissues and occlusion have beenintroduced and will be covered in greater detaillater in this series. However, the emphasis hasbeen on planning for the future rather than pro-viding a short-term fix for single teeth in isola-tion. The needs and expectations of the patient

are key elements in the treatment planningprocess as is the importance of minimising fur-ther damage to the dentition and reducing therisk of disease in the future. These essential fac-tors could perhaps be summarised in one over-riding question before embarking on a course oftreatment : ‘will this patient’s oral health be bet-ter off when I have finished?’. If there are anydoubts about the answer to this question, theplan should be modified and an alternativeapproach considered.

1. Kelly M, Steele J, Nuttall N, Bradnock G, Morris J,Nunn J, Pine C, Pitts N, Treasure E, White D. AdultDental Health Survey: Oral Health in the UnitedKingdom in 1998. (2000) London: TSO.

2. Saunders W P, Saunders E M. Prevalence of periapicalpathoses associated with crowned teeth in a Scottishsub-population. Br Dent J 1998; 185: 137-140.

3. Valderhaug A, Jokstad, Ambjornsen E, Norheim P W.Assessment of the periapical and clinical status ofcrowned teeth over 25 years. J Dent 1997; 25: 97-105.

4. Bergenholtz G. Iatrogenic injury to the pulp in dentalprocedures: Aspects of pathogenesis, managementand preventive measures. Int Dent J 1991; 41: 99-110.

5. Steele J G, Sheiham A, Marcenes W, Fay N, Walls A WG. Clinical behavioural risk indicators for root caries inolder people. Gerodontol 2001; 18: 95-101.

6. Steele J G, Walls A W G, Murray J J. Partial dentures asan independent indicator of root caries risk in a groupof older adults. Gerodontol 1998; 14: 67-74.

7. Locker D. Incidence of root caries in an olderCanadian Population. Comm Dent Oral Epidemiol1996; 24: 403-407.

8. Dummer P M H. The quality of root canal treatmentprovided by General dental Services working withinthe general dental services of England and Wales. Part 1. Dent Profile 1997; 17: 1-5.

9. Dummer P M H. The quality of root canal treatmentprovided by General dental Services working within thegeneral dental services of England and Wales. Part 2.Dent Profile 1998; 19: 8-10.

10. Gough M B, Setchell D J. A retrospective study of 50treatments using an appliance to produce localisedocclusal space by relative axial tooth movement. Br Dent J 1999; 187: 134-139.

11. Dyer K, Ibbetson R, Grey N. A question of space:Options for the restorative management of wornteeth. Dent Update 2001; 28: 118-123.


Crowns and extra-coronal restorations:Endodontic considerations: the pulp, the root-treated tooth and the crownJ. M. Whitworth1 A. W. G. Walls2 and R. W. Wassell3

Endodontic considerations is the fourth in the series on crowns and other extra-coronal restorations. This article focusesstrongly on contemporary biological principles, and is not intended to provide a comprehensive review of commerciallyavailable materials and techniques. Principles are illustrated in a variety of clinical case scenarios.

1Senior Lecturer,2Professor,3Senior Lecturer in Restorative Dentistry,Department of Restorative Dentistry, The Dental School, Framlington Place,Newcastle upon Tyne NE2 4BW*Correspondence to: Dr J. M. WhitworthE-mail: [email protected]


● A consideration of the impact of crowning on the dental pulp, and the special issues involvedin crowning root-treated teeth.

● The damaging effects of crown preparation.● Assessment and preparation of root-treated and non root-treated teeth to minimize

endodontic complications.● Special considerations in the temporisation and restoration of root-treated teeth.

I N B R I E F

Crowns should not be made without considera-tion of the teeth which lay the foundations forthem. In this article, important principles areoutlined for the assessment of root-treated andnon root-treated teeth before crowning, avoid-ing endodontic complications during crown fab-rication, and special considerations in the tem-porisation and restoration of root-treated teeth.

Pulp morbidity in crowned teethDental pulp is the highly vascular, richly inner-vated soft tissue structure whose principal role istooth formation. But even after teeth are eruptedinto the mouth and fully formed, the dental pulpis not a redundant organ. Pulp tissue retains theimportant function of supporting its secretoryodontoblasts which lay down reparative dentinein defensive response to dental injuries through-out life. There is also some evidence that the pulpmay be involved in a pressure-receptive func-tion, limiting the possibility of damaging func-tional overload on teeth.1

If this were not sufficient justification to pre-serve healthy pulps, then the desire to dopatients no harm and to avoid the pain, swellingand suffering which often accompanies theinjury and demise of a pulp surely must be.

An insulating coverage of dentine and animpervious layer of enamel protect the pulps ofhealthy, intact teeth from injury. Crown prepara-tion places the pulp at risk in a number of ways.High speed stripping of hard tissue poses thethreat of pulpal overheating, with disturbance ofmicrocirculation, vascular stasis, thrombosis,reduced blood flow and internal bleeding.2 It also

opens a multitude of dentinal tubules that com-municate directly with the pulp. The deeper thedentine is cut, the more permeable it is,3 and themore vulnerable the pulp becomes to chemical,physical and microbial irritants. The microbialthreat presented by the oral flora is by far themost serious, and is capable of heralding intenseinflammatory changes, with micro-abscess for-mation and progressive pulpal necrosis.4,5

Although the pulp shows considerableresilience and is often capable of recoveringfrom irritation, the injuries induced can becomesignificant in the long term.5 Scarring as aresult of inflammation and repair interfereswith the nervous and vascular supply to the tissue6 and jeopardises its resistance to furtherinsult. It is important in this respect to recog-nise that crowns are rarely made for pristine,intact teeth. Rather, they are made to protectand restore teeth which have been damaged bywear, trauma, or cycles of caries and repair.After a lifetime of cumulative insult, crownpreparation can be the final straw, bringingpulpal breakdown (Fig. 1a), and the need forroot canal treatment.7

It is uncertain how many teeth lose vitality asa direct consequence of crown preparation.Bergenholtz and Nyman’s8 much quoted studyshowed that 9% of crowned teeth, comparedwith only 2% of uncrowned controls lost vitalityduring long-term review. None of this wasattributable to caries or other obvious causes,but the crowned teeth in this study did haveadvanced periodontal destruction, and wereinvolved in extensive, cross-arch bridgework.

4










restorations

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Extrapolation to the case of uncomplicated, single crowns is therefore difficult.

Even higher levels of pulp death were recordedby Felton et al.,9 where 13.3% of teeth restoredwith full coverage crowns, compared with 0.5%of unrestored controls lost vitality during the 3–30 year review period. But it was not possibleto derive from this report how many teeth hadsuffered further pulpal insult, such as recurrentcaries, which may have artificially inflated theadverse effects reported.

A more realistic estimation may be in theorder of 4–8% in the 10 years following activetreatment.10–13 This estimation does, however,assume that efforts were made to identify andmanage all obvious pulpal pathosis beforecrowns were made; an assumption that cannotalways be taken for granted in practice andwhich increases the likely incidence of unex-pected endodontic problems that need attentionat a later date.

The dangers of root-treating crowned teethExtra-coronal restorations do not rule out root-treatment or retreatment (Fig. 1b), but it shouldbe noted that working through a crown is alwaysmore difficult, and that damage can be done.

Just piercing the glaze of a porcelain crowndramatically reduces its strength,14 whilst cut-ting through a metal ceramic crown canweaken the porcelain bond and predispose tofracture. Vibration can disturb the cement luteof a casting and predispose leakage or loss,whilst rubber-dam clamps may crack and pitcervical porcelain15 and occasionally cause acrown to debond.

Once through the crown, the search for thepulp can be hazardous. Metal copings and coresobscure the pulp and prevent its location andassessment from preoperative radiographs. Thealignment and rotation of the crown may alsonot correspond to the underlying tooth, causingloss of orientation and misdirected cutting.Added to this, problems are compounded by limited entry of light and poor visibility.

All of this can leave the operator severelyweakening the core and vertical walls of thetooth in search for the pulp chamber and canal

Buccal

Fig. 2a Overcut and misdirected access through a crown grossly weakens thevertical walls of the preparation

Fig. 1a Pulpal breakdown and acuteapical periodontitis affecting arecently crowned mandibular firstmolar

Fig. 1b Root-treatment completedthrough the crown without seriousdamage

Fig. 2b Disorientated by the presence of a crown which had modestly realigned thetooth, the access cavity into this lower molar completely bypassed the pulp chamber tothe mesial and lingual. There were no less than five separate perforations

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openings (Fig. 2a). Catastrophic errors such asperforation are also possible (Fig. 2b).

Of equal importance is the damage that canbe done to patient confidence and trust if arecently crowned tooth becomes troublesomeand has to be accessed or the restorationremoved for endodontic treatment.16 As thecomplexity of the crown and bridgeworkincreases, so the consequences become moreserious. Replacing a single crown damagedduring access is one matter; replacing a largebridge which has suffered irreparable damageto one of its abutments is quite another.

It is certain that a small number of teeth willalways develop unexpected endodontic prob-lems after crowning,11 but it is also certain thatmany such instances can be avoided by carefulpreoperative workup.

PREVENTATIVE ENDONDONTICS — AVOIDINGTHE AVOIDABLEAll teeth scheduled to be crowned, whether theyare presumed to have healthy pulps or previous-ly root-treated should be thoroughly assessed atthe planning stage. In this way, potentialendodontic problems can be identified andaddressed and future embarrassment and trau-ma kept to a minimum.

Teeth presumed to have healthy pulpsPulps cannot be visualised directly to assesstheir health. Neither is there a single test whichwill reliably deliver this information in all cir-cumstances. We rely therefore on a combina-tion of fairly crude methods, including painhistory, clinical examination, special tests andradiographs to build a picture of pulp status. All teeth due to be crowned should betested systematically to give as clear a pictureas possible.

Pain historyA brief pain history reveals areas of the mouthsensitive to hot, cold or sweet, and teeth sensi-tive to chewing pressure, which require furtherexamination. Review of the case notes mayidentify teeth with a history of trauma, pulpalexposure or pulpitic symptoms. But this is notenough. Despite textbook accounts of classicaltoothaches, most injured pulps die quietly, and itis also known that pulpal pain is poorlylocalised. The absence of reported symptoms istherefore not proof of health, and further exami-nation is needed to identify inflamed andnecrotic pulps in need of treatment.

Clinical examinationIndications of pulp condition may come fromthe identification of caries, large or leakingrestorations, non-carious tooth tissue loss andtraumatic injury. Inspection and palpation ofsoft tissues overlying the apices of teeth to becrowned may reveal signs of endodontic patho-sis, including swelling, redness and dischargingfistulae. Non-vital teeth may appear slightlydarker than their neighbours, and periapically

involved teeth may be tender to biting pressureor gentle percussion.

Special testsCharacteristically, pulpal symptoms are diffi-cult for patients to localise, and require sys-tematic provocation and reproduction to iden-tify the offending tooth with certainty. Ifsensitivity is reported to hot or cold, the teethshould be challenged with that stimulus. Coldcan be applied with an ethyl chloride soakedcotton pledget, though ice sticks or proprietaryrefrigerants such as Endo-Frost (Reoko) cangive a more profound cold challenge to stimu-late the pulps of old or heavily restored teeth.Heat can be applied with a stick of warm guttapercha ‘temporary stopping’, taking care tocoat the tooth first with petroleum jelly to pre-vent the hot material from adhering. Electronictouch and heat instruments used in thermo-plastic gutta percha filling techniques can alsobe used to deliver a known and reproduciblethermal challenge.

As a general rule, thermal tests are morediscriminating of pulp condition than electri-cal.17 They should be repeated, and contra-lateral and adjacent teeth tested for reference.An exaggerated and lingering response mayindicate irreversible pulpal inflammation,whilst a consistent absence of response maysuggest pulp necrosis.

Pulp sensitivity tests are essential inpre-operative assessment, but their resultsshould not be taken in isolation, and shouldalways be interpreted with caution.

RadiographsPeriapical radiographs should be of diagnosticquality and taken by a paralleling technique. Ifthere is a discharging fistula, a gutta percha conesize 25 or 30 should be inserted to source theinfectious focus (Fig. 3). More than one film,taken at different angles, may be needed to visu-alise all roots and all root canals, and should beexamined for apical and lateral lesions ofendodontic origin. If root-treatment is indicated,an assessment should be made of the degree ofdifficulty this presents, and whether a pre-dictable, quality result is likely.

The size of a healthy, vital pulp should also benoted, especially if the reason for the crown is torealign the tooth. Heavy tooth reduction in suchcases may result in embarrassing unexpectedpulpal exposure.

Fig. 3 A gutta percha point insertedinto a discharging fistula locates itssource at the apex of the premolarbridge abutment

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Previously root-treated teethPreviously root-treated teeth also need criticalappraisal before crowning.

Clinical signs and symptoms of endodonticfailure obviously demand endodontic retreat-ment or surgical revision before a crown ismade, but even in the absence of symptoms,technically inadequate, short or poorly con-densed root fillings (Fig. 4) do not represent pre-dictable foundations and should be revised.Even if the resultant root filling is not betterextended, recleaning the canal system can onlyimprove the internal environment of the tooth,and make a better foundation for restoration.

A further group of unreliable teeth is thosewhere the root filling has been open to themouth through defective restoration margins orrestoration loss (Fig. 5). Saliva and micro-organ-isms migrate quickly alongside exposed root fill-ings,18 and whilst firm rules are difficult tomake, it is probably wise to revise all root fillingsthat have been exposed to the mouth for morethan a month, even if they appear technicallysatisfactory on radiographs. The appearance ofthe film in such cases denies the biological statusof the canal system.

Treatment planning and guidelinesTeeth identified as having necrotic pulps, orshowing signs of irreversible pulpal inflamma-tion should be root-treated before crowns areprepared. Elective root-canal treatment shouldalso be planned for teeth whose pulps are at seri-ous risk of exposure during crown preparation,and for teeth that cannot be restored withoutusing the pulp chamber and root canals forretention and support. Pulps accidentallyexposed during crown preparation should not becapped; root canal therapy should proceed.

Technically inadequate root fillings, androot fillings which have been exposed to themouth, should be replaced before crowning.This option is always preferable to retreatmentor corrective surgery after the event. Decisive-ness is also called for in the case of teeth whichare unrestorable, or which are of very poorprognosis. Procrastination rarely rewards thepractitioner and such teeth should be plannedfor extraction.

If there is doubt in a complex case over diag-nosis, or the ability of the practitioner to lay areliable endodontic foundation, referral for aspecialist opinion should be considered.

Detailed accounts of contemporary root-treatment and retreatment techniques arebeyond the scope of this article, and readers arereferred to standard texts.19,20 However, impor-tant guiding principles should be stated. Root-treatment or retreatment should sacrifice a mini-mum of structurally important tooth tissue,which is compatible with quality care. Accesscavities should not be over-cut and root canalsshould not be over-flared; to do so simply risksweakening the tooth further. Anterior teeth mayin fact be best accessed though the incisal andlabial surfaces to preserve tissue21 (Fig. 6).

Posterior teeth may be reduced out of occlu-sion at the start of treatment to minimise the riskfrom damaging wedging forces splitting theaccessed tooth, and the additional support of awell-fitting orthodontic band cemented withglass-ionomer cement may be considered forbadly broken-down and vulnerable teeth (Fig. 7).

After root-treatment is complete, provisionalrestorations should be in place for the minimumtime possible and should seal against the ingressof saliva and micro-organisms to the canal sys-tem, which will predispose failure. Temporaryrestorations in place for any time should alsoprovide the necessary protection and support toprevent tooth fracture. Further considerationwill be given later to the temporisation andrestoration of root-treated teeth.

Fig. 4 Pain and swelling associatedwith a tooth which was recentlycrowned without revising theunsatisfactory endodonticfoundation

Fig. 5 A technically satisfactoryroot-filling exposed to the mouththrough defective restorationmargins. Oral microbes will rapidlyinvade the canal system andcompromise the root-treatment.Endodontic retreatment should beconsidered before a new crown ismade

(a) (b)

Buccal

Fig. 6 a) Conventional access does not compromiseaesthetics, but sacrifices much lingual tissue.b) Labial/incisal access preserves tissue in teethscheduled to be crowned. The access can be aestheticallytemporised with composite resin

Fig. 7 A well-fitting orthodontic band provides supportand protection for a vulnerable posterior tooth

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MINIMISING DAMAGE TO TEETH WITHHEALTHY PULPSHaving now identified and treated those teethwith inflamed, necrotic or at risk pulps, theremaining teeth should be treated with allrespect to preserve their pulps in health. Fric-tional heat generated during crown preparationshould be strictly controlled. Although dentineis a good thermal insulator, damaging rises intemperature can occur, especially in prepara-tions involving substantial tissue removal. Highvolume, well focused water-cooling should beused at all times,22,23 and cutting strokes shouldbe intermittent and light.

Open dentinal tubules should also be managedwith care and respect. Cut dentine should not beover-dried with the 3 in 1 syringe, and smearplugs should not be removed by the carelessapplication of acidic astringents used in gingivalhaemorrhage control.

Bacteria and their metabolic by-products arethe greatest danger to pulp health,24,25 and itbehoves the practitioner to ensure that temporaryand permanent crowns fit accurately and providethe best possible marginal seal. The pre-sealing ofdentine with a resin-bonding agent may be ameans of reducing dentine permeability andenhancing pulp protection,26 though there arefew published data to date which specificallydemonstrate the effectiveness of this action inpreserving the pulps of crown-prepared teeth.

RESTORATION OF THE ROOT-TREATED TOOTH

General considerationsRoot-treated teeth are in a vulnerable state untilthey are permanently restored.

The risks they face fall into two major cate-gories:1. Fracture of remaining tooth tissue 2. Reinfection of the root canal from the mouth

These two considerations underpin theapproach to temporising and restoring all root-treated teeth.

Protecting root-treated teeth from fractureIt is widely held that root-treated teeth are brittle.Historically, this has been attributed to desicca-tion, or other physical changes in the dentine ofpulpless teeth, which reduce its toughness.

More recently, it has been recognised that thekey change is the loss of structural tissue whichis capable of holding the tooth together underfunctional load. This is especially so for posteri-or teeth, where wedging forces come to bear(Fig. 8). Conservative access preparation in anotherwise sound tooth may not be especiallydamaging, but endodontic access in combina-tion with the earlier loss of one or both marginalridges leaves the tooth at serious risk, even if itwas reduced from direct occlusal contact beforeendodontic treatment began.

For this reason, some form of cuspal coverageshould always be provided for root-treated pos-terior teeth,27 unless the endodontic access is the

LingualBuccal

Fig. 8 Wedging forces on theunprotected cuspal slopes of a root-treated posterior tooth

(a) (b)

(c)

(d)(e)

(f)

Ferrule

Ferrule

Fig. 9 The development of protective ferrules for anterior teeth: a) Moderate loss of tooth tissue — the post and core provide no protection, a ferrule is provided by thecrown; b) Moderate loss of tooth tissue — bevelling of the residual tooth tissue allows the core as well as the crown to provide protective ferrules; c) Decoronated,root-treated anterior tooth which is vulnerable to fracture and requires protection; d) No protective ferrule provided by the core, or by the crown; e) No protectionprovided by the core, but the crown extends onto tooth and provides a protective ferrule; and f) Protective ferrule provided by a cast post and diaphragm

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only coronal tissue loss, and there are no dam-aging functional or parafunctional loads on thetooth.

Incisors and canines are spared the wedgingcuspal deflections of posterior teeth, but they toocan suffer longitudinal fracture if significanttooth tissue has been lost, and a protective fer-rule is not incorporated into the coronal restora-tion. A ferrule is a band of metal which totallyencircles the tooth, extending 1–2 mm ontosound tooth tissue to guard against longitudinalfracture.28 Figure 9 shows examples of ferrulesfor moderately and severely broken down ante-

rior teeth. For such teeth ferrules should beincorporated as routine.

Posts: a means of protecting teeth fromfracture?Another popular misconception is that thecementation of a metallic post in a root-treatedtooth provides reinforcement. In reality, the postoffers no reinforcement, and dentine removal toaccommodate it may in fact weaken the toothfurther and create an area of stress concentra-tion at the terminus of the post channel (Fig. 10).

The chief function of a post is to retain thecore. If adequate retention for the core can bederived from the use of natural undercuts in thepulp chamber and canal entrances (Fig. 11), thena post should not be used.

If a post really is required, then considerationshould be given to how it can provide retentionfor the core without weakening or stressing thetooth, or compromising the effectiveness of theroot filling.

Practitioners are not short of choice in postselection, and it is certainly beyond the scope ofthis article to consider the many systems avail-able in any detail. A review of commerciallyavailable systems by Edmunds and Dum-mer29–32 ran to no less than four articles, andnew systems continue to appear.

The literature contains many in vitro reportsof post performance and failure characteristics,but as in many other areas of dentistry, well-controlled clinical studies are few and farbetween. Objective judgements cannot thereforebe made on the performance of many of theavailable post systems, though certain principalconsiderations can be derived.

It is likely that many post systems will pro-vide satisfactory retention for the coronalrestoration, but the potential cost of enhancingretention should be considered. Dentists shouldperhaps focus less on factors which maximiseretention and more on factors that affect resist-ance to root fracture,27 ie the preservation andprotection of tooth tissue, and the avoidance ofstressing restoration components.

Avoiding problems with posts

Post lengthThere is little doubt that long posts are moreretentive than short posts. Endodontic postsshould therefore be as long as possible, and it isimportant to note that this is achieved not onlyby extending the post apically, but also by pre-serving tooth tissue coronally (Fig. 12). There isno place for the decoronating ‘rooftop’ prepara-tion in the restoration of root-filled teeth. This isparticularly so when evidence suggests that atleast 4–5 mm of gutta percha should remain api-cally to ensure that the seal of the root filling isnot compromised.11,33

Retaining coronal dentine also allows forwrap-around coverage by the subsequentcrown, which provides the essential ‘ferruleeffect’ discussed earlier (Fig. 12c).

(a)

(b)

(c)

Figure 10 a) Wide and tapered rootcanal. b) Preparation for a parallelpost removes more tissue andweakens the root further. Stressesare also concentrated at the sharpangles of the post terminus. c)Custom cast metal post requireslittle or no further loss of tissue

Fig. 11 Canal entrances and pulpchamber anatomy used to retain anall amalgam core in a molar.Composite resin may be used in asimilar way for anterior teeth

(a)

(b)

(c) Additional postlength

Fig. 12 a) Fractured and root-treated incisor to be restoredwith a post retained crown. b) Rooftop preparationdamagingly removes allremaining coronal toothtissue, and may compromisethe ability to create aprotective ferrule. c) Conservative preparationpreserves tooth tissue,lengthens the post, and allowsthe development of protectiveferrules

PRACTICE


Post shapeAll other factors being equal, parallel-sidedposts, such as the Parapost (Fig. 13) are moreretentive than tapered posts.34 However, thepreparation of a parallel-sided post channel, andsubsequent cementation of a square-ended par-allel post may produce increased stress in thenarrow and tapering root-end35 (Fig. 14a) andpredispose to root fracture. Systems, which arebevelled apically may therefore be preferred(Fig. 14b). But once again, the preservation oftooth tissue is important to the long-termintegrity of the tooth, and tissue should not besacrificed in order to create a parallel-sided postchannel if a well-adapted tapered post can beplaced with less sacrifice of dentine.

Tapered posts such as the PD system have agood record of clinical success.36 Concerns haveoften been raised over the generation of wedg-ing stresses by tapered (including customisedcast) posts, and the tendency to promote rootfracture. However, such forces are not active inthe same way as those generated by self-tap-ping screw systems, and it may be that manycases of root fracture associated with taperedposts reflect the type of cases in which suchposts are often used, ie the wide, thin-walledtapered canal. Again, the importance of provid-ing a protective coronal ferrule cannot be over-emphasised.

In their study of parallel versus tapered postsystems, Torbjornet et al.34 noted that in fact thetype of post may be of minimal importance tothe risk of root fracture if the tooth is covered bya complete crown with a good ferrule effect atthe crown margin area. Their comments werenot, however, directed to posts involving activemethods of retention.

Customised cast posts are especially versa-tile and can often be fabricated with the mini-mum of additional canal preparation. Suchposts have a strong history of clinical suc-cess28,37 especially once again when a coronalferrule is provided.

In summary, parallel-sided posts are preferredto tapered posts, but each case should be carefullyconsidered on its merits, and dentine should not be unnecessarily sacrificed to dogmaticallysatisfy the desire to place a moderately moreretentive parallel post.

Surface characteristics: threaded versus non-threaded postsThere is little doubt from the literature thatthreaded post systems offer the maximummechanical retention. But the retention theyprovide is often by active engagement of elasticdentine, producing stress concentration aroundthe threads, and increasing the risk of root frac-ture.38 This is especially so if posts are self-tapping, and is amplified if the post also has awedge-like, tapered design.Popular commercial threaded posts include:• Radix Anker (Fig. 15) • Dentatus (Fig. 16) • Kurer Anchor (Fig. 17)

Concerns have been expressed about allthreaded post systems, and it is notable thatmost manufacturers now recommend that thepost is used first to cut a thread, and is thenremoved and reinserted with cement, or is sim-ply derotated a quarter turn to reduce stresseswhich are generated during initial insertion. TheKurer post system, in which the canal is firsttapped before post insertion, is probably theleast stressing of threaded post systems. Anothermethod of limiting stress is seen in the Flexipost,which incorporates a split-pin mechanism, thesplit pin collapsing upon itself as the post isinserted to reduce pressure on the canal walls.39

Generally speaking, threaded posts are notpreferred and Meta analysis of the limited clinicalevidence available suggests that their perform-ance is inferior to that of customised cast posts.37

If enhanced retention is required in a special situ-ation, then threaded posts are no longer the onlyoption. Increasingly, resin-bonding agents maybe employed with serrated, and preferably sand-blasted metal or fibre posts, reducing the potential

Fig. 13 Parapost — parallel, serratedpost

(a) (b)

Fig. 14 a) Stress concentration atthe base of a parallel postpreparation. b) Reduced stressconcentration with a chamfered tip

Fig. 15: Radix Anker — parallel, self-tapping or pre-tapped post

Fig. 16 Dentatus screw — tapered,self-tapping post

Fig. 17 Kurer Anchor — parallel,threaded post for which the rootcanal is pre-tapped

PRACTICE


for stress, and enhancing the possibility of devel-oping an hermetic coronal seal.28

Minimising danger during post spacepreparationIdeally, post space preparation is completed atthe appointment when the root canal is filled. Atthis time, the practitioner is most familiar withthe canal system and reference points. He is alsoable to make post space with the rubber dam inplace to minimise microbial entry, and can fur-ther condense the apical segment of the root fill-ing after the coronal gutta percha has beenremoved. Gutta percha removal and post chan-nel preparation should not be undertaken in asingle act with the aggressive end-cutting twistdrills provided with proprietary post systems. Todo so is to risk losing alignment and perforatingthe root for the sake of a few seconds of time.Gutta percha should first be removed to the pre-determined length using burs with non-cuttingtips (eg Gates Glidden) or with hot instrumentsbefore the channel is shaped and enlarged pro-gressively with measured twist drills.

Guarding against coronal microleakageCoronal microleakage is a major cause ofendodontic failure.40 Saliva and organisms fromthe mouth migrate rapidly alongside poorlyadapted restorations and even root fillingswhich appear well condensed.18 The periradicu-lar tissues will be inflamed by such reinfectionand the reactivation of micro-organisms lyingdormant after initial treatment. A well sealingcoronal restoration is critical therefore toendodontic success, and it is again stressed thatthis applies as strongly to temporary restora-tions as it does to permanent ones. Posteriorteeth can be temporised with cuspal-coverageamalgam restorations, which will prove durableand well sealing for many months or years. Butthe same cannot be said for anterior temporarypost crowns, which should be in place for theminimum time possible.

Restorations should be well adapted, andevery use should be made of modern adhesivesystems in an effort to control salivary entry.

TREATMENT GUIDELINESIn summary, the guiding principles in restoringroot-treated teeth are:1. Preserve as much tooth tissue as possible2. If a post is needed make sure it is long

enough to be retentive and sufficientlystrong to resist distortion

3. Avoid twist drills for the removal of guttapercha

4. Avoid active restorations or restoration com-ponents which induce internal stresses

5. Provide the necessary coronal coverage forprotection

6. Strive for the best possible fluid and bacteria-tight seal

These principles will now be illustrated for avariety of restorative scenarios.

Minimal loss of coronal tissueIncisors and canines which have lost tissue dur-ing access preparation, in addition to tissue losscaused by caries in one or both proximal sur-faces can often be restored to satisfactorylong-term function and aesthetics with dentineand enamel-bonded composite resin. Gutta per-cha should be cleared from the interior of thecrown with hot instruments or Gates Gliddendrills. Powerful organic solvents such as chloro-form or halothane are not recommended in postchannel preparation, as it is impossible to control their advance into the root canal wherethey can rapidly dissolve gutta percha and sealerat a deeper level than anticipated. All traces ofsealer should be removed from the coronal tissues to prevent later discolouration. Concernshave been expressed about zinc oxide-eugenolsealers in particular, traces of which may inter-fere with the polymerisation of restorative resins.Alcohol rapidly sequesters excess eugenol and isnot known to threaten the integrity of gutta per-cha root fillings. It may be wise to rinse the pulpchamber with alcohol to remove traces ofeugenol before attempting to bond.41

Premolars and molars with only minimalaccess cavities and no other coronal tissue losscan be restored with amalgam or compositeresin in combination with a resin bonding sys-tem to enhance the marginal seal. The restora-tive material should be extended 2–3 mm intocanal entrances by carefully removing root fill-ing material with hot instruments or Gates Glid-den drills. However, large Gates should not beextended deeply into canals with the risk ofunnecessary dentine removal, and even stripperforation. The first increments of core materialshould be packed with a long, narrow plugger(eg Mortensen Condenser) to ensure good adap-tation into these retentive features.

Moderate loss of tooth tissueAdvances in bonding technology and improve-ments in the physical and aesthetic properties ofcomposite resins continue to increase the poten-tial for simple, plastic restoration of anteriorteeth. Depending upon aesthetic and functionaldemands, root-treated teeth with proximal andincisal tissue loss, in addition to the access cavi-ty may often be restored without the need for acrown. Bonding composite resin soon after com-pletion of the root filling has the added advan-tage of securing an early, hermetic seal againstcoronal microleakage.

It is impossible in this context to lay downfirm rules on the precise degree of tissue lossthat can be successfully restored in this non-destructive way. But it should also be noted thatteeth restored with simple plastic restorationscan always be revisited at a later stage for amore advanced and destructive restoration ifthe need arises.

Anterior teeth, which have some coronal tis-sue remaining, but where this is consideredinsufficient for long-term restoration with com-posite resin, either on functional or aesthetic

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grounds, require core build-up and a crown. Thecore may not always need a post for retention.

Gutta percha and sealer are first cleared fromthe crown and coronal 2–3 mm of the root canal.The remaining coronal tooth tissue is then pre-pared to receive a crown. Under no circum-stances should the tooth be decoronated to cre-ate a ‘rooftop’ preparation (Fig.12a, b). Weak,undermined coronal tissue and spurs of tissue,which are taller than they are wide, should bereduced and the remaining, well-supported tis-sue bevelled. Every effort should be made to pre-serve as much coronal tissue as possible. If thereis adequate retention and support available forthe core material, then dentine-bonded compos-ite is cured into the chamber and extended tocomplete the preparation coronally.

If tissue loss is more severe, then a post isrequired (Fig. 12c). This may either be cast, orprefabricated. Gutta percha is removed from thecanal, leaving 4–5 mm of filling material api-cally. An initial path is made with hot instru-ments, or with Gates Glidden drills, numbers 2and 3, which should be running at the maxi-mum speed achievable with the slow speedhandpiece to generate frictional heat which willsoften the gutta percha and ease its removalwithout disturbing the apical root filling.42

Having created a path, twist drills appropriateto the post system selected are used to enlargeand shape the channel. Excessive dentineshould not be removed to accommodate snuglya preformed parallel post in a flared canal. Inthis situation, a tapered or customised cast postor a fibre post is often preferred.

Impressions may then be taken for the pro-duction of an indirectly constructed casting, or adirect pattern fabricated in the mouth. Metalcastings have the advantage that features can bebuilt in to provide a protective ferrule, and thatthey can be customised to minimise the need fordentine removal. The chief disadvantage of thisapproach is that the tooth will need temporisa-tion with a temporary post crown, which isunlikely to provide an hermetic coronal seal dur-ing the time required to fabricate the post.43

For this reason, it may be preferable to restorethe tooth immediately with a prefabricated post,and composite core.43,44 An immediate, and per-manent coronal seal is then secured. This benefitshould be balanced in heavy loading situationswith consideration of the strength of the coreand the post-core interface. However the devel-

opment of a ferrule by extending crown marginswell onto sound tooth tissue should minimisethe physical demands on the composite core.Such extension may be subgingival or involve acrown lengthening procedure or forced eruptionto obtain supragingival margins.

Posterior teeth, which have lost one or bothmarginal ridges in addition to tooth loss forendodontic access, require cuspal coverage.Amalgam or composite cores can again often beretained without the need for posts. If a post isconsidered necessary, it should usually be placedin the straightest and most bulky root, common-ly the distal in lower molars, and the palatal inupper molars. Premolar roots should be judgedon their merits. Even if a post is placed, root-filling materials should always be removed fromthe entrances of other canals to provide supple-mentary retention for the core, and resistance torotational torque.

A cuspal coverage restoration can then beprepared which fits the functional and aestheticdemands of the situation. This may be as con-servative as a cast metal onlay, or three-quartercrown, or as extensive as a full-coverage metalor ceramo-metal crown (Fig. 18). Extracoronalrestorations should be extended at least1–2 mm onto sound tooth tissue to provide all-enveloping protection against fracture of theunderlying tooth.

Adhesively retained plastic restorations can-not be relied upon as long-term internalsplints. If there is no plan to crown a weakenedposterior tooth, either because the root-treat-ment is on probation, or for financial reasons,then physical cuspal coverage and protectionmust be provided by the core material. Cuspsadjacent to lost marginal ridges should bereduced in height by 3 mm and overlaid withdentine bonded amalgam or composite resin.Such restorations can provide cost-effective,durable service for many years.45

Extensive loss of tooth tissueTeeth with little or no coronal tissue remainingrequire special measures to provide a protectiveferrule. Again, surgical crown lengthening orforced eruption may be needed to allow the place-ment of restoration margins on sound tissue.

Anterior teeth may be prepared to receive acast metal post and diaphragm (Fig. 19), withextension of the metal casting over the bevelledor chamfered margins of the root-face to provide

(a) (b) (c)

Buccal

Fig. 18 Cuspal protection and development of protective ferrules for posterior teeth with varyingamounts of tissue loss: a) Simple metal onlay, b) Three-quarter crown, c) Full coverage crown

Fig. 19 Cast post and core with a diaphragm to cover and support a damaged incisor root

PRACTICE


an effective ferrule. Alternatively, a convention-al cast or prefabricated post and core may beplaced, before constructing a crown with a metalcollar extending 1–2 mm onto tooth tissue toafford the necessary protection.

Immature or hollowed-out roots can presentspecial problems for rehabilitation. One methodinvolves packing dentine bonded compositeresin into the widened canal, and curing it inplace with light-transmitting posts to effectivelyreline and internally splint the root (Fig. 20).46

Alternatively, self-curing composite can beused. Conventional or fibre44 posts may then becemented into the ‘relined’ root before coronalcore build-up, or advantage taken of the cohe-sive strength of composite in thick section tobuild a continuous mass of composite formingthe post and core. The final crown should againincorporate a ferrule extending well onto soundtooth tissue for protection.

Posterior teeth which cannot be built up withpost and adhesive-retained plastic materialsmay occasionally require the fabrication of asectional casting. These restorations are diffi-cult, exacting, and costly to manufacture andare testament to the engineering skills of practi-tioner and technician. Separate post and coreelements with different paths of insertion thatlink on placement, or a single core unit withchannels for multiple post placements at differ-ent angles may be manufactured (Fig. 21).

The extracoronal restoration should again beextended onto sound tooth tissue to provide asupporting ferrule for the underlying tooth.

In conclusion:1. All teeth to be crowned should be carefully

assessed as to their pulpal and endodonticstatus, and reliable foundations laid

2. Teeth with healthy pulps should be preparedand temporised with due care to preservepulpal health

3. Root-treated teeth are at risk of fracture andof coronal microleakage. Control of thesefactors should underpin the design of alltemporary and permanent restorations forsuch teeth.

1. Randlow K, Glanz P O. On cantilever loading of vital and nonvital teeth: an experimental clinical study. Acta OdontolScand 1986; 44: 271-277.

2. Zach L. Pulp lability and repair: effect of restorativeprocedures. Oral Surg 1972; 33: 111-121.

3. Pashley D H. Clinical considerations of microleakage. J Endod1990; 16: 70-77.

4. Cox C F, Subay R K, Suzuki S, Suzuki S H, Ostro E.Biocompatibility of various dental materials: pulp healingwith a surface seal. Int J Periodont Rest Dent 1996; 16: 241-251.

5. Bergenholtz G. Iatrogenic injury to the pulp in dentalprocedures: aspects of pathogenesis, management andpreventive measures. Int Dent J 1991; 41: 99-110.

6. Seltzer S, Bender I B. The Dental Pulp. 3rd Edition. pp324-348.Philadelphia: Lippincott, 1984.

7. Abou-Rass M. The stressed pulp condition: An endodontic-restorative diagnostic concept. J Prosthet Dent 1986; 48:264-267.

8. Bergenholtz G, Nyman S. Endodontic complicationsfollowing periodontal and prosthetic treatment of patientswith advanced periodontal disease. J Periodontol 1984; 55:63-68.

9. Felton D, Madison S, Kanoy E, Kantor M, Maryniuk G. Long

(a) (b)

(c) (d)

Light

Fig. 20 Rehabilitation of a severely weakened tooth by relining withcomposite resin: a) Hollowed-out, fragile root; b) Incremental additions ofcomposite cured with a light-transmitting post; c) Space for a conventionalpost within the composite resin ‘reline’. Tooth margins prepared for thecrown to provide a ferrule; and d) Continuous composite post and coretakes advantage of the materials cohesive strength in thick section. Toothmargins prepared for the crown to provide a ferrule

Fig. 21 a) Metal post and core for adecoronated posterior tooth. Themargins of the preparation arechamfered to provide a ferrule. The metal casting contains twoshort posts for the buccal roots. A separate wrought post will beinserted through it into the palatalroot. b) The casting in place, withthe wrought, parallel postcemented through it and into thepalatal root

a

b

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term effects of crown preparation on pulp vitality. J Dent Res1989; 68 (special issue): 1009.

10. Landolt A, Lang N P. Erfolg und misserfolg beiextensionsbrucken. Schweiz Monat Zahnmed 1988; 98:239-44. [German with English abstract]

11. Reichen-Graden S, Lang N P. Periodontal and pulpalconditions of abutment teeth. Schweiz Monat Zahnmed1989; 99: 1381-1385.

12. Gonzalez G , Wier D J , Helm F , Marshall S J, Walker L, StofferW. et al. Incidence of endodontic treatment in teeth with fullcoverage restorations. J Dent Res 1991; 70 (special issue): 446.

13. Valderhaug J, Jokstad A, Ambjornsen E, Norheim P W.Assessment of the periapical and clinical status of crownedteeth over 25 years. J Dent 1997; 25: 97-105.

14. McLean J W. The Science and Art of Dental Materials. Vol. 1. p 59. Chicago: Quintessence, 1979.

15. Madison S, Jordan R D, Krell K V. The effect of rubber damretainers on porcelain fused-to-metal restorations. J Endod1986; 12: 183-186.

16. Dental Protection. Annual Report of the Board 1996; 8: 22.17. Cohen S. In Cohen S, Burns R C. (eds). Pathways of the Pulp.

6th edn. pp2-24. St Louis: Mosby, 1994.18. Khayat A, Lee S J, Torabinejad M. Human saliva penetration

of coronally unsealed obturated root canals. J Endod 1993;19: 458-461.

19. Walton R, Torabinejad M. Principles and Practice ofEndodontics. 2nd edn. Baltimore: Saunders, 1996.

20. Pitt-Ford T R. Harty’s Endodontics in Clinical Practice. 4thedn. Oxford: Wright, 1997.

21. Clements R E, Gilboe D B. Labial endodontic access openingfor mandibular incisors: Endodontic and restorativeconsiderations. J Can Dent Assoc 1991; 57: 587-589.

22. Langeland K, Langeland L K. Pulp reactions to cavity andcrown preparation. Aust Dent J 1970; 15: 261-276.

23. Langeland K, Langeland L K. Cutting procedures withminimized trauma. J Am Dent Assoc 1968; 76: 991-1005.

24. Bergenholtz G, Cox C, Loesche W J, Syed S A. Bacterialleakage around dental restorations: its effect on the dentalpulp. J Oral Pathol 1982; 11: 439-450.

25. Goldman M, Laosonthorn P, White R R. Microleakage: fullcrowns and the dental pulp. J Endod 1992; 18: 473-475.

26. Cox C F, Suzuki S. Re-evaluating pulp protection: Calciumhydroxide liners vs. cohesive hybridization. J Am Dent Assoc1994; 125: 823-831.

27. Sorensen J A, Martinoff J T. Intracoronal reinforcement andcoronal coverage: a study of endodontically treated teeth.J Prosthet Dent 1984; 51: 781-784.

28. Morgano S M. Restoration of pulpless teeth: an applicationof traditional principles in present and future contexts. JProsth Dent 1996; 75: 375-380.

29. Edmunds D H, Dummer P M H. Root canal retainedrestorations: 1. General considerations and custom-madecast posts and cores. Dent Update 1990; June: 183-188.

30. Dummer P M H, Edmunds D H. Root canal retained

restorations: 2. Prefabricated post and core systems —(i) Non-threaded posts. Dent Update 1990; July/August: 244-249.

31. Dummer P M H, Edmunds D H. Root canal retainedrestorations: 2. Prefabricated post and core systems —(ii) Threaded posts. Dent Update 1990; September: 286-289.

32. Edmunds D H, Dummer P M H. Root canal retainedrestorations revisited. Dent Update 1993; January/February:14-19.

33. de Cleen M J H. The relationship between the root canalfilling and post space preparation. Int Endod J 1993; 26: 53-58.

34. Torbjorner A, Karlsson S, Odman P A. Survival rate and failurecharacteristics for two post designs. J Prosthet Dent 1995;73: 439-444.

35. Standlee J P, Caputo A A, Collard E W, Pollack M H. Analysis ofstress distribution by endodontic posts. Oral Surg 1972; 33:952-960.

36. Weine F S, Wax A H, Wencus C S. Retrospective study oftapered smooth post systems in place for 10 years or more.J Endodont 1991; 17: 293-297.

37. Creugers N H J, Mentink A G B, Kayser A F. An analysis ofdurability data on post and core restorations. J Dent 1993;21: 281-284.

38. Deutsch A S, Cavallari J, Musikant B L, et al. Root fracture andthe design of prefabricated posts. J Prosthet Dent 1985; 53:637-640.

39. Cohen B I, Musikant B L, Deutsch A S. A 10-year literaturereview of a split-shanked threaded post. Compend Cont EducDent 1995: 16: 630-631.

40. Saunders W P, Saunders E M. Coronal leakage as a cause offailure in root-canal therapy: a review. Endod Dent Traumatol1994; 10: 105-108.

41. Tjan A H L, Nemetz H. Effect of eugenol-containingendodontic sealer on retention of prefabricated posts lutedwith adhesive composite resin cement. Quintessence Int1992; 23: 839-844.

42. Haddix J E, Mattison G D, Shulmann C A et al. Postpreparation techniques and their effect on the apical seal.J Prosth Dent 1990; 64: 515-519.

43. Fox K, Gutteridge D L. An in vitro study of coronalmicroleakage in root-canal-treated teeth restored by thepost and core technique. Int Endod J 1997; 30: 361-368.

44. Ferrari M, Vichi A, Mannocci F, Mason PN. Retrospectivestudy of the performance of fiber posts. Amer J Dent 2000;13 (special issue): 9B-13B.

45. Nayyar A, Walton R E, Leonard L A. An amalgam coronal-radicular dowel and core technique for endodonticallytreated posterior teeth. J Pros Dent 1980; 43: 511-515.

46. Saupe W A, Gluskin A H, Radke R A. A comparative study offracture resistance between morphologic dowel and coresand a resin-reinforced dowel system in the intraradicularrestoration of structurally compromised roots. QuintessenceInt 1996 ; 27: 483-491.

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BRITISH DENTAL JOURNAL VOLUME 192 NO. 7 APRIL 13 2002 377

Crowns and other extra-coronal restorations:Occlusal considerations and articulator selection J. G. Steele1 F. S. A. Nohl2 and R. W. Wassell3

For many dentists, occlusion carries an air of mystique. It even seems sometimes that a perverse pleasure is derived in makingthe whole subject more complicated than it really is. As a clinician, you need to be able to decide what you expect from yourproposed restoration, and to identify situations where you may need to alter the existing occlusal scheme. At a fundamentallevel, you also need to provide the laboratory with appropriate clinical records to ensure that when you fit them, adjustmentsto the expensively prepared restorations are minimal. This requires a sound understanding of the basics.

● Clarification of some of the terminology and concepts relating to occlusion as it is used ineveryday practice, making clear why these concepts matter

● Undertaking a simple pre-operative examination of the occlusion as a matter of routine● Helping clinicians identify cases where articulated study casts will help plan treatment and

design restorations● Advice is provided about selecting an appropriate articulator and taking appropriate records

at the treatment stage.

I N B R I E F

This fifth article in the series will try to presentimportant occlusal concepts in a way whichrelates directly to the provision of successfulcrowns. It is not a comprehensive guide to occlu-sion, or a manual of techniques for extensivefixed prosthodontics. There are several usefulbooks and articles dedicated to the subject andsome of these are specifically referenced (if atechnique is particularly well described) or arelisted in the further reading section. However, wehope that this article should allow you to avoidmost of the problems associated with the provi-sion of crowns. Occasionally some pre-operativeocclusal adjustment is needed. Our experience isthat this is best taught ‘hands on’ and we wouldrecommend attending an appropriate coursebefore attempting more complex adjustments.

BASIC CONSIDERATIONS — WHAT MATTERS?One of the essential starting points with occlu-sion is to make sure that the terminology is clear.There are any number of occlusal terms, many ofwhich overlap. There are only a few that reallymatter and these need to be understood if what isto follow is to make any sense.

The intercuspal position (ICP or IP)Synonyms: centric occlusion (CO), maximumintercuspation

What is it?Most dentate patients, when asked simply to‘bite together on your back teeth’, close immedi-ately into a comfortable, reproducible “closed”position where the maximum number of toothcontacts occur. This is the intercuspal position

(ICP). Travel into this position is partly guided bythe shape of teeth and partly by conditionedneuromuscular co-ordination.1 ICP is the most‘closed’ position of the jaws.

Why does this matter?ICP is usually the position in which verticalocclusal forces are most effectively borne by theperiodontium with teeth likely to be loaded axi-ally, which helps to stabilise their position.Indeed it is the end point of the chewing cyclewhere maximum force is exerted. In everydaypractice this is the position of the jaws in whichrestorations are made.

Guidance (from the teeth)What is it?When a patient moves their mandible from sideto side so that the teeth in opposing jaws slideover each other, the path taken is determinedpartly by the shapes of the teeth which makecontact, as well as by the anatomical constraintsof the temporomandibular joints (TMJs) andmasticatory neuromuscular function. Each has abearing on the other, and, for want of a betterterm, they should work in harmony. In these cir-cumstances the teeth provide guidance for themovement of the mandible. The shape and formof the temporomandibular joints also guide themovement of the mandible (sometimes calledposterior guidance). Guidance teeth can be anyteeth, anterior or posterior.

When the patient slides the mandible out toone side, the side they move the mandibletowards is called the working side (because it isusually the side on which they are about to

5

1*,3Senior Lecturer in Restorative Dentistry,Department of Restorative Dentistry, The Dental School, Framlington Place,Newcastle upon Tyne NE2 4BW2Consultant in Restorative Dentistry, TheDental Hospital, Framlington Place,Newcastle upon Tyne NE2 4AZ*Correspondence to: J. G. SteeleE-mail: [email protected]











restorations

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378 BRITISH DENTAL JOURNAL VOLUME 192 NO. 7 APRIL 13 2002

chew). The other side, the side the mandible ismoving away from, is called the non-workingside. So for example, an excursion to the right(as may occur during chewing) will make theright side the working side and the left the non-working side, whilst during an excursion to theleft the reverse will be true. During these excur-sions the upper and lower guidance teeth will bein contact and partly dictate the movement ofthe mandible. Canine guidance is where theupper and lower canines on the working side arethe only teeth in contact during a lateral excur-sion, causing all of the posterior teeth to disclude(Figs 1 and 2). When several pairs of teeth, usu-ally premolars or premolars and canines (andsometimes molars) on the working side share thecontacts during excursions group function issaid to take place (Fig. 3). Other patterns of guid-ance can take place, using almost any combina-tion of teeth.

Incisors and canines usually provide protru-sive guidance, when the mandible slides for-ward, but where there is only a limited overbitethe posterior teeth may be involved.

Why does it matter?Guidance teeth are repeatedly loaded non-axially(laterally) during excursions. As a result heavilyrestored or crowned teeth may be at risk of frac-

ture or decementation, particularly if these loadsare heavy. Other manifestations of problemswith guidance include:

• Fractured teeth or restorations• Accelerated local wear• Tooth migration• Tooth mobility• TMJ dysfunction

To avoid these it is important to identifywhich teeth provide guidance before you starttooth preparation. If the guidance is satisfactory,and the guidance tooth or teeth are strongenough to withstand the likely loading in thelong term, it is usually best to try to re-establishthe same guidance pattern in the new restora-tion. Techniques for doing this are describedlater in the article. Occasionally, you may feelthat a tooth that you are about to crown is insuf-ficiently robust to carry a guidance contact andthe guidance is best moved onto other teeth. A specific example of this would be where a broken down guidance canine is restored with apost retained crown. There may be a risk of rootfracture of the tooth in the longer term becauseof the heavy lateral forces. In a case like this, bytaking a little care with the shape of both prepa-ration and crown, guidance can often be trans-ferred from the canine to the premolars, if theyare in a better position to accept the heavy loads.

Other practical reasons for identifying guid-ance teeth include:• The need to provide clearance from the oppos-

ing tooth during preparation, not just in ICP,but also along the guidance track. If you donot do this you can end up unwittingly trans-ferring all the guidance forces on to your newcrown.

• The need to select and prescribe an appropri-ate material to restore the guidance surface(metal is usually best if possible).

These aspects are discussed in detail later inthe series.

The message is that getting guidance right isone of the most important aspects of crown pro-vision; problems can, and will, occur unlessguidance is correctly managed on teeth to becrowned.

Interferences What are they?Interferences are any tooth-to-tooth contact(s),which hamper or hinder smooth guidance inexcursions or closure into ICP. An interferenceon the side to which the mandible is moving iscalled a working side interference. An interfer-ence on the side from which the mandible ismoving is called a non-working side interfer-ence (NWSI) or balancing side interference.There is a distinction to make between NWSIsand non-working side contacts: in the lattercase, excursions are guided equally by workingand non-working tooth contacts, akin to the balanced articulation often taught as an idealcomplete denture occlusion. However, where

Fig. 1 Right canine guidance duringright lateral excursion. Posteriorteeth are discluded but contactremains between the lateral incisors

Fig. 2 Left canine guidance duringleft lateral excursion. Here thecanines are the only teeth in contact

Fig. 3 Shared contact between manyposterior teeth in right lateralexcursion. Guidance is described asbeing group function

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there is a NWSI it acts as a cross arch pivot, dis-rupting the smooth movement and separatingguidance teeth on the working side (Fig. 4).

Why does it matter?There has been much written about the signifi-cance or otherwise of interferences, particularlyNWSIs, in relation to initiating parafunction andTMJ dysfunction. Warnings of the direst conse-quences to the stomatognathic system andbeyond from NWSIs are frankly misleadingthough. Many people function perfectly happilywith a mouthful of NWSIs. However, when con-templating crowns there are important implica-tions. Most NWSIs are on molars so teeth orrestorations directly involved are subject to highand often oblique occlusal forces with the con-sequent risk of fracture or uncementing.

As a general rule, it is best to remove interfer-ences before tooth preparation if the interfer-ence is on a tooth which is to be prepared. Thisapplies to all types of interference — working,non-working and protrusive. In practice it is bestto do this at a separate appointment prior totooth preparation. This will allow the patienttime to adapt to a new pattern of excursive guid-ance, and you time to refine the guidance if nec-essary. The process of dealing with a non-work-ing side interference prior to preparation isshown in Figure 4. If you do this, it is importantto identify suitable teeth on the working side totake over the guidance once the interference hasbeen eliminated. If there are no teeth to take overthe guidance, it may be impossible to eliminatethe NWSI. If you are in any doubt it would bebest to seek advice before cutting the tooth.

Where interferences exist on teeth that arenot themselves to be prepared, the need foradjustment may be less important. Many peoplehave asymptomatic interferences and seem to beable to lead a normal existence and we certainlywould not advocate the removal of all interfer-ences as a public health measure.

One final point is that it is disturbingly easyto introduce new interferences when you placerestorations, even where there were no interfer-ences previously. If you check your preparationsfor adequate clearance, not only in ICP but inlateral and protrusive excursions as well, thechances of this occurring should be minimised.Obviously, there is the opportunity to removeminor interferences on the final restorationbefore cementation.

Retruded Contact Position (RCP)Synonyms: centric relation (CR), centric relationcontact position (CRCP), retruded axis position(RAP), terminal hinge position.

What is it?This is the position of the mandible when thefirst contact between opposing teeth takes place,during closure on its hinge axis (or retruded arcof closure), that is with the condyles maximallyseated in their fossae. This condylar position isone of health. Generally, as the mandible hinges

closed with the condyles in this position, there isa contact between a pair of teeth somewherearound the mouth (Fig. 5). The mandible willthen close, from this retruded contact, down intoICP, usually sliding forward and laterally(Fig. 6). If you want to try to manipulate apatient’s mandible into this position it is impor-tant that they are relaxed (Fig. 7), otherwise itcan be very difficult and you will feel resistanceto free movement of the mandible. For about10% of people ICP will be the same as RCP2 andin these cases if you hinge the mandible until theteeth are in contact they will go straight into ICPwith no deflective contact.

Why does it matter?There is no magic quality about RCP, but thereare a number of reasons why RCP and the asso-ciated slide into ICP may be relevant when pro-viding crowns. Box 1 contains a more detailed

Fig. 4 A non-working side interference between the left first molars and the possible consequencesof carrying out crown preparation without appreciating its presence (transverse section): a) Duringa right lateral excursion (see black arrow) the left first molars act as a cross-arch pivot lifting theteeth out of contact on the working side; b) The maxillary first molar has been prepared for acrown. Occlusal reduction has eliminated the pivot, allowing the teeth on the working side tocontact during lateral excursion. However, clearance between the preparation and opposing teeth isnow inadequate which may cause problems with the provisional restoration. Worse still, thedefinitive restoration may require gross adjustment resulting in its perforation; c) You can avoidthese problems by removing the non-working side contact prior to tooth preparation (blue linerepresents tooth recontoured in this way)

Fig. 5 A retruded contact betweenmaxillary molar and mandibularpremolar. Most retruded contactscause no problems. This one resultedfrom over-eruption of the molar andthe interference was associated withaccelerated wear of the incisors

a)

b)

c)

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analysis of the situations where it may be aproblem. In many, but not all cases, managingRCP is of less significance than managing guid-ance effectively, but there are times whenadjustments need to be made and, clearly, it isalways better to identify potential problems thanblindly to hope for the best. To this end it isalways worth examining RCP pre-operatively sothat you at least know where it is. On the veryfew occasions where it is likely to be a major fac-tor affecting your restorative procedure, castsmounted in RCP (or in the hinge axis just before

the teeth actually contact) on a semi-adjustablearticulator will facilitate occlusal examinationand allow trial adjustments.

Establishing and recording RCP, and particu-larly re-organising the occlusion, are often diffi-cult and we would refer you to Further Readingbelow if you wish to follow these subjects up inmore detail.

PRACTICAL ASPECTS OF OCCLUSION:RECORDS FOR PLANNING CROWNS

Records for planning crowns: The occlusalexamination Before providing crowns it really is mandatoryto undertake some sort of an occlusal examina-tion. The following observations take momentsto gather and are worth the effort:

• Check ICP (for reproducibility and contacts onthe teeth to be restored)

• Check RCP (to establish whether there is anydeflective contact or slide which you ought toknow about). For technique see Reference 3.

• Check the relationship of the teeth in lateraland protrusive excursions (to determinewhether your crowns will be involved in guid-ance or if you need to consider removing aninterference).

A more detailed and lengthy examination,where all of the contacts are marked using goodquality ultra thin articulating tape (Fig. 8) issometimes indicated, particularly where therehas been a history of temporomandibular dys-function or where there is a specific occlusalproblem which you need to address. The variouscomponents, which may be included in a fullocclusal examination, are given in Box 2.

Records for planning crowns: hand-held studycastsHand-held casts can be a very useful aid toexamination and treatment planning. Toglean maximum information from them they

Fig. 6 The slide fromretruded contactposition (RCP) to theintercuspal position(ICP). Here, in RCP themolars make firstcontact as shown bythe arrows. Themagnitude anddirection of the slidecan be estimated atboth the molar andincisor regions

RCP

ICP

Fig. 7 One method of manipulatingthe mandible of a relaxed patientinto the hinge axis position. Thistechnique is called bimanualmanipulation and is designed to seatthe condyles fully in their fossaewith the disks interposed

BOX 1: RCP AND CROWNSMost crowns and other extra-coronal restorations will be made to conform with the patient's ICP and usually a slide from RCP to ICPwill be of no major relevance when providing crowns. In some circumstances additional management may be appropriate. Thefollowing are the situations where adjusting the contact in RCP is likely to be most important:

1. As a general rule, when RCP involves a tooth you are about to prepare it is often best to remove the deflective contact at anappointment before you start tooth preparation.

2. When re-organising the occlusion at a new vertical dimension you really have no option but to construct the new occlusion, if not at RCP itself , certainly around centric relation (with the condyles in the hinge axis). This represents the only reproduciblestarting point.

3. If you need space but you wish to avoid increasing the vertical dimension, there may be the scope to ‘distalise’ the mandible tocreate space lingually for anterior crowns (only possible where there is a bodily translation between RCP and ICP).

4. If you are about to restore anterior teeth and the RCP contact results in a strong anterior thrust against the teeth to be prepared.Although we would usually advise removal of a deflective RCP interference before preparation if it is on a tooth you are about toprepare for a crown, many dentists do no not carry out any such adjustment and no problems result. This is probably because cuttingthe crown preparation effectively removes the contact. By removing it before preparation though you can ensure sufficient removalof tissue to allow space for the crown without re-introducing it in an uncontrolled way when the restoration is made. The principlesinvolved are the same as those for removing non-working side interferences (see Fig. 4).

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need to be made from good quality impres-sions, which have been handled and pouredcorrectly. We will return to this important butunderrated subject later (see below: ‘OpposingCasts’).

Hand-held study casts enable:• A judgement to be made regarding the ease of

obtaining a stable ICP. This helps to determinewhether or not an interocclusal record isrequired for the working casts upon which therestoration(s) will be made.

• An unimpeded view of ICP. It is possible toview aspects such as the lingual, which it would not be possible to see at the chair-side.

• Careful evaluation of clinical crown heightand the availability of inter-occlusal space forrestorative material. These two factors canhelp make the decision on how to facilitate therestoration of short teeth (see Part 3 ‘Pre-oper-ative assessment’ in this series).

However tempting it may be to assume other-wise, hand-held casts provide no informationabout excursive tooth contacts or RCP, beyondthe distribution of wear facets.

Records for planning crowns: Articulated studycastsAccurate casts of the dental arches mounted in asemi adjustable articulator are the most impor-tant tools of the trade when constructing artifi-cial crowns. The need for an articulator and thepositions in which you mount the casts dependon what you need to do (see Box 3). Articulatorsare surrounded by an aura of mystery, but at theend of the day they are a tool to help give yourpatient a successful restoration and to help youto save time, money and hassle. The quality ofthe final result is much less dependent on thearticulator you use than it is on the care youexercise to make and mount the casts that youput in it.

There is little merit in examining study castsfor planning purposes on a simple hinge or other‘non-anatomical’ articulator because the abilityto replicate physiological movements will be, atbest, crude, and at worst, wholly misleading. Anon-anatomical articulator will allow casts to beput into a reproducible ICP, which may be helpfulif there are insufficient contacts to make hand-held casts stable, but that is the limit of what asimple hinge articulator can do.

BOX 2: COMPONENTS OF AN OCCLUSAL EXAMINATIONA full occlusal examination including all of the components below is rarelyindicated, but different components may be required at different times.Visual assessments of ICP, RCP and guidance teeth are always indicatedwhere you are planning crowns. The following may also be useful:

1. Examining for signs of temporomandibular dysfunction:

• Palpation of muscles of mastication for tenderness.

• Palpation of the temporomandibular joints and detailedassessment of any clicks or deviations in mandibular movement.

• Assessment of the range of mandibular movement.

2. Checking for facets, fremitus, mobility and drifting (particularly wherethere are problems with individual teeth).

3. Marking up of the dry teeth in different coloured tapes (Fig. 8) toshow contacts in all excursions using a high quality articulating tape(where there are issues affecting the entire occlusal scheme).

BOX 3: WHEN TO USE A SEMI-ADJUSTABLE ARTICULATOR DURING THE MANUFACTURE OF CROWNS1. If you wish to ensure appropriate guidance with your new restorations, particularly where

multiple crowns are involved.

2. If you plan to increase the vertical dimension at all.

3. In any case where either you are going to remove so many of the occlusal contacts that ICPwill effectively be lost and/or where you plan to make a new ICP based on RCP (sometimesknown as a reorganised occlusion).

4. Where you plan to remove occlusal interferences (the study casts can enable a trialadjustment to be carried out).

5. When there is a need to provide an occlusal stabilisation appliance (occlusal splint), eitherbefore treatment to stabilise jaw position or after treatment to protect restorations from theeffects of bruxism.

Fig. 8 Tools for examining occlusal contacts: foilshimstock held in forceps and high quality ultra-thinarticulating tapes held in Millers forceps

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The combination of a facebow record (whichlocates the approximate position of the condylarhinge axis in relation to the upper arch) and a jawrelation record (which then locates the lower castto the upper), enables movements of casts articu-lated on a semi-adjustable articulator to be rea-sonably anatomical. You can simulate the move-ments of the teeth in lateral and protrusiveexcursions, and around the hinge axis and be con-fident that what you see is close to what is reallyhappening in the mouth. However, whilst theinstrument is key, the quality of the casts and the care with which they are mounted are just ascritical. There is no room for carelessness at thisstage, wrongly articulated casts are probablyworse than no casts at all as they may result infalse assumptions about treatment. Similarly, inac-curacies with the original impressions can result inprofound errors and the use of an accurate and stable impression material (such as addition curedsilicone) may be appropriate in cases where adetailed occlusal analysis is necessary. Details ofhow to record a facebow record and a retrudedhinge axis inter-occlusal record can be found inReferences 3 and 4. Some simple tips on accurateimpression recording can be found in Box 4.

Although you can see and reproduce move-ments with carefully articulated casts, you mayoften want to go on to the next stage and preparea diagnostic wax up.

Records for planning crowns: the diagnosticwax-upIn addition to its uses in planning changes inappearance (see Aesthetic Control — the sixtharticle in this series), a diagnostic wax-up can bean absolutely invaluable technique where youare changing the occluding surfaces of severalteeth with crowns or resin bonded restorationsand allows you to plan the following:

• The new static occlusal contacts (in ICP) andthe shape of the guidance teeth

• The impact that the modified occlusion has onappearance

• The best options for creating interocclusalspace for restoration(s) or optimising crownheight by periodontal surgery (see Part 3: ‘Pre–operative Assessment’ in this series).

You can also use the completed wax-up as atemplate to determine the form of temporaryand final restorations.

PRACTICAL ASPECTS OF OCCLUSION:RECORDS FOR MAKING CROWNSWhen the diagnosis stage has been completed,the crowns or restorations still need to be made,and various records are essential at this stagetoo. This section discusses the choice of articula-tor and the need to obtain accurate occlusalrecords, including the simplest things such as

BOX 4: TAKING A GOOD OPPOSING IMPRESSION1. Choose stock impression trays with adequate rigidity which, perhaps in

combination with an adhesive, have sufficient retentive features to holdset alginate securely.

2. If the restoration is on a very posterior tooth you may need to extend thetray with a rigid material (green stick or compo but not carding wax) toensure that the alginate is supported in important posterior areas.

3. Make sure adhesive has been air dried in the tray before loading alginate.Lots of wet adhesive acts as a lubricant not an adhesive, and if thealginate pulls away from the tray you are left with an enormous but notvery obvious distortion in the final cast.

4. Dry the teeth with gauze or cotton wool to remove saliva, which aidstremendously in recording occlusal detail and producing of a smooth,hard cast surface. It can be done as the alginate is being mixed.

5. Smear a little alginate firmly onto the dry occlusal surfaces with yourfinger as the assistant finishes loading the tray.

After removal6. Check to make sure the material has not pulled out of the tray (Fig. 9).

If it has, retake it and do not assume it will be all right or be tempted topress dislodged material back into place; you will pay in the time spentadjusting the final crown.

7. Cut off the heels of the impression flush with the tray if you do not needthem, they are prone to distort when you lay the impression down onthe bench or even when it is loaded with wet stone. This takes seconds.

8. If you get the chance, check the casts, if not, ask the technician to do thisand to flick off any air bubbles on critical surfaces (Fig. 10).

9. In really critical cases, consider using silicone to obtain a very accurateopposing cast especially if there is to be a delay before pouring up.

Fig. 10 Large stone blebs on the palatal surfaces of upperanterior teeth as a result of air bubbles in the impression

Fig. 9 Impression material has been pulled away from thetray on withdrawal from the mouth. This will equate to at least a 1mm occlusal error

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opposing impressions, which are a frequentsource of error. Finally, it introduces ways ofcontrolling guidance on front teeth.

The articulator When manufacturing the final crowns, in theinterests of simplicity and cost it would seemsensible to use the simplest cast relating devicethat will not compromise the final restoration.5

Small numbers of restorations, which are notinvolved in excursive contacts, can very reason-ably be made on a non-adjustable articulatorand then any adjustments made in the mouthbefore final cementation. However, crownsinvolved in excursions benefit from the use ofan articulator with anatomical dimensions sothat the excursive movements can be made andthe shape of the crown adjusted in the lab withreasonable accuracy, saving chairside time. Thisbecomes particularly important, and cost effec-tive, when several restorations are being createdat the same time. Highly sophisticated semi-adjustable and fully adjustable articulators areavailable for this purpose, but the majority ofcases can be managed quite satisfactorily usinga less sophisticated, fixed average value articu-lator in combination with a facebow.

It may not be possible to check occlusion onadhesive restorations prior to cementation, eitherbecause the act of checking may damage porce-lain, which is delicate until cemented, or becausethey will not stay in place during excursions. Inthese cases, controlling the role of the restorationsin guidance can be critical to their long-term sur-vival. A semi-adjustable articulator can beinvaluable in situations such as these because itallows the technician to secure restorations ontothe working cast and do the critical adjustmentsin the lab so that all you need to do is cementthem with little or no adjustment afterwards.

Opposing castsIn any discussion about articulators, it is disturbingly easy to forget the importance of anaccurate cast to oppose the working cast. Theopposing impression is often the last thing we doand, after a long session preparing teeth, makingtemporaries and taking impressions it tends to bea bit of an afterthought. However, a poor oppos-ing impression is very easy indeed to achieve andyet can cost a great deal of precious time subse-quently. A cast made with a distorted impressionor a porous impression resulting in plaster blebson occlusal surfaces will not fit comfortably intoICP. If such a cast is used in the lab it can result ina crown which looks perfectly good on the castbut which may be very high in the ICP and whichcan take a great deal of time to get right prior tofit. It is easy to record bad opposing impressions,but good ones are just as easy. Attention to thefew steps listed in Box 4 takes, literally, no extratime but can save a lot of heartache. In an idealworld every opposing impression would berecorded in a dimensionally accurate and stablematerial such as an addition cured silicone, but

this is probably not economically realistic. Box 4describes the use of alginate for an opposingimpression. In cases involving multiple restora-tions though, a very stable and accurate materialmay be cost effective in the long term.

Interocclusal records (IOR)Once you have your working impression andopposing impression you then need to decidewhether you need to provide additional informa-tion to the lab to allow them to mount the casts;an inter-occlusal record (IOR). There is a commonperception that providing an intercuspal record(such as a wax or silicone ‘bite’) will improve theaccuracy of mounted casts. The truth is that inmany cases it does precisely the opposite.6

For a patient with a stable intercuspal posi-tion, the loss of interocclusal contact created bypreparation of a tooth for a single unit restora-tion, is unlikely to detract from the ease withwhich working and opposing casts can be locat-ed in ICP. In this circumstance, placing a layer ofwax or silicone between the casts to help tolocate them can often result in them failing toseat into ICP at all, and there is a very seriousrisk of the record introducing inaccuracies,rather than acting as the ‘insurance policy’ youintended. It is worth taking the opportunity ofexamining the ease with which any study castscan be located by hand before deciding whetheran IOR is needed. Often (perhaps even usually)you are better with nothing at all.

Sometimes an IOR is required to stabilisecasts, particularly where the teeth that are pre-pared are key support teeth in an arch. Thechoice of materials is generally between hardwax alone, hard wax (as a carrier) used with zincoxide/eugenol, silicone elastomers and acrylicresins. The fundamental requirement is to obtainenough detail in a dimensionally stable record-ing material to enable casts to be confidentlylocated in the laboratory whilst not recording somuch detail that it stops the casts seating.Occlusal fissure patterns reproduced accuratelyin the IOR may well not be reproduced to thesame extent in the cast, preventing full seatingof the casts in the record. Furthermore, an IORwhich contacts soft tissues in the mouth andcauses their displacement (which is obviouslynot reproduced in the stone cast) will result in anIOR which will not seat accurately (Figs 11 and12). In order to meet the requirements for suc-cess, an IOR should:

1. Record the tips of cusps or preparationsBUT

2. Avoid capturing fissure patterns as much aspossibleAND

3. Avoid any soft tissue contact

The key to a successful record is not so muchthe type of material used, but how it is used. Thesmaller the amount you use, the less it is likely tocause a problem. A small, trimmed record,restricted to the area of the preps themselves,

PRACTICE


with all interproximal tags and undercutsremoved is the ideal. It is usually necessary totrim the record to achieve these requirementsand silicone materials trim very easily with ascalpel (Fig. 13). One final consideration is thatstone casts and dies can easily be abraded byIORs especially those made of acrylic resin andspecial care is required in the laboratory whenthese materials are used in combination.

Whether or not you have used an inter-occlusal record, you can easily verify the accu-racy of mounted casts in the ICP using foil shim-stock. When held tightly together the cast shouldhold the shim between the same teeth as they doin the mouth.

Copying tooth guidanceWhen a restoration is being provided whichincludes a guiding surface for mandibularexcursions (lateral, protrusive or anything inbetween), the technician needs to know whatform the contacting surface is to take. Failure toappreciate this risks introducing excursive con-tacts that are not in harmony with the otherguidance teeth, the condylar movements andmusculature. This is discussed fully earlier in thearticle, but common manifestations of dishar-mony include:

• Decementing and fractured crowns• Tooth mobility• Tooth migration • TMJ dysfunction

Because they are intimately involved inspeech formation, getting guidance wrong onthe palatal surfaces of upper anterior teeth mayalso come to light as phonetic problems.

The guidance of a new restoration can oftenbe made simply to fit in with that provided byadjacent teeth (for example in group function). In

this case the new functional surface is relativelystraightforward to achieve on a semi-adjustablearticulator or even at the chairside. However, ifseveral teeth are to be prepared there may be noexisting guiding surfaces left intact after prepa-ration, so all clues to guidance are lost (Fig. 14).Where satisfactory guidance is present beforeyou start, there are several ways of copying itbefore you prepare the teeth. A commonly usedtechnique is to use a putty matrix made on a castof the tooth surfaces to be copied, but this tech-nique will often not provide the tight controlover tooth shape which is required. The two mosteffective methods to address this problem neces-sitate the use of a facebow and semi-adjustablearticulator to allow anatomical movements inexcursions. They are:

1. The ‘crown about’ method: Alternate teeth arerestored, thus maintaining the shapes of func-tional surfaces, which continue to provideguidance for the articulated casts. This tech-nique is well described in Reference 7.

2. The custom incisal guide table: This is anotherway of copying satisfactory anterior guidancefrom teeth or trial restorations that providecrucial excursive contacts. Unlike the previousmethod, it enables guidance to be copied forjust one restoration. It involves moving studycasts (on a semi-adjustable articulator)through the full range of lateral and protru-sive excursions with a mound of un-setacrylic on the guide table, so that the tip of thearticulator guide pin shapes the acrylic dough

Fig. 11 A very accurate full archocclusal record in a silicone material.The excessive detail may well hinderseating on a stone cast. See Fig. 12

Fig. 12 The combination of a verydetailed silicone occlusal record andless detailed stone cast, particularlyof the occlusal fissures, has meantthat the record will not seat. Therewere sufficient tooth contacts in theintercuspal position not to need aninterocclusal record for this case!

Fig. 13 A carefully trimmed interocclusal recordrestricted to the area of tooth preparation. The resultingcrowns needed no adjustment

Fig. 14 Preparation of the teeth has resulted in loss of allguiding surfaces on the anterior teeth

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into (once set) a permanent record of themovements of the mandible (Fig. 15). Whenthe working casts are articulated the acrylicguide table guides the articulator through thesame movements that were present in thestudy casts, and the palatal surfaces of theupper teeth can be shaped to conform precise-ly to this. This technique is described in detail,with illustrations, in Reference 4.The extra effort involved in using these tech-

niques is not enormous, and where several ante-rior teeth are to be crowned we would stronglyrecommend using one or other of them.

1. Okeson J P. Management of temporomandibular disorders andocclusion. 1998. 4th Edition. St Louis: Mosby, pp47-51.

2. Posselt U. Studies in the mobility of the human mandible. ActaOdontol Scand 1952; 10 : 109.

3. Wise D. Occlusion and restorative dentistry for the generalpractitioner. Part 2- Examination of the occlusion andfabrication of study casts. Br Dent J 1982; 152: 160-165.

4. Howat A P, Capp N J, Barrett N V J. A colour atlas of occlusionand malocclusion. 1991. London: Wolfe Publishing, pp137-139.

5. Loos L G. Clinical criteria used to select an articulator.Compend Contin Educ Dent 1993; 14: 80-88.

6. Walls A W G, Wassell R W, Steele J G. A comparison of twomethods for locating the intercuspal position (ICP) whilstmounting casts on an articulator. J Oral Rehabil 1991; 18: 43-48.

7. Wise D. Occlusion and restorative dentistry for the generalpractitioner. Part 9 – Restoration of anterior teeth. Br Dent J1982; 152: 407-413.

Fig. 15 A custom acrylic guide table for use witha semi-adjustable articulator. This is an excellentmethod of copying tooth guidance into definitiverestorations

Further reading1. Wise M. Occlusion and restorative

dentistry for the general practitioner.Series of ten articles published in theBr Dent J 1982; 152: 117; 152: 160;152: 197; 152: 235; 152: 277; 152:316; 152: 347; 152: 381; 152: 407;153: 13.

2 Howat A P, Capp N J, Barrett N V J. Acolour atlas of occlusion andmalocclusion. 1991. London: WolfePublishing.

3 Shillingburg H T Jr, Hobo S, Whitset LD, Jacobi R, Brackett S E.Fundamentals of fixedprosthodontics. 3rd ed. 1997.London: Quintessence Publishing Co.

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Crowns and other extra-coronal restorations:Aesthetic controlF. S. A. Nohl1 J. G. Steele2 and R. W. Wassell3

A pleasing dental appearance is the subjective appreciation of the shade, shape and arrangement of the teeth and theirrelationship to the gingiva, lips and facial features. Achieving such a pleasing appearance in our patients is not always easybut is critical, not least because our work is effectively on display and this has implications for patients’ perceptions of ourpractice. To be successful, thorough assessment, careful planning and precise clinical execution is required. Every bit asimportant though, is good communication, both with the dental laboratory and particularly with the patient. In few areas ofdentistry can effective communication be as critical as it is here.

1Consultant in Restorative Dentistry, The Dental Hospital, Framlington Place,Newcastle upon Tyne NE2 4AZ2,3Senior Lecturer in Restorative Dentistry,Department of Restorative Dentistry, The Dental School, Framlington Place,Newcastle upon Tyne NE2 4BW*Correspondence to: F. S. A. NohlE-mail: [email protected]


● Identification and definition of the patient’s aesthetic problem● Consideration of the balance between aesthetics and tooth destruction for conventional and

adhesive restorations● An awareness of the aesthetic limitations of restorations and an attempt to ensure that the

patient’s expectations are realistic● Incorporation of procedures leading to better aesthetics at each clinical stage● Confidence in determining shade and communicating effectively with the laboratory

I N B R I E F

Retention of natural teeth into old age is nowcommonplace and whilst usually desirable, ithas brought with it considerable additionalproblems. Making well-aligned white teeth in acomplete denture is usually straightforward, butmatching a single crown or veneer to a group ofnatural incisors is a different matter altogether.This problem is illustrated by data from the 1988survey of adult dental health in the UnitedKingdom1 which showed that having just one ortwo crowns was more likely to be associatedwith dissatisfaction with the appearance thanhaving none or many.

In each case, planning tooth preparationinvolves the dentist in a cost:benefit analysis,where the cost of improved aesthetics is judgedin terms of removal of tooth tissue and in thepotential for damage to the pulp and periodon-tium. However the benefit of stunning porcelainwork is easiest to achieve where a thick layer ofmaterial can be used to develop the optimumoptical properties, but this usually requires moretooth tissue to be removed. This is a theme whichruns right through all aesthetic considerationsand should underpin what follows. This conceptdoes not sit comfortably with a dogmaticapproach with hard and fast rules about thedimensions of a preparation. The clinician willchoose to alter the cost:benefit balance in differ-ent ways in different cases. The choice might bemade to sacrifice aesthetics for long-term health,or to take a risk with long-term pulp health tomaximise aesthetics; for example with a heavilyprepared ceramo-metal restoration.

Aesthetic improvements are most importantfor anterior teeth and may often be the sole

reason for providing the restoration. The type ofmaterials used clearly have an important bearingon both the appearance and the amount ofpreparation and are an important part of the aes-thetic cost:benefit equation. Table 1 lists the aes-thetic restorations commonly available. Whilstthey are much less destructive of tooth tissuethan traditional ceramo-metal crowns, adhesiverestorations such as the porcelain laminateveneer and dentine bonded crown do have limi-tations: specifically the problem of masking thecolour of darkly stained teeth, problems of tem-porisation and the inability to cement restora-tions provisionally. Veneers are covered in detailin a separate article in this series (Part 12). Furthermore whilst there have been significantimprovements in indirect composite technology

6










restorations

Table 1 Laboratory made aesthetic restorations

• All porcelain

Conventional porcelain jacket crown

High strength porcelain jacket crown

Full coverage porcelain veneer (dentinebonded crown)

Porcelain labial veneerPorcelain onlay

• All composite

CrownVeneerOnlay

• Ceramo-metal crown

• Composite bonded to metal crown

• Partial coverage metal crown

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there is as yet little clinical evidence of their sta-bility and longevity.

The key decisions are similar for anterior orposterior teeth, but there is usually less roomfor aesthetic compromise at the front of themouth. On posterior teeth it may be feasible tosacrifice optimum aesthetics by restricting theuse of porcelain only to the most visible sitesand consequently cutting a less damagingpreparation. For example on a short tooth, cre-ating space for occlusal porcelain and metalrather than for metal alone could make the dif-ference between success and failure of reten-tion. Furthermore, tooth preparation carrieswith it the risk of pulp damage.2

A conservative approach would equate notonly to less pulp morbidity, but more toothremaining should the need arise to remake therestoration. On a posterior tooth it may also bepossible to use a three quarter crown, whichleaves the bulk of the buccal surface intact. Onceagain, whatever the materials chosen, it isimportant that the patient fully understands theadvantages and limitations of the restorativesolution. This article aims to address all of theseissues. The field of dental aesthetics is highlysubjective and, as a result, difficult to research.The advice we offer and the recommendationswe give are necessarily based more on experi-ence than scientific analysis.

IDENTIFYING THE PROBLEMThe first and fundamental key to obtaining asuccessful aesthetic result is to establish the pre-cise nature of the patient’s demands, at the out-set of treatment. What is perceived as ‘natural’ orpleasing to the dentist or technician may bemuch too ‘natural’, and far from pleasing for thepatient. This may sound obvious, but without adetailed assessment it is easy to fail to make aprecise diagnosis of the patient’s desires, and soend up treating something which is not a prob-lem for the patient, or creating technically beau-tiful restorations which the patient perceives tobe aesthetically unsatisfactory, with all the angstand cost that this implies.

Table 2 shows the many factors, which mustbe considered in defining the patient’s aestheticproblem. On the basis of a thorough assessment,the dentist must decide whether:• The patient’s expectations are realistic• The proposed treatment options are in the

patient’s best interests• The dentist, with the support of the laboratory,

has the skill to carry out the treatment.

One of the greatest challenges for dentistsproviding crowns and veneers is the need tomatch expectations to what is technically andaesthetically achievable.

Matching expectations with realityMost patients appreciate a full and frank discus-sion about what is achievable. Time spent at thisstage can save a lot of heartache and expenselater on. There are reversible means of helping

patients understand the scope and possible con-sequences of treatment:• Wax mock-ups (diagnostic wax-ups) on stone

casts can be very useful for demonstratingtreatment options and act as blue-prints forcarrying out clinical and laboratory work(Figs 1 and 2). Some dentists prefer these mod-els to be created in tooth-coloured wax whilstothers express concern that patients may notfully appreciate that these models are to assessshape not colour and for this reason deliber-ately use non tooth-coloured wax.

• Composite resin can be used on teeth in itsuncured state to indicate the potential forshade and additive shape changes to naturalteeth (Figs 3 to 5).

• When viewed against the darkness of themouth, black, water-soluble ink can give arough idea of the effect of subtractive shapechanges such as shortening overerupted lowerincisors (Fig. 4).

• Computer software, which can manipulatephotographic images, is becoming common-place in the dental surgery and can be usefulin discussions of treatment options. However,

Table 2 Factors contributing to dental aestheticsDental Mucogingival Facial General

• Shade • Margin level • Lip levels at rest • Age

• Contour • Margin pattern • Lip levels in function • Occupation

• Texture • Shape of papillae • Centre line • Gender

• Shape • Colour • Personality

• Position • Thickness

• Arrangement

• Interproximal contacts

• Incisal embrasures

• Gingival embrasures

• Incisal level

• Posterior occlusal level

Fig. 1 Short anterior teeth causingan aesthetic problem

Fig. 2 Diagnostic wax-up of Case infigure 1

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it is probably fair to say that it is much moredifficult to provide restorations exactly as cre-ated on a screen than to use a wax model as aguide, especially if the patient has been givena colour enhanced print of the expected out-come! Unlike the techniques described above,computer manipulation of images has nophysical limits and may as a result createunrealistic expectations.

• Photographs of previous cases may helppatients to understand both the possibilitiesand the limitations. Restricting these to theones with the best results may be a mistake.Where there are clearly going to be aestheticlimitations, it is probably best to illustratethem with realistic examples. The patient maybe more likely to agree to treatment if theycan picture the outcome, rather than imagineit from a verbal description.

• Provisional restorations can allow the subtlerelationship between the shape and form ofthe teeth and the soft tissues and facial fea-tures to be evaluated and decided before thefinal restoration is constructed. They canalso ensure that tooth preparation forrestorative material is adequate (by estab-lishing the desired shape and form and then

measuring the thickness of temporaries withcallipers).

Unrealistic expectationsSome patients may demand changes inappearance which are objectively difficult toappreciate and still more difficult to realise. Inmost cases this is simply a problem of commu-nication, but unrealistic expectations and ahistory of multiple previous treatmentsaddressing appearance may be a warning of apatient with Body Dysmorphic Disorder (BDD)or Dysmorphophobia3: a preoccupation with adefect in appearance which is either imaginedor excessive in relation to a minor defect andwhich causes significant distress in social,occupational and other areas of life. BDD isprobably rare but is an extraordinarily difficultproblem to deal with. It is unlikely thatdemands to change appearance will be satis-fied for this group of patients. A second opin-ion is a perfectly acceptable course of action ifin doubt.

FINAL PLANNING AND CLINICAL PROCEDURESHaving decided on the restoration type, itremains to finalise margin features and carry outthe clinical stages, ultimately leading to cemen-tation.

Supra or sub-gingival: where should the crownmargin go?Factors identified in the assessment shouldhelp to determine the location of crown mar-gins. Where the margins are not visible, thereare good biological arguments for placing allmargins supra-gingivally.4 These sites wouldinclude all margins on molars, lingual andinterproximal sites, and buccal aspects ofanteriors and premolars where functional lippositions obscure the gingival margin. Notonly will this facilitate finishing and mainte-nance but should also favour periodontalhealth. The appearance of a supragingivalmargin can be optimised by ensuring that thefinish line is in harmony with the level of thegingival margin.

Where exposed crown margins are likely tocreate an aesthetic problem, margin placementup to 1 mm into the gingival sulcus can beacceptable.5 However, great caution should be exercised as gingival margins are at risk ofrecession, particularly when there are prominentroots, thin gingival tissues or in the presence ofperiodontitis. Though periodontal attachmentloss occurs to a greater or lesser degree throughlife,6 trauma during clinical stages and poorlyfitting temporary and final restorations willincrease the risk of gingival inflammation andsubsequent recession. The need to optimiseretention by increasing preparation length maybe better addressed by removing periodontalattachment surgically (‘crown lengthening’),than by making the margin encroach deeply intothe gingival sulcus and attachment apparatuswith the risk of inflammation.

Fig. 3 Case requiring changes inupper lateral incisor length andlevelling of lower incisal plane

Fig. 4 Trial alterations to case infigure 3: uncured composite addedto upper incisal edges and water-soluble ink to lower left incisors andcanine

Fig. 5 Final restoration of upperlateral incisor edges of case in figure 3 (directly applied compositeresin used in this instance)

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Shoulder or chamfer: what should thepreparation finish line be like?There is a forceful argument that where possible,ceramo-metal crowns should have metal mar-gins because this produces the most predictablemarginal seal7 but as discussed earlier in thisseries (Part 2 ‘Materials Considerations’), this is acontentious issue. However by avoiding themetal collar, a porcelain butt fit, created on ashoulder finish line, will generally allow for better aesthetics in critical areas. A restorationwhose margin is in porcelain may allow light topass into porcelain from the gingival aspect as itdoes into intact teeth contributing to a lifelikeappearance.8

How much metal: where should the porcelain-metal junction on ceramo-metal crowns be?There is no biological or technical benefit inusing porcelain at sites that are not visible. Con-sideration given to the precise location of porce-lain-metal junctions for ceramo-metal crowns atthe planning stage gives the potential to opti-mise conservation of tooth structure yet stillmaintain satisfactory aesthetics. Volume to vol-ume, the extent of reduction for metal alone issubstantially less than for metal and porcelain:different depths of tooth reduction can be usedat different sites depending on the coveringmaterial(s). Tooth preparation then becomes anordered technical exercise to satisfy the need fordifferential space attainment. It should be obvi-ous to the technician examining the resultingdie where to locate porcelain-metal junctions(Fig. 6). There are laboratory cost implications toprovision of ceramo-metal crowns of this sort. Itis necessary to wax a full contour restoration onthe die, mark the porcelain-metal junction andthen cut back space in the wax pattern for porce-lain rather than simply to create a thin metalcoping over the whole preparation which is cov-ered by porcelain.

Shade matchingShade matching is something many of us finddifficult and is often done last whereas in fact itshould be done first! It is not an exact science,involving as it does a good deal of subjectivejudgement. Although an accurate reproductionof shade is an obvious goal, it cannot bedivorced from consideration of shape, surfacetexture and special characteristics, which aredescribed later. Teeth possess a range of opticalfeatures seemingly designed to make shadematching difficult! Teeth:• Are non-uniform in colour• May have complex visible internal and sur-

face features• Are semi-translucent• Exhibit a degree of fluorescence• Change shade and shape with age

In addition, a good shade match to porcelainin one light condition may be a poor one underdifferent lighting: a phenomenon termedmetamerism. Despite these obstacles, the best

porcelain restorations go a long way to repro-ducing nature using a combination of skilfulartistry and optical trickery. Before recordingand prescribing shade it is useful to have a basicunderstanding of the science and dimensions ofcolour and texture so that shades can be inter-preted and communicated precisely.

Dimensions of colourColour can be described in terms of three dimen-sions:1. Hue: The name of the colour eg blue, red etc.2.Value: An achromatic measure of the light-

ness or darkness of a particular colour suchthat high value refers to a shade which islight and low value to one which is dark.Two completely different colours can haveexactly the same value. To help understandthis, imagine the effect of black and whitetelevision on colours.

3. Chroma: The strength or saturation of a colourof particular hue. Imagine increasing thechroma of a small amount of colour pigmentdiluted in water by adding more of the samepigment.

Shade guides in common use (Figs 7 and 8)are not designed for a systematic assessment ofthe dimensions of colour and have been criti-cised for not including a broad enough range ofshades. Two commonly used guides (Vita Luminand Ivoclar Chromoscope) are composed ofgroups based essentially on hue (Vita Lumin: A= reddish brown, B = reddish yellow, C = greyshades, D = reddish grey, Ivoclar Chromascop: 1 series = cream, 2 series = orange, 3 series =light brown, 4 series = grey, 5 series = darkbrown), with sub-classes of varying value andchroma. The Vitapan 3D Master system uses asimple but methodical approach to shade deter-mination based on the three dimensions ofcolour. In common with previous work,9 the

Fig. 6 Die with reduction to matchveneering material(s): the largerbuccal axial reduction is needed toaccommodate metal and porcelain,the smaller palatal reduction is formetal alone

Fig. 7 Vita Lumin shadetabs with stained necksremoved in order ofdecreasing value

Fig. 8 IvolcarChromascop shadeguide

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Vitapan 3D Master system emphasises value asthe most important dimension in colour match-ing for porcelain restorations. Whichever guideis used, it is useful to understand colour termi-nology as it forms a language for communicat-ing additional information about colour to thelaboratory.

Surface textureThis quality describes surface contour both at a‘macro’ level, such as developmental lobes andridges, as well as fine surface detail such asperikymata. The lustre of a restoration describesthe level of glaze produced in the porcelain ovenor by various rotary instruments and polishingtechniques. Lustre can effect value perceptionsuch that high lustre raises value. It is thereforean important feature to match and one which isoften neglected. At the very least, terms such ashigh, medium or low lustre can be used on theprescription, and are more effective if they arelinked to a standardised reference guide whichcan be used both in the surgery and in the dentallaboratory. The technician can often get a goodindication of other surface features from sur-rounding teeth.

Special characteristicsThese include fracture lines, white spots andtranslucency. The best looking special character-istics are incorporated during incrementalporcelain application. Surface stains can be usedto produce some of these effects but are prone towearing away with time.

Choosing and prescribing a shadeAs well as factors inherent in teeth themselves,barriers to accurate shade matching ofteninclude inappropriate viewing conditions, shadeguides that tend to be made from thick layers ofhigh fusing porcelain and even colour blind-ness.10 Against this background, it is perhaps notsurprising that different dentists tend to matchthe same tooth differently from each other, andeven from previous attempts to match the sametooth themselves.11 Choosing a shade will benefitfrom adherence to a protocol based on sound rea-soning. Table 3 gives a method for assessingshade and surface texture12 which we think ishighly appropriate and is easy to apply. Occasion-ally custom-made shade tabs with varyingamounts of porcelains and a range of surface tex-tures and special characteristics are helpful. Elec-tronic optical devices have been produced toassist making an objective assessment of shadebut their usefulness remains to be fully evaluated.

Tooth preparationAchieving optimum aesthetics depends heavilyon providing the technician with adequate spacefor the incremental application of porcelain (Fig. 9). The considerations have already beendiscussed above, but when it comes to the prac-ticalities, the extent of tooth preparation is bestvisualised intra-orally by reference to a prepara-tion guide. A small putty mould, made over the

Table 3 A scheme for shade determination

1. Determine shade at the start of an appointment before the risk of eye fatigue and tooth dehydration with resultant shade change (especially after use of rubber dam).

2. Use either natural light (not direct sunlight) or a colour corrected artificial light source.

3. Drape the patient with a neutral coloured cover if clothing is bright and have the patient remove brightly coloured makeup.

4. Assess value by squinting. The reduced amount of light entering the eye may allow the retinal rods to better distinguish degrees of lightness and darkness. (Vita Lumin shade tabs set in order of value facilitates this [Fig. 7])

5. Make rapid comparisons with shade tabs (no more than 5 seconds each viewing). Gazing at a soft blue colour between attempts is said to reduce blue fatigue, which can result in accentuation of yellow-orange sensitivity.

6. Choose the dominant hue and chroma within the value range chosen. The canines have high chroma and may be a useful guide to assessing hue.

7. Compare selected tabs under different conditions eg wet vs dry, different lip positions, artificial and natural light from different angles.

8. Select a shade which is higher in value (lighter) if in doubt. Surface stains can reduce these dimensions but not easily increase them.

9. Look carefully for colour characterisation such as stained imbrication lines, white spots, neck colouration, incisal edge translucency and halo effect (a thin opaque line sometimes seen within a translucent incisal). Shade tabs exactly representing the pure porcelains and stains available can be useful for this task. Simple diagrams are invaluable.

10. Determine surface lustre.

Fig. 10 Putty mould sectioned andnumbered on a diagnostic wax-up

Fig. 9 Space requiredfor metal coping andlayers of porcelain fora ceramo-metal crown

ToothCement luteMetal copingOpaquerOpacious dentineDentineEnamelTranslucent

PRACTICE


tooth before preparation, and then cut in cross-section is invaluable if the shape of the tooth isto be maintained. A putty mould (Figs 10 and 11)or vacuum formed matrix made from a diagnos-tic wax-up is required if the shape of the tooth isto be changed (Fig. 12). Depth cuts to guidetooth reduction may be a useful guide to ensureadequate reduction, but are not very helpfulwhen shape changes are planned. Matrices areparticularly helpful on the buccal surfaces ofupper anterior teeth which are curved whenviewed from the mesial or distal. There is a ten-dency to prepare the buccal surface in a single plane, ignoring the curvature (Fig. 13).The aesthetic result will either be a bulky crown(if the full thickness of porcelain is placed), or acrown where the contour is correct, but wherethe core porcelain is inadequately masked. Toachieve a good aesthetic result the buccal sur-face preparation should follow the natural cur-vature of the tooth (see Fig. 11).

Clinical recordsAs well as the role of the facebow record inhelping to make movements of casts on an

articulator anatomical, it ensures that articulat-ed working casts are orientated to the base ofarticulator in the same way that the patient’steeth are orientated with respect to the floor (ifthe patient’s head is upright and the anatomicalfeatures used as reference points are normallyrelated!). This helps the technician ‘see’ therestorations orientated as they would be whenobserving the patient. Very occasionally anear-bow recording can give an erroneous inter-pretation of the relationship of the occlusal tothe horizontal plane. This discrepancy occursas a result of the patient’s ears being at differ-ent levels and may need to be compensated forwhere multiple anterior crowns are prescribed.

Try-in and cementationWhere shade matching has been difficult forconventional crowns, there is merit in trialplacement of moistened restorations before giv-ing them their final surface finish. Surface stainsand changes in surface form can be prescribed atthis stage (Fig. 14). It should be remembered thatsurface stains might eventually be lost.

After glazing, a period of trial cementationleaves scope for a further period of assess-ment by both patient and dentist. If restora-tions are subsequently returned to the labora-tory for adjustment, steps must be taken todehydrate porcelain before firing to avoid therisk of fracture. Furthermore, temporarilycemented definitive crowns can be difficult toremove. Zinc oxide and eugenol-basedcement must have its mechanical propertiessignificantly reduced by adding a modifier asdescribed later in this series (Part 11 ‘Try-inand cementation’).

Pigmented luting agents allow subtle manip-ulation of shade for adhesive porcelain restora-tions. Some systems provide water-based trial

Fig. 11 Putty matrix in-situ to helpvisualise appropriate tooth reduction

Fig. 12 Vacuum formed matrix in-situ

Fig. 13 Crown preparation with no second plane ofreduction

Fig. 14 Ceramo-metal crown at the pre-glaze stage.Along with written instructions, pencil marks help toindicate changes required

1. Todd J E, Lader D. Adult Dental Health1988: United Kingdom. Office ofPopulation Censuses and Surveys,1991.

2. Saunders W P, Saunders E M.Prevalence of periradicularperiodontitis associated withcrowned teeth in an adult Scottishsubpopulation. Br Dent J 1998; 185:137-140.

3. Cunningham S J, Bryant C J, ManisaliM, Hunt H P, Feinman C.Dysmorphophobia: recentdevelopments of interest to themaxillofacial surgeon. Br J Oral andMaxillofac Surg 1996; 34: 368-374.

4. Valderhaug J, Birkeland J M.Periodontal conditions in patients 5 years following insertion of fixedprostheses. J Oral Rehabil 1976; 3:237-243.

5. Freilich M A, Niekrash C E, Katz R V,Simonsen R J. Periodontal effects offixed partial denture retainermargins: configuration and location.

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cements to facilitate the choice of colour. Manu-factures’ instructions should be followed.

COMMUNICATION WITH THE LABORATORYThe dentist must accept ultimate responsibilityfor all aspects of completed laboratory work.On the face of it this might suggest that a total-ly prescriptive one-way communication isrequired. Not surprisingly such an attitude canlead to feelings of frustration and dissatisfac-tion to all concerned. It does not have to belike this! Trained technicians are highly skilledin a unique blend of art, craftsmanship andscience (as can be appreciated very rapidly byany dentist attempting to wield wax or porce-lain!). Better then to foster a team approachand central to a conflict free relationship is theestablishment of dialogue and clearly definedroles for dentist and technician. To this endthere is little to beat a personal visit to the lab-oratory and subsequently it is helpful to beavailable to speak to technicians and to shareideas. Certainly it is important at the very leastto provide a clear written prescription whichshould include a diagram to enable regional

variations in shade and special characteristicsto be understood. Where there are difficultiesin recording shade a wise dentist will involvethe technician in the decision. Compliments aswell as constructive criticism will help techni-cians evaluate their work, and anyone whotakes pride in their work will appreciate theopportunity to see the final result of a job welldone. It is probably fair to say that qualityclinical work will be rewarded with higherquality restorations.

CONCLUSIONA complete understanding of a patient’s aes-thetic problems is the key to treatment plan-ning. Only then can an attempt be made tomatch expectations with realities and to pro-vide appropriate restorations. This processdepends heavily on an understanding of thelimitations of the techniques and materialsavailable.

Manufacturers’ details:Ivoclar-Vivadent Ltd, Meridian South, Leicester LE3 2WYVITA Zahnfabrik, H Rauter GmbH & Co KG, Postfach, D-79704, Bad Säckingen, Germany

J Prosthet Dent 1992; 67: 184-190.

6. Abdellatiff H M, Burt B A. Anepidemiological investigation into therelative importance of age and oralhygiene status as determinants ofperiodontitis. J Dent Res 1987; 66: 13-18.

7. Bishop K, Briggs P, Kelleher M. Margindesign for porcelain fused to metalrestorations which extend onto theroot. Br Dent J 1996; 180: 177-184.

8. Lehner C R, Manchen R, Scharer P.Variable reduced metal support forcollarless metal ceramic crowns: a newmodel for strength evaluation. Int JProsthodont 1995; 8: 337-345.

9. Sproull R C. Color matching in dentistry.Part II: Practical applications of theorganization of color. J Prosthet Dent1973; 29: 556-566.

10. Moser J B, Wozniak W T, Naleway C A.Colour vision in dentistry: a survey. J Am Dent Assoc 1985; 110: 509-510.

11. Culpepper W D. A Comparative study ofshade-matching procedures. J ProsthetDent 1970; 24: 166-173.

12. Sorensen J A, Torres T J. Improved colormatching of metal-ceramicrestorations. Part I: A systematicmethod for shade determination. J Prosthet Dent 1987; 58: 133-139.

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BRITISH DENTAL JOURNAL VOLUME 192 NO. 9 MAY 11 2002 499

Crowns and other extra-coronal restorations:Cores for teeth with vital pulpsR. W. Wassell1 E. R. Smart2 and G. St. George3

Cores for teeth with vital pulps is the seventh in the series of crowns and other extra-coronal restorations. A core is definedas ‘that part of a preparation for an indirect restoration consisting of restorative material’. This article questions the need forroutine pin placement and addresses the following issues — removal of existing restorations, the need for a core, corematerials, core retention, and problem solving.

1*Senior Lecturer, 2Lecturer, Department ofRestorative Dentistry, The Dental School,Newcastle upon Tyne NE2 4BW 3SpecialistRegistrar in Restorative Dentistry, EastmanDental Hospital, 256 Grays Inn Road,London WC1X 8LD*Correspondence to: Dr R.W. Wassell, Dept.of Restorative Dentistry, The DentalSchool, Newcastle upon Tyne NE2 4BWE-mail: [email protected]


● Core placement nowadays demands more use of adhesives (coupled with retentive cavitypreparation) and less use of pins

● It is good practice to remove existing restorations of unknown provenance to facilitate cavityinspection and ensure core retention

● Cores act either as a simple space filler or a structural build-up. The less tooth structure thegreater the mechanical demands on the core, the material for which must be chosen carefully

● Dentists placing pins need to be aware of how to prevent and manage pin placement problems

I N B R I E F

For almost half a century pinned amalgamrestorations have been taught as the basis forcores in posterior vital teeth. Many schools,including our own, continue this venerable tra-dition in the knowledge that when skilfully car-ried out in optimal circumstances it seems towork well. However, clinical research on pinplacement shows that the technique is not with-out problems.1 Of 429 pin placements by staffand students at Leeds Dental School 19%showed complications — most frequently a loosepin or inadequate penetration of the pin into thepin channel. However, 10% of complicationswere serious involving either perforation of thepulp or periodontium, or tooth fracture. Inexpe-rienced operators are especially prone to prob-lems with almost half of the pins (41%) placed bysecond year students proving unsatisfactory.Experienced operators are not immune to diffi-culty; a survey of 37 practitioners placing 1394pins over a 3-month period reported difficultiesin 1 in 20 placements.2

A considerable amount of in-vitro evidence(see Table 1) shows the potential pins have forcausing crazing of the dentine (look at Fig. 1)and pulpal sensitivity.

In addition, an animal study12 showedsevere pulpal inflammation where pins wereplaced within 0.5 mm of the pulp. Some self-shearing pins cause stressing and crazing whenthey ‘bottom out’ in the pin channel,4 whichmay explain the symptoms some patientsdevelop following pin placement. Of course,symptoms may also arise from undetected pulpal or periodontal perforations.

Fortunately there are other methods of retain-ing a core including the use of existing cavity

preparation features, additional slots and boxesand, most importantly, adhesion. Much of theskill in placing a core involves the selection ofthe most appropriate material and technique, butwhatever type of core is placed an importantprinciple is that existing restorations should firstbe removed.

REMOVAL OF EXISTING RESTORATIONSFigure 2 shows a tooth that clearly is a potentialcandidate for a crown. Only one cusp remainsplus the remnants of another. The tooth has beenrepaired on a number of occasions and there isveritable jigsaw of amalgam ‘fillings’. There isalso a suggestion of mesial caries. No dentists intheir right mind would pick up a handpiece andstart to prepare this tooth for a crown. The mostlikely outcome would be a complete collapse ofwhat is there with the potential for a rather diffi-cult reassembly. Furthermore, if what is on dis-play is representative of previous treatment, thepossibility exists of there being residual as wellas recurrent caries and even perhaps latentinvolvement of the pulp chamber.

Figure 3 shows a bitewing radiograph of thetooth. This adds to the diagnostic complexity ofthe case. What is holding the amalgam in place?There is no evidence of pins or other retentivedevices and there are signs that the pulp cham-ber has been visited in the past. The clear evi-dence of dentine bridges indicate a successfulpulpotomy and the question arises whether thisshould be accepted or in view of the potentialrisk of future pulp problems, should the tooth beroot filled? (In this case the fillings were replacedwith a more acceptable amalgam restoration andthe tooth put on probation with a view to

7










restorations

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500 BRITISH DENTAL JOURNAL VOLUME 192 NO. 9 MAY 11 2002

re-assessing the endodontic condition andcrowning it later.)

Before a crown preparation is undertaken,look closely at the tooth and form a mental pic-ture of what will be left after cutting. Considerthe existing restorations; decide on whether theyare sound and whether they will remain in placeduring the cutting procedure. A history of therestoration should be determined but if this isnot possible, especially if another operator hasdone the previous treatment, then considerationshould be given to replacing the restoration.From personal experience, such restorationshave been found on removal to have caries,cracked cusps and in some cases, latent pulpexposure beneath them. It is better to removesuch fillings rather than be faced with an embar-rassing endodontic emergency after the crownhas been fabricated and cemented. It also givesyou the chance to inspect (preferably undermagnification) what remains of the tooth anddecide whether this can be satisfactorily adaptedto retain the core.

In summary, removal of existing restorationsallows proper assessment of:• The tooth’s structural integrity (bearing in

mind the need for mechanical core retentionfollowing tooth reduction during crownpreparation)

• Pulpal exposure• Underlying caries

THE NEED FOR A COREIt is useful to think of a core as either a ‘build-up’, which contributes significantly to thestrength and retentiveness of the crownpreparation, or ’filler’ that simply alters theshape of the preparation, usually with the pur-pose of eliminating undercuts.13 Fillers areoften used on anterior teeth where class IIIand V restorations need to be replaced duringtooth preparation.

Occasionally it may be necessary to elec-tively devitalise a tooth and provide a post andcore, often in combination with crown length-ening, to give adequate retention for therestoration (eg where only a root face exists).However a resin bonded indirect restorationmay provide a viable alternative to devitalisa-tion, especially where the intended crown isshort occluso-gingivally. In this situation theadhesive would be subjected to less heavy peeland sheer stresses during function than if thecrown were long.

It is worth emphasising that crown prepara-tions do not always have to be built up with acore to an ‘ideal shape’. Instead, existing cavityfeatures can be refined to give suitable groovesand boxes. This approach is particularly usefulfor crown preparations affected by short clinicalcrown height (Fig. 4).

Cores may be placed either at the time oftooth preparation or beforehand. Where apatient requires only a single restoration thecore and preparation can be made simultaneous-ly in which case it is easiest to contour the coreto resemble the intended preparation (Fig. 5).However, placing the core in advance gives abetter opportunity to assess the integrity of atooth and its pulp chamber and choose the mostappropriate indirect restoration for the amountand configuration of remaining tooth tissue.Cores placed in advance should be properly contoured to provide occlusal stability, patientcomfort and freedom from food packing (Fig. 6).This approach becomes more crucial wherepatients require multiple cores. Once such coresare placed, the patient can be reviewed to ensurecompliance with oral hygiene instruction,dietary advice and disease control. If thepatient’s response is unsatisfactory the provision

Table 1. In vitro studies demonstrating problems with retentive pin placementProblem Reason for problem Study

Dulling of twist drill during pin channel preparation caused by Newitter et al.,an adherence of smear debris behind the drill’s cutting edge. 19893

Stresses within dentine resulting from differences in Standlee et al., diameter between the drill and the pin. Potential for 19714

damage increases with the number of pins inserted and . Kera et al.,the pin diameter 19785

Pin placement strain caused by pins which have a core Bione and Wilson, diameter (ie at the inner aspect of the thread) greater 19866

than the drill diameter

Any fluid in the pin channel during pin insertion Hummert andcan contribute to cracking. Kaiser, 19927

Extensive cracks occurring with larger sized pins frequently Webb et al., communicate with the pulp chamber. 19898

Heat generated during pin channel preparation, which is Cooley andgreater with larger diameter drills at higher speeds and Barkmeier, 1980 9

deeper penetration.

Microleakage, which is worst with cemented pins and least Chan, 197410

with threaded pins. Cavity varnish does not reduce the problem

Some pins only partially penetrate the pin channel. Barkmeier and This will reduce retention and leave a dead space prone Cooley, 197911

to bacterial invasion.

Fig. 1 Section of threaded pinsplaced into dentine. Notice thedentine crazing (A) and the failure of a pin to fully penetrate the pinchannel (B)

Fig. 2 Would you use this amalgamas a core?

Fig. 3 Bitewing radiograph of tooth in Fig. 2 revealing anumber of underlying problems

Crazing of dentine

Pulpal inflammationfollowing pinplacement

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of crowns may be suitably deferred. Core build-up materials need therefore to be chosen withcare, as there is always the possibility that the core may in itself become the definitiverestoration.

CORE MATERIALSThe material requirements of a core will differdepending on whether it is to be used as a build-up or filler. As a rule of thumb if sufficient toothremains to provide a strong and retentive prepa-ration then the core acts simply as filler. Shouldyou be in any doubt it is better to choose astrong build-up material than risk mechanicalfailure of weak filler.

AmalgamAdvantages• Not especially technique sensitive• Strong in bulk section• Sealed by corrosion products• Can be ‘glued’ into place with cements and

resinsDisadvantages• Best left to set for 24 hours before tooth

preparation• Weak in thin section• Mercury content may be of concern to some

patients and dentists• Potential electrolytic action between core and

metal crown• Not intrinsically adhesive

Recommendations• Excellent core build-up material for posterior

teeth• Excellent interim restoration for posterior

teeth• Adhesives and preparation features can often

substitute for pin retention

Amalgam has traditionally been regarded asthe best build-up material under conventionallycemented crowns as it has good bulk strengthand is sealed by its own corrosion products. It isnot especially technique sensitive providing thatduring placement it is well condensed and is notgrossly contaminated by blood or saliva.Although it is possible to find rapidly settingamalgams it is usually better to delay crownpreparation for at least 24 hours.

Amalgam’s main disadvantage lies in its mer-cury content which a minority of patients anddental practitioners find unacceptable. Also, thepotential electrolytic action between the metalsubstructure of the future crown and the under-lying amalgam is to some individuals a cause ofconcern14 as it may liberate metallic ions. How-ever, the intervening cement will act as an insu-lator and limit ion liberation, so unless thepatient is suffering from lichen planus this isunlikely to be a problem.

Amalgam is weak in thin section and for thisreason has no role to play in the provision ofcores in anterior teeth. In posterior teeth the coremay flake away if left in insufficient bulk follow-ing tooth preparation. Its retention is mainlymechanical, but increasingly adhesives are beingused as will be discussed in the next section.

Fig. 4a A core is not always necessary. With teeth ofshort clinical crown height consider using internalpreparation features rather than building up to a‘classical’, textbook preparation

Fig. 4b The boxes and grooves of the preparation will beengaged by the fit surface of the crown providing asubstantial increase in retention

Fig. 5 It is sometimes convenient tobuild a core to resemble theintended preparation. (Courtesy ofProfessor Ian Barnes)

Fig. 6 Fully contoured cores areessential when longer-term interimrestorations are needed

PRACTICE


CompositeAdvantages• Strong • Can be used in a thinner section than amalgam• Fast setting (either light or chemically cured)• Does not always need a matrix during place-

ment• Avoids mercury controversyDisadvantages• Highly technique sensitive• Relies on multi-stage dentine bonding requir-

ing effective isolation• Dentine bond can be ruptured by polymerisa-

tion contraction• Minor dimensional changes caused by the

coefficient of thermal expansion (three timeshigher than the tooth) and water absorption —not usually clinically significant

• Can be difficult to distinguish between toothand core during preparation

Recommendations• Excellent build-up material for posterior and

anterior teeth if isolation assured• Aesthetic interim restoration, but takes far

longer to place than amalgam• Pin retention rarely necessary

Although composite is as strong as amal-gam15 it has only recently been accepted as agood core material, albeit a less forgiving one.Without dentine bonding agents microleakage16

is a significant problem. Dentists who placedcomposite cores before the advent of dentinebonding agents will remember the resultingcaries and pulpal problems — although surpris-ingly, this problem was never documented sci-entifically.

Effective bonds between composite and toothare now possible, but only where moisture contamination and shrinkage can be properlycontrolled. The term ‘wet bonding’, whereby thedentine is left damp following etching and rins-ing to encourage better penetration of theprimer, should not lull us into a false sense ofsecurity. Experience shows blood and salivacontamination will render bonding useless. Wetherefore recommend the use of rubber dam andincremental placement of light cured compositeto reduce shrinkage problems. Chemically curedcomposite can be placed as a single increment asshrinkage stresses are partially dissipatedthrough the much longer setting time. Where

totally effective isolation cannot be achieved, asin many subgingival situations, the use of com-posite cores is contraindicated.

With an amalgam core there is usually littleproblem in identifying whether the finish linefor a crown preparation lies on a core or onsound tooth tissue. With tooth coloured coressuch discrimination can be difficult, even if acontrasting shade of composite is chosen. Toovercome this difficulty composites have beenintroduced with titanium filler particles. Theseare chemically setting materials, which despitetheir impressively strong appearance havelower values of diametral strength, compres-sive strength and fracture toughness than reg-ular light cured composite.15,17 They are how-ever stronger than GIC.

Glass ionomerAdvantages• Intrinsically adhesive• Fluoride release — but this does not guarantee

freedom from 2o decay (Fig. 7)• Similar coefficient of thermal expansion to

toothDisadvantages• Considerably weaker than amalgam and com-

posite• Tendency to crack worsened by early instru-

mentation• Silver containing materials offer little

improvement in physical properties• Some materials radiolucentRecommendations• Excellent filler but relies on having sufficient

dentine to support crown• Where used as a build-up, best to leave tooth

preparation until next appointment• Good material on which to bond restorations

with resin cement

Some dentists favour glass ionomers cementsfor cores, in view of the apparent ease of place-ment, adhesion, fluoride release, and matchedcoefficient of thermal expansion. Silver contain-ing GICs18 (eg the cermet, Ketac Silver, EspeGMbH, Germany) or the ‘miracle mix’ of GIC andunreacted amalgam alloy have been especiallypopular. Some believe the silver within the mate-rial enhances its physical and mechanical proper-ties, however, in-vitro studies are equivocal19,20

and a study of a cermet used to fill deciduousteeth showed that it performed less well than aconventional GIC.21 In the days when manyGICs were radiolucent, the addition of silverconferred radiopacity without which it would bedifficult or impossible to diagnose secondarycaries. Nowadays, many conventional GICs areradiopaque and are easier to handle than the silver containing materials. Nevertheless, manyworkers regard GICs as inadequately strong tosupport major core build-ups.15,17,22–24 Hencethe recommendation that a tooth should have atleast two structurally intact walls if a GIC core isto be considered.25 In our view it is best to regardGIC as an excellent filler but a relatively weak

Fig. 7 GICs release fluoride, but thisis not always effective in preventingdecay as seen in this radiograph of a10-year-old Ketac Silver core in thelower first molar

PRACTICE


build-up material (Fig. 8). In order to protect aGIC core the crown margin should, whereverpossible, completely embrace 1–2 mm of soundtooth structure cervically. Extension of thecrown margin in this way is termed the ‘ferruleeffect’26 and should ideally be used for all cores.

Resin modified glass ionomersAdvantages• Command set• Stronger than regular GICs• Either intrinsically adhesive or with simplified

bonding system• Fluoride releaseDisadvantages• Most are weaker than amalgam and composite• Hydrophilic resins cause swelling and can

crack overlying porcelain • Some materials can inhibit surface setting of

addition silicone impressions• Not reliable like amalgam and composite as an

interim restorationRecommendations• Useful filler but confers few advantages over

regular GIC

These materials come in a number of presen-tations, which can be used for a variety of pur-poses including fillings, cementation and coreplacement. They have been developed to provideproperties intermediate between regular GICsand light cured composites. There is a spectrumof such materials. At one end are those that startto set in the same way as a GIC following mixingbut are rapidly hardened by light curing theincorporated resin (eg Fuji II LC, GC InternationalCorp, Leuven, Belgium). At the other end of thespectrum are the ‘compomers’, which have aninitial setting reaction similar to composites (egDyract, Dentsply, Weybridge, UK). The GIC reac-tion does not occur until later when moisturefrom the mouth is absorbed into the set resinmatrix where it activates incorporated poly-acids. The resins used in these materials arehydrophilic and swell slightly following waterabsorption. This expansion has the potential tofracture ceramic restorations overlying coresand cements made with resin modified glassionomers or compomers.27

Dentists have received these materials withsome enthusiasm not least because the adhesivesystems are easier to use than for compositeresin and unlike GICs their rapid set does notdelay tooth preparation. As well as good han-dling properties there is also the advantage offluoride release.

Vitremer (3M, St Paul, USA) is an RMGI withgood strength properties,28 specifically advocatedas a core material. In a short-term clinical trial itbehaved satisfactorily under gold crowns, with-out the need for pin placement.29 However, dur-ing the 3 months before crown preparation athird of the cores developed significant surfacedefects, which, although eliminated by crownpreparation, suggest the material unsuitable forlong-term interim restorations needed to estab-

lish occlusal stability. The authors were also atpains to emphasise the need for ferruling thecrown preparation onto sound dentine in thesame way as for regular GICs. Also of some con-cern, Vitremer prepared with a diamond bur hadan inhibitory effect on the setting of a polyvinylsiloxane impression material.30 This unset mate-rial may result in die inaccuracies.

At the present time we are not wildly enthusi-astic about using these materials for cores otherthan as fillers.

CORE RETENTIONIn this section we consider techniques of secur-ing the core, which may be used either singly orin combination. These include:

• Cavity modifications• Resin bonding• Cement bonding• Pins

There is little specific evidence of how wellthese methods work to retain cores but muchinformation can be derived from laboratorystudies and clinical studies of large amalgam orcomposite restorations.

Cavity modificationsAnyone who has had a core detach within acrown (Fig. 9) will know that it is unwise to placecomplete faith in either glues or pins. To gainmechanical retention for the core it is alwaysworth capitalising on existing cavity featuressuch as boxes or an isthmus. Where there is onlya small amount of tooth tissue remaining it isalso worth considering crown lengthening toensure the crown margin is ferruled onto soundtooth structure.

Improved interlock between core and toothcan often be obtained by cutting new boxes orgrooves, or by reducing and onlaying weakenedcusps with core material. Where cusps areonlayed in this way the material must be suffi-ciently thick so that the core is not catastrophi-cally weakened during occlusal reduction of thecrown preparation. As a rough guide cuspsshould be reduced in height where they are lessthan 1 mm thick or the wall thickness to heightratio is less than 1:1.31 Another useful tip is toresolve sloping walls into vertical and horizontalcomponents. This approach will improve theresistance for both cores and castings. Whencutting these auxiliary features one clearlywants to conserve tooth structure, but it isworth sacrificing non-critical amounts to makethe work reliable. Problems with pulpalinvolvement may occur if such features are cutinto the ‘heart’ of the tooth — a term used byShillingburg31 to describe the central volume ofdentine beneath which lies the pulp. The heartmay be avoided by not cutting any featuresmore than 1.5 mm from the amelo-dentinaljunction (ADJ) in a transverse plane.

Most dentists are familiar with the use ofproximal grooves to retain Class II amalgamrestorations. Not so many dentists know that

Fig. 8 GICs work best where theretention and resistance for thedefinitive restoration is derivedmainly from surrounding toothtissue. Here a silver cermet acts asexcellent space filler in a gold onlaypreparation

Fig. 9. A detached core still attachedto its crown, despite a goodlynumber of pins. Notice that thecrown does not provide a ‘ferruleeffect’ and the absence of cavityretention features.

PRACTICE


grooves can be used as an alternative to pins toretain large amalgam and composite restora-tions. Such grooves are cut into the base ofcusps or into the gingival floor of boxes (Fig.10). A small round bur (eg ½ or 1 depending ontooth size) can be used. The depth of the grooveneeds to be sufficient to offer resistance to with-drawal of the head of the bur when it is used togauge the presence of undercut. This usuallymeans cutting to between two thirds and thecomplete depth of a round-headed bur. Groovesneed to be positioned to within 0.5 mm of theamelo dentinal junction. Newsome has writtenan excellent account of the practicalprocedure.32

The use of grooves (sometimes termed ‘slots’)has been tested in–vitro and in-vivo. A circum-ferential groove used to retain a full coronalamalgam compared with four dentine pinsshowed no significant difference in dislodgingforce in one in-vitro study33 but was less resist-ant in another.34 The majority of pinned cores inthese studies failed through amalgam slippageand pins bending. Where the slippage of amal-gam had been slight this would have been diffi-cult to detect clinically. By contrast failure of thegrooved cores was all or none.

Short-term clinical trials show that grooveretained amalgams perform at least as well as pinned amalgams.35,36 However, groovesare associated with less pulpal inflammationthan pins.12

The above studies were carried out withoutadhesives or dentine bonding. A combination ofgrooves and bonding should be even better.

Resin bondingResin adhesives were devised to bond compositerestorations to enamel and dentine. These mate-rials have been developed into luting agents foradhesively retained bridges and also bondingagents for amalgam restorations. Examples ofamalgam bonding agents include:

Amalgam bonding agent Adhesive resin

Panavia EX and Panavia 21 Phosphate ester of Bis GMA

All Bond 2 NPG GMA

Amalgam Bond and 4 META/TBB-MMA, Amalgam Bond Plus HEMA

In-vitro studies show that when properlyplaced these materials can enhance bond

strength34,37–39 and reduce microleakage.40 AllBond 2 has some of the highest bondstrengths.38 However, some bonding agents areineffective, or relatively so, under conditions ofin-vitro testing. For instance, Panavia 21 hadsuch low bond strengths with one variety ofamalgam it was considered ineffective for reten-tion.41 Worse still, all specimens made withAmalgam Bond debonded prior to testing.38 Butit needs to be emphasised that better results havebeen obtained with Amalgam Bond Plus39,42 — abonding agent, which provides a similar sheerstrength to pin retained amalgams.39,42 Howevermechanical retention, in the form of gooves orboxes,43 should be provided where possiblerather than rely entirely on the adhesive.42

A clinical study has also compared the per-formance of pinned amalgam restorations withthose retained by resin adhesive. After 2 yearsboth types of restorations performed equallywell with no deterioration or loss of retention ineither group.44

The disadvantage of most resin adhesives isthat they require a multi-stage placement tech-nique involving etching, washing, and primingbefore the amalgam is condensed onto the wetadhesive. Although some single-bottle primer/adhesive systems show promise for bondingamalgam,45 extra time and effort is still neededand contamination during critical stages willdestroy the bond.

Cement bonding (Baldwin technique)Another way of getting amalgam to bond todentine is to pack it onto a thin, wet layer ofcement. This method, called the Baldwin Tech-nique,46 was reported in 1897 — the very sameyear as grooves were described.47 Whilst thisapproach may seem old fashioned it should beremembered that screw pins for dentine alsooriginated from that time.48

Baldwin used wet zinc phosphate cement toimprove the seal of newly placed amalgamrestorations. The technique never really caughton, possibly because traditional teaching insiststhat a cement base must be set to prevent its dis-placement by the condensed amalgam. Also, ifzinc phosphate were extruded to the cavity mar-gins in any thickness it would be vulnerable todissolution. Nevertheless, wet cement has beenrecommended to assist with core retention bysome highly reputable dentists.49

The evidence for using GICs as an amalgamcore adhesive is currently only laboratory based.GICs form a good bond to dentine and an evenbetter one to amalgam.50,51 In bonding amalgamto dentine GICs and resins give similar results.Evaluations37,52,53 have included shear, tensileand fracture strength tests for a variety of GICs(the luting agent, Ketac Cem; the base/liningmaterial, Vitremer, and the filling material Fuji II).

Unfortunately there are no comparative data,either laboratory or clinical, to guide us in theselection of the most suitable GIC. Therefore,whilst the technique holds great promise wehave to rely on empirical and derived informa-

Fig. 10 Groove preparation in anupper molar. A ½ round bur is beingused to groove the cervical floor ofthe box. A: A number 1 round burhas been used in the lingual cusp B: Grooves should be sufficientlydeep to offer some resistance to burwithdrawal

PRACTICE


tion. For instance it would be best to avoid GICsdesigned for use as base materials, some ofwhich have been shown to be soluble when usedin the sandwich technique.54

Table 2 offers a clinical guide to using GIC asa bonding agent for amalgam.

PinsIt is not the purpose of this article to put pinmanufacturers out of business, merely to drawattention to potential problems of pin placementand to emphasise viable alternatives. However,many dentists will still feel the need to placepins, perhaps on a belt and braces basis. Wherethe urge cannot be resisted the advice in Table 3may help reduce problems.

PROBLEM SOLVINGMany, although not all core placement problemsrelate to the use of pins. Problems with pins canbe avoided by using alternative techniques.

Inadequate isolationCores are often required for heavily brokendown teeth with subgingival involvement whereoozing gingival margins cause problems withvisibility, caries removal and moisture control.At the simplest level a well-placed matrix bandalong with cotton wool rolls and aspiration mayprovide sufficient isolation, but not always.Rubber dam too may fall short of providingexcellent isolation. However, spaces betweentooth and rubber can be remedied by syringingin a caulking material such as Ora Seal Putty(Optident Ltd, Skipton, UK). This material mustbe kept clear from the cavity margin. Electro-surgery can be used first to remove any gingivalexcess and gingival bleeding controlled with astyptic agent such as Astringident (Optident Ltd,Skipton, UK).

Pin in periodontium or pulpOne event guaranteed to leave most dentists hotunder the collar is when the drill suddenly givesand the patient gives a cry of pain or blood oozesout of the pin channel. Alternatively the pincontinues to worm its way in, perhaps perforat-ing a thin dentine wall separating the pin chan-nel from either pulp or periodontium. Such per-forations can be difficult to treat and beforetaking action it is important to first confirm thesite of the perforation. A periapical radiographmay help, but an electronic apex locator, whichwill read ‘beyond the apex’ if the pin or pinchannel is in the periodontium, will give aninstant and reliable diagnosis.57 For both peri-odontal and pulpal perforations the importantprinciple of treatment is to prevent microbialingress. Treatment decisions are necessarilyempirical in the absence of controlled clinicalstudies.

Periodontal perforations are perhaps mostdifficult to remedy and if left can result inchronic infection and long-term patient dis-comfort. A perforation within the gingival sulcus should be included within the margin of

the intended restoration or else it will become asubgingival plaque trap. A crown lengtheningprocedure involving gingivectomy or raising aflap (if necessary with bone recontouring) isusually needed to access the pin. The pin canthen be either trimmed level with the root sur-face1,58 or the open perforation sealed. If the pinhas been placed into the periodontal ligament itmay be possible to remove it ultrasonically fromthe coronal aspect, although with a freshlyplaced pin this can take considerable time.59 Ifthe pin can be removed the new root canal per-foration sealant, MTA (Mineral Trioxide

Table 2 The Baldwin Technique modified for use with GIC cement and amalgam

• Optimise mechanical retention with grooves, boxes etc.

• Use Vitremer or a GIC luting agent with a longer working time

• To prevent the set cement from sticking to the matrix band apply a thin layer of petroleum jelly to its inner aspect. This must be done before fitting the matrix band or the cavity will be contaminated

• Ensure good isolation but do not over-dry the cavity as this may result in post operative sensitivity

• Apply a thin layer of cement over the entire cavity surface

• Condense the amalgam into the deepest areas first (eg boxes and grooves) encouraging the wet cement to be extruded up to the occlusal surface

• When the cavity has been packed full remove the last increment of cement-contaminated amalgam and repack with a fresh increment

Table 3 How to reduce pin placement problems

Pin selection• Avoid large pins for initial pin placement as they are not

significantly more retentive than medium pins

• Select a pin with a stop to prevent ‘bottoming out’, and abuttress thread to maximise retention (Fig. 11) and minimisedentine stress55 (eg Unity Pins, Whaledent, Mahwah, USA)

Number and location of pins• Restrict the number of pins — normally no more than one

per missing cusp

• Place pins in locations non-hazardous to pulp andperiodontium:

• 1 mm from the ADJ

• Towards the corners of molars56

• Avoiding the central palatal area in upper molars (riskof pulp perforation of the palatal root)

• Avoiding the centre of proximal boxes, especially inmaxillary first premolars and other teeth with proximalroot concavities (Fig. 12)

• Avoiding areas apical to the cemento-enamel junction

Pin channel preparation• Ensure the twist drill is not dull and runs

in the correct direction

• Angulate the pin drill by aligning itagainst the outer surface of the tooth

• Run drill at moderate speed withoutexcessive pressure

• Partially withdraw drill after cutting thefirst 1 mm to allow debris to clear

Pin placement• Use a speed reducing hand piece or run the hand piece at low speed

• Use low pressure and let the pin find its way in

• If necessary, bend the pin inwards to ensure clearance between the pin and matrix band

Fig. 11 The pin on the left has manydesirable features. In particular,notice the stop half way along itslength to prevent it ‘bottoming out’in the pin channel. The roundedcontours above the stop aredesigned to reduce stressconcentration in the core

Fig. 12 Pins can easily perforate the periodontiumin areas of root concavity of molars and premolars

PRACTICE


Aggregate, Dentsply, Tulsa, USA), has thepotential to provide an excellent, biocompatibleseal.60 The occlusal portion of the pin channelmay need to be opened up sufficiently to allowmoisture control and the material to be con-densed properly. If the pin is not retrievable itmay be left and the tooth put under probationwith a view to later crown lengthening surgeryshould the tooth give symptoms. Certainly, itwould be unwise to place an expensive indirectrestoration on such a tooth until its prognosiswas confirmed.

Pulpal perforations are usually easier tomanage than periodontal perforations. Manyteeth requiring large cores have a questionablepulpal status, and, if perforated are best roottreated, especially if the tooth is crucial to thetreatment plan and the outcome of the roottreatment can be assured. However, if there isno rush to provide an indirect restoration thetooth can be kept under probation and a pulpcapping technique used. Many materials seemwell tolerated by the pulp providing a bacterio-logical seal can be established and maintained.Some authorities suggest that even the pinitself can act as a ‘pulp cap’61,62 but few den-tists sterilise their pins before placement andthe space between dentine and the inner diam-eter of the thread will allow bacteria to spiraldown into the pulp. At the very least the pinand surrounding dentine will need to be sur-face sealed with a dentine-bonding agent.Alternatively, the pin may be removed ultrasonically (if it has already been placed) and thepulp capped with calcium hydroxide or MTA,60

followed by sealing with composite and den-tine bonding agent.

Of course, groove preparation also has thepotential to perforate either the pulp or peri-odontium, but this is likely to be a less frequentevent than pin perforation and should it occuraccess for repair is very much easier.

Loose pinA pin may become loose immediately afterplacement, during pin length reduction or whilstreplacing an old pin retained restoration. If amedium sized pin has been used it can bereplaced with a large pin. Alternatively, the wis-dom of replacing the pin can be reviewed, thepin channel converted into a groove and thecore bonded adhesively.

A pin that is too tall will protrude through theocclusal surface of the core. Where the pin isshortened with a bur it can very easily comeunscrewed, especially if the bur is kept in linewith the pin, which causes an anticlockwise fric-tional force. The chatter from a tungsten carbidebur is also effective in dislodging pins. We rec-ommend holding the tip of a long tapered dia-mond bur at right angles to the pin and cuttingfrom the side rather than grinding from above.An airotor handpiece should be used with a lighttouch. In this way the frictional forces generatedtend to act on the pin in a clockwise direction.Needless to say this procedure should be done

with water coolant and with aspiration to catchthe fragment of cut pin.

Tooth fracture during pin placementRoot treated or brittle teeth can chip and frac-ture especially if the pin is placed too close tothe ADJ. It emphasises that dentine is deformedby pin placement resulting in stresses whichcan crack the tooth. If the tooth can be saved,the fractured area should be covered by therestoration.

Matrix band placementThe placement of a matrix band can often provea challenge to even the most experienced ofdentists. However, ensuring the proximal con-tacts are open and that there are no spicules oftooth at the gingival margin can facilitate place-ment. The choice of band is highly personal andthe best matrix system is ‘the one which worksbest for you.’ However occasions arise when amore sophisticated matrix system is needed,such as the Automatrix system (LD Caulk Co,Milford, USA), which has the advantage of notbeing encumbered by a matrix retainer. In com-mon with other systems it does need to bewedged and where there are multiple cusps to bereplaced it may sometimes need to be stabilisedusing greenstick, taking precautions not to scaldthe patient with hot composition.

Despite great advances in operative dentistrythe copper ring (PD Copper Bands, Vevey,Switzerland) is sometimes the only way of fittinga matrix to a tooth, especially where all the cuspshave been lost. It is best to choose a band that is,if anything, slightly too small, trim it to size withBee Bee scissors and squeeze it to the approxi-mate outline of the tooth. It can then be adaptedsection by section around the tooth stretchingthe band slightly with an amalgam plugger orhalf Hollenback instrument. If the band is tootight to fit it can be annealed and stretched fur-ther. Annealing involves heating the copper tocherry red heat in a gas flame and quenching.Although many would consider leaving such aband in place to allow the amalgam to be sup-ported whilst it sets there is a risk of unseen,excess material being left and this could lead togingival inflammation. It is usually an easy mat-ter to slit the band with a tapered diamond burand remove it at the time of placement. Ortho-dontic bands or aluminium temporary crownshells with their occlusal surfaces removed canact as a substitute for copper rings,63 but alu-minium reacts with mercury to form a flocculentairborne precipitate.

Early core fracture We have all had the frustration of early corefracture occurring whilst the matrix band isremoved or when the patient bites onto a par-tially set amalgam. If only a small portion of thecore breaks away it is often possible to shape theremaining amalgam to provide a retentive cavi-ty and pack fresh amalgam — sometimes withoutthe need for the matrix being replaced. A more

PRACTICE


catastrophic fracture will of coursenecessitate the complete replacementof the core.

CONCLUSIONA well-placed core is the foundationfor a successful restoration. Successdepends on selecting the most appro-priate material and ensuring that it isproperly retained. Pin retention is notwithout problems and in most situa-tions can be avoided. Retention can beassured through cavity modifications,crown lengthening (to provide a fer-rule effect on sound tooth structure)and adhesives.

1. Newsome P R, Youngson C C.Complications of pin placement. A survey of 429 cases. Br Dent J 1987;163: 375-378.

2. Wilson N H. The pattern of usage ofdentine pins. Eur J Prosthodont RestorDent 1996; 4: 137-139.

3. Newitter D A, Gwinnett A J, Caputo L.The dulling of twist drills during pinchannel placement. Am J Dent 1989;2: 81-85.

4. Standlee J P, Caputo A A, Collard E W.Retentive pin installation stresses.Dent Pract Dent Rec 1971; 21: 417-422.

5. Khera S C, Chan K C, Rittman B R.Dentinal crazing and interpindistance. J Prosthet Dent 1978; 40:538-543.

6. Bione H M, Wilson P R. The effect ofthe mismatch between the corediameter of self-threading dentinepins and the pinhole diameter. AustDent J 1998; 43: 181-187.

7. Hummert T, Kaiser D. In vitroevaluation of dynamic fluiddisplacement in dentinal tubulesactivated on pin placement. JProsthet Dent 1992; 68: 248-255.

8. Webb E L, Straka W F, Phillips C L.Tooth crazing associated withthreaded pins: a three-dimensionalmodel. J Prosthet Dent 1989; 61:624-628.

9. Cooley R L, Barkmeier W W.Temperature rise in the pulp chambercaused by twist drills. J Prosthet Dent1980; 44: 426-429.

10. Chan K C, Denehy G E, Ivey D M.Effect of various retention pininsertion techniques on dentinalcrazing. J Dent Res 1974; 53: 941.

11. Barkmeier W W, Cooley R L. Self-shearing retentive pins: a laboratoryevaluation of pin channelpenetration before shearing. J AmDent Assoc 1979; 99: 476-479.

12. Felton D A, Webb E L, Kanoy B E, Cox C F. Pulpal response to threaded pinand retentive slot techniques: a pilotinvestigation. J Prosthet Dent 1991;66: 597-602.

13. Christensen G J. When to use fillers,build-ups or posts and cores. J AmDent Assoc 1996; 127: 1397-1398.

14. Gross M J, Harrison J A. Someelectrochemical features of the invivo corrosion of dental amalgam. J Appl Electrochem 1989; 19: 301-310.

15. Cho G C, Kaneko L M, Donovan T E,White S N. Diametral andcompressive strength of dental corematerials. J Prosthet Dent 1999; 82:272-276.

16. Hormati AA, Denehy GE.Microleakage of pin-retainedamalgam and composite resin bases.J Prosthet Dent 1980; 44: 526-530.

17. Ziebert A J, Dhuru V B. The fracturetoughness of various core materials. J Prosthodont 1995; 4: 33-37.

18. Burke F J T, Watts D C. Cermet — Anideal core material for posteriorteeth? Dent Update 1990; 17: 364-369.

19. Beyls H M, Verbeeck R M, Martens L C,Lemaitre L. Compressive strength ofsome polyalkenoates with or withoutdental amalgam alloy incorporation.Dent Mater 1991; 7: 151-154.

20. Walls A W, Adamson J, McCabe J F,Murray J J. The properties of a glasspolyalkenoate (ionomer) cementincorporating sintered metallicparticles. Dent Mater 1987; 3: 113-136.

21. Kilpatrick N M, Murray J J, McCabe J F.The use of a reinforced glass-ionomercermet for the restoration of primarymolars: a clinical trial [see comments].Br Dent J 1995; 179: 175-179.

22. Kovarik R E, Breeding L C, CaughmanW F. Fatigue life of three corematerials under simulated chewingconditions. J Prosthet Dent 1992; 68:584-590.

23. Haller B, Gotze W, Weiss G.Parapulpal pins and their effects onthe fracture resistance of pin-retained cores. J Oral Rehabil 1991;18: 459-469.

24. Combe E C, Shaglouf A M, Watts D C,Wilson N H. Mechanical properties ofdirect core build-up materials. DentMater 1999; 15: 158-165.

25. DeWald J P, Arcoria C J, Ferracane J L.Evaluation of glass-cermet coresunder cast crowns. Dent Mater 1990;6: 129-132.

26. Nayyar A, Walton R E, Leonard L A. Anamalgam coronal-radicular doweland core technique forendodontically treated posteriorteeth. J Prosthet Dent 1980; 43: 511-515.

27. Sindel J, Frankenberger R, Kramer N,Petschelt A. Crack formation of all-ceramic crowns dependent ondifferent core build- up and lutingmaterials. J Dent 1999; 27: 175-181.

28. Kerby R E, Knobloch L, Thakur A.Strength properties of visible-light-cured resin-modified glass-ionomercements. Oper Dent 1997; 22: 79-83.

29. Wilson N H, Cowan A J, Crisp R J,

Wilson M A. A short-term clinicalevaluation of a tricure glass-ionomersystem as a transitional restorationand core buildup material.Quintessence Int 1999; 30: 405-411.

30. Moon M G, Jarrett T A, Morlen R A,Fallo G J. The effect of variousbase/core materials on the setting ofa polyvinyl siloxane impressionmaterial. J Prosthet Dent 1996; 76:608-612.

31. Shillingburg H T, Jr., Jacobi R, BrackettS E. Preparation modifications fordamaged vital posterior teeth. DentClin North Am 1985; 29: 305-326.

32. Newsome P R H. Slot Retention: Analternative to pins in the largeamalgam restoration. Dent Update1988; 15: 202-207.

33. Outhwaite W C, Garman T A, PashleyD H. Pin vs. slot retention in extensiveamalgam restorations. J ProsthetDent 1979; 41: 396-400.

34. Pashley E L, Comer R W, Parry E E,Pashley D H. Amalgam buildups:shear strength and dentin sealingproperties. Oper Dent 1991; 16: 82-89.

35. Garman T A, Outhwaite W C, HawkinsI K, Smith C D. A clinical comparisonof dentinal slot retention withmetallic pin retention. J Am DentAssoc 1983; 107: 762-763.

36. Tewari S, Govila C P, Paharia Y N. A clinical evaluation of dentinal slot,amalgapin & T.M.S. retention inamalgam restorations. Fed Oper Dent1990; 1: 14-17.

37. Covey D A, Moon P C. Shear bondstrength of dental amalgam bondedto dentin [published erratum appearsin Am J Dent 1991 Apr;4(2):94]. Am JDent 1991; 4: 19-22.

38. DeSchepper E J, Cailleteau J G, RoederL, Powers J M. In vitro tensile bondstrengths of amalgam to treateddentin. J Esthet Dent 1991; 3: 117-20.

39. Hadavi F, Hey J H, Strasdin R B,McMeekin G P. Bonding amalgam todentin by different methods. J Prosthet Dent 1994; 72: 250-254.

40. Charlton D G, Moore B K, Swartz M L.In vitro evaluation of the use of resinliners to reduce microleakage andimprove retention of amalgamrestorations. Oper Dent 1992; 17:112-119.

41. Phrukkanon S, Burrow M F, Tyas M J.Bonding of amalgam and a galliumalloy to bovine dentin. Oper Dent1998; 23: 195-202.

42. Imbery T A, Burgess J O, Batzer R C.Comparing the resistance of dentinbonding agents and pins in amalgamrestorations. J Am Dent Assoc 1995;126: 753-759.

43. Fischer G M, Stewart G P, Panelli J.Amalgam retention using pins, boxes,and Amalgambond. Am J Dent 1993;6: 173-175.

44. Belcher M A, Stewart G P. Two-yearclinical evaluation of an amalgamadhesive. J Am Dent Assoc 1997;128: 309-314.

45. Cobb D S, Denehy G E, Vargas M A.Amalgam shear bond strength todentin using single-bottleprimer/adhesive systems. Am JDent 1999; 12: 222-226.

46. Baldwin H. Cement and amalgamfillings. Br J Dent Sci 1897; vol. XL:193-234.

47. Kirk E C. The American Textbook ofOperative Dentistry. Philadelphia:Lea Brothers, 1897.

48. Ottolengui R. Methods of fillingteeth. 2nd ed.pp.112. London:Claudius Ash & Sons Limited, 1899.

49. Vale W A. Everyday procedures indentistry - abutment preparationsfor fixed bridgework. Br Dent J1953; 94: 93-98.

50. Aboush Y E, Jenkins C B. Thebonding of glass-ionomer cementsto dental amalgam. Br Dent J 1989;166: 255-257.

51. Aboush Y E, Elderton R J. Bondingof a light-curing glass-ionomercement to dental amalgam. DentMater 1991; 7: 130-132.

52. al-Moayad M, Aboush Y E, EldertonR J. Bonded amalgam restorations:a comparative study of glass-ionomer and resin adhesives. BrDent J 1993; 175: 363-367.

53. Chen R S, Liu C C, Cheng M R, Lin CP. Bonded amalgam restorations:using a glass-ionomer as anadhesive liner. Op Dent 2000; 25:411-417.

54. Welbury R R, Murray J J. A clinicaltrial of the glass-ionomer cement-composite resin ‘sandwich’Quintessence Int 1990; 21: 507-512.

55. Butchart D G. A new self-threadingdentine pin. Br Dent J 1983; 155:83-84.

56. Gourley J V. Favorable locations forpins in molars. Oper Dent 1980; 5: 2-6.

57. Knibbs P J, Foreman P C, Smart E R.The use of an analog type apexlocator to assess the position ofdentine pins. Clin Prev Dent 1989;11: 22-25.

58. Cooley R L, Lubow R M, Wayman BE. Treatment of pin perforations.Gen Dent 1982; 30: 148-153.

59. Wilson P R, Bione H M. Ultrasonicremoval of dentine pins. J Dent1993; 21: 285-288.

60. Torabinejad M, Chivian N. Clinicalapplications of mineral trioxideaggregate. J Endo 1999; 25: 197-205.

61. Dolph RW. Intentional implantingof pins into the dental pulp. DentClin North Am 1970; 14: 73-80.

62. Abraham G C, Baum L. Intentionalimplantation of pins into thedental pulp. J Southern Calif DentAssoc 1972; 40: 914-920.

63. Chapman K W. A matrix bandtechnique for large amalgam cores. J Am Dent Assoc 1981; 102: 56-57.

PRACTICE


Crowns and other extra-coronal restorations:Preparations for full veneer crowns F. M. Blair1 R. W. Wassell2 and J. G. Steele3

Preparations for full veneer crowns is the eighth in the series on crowns and other extra-coronal restorations. Whilsthandpiece skills are important, many other factors combine to ensure provision of a satisfactory full veneer crown (alsotermed ‘full coverage crown’). Our aim in writing this article is to consider the principles which influence crown preparation,seasoned with clinical advice our undergraduate and postgraduate students have found useful.

1Consultant in Restorative Dentistry,Birmingham Dental Hospital, St Chad’sQueensway B4 6NN2*,3Senior Lecturer in Restorative Dentistry,Department of Restorative Dentistry, The Dental School, Framlington Place,Newcastle upon Tyne NE2 4BW*Correspondence to: R. W. WassellE-mail: [email protected]


● The principles of crown preparation and crown selection● Guidelines for tooth reduction and margin design● Preparation of the taper● Strategies for enhancing resistance and retention● Tooth preparation

I N B R I E F

In the second article of this series we consideredthe broad range of materials from which crownsare made. The four main categories of crownwere metal, ceramic, metal-ceramic and com-posite. Tooth preparations need to reflect the dif-ferent requirements of these materials in termsof amount of space needed to accommodate thecrown and its marginal configuration. Toothpreparations also need to be free from undercutswhilst reliably retaining the crown.

The principles of crown preparation describedby Shillingburg1 (Table 1) determine the shapeand form of our preparations, but they cannot beconsidered in isolation as there has to be a bal-ance between them. For example, minimalpreparation can result in the production of abulky, unaesthetic crown which can in turncause periodontal or occlusal problems (Fig. 1).Conversely over preparation can be compensat-

ed by making a thicker and perhaps very aes-thetic crown, but the strength and pulpal vitalityof the underlying tooth may be compromised. Inreality, preparations should be planned accord-ing to each individual case and in each case thebalance will be different. Clearly, patients needto be involved in deciding what is best for them.This approach differs fundamentally from sim-ply cutting ‘off the shelf’ preparations basedentirely on text-book diagrams.

CROWN SELECTIONThe usual indications for full veneer crowns are:

• To protect weakened tooth structure• To restore the tooth to function• To improve or restore aesthetics• For use as a retainer for fixed bridgework

However, recent developments in dental

8





articulator selection6. Aesthetic control7. Cores for teeth with





restorations

Table 1 Seven key principles of preparation (derived from Shillingberg1)Seven key principles Function

Conservation of To avoid weakening the tooth unnecessarilytooth tissue To avoid compromising the pulp

Resistance form To prevent dislodgement of a cemented restoration by apical or obliquely-directed forces

Retention form To prevent displacement of a cemented restoration along any of its paths of insertion, including the long axis of the preparation

Structural durability To provide enough space for a crown which is sufficiently thick to prevent fracture, distortion or perforation

Marginal integrity To prepare a finish line to accommodate a robust margin with close adaptation to minimise microleakage

Preservation of the To shape the preparation such that the crown is not over periodontium contoured and its margin is accessible for optimal oral hygiene

Aesthetic considerations To create sufficient space for aesthetic veneers where indicated

PRACTICE


materials mean that the use of less destructivealternatives such as veneers, onlays, and suit-ably designed plastic restorations are often anoption. Where crowning is in the patient’s bestinterests the type of crown provided will usuallydepend on the functional requirements, thestrength and vitality of the remaining tooth andthe patient’s aesthetic demands. Occasionally,the choice may be limited by a patient’s con-

cerns, real or imagined, over biocompatibility ofproposed materials.

Full veneer metal crowns (VMC) have theadvantage of requiring relatively little toothpreparation. They are generally limited by aes-thetics to the back of the mouth, but bear inmind that some people may favour an anteriordisplay of gold.

Porcelain jacket crowns (PJC) are indicated inareas of critical aesthetics, but problems of frac-ture under occlusal load limit their use to anteri-or teeth.

High strength porcelain crowns (HSPC), usingmodern ceramics without the need for a metalsubstructure, may be used on posterior teeth andin certain selected cases for anterior bridgeworkespecially where the use of metal is an issue.However, not all systems are suitable for thesepurposes. The preparation design for a posteriorHSPC is important2,3 and relatively destructive.Enough tooth tissue needs to be removed to permit an adequate thickness of porcelain whilethe degree of taper should be greater than for aconventionally cemented crown, and all cornersand line angles need to be rounded to allowunhindered, passive seating of the crown. Atleast one system (Procera AllCeram, Nobel Bio-care, Göteburg, Sweden) provides burs designedto cut a deep chamfer at the finish line to allowthe profile of the die to be scanned with a digitalprofilometer for the production of a highstrength ceramic core.

Resin bonded porcelain crowns (RBPC) aresimilar to porcelain veneers but provide cov-erage on all surfaces and are very conservativeof tooth tissue. The strength of these restora-tions is largely reliant on the resin bond asthere is no reinforcing ceramic core. Excellentaesthetics are possible and a much lessdestructive preparation can be used than foreither PJCs or porcelain fused to metal crowns(look at Fig. 2). RBPCs may be particularly use-ful in younger patients who have large, vul-nerable pulps. Long term follow-up is lackingbut a similar success rate to veneers might beexpected. In our experience this type ofrestoration is not suited to areas of heavyocclusal load (eg where tooth wear has beencaused principally by parafunction) as theporcelain may fracture.

Porcelain fused to metal crowns (PFM), alsocalled metal-ceramic crowns, are undoubtedlymost versatile combining strength with aesthet-ics. Tooth preparation to accommodate bothmetal and porcelain can be very destructive(Fig. 3) so the design, particularly the distribu-tion of porcelain, needs to be carefully thoughtthrough. Metal surfaces require less toothdestruction, are easier to construct and adjust,and do not chip like porcelain. Porcelain cover-age should only be used where it is required foraesthetics, or possibly when opposing otherporcelain surfaces (as metal does not wear wellagainst porcelain, especially the metal backs ofanterior crowns).

If full porcelain coverage is required for a

Fig. 1 Under preparation results in pooraesthetics or an over built crown (dottedline) with periodontal and occlusalconsequences. Over preparation resultsin pulp and tooth strength beingcompromised

Fig. 2 Preparation features for three different types of crown for anupper central incisor: a) Porcelain fused to metal; b) Porcelain jacket;and c) Resin bonded porcelain crown (remaining enamel shown lightblue). Questions — which crown or crowns: Provide the bestaesthetics? Has the least destructive preparation? Is least destructiveto opposing teeth? Allows anterior guidance to be developed easily? Isbest suited to bruxists? (see text for answers)

Fig. 3 Sections (axial and transverse) through an upper first premolar showing howa logical use of metal lessens the amount of tooth reduction: a) Porcelain restrictedto buccal cusp; b) Porcelain covering proximal and occlusal surface; and c) Fullporcelain coverage

PRACTICE


PFM the amount of tooth preparation may bereduced slightly by using a special metal com-posite coping (Captek, Schottlander, Letch-worth, UK). This type of coping is 0.1–0.2 mmthinner than a cast coping made from a noblemetal alloy.

Composite crowns which use newly devel-oped laboratory materials have not been fullyevaluated. Specific indications and limitationshave yet to be defined. Manufacturers are cur-rently recommending that the tooth be preparedin the same way as for a HSPC.

The major factor to be considered beforeselecting the most suitable type of crown is theamount of tooth destruction you are willing toallow in order to give the aesthetics you want.You may also want to consider the need for the crown to incorporate special features, for example:

• A metal anterior guidance surface• Rest seats and other features to retain a partial

denture • Metal occlusal surfaces for a bruxist

GUIDELINES FOR AMOUNT OF TOOTHREDUCTION AND MARGIN DESIGNTooth preparation represents a balancebetween, conserving tooth structure and pulphealth on the one hand, whilst on the other,achieving an aesthetic and strong crown.Guidelines for the amount of tooth reductionfor different types of crown have evolvedlargely as a result of experience rather thanscientific evidence. Tables 2 and 3 give anindication of the amount of occlusal and cer-vical reduction for different types of crownsfor posterior and anterior teeth. It is worthemphasising that there will be times when lesstooth should be removed in the interests ofconserving tooth structure and pulp health.There are also occasions when little or notooth reduction is needed, eg from theocclusal surface when the vertical dimensionis to be increased, or, from the buccal surfacewhere the tooth is already worn and is to bere-contoured by the crown.

The amount of reduction can be gaugedusing depth cuts coupled with a knowledge ofthe appropriate bur end diameter (look at Fig. 4for examples of typical bur end diameters). Apreparation reduction matrix, formed on adiagnostic wax-up (described in the sixth arti-cle in the series), can be invaluable when theshape of the intended crown differs from thatof the original tooth.

The metal margins of VMCs and PFMs can beconfigured in a variety of ways (Fig. 5). It is bestto avoid the knife edge as the finish line can bedifficult for the technician to detect. Chamfermargins and shoulder with bevel margins allowa fine edge of metal to be cast which, if a suitablealloy is chosen, can be burnished by the techni-cian to improve marginal fit. However, there isalways the danger that the die may be damagedby burnishing. Furthermore, the margin should

not be too acute. Theoretical arguments havebeen made in favour of acute margins compen-sating for seating discrepancy4 but only beforecementation. Once cemented, an acutely bev-elled margin may prevent the crown from seat-ing fully,5,6 presumably by restricting cementextrusion.

Table 2 Suggested preparation features for posterior crownsCrown type Posterior crowns — preparation features

Occlusal reduction* Finish line depth and configuration

VMC 1 mm non-functional cusps 0–1.0 mm

1.5 mm functional cusp Chamfer, knife-edge,

shoulder or shoulder with bevel

HSPC 2 mm non-functional cusps 0.8-1.0 mm

2.5 mm functional cusps Shoulder or heavy chamfer

PFM As for VMC if metal surface 1.2 mm labial shoulder† or chamfer

2 mm non-functional cusps 0.5 mm lingual chamfer

2.5 mm functional cusps

*Where tooth is tilted or where vertical dimension is to be increased, the amount of occlusal reductionrequired will vary†Too deep a reduction for diminutive teeth or for long clinical crowns where a metal collar is preferable

Table 3 Suggested preparation features for anterior crownsCrown type Anterior crowns — preparation features

Occlusal reduction* Finish line depth and configuration

PJC 2 mm incisally 0.8–1.0 mm shoulder

1 mm lingual aspect

RBPC 2 mm incisally > 0.4 mm chamfer

0.5–1.0 mm lingual aspect

PFM 2 mm incisally 1.2 mm labial shoulder†

0.5–1.0 mm lingual aspect or heavy chamfer

(porcelain guidance requires 0.5 mm lingual chamfergreater clearance)

*Where the vertical dimension is to be increased, the amount of occlusal reduction required will be lessor non-existent† Too deep a reduction for diminutive teeth eg lower incisors or for long clinical crowns where a metalcollar is preferable

Fig. 4. Burs used for preparation of full veneer crowns at NewcastleDental Hospital (from left to right): a) Flat-end tapered diamond(occlusal and axial reduction) end Ø = 0.8 mm; b) Long, round-endtapered diamond (as for a. and also shoulder production) end Ø =1.1 mm; c) Long needle diamond (initial proximal reduction); d) Chamfer diamond (chamfer production) end Ø = 1.0 mm;e) Chamfer tungsten carbide (chamfer and preparation finishing); and f) Large flame or ‘rugby ball’ diamond (lingual concavityproduction)

PRACTICE


As a general rule when using porcelain orPFMs, adequate clearance is required to achievegood aesthetics. Traditionally, this is achievedwith a shoulder or heavy chamfer of 0.8–1 mmwidth for PJCs and 1.2 mm width for PFMs.However, shoulders of these depths may compro-mise tooth strength and pulp health especially fordiminutive teeth such as mandibular incisors. Asimilar problem occurs on teeth with long clini-cal crowns because of the narrowing of theirdiameter in the cervical region. In a long prepa-ration extending onto or beyond the cemento-enamel junction, considerable tooth tissue mustbe removed to eliminate undercuts. Solutionsinclude using a minimal shoulder, a metal collaron a chamfer (as illustrated in Fig. 6) or placingthe margin at the cemento-enamel junction.Under normal circumstances these options donot compromise aesthetics, being hidden by thelip. Clearly, patients need to understand the ben-efits of a less destructive preparation and, as stat-ed previously, must be involved in the decisionmaking process pre-operatively.

Some operators use what they describe as amini-shoulder, 0.5–0.7 mm wide, for both

ceramic and metal margins. This approach canproduce acceptable results for metal margins,although it is easier for the technician to adaptthe wax and finish to a chamfer. A much greaterproblem occurs when ceramics are used as theinevitable consequence is for the crown to beover-bulked resulting in compromised aestheticsand a poor gingival emergence profile.

PREPARATION TAPERThe subject of taper is contentious. In the firstplace it means different things to differentpeople. For our purpose it has the same meaningas ‘convergence angle’ ie the angle betweenopposing preparation walls. To avoid confusionwhen reading different publications, it is impor-tant to appreciate that ‘taper’ may also bedefined as ‘the angle between a single prepara-tion wall and the long axis of the preparation’.Taper defined by the first definition will be twicethe angle defined in the second.

As regards the question of ideal convergenceangle, text books have traditionally based theirrecommendations (variously between 3 and 14o)7–9 on the results of experimental studies10,11

which show a decrease in retention of conven-tionally cemented crowns as taper is increased.Experimental studies have also shown thatpreparations with tapers greater than 20o displaya significant fall in resistance to oblique displac-ing forces12 and show increased stress concen-tration within the cement13 which may rupturethe cement lute.

On the other hand, clinical measurementsof taper have been made indirectly on stonecasts. 14–18 These studies showed mean values ofabout 20o with a considerable variability aroundthe mean. Furthermore, greater tapers wereachieved on mandibular molars than on maxil-lary incisors19 possibly because of differences intooth shape and problems with access. Thesefindings suggest that clinicians, even those whoare technically gifted, frequently cut a greatertaper than text books recommend.

The issue of taper may not be quite as criticalfor single crowns as was once thought, but as aworking rule operators should strive to producethe least taper compatible with the eliminationof undercut. It is helpful to know that manytapered burs have a 5–6o convergence anglewhich can be used to survey preparation taperby holding the handpiece in the same plane forall axial surfaces.

Resin bonded crowns are the importantexception to the rule of minimizing taper, espe-cially RBPCs which may benefit from havingtapers of about 20o to avoid generating highseating hydrostatic pressures during lutingresulting in crown fracture.

STRATEGIES FOR ENHANCING RESISTANCEAND RETENTIONWhile many factors influence resistance andretention, one of the most important is thenature of the cement lute. Conventionalcements are strong in compression and weak in

Fig. 5. Finish lines with marginal configurations for PFMs: a) Shoulder withporcelain butt fit; b) Deep chamfer with metal collar; c) Shoulder plus chamfer(bevel) with metal collar; d) Knife edge with metal margin; and e) Chamfer withmetal margin

Fig. 6. Teeth with long preparationsare prone to pulpal exposureespecially if a deep finish line is cut.(a) Teeth with narrow roots are mostvulnerable (b) A metal collar reducesthe need for a deep finish line

a b

PRACTICE


tension, so, wherever possible, preparationsshould be designed to limit tensile and shearstresses in the lute, especially when obliqueforces are applied to the crown (Fig. 7). Cementselection will be considered in detail in Part 11of the series.

Preparations which are either short or over-tapered or both are vulnerable to crown dece-mentation. Often one is confronted with havingto replace a crown where the preparation isover tapered and simply re-preparing the toothmay be excessively destructive. This sectionconsiders aspects of preparation design andother methods which can be used to preventdecementation.

Any dentist who has seen a number of dece-mented crowns will have an idea of what an unretentive preparation looks like. However,there are no absolute guidelines for preparationdimensions which risk decementation. An in-vitro study20 has shown a significant increasein resistance and retention as axial walls extendfrom 2—3 mm in height and recommended3 mm as the minimum preparation height. As aworking rule this seems reasonable but there willbe a multitude of exceptions and caveatsdepending on factors such as taper, cementselection and occlusal loading.

Where the preparation is over-tapered it ispossible to up-right the axial walls at the base ofthe preparation, but this can result in a deepshoulder, excessive destruction of tooth tissueand possible compromise of pulpal health. If thebulk of remaining core or tooth tissue permits, aseries of near parallel steps can be made in theover-tapered axial walls which results in a muchless destructive preparation.

Other less destructive approaches of dealingwith an unretentive preparation are:

• Retentive preparation features ie grooves andboxes

• Resin cements• Surgical crown lengthening• Pins and cross-pinning

Grooves and boxes provide increased reten-tion by presenting additional near-parallel sidedwalls to the preparation and limiting the path ofinsertion. Resistance is improved by preventingrotation of the restoration (Fig. 8). They are usedmainly for metal and metal-ceramic restorationsbut are generally impractical for all ceramiccrowns.

Grooves Grooves can be placed in one or more of theaxial walls using a minimally tapered bur. Thebur chosen should be of sufficient diameter toprovide a groove that will not be blocked out onthe die with die-spacer (see flat-end tapered dia-mond in Fig. 4). The groove should be placedwithin a sound bulk of tooth tissue or core notleaving any weak surrounding areas which areliable to fracture. The tooth may also be less vul-nerable to the effects of micro-leakage if thebase of the groove is kept 0.5 mm clear of the

finish line. To be effective the groove should besunk to at least half the bur’s diameter. Whenplacing grooves in tapered axial walls be verycareful to ensure that the walls of the groove donot conflict with the path of insertion. Thisrequires the bur to be held parallel to the path ofinsertion and not parallel with the tapered axialwall. Providing these precautions are taken,grooves provide a simple, effective means ofimproving resistance.

BoxesBoxes function similarly to grooves in providingincreased resistance and retention, but are less con-servative so it is difficult to justify them being cutinto sound tooth structure unless there are otherreasons for their presence (eg provision of a crownwith an intra-coronal attachment). Nevertheless, atooth may have previously contained a restorationwith a box form. Instead of using the box to retain acore the box can be incorporated into a crownpreparation. This is a useful approach where thecore would otherwise be thin and weak. You may

Fig. 7 Preparations (a) and (b) have similar retention (similar axial surface areaand convergence angle) but (a) has much greater resistance. Application ofoblique force to occlusal surface results in potential rotation of crown (a) around a fulcrum (f). Rotation is resisted by the cement above the arc of rotation of thebase of the crown being thrown into compression — represented by arrow.Rotation of the shorter crown (b) results in tensile forces fracturing the cementlute. It is therefore important that the radius of rotation intersects the opposingaxial wall.28

Fig. 8 Placement of axial groovesincreases resistance by effectivelyreducing the radius of rotation

PRACTICE


need to take care to ensure the resulting crown isnot so bulky that casting porosity or thermal sensi-tivity becomes a problem.

Boxes need not necessarily be sited solely onaxial walls For example, it is sometimes veryuseful to cut an intra-coronal box, resemblingan occlusal inlay, into the occlusal aspect of asubstantial core. Clearly, this approach wouldbe inappropriate if it weakened the core appre-ciably.

Resin cements Resin cements (to be described in Part 13 of theseries) provide a relatively simple option to over-come the low tensile strength and poor adhesionof conventional cements. Resin cements havemuch higher tensile strength21 and when used incombination with dentine bonding agents are lesssensitive to repetitive dislodging forces.22,23 Theyare, however, technique sensitive and are not sup-ported by long term clinical data. Current clinicalwisdom is, where possible, to combine soundretentive design with resin cementation.

Crown lengthening Exposure of a greater height of clinical crownmay involve either gingivectomy (with a scalpelor electrosurgery) or flap surgery with osseousrecontouring. It is an invaluable means ofenhancing retention, but can be a substantialundertaking and has to be balanced against thedisadvantage of patient discomfort. Details oftechnique are described elsewhere.24 Crownlengthening needs to be planned in advance oftooth preparation (Fig. 9). If the ultimate posi-tion of the gingival margin is critical then goodprovisional restorations should be provided andworn for 2–3 months before the final impression

to allow the gingival tissues to recover fully totheir final form.

Pins and cross-pins Pins and cross pins are rarely used as they aretechnically demanding and have become evenless popular since the introduction of resincements. Pins may be considered where thereis a good bulk of tooth tissue. They are incor-porated during the construction of therestoration and are sunk through the occlusalsurface.

Cross-pins are placed following cementationand are screwed transversely through the axialsurface of the crown into the underlying prepa-ration.

TOOTH PREPARATIONTo avoid any unwanted surprises, the struc-tural, endodontic, periodontal, aesthetic andocclusal factors outlined in the previous partsof the series should be checked before bur isput to tooth.

There are a variety of burs which can beused for crown preparations and operators willhave their own preferences. Some operatorsmay prefer a flat ended bur for shoulder pro-duction. However, the round ended bur has thebenefit of producing a rounded junctionbetween the finish line and axial walls whichwill help reduce stress concentration in thisvulnerable part of the preparation and is lesslikely to cut steps.

Most practitioners in the UK use air rotorhandpieces, although in continental Europeand elsewhere speed increasing handpiecesare favoured. Whatever option is chosen awater spray is absolutely essential to avoid

a b

c d

Fig. 9 When clinical crown height is short, plan ahead. a) Upper anterior teeth needing crowning with PFMs b) Crownlengthening using apically repositioned flap and osseous recontouring c) Preparations made 3 months after surgery.Note no incisal reduction needed as vertical dimension to be increased d) Preparations viewed occlusally (mirror view)showing retention grooves in cingulum of UL2 (22)

PRACTICE


pulpal damage and endodontic complications.Following air rotor preparation, a speedincreasing handpiece can be useful for finish-ing the preparation, defining finish lines andplacing retention devices such as grooves orboxes.

Preparation sequenceThere are definite advantages in following a setorder of tooth reduction and ensuring that eachelement of reduction is complete before startingthe next. For instance, if the occlusal surface isprepared first there will be better access for themore difficult proximal preparation. Depth cutsplaced before embarking on larger areas of toothreduction help ensure controlled removal oftooth tissue but where the proposed crown is tobe shaped differently from the original tooth apreparation matrix, as described in Part 6 of theseries, is more helpful. During axial preparationit is best to complete the most difficult wall firstso if any alignment modifications are requiredthey can be made in more accessible areas.

Each stage of reduction has its own specialconsiderations and these will now be discussedin the sequence of preparation that we wouldrecommend.

Posterior preparationsOcclusal reduction Before any reduction is carried out it is importantto assess the occlusion and note any space alreadyavailable between opposing teeth. For example, a mesially tilted molar may require little or noreduction of its mesial occlusal surface so depthcuts can be confined to the distal occlusal area. Ifthe reduction follows the cuspal contours you willget maximum axial wall height available for resist-ance and retention, but clearly this is not so criticalwhere long axial walls are to be prepared. Thefunctional cusp bevel, shown in Fig. 10, is a usefulfeature of the occlusal reduction and ensures spacefor adequate bulk of crown material in a site ofheavy occlusal contact. The functional cusp (orholding cusp) must be identified and, after bevel-ling, adequate clearance should be confirmed in allexcursive movements. This small simple step helpsavoid the creation of an occlusal interference orperforation of a crown’s occlusal surface.

Lingual reduction Lingual access may be difficult. However if thelingual surface is the first axial surface to beprepared, it reduces the likelihood of produc-ing an over tapered preparation especially ifthe bur is held parallel to the long axis of thetooth. Subsequent alignments to the preparedlingual wall are then carried out on moreaccessible surfaces.

Buccal reduction Good retention relies on near parallelism cervi-cally of the buccal and lingual axial walls. How-ever, it is also important to ensure that thepreparation is in harmony with the buccal con-tours of the adjacent teeth so that sufficient

space is available for good aesthetics. It is worthviewing the tooth from both occlusal and buccalaspects to ensure the correct planes of adjust-ment have been made.

On molars the preparation may extendtowards the furcation region where a concavityof the buccal surface can be found. To avoid cre-ating a crown with an over-bulky buccal sur-face, the finish line should be cut to its full depthin this area and the concavity should be extend-ed up the buccal axial wall to the occlusal sur-face. This approach, which may also need to beused on other axial walls with cervical concavi-ties, eg the mesial aspect of upper first premo-lars, results in a preparation having a kidneyshaped appearance when viewed occlusally.

Proximal reductionDuring proximal reduction many adjacent teethare damaged.25 If a fine tapered bur is used forthe preliminary cut it can be kept safe by ensur-ing a fine sliver of tooth or core material remainsbetween the preparation and the adjacent tooth.This sliver can then be flicked away before refin-ing the reduction with a bur of larger diameter.To ensure clearance of the proximal contact, tryto keep the tip of the bur at the level of the pro-posed finish line. There is no doubt that this isthe most difficult stage of the preparation.

Once the basic preparation is complete, checkthe path of insertion and taper. Again, if youview the preparation from both occlusal andbuccal aspects you should ensure that no under-cut goes undetected. A surface reflecting mirroris especially useful for such inspection. Whenviewing occlusally do so with one eye closedbecause an undercut can be perceived as a nearparallel taper when seen with two eyes. Takespecial care to check the junction between prox-imal and buccal/lingual reductions which are acommon site for undercuts.

On PFM preparations, where the deeperreduction for porcelain and metal meets the shal-lower reduction for metal (this is shown from theocclusal aspect in Fig. 3a) there is often a distinctstep in the axial wall. This feature is termed

Fig. 10 Functional cusp bevel (FCB)of the holding cusps required forocclusal clearance

PRACTICE


rather confusingly the ‘wing’, resulting in gener-ations of students creating a bizarre preparationresembling a small bird in flight, but correctly cutit is a useful feature providing some increase inresistance and helps guide the technician in theposition of the porcelain-metal junction. Shoulda metal proximal contact be required the ‘wing’should lie buccal to the contact. Conversely, if aporcelain proximal contact is planned the wingshould lie lingual to the contact. To avoid under-cut it is important that the wing is made parallelto the buccal axial reduction.

Finish line Ideally, this should be placed supra-gingivally26,27

on sound tooth tissue, but in reality this is oftennot possible. Sometimes aesthetics dictates amargin is placed subgingivally and in these situ-ations it should extend by 0.5–1 mm, but cer-tainly no more than half the depth of the gingi-val sulcus, to ensure the epithelial attachment isnot compromised. Packing of retraction cord inthe gingival sulcus prior to preparing the finishline will allow displacement of the gingival mar-gin for access and help minimise gingival trau-

ma during preparation. Whether a finish lineshould ever be placed on core rather than toothis an area of contention. Some consider the prac-tice acceptable if the core has a perfect margin.However, in practice it is rarely possible to guar-antee the condition of the core margin and wewould generally recommend the preparationshould be extended sub-gingivally to finish ontosound tooth tissue. Where the finish line is likelyto be extensively sub-gingival a crown lengthen-ing procedure can often facilitate crown provi-sion and ensure a more accessible crown margin.

Whatever type and location of finish line ischosen it is important that:

• It has no unsupported lips of tooth structure atthe edge which can either break away on thedie causing problems with crown seating orbreak away from the tooth resulting in a mar-ginal gap. Use hand instruments to plane theseaway

• It is in harmony with the outline of the gingi-val margin which will optimise aesthetics ofcrowns finished at or above gingival level,and, for sub-gingival margins, will ensurethere is no localised over-cutting

• The technician will be able to detect it on the die• There is sufficient space to provide a crown

with an appropriate gingival emergence profile.

Additional retention featuresAt this stage consider whether, as described ear-lier, there is a need for grooves or boxes toenhance resistance and retention in a short ortapered preparation.

Final finishing Complete smoothing of the preparation is notessential, may diminish micro-mechanicalretention and may also risk overheating the pulpif it is not done under water spray. However, thefinal preparation must be free from irregulari-ties, sharp line angles and corners. These causedifficulty in casting models and result in dieswhich are vulnerable to damage leading to anill-fitting crown. An area which is often neglect-ed is the axio-occlusal line angle but it is impor-tant that all line angles and corners are carefullyrounded. This is especially so for preparationsfor all ceramic restorations to reduce areas ofpotential stress concentration.

Anterior preparationsWith the exception of provision of a functionalcusp bevel, anterior preparations need similarconsideration to posterior preparations. However,a few additional points need highlighting:

• Incisal reduction is best carried out first. Thiswill improve subsequent preparation accessand helps to ensure correct proportioning ofaxial reduction planes

• The reduction of the labial surface should bein two or more planes to achieve good aes-thetics and conserve tooth tissue. Long clini-

Fig. 11 Single plane reduction (a) can result in either shine through of the porcelaincore (feint outline) or pulpal damage. Problem solved by two plane reduction (b).Three plane reduction (c) needed for long clinical crowns giving a slightly morebuccal path of insertion

Fig. 12 Ensure occlusal clearance inboth ICP and excursions

Fig. 13 Full veneer crown preparation LL6 (36) and three quarter preparation atLL5 (35): a) Occlusal reduction carried out on both teeth. In the same way eachaxial surface was completed sequentially. Note the depth grooves on the occluso-buccal aspect of LL6 (36); b) The completed preparations showing the mesial finishline on a sound amalgam core

a b

PRACTICE


cal crowns will often need to undergo threeplane reduction; Figure 11 illustrates this

• The palatal reduction needs to reproduce thenatural concavity of maxillary teeth if space isto be provided for the development of anteriorguidance; Figure 12 illustrates this. Unlessclearance during lateral and protrusive move-ments has been checked, it is very easy to endup with a crown which occludes satisfactorilyin the intercuspal position, but which inter-feres during excursions.

Multiple preparationsPreparation of multiple teeth can be stressful forboth patient and dentist. Where possible try tosequence treatment so that no more than fourteeth are prepared at one sitting or perhaps six ifdealing with the maxillary anteriors.

If you are able to prepare multiple teeth witha mutual path of insertion it will facilitate theconstruction of provisional restorations andhelps with cementation of definitive crowns.However, this must not be done at the expense ofexcessive tooth reduction. To ensure a mutualpath of insertion it is usually best to prepareeach surface sequentially for all the teeth ratherthan fully complete each preparation beforemoving onto the next (Fig. 13).

CONCLUSIONCrown preparations are destructive to underly-ing tooth tissue and can affect the pulp. There-fore, the type of crown selected should havethe least destructive preparation in keepingwith the patient’s functional and aestheticrequirements. Where appropriate the use ofless destructive, adhesively retained restora-tions should be considered.

The authors would like to thank Mr Alan Waller, Audio-Visual Department, for help with the diagrams. Thanks alsoto Drs Eoin Smart and Ian Macgregor for their constructivecriticism and careful proof reading.

1. Shillingberg H T, Hobo S, Whitsett L, Jacobi R, Brackett S E.Fundamentals of fixed prosthodontics. 3rd ed. Chicago:Quintessenence, 1997.

2. Burke F J T. Fracture resistance of teeth restored with dentin-bonded crowns: the effect of increased tooth preparation.Quintessence Int 1996; 27: 115-121.

3. Broderson S P. Complete-crown and partial-coverage toothpreparation designs for bonded cast ceramic restorations.Quintessence Int 1994; 25: 535-539.

4. Rosner D. Function placement and reproduction of bevels forgold castings. J Prosthet Dent 1963; 13: 1160-1166.

5. Ostlund L E. Cavity design and mathematics: Their effect ongaps at the margins of cast restorations. Operative Dent1985; 10: 122-137.

6. Gavilis J R, Morency J K, Riley E D, Sozio R B. The effect ofvarious finish line preparations on the marginal seal and

occlusal seat of full crown preparations. J Prosthet Dent1981; 45: 138-145.

7. Dykema R W, Goodacre C J, Phillips R W. Johnston’s ModernPractice in Crown and Bridge Prosthodontics. pp.24.Philadelphia: W.B. Saunders Co., 1986.

8. Shillingburg H T, Hobo S, Fisher D W. Preparations for CastGold Restorations. pp.p16. Chicago: Quintessence PublishingCo., 1974.

9. Tylman S D, Malone W F P. Tylman’s Theory and Practice ofFixed Prosthodontics. pp.103. St. Louis: C. V. Mosby Co., 1978.

10. Jorgensen K D. Relationship between retention andconvergence angle in cemented veneer restorations. ActaOdontol Scand 1955; 13: 35-40.

11. Kaufman E G, Coehlo D H, Colin L. Factors influencing therentention of cemented gold castings. J Prosthet Dent 1961;11: 487-502.

12. Dodge W W, Weed R M, Baez R J, Buchanan R N. The effect ofconvergence angle on retention and resistance form.Quintessence 1985; 16: 191-194.

13. El-Ebrashi M K, Craig R G, Peyton F A. Experimental stressanalysis of dental restorations. Part III. The concept of thegeometry of proximal margins. J Prosthet Dent 1969; 22:333-345.

14. Ohm E, Silness J. The convergence angle in teeth prepared forartificial crowns. J Oral Rehabil 1978; 5: 371.

15. Eames W B, O’Neal S J, Monteiro J, Roan J D, Cohen K S.Techniques to improve the seating of castings. J Am DentAssoc 1978; 96: 432.

16. Mack P J. A theoretical and clinical investigation into thetaper achieved on crown and inlay preparations. J OralRehabil 1980; 7: 255.

17. Norlander J, Weir D, Stoffer W, Ochi S. The taper of clinicalpreparations for fixed prosthodontics. J Prosthet Dent 1988;60: 148-151.

18. Noonan J E, Goldfogel M H. Convergence of the axial walls offull veneer crown preparations in a dental schoolenvironment. J Prosthet Dent 1991; 66: 706-708.

19. Kent W A, Shillingburg H T, Duncanson M G. Taper of clinicalpreparations for cast restorations. Quintessence Int 1988;19: 339-345.

20. Maxwell A W, Blank L W, Pelleu G B. Effect of crownpreparation height on the retention and resistance of goldcastings. Gen Dent 1990; May-June: 200-202.

21. Michelini F S, Belser U C, Scherrer S S. Tensile bond strengthof gold and porcelain inlays to extracted teeth using threecements. Int J Prosthodont 1995; 8: 324-331.

22. Wiskott H W, Nicholls J I, Belser U C. The relationshipbetween abutment taper and resistance of cemented crownsto dynamic loading. Int J Prosthodont 1996; 9: 117-139.

23. Wiskott H W, Nicholls J I, Belser U C. The effect of toothpreparation height and diameter on the resistance ofcomplete crowns to fatigue loading. Int J Prosthodont 1997;10: 207-215.

24. Smith D G. Toothwear: Crown lengthening procedures. In:Barnes I E, Walls A W G, editors. Gerodontol pp.109-117.Oxford: Wright, 1994.

25. Moopnar M, Faulkner K D. Accidental damage to teethadjacent to crown-prepared abutment teeth. Aust Dent J1991; 36: 136-140.

26. Silness J. Periodontal conditions in patients treated withdental bridges. II. The influence of full and partial crowns onplaque accumulation, development of gingivitis and pocketformation. J Perio Res 1970; 5: 219-224.

27. Silness J. Periodontal conditions in patients treated withdental bridges. III. The relationship between the location ofthe crown margin and the periodontal condition. J Perio Res1970; 5: 225-229.

28. Hegdahl T, Silness J. Preparation areas resisting displacementof artificial crowns. J Oral Rehabil 1977; 4: 201-207.

Details of burs mentioned in articleBur No Supplier

Flat-end tapered diamond 554 Kent - AD Burs Ltd, Gloucester, UK

Long, round-end tapered diamond 503 Kent - AD Burs Ltd, Gloucester, UK

Long needle diamond 557 Kent - AD Burs Ltd, Gloucester, UK

Chamfer diamond 877/010 Komet-Brasseler gmbh, Lemgo, Germany

Chamfer tungsten carbide 282/101 Komet-Brasseler gmbh, Lemgo, Germany

Large flame (‘rugby ball’) diamond 257/023 Horico-Hopf Ringled & Co GmbH, Berlin, Germany

PRACTICE

BRITISH DENTAL JOURNAL VOLUME 192 NO. 11 JUNE 15 2002 619

Crowns and other extra-coronal restorations:Provisional restorations R. W. Wassell1 G. St. George2 R. P. Ingledew3 and J. G. Steele4

The important role of provisional restorations is often overlooked. This may be because they are left until the end of anappointment when time for construction is short or because they generally do not need to last for long. However, not onlycan good provisional restorations help produce better final restorations, they can also save a lot of time and expense atsubsequent appointments. In fact time spent in their construction will be more than repaid in time saved doing additionalprocedures, adjustments and remakes later on.

1*,4Senior Lecturer in Restorative Dentistry,Department of Restorative Dentistry, TheDental School, Newcastle upon Tyne NE24BW; 3Senior Dentist, Boots Dental Care,54-58 High Street, Maidenhead, BerkshireSL6 1PY; 2Higher Specialist Trainee,Eastman Dental Hospital, 256 Grays InnRoad, London WC1X 8LD*Correspondence to: Dr R. W. Wassell,Department of Restorative Dentistry, The Dental School, Newcastle upon Tyne NE2 4BWE-mail: [email protected]


● The functions of provisional (sometimes termed temporary) restorations● Diagnostic uses of provisional restorations● Provisional restorations for conventional preparations including: the concept of short,

medium, and long-term temporisation; materials; and direct and indirect provisionalrestorations

● Provisional restorations for adhesive preparations● Problem solving

I N B R I E F

This article discusses the need for provisionalrestorations, the types and materials available.Provisional restorations for adhesive restora-tions often pose a difficult problem and theseare considered separately. You may alsoencounter other difficulties with provisionalrestorations so we end the article on problemsolving.

Functions of provisional restorationsProvisional restorations are used in the inter-im between tooth preparation and fitting adefinitive restoration. The various functionsthey fulfil are described in Table 1. Withcrown preparations provisional restorationsare generally essential to cover freshly cutdentine and prevent tooth movement. Withadhesive preparations protection of exposeddentine is usually less of an issue and often aprovisional restoration is not needed, butthere are still occasions where it is importantto prevent unwanted tooth movement ormaintain aesthetics.

Provisional restorations can be invaluable fortesting out aesthetic and occlusal changes beforethey are incorporated in the definitive restora-tion. They can also help stabilise the periodontalcondition prior to definitive restoration. Thesediagnostic uses will be considered in more detailin the next section.

As well as the prophylactic and diagnosticuses, provisional restorations have other practi-cal applications. For example, callipers may beused to test the thickness of a provisionalrestoration to ensure sufficient tooth preparationto accommodate the proposed restorative mate-rial (Fig. 1). Occasionally, a provisional restora-tion may be used to provide a coronal build upfor isolation purposes during endodontic treat-

ment. A period of long-term provisional restora-tion may also be advisable to assess teeth ofdubious prognosis. Finally, a provisionalrestoration may find a use as a matrix for corebuild ups in grossly broken down teeth, simplyby removing the coronal surface to allow place-ment of restorative material.

DIAGNOSTIC USESProvisional restorations, especially those usedfor conventional preparations, are invaluable insituations where aesthetic, occlusal or periodon-tal changes to a patient’s dentition are planned.The principles behind such changes are dis-cussed later.

Aesthetic changesProposed changes to the shape of anterior teethare best tried out with provisional restorations toensure patient acceptance, and, approval fromfriends and family; clearly, it is easier to trim oradd acrylic than it is porcelain. Once happy, an

9










restorations

Fig. 1 Provisional restorations have many functions otherthan just protection of the prepared tooth. Here anSvensen gauge is used to assess sufficient toothreduction to accommodate the proposed restoration

PRACTICE

620 BRITISH DENTAL JOURNAL VOLUME 192 NO. 11 JUNE 15 2002

alginate is recorded so that the technician cancopy the shape into the definitive restoration. Ifonly one or two teeth are involved it is perfectlypossible to contour provisional restorations atthe chair side. It is however important to balancethe time spent doing this against the advantagesof having a diagnostic wax up and matrix madein the laboratory (Fig. 2). Alternatively, indirectprovisional restorations can be prescribed whichwill be described later.

Occlusal changesA patient’s tolerance to changes in anteriorguidance or increased occlusal vertical dimen-sion is best tried out with provisional restora-tions. Again, a diagnostic wax up is advisable,and, with occlusal changes, the importance ofusing casts mounted on a suitable articulatorcannot be overstated (Fig. 3). Direct or indirectprovisional restorations are then constructedfrom these and cemented temporarily afteradjusting to provide even occlusal contact inthe intercuspal position and guidance or dis-clusion if required. The patient can then beexamined at a further appointment and theocclusal surfaces copied as long as the follow-ing criteria are met:

• Restorations are still cemented• Occlusal contacts have been maintained, with

no drifting of teeth• Teeth are not mobile, or existing mobility is

not increasing• There is no discomfort

Guidance surfaces can be copied betweenprovisional and definitive restorations by usinga custom guidance table as described in Part 5of the series.

Where it is decided to increase the patient’socclusal vertical dimension, provisional restora-tions provide a way of assessing tolerance to theincrease, as well as assessing aesthetics and theoverall occlusal scheme. However it is often wiseto make an initial assessment of such changeswith a more reversible method, such as a splint,before the teeth are prepared.

Periodontal changesIt may be necessary as part of a patient’s periodon-tal treatment to remove overhanging restorationsto allow access for cleaning and resolution ofinflammation. Long-term wear of properly fittingand contoured provisional restorations allows thehealth of the gingival margin to improve and itsposition to stabilise before impressions are record-ed for definitive restorations.

Following periodontal or apical surgery thetissues will also need time to stabilise before thefinal finish line is cut for definitive crowns.Where surgical crown lengthening is used toincrease clinical crown height, it is best to allow6 months before definitive restoration, especial-ly if the aesthetics are critical.1 If provisionalrestorations are provided soon after crownlengthening it is important to avoid taking thepreparations subgingivally as this may set up achronic gingivitis which is difficult to resolve.

Changes in tooth shape — avoiding problemsFor the majority of people, minor adjustments intooth shape are unlikely to cause any problems,but for others, eg singers and wind instrumentmusicians, the eventual restorations, if poorlyplanned, may interfere with the patient’s‘embouchure’. This term describes the fine mouthmovements and lip/tooth contact required forspeech production or sound generation in thecase of a musical instrument. Also the incorpo-

Table 1 The functions of provisional restorations

1) Comfort/tooth vitality To cover exposed dentine to prevent sensitivity, plaque build up,subsequent caries and pulp pathology.

2) Occlusion and positional stability To prevent unwanted tooth movement by the maintenance ofintercuspal and proximal contacts. It may be necessary toestablish a holding contact on the provisional restoration.Depending on the patient’s occlusal scheme, the provisionalrestoration may need to provide guidance in protrusive andlateral excursions, or disclude to prevent working or non workinginterferences. Interproximal contacts also need to be maintainedto prevent food packing.

3) Function Attention to 1) and 2) will ensure the patient continues tofunction adequately.

4) Gingival health and contour To facilitate oral hygiene and prevent gingival overgrowthprovisional restorations require accurate margins and cleansablecontours. They can be used in the interim where the level of thegingival margin has yet to stabilise (e.g. after crown lengtheningor removal of a crown with defective margins).

5) Aesthetics To provide an adequate interim appearance provisionalrestorations should either mimic the tooth just prepared, or thefinal intended restoration.

6) Diagnosis To assess the effect of aesthetic and occlusal changes. The abilityto re-shape can also be used to overcome phonetic problemsbefore construction of the definitive restoration.

7) Other practical uses To measure tooth reduction, to isolate during endodontics, toassess prognosis, to act as a matrix for core construction.

Fig. 2 A diagnostic wax-up is aninvaluable way of planning changes,which can be tried out with theprovisional restorations. The patientwould be more impressed by toothcoloured wax!

Fig. 3 Casts need to be mounted on a semi-adjustablearticulator to wax occlusal changes

PRACTICE


ration of wider cervical embrasure spaces, tofacilitate interproximal cleaning, may occasion-ally cause embarrassment because of air leak-age. Therefore it makes sense to copy the fea-tures of successful provisional restorations, toavoid patient dissatisfaction and expensiveremakes.

PROVISIONAL RESTORATIONS FORCONVENTIONAL PREPARATIONSThe variety of provisional restorations availablecan seem bewildering. To help you through themaze we need first to consider the materials andthen the techniques by which provisionals canbe made. Most provisionals are made directly atthe chair side, but for long term wear or diagnos-tic use there can be advantages in having themlaboratory-made.

It is worth emphasising that the length oftime between preparation of teeth and cemen-tation of final restorations can vary from afew days for straightforward cases (short-term), to several weeks (medium-term) oreven, in the case of complex reconstruction,several months (long-term). The longer provi-sional restorations are in the mouth, thegreater are the demands on the material fromwhich they are made.

MaterialsMaterials used in the surgery comprise pre-formed crowns (made of plastic or metal), selfcured or light cured resins or resin compositesand cements. Laboratory formed temporaries aregenerally made in self cured or heat curedacrylic, or cast metal.

Pre-formed crownsAlso known as proprietary shells, these come ina series of sizes but usually need considerableadjustment marginally, proximally andocclusally. Plastic shells are made from polycar-bonate or acrylic, and, with good aesthetics, arecommonly used for anterior teeth including pre-molars. Metal shells may be made from alumini-um, stainless steel or nickel chromium and areonly used on posterior teeth. Both plastic andmetal shells can be relined with self cured resinto improve their fit.

Self or light cured resinsA variety of materials is available for eitherdirect or indirect techniques:

• Polymethyl methacrylate (self or heat cured) (eg Vita K&B Acrylics)

• Polyethyl methacrylate (eg Snap, Trim)• Bis acryl composite (eg Protemp, Quicktemp)• Urethane dimethacrylate (light cured) (eg

Provipont DC)• Restorative composite

Polymethyl methacrylate is strong, has ahigh wear resistance, is easy to add to, and hasgood aesthetics, which is maintained overlonger periods.2 However, it does have threemain disadvantages:

i) Polymerisation shrinkage which can affectfit

ii) Polymerisation exotherm which can damagepulp3

iii) Free monomer may cause pulp and gingivaldamage

It is, however, a good material for indirect pro-visional restorations. Self cured acrylic can bepolymerised under pressure in a hydroflask whicheffectively reduces porosity.4 Alternatively, heatcured acrylic can be used.

Polyethyl methacrylate (Fig. 4a) is suitablefor intra-oral use as it shrinks less and is lessexothermic than polymethyl methacrylate.However, strength, wear resistance, aestheticsand colour stability are not as good. Some pre-sentations come with a good colour range (egTrim II) while others have only light and darkshades (eg Trim).

Bis-acryl composites (Fig. 4b) produce lessheat and shrinkage during polymerisationthan other resins, resulting in a better margin-al fit.5 Despite being reasonably strong theyare brittle in thin section and difficult to addto. Aesthetically they are reasonable, but fewshades are available and they stain easily if theunpolymerised surface layer is not removedwhich may be accomplished with alcohol andpolishing. They are more colour stable thanpolyethyl methacrylate materials and aretherefore better suited for use as long-termprovisionals.6

Most recently, visible light cured resins havebeen introduced based on urethane dimethacry-late eg Provipont D.C. These relatively expensiveresins have good mechanical properties and,being light cured, the operator has some controlover the material’s working time. Generallymore shades are available than bis acryl com-posite and the colour is relatively stable, but isstill prone to staining. Marginal fit can be poor,

Fig. 4 Examples of resins used forprovisional restorations: (a) Polyethyl methacrylate (powder-liquid presentation); and (b) Bis-acryl composite (syringe mixed)

a

b

PRACTICE


but the material can be added to. Light curedmaterials, especially unfilled ones, have a higherexotherm than chemically cured materialsbecause of their greater speed of reaction.7 Thisexotherm may have implications for pulpalhealth.

Restorative composites, normally used forfilling teeth, can be used as a provisional materi-al for adhesive preparations, as described later.Other restorative composites are designed forlaboratory fabrication but may be useful forlong-term provisional restorations.

Cast metalAlloys used include nickel chromium, silver andscrap gold. Copings can be cast with externalretention beads for acrylic or composite. In lessaesthetically critical areas of the mouth, metalmay be used on its own. Cast metals are verydurable, but rarely used unless provisionalrestorations have to last a long time.

Provisional cementsProvisional restorations are usually cementedwith soft cement. Traditionally, a creamy mix ofzinc oxide eugenol was used, but nowadaysmost dentists prefer proprietary materials suchas Temp Bond (Fig. 5). This material comes witha modifier, which is used to soften the cement, asdescribed later in the article, to ease removal ofthe provisional restoration from more retentivepreparations. Temp Bond NE is a non-eugenolcement which may be used for patients witheugenol allergy or where there is concern overthe possible plasticising effect of residualeugenol on resin cements and dentine bondingagents. Certainly, surface hardness11 and shearbond strength of resin12 to resin can both beaffected by eugenol and it is worth noting thateugenol cements can significantly reduce thebond of resin cements to composite cores.16

However, resin bond strengths to enamel13 anddentine14 are not affected if the eugenol residueis removed with pumice and water before condi-tioning. Microleakage15 is also unaffected by theuse of eugenol.

Occasionally, hard cement is needed to retaina provisional on a short preparation. This is con-sidered later in the ‘problem solving’ section.

Direct provisional restorations — techniquesMost provisional restorations will be madedirectly in the mouth. As mentioned earlier it isworth taking time in their construction. As a ruleof thumb, the time taken to temporise a toothshould be similar to the time taken to prepare it.This approach ensures sufficient time is devotedto good fit and contour. The techniques availableare listed overleaf:

Fig. 5 Temp Bond and Temp Bond NE: The modifier (central tube) can be mixed with Temp BondBase and Catalyst to ease crown removal with retentive preparations. Regular Temp Bond containseugenol, which can soften composite cores. Temp Bond NE (shown to right of photograph) does notcontain eugenol and will avoid this problem

Fig. 6 (a) A familiar polycarbonateshell crown relined with Trim; and(b) The provisional is carefullytrimmed to help maintain gingivalhealth

a

b

Fig 7 (a) Aluminium shell crowns are convenient, butsuitable only for short term use on posterior teeth; and(b) Crimping of the crown margins will improveretention and fit

a

b

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• Shells (proprietary or custom)• Matrices (either formed directly in the mouth

or indirectly on a cast)• Direct syringing

To avoid confusion with terminology, a shellis incorporated into the provisional restorationwhereas a matrix is merely used to form it.

Proprietary shellsShells can be divided into proprietary and cus-tom made. Proprietary shells made of plastic(Fig. 6) or metal (Fig. 7) are used commonly inpractice when only one or perhaps two prepara-tions are involved.

Proprietary plastic shells: A crown with thecorrect mesio-distal width is chosen and placedon the tooth preparation. The cervical marginsare trimmed to give reasonable seating and adap-tation. The preparation is then coated with petro-leum jelly and the crown, containing a suitableresin eg Trim, is reseated. While the resin is stillincompletely set, the proximal excess is removedusing a sharp bladed instrument such as a halfHollenback amalgam carver. The crown is thenremoved and replaced several times to preventresin setting in undercuts. Finally, the crown isadjusted and polished using steel or tungstencarbide burs and Soflex discs. Diamond burs arebest avoided, as they tend to melt the shell andresin because of the heat generated.

Proprietary metal shells: Aluminium crownsare really only suitable for short-term use asthey are soft resulting in wear and deformation.Furthermore, they can produce galvanic reac-tions in association with amalgam restorations.Their fit is usually poor unless considerable timeis spent trimming and crimping the margins fol-lowed by relining with a resin. Stainless steel ornickel chromium crowns may occasionally beused on molar teeth opposed by flat cusps whereheavy occlusal loading would quickly wear orbreak a resin crown.

Custom shellsSome operators favour custom shells for multi-ple tooth preparations. The shell is made inadvance of tooth preparation so the desiredexternal contours are pre-formed, but reliningand careful marginal trimming are necessaryprior to fitting. Custom shells are of two types,either beaded acrylic or ‘Mill Crowns’. Both offerthe advantage of being able to use the superiorproperties of polymethyl methacrylate, whilstavoiding pulpal damage by constructing theshell out of the mouth.

The beaded acrylic shell is formed within animpression taken of the teeth prior to preparationor of a diagnostic wax up. A thin shell of poly-methyl methacrylate is constructed in the impres-sion by alternately placing small amounts ofmethyl methacrylate monomer followed by poly-mer, taking care not to make the shell too thick,otherwise it will need time-consuming adjust-ment later. Once set, it is trimmed and then relinedin the mouth as with polycarbonate crowns.

Mill Crowns are formed by first cutting mini-mal crown preparations on a stone cast. A pre-preparation matrix is then filled with poly-methyl methacrylate and placed over thepreparations. The trimmed and adjusted provi-sional crowns are again relined in the mouth.

MatricesMany operators prefer matrices (Fig. 8) to shellcrowns for making single or multiple provision-al crowns. This is because matrices closely dupli-cate the external form of satisfactory existingteeth, or, if changes are required, a diagnosticwax up. If the matrix is carefully seated minimaladjustments are generally needed other thantrimming flash at the crown margin.

There are three main types of matrix:

• Impression (alginate or elastomer)• Vacuum formed thermoplastic• Proprietary celluloid

The simplest way of making a matrix is torecord an impression of the tooth to be preparedeither in alginate or silicone putty. Impressionmatrices are quick, easy and inexpensive, andcan be formed while the local anaesthetic isallowed to take effect. When impression matri-ces are used some judicious internal trimmingmay be helpful to improve seating and bulk outcritical areas of the provisional restoration.These aspects are covered later when we dealwith problem solving. Alginate matrices arebest at absorbing the resin exotherm3 —although the temporary should have beenremoved before this stage of set. Elastomericimpression matrices have the advantage ofbeing reusable, allowing them to be disinfectedand stored in case they are required again.Polyvinylsiloxane putty impressions are fre-quently used because of their ease of handlingand long-term stability.

Fig. 8 (a) A commonly used matrix isan alginate impression of theunprepared tooth; and (b) The flashmust be removed and the linkedprovisionals trimmed prior tocementation

a

b

PRACTICE


If a tooth is broken down or its shape needsto be changed, it will first need to be built up.Soft red wax can be used for this purpose, butanyone who has tried this will know that itdoes not stick well. Adhesion can be improveddramatically by first painting the tooth with acoat of varnish (eg copal ether or glassionomer varnish) and allowing it to dry. Theopposing teeth are smeared with petroleumjelly to allow their form to be stamped into thewax should it be necessary to build up theocclusal surface. Rather than try and achieveperfection with an intra-oral wax-up it is bet-ter to aim for a slight over-contour, which canbe corrected by trimming the provisionalrestoration. It is best to avoid putty for makingthe matrix as it can distort or displace the waxwhen the impression is seated.

There is no doubt that the above technique isinvaluable, however, when dealing with multi-ple crowns it is a much better strategy to firstcarry out a diagnostic wax up on mounted casts(Fig. 3). The intended aesthetics and occlusioncan be formed much more efficiently andpatients appreciate being able to see a ‘blueprint’ of the definitive restorations on the articu-lator. Moreover, the wax up can be used to forma suitable indirect matrix.

Indirect matrices can be made from impres-sion material (Fig. 9a) or you can ask yourlaboratory for a vacuum formed matrix madeof clear plastic (Fig. 9b). If you decide to makean indirect matrix from impression materialremember to first soak the cast for five min-utes. In this way you will avoid the embarrass-ment of sticking impression to cast. Immer-sion in warm water (not hot) has theadvantage of speeding up the impressionmaterial’s setting time.

Vacuum formed matrices are made of clearvinyl sheet produced on a stone duplicate of the

waxed up cast. This is necessary to avoid melt-ing the wax when the hot thermoplastic materialis drawn down. Not everyone is enthusiasticabout using a vacuum formed matrix becausethey are flexible and can distort when seated,especially if there are few or no adjacent teeth toaid location. Where it is necessary to rely on thesoft tissues for matrix location we prefer to usean impression matrix.

Whilst vacuum formed matrices are not with-out problems, being made of clear plastic theyare indispensable for moulding light curedresins. A proprietary celluloid matrix can beused if only a single provisional crown is to beformed using light cured resin.

Whatever matrix is chosen care must betaken in its use. After tooth preparation, a thinsmear of petroleum jelly is placed over thereduced tooth and adjacent teeth. The matrix isblown dry and the mixed resin is syringed intothe deepest part of the appropriate tooth recess,taking care not to trap air, especially at theincisal angles. After reseating, the matrix is leftuntil the resin reaches a rubbery stage. It is thenremoved and interproximal excess removed inthe same way as for the proprietary shell. Settingcan be monitored to some extent by testing theconsistency of a small portion of materialsyringed onto the front of the seated impression.Following removal, the crowns are trimmed,polished and cemented.

Direct syringingWhen no shell temporary can be found to fitand, for whatever reason, no matrix is availableit can be useful to syringe material directlyaround a preparation. For this purpose the poly-ethyl methacrylate materials are best as they canbe mixed to sufficient viscosity not to slump butare still capable of being syringed. This propertywhereby a material undergoes an apparentdecrease in viscosity at high rates of shear, aswhen passed through a syringe nozzle, is called‘shear thinning’. It is also seen with the polyethermaterial, Impregum.

When syringing, start at the finish line andspiral the material up the axial walls. Overbuildthe contours slightly as it is easier to trim awayexcess than to have to add later.

Indirect provisionalsMany dentists will not have used indirect provi-sional restorations and may find it hard to justifylaboratory costs. However, indirect provisionalsoffer certain advantages with complex casesneeding long-term temporisation. Firstly, materi-als which are stronger and more durable can beused eg heat cured acrylic or self cured acryliccured in a hydroflask. Secondly, if aesthetic orocclusal changes are to be made these can bedeveloped on an articulator. Indirect provisionalscan certainly save clinical time, especially withmultiple restorations and most particularly wherethere is to be an increase in vertical dimension,especially where the patient is a bruxist (Fig. 10).

Whether or not major changes are indicat-

Fig. 9 Where aesthetic or occlusalchanges are proposed, provisionalcrowns can be formed in the mouthwith laboratory-made matrices: (a) A putty or alginate matrix can beformed directly on the wax-up(remember to soak cast first); and(b) A vacuum formed matrix canalso be made, but on a stoneduplicate of the wax-up to preventthe wax from melting (matrix shownprior to trimming)

a

b

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ed, it is best to decide on the type of provi-sional restoration during treatment planning.If indirect restorations are chosen, sufficienttime can be scheduled either to make themwhilst the patient waits or an additionalappointment can be made to fit those made inthe laboratory.

Some operators favour making indirect pro-visional restorations from self cured acrylic atthe same appointment the teeth are prepared. Ifthe surgery has an on site technician (or suit-ably trained nurse) this can be a very efficientway of working as it allows the dentist to dosomething else while the provisional restora-tions are being made. An alginate impression isrecorded of the prepared teeth and this is castup in quick setting plaster. The plaster model isthen coated with cold-mould seal and a suit-able shade of self cured acrylic mixed up. Theacrylic is then flowed into a matrix made fromthe diagnostic wax up (eg silicon putty or vacu-um formed) which is then seated onto the castand its position stabilised with elastic bands,taking care not to distort it. Polymerisationtakes place within a hydro flask followingwhich the matrix is removed; the relatively softplaster dug out and the acrylic flash trimmedback to the margin. Additions may need to bemade to the margins of the crowns where poly-merisation shrinkage has produced a gap. Thiscan be done in the mouth or on a cast. If donein the mouth, then the tooth needs lubricatingwith petroleum jelly and the crowns removedbefore excessive heat is generated.

Clearly, an extra appointment will be neces-sary if indirect provisional restorations aremade in an outside laboratory. In the interimdirect provisional restorations will also beneeded. This approach can be very effective,however, where an increase in vertical dimen-sion is prescribed. A number of strategic teethcan be prepared and interim provisionalrestorations made directly to conform to theexisting occlusion (eg from polyethylmethacrylate or Bis acryl composite). Onreturn, the indirect provisional restorationscan be used to establish the increased verticaldimension on multiple teeth. Often these casesrequire minimal or no occlusal preparation asocclusal clearance is provided by the increasein vertical dimension. When this happens theinterim provisional restoration will resemble atube with no occlusal surface, which is usuallyacceptable for short periods.

PROVISIONAL RESTORATION OF ADHESIVEPREPARATIONSProvisional restorations for conventional toothpreparations (eg full veneer crowns, 3/4 crowns,and onlays) obtain retention in a similar way tothe final restorations ie via cement on prepara-tions with long, minimally tapered axial walls.The lack of conventional retention provided bymost adhesive preparations results in temporarycements being ineffective. A number of strate-gies can be used to deal with this problem, some

of which are more appropriate for certain situa-tions than others:

• No temporary coverage may be necessary egwith veneer preparations involving minimaldentine exposure and not removing intercus-pal or proximal contacts. Where space hasbeen created with a Dahl appliance, the appli-ance can be used in the interim to retain theteeth in position

• A simple coat of zinc phosphate cement toprotect exposed dentine eg in tooth prepara-tions which are not aesthetically critical andwhere the occlusion is either not involved orthe restoration can be returned rapidly fromthe laboratory and fitted before significanttooth movement occurs

• Composite resin bonded to a spot etched onthe preparation eg veneer preparations whichare aesthetically critical or occlusally criticalor have sensitive, fresh cut dentine. The provi-sional restoration is fabricated using eitherdirectly placed composite which is time con-suming, or composite and a clear vacuumformed matrix made from a preoperative cast.For longer lasting provisionals, acrylicveneers may be made in the laboratory. Limit-ing the area of bonding facilitates compositeremoval, but the bonded area must be groundback to tooth substance when the definitiverestoration is fitted. If the composite is cutwithout water spray it is easier to distinguishbetween the powdery surface of the groundcomposite and the glassy appearance of theunderlying tooth

• Composite resin bonded to the opposing toothto maintain occlusal contact and preventover-eruption (eg shims or veneers wheresome additional occlusal reduction isrequired). After the definitive restoration isplaced, the opposing composite is removed

Fig. 10 Metal and acrylicprovisionals used in the occlusalreconstruction of a bruxist: (a) A relined NiCr shell at tooth LL7 (37) where the previous acrylicprovisional crown had fracturedrepeatedly; and (b) The upper archhas acrylic heat cured onto beadedmetal copings in an attempt toimprove fracture resistance —however a good bond between metaland acrylic is needed for success

a

b

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• Conventional provisional restorationscemented with either a non-eugenol tempo-rary cement or a hard cement such as zinc car-boxylate. This approach may be used foradhesive restorations having some mechani-cal retention eg an inlay or resin bondedcrown. The choice of cement will depend onhow retentive the preparation is.

It is sensible to expect provisionals for adhe-sive preparations to be effective only in the shortterm. Certainly, their diagnostic usefulness intesting changes in aesthetics and occlusion ismuch more limited than with provisionalrestorations for conventional preparations.

PROBLEM SOLVINGA number of problems are encountered whenmaking provisional restorations. Some of theseare discussed below:

Insufficient bulk of materialThe axial walls of resin provisionals are oftenthin which makes them prone to damage duringremoval from the mouth. This is particularly thecase when minimal amounts of tooth areremoved eg preparations for gold crowns. To

prevent damage, the provisional should be madetemporarily wider by relieving the appropriatepart of the impression with a large excavator(Fig. 11). The excess resin can be contoured afterit has completely set.

Gross occlusal errors, air blows and voidsAn impression matrix not being reseated fullyoften causes gross occlusal errors and may occurfor two reasons:

• Fins of interproximal impression materialbeing displaced and sandwiched between theimpression and the occlusal surface — trimaway any suspect areas from the inside of theimpression with a scalpel or scissors beforereseating

• Hydrostatic pressure built up within the unsetresin during reseating of the impressionmatrix — consider cutting escape vents cutfrom the crown margin to the periphery of theimpression with a large excavator.

Avoid voids by syringing material directlyonto preparations. Of course, material is alsoloaded into the impression, ensuring the tip isalways in the resin, to prevent the incorporationof air.

Locking in of provisional restorationsProvisional restorations are often locked in byextruded material engaging the undercutsformed by the proximal surfaces of adjacentteeth. The technique of cutting out a triangularwedge of material from the gingival embrasurespace with a half Hollenback instrument hasalready been mentioned. This must be done whilethe material is still soft and before any attempt ismade to remove the provisional restoration fromthe preparation. Once removed, any thin flashshould be trimmed with a pair of scissors, and thecrown reseated a number of times to ensure itdoes not lock into place when set.

Provisional inlays can be a particular prob-lem because of the difficulty in removing excessresin proximally on posterior teeth. A time sav-ing technique is possible using two light curedresins,8 a soft one for the cavity floor and base ofthe box (eg Fermit), and a harder one for theocclusal surface (eg Provipont DC). A vacuumformed, transparent matrix is also needed. Thesofter resin is condensed into the cavity using aconventional matrix band to limit proximalextrusion. The cavity is filled to the level of theproximal contacts after which the resin surfaceis pock marked with a condensing instrument toprovide mechanical retention for the harderresin. Following light curing the matrix band isremoved. The coronal portion of the restorationis formed from the harder resin by means of thevacuum formed matrix. After light curing, thematrix is removed along with the provisionalrestoration, which can be further cured from thefitting surface. The amount of flash produced bythis method is minimal, and the manufacturingof the inlay up to the point of trimming can becompleted in less than 4 minutes.8

Marginal discrepanciesAlthough polymerisation shrinkage can causemarginal discrepancies, it is our opinion thatmost problems are caused simply by distor-tion of the margin when the provisionalcrown is first removed from its preparation.Such distortion results from excess materialengaging proximal undercuts and can easilybe prevented by following the advice in theprevious section.

Should a marginal discrepancy occur witha provisional resin crown, the simplest solu-tion is to reline it. A useful tip is to flare outthe inside of the crown margin with a bur. Thisapproach provides a greater bulk of relinematerial and more area for it to bond. To facil-itate seating it is best not to fill the wholecrown with resin, but to confine the relinematerial to the inner aspect of the crown mar-gin, thus reducing hydrostatic pressure.

Multiple crownsWhen several adjacent preparations require provi-sional restorations, reseating an impression con-taining resin invariably results in all the restora-tions being joined together as material passesthrough the thin and often torn interproximal

Fig. 11 Where the resultingprovisional restoration would be toothin, the inside of the alginatematrix can be trimmed to give agreater bulk of resin

PRACTICE


area. Splinting teeth together in this way has theadvantage of preventing drift due to poor inter-proximal and occlusal contacts. However, it isextremely important to ensure the gingivalembrasures are opened sufficiently to give goodaccess to tooth brushing (Fig. 12a-c). This is bestaccomplished with a flame shaped bur (Fig. 12d).

Ideally, provisional crowns should be sepa-rate, but separation can result in unwanted gapsbetween them. One way9 of overcoming this isto place small pieces of celluloid strip, roughly1cm long, between the teeth to be prepared. Thestrips should have holes punched in their buccaland lingual portions with a rubber dam punchto aid retention in the over-impression. A smallamount of alginate is smeared over the celluloidstrip’s retentive holes before seating the tray.When the impression is removed, the strips stayembedded in the alginate and separate the resincrowns while they are made.

Premature decementationPremature loss of provisional restorations isfrustrating for both patient and dentist. Thisproblem can be largely avoided by ensuring har-mony with the occlusion. A few seconds spentmarking up and adjusting occlusal contacts willsave time in the long run.

Occasionally, it is necessary to use a strongercement, such as zinc polycarboxylate, especiallywhere retention is limited.

Partial denture abutmentsA provisional crown used as a partial dentureabutment is made best from an acrylic resin(eg Trim) as additions are easy to make. Thefollowing technique is recommended: The pro-visional crown should initially be kept clearfrom where rest seats and guide planes are tocontact. Fresh resin is then placed in theseareas before reseating the partial denture with

its components lubricated with petroleum jelly.After the resin has set, the denture is removedand the crown is finished.

Eugenol containing temporary cements andadhesionAs discussed previously, eugenol-containingcements should be avoided where it is intendedto cement the definitive restoration to an under-lying composite core.

Removing temporary crownsAlthough it is desirable for provisional crownsto remain cemented during function, theyshould still be easily removed when impres-sions are taken, adjustments are needed, ordefinitive restorations need cementing. Whenpreparations are of optimal height and taper,the use of even comparatively weak tempo-rary cements may make removal difficult,especially when the provisional restorationneeds to remain undamaged for recementa-tion or when definitive crowns are cementedon a temporary basis.

To make removal easier, the cement shouldbe applied in a ring around the inner aspect ofthe margin. Alternatively, the manufacturer’s

Fig. 12 Provisional restorations andgingival embrasures: (a) No gingivalembrasure space provided betweenthe maxillary incisors; (b) Bleedingfrom the resulting gingivalinflammation prevented impressionsbeing recorded; and (c) Patientsmaintain gingival health best wherethere are open gingival embrasures(as shown in this provisional bridge)to allow toothbrush penetrationinterproximally; and (d) Gingivalembrasures under linked provisionalsneed to be opened out with a flameshaped bur

a b

c d

Fig. 13 A handy tip to facilitateremoval of set cement (either hardor soft); loop floss under eachconnector of linked crowns andapply petroleum jelly to externalsurfaces

1. Wise M D. Stability of gingival crestafter surgery and before anteriorcrown lengthening. Br Dent J 1985;53: 20-23.

2. Crispin B J, Caputo A A. Color stabilityof temporary restorative materials. JProsthet Dent 1979; 42: 27-33.

3. Moulding M B, Teplitsky P E.Intrapulpal temperature during directfabrication of provisionalrestorations. Int J Prosthodont 1990;3: 299-304.

4. Donovan T E, Hurst R G, Campagni WV. Physical properties of acrylic resinpolymerized by four differenttechniques. J Prosthet Dent 1985; 54:522-524.

5. Tjan A H, Castelnuovo J, Shiotsu G.Marginal fidelity of crowns fabricatedfrom six proprietary provisionalmaterials. J Prosthet Dent 1997; 77:482-485.

6. Lang R, Rosentritt M, Leibrock A, BehrM, Handel G. Colour stability ofprovisional crown and bridgerestoration materials. Br Dent J 1998;185: 468-471.

7. Gulati A J. Physical properties ofprovisional restorative materials[MSc]. University of Newcastle UponTyne, 1996.

8. Nicholson J W, Chan D C. Two-step

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provisional technique for onlaypreparations. J Esthetic Dent 1992; 4:202-207.

9. Liebenberg W H. Improvinginterproximal access in directprovisional acrylic resin restorations.Quintessence Int 1994; 25: 697-703.

10. Shillingburg H T, Hobo S, Whitsett L D.Provisional Restorations.Fundamentals of FixedProsthodontics. 4 ed pp.225-256.Chicago: Quintessence international,1998.

11. Civjan S, Huget E F, De Simon L B.Compatibility of resin compositeswith varnishes, liners and bases. JDent Res (Special issue) 1973; 52: 65(Abstract no.27).

12. Dilts W E, Miller R C, Miranda F J,Duncanson M G J. Effect of zinc oxide-eugenol on shear bond strength ofselected core/cement combinations. J Prosthet Dent 1986; 55: 206-208.

13. Schwartz R S, Davis R D, Mayhew RW. The effect of a ZOE temporarycement on the bond strength of aresin luting agent. Am J Dent 1990; 3:28-31.

14. Schwartz R, Davis R, Hilton T J. Effectof temporary cements on the bondstrength of a resin cement. Am J Dent1992; 5: 147-150.

15. Woody T L, Davis R D. The effect ofeugenol-containing and eugenol-free temporary cements onmicroleakage in resin bondedrestorations. Operative Dent 1992;17: 175-180.

16. Millstein P L, Nathanson D. Effects oftemporary cementation onpermanent cement retention tocomposite resin cores. J Prosthet Dent 1992; 67: 856-859.

modifier should be added to the cement (Fig.5). Equal lengths of base and catalyst with athird of a length of modifier will soften thecement appreciably. Therefore, the proportionof modifier needs to be gauged for each case.Either finger pressure or instruments such astowel clips can then remove the restorations,without risking damage to the preparationmargin.

Removal of excess cementTemporary cement removal is facilitated bypre-applying petroleum jelly to the outside ofthe restorations and placing floss under eachconnector of linked crowns before seating(look at Fig. 13). Once set, the excess cement iseasily removed with the strategically posi-tioned floss.

CONCLUSIONQuality restorative dentistry needs qualityprovisional restorations for predictableresults. Dentists therefore need to be familiarwith the range of materials and techniques forshort term, medium-term and long-term tem-porisation. Forethought and planning are alsoneeded to ensure the most appropriate provi-sional is used, especially when multiple teethare to be prepared or where occlusal or aes-thetic changes are envisaged. Such changesare best tried out with provisionals so thatmodifications can easily be made intra-orallyand when satisfactory copied into the defini-tive restorations. In this respect an initialdiagnostic wax-up is invaluable to facilitatethe construction of laboratory formed provi-sionals or matrices.

List of products mentioned in the text:

Fermit Ivoclar-Vivadent UK Ltd, Leicester, UK

Protemp II (hand-mix) ESPE, Seefeld, Germany

Protemp Garant (syringe mix) ESPE, Seefeld, Germany

Provipont DC Ivoclar-Vivadent UK Ltd, Leicester, UK

Quicktemp Davis, Schottlander & Davis Ltd, Letchworth, UK

Snap Parkell, Farmingdale, USA

Soflex Discs 3M Dental Products, St Paul, USA

Temp Bond Kerr UK Ltd, Peterborough, UK

Temp Bond NE Kerr UK Ltd, Peterborough, UK

Trim II Harry J. Bosworth Co, Illinois, USA

Vita Autopolymerizing Panadent, London, UKK+B Acrylics

PRACTICE


Crowns and other extra-coronal restorations:Impression materials and technique R. W. Wassell1 D. Barker2 and A. W. G. Walls3

Well-fitting indirect restorations can only be made if there are accurate models of the oral tissues available, made from highquality impressions. Waiting for an impression to set may be more stressful for the dentist than the patient. Should theimpression need to be repeated there is the embarrassment of having to explain this to the patient, the cost implications ofmaterial and time wasted and the aggravation of running late for the next appointment. Yet, if a ‘Nelsonian’ eye is turned to adefective impression we can only expect a substandard restoration in return.

1Senior Lecturer in Restorative Dentistry,2Higher Specialist Trainee, 3Professor,Department of Restorative Dentistry, The Dental School, Newcastle upon TyneNE2 4BW; *Correspondence to: Dr R. W. Wassell,Department of Restorative Dentistry, The Dental School, Newcastle upon Tyne NE2 4BWE-mail: [email protected]


● An overview of impression materials● A rationale is put forward for the choice of elastomeric materials ● Solutions are provided to common problems encountered by the clinician in producing good,

accurate impressions● An emphasis on the need for feedback between the laboratory and the dentist

I N B R I E F

Sometimes impression defects only come to lightafter the cast has been poured. Thus laboratoryinspection is an important part of quality con-trol, but many technicians find it difficult to feedback to their dentists for fear of the messengerbeing shot. Feedback between laboratory anddentist is critical to the establishment of an openand honest relationship. Hopefully, an improvedunderstanding of impression materials coupledwith techniques to overcome problems willencourage higher standards and the confidenceto accept appropriate advice.

The first part of this article considers thefactors influencing choice of impression materi-al. To have a practical understanding of impres-sion materials it is not necessary to have a PhDin dental materials science. Nevertheless, toselect an appropriate material it does help tohave a feel for the classification of impressionmaterials as well as concepts such as workingtime, setting time, permanent deformation anddimensional stability. There are certainly otherimportant factors that will influence your deci-sion such as ease of manipulation, taste andtackiness but these have so far eluded quantita-tive measurement.

The putty-wash technique has proven popu-lar not least for its ease of handling, but there aredrawbacks that can have significant effects onits accuracy and these will be discussed.

The final part of the article will consider howto overcome the problems routinely encounteredin recording impressions.

CLASSIFICATION OF IMPRESSION MATERIALSImpression materials are commonly classifiedby considering their elastic properties once set.Therefore, they can be broadly divided into

non-elastic and elastic materials as shown inFigure 1. Non-elastic impression materials aregenerally not used for obtaining impressions ofcrown preparations because of their inability toaccurately record undercuts. The elastic impres-sion materials can be divided into two groups:the hydrocolloids and the synthetic elastomers.Table 1. details the physical properties of themajor groups of elastic impression materialsavailable.

Hydrocolloid impression materialsThe two types of hydrocolloids used in dentalimpressions are agar and alginate. Agar is areversible hydrocolloid because it can passrepeatedly between highly viscous gel and lowviscosity sol simply through heating and cool-ing. However, alginate once converted to the gelform cannot be converted back into the sol, andis therefore said to be irreversible hydrocolloidmaterial.

Agar and alginate may be used independentlyor in combination to record crown impressions.Agar was first introduced into dentistry forrecording crown impressions in 1937 by Sears1

and was the first elastic impression materialavailable. It is not commonly used in dentalpractice today however, because of the need forexpensive conditioning baths and water cooledtrays. Alginate, unlike agar, does not require anyspecial equipment. Being easy to use and inex-pensive it is popular for less critical applicationseg opposing casts and study models.

Alginate and agar produce impressions withreasonable surface detail. They are both rela-tively hydrophilic and are not displaced fromwet surfaces as easily as the elastomers.2 How-ever, in respect of recording crown prepara-

10










restorations

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tions these materials have two major disad-vantages. Firstly, very poor dimensional sta-bility because of the ready loss or imbibition ofwater on standing in dry or wet environmentsrespectively. Secondly, low tear resistancewhich can be a real problem when attemptingto record the gingival sulcus.

Some work supports the use of combinedreversible and irreversible hydrocolloidimpression systems.3,4 These systems are usedin a way similar to the putty-wash techniquefor silicone rubbers described later in this arti-cle, with the agar injected around the prepara-tion to capture surface detail and the more vis-cous alginate in the impression tray. Theadvantages of this combination system com-pared with agar or alginate used individually isthe minimisation of equipment required torecord an agar impression (no water cooledtray is needed) and the fact that agar is morecompatible with gypsum model materials thanalginate. It is also relatively cheap in compari-son to many synthetic elastomers. Lin et al.5

demonstrated that the accuracy of this combi-nation system is better than either thereversible or irreversible materials used sepa-rately and is comparable to that of polysul-phide impression materials. However, theproblems of low tear resistance and poordimensional stability still apply resulting inthe need for impressions to be cast up immedi-ately. For these reasons, most practitionerstend to reject the hydrocolloids in favour ofthe synthetic elastomers to produce accurateand stable impressions.

The synthetic elastomersFirst introduced in the late 1950s, syntheticelastomeric impression materials quicklybecame popular as dental materials becausethey significantly reduced the two main prob-

lems associated with the hydrocolloids, namelypoor dimensional stability and inadequate tearresistance.

PolysulphidesThe polysulphide impression materials have thelongest history of use in dentistry of all the elas-tomers. Interestingly, they were first developedas an industrial sealant for gaps between sec-tional concrete structures.6 They are available ina range of viscosities namely, light bodied (lowviscosity), medium or regular bodied and heavybodied (high viscosity).

These are now relatively unpopular materials.The setting reaction of polysulphides tends tobe long with setting times often in excess of10 minutes (acceleration is possible by addinga small drop of water to the mix). They arealso messy to handle and have an objection-able odour.

Dies resulting from polysulphide impressionsare generally wider and shorter than the toothpreparation. This distortion, which worsens thelonger the delay in pouring up, is the result ofimpression shrinkage which is directed towardsthe impression tray — hence the wider die.Shrinkage occurs firstly as a result of a contin-ued setting reaction after the apparent settingtime, and secondly through the evaporation ofwater produced as a by-product of the settingreaction. A special tray, providing a 4 mm uni-form space, is needed to reduce distortion fromthe shrinkage of a large bulk of material. Therecommended maximum storage time of the setimpression is about 48 hours.6

A significant advantage of polysulphide,however, is its long working time. This is espe-cially useful when an impression of multiplepreparations is required and some dental schoolsstock a few tubes to help students deal with thisdifficult situation. Another advantage of thesematerials is that they possess excellent tearresistance, undergoing considerable tensilestrain before tearing. Unfortunately, their elasticproperties are not ideal and some of this strainmay not be recovered (high value for stressrelaxation 2 minutes after setting time — seeTable 1). To optimise the recovery of these vis-coelastic materials, the impression should beremoved with a single, swift pull as the strainimparted on the material is a function of thetime for which the load is applied. This methodof removal of impressions should be adoptedwhen using any impression material, irrespec-tive of its elastic properties.

PolyethersA popular polyether impression material,Impregum (Espe GmbH, Germany), was the firstelastomer to be developed specifically for use indentistry and introduced in the late 1970s. Ini-tially available only in a single ‘regular’ viscosi-ty, slight modification of the viscosity is possiblewith the use of a diluent. More recently a heavylight bodied system has been intoduced (Perma-dyne, Espe GMbH, Germany).

Fig. 1 Classification of impression materials

IMPRESSION MATERIALS

Non-elastic materials Synthetic elastomers

Polysulphides

Polyether

Silicones

CondensationCondensation Addition

HydrocolloidsImpression plasterImpression compoundImpression waxes

ReversibleIrreversible

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Polyether impression materials tend to have afast setting time of less than 5 minutes and, forthis reason, have been popular for the recordingof single preparations in general practice. Incontrast to polysulphides, they undergo an addi-tion cured polymerisation reaction on settingwhich has no reaction by-product resulting in amaterial with very good dimensional stability.The set material may however swell and distortbecause of the absorption of water on storage inconditions of high humidity. Impressions shouldtherefore be stored dry. They should also not bestored in direct sunlight. Ideally, impressionsshould be poured within 48 hours of them beingrecorded.6 An advantage of their relativehydrophilicity is that polyether impressionmaterials are more forgiving of inadequatemoisture control than the hydrophobic polysul-phides and silicone rubbers.

Polyether impression materials have ade-quate tear resistance and very good elastic prop-erties. However they do have a high elastic mod-ulus and consequently are relatively rigid whenset, hence considerable force may be required toremove the impression from both the mouth andthe stone cast (Table 1, stress to give 10% com-pression). This may preclude their use in caseswhere severe undercuts are present.

Silicones Silicone impression materials are classifiedaccording to their method of polymerisation onsetting, viz. condensation curing (or Type I) sili-cones and addition curing (or Type II) silicones.

Silicone rubbers are available in a similarrange of viscosities to the polysulphides (ielight, medium and heavy). However, the range issupplemented by a fourth viscosity; a very highviscosity or ‘putty’ material. The high fillerloading of the putty was initially devised toreduce the effects of polymerisation shrinkage.The putty is commonly combined with a lowviscosity silicone when recording impressions, aprocedure known as the ‘putty-wash technique’which will be discussed in some detail later inthe article.

Condensation curing silicones were intro-duced to dentistry in the early 1960s. As with thepolysulphides, the setting reaction produces avolatile by-product, but with type I silicones it isethyl alcohol, not water. Loss of the by-productleads to measurable weight loss accompanied byshrinkage of the impression material on storage.

The dimensional changes of condensation sili-cones are slightly greater than those of polysul-phides, but the changes in both types of materialare small in comparison to the changes whichoccur with alginate. Nevertheless, to produce themost accurate models, regular and heavy bodyimpressions should be cast within 6 hours ofbeing recorded.6 This may be a problem if thelaboratory is not close to the practice.

In contrast, addition cured silicone rubbersare considered the most dimensionally stableimpression materials. Like polyethers, they set,not unexpectedly, by an addition cured poly-merisation reaction. No by-product is producedduring cross-linkage resulting in an extremelystable impression which has been shown toremain unchanged over a substantial period oftime, hence allowing impressions to be poured atleisure some days after they were recorded.

As with polysulphides, silicone rubbers arevery hydrophobic so unless the teeth are proper-ly dried ‘blowholes’ are likely to be produced inthe set impression.

Both types of silicone rubber have the bestelastic properties of any impression material, therecovery of strain being said to be almost instan-taneous (Table 1, stress relaxation at 2 minutesafter setting time). Like the other elastomers,they have adequate tear resistance. They arenon-toxic and absolutely neutral in both colourand taste.

A great deal of recent research has been cen-tred around the production of hydrophilic sili-cone rubbers. Some commercial addition curedproducts have recently been introduced (egTake 1 Kerr US, Misssouri USA). A study byPratten and Craig7 showed one of these‘hydrophilic’ addition silicone materials to havea wettability similar to that of polyethers. Otherstudies have also shown that treatment ofimpression materials with topical agents,including surfactants, results in a decrease inthe number of voids found in the final impres-sion and the dies poured from them.8—11

THE PROBLEMS OF PUTTY-WASHThe putty-wash technique is probably that mostcommonly used in general dental practice. Aswith most techniques it has its problems, themost common of which is invisible when theimpression is recorded only becoming apparentwhen the restoration is tried in and fails to seatsatisfactorily.

Table 1 Properties of elastomeric impression materials Property Polysulphides Condensation Addition Polyethers

silicones silicones

Coefficient of thermal expansion (10–6 oC–1) 270 190 190 300

Volumetric polymerisation concentration (%) 0.3 1.0 0.4 0.4

Weight Loss at 24h (%) 0.15 0.9 0.05 0.02

Stress to give 10% compression (MPa) 70 110 160 400

Stress relaxation (%) at 2 mins after setting time 45 10 5 11

Tear strength (MPa) 0.5 1.6 2.8 1.5

Elongation at break (%) 500 350 150 300

PRACTICE


As has been mentioned already, putties weredeveloped initially to reduce the shrinkage ofcondensation silicones, but the heavy filler load-ing is not needed for addition silicones sincetheir polymerisation contraction and dimen-sional stability are in any case excellent. Pre-sumably, addition silicone putty-wash impres-sions are preferred principally for their handlingcharacteristics.

There are essentially three ways of recordinga putty-wash impression:

• One stage impression — putty and wash arerecorded simultaneously (also called twin mixor laminate technique)

• Two stage unspaced — putty is recorded firstand after setting relined with a thin layer ofwash

• Two stage spaced — as for two stage unspacedexcept a space is created for the wash. Thisspace may be made by: • Polythene spacer over the teeth prior to

making the putty impression• Recording the putty impression before tooth

preparation• Gouging away the putty and providing

escape channels for the wash.

The problem that causes invisible, but some-times gross distortions, is recoil. Recoil canresult in poorly fitting restorations and makes amockery of using what should be accurate mate-rials. Recoil works in the following way. Consid-erable forces are needed to seat putty impres-sions, which can result either in outward flexionof the tray wall or the incorporation of residualstresses within the material. On removing thetray from the mouth the tray walls reboundresulting in dies, which are undersized bucco-lingually.12 This has been demonstrated clearlywith plastic stock trays used with the one stagetechnique.

Although putties of lower viscosity are avail-able they produce similar distortions with plastictrays.13–15 Rigid metal trays however can min-imise such distortions and are to be recommend-ed for putty-wash impressions.

The two stage technique is not immune todistortion which may occur as follows:

1. Where it is used unspaced hydrostatic pres-sures can be generated during the seating ofthe wash impression, which can cause defor-mation and subsequent putty recoil2 onremoval. This problem can occur even withrigid trays. It may be reduced but not neces-sarily eliminated by spacing.

2. The putty impression may not be reseatedproperly causing a stepped occlusal surfaceof the cast and a restoration requiring exces-sive occlusal adjustment. It is often difficultto reseat an impression where the materialhas engaged undercuts especially interproxi-mally. As such, unspaced or just locallyrelieved impressions are most at risk.

In summary, the most convenient and reli-able way of recording a putty-wash impression

is to use the one stage technique with additionsilicone putty in a rigid metal tray. There is nodoubt that plastic stock trays are convenientbut whilst unreliable with putty-wash they canproduce accurate results with a combination ofheavy and light bodied addition silicones.12–14

Special trays are only needed for heavy lightbodied addition silicone impressions wherestock trays are a poor fit.

DISINFECTION OF IMPRESSION MATERIALSIt has long been recognised that a potentialexists for cross-infection as a result of contami-nated dental impressions.16–18 Consequentlysuch impressions pose a hazard to laboratorypersonnel; it is therefore important that allimpressions are disinfected prior to being trans-ferred to a laboratory.

A study by Blair and Wassell (1996)19 con-sidered a number of solutions used for disin-fecting impression materials. It highlighted thatthere is no universally recognised impressiondisinfection protocol available but showed thatthe use of a disinfectant of some description, atleast in dental hospitals, had increased from1988.20 The recommendations of the study aresupported by the British Dental Association;21

namely that all impressions should at leastundergo a disinfecting procedure by immersionin 1% sodium hypochlorite for a minimum often minutes.

PROBLEM SOLVING At Newcastle senior members of staff check allimpressions for indirect restorations on removalfrom the mouth and again in the laboratory. It issurprising how often an impression, appearingsatisfactory to a cursory glance, is fatally flawedwhen viewed alongside the resulting cast. Wewould encourage dentists to audit their ownwork in this way with peer review providing thebest stimulus for improvement.

Visible flaws related to impression techniquewhich occur commonly include:

• Finish line not visible• Air bubbles in critical places• Voids or drags• Unset impression material on surface of

impression and cast

Invisible impression flaws, resulting in anapparently good fit of the restoration on the diebut a poor fit on the tooth, may also occurbecause of:

• Tray and impression recoil (as described forthe putty-wash technique)

• Detachment of impression from tray• Permanent deformation

Where multiple preparations are recorded thelikelihood of an impression defect occurring isincreased and it is useful to have strategies tocope with this problem.

Whilst we cannot cover every eventuality wehope that the advice given below will help inreducing problems. Specific techniques have

PRACTICE


been summarised in the tables. For full accountsof potentially damaging — but useful tech-niques, such as electrosurgery and rotary curet-tage, the reader is referred elsewhere.22

Visible flawsFinish line not visibleIf the technician cannot identify the finish lineon the impression, the resulting crown willinevitably have a poor fit with a compromisedprognosis. It is therefore of some concern thatrecent studies report impression defects at thefinish line in over a third of cases.23,24 Thesedefects are usually the result of inadequate gin-gival management in the following circum-stances:

Gingival inflammation and bleeding. Everyeffort should be made to ensure that toothpreparations are being carried out in a healthymouth which means patients should have effec-tive periodontal treatment prior to recordingimpressions for definitive restorations. Bleedingfrom inflamed gingivae will displace impressionmaterial resulting in an inaccurate cast. Further-more, if inflammation has not been controlledand a sub gingival margin placed, there is a riskof gingival recession leaving the margin as anunsightly tide line. Where the potential for asuccessful outcome is low, it is often sensible todelay taking the impression until the gingivalcondition is resolved.

Certainly there are times when contours andledges on pre-existing defective restorationsmake it impossible for the patient alone toresolve the inflammation. Prior to recording theimpression the defective part, or more usuallythe whole restoration, should be removed and awell contoured provisional restoration placed. Inorder to obtain a satisfactory margin on the pro-visional restoration some localised electro-

surgery and gingival retraction may be required(see later). Leading up to the removal of thedefective restoration and during the time of tem-porisation, it may be helpful to prescribe anantimicrobial rinse (eg Chlorhexidine gluconate0.12%) for 2 weeks.

Subgingival finish line. The more subgingi-val a preparation the more difficult it is torecord the finish line adequately. Preparationsfinished at the gingival margin can occasional-ly be recorded without gingival retraction, butretraction cord will often give a more pre-dictable result. Preparations finished within thegingival sulcus will certainly require gingivalretraction. Any one or a combination of meanscan achieve retraction:

• Retraction cord (plain or impregnated) with orwithout accompanying solution

• Two-cord technique (described in Fig. 2)• Rotary curettage• Electrosurgery• Copper ring

The techniques (summarised in Table 2)become potentially more invasive towards thebottom of the list but may be essential to man-age more difficult cases. We find the most con-sistently helpful approach for subgingivalimpressions to be the ‘two-cord’ technique25

used with ferric sulphate solution (Table 3)where necessary in combination with electro-surgery (Fig. 3). The principal advantage of thetechnique is that the first cord remains in placewithin the sulcus thus reducing the tendency ofthe gingival cuff to recoil and displace partiallyset impression material. This approach not onlyhelps to control gingival haemorrhage and exu-date but also overcomes the problem of the sul-cus impression tearing because of inadequatebulk — an especially important consideration

Fig. 2 Two cord technique: (a) A piece of fine retractioncord is placed in the gingival sulcus; (b) A thicker cordis placed over the first leaving a tag for removal; (c) The thicker cord is removed after washing (noteclearly defined sulcus); and (d) The resulting impressionof the lower first premolar

a

c d

b

PRACTICE


with the hydrocolloids, which have low tearstrength. The main disadvantage of the two-cordtechnique is failing to remove the first cordthereby inflicting a painful, florid gingival reac-tion. It is worth pointing out that where ferricsulphate solution is used, it must be appliedfirmly in order to stabilise the coagulum. Thisapproach differs to the gentle technique usedwith other solutions.

Preparations that extend beyond the epithe-lial attachment may have finish lines adjacent tothe alveolar bone. In such cases the above tech-niques are unsuitable and surgical crownlengthening with osseous recontouring may beindicated to ensure that the gingival attachmenthas an adequate biological width (ie 2–4 mm ofgingival tissue above the alveolar crest).26

Brägger et al.27 have shown that whilst in mostpatients the gingival margin is stable followingcrown lengthening, in 12% of sites 2–4 mm ofrecession occurred between 6 weeks and 6months. These findings emphasise the need, inaesthetically critical areas, to delay recordingthe impression until the stability of the gingivalmargin is assured.

Localised gingival overgrowth. This annoyingproblem is often seen when replacing crownswith open margins where an ingrowth ofinflamed gingiva prevents access to the finishline. It also occurs after a patient loses a crownwith subgingival margins or a poor quality tem-porary crown is cemented prior to the impressionstage. In these circumstances packing with retrac-tion cord can be onerous and may be useless. A

Table 2 Techniques to capture subgingival finish lines

Description Indications Comments Hazards

Retraction Standard method of retraction Gingival or subgingival Single cord often results in inadequate Trauma and recession fromcord using twisted or knitted cord finish lines. gingival retraction. Two cord technique excessive packing pressure.

where first small diameter cord is left in Cord contaminated by gloves place during impression recording may prevent impression of improves definition. Wetting the cord gingival sulcus from settingjust before removal helps control Florid inflammation if first haemorrage even when solutions used cord not removed.(see below). Occasionally the first cord provides sufficient retraction and the second cord is not needed. Obviously, no cord tag should protrude from the sulcus whilst the impression is recorded.

Chemical Used to soak retraction cord prior Best used routinely with retraction Alum and epinephrine similarly Concerns over 'epinephrinesolutions to insertion and may be applied cords — plain cords result in haemostatic,39 retractive40 and both syndrome' (raised heart rate,

topically to stop gingival bleeding. bleeding on removal in >50% give minimal postoperative respiratory rate and bloodSolutions include: cases.38 Impregnated cords inflammation.41 Clinically, ferric sulphate pressure) when epinephrineEpinephrine (1:1000 conc.) twice as effective if first appears better haemostatic agent but solution used on laceratedAlum (eg Aluminium soaked in solution.39 needs to be rubbed firmly onto bleeding gums in susceptible patients.42

potassium sulphate) With ferric sulphate the initially gingival sulcus. Solutions need to be Concentrated solutions ofFerric Sulphate (15.5%) soaked cord can be removed from washed off before impression recorded. Alum can cause severe

the sulcus and further solution inflammation and tissueapplied with a special applicator necrosis.43 Solutions willto help stabilise the coagulum. concentrate if top left off bottle.

Ferric sulphate can stain the gums yellow-brown for a few days.

Electrosurgery Controlled tissue destruction by Uses: Current types: Contra-indicated in patientsrapid heating from radio 1. Widen gingival sulcus Troughing- 'cut/coag' setting (fully with cardiac pacemakers.22

frequency (>1.0 MHz) electrical (troughing) before cord placed. rectified, filtered) Modern pacemakers arecurrent passing from wire tip NB Avoid using on thin relatively well shielded44 but(high current density) through gingiva as unwanted recession still good practice to evacuatepatient's body into large area can result. pacemaker patients from collecting electrode (low 2. Gingivectomy for overgrown Gingevectomy— 'cut' setting adjacent areas.To avoid current density). tissue or to crown lengthening (fully rectified) unwanted arcing and tissue

3. Coagulation (ball electrode) Coagulation– 'coag' setting burns use plastic mirrorsbut produces most tissue (unrectified, damped) and check integrity of tipdestruction and slow healing. insulation. Similarly, do not

touch against metal restorations. Keep collecting electrode away from rings and buckles etc.

Rotary Use of chamfered diamond For subgingival preparations in Palatal tissues respond better than A slight deepening of the curettage bur to remove epithelial healthy gingivae. Gingival sulcus thinner buccal tissues..47 Not suitable sulcus may result.47 Poor (Gingettage) tissue within healthy sulcus to depth must not exceed 3 mm and technique if a periodontal probe in tactile sensation during

expose subgingival finish there should be adequate the sulcus can be seen through the instrumentation gives highline during its preparation.45 keratinised gingivae.46 gingiva. potential for overextension

and damage.

Copper A closely adapted copper ring Often used in an attempt to Dies from composition impressions Attempts to locate a die from a ring is used as a vehicle to carry retrieve the situation where are either electro formed or made in copper ring impression into the

impression material subgingivally. multiple preparations are amalgam. Elastomeric copper ring relevant recess in a full arch A rigid impression material recorded in an elastomeric impressions can also be cast in elastomeric impression are (composition) or an elastomer impression and a localised stone. rarely accurate and bestmay be used. impression defect has occurred avoided for bridgework.

PRACTICE


better approach is to remove the overgrown tissuewith electrosurgery and then use a fine, straight-wire electrode to create a sulcular trough intowhich retraction cord may be placed (Fig. 3).

Retraction cord displaced from sulcus. In thiscircumstance the gingival tissues conspire toeject the cord from the sulcus almost immediate-ly after placement. This frustration may occurwhere healthy gingivae are tightly bound to thetooth or where there are adjacent tooth prepara-tions. In the latter situation placement of cordinto one proximal sulcus compresses the papillaand displaces the cord already packed into theadjacent sulcus. Again, electrosurgery to createa trough into which the cord may be packed isthe easiest way to deal with the problem. It isinappropriate to use electrosurgery where thegingival tissues are thin, eg buccal aspects oflower incisors, as unwanted gingival recessionmay result. Where the tissue is thin a smalldiameter retraction cord should be packed care-fully using a sustained, controlled force. A halfHollenback amalgam carver or proprietary cordpacking instrument is useful for the purpose. Asecond instrument can be used to help retaincord already packed.

Air bubbles in critical placesAir bubbles in impressions form either as a resultof mixing, tray loading, syringing or tray seat-ing. Compared with spatulation, syringe mixsystems significantly reduce incorporated bub-bles but are not foolproof.28 Prior to placing themixing nozzle, a small amount of materialshould be extruded from the cartridge to ensureno blockage present. A partial blockage willmake extrusion difficult and detrimentally alterthe base-catalyst ratio. A complete blockage cancause the cartridge to rupture. Blockages canusually be cleared with a Briault probe.

Syringing impression material around apreparation requires a certain amount of skill.

Air can easily be trapped at the gingival sulcusas the syringe tip circumnavigates the tooth anda good tip is to keep the syringe tip in theexpressed material during syringing. Anotherpiece of good advice where access for the tip isrestricted is to start syringing from the most dif-ficult area — usually the disto-lingual. Firstexpress the material into the distal interproximalsulcus so that it extrudes through to the buccal.Continue along the lingual sulcus and thenexpress material into the mesial interproximalsulcus again extruding through to the buccal.Next place the syringe tip into the disto buccalextrusion and syringe along buccally finishingwith the tip in the mesial extrusion. Finallysyringe up onto the occlusal surface and thenuse a three in one syringe to blow the light bodyevenly over the preparation.

Voids and dragsTo obtain a void free impression it is necessaryfor the material to wet the teeth and soft tissues.In addition, the tray must effectively constrain

Table 3 Gingival retraction using ferric sulphate solution

• Ensure adequate isolation and moisture control — a flanged salivary ejector is needed for impressions

of lower posterior teeth.

• Consider need for electrosurgery (either troughing or gingivectomy or both), one or two cord

technique. If gingival inflammation needs to be resolved, temporise with well fitting margins

• Soak cord in ferric sulphate solution (15.5% w/v) and pack

• Apply further solution using syringe applicator or pledget of cotton wool (beware — solution

tastes foul)

• After 5 minutes wash cord well and remove carefully so that lining of sulcus is not stripped out

• Continue to wash preparation with atomised spray and dry well, especially the more inaccessible parts

of the preparation. The inner aspect of the sulcus will often appear black with stabilised coagulum.

Remove any coagulum adhering to tooth preparation or finish line

• Only start mixing the impression if the gingivae are adequately retracted and dry

• If bleeding starts, reapply ferric sulphate solution and repack with soaked cord for a further 5 to 10

minutes before reattempting impression

Fig. 3 ‘Troughing’ with electrosurgeryprior to packing retraction cord (a, b).The buccal tissues are relatively thinand great care is needed to avoidrecession. Where subgingival finishlines need to be uncovered (c,d)electrosurgery is invaluable. Sufficienthaemostasis can usually be achievedwith ferric sulphate solution to allowthe impression to be recorded

a b

c d

PRACTICE


the material to prevent it from flowing awayfrom critical areas thus inducing impressiondrags that are commonly seen on the distalaspects of teeth adjacent to edentulous spacesand in undercut regions. Preparations andocclusal surfaces must be adequately dried witha three in one syringe or the relativelyhydrophobic elastomers will be repelled and,much like a skidding lorry on a wet motorway,aquaplane away from the tooth. We have foundthe new generation of hydrophilic addition sili-cones (eg Take 1, Kerr US, Misssouri USA) tooffer much improved performance in overcom-ing these effects.

All experienced dentists will recognise thescenario of repeating an impression only to findthat the offending void or drag has reappearedin the same place. The cause is often a poorlyadapted tray and the answer is to either adaptthe stock tray with a rigid material (eg com-pound) to give more consistent spacing in thecritical area or have a special tray made up. Spe-cial trays are best avoided for putty-washimpressions since there is a significant risk of therigid, set impression locking into undercuts andthen having to be cut free from the patient’smouth.

Gingival control has already been consideredbut it is worth re-emphasising that crevicularfluid and haemorrhage will displace impressionmaterial and result in voids and rounded, indis-tinct finish lines.

Yet another cause of voids is prematuresyringing of impression material intra-orallyprior to seating the tray. The set of the syringedmaterial is accelerated by the warmth of themouth, resulting in a poor bond betweensyringed and tray materials and the appearanceof a fissure at the interface between them. Thistype of void may be exacerbated by salivarycontamination of the syringed material. Theskilful use of cotton wool rolls, flanged salivaryejector and high volume aspiration is critical toeffective moisture control.

Unset impression materialThis problem usually does not become appar-ent until the impression is cast-up and a tell-tale smear of unset impression material is seenon the surface of the die and the surroundingteeth; the affected stone cast often has a char-acteristic granular appearance. Alternatively,the putty in a putty-wash impression mayrefuse to set. The most likely cause of boththese problems is contamination of the impres-sion by ingredients of latex rubber gloves,which poison the choroplatinic acid catalyst ofaddition silicones.29,30 Not all brands of latexgloves are responsible31 and the simple expe-dients are to change brands or to use non-latexgloves (eg polyethylene) for impression proce-dures. Where the string variety of retractioncord is used, twisting it tight. It in gloved handsto make it more easily packable also has thepotential to contaminate and prevent impres-sion setting.32 This is less of a problem with

knitted or woven cords, which should not betwisted prior to insertion. Perhaps surprisingly,retraction solutions have not been shown toeffect impression setting significantly.32

Invisible flawsImpression and tray recoilA visible impression flaw may be made invisibleby attempting a localised reline with a little lightbodied material. It may be tempting but is notgood practice; seating pressures can result inimpression recoil and significant distortion.33

Moreover, the addition may bond poorly andsubsequently peel away. If an impression isunsatisfactory it should be retaken.

As already discussed the use of putty-wash innon-rigid trays can result in tray wall recoil andundersized dies.

Detachment of impression from trayDetachment of the impression from the tray canresult in gross distortion of the cast. It may occuron removal from the mouth and may often gounnoticed. Prevention of detachment relies onthe proper use of adhesive and having a traywith adequate perforations.34 It is a good idea toselect the tray and apply adhesive before thetooth is prepared. Doing so will allow time forthe adhesive’s solvent to evaporate and for ade-quate bond strength to develop.35 Painting thetray immediately before recording the impres-sion is not a good idea. This advice applies toelastomers and alginates. Alginates are moreeasily debonded from the tray so it is good prac-tice to use a scalpel to cut away excess alginatefrom the tray heels to facilitate inspection of thisvulnerable area. The excess needs to be removedbefore putting the impression down or theimpression will distort.

Elastomeric impressions may require to bepoured up more than once, especially if criticalair blows in the stone affect the resulting die.The repour will be grossly inaccurate if impres-sion material has lifted away from the traybecause of the lack of adhesive.36

Where a special tray is made it is importantthat the wax spacer does not come into contactwith the tray acrylic; contamination willreduce the strength of the adhesive bond. Tech-nicians may need to be instructed to place alayer of aluminium foil over the surface of thewax before forming the tray.35 Furthermore, aself cured acrylic tray should be made at least aday in advance to allow for its polymerisationcontraction.

Permanent deformationWithdrawal from an undercut will test animpression’s elastic recovery. As already men-tioned the addition silicones have good resist-ance to permanent deformation, however, thereare situations where an impression can bedeformed and the small but significant defor-mation is unlikely to be detected. In this respectgingival embrasure spaces cause especial diffi-culty in two situations. Firstly, significant gin-

PRACTICE


gival recession with the loss of the interproxi-mal papilla will lock set impression materialinto the space. The impression will either betorn on removal from the mouth or deformed orboth. This problem is best dealt with by block-ing out embrasure spaces with soft red wax or aproprietary blocking out material. Secondly,where there is a significant triangular inter-proximal space below the preparation finishline it is best to extend the finish line gingival-ly. The space is thereby opened up to allow theimpression to be withdrawn without tearing ordistortion.

Special trays should be given sufficient spac-ing (at least two layers of baseplate wax) to givesufficient thickness of impression material toresist undue stress and strain on removal fromundercut areas.

Finally, the elastic properties of materials arenot fully developed at manufacturers’ stated set-ting times. So it is worth remembering that a sig-nificant improvement in resistance to perma-nent deformation occurs if addition siliconeimpressions are left a further minute or twobefore removal from the mouth.13

The problem of recording multiple preparationsIt is always distressing when in an attempt torecord multiple preparations one or two areas ofthe impression have a critical defect. There areseveral strategies for dealing with the situation:

• Retake the whole impression• Record a separate impression of the prepara-

tion (or preparations) having the impressiondefect. A copper ring may be used as describedpreviously in Table 2. The resulting die is thenlocated in the defective region of the firstimpression before pouring up the master die.Not surprisingly, it can be difficult to locatethe die reliably and there is a risk of causingocclusal or proximal contact discrepancies.

• Retake sufficient impressions to ensure thatthere is an adequate impression available ofeach preparation. The patient is re-appointedfor a transfer coping pick-up impression.37 Inthe interim individual dies are made by copperor silver plating (Table 4). On each die isformed an accurately fitting acrylic (Duralay,Reliance Dental Co, Illinois, USA) transfercoping. At the next appointment the transfercopings are tried onto the preparations andthe fit checked. Copings having the same pathof insertion are linked together with wire and

acrylic so that stability of coping position isensured within the pick-up impression. Alter-natively, excrescences of acrylic can be addedto a coping to ensure it is retained within thepick-up impression. After recording the pick-up impression individual dies can be securedwithin their copings using sticky wax beforethe master cast is poured. The technique canbe used with stone dies, but there is a risk ofthe die being abraded by the construction ofthe acrylic coping. This problem can be over-come by double pouring each die. The copingis made on one die, which is then discarded,and the other die is used for the master cast.

Where it is clearly going to be a problem torecord many preparations on a single impressionthis should be taken into account and plannedfor. There are few cases that cannot be brokendown into smaller more manageable stages evenif this means using provisional restorations tostabilise the occlusion while say four or sixdefinitive anterior crowns are constructed.When it is absolutely necessary to record simul-taneously more than six teeth in one arch it iswise to use the transfer coping and pick-upimpression technique from the outset.

CONCLUSIONThe ability to record consistently good impres-sions is both a science and an art. We hope thisarticle has shed light on both aspects. It is worthbearing in mind that the impression influencesnot only the quality of the subsequent restora-tion but also the technician’s perception of thedentist’s skill. As none of us can achieve perfec-tion every time there is much to be said forencouraging technicians to feed back when theyreceive a substandard impression.

1. Sears A W. Hydrocolloid impression technique for inlays andfixed bridges. Dent Digest 1937; 43: 230-234.

2. Brown D. An update on elastomeric impression materials. Br Dent J 1981; 150: 35-40.

3. Heisler W H, Tjan A H L. Accuracy and bond strength ofreversible with irreversible hydrocolloid impression systems:a comparative study. J Prosthet Dent 1992; 68: 578-584.

4. Appleby D C, Parneijer C H, Boffa J. The combined reversiblehydrocolloid/ irreversible hydrocolloid impression system. J Prosthet Dent 1980; 44: 27-35.

5. Lin C, Zeiber G J. Accuracy of impression materials forcomplete arch fixed partial dentures. J Prosthet Dent 1988;59: 288-291.

6. Craig R G. Restorative Dental Materials. 10th ed.pp.281-332.London: Mosby, 1997.

7. Pratten D H, Craig R G. Wettability of a hydrophilic additionsilicone impression material. J Prosthet Dent 1989; 61: 197-202.

8. Vassilakos N, Fernandes C P, Nilner K. Effect of plasmatreatment on the wettability of elastomeric impressionmaterials. J Prosthet Dent 1993; 70: 165-171.

9. Millar B J, Dunne S M, Robinson P B. The effect of a surfacewetting agent on void formation in impressions. J ProsthetDent 1997; 77: 54-56.

10. Ozden N, Ayhan H, Erkut S, Can G, Piskin E. Coating ofsilicone-based impression materials in a glow-dischargesystem by acrylic acid plasma. Dent Mat 1997; 13: 174-178.

11. Boening K W, Walter M H, Schuette U. Clinical significance ofsurface activation of silicone impression materials. J Dent1998; 26: 447-452.

12. Wassell R W, Ibbetson R J. The accuracy of polyvinylsiloxaneimpressions made with standard and reinforced stock trays. J Prosthet Dent 1991; 65: 748-757.

Table 4 Copper and silver-plated dies; whichimpression materials can be electroplated

Copper plate Silver plate

Addition silicone Yes Yes

Condensation silicone No Some

Polyether No Yes

Polysulphide No Yes

Impression compound Yes No

PRACTICE


13. Abuasi H. Accuracy of polyvinyl siloxane impressions. [PhD].University of Newcastle upon Tyne, 1993.

14. Abuasi H A, Wassell R W. Comparison of a range of additionsilicone putty-wash impression materials used in the one-stage technique. Eur J Prosthodont Restor Dent 1994; 65:748-757.

15. Carrotte P V, Johnson A, Winstanley R B. The influence of theimpression tray on the accuracy of impressions for crownand bridgework. Br Dent J 1998; 185: 580-585.

16. Ray K C, Fuller M L. Isolation of Mycobacterium from dentalimpression material. J Prosthet Dent 1963; 13: 390-396.

17. Leung R L, Schonfeld S E. Gypsum casts as a potential sourceof microbial cross-contamination. J Prosthet Dent 1983; 49:210-211.

18. Powell G L, Runnells R D, Saxon B A, Whisenant B K. Thepresence and identification of organisms transmitted todental laboratories. J Prosthet Dent 1990; 64: 235-237.

19. Blair F M, Wassell R W. A survey of the methods ofdisinfection of dental impressions used in dental hopitals inthe United Kingdom. Br Dent J 1996; 180: 369-375.

20. Watkinson A C. Disinfection of impressions in UK dentalschools. Br Dent J 1988; 164: 22-23.

21. BDA. Advice sheet A12: Infection Control in Dentistry. pp12:British Dental Association Advisory Service, 1996.

22. Shillingburg H T, Hobo S, Whitsett L D, Jacobi R, Brackett S E.Fundamentals of fixed prosthodontics. 3rd ed. pp257-279.Chicago: Quintessence, 1997.

23. Carrotte P V, Winstanley R V, Green J A. A study of the qualityof impressions for anterior crowns received at a commerciallaboratory. Br Dent J 1993; 174: 235-240.

24. Winstanley R B, Carrotte P V, Johnson A. The quality ofimpressions for crowns and bridges received at commercialdental laboratories. Br Dent J 1997; 183: 209-213.

25. Cloyd S, Puri S. Using the double cord packing technique oftissue retraction for making crown impressions. Dent Today1999; 18: 54-59.

26. Ingber F J S, Rose L F, Coslet J G. The ‘Biologic Width’ - Aconcept in periodontics and restorative dentistry. Alpha-Omegan 1977; 10: 62-65.

27. Brägger U, Lauchenauer D, Lang NP. Surgical lengthening ofthe clinical crown. J Clin Periodont 1992; 19: 58-63.

28. Chong Y H, Soh G, Lim K C, Teo C S. Porosities in fiveautomixed addition silicone elastomers. Operative Dent1991; 16: 96-100.

29. Noonan J E, Goldfogel M H, Lambert R L. Inhibited set of thesurface of addition silicones in contact with rubber dam.Operative Dent 1985; 10: 46-48.

30. Kahn R, Donovan T, Chee W. Interaction of latex gloves andpolyvinylsiloxane impression materials: a screening survey.Int J Prosthodont 1989; 2: 342-346.

31. Baumann M A. The influence of dental gloves on the settingof impression materials. Br Dent J 1995; 179: 130-135.

32. de Camargo L M, Chee W W L, Donovan T E. Inhibition ofpolymerization of polyvinyl siloxanes by medicaments used ongingival retraction cords. J Prosthet Dent 1993; 70: 114-117.

33. Bomberg T A, Hatch R A. Correction of defective impressionby selective addition of impression material. J Prosthet Dent1984; 52: 38-40.

34. MacSween R. Peel bond strengths of five impression materialtray adhesives. J Can Dent Assoc 1991; 57: 654-657.

35. Davis G B, Moser J B, Brinsden G I. The bonding properties ofelastomer tray adhesives. J Prosthet Dent 1976; 36: 278-285.

36. Tjan A H. Comparing effects of tray treatment on theaccuracy of dies. J Prosthet Dent 1987; 58: 175-178.

37. Howat A P, Capp N J, Barrett N V J. A colour atlas of occlusion& malocclusion. pp182. Aylesbury: Wolfe Publishing Ltd,1991.

38. Pelzner R B, Kempler D, Stark M M, Lum L B, Nicholson R J,Soelberg K B. Human blood pressure and pulse rate responseto r-epinephrine retraction cord. J Prosthet Dent 1978; 39:287-292.

39. Weir D J, Williams B H. Clinical effectiveness of mechanical-chemical tissue displacement methods. J Prosthet Dent 1984;51: 326-329.

40. Bowles W H, Tardy S J, Vahadi A. Evaluation of new gingivalretraction agents. J Dent Res 1991; 70: 1447-1449.

41. de Gennaro G G, Landesman H M, Calhoun J E, Martinoff J T.A comparison of gingival inflammation related to retractioncords. J Prosthet Dent 1982; 47: 384-386.

42. Donovan T E, Gandara B K, Nemetz H. Review and survey ofmedicaments used with gingival retraction cords. J ProsthetDent 1985; 53: 525-531.

43. Shaw D H, Krejci R F, Cohen D M. Retraction cords withaluminium chloride: effect on the gingiva. Operative Dent1980; 5: 138-141.

44. Riordan A T, Fosko S W. Electrosurgery and cardiac devices. JAm Acad Dermatol 1997; 37: 250-255.

45. Ingraham R, Sochat R. Rotary gingival curretage —A technique for tooth preparation and management of thegingival sulcus for impression taking. Int J PeriodontolRestorative Dent 1981; 1: 9-33.

46. Brady W F. Periodontal and restorative considerations inrotary curettage. J Am Dent Assoc 1982; 105: 231-236.

47. Kamansky F W, Tempel T R, Post A C. Gingival tissue responseto rotary gingival curretage. J Prosthet Dent 1984; 52: 380-383.

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BRITISH DENTAL JOURNAL VOLUME 193 NO. 1 JULY 13 2002 17

Crowns and other extra-coronal restorations:Try-in and cementation of crowns R. W. Wassell1 D. Barker2 and J. G. Steele3

Having successfully negotiated the planning, preparation, impression and prescription of your crown, the cementation stagerepresents the culmination of all your efforts. This stage is not difficult, but a successful outcome needs as much care as thepreceding stages. Once a restoration is cemented there is no scope for modification or repeat. You have to get it right first time.Decemented crowns often have thick layers of residual cement suggesting problems with either initial seating or cement handling.When the fate of restorations costing hundreds of pounds depends on correct proportioning of cements and the quality ofthe mix, the value of a well-trained and experienced dental nurse is easy to see. Both dentist and nurse need a workingknowledge of the materials they are handling.

1,3Senior Lecturer in Restorative Dentistry,2Higher Specialist Trainee, Department ofRestorative Dentistry, The Dental School,Newcastle upon Tyne NE2 4BW; *Correspondence to: Dr R. W. Wassell,Department of Restorative Dentistry, The Dental School, Newcastle upon Tyne NE2 4BWE-mail: [email protected]


The stages of fitting and cementation of crowns are considered including:● The clinical stages of try-in and adjustment of restorations prior to cementation● Conventional and adhesive cements● A rationale for the use of different cements based on their various physical and

chemical properties● Crown cementation with conventional cements

I N B R I E F

Crown seating relies on a satisfactory try-in andthis subject will be covered first. We will thenconsider the topics of cements, their selectionand usage.

TRY-IN PROCEDURE FOR CROWNSThis involves three stages: firstly pre-operativeevaluation of crown on its die, secondly seatingon the tooth and finally evaluation of the seatedcrown.

Checking the crown on the die It is always worth checking the fit of the crownon the cast before trying it in the patient. In thisway problems involving marginal fit, aestheticsand articulation can be anticipated prior to tryin. Always check the fit surface of the crown fordefects and the die for damage, preferably witha good light and under magnification (Table 1).Detecting these problems before try-in willallow you either to address the problem your-self or to negotiate with your laboratory from aposition of strength. It is surprising how oftenclear ledges or deficiencies can be detected atthis stage. Subsequent stages then rely on thecrown being checked in the mouth and thenoften back on the die again when there is aproblem with the fit.

Seating the crownHaving checked the crown on the cast, any tem-porary restoration is removed and the prepara-tion is carefully cleaned of all residues of tempo-rary cement, especially in retention grooves. Thetry-in procedure can normally (though notalways) be accomplished without the need forlocal anaesthetic. This is advantageous in that

the patient’s tactile sense is not impaired whichis valuable in assessing the occlusion and tight-ness of proximal contacts.

The crown should be tried in without forcingit onto its preparation; if it fails to seat there area range of reasons why this may have happened.It pays to use a systematic approach to localiseproblems:

1. First, ensure there is no retained temporarycement or trapped gingival tissue

2. Then check and adjust tight proximal contacts(see following section) as these often preventseating. Also check the original cast for dam-age to the stone in these contact areas

3. Then re-check the crown for the most obviouslaboratory errors, including casting blebs,damaged or chipped dies or grossly overex-tended margins. Casting blebs can be removedwith a bur. Over-extended margins should beadjusted from the axial surface, not fromunderneath (Soflex discs are useful for thispurpose — see Fig. 1). To avoid the abrasivedragging metal over the margin, run the discso that the abrasive travels in the direction ofthe occlusal surface, not towards the margin

4. Where the crown still does not seat burnishmarks on the internal walls of a sandblastedmetal crown may indicate where it is binding.The identification of these points may be aidedby the use of disclosing wax or aerosol sprays.Any imperfections may be lightly ground witha bur or stone before retrying the crown.

With modern day addition silicones, impres-sion distortion is an unusual cause of ill-fittingrestorations but may be responsible where noapparent fault can be detected on restoration or

11










restorations

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18 BRITISH DENTAL JOURNAL VOLUME 193 NO. 1 JULY 13 2002

die. Problems can occur where an impression isremoved too soon or where unset impressionmaterial results from glove contamination (lookout for the tell tale smear of unset impressionmaterial on the surface of the cast). If you cannotget the crown to seat easily and can find noobvious reason, you may rightly suspect thatthere is an impression distortion. In that case,save time and just take a new one.

Finally, it is important to distinguish betweena crown that rocks on its preparation because ofbinding somewhere on the fit surface and onethat just has a loose fit resulting from the use ofdie spacer. In fact, tightly fitting crowns conferno additional retention after cementation1 andmay interfere with seating.

Assessment of the seated crown There is no point in making a detailed assess-ment of proximal contacts, marginal fit, aesthet-ics and occlusion until the crown is seated fully.

Once you have got to this stage, providing youand the laboratory have taken care with preced-ing clinical and technical stages, minimal or noadjustment should be necessary. Again it isimportant to adopt a systematic approach.

Proximal contactsThe tightness of proximal contacts can be test-ed with dental floss and should offer someresistance but not make its passage too diffi-cult. If these are too tight they can be ground alittle at a time and polished. This requires thegreatest care as it is easy to open the contactaccidentally, and it is very problematic tryingto rebuild it at this stage. Prior to adjustment itmay be helpful to mark the proximal contactby sandwiching a small piece of articulatingpaper between crown and tooth either on thecast or in the mouth.

Open contact points occur less frequentlyand can only be modified by returning the

Table 1. Check list of laboratory errors affecting marginal fitError Cause Remedy

Tight proximal Imprecise die location or abrasion of Check for displacement of the diescontacts the adjacent stone contact points when the crown is seated on the

working cast. Identify tight contact byinterposing articulating paper, grindand polish

Casting blebs on Air bubbles trapped during investment Identify under magnification andfit surface remove with small round bur

Over-extended Poor impression, poor die trimming, Trim from axial surface (Fig. 1) andcrown margins surplus untrimmed wax or porcelain polish — consider returning crown to lab

Under-extended Poor impression, poor die trimming, If under-extension obvious andcrown margins difficulty identifying finish line impression satisfactory have crown

remade. Alternatively retakeimpression

Damaged dies Finish line chipped because of careless Always try and determine why the diehandling or abraded when casting reseated is damaged. If the crown does not fitwith blebs or overextended margins after adjusting blebs or over-extended

margins return it to laboratory

No die spacer Technician not aware of technique or Lack of spacer results in a tightly(Space needed to forgot to apply fitting crown which may not seataccommodate during try in and may ‘lift’ furthercement lute) after cementation

a) b) c)

Fig. 1a) Marginal discrepancy causedby an overextended lingual margin;b) Reduce the overextension fromthe axial surface not fromunderneath; and c) Seatingachieved with the axial bulkremoved, incorrect adjustmentresults in a deficient margin

PRACTICE


crown to the laboratory for addition of porce-lain or gold solder.

Assessment of marginal fitCrowns really must have an accurate marginalfit if you intend them to survive in the long term.There is a full spectrum of fit, from perfection(which is probably rarely achieved) to open mar-gins around the entire crown. It is up to you,your own clinical standards and perhaps yourpatient at what point on that spectrum youdecide that the marginal fit becomes unaccept-able. A combination of clinical experience andempirical data suggest a marginal opening of100 µm2 is at the borderline of acceptability,especially where the margin can only beprobed.3 Maintaining the highest possible clini-cal standards at this stage has long-term bene-fits; ill-fitting margins will render the toothmore susceptible to cement dissolution, plaqueretention and recurrent caries. Defective subgin-gival margins compromise gingival health by analteration in local bacteria.4

Poor fit can present as a gap or an overhang-ing margin (positive ledge) or deficient margin(negative ledge). Overextended margins andpositive ledges may be corrected by adjustingthe crown from its axial surface until it is pos-sible to pass a probe from tooth to crown with-out it catching. A larger problem, necessitatingthat the crown be remade if it is unacceptable,occurs when a margin remains deficient or hasa negative ledge.

Gold restorations should have their accessiblemargins burnished before cementation. The setcement is likely to be cracked if the margin isburnished afterwards. There is no evidence thatburnishing improves longevity but it can insurethat a discerning patient does not catch a finger-nail at the margin! The procedure involves drag-ging the gold from restoration to tooth using arotary instrument such as a green stone or steelfinishing bur. Where only minimal burnishing isneeded a sharp hand instrument such as a pro-prietary gold knife or half Hollenback instru-ment is useful. Once burnished the restorationmust be re-polished taking care to avoid the fineedge of marginal gold.

Aesthetics For porcelain crowns, some adjustment ofshape and shade is possible at this stage but itis best to ensure minimal adjustment by fol-lowing the advice given in Part 6 of the series.

Grinding with diamond burs can alter crowncontours and porcelain additions can be made tometal ceramic crowns if necessary. Shades thatare slightly too light may be darkened by theaddition of stain and re-firing while all ceramiccrowns with no cores may have their shademodified slightly by the luting composite in thesame way as veneers. If you anticipate the needfor substantial adjustments, a try-in at biscuitbake can be specified so that the crown can beglazed when you are happy with the finalappearance. Obviously the patient will need to

see the restoration and comment on it prior tocementation. If the shape and shade are clearlyunacceptable it is better to acknowledge thisrather than risk a dissatisfied patient with thepossibility of having to cut off the restorationand repeat all the preceding stages.

Assessment of the occlusionThe occlusion is the last assessment to make, butthere is no point thinking about making anyadjustment to the occlusal surface until thecrown is finally seated. It is then best to removeit and identify a pair of adjacent occluding teeth,termed index teeth, which, after re-seating, canbe used to assess visually, and with shim stock,how much adjustment is needed.

Visually checking the occlusion gives only agross indication of the amount of adjustmentneeded; articulating foils and shim stock areneeded too. Figure 2 shows shim stock, which isa 10 µm thick Mylar film, held in mosquito for-ceps and used as a feeler gauge between occlud-ing teeth. Shim stock can be bought from dentalsuppliers or can be made, very inexpensively, inthe dental surgery by passing a ‘space blanket’(the sort of thing marathon runners wrap aroundthemselves after a race, available at campingshops) through an office shredder and then cut-ting it into 2 cm lengths.

Miller’s forceps, used to support articulating

Fig. 2 Occlusal assessment. With therestoration out select a pair of indexteeth, which hold shim stock (a).Mark with black articulating foil and adjust ICP (b). Mark with redand adjust excursive interferences and deflections (c). Adjustmentcomplete (d)

a

b

c

d

PRACTICE


foils so that they do not crumple during intra-oral placement, are also an advantage (againlook at Fig. 2). Some articulating papers resem-ble blotting paper in consistency and thickness.They are prone to leave false marks and mayalter the patient’s position of closure. Thesepapers can be as thick as 200 µm, which is overten times as thick as the best thin foils such asGHM (GHM Occlusion Prüf Folie, Germany),5

which are infinitely preferable. Despite theirslightly higher cost, the accuracy and precisionwith which they will mark a restoration can savea great deal of time and effort provided the teethare dry.

With posterior teeth, both restoration andadjacent teeth should hold shim stock firmly inthe intercuspal position (ICP). With anteriors, ifthe other incisors hold shim stock lightly therestoration should be made to do so too. Failureto do so can result in the crowned tooth beingoverloaded, which in turn can cause pain,mobility, fracture or displacement. As well asusing shim stock and articulating foils, it is alsoworth listening to the occlusion with and with-out the crown in place as small occlusal discrep-ancies can readily be heard with the teeth beingtapped together.

Dentists will often have a favourite bur forocclusal adjustment. We prefer a large flameshaped diamond in an air rotor or speedincreasing handpiece. Occasionally, it may benecessary to adjust the tooth opposing arestoration to avoid crown perforation or expo-sure of rough opaque porcelain. Such adjust-ments should be planned with the patient’s con-sent and not sprung on them part way throughthe procedure. A thickness gauge (eg SvensenGauge) is invaluable for predicting areas vul-nerable to perforation.

Once ICP has been re-established the excur-sions can be checked, preferably with a differentcoloured foil (eg red). ICP contacts are then re-marked with the original colour (eg black) allow-ing the excursive contact to be differentiatedand refined. The decision about whether therestoration is to be involved in guiding jawmovement (which it often is with anterior teeth)or whether there should be disclusion (as oftenoccurs posteriorly) should have been made wellbefore this stage and it is important you have aclear idea of the pattern of occlusal contact youare trying to achieve. Finally, it is worth guidingback the mandible into the retruded path of clo-sure to ensure the restoration is not introducinga new deflective contact.

Occasionally, a restoration will be short ofocclusion. This is used as a deliberate ploy insome laboratories to eliminate the clinical needfor occlusal adjustment. A thin card spacerplaced over the cast of the opposing tooth cre-ates clearance. This may seem an innocuous,perhaps even a desirable practice, but can resultin serious immediate problems presenting wheremultiple restorations are fitted. The lack of con-tacts can affect occlusal stability whilstdestructive interferences may occur as teeth

erupt back into occlusion. It may often beacceptable to monitor infra-occluding restora-tions and adjust interferences as necessary. Inmore critical cases occlusal additions of porce-lain or solder may be indicated or the restora-tion remade. The point is that it should not benecessary to do this at all because all of theseproblems can be avoided by taking care at thepreceding stages, especially with impressions(including opposing impressions), jaw registra-tion and temporary restorations.

Finishing and polishingThe final stage prior to cementation is polishing.A rough surface, especially in porcelain, willrapidly wear the opposing tooth6 and so it isvery important to use a sequence of abrasivesdesigned for the material in question to achievea smooth surface. Details of the materials we useare shown at the end of the article. Metal sur-faces can be finished with finishing burs fol-lowed by rubber abrasive points (Kenda: blue,brown and green). Abrasive discs (eg Soflex) areuseful for flat areas such as proximal contactpoints and can be used on either metal or porce-lain. Porcelain can be also be finished with com-posite finishing diamonds (Premier: yellow andwhite stripe), but a light touch and water spray isneeded to avoid stripping off the diamond coat-ing. Further finishing is achieved with rubberabrasive points (Kenda: white) followed by a feltwheel or rubber cup charged with diamond pol-ishing paste (Super Diglaze). Alternatively, ametal ceramic crown can be reglazed.

CEMENTSWhen the fit of the crown is considered satisfac-tory and all adjustments have been made, thecrown can be cemented using one of the materi-als described in this section.

Cements may be classified as soft or hard.Soft cements can be used for provisional cemen-tation of definitive crowns when a trial assess-ment period is needed, for example if the occlu-sion or aesthetics is being significantly altered.Hard cements are used for definitive cementa-tion. This article deals principally with hardcements.

Hard cementsMany types of hard cement have been devel-oped and continue to be available. This diversi-ty of choice suggests no one cement meets allof the requirements of an ‘ideal cement’ how-ever some are more suitable than others for cer-tain applications.

There are essentially three types of hardcement: conventional, resin or a hybrid of thetwo. Conventional cements (eg zinc phosphate,zinc polycarboxylate and glass ionomer) rely onan acid-base reaction resulting in the formationof an insoluble salt (the cement) and water. Resincements set by polymerisation.

The mechanisms by which cements securerestoration to prepared tooth include non-adhe-sive luting, micro-mechanical bonding and

ASSESSMENT OF THEOCCLUSIONIt is important you have aclear idea of the patternof occlusal contact youare trying to achieve

PRACTICE


molecular adhesion. The mechanisms of non-adhesive luting and micro-mechanical retentionare the main methods of action of conventionalcements. Molecular adhesion on the other handis more significant in the case of resin cementsand hybrid cements. Although some convention-al cements have adhesive properties, such as zincpolycarboxylate and GIC, these are limited by thecement’s tensile strength. Furthermore, adhesionto noble metals is negligible but can be improvedin the case of GICs by the use of tin-plating. Tex-turing the fitting surface of the crown, as aftersandblasting, increases the resistance of thecement to dynamic lateral loading.7

We will now go on to discuss the advantagesand disadvantages of each of the major groupsof cements and make recommendations fortheir use.

ZZiinncc pphhoosspphhaattee cceemmeennttAdvantages• Long track record• Good compressive strength (if correctly pro-

portioned)• Good film thickness• Reasonable working time• Resistant to water dissolution• No adverse effect on pulp although initially

acidicDisadvantages• Low tensile strength• No chemical bonding• Not resistant to acid dissolutionRecommendations• Good default cement for conventional crowns

and posts with retentive preparations• Working time can be extended for cementa-

tion of multiple restorations by incrementalmixing and cooled slab

Zinc phosphate has the longest track recordand has remained popular for luting purposesdue mainly to its high compressive strength, rel-atively long working time and ability to form alow film thickness between crown and tooth.Look at Table 2 for the relative physical proper-ties of cements and it is clear that in commonwith the other conventional cements, zinc phos-phate possesses high compressive strength but

low tensile strength. It is this low tensilestrength, which dictates the importance ofpreparation geometry in reducing the develop-ment of disruptive tensile stresses within thecement lute resulting in loss of retention of therestoration.

The retaining action of zinc phosphatecement is one of micromechanical interlockingbetween surface irregularities of the crown andtooth. It does not bond to tooth substance orcrown material.

It is normally supplied as a powder (essential-ly zinc oxide) and liquid (phosphoric acidbuffered with zinc and aluminium ions), whichare mixed together by hand. The proportions ofpowder and liquid are not normally measuredand therefore care must be taken to produce a‘mix’, which provides a cement of low initial vis-cosity to form a thin film, but with sufficientpowder incorporated to give adequate strengthonce set.8 The powder should be incorporated inincrements to prevent the cement setting tooquickly as a result of the exothermic reaction. A slab cooled in the refrigerator can furtherextend working time. The consistency of zincphosphate may be checked by lifting the cementon the spatula and holding it over the slab. Itshould string out slightly between the spatulaand slab before running back onto the slab. If itrequires to be pushed off the spatula it is toothick and conversely, if it runs off too quickly itis not thick enough. Given how critical this is, itis worth making sure that you and your chairside assistant both understand fully what isrequired, and are prepared to stop and mix againif a problem arises.

All cements are to some extent soluble.Zinc phosphate has a low solubility in waterbut erosion leading to loss of the cement luteand failure of the restoration is not normallyassociated with this cement, crowns tendingto be lost more because of a poor retentivedesign of the preparation. However, cementerosion is seen in patients with acid regurgita-tion (Fig. 3).

Historically, zinc phosphate cements havebeen identified as having a potential irritanteffect on the pulp.9 This has been attributed tothe low pH of the cement at the time of cementa-

Table 2 Cement propertiesCement Compressive Tensile Bond Film Solubility Setting time at

strength strength strength thickness in water 37°C and 100%(MPa) (MPa) to dentine (µm) (% in 24h) humidity (mins)

(MPa)

Zinc phosphate 96–133 3.1–4.5 0 25 max 0.2 max 5–9

Zinc polycarboxylate 57–99 3.6–6.3 2.1 25–48 < 0.05 7–9

GIC 93–226 4.2–5.3 3–5 22–24 0.4–1.5 6-8

RMGI 85–126 13–24 10–12 10–22 0.07–0.4 5.5–6.0

without DBA

14–20

with DBA

Resin cement 180–265 34–37 18–30 13–20 0.13 4–5with DBA

PRACTICE


tion, but preparation trauma, temporisation andbacterial contamination may also have beenresponsible. Although zinc phosphate is acidicon mixing (pH 2–3.5 depending on brand) thisacidity reduces over the first 24 hours and sta-bilises at a near neutral pH of 6.5. Despite thisacidity Brannstrom and Nyborg10,11 found noirritating effect on the pulp per se and, in prac-tice, this potential irritant effect does not seem tobe significant.

At one time cavity varnish was advocated tocoat a preparation prior to cementation to pro-tect it from cement but this adversely affectedretention.12,13 Nowadays a dentine-bondingagent could be used and, anecdotally, this hashelped with some teeth, which have been sensi-tive after preparation, but controlled studies areneeded to assess the long-term consequences. Ifthe dentine is to be etched it is essential that theprimer seals all the open tubules or sensitivitymay worsen and bacterial invasion may jeopar-dise the pulp.

ZZiinncc ooxxiiddee eeuuggeennooll cceemmeennttssCements based on zinc oxide and eugenol areclassical soft cements. Attempts have been madeto create a more permanent cement by adding o-ethoxy-benzoic acid (EBA) to zinc oxide-eugenol and by reinforcing it with aluminiumoxide and polymethylmethacrylate. Based on in vitro tests, this type of cement was reported tohave good strength and be less soluble than zincphosphate cement.14,15 Unfortunately, its per-formance was much poorer in vivo and studieshave shown that it deteriorates much more rap-idly in the mouth than other cements.16,17 It can-not be recommended as a definitive lute forrestorations.

PPoollyyccaarrbbooxxyyllaattee cceemmeennttssAdvantages• Reasonable track record• Good compressive strength (if correctly pro-

portioned)• Adequate working time• Bonds to enamel and dentine• Adequate resistance to water dissolution (but

less good than zinc phosphate)• No adverse effect on pulp and less acidic than

zinc phosphate on mixing

Disadvantages• Low tensile strength• Can deform under loading• Can be difficult to obtain low film thickness• Not resistant to acid dissolutionRecommendations• Traditionally used for vital or sensitive teeth,

but no evidence to support efficacy (dentinebonding agents used to seal preparation priorto cementation may be a better option)

• Occasionally useful to retain an unretentiveprovisional crown

Zinc polycarboxylate (or zinc polyacrylate)has a relatively long history as a luting cement.Unlike zinc phosphate, polycarboxylate cementdoes bond to tooth tissue, its bond strength toenamel being greater than that to dentine. It alsobonds to stainless steel so dental instrumentsmust be cleaned before the material sets to pre-vent a tenacious bond forming.

The tensile strength of polycarboxylate com-pares favourably to that of zinc phosphatealthough its compressive strength tends to belower and it is difficult to achieve an equally lowfilm thickness.

Zinc polycarboxylate cement is not as acidicon mixing (approx. 4.8) as zinc phosphate. Thereappears to be little irritation to the pulp18 possiblybecause there is little penetration of the largepolyacrylic acid molecules into the dentinetubules. The liquid for the cement is either a vis-cous solution of polyacrylic acid or water. If wateris used the acid is contained in its anhydrous statewithin the zinc oxide powder. More recentlydeveloped polycarboxylate cements contain fluo-ride salts, which may aid caries prevention.

Correctly mixed polycarboxylate cement hasa consistency similar to that of honey and thecement may appear too viscous to allow properseating. However, this is normal and should notbe of concern since the cement undergoes ‘sheerthinning’ which reduces the apparent viscosityduring the seating of the crown.19

GGllaassss iioonnoommeerr cceemmeennttssAdvantages• As for polycarboxylate cement but cement has

similar acidity to zinc phosphate on mixing• Fluoride releaseDisadvantages• Sensitive to early moisture contamination• Low tensile strength• Not resistant to acid dissolution• Has been accused of causing post-operative

sensitivity but a controlled trial reports it is noworse than zinc phosphate

Recommendations• Used empirically for conventional crowns

where patient has had a previously high cariesrate

• May be used as an alternative ‘default cement’to zinc phosphate

Conventional glass ionomer cements werefirst introduced into dentistry as a filling materi-

Fig. 3 Erosion of zinc phosphatecement seen in a patient with acidreflux

PRACTICE


al in 1972.20 Like polycarboxylates, glassionomers may be supplied as a powder andaqueous acid (polyalkenoic) or powder andwater. The aluminosilicate glass powder of GICluting cements has smaller particles than GICfilling materials to reduce film thickness, whichmay be similar to or lower than that of zincphosphate cements.

For luting purposes, mixing is generallycarried out by hand and where provided themanufacturer’s powder and liquid dispensersshould be used. Encapsulated glass ionomerluting cements are also available and have theadvantage of providing the correct powder-liquid ratio.

GIC compares favourably with zinc phos-phate with regard to compressive and tensilestrength (Table 2). GIC has a significant advan-tage to zinc phosphate in that it forms a con-siderable bond to tooth tissue by reaction withthe calcium salts in the tooth structure andreleases fluoride ions.

The higher solubility in water of GIC in com-parison with zinc phosphate and zinc polycar-boxylate cements has been identified as a prob-lem when the cement is used for luting purposes.This solubility is adversely affected by earlymoisture contamination and the cement lutemargins should be protected with a varnish fol-lowing cementation, although this may be diffi-cult when the crown margin is subgingival. Sol-ubility is not a great problem clinically once thecement is set. Another disadvantage of GIC isthat its pH during setting is even lower than thatof zinc phosphate and some concern has beenexpressed regarding post-cementation hyper-sensitivity.21 However, a randomised, doubleblind trial of GIC versus zinc phosphate showedno significant difference in sensitivity,22 but itshould be noted that cementation procedureswere carefully controlled, including the use ofencapsulated mixing. Dentine desiccation mayon occasion be responsible for sensitivity; someauthorities are convinced that dentinal fluid isdrawn into the setting cement, which may causeproblems if the preparation is over-dried with anair syringe.

RReessiinn mmooddiiffiieedd ggllaassss iioonnoommeerr cceemmeennttss aannddccoommppoommeerrssAdvantages• Good compressive and tensile strengths (if

correctly proportioned)• Reasonable working time• Resistant to water dissolution• Fluoride releaseDisadvantages• Short track record• May expand and crack overlying porcelain

because of water absorptionRecommendations• Worth trying for metal or metal ceramic

crowns especially where preparation reten-tion is borderline

• Currently unclear which RMGI cements canbe used safely with ceramic crowns

Resin modified glass ionomer (RMGI)cements are a hybrid of traditional glassionomer cement with small additions of lightcuring resin23 and generally have the advan-tages of both in that they are purported tocombine the strength and insolubility of resinwith the fluoride release of GIC. They wereintroduced with the aim of overcoming themoisture sensitivity and the low strength ofconventional glass ionomers. Examplesinclude Vitremer and Fuji Plus. Compomers arealso composed of resin and glass ionomer butare more closely related to composites with theglass ionomer setting reaction occuring slowlyas moisture is absorbed into the set resinmatrix. Examples include Dyract Cem andDyract Cem Plus.

The use of RMGIs for luting purposes isbecoming more popular because of their rela-tively high bond strength to dentine, and theirability to form a very thin film layer. RMGIsleach fluoride, but it is unclear how useful thisis in preventing secondary caries formation.Several RMGIs are available but whilst theymay seem promising clinical studies are stillin their infancy. There have also been anec-dotal reports of high strength porcelaincrowns fracturing following cementation withRMGIs, possibly as a result of expansion ofthe cement from water sorption.24 These havebeen supported by an in-vitro study showingthat ceramic crowns crack between 3 and 12months after cementation with both RMGIsand compomers.25 RMGIs contain the resinHEMA and absorb significantly more waterthan composites. In view of these concerns itis best to restrict the use of RMGIs and com-pomers to metal restorations. Furthermore,when cementing porcelain or compositerestorations a better aesthetic result can beachieved with composite resin cements.

RReessiinn cceemmeennttssAdvantages• Good compressive and tensile strengths• High tensile strength (relative to convention-

al cements)• Resistant to water dissolution• Relatively resistant to acid dissolution• Can enhance strength of ceramic restoration

if bond obtainedDisadvantages• Film thickness varies substantially between

materials• Excess material extruded at margin may be

difficult to remove especially proximallyRecommendations• Must be used with or incorporate an effective

dentine bonding agent• Material of choice for porcelain veneers,

ceramic onlays and resin bonded ceramiccrowns

• May be used to improve retention wherepreparation geometry sub-optimal, but clini-cal studies needed to determine long-termsuccess

RESIN MODIFIED GLASSIONOMER CEMENTSReports of high strengthporcelain crowns fracturing followingcementation withRMGIs, possibly as aresult of expansion ofthe cement

GLASS IONOMERCEMENTSA randomised, doubleblind trial of GIC versuszinc phosphate showedno significant differencein sensitivity

PRACTICE


Resin cements are composites composed of aresin matrix, eg bis-GMA or urethanedimethacrylate, and a filler of fine inorganicparticles. They have been available as direct fill-ing materials since the early 1950s26 but it wasnot until the early 1970s that a composite resinwas introduced for crown and bridge cementa-tion.27 Resin luting cements differ from restora-tive composites primarily in their lower fillercontent and lower viscosity. Following on fromtheir successful use in the cementation of resinbonded bridges and veneers, their popularity hasbeen increasing in recent years for crowncementation because of their use in conjunctionwith dentine bonding agents (DBA). Howevereven when DBAs are used, resin cements are notwithout problems (see later).

Composite resin cements are available as selfcured, light cured and dual cured materials. Theself cured materials are typically used as lutingcements because of the inability, or at best diffi-culty, of light to pass through porcelain andmetal restorations. Examples include Panavia,21

All Bond 2 luting cement and Superbond.Mechanical and physical properties of resin

cements compare favourably with the othercements discussed above (Table 2). In particular,tensile strength is about ten times that of zincphosphate, which in combination with the highbond strength explains why preparation geome-try is of less importance to retention than withconventional cements. This makes resin cementsuseful for bonding restorations on tooth prepa-rations that would not be retentive enough tosucceed with conventional cements. Moreover, awell-bonded composite lute will confer muchgreater strength to an overlying porcelainrestoration than a weaker conventional cement.This feature has been demonstrated in vitro withporcelain veneers28 and in vivo with porcelaininlays29 which were almost five times more like-ly to fracture when cemented with conventionalglass ionomer. It should be noted that effectiveresin bonding to some high strength porcelaincores (eg In-Ceram) could not be achieved by theusual etching with hydrofluoric acid because ofthe lack of pores in the material.30

Problems with the use of resin cements forluting full crowns include excessive film thick-ness with some materials,31,32 marginal leakagebecause of setting shrinkage, and severe pulpalreactions when applied to cut vital dentine.However, this latter problem may be relatedmore to bacterial infiltration than to any chemi-cal toxicity. The use of DBA under resin cementis critical to its success unless the preparationhas been cut only into enamel. Pulpal responseis reduced by the use of DBAs, presumably bysealing dentine tubules and reducing microleak-age.33 Adhesive resin cement was found to pro-duce a better marginal seal than zinc phosphatecement.34 However, even if the problems ofmicroleakage and film thickness could besolved, the problem of adequately removinghardened excess resin from inaccessible marginsmay preclude the use of resin cement for crowns

with subgingival margins. Indeed, proximalextrusions of resin cement are often radiolucentand may remain undetected.35

CROWN CEMENTATION When a crown has been successfully tried-in andthe cement chosen, cementation may then takeplace. This section will consider conventionalcementation. Cementation with resin cements iscovered more fully in Parts 12 and 13 of thisseries on porcelain veneers and resin bondedmetal restorations respectively.

Trial cementationMost dentists are in the habit of fitting crownsand then cementing them with hard cement.Whilst this approach is usually satisfactory thereare times where it is difficult to predict apatient’s response to changes in aesthetics orocclusion. If such a patient returns unhappy theoffending crowns must be cut off — a distressingexperience for all concerned. In cases of doubt itis useful to have a period of trial cementationusing soft cement, but you must ensure that thedefinitive restoration can be removed withoutdamage to it or the underlying preparation. Tomake removal easier the cement should beapplied in a ring around the inner aspect of thecrown margin. It is important that the manufac-turer’s modifier is added to the cement. Equallengths of base and catalyst with a third of alength of modifier will soften cements such asTemp Bond. Alternatively, a ‘non-setting’ zincoxide eugenol material (eg Optow Trial Cement)can be used for short periods of soft cementationwhere preparations are retentive. This materialhas the advantage that it is easily pealed out ofthe crown like a membrane, but it cannot berelied upon for more than a few days retention.

Restorations can be removed either by fingerpressure or by the application of a matrix band.In cases of difficult removal a Richwil crownremover can be helpful. This crown remover issimply a material, not unlike a sticky sweet,which is softened in hot water, positioned overthe crown and the patient asked to bite. Once thematerial has hardened the crown is removed byasking the patient to snap open. Another way ofapplying a dislodging force to a soft-cementedcrown is to use an impact mallet. The problemwith this technique is finding a point of applica-tion on the crown. One solution to this problemis to incorporate small lugs resembling mush-rooms on the lingual aspect of the crown’s metalwork.36 The lugs are removed, of course, prior tohard cementation.

Controlling cement film thicknessThe interposition of a cement lute inevitablyaffects crown seating. Consequently, the art ofcementation is to choose a cement with aninherently low film thickness and use techniqueswhich allow it to escape whilst the crown isbeing seated.

Cement flow can be hindered by preparationfeatures, which cause a build up of hydrostatic

RESIN CEMENTSThe tensile strength ofresin cements is aboutten times that of zincphosphate

RESIN CEMENTSThe Richwil crownremover is not unlike asticky sweet

PRACTICE


pressure.37 Thus, retentive preparations, whichare long, near parallel and have a large surfacearea, are most at risk of not seating fully. Thisproblem can be overcome by die spacing andcontrolled cement application or by venting thecrown. These techniques need to be used for allcrowns not just apparently retentive ones.

Die spacing is the most common method ofachieving space for the cement lute.38 It involvespainting several layers of die relief agent overthe whole of the die but avoiding the finish line.The increased cement space results in more rapidseating with decreased deformation of therestoration.39–41 Die spacing results in a slightlyloose fit of a crown on its preparation, but itseffect on retention is unclear with some studiesreporting an increase in retention37 while othersreport a decrease or no effect. A recent studyconcluded that decreasing the width of thecement layer increases the resistance to dynamiclateral loading.7 This variability may occurbecause of differences in cement film thickness.A very thin cement lute may have higher stressconcentrations than a slightly thicker one.42

However, too thick a cement lute is also undesir-able as it is liable to fracture.

Another factor which influences the verticalseating of crowns and hence marginal adapta-tion is the amount of cement loaded into thecrown prior to cementation. A study on theeffect of volume of zinc phosphate cement,reported that lesser amounts of cement placedwithin a crown resulted in smaller marginal dis-crepancy and better occlusal accuracy.43 Indeed,a crown treated in such a way seated almost 70%better than an identical crown completely filledwith cement. However, care must be taken inapplying cement in this way not to exceed theworking time or the cement may be too viscousat the time of seating.

Venting is an effective8 but less popularmethod of reducing cement film thickness. Exter-nal venting involves creating a perforation in theocclusal surface of the crown, which is sealed witha separate restoration after cementation. Withinternal venting an escape channel is createdeither in the axial wall of the preparation or the fitsurface of the crown to help cement escape.

The amount of force required to allow maxi-mum seating of cast crowns has been shown tobe cement specific.44 Seating forces are discussednext in relation to cementation technique.

TechniqueIsolate the preparation and ensure good mois-ture control. If the gingivae have overgrown thefinish line use either retraction cord withhaemostatic agent or if more severe use electro-surgery. A breakdown in technique at any of thefollowing stages will predispose to failure:

• Clean the preparation and crown with waterspray

• Air dry but do not desiccate preparation• Mix cement according to manufacturer’s

instructions

• Coat the fit surface with cement - do not over-fill

• Only apply cement to preparation if cement-ing a postThe crown should be seated quickly with firm

finger pressure until all excess cement has beenexpressed from the margins. Seating force mustbe adequate to ensure complete seating of thecrown onto the preparation, but sudden exces-sive force may result in elastic strain of the den-tine, creating a rebound effect, which results inthe crown being partly dislodged when the forceis removed.34 Karpidis and Pearson (1988)45

revealed that crowns seated on preparations inbovine dentine with a force of 300 N/cm2 couldbe removed more easily than those cementedwith half the force.

Depending on the angulation of the tooth,pressure may then continue to be exerted ontothe crown by the dentist or by the patient bitingonto a cotton roll. Some operators prefer awooden orange stick or similar implement forcementing posterior restorations as this canreduce film thickness. However care must betaken as these are rigid and may only contactpart of the occlusal surface of the crown result-ing in tipping. Pressure should be maintained forabout one minute. Maintaining pressure beyondthis time has no appreciable additional effect.46

It is worth checking the accuracy of the fit at thisstage using a sharp probe on the margin and ifnecessary fine gold margins can be burnishedbefore the cement sets.

Adequate moisture control should be main-tained until the cement has set to prevent mois-ture contamination of the unset material at thecrown margin. In the case of conventionalcements, excess cement should be left until afterthe cement sets. For resin-based cements,removal of excess before setting is recommend-ed as it can be very difficult to remove followingsetting but may still remain despite our bestefforts.47 Some operators apply a smear of petro-leum jelly to the outside of the crown which alsohelps with removal of set cement, but if you dothis take great care to prevent contamination ofthe fit surface. A common failing is for excesscement to be left, especially interproximally.Your nurse can help by having a piece of flossready. This can be made more effective by tyinga knot in the middle of the floss and passing itthrough the interdental space. Following cleanup, a final evaluation of the cemented crown canbe made including rechecking the occlusion.

1. Kaufman E G. The retention of crowns before and aftercementation. NY Univ J Dent 1967; 25: 6-7.

2. McClean J W, von Fraunhoffer J A. The estimation of cementfilm thickness by an in vivo technique. Br Dent J 1971; 131:107-111.

3. Christensen G J. Marginal fit of gold inlay castings. J ProsthetDent 1966; 16: 297-305.

4. Lang N P, Kiel R A, Anderhalden K. Clinical andmicrobiological effects of subgingival restorations withoverhanging or clinically perfect margins. J Clin Perio 1983;10: 563-578.

5. Kelleher M G D, Setchell D J. An investigation of markingmaterials used in occlusal adjustment. Br Dent J 1984; 156:96-102.

TECHNIQUEForce must be adequateto ensure complete seating, but suddenexcessive force mayresult in elastic reboundand the crown beingpartly dislodged

PRACTICE


6. Monasky G E, Taylor D F. Studies on the wear of porcelain,enamel and gold. J Prosthet Dent 1971; 25: 299-306.

7. Wiskott H W, Belser U C, Scherrer S S. The effect of filmthickness and surface texture on the resistance of cementedextracoronal restorations to lateral fatigue. Int J Prosthodont1999; 12: 255-262.

8. Kaufman E G, Colin L C, Schlagel E, Coelho D H. Factorsinfluencing the retention of cemented gold castings: thecementing medium. J Prosthet Dent 1966; 16: 731-739.

9. Langeland K, Langeland L K. Pulp reactions to crownpreparation, impression, temporary crown fixation andpermanent cementation. J Prosthet Dent 1965; 15: 129-143.

10. Brännström M, Nyborg H. Bacterial growth and pulpalchanges under inlays cemented with zinc phosphate cementand Epoxylite CBA 9080. J Prosthet Dent 1974; 31: 556-565.

11. Brännström M, Nyborg H. Pulpal reaction to polycarboxylateand zinc phosphate cement used with inlays in deep cavitypreparations. J Am Dent Assoc 1977; 94: 308-310.

12. Smith D C, Ruse N D. Acidity of glass ionomer cementsduring setting and its relation to pulp sensitivity. J Am DentAssoc 1986; 112: 654-657.

13. Chan K C, Svare C W, Horton D J. The effect of varnish ondentinal bonding strength of five dental cements. J ProsthetDent 1976; 35: 403-406 .

14. Brauer G M, McLaughlin R, Huget E F. Aluminium oxide as areinforcing agent for zinc oxide-eugenol-o-ethoxy-benzoicacid cements. J Rest Dent 1968; 47: 622-628.

15. Phillips R W, Swartz M L, Norman R D, Schnell R J, Niblack B F.Zinc oxide and eugenol cements for permanent cementation.J Prosthet Dent 1968; 19: 144-150.

16. Osbourne J W, Swartz M L, Goodacre C J, Phillips R W, GaleE N. A method for assessing the clinical solubility anddisintegration of luting cements. J Prosthet Dent 1978; 40:413-417.

17. Mesu F P, Reedijk T. Degradation of luting cements measuredin vitro and in vivo. J Rest Dent 1983; 62: 1236-1240.

18. Going R E, Mitchem J C. Cements for permanent luting: asummarising review. J Am Dent Assoc 1975; 91: 129-137.

19. Lorton L, Moore M L, Swartz M L, Phillips R W. Rheology ofluting cements. J Rest Dent 1980; 59: 1486-1492.

20. Wilson A D, Kent B E. A new translucent cement for dentistry.Br Dent J 1972; 132: 133-135.

21. Smith D C, Ruse N C. Acidity of glass ionomer cementsduring setting and its relation to pulp sensitivity. J Am DentAssoc 1986; 112: 654-657.

22. Kern M, Kleimeier B, Schaller H G, Strub J R. Clinicalcomparison of postoperative sensitivity for a glass ionomerand a zinc phosphate luting cement. J Prosthet Dent 1996;75: 159-62.

23. Sidhu S K, Watson T F. Resin-modified glass ionomermaterials. A status report for the American Journal ofDentistry. Am J Dent 1995; 8: 59-67.

24. Kanchanavista W, Arnstice H M, Pearson G J. Water sorptioncharacteristics of resin-modified glass-ionomer cements.Biomater 1997; 18: 343-349.

25. Leevailoj C, Platt J A, Cochran M A, Moore B K. In vitro study offracture incidence and compressive fracture load of all-ceramic crowns cemented with resin-modified glass ionomerand other luting agents. J Prosthet Dent 1998; 80: 699-707.

26. Schouboe P J, Paffenbarger G C, Sweeney W J. Resin cementsand posterior type direct filling resins. J Am Dent Assoc 1956;52: 584.

27. Lee H, Swartz M L. Evaluation of a composite resin crown andbridge luting agent. J Rest Dent 1976; 51: 756.

28. Brandson S J, King P A. The compact fracture resistance ofrestored endodontically treated anterior teeth. J Rest Dent1992; 72: 1141.

29. Åberg C H, van Dijken J W V, Olofsson A-L. Three-yearcomparison of fired ceramic inlays cemented with compositeresin or glass-ionomer cement. Acta Odontol Scand 1994;52: 140-149.

30. Awliya W, Oden A, Yaman P, Dennison J B, Razzoog M E.Shear bond strength of a resin cement to densely sinteredhigh-purity alumina with various surface conditions. ActaOdontol Scand 1998; 56: 9-13.

31. White S N, Yu Z, Kipnis V. Effect of seating force on filmthickness of new adhesive luting agents. J Prosthet Dent1992; 68: 476-481.

32. White S N, Kipnis V. Effect of adhesive luting agents on themarginal seating of cast restorations. J Prosthet Dent 1993;69: 28-31.

33. Qvist V, Stolze K, Qvist J. Human pulp reactions to resinrestorations performed with different acid-etch restorativeprocedures. Acta Odontologica Scandinavia 1989; 47: 253-263.

34. Tjan A H L, Dunn J R, Brant B E. Marginal leakage of cast goldcrowns luted with an adhesive resin cement. J Prosthet Dent1992; 67: 11-15 .

35. O’Rourke B, Walls A W, Wassell R W. Radiographic detectionof overhangs formed by resin composite luting agents. J Dent 1995; 23: 353-357.

36. Pameijer J H N. Periodontal and occlusal factors in crown andbridge procedures. pp394. Amsterdam: Centre for Postgraduate Courses, 1985.

37. Carter S M, Wilson P R. The effect of die-spacing on crownretention. Int J Prosthodont 1996; 9: 21-29.

38. Grajower R, Zuberi Y, Lewinstein I. Improving the fit ofcrowns with die spacers. J Prosthet Dent 1989; 61: 555-563.

39. Wilson P R, Goodkind R J, Sakaguchi R. Deformation ofcrowns during cementation. J Prosthet Dent 1990; 64: 601-609.

40. Wilson P R. The effect of die spacing on crown deformationand seating time. Int J Prosthodont 1993; 6: 397-401.

41. Wilson P R. Effect of increasing cement space oncementation of artificial crowns. J Prosthet Dent 1994; 71:560-564.

42. Kamposiora P, Papavasilious G, Bayne S C, Felton D A. Finiteelement analysis estimates of cement microfracture undercomplete veneer crowns. J Prosthet Dent 1994; 71: 435-441.

43. Tan K, Ibbetson R. The effect of cement volume on crownseating. Int J Prosthodont 1996; 9: 445-451.

44. Wilson P R. Low force cementation. J Dent 1996; 24: 269-273.

45. Karipidis A, Pearson G J. The effect of seating pressure andpowder/liquid ratio of zinc phosphate cement on theretention of crowns. J Oral Rehabil 1988; 15: 333-337.

46. Jorgensen K D. Structure of the film thickness of zincphosphate cements. Factors affecting the film thickness ofzinc phosphate. Acta Odontol Scand 1960; 18: 479-501.

47. Mitchell C A, Pintado M R, Geary L, Douglas W H. Retentionof adhesive cement on the tooth surface after crowncementation. J Prosthet Dent 1999; 81: 668-677.

List of materials mentioned in textAll Bond 2 dentine bonding agent and luting cement: Bisco Inc, Itasca, IL60143, USA

Dyract Cem and Dyract Cem Plus: Dentsply UK Ltd, Hamm Moor Lane, Addlestone, Weybridge, Surrey KT15 2SE

Fuji Plus: GC, Tokyo, Japan

Premier composite finishing diamonds (white and yellow stripe): Panadent Ltd, 15 Great Dover Street, London SE1 4YW

Kenda polishing points (blue, brown and green for metal; white for porcelain and composite): Austenal Ltd, 4 Crystal Way, Harrow, Middlesex HA1 2HG

Optow Trial Cement: Teledyne Getz, Elkbrook Village, Illinois, USA

Panavia 21: Cavex Holland BV, Haarlem, Holland; supplied by J&S Davis, Summit House, Summit Road, Potters Bar, Hertfordshire EN6 3EE

Richwil Crown and Bridge Remover: Horizon Dental, PO Box 14, Disley, Stockport SK12 2RP

Soflex discs: 3M Health Care Ltd, 3M House, Morley Street, Loughborough

Super Bond: Sun Medical Co Ltd, Moriyama, Japan

Super Diglaze, sub-micron diamond polishing paste: Distributed in UK and Eire by Panadent Ltd, 15 Great Dover Street, London SE1 4YW

Temp Bond: Kerr UK Ltd, Peterborough, UK

Vitremer luting cement: 3M Dental Products, St Paul, Mn, USA

PRACTICE


Crowns and other extra-coronal restorations:Porcelain laminate veneersA. W. G. Walls1 J. G. Steele2 and R. W. Wassell3

Porcelain veneers are resin-bonded to the underlying tooth and provide a conservative method of improving appearance ormodifying contour, without resorting to a full coverage crown. The porcelain laminate veneer is now a frequently prescribedrestoration for anterior teeth. The sums spent by the Dental Practice Board on this type of treatment increased from quarterof a million pounds in 1988/89 to over seven million in 1994/95,1 representing some 113,582 treatments. Since that time thenumber has stabilised at over 100,000 veneers prescribed each year.2 The objective of this paper is to give a practical guideon providing these restorations.

1*Professor of Restorative Dentistry,2,3Senior Lecturer in Restorative Dentistry,Department of Restorative Dentistry, TheDental School, Newcastle upon Tyne NE2 4BW; *Correspondence to: Prof A. W. G. Walls,Department of Restorative Dentistry, TheDental School, University of Newcastleupon Tyne, Newcastle upon Tyne NE2 4BWE-mail: [email protected]


● The development of porcelain veneers● Longevity and factors affecting it● Tooth preparation, and management of existing restorations● Impression recording and temporisation (in those few cases which require it)● Try in, bonding and finishing● Non-standard porcelain veneers

I N B R I E F

Whenever possible guidelines for provision ofporcelain laminate veneers are based on datafrom the dental literature, but where this is notpossible they will be based on our clinical expe-rience and practice. Veneers are often placed onthe buccal aspect of maxillary anterior teeth butother applications are possible and these aredescribed at the end of the article.

HISTORY The concept of veneering was first described inthe dental literature some time ago,3 although itis only with the advent of efficient bonding ofresins to enamel and dentine and the use ofetched, coupled porcelain surfaces that aestheti-cally pleasing, durable and successful restora-tions can be made.4 These restorations are nowan accepted part of the dentist’s armamentari-um.5–7 Custom-made acrylic resin veneers pre-ceded them, but these showed unacceptable lev-els of failure and of marginal stain.8 Alternativeveneering materials are still available, usuallyeither direct or indirect composite resin materi-als. However, these may suffer from degradationof surface features and accretion of surface stainwith time.9–11

Porcelain veneers have traditionally beenmade from aluminous or reinforced feldspathicporcelains, which have relatively poor strengthin themselves but produce a strong structurewhen bonded to enamel. Porcelain veneers canbe made from most of the high strength ceramicsdiscussed in the second article of the series. Suchmaterials may hold promise for the future. Astudy of 83 IPS Empress veneers placed over a 6-year period in private practice reported onlyone failure, but as yet there are no clinical data

making a direct comparison between these andthe traditional materials.

That the strength of traditional porcelain isgenerally adequate for anterior porcelainveneers is supported by a number of clinicalstudies. Some authors9,12–17 have reported lowrates of failure because of the loss of retentionand fracture (0–5%) with short and medium-term studies of up to 5 years. Indeed, a long-termfollow-up18 of veneers placed over a 10-yearperiod shows a survival rate of 91% at 10.5 years(calculated with the Kaplan Meyer method).These excellent results may, amongst otherthings, reflect careful case selection, but it isworth noting that other authors,5,13,19,20 havereported much higher rates of failure of between7–14% over 2–5 years. Such studies suggest thatthe risk factors for veneer failure are:

• Bonding onto pre-existing composite restora-tions (which is considered later)

• Placement by an inexperienced operator• Using veneers to restore worn or fractured

teeth where a combination of parafunction,large areas of exposed dentine and insufficienttooth tissue exist.

Another risk factor, shown up by in-vitrowork, is the tendency for thermal changes incombination with polymerisation contractionstresses to cause cracking of the veneer when theporcelain is thin and the luting compositethick.21 A thick composite lute may occur as aresult of a poorly fitting veneer or the use ofcopious die spacer in an attempt to mask under-lying tooth discolouration. Least cracking wasseen with a ceramic and luting composite thick-ness ratio above 3.

12










restorations

PRACTICE


In-vivo, minor chipping and cracking may besmoothed or repaired without the need toremove the whole veneer. Dunne and Millarreported that the incidence of such repairabledefects (8%) was similar to the number ofveneers requiring total replacement (11%).5

It is useful to be aware of the above data whenpatients ask how long their proposed veneers arelikely to last. It may also be prudent to warn themthat although most (80–100%)7 patients remainsatisfied with the aesthetics, veneers are prone tomarginal staining, the amount of which will varyfrom patient to patient. Staining may be causedby one or more of the following:

• Microleakage at the cervical margin, especial-ly where located in aprismatic enamel or,worse still, dentine

• Wear and submargination of the luting com-posite, especially with an open margin

• Marginal excess of luting composite

To some extent, these factors can be con-trolled or influenced by careful attention to clin-ical technique.

CLINICAL TECHNIQUEA key element in success with porcelain veneersis carefully controlled but appropriate tooth tis-sue reduction.22–24 The aims of tooth prepara-tion are to:

• Provide some space into which the techniciancan build porcelain without over-contouringthe tooth

• Provide a finished preparation that is smoothand has no sharp internal line-angles whichwould give areas of high stress concentrationin the restoration

• Maintain the preparation within enamelwhenever possible

• Define a finish line to which the techniciancan work.

It may be possible to prepare veneer prepara-tions without local anaesthetic. However, in ourexperience, sub-gingival margin placement,inadvertent dentine exposure and the unpleas-ant coldness from the water spray and aspiratorusually make its use advisable.

Depth of preparationIt is desirable for the tooth preparation to remainwithin enamel so careful control of preparationdepth is important. Obviously, the enamel thick-

ness varies from the incisal edge to the cervicalmargin. Hence the preparation depth will need tovary over the length of the tooth to avoid (if pos-sible) exposing dentine. The preparation depthshould be of the order of 0.4 mm close to the gin-gival margin, rising to 0.7 mm for the bulk of thepreparation. This is best achieved by using adepth mark of some sort. In our experience formaldepth grooves can be of limited value in this areaas there is a tendency for the bur to catch and runinto the groove during buccal reduction, accentu-ating the groove. The alternative is to use depthpits prepared on the surface of the tooth using a 1mm diameter round bur sunk to half its diameter(Fig. 1). The buccal surface reduction can then beundertaken to join the base of the pits. The reduc-tion should mimic the natural curvature of thetooth in order to provide an even thickness ofporcelain layer over the tooth surface, hence itshould be in at least two planes.24

When the tooth concerned is markedly dis-coloured, it is sensible to undertake a greater levelof reduction to give the technician more chance tomask the underlying stain without over-contour-ing the tooth. This will have obvious disadvan-tages, as the preparation is likely to extend intodentine with greater depth of tooth reduction.

Nattrass et al.25 have demonstrated that evenwith experienced operators and careful controlof cutting instruments there is a tendency fordentine to be exposed in the cervical and proxi-mal regions of the preparations, where theenamel is thinnest. This should be borne in mindwhen deciding on the type of luting agent to beused in veneer placement. They also found thatthere was a tendency for variations in toothpreparation depth across their samples with leastreduction in the mid-incisal region. There is nosuggestion in the literature as yet that this caus-es any long-term damage to the tooth or affectsthe longevity of the veneer.

Incisal edge reductionOne important decision to make before com-mencing the preparation is whether or not theincisal edge of the tooth is to be reduced. Thereare four basic preparation designs that havebeen described for the incisal edge (Fig. 2):

• Window, in which the veneer is taken close tobut not up to the incisal edge. This has theadvantage of retaining natural enamel over theincisal edge, but has the disadvantage that theincisal edge enamel is weakened by the prepa-ration. Also, the margins of the veneer wouldbecome vulnerable if there is incisal edge wearwhilst the incisal lute can be difficult to hide.

• Feather, in which the veneer is taken up to theheight of the incisal edge of the tooth but theedge is not reduced. This has the advantagethat once again guidance on natural tooth ismaintained but the veneer is liable to be frag-ile at the incisal edge and may be subject topeel/sheer forces during protrusive guidance.

• Bevel, in which a bucco-palatal bevel is pre-pared across the full width of the preparation

Fig. 1 A 1 mm round diamond burbeing used to create depth marks onthe buccal surface of UR1 (11)

PRACTICE


and there is some reduction of the incisallength of the tooth. This gives more controlover the incisal aesthetics and a positive seatduring try in and luting of the veneer. The mar-gin is not in a position that will be subjected todirect shear forces except in protrusion. How-ever, this style of preparation does involvemore extensive reduction of tooth tissue.

• Incisal overlap, in which the incisal edge isreduced and then the veneer preparationextended onto the palatal aspect of the prepa-ration. This also helps to provide a positiveseat for luting whilst involving more exten-sive tooth preparation. This style of prepara-tion will also modify the path of insertion ofthe veneer which will have to be seated fromthe buccal/incisal direction rather than thebuccal alone. Care needs to be taken to ensurethat any proximal wrap around of the prepa-ration towards the gingival margin does notproduce an undercut to the desired path ofinsertion for the veneer. It may be necessary torotate such veneers into place by locating theincisal edge first then rotating the cervicalmargin into position.

There is little data available upon which tobase a decision over incisal edge preparation.Hui et al.26 demonstrated that veneers in win-dow preparations were best able to resist incisaledge loading and that an overlap design frac-tured at the lowest loads. However, the magni-tude of loading at which the overlap designveneers failed was much greater than thatencountered clinically for such teeth. Further-more, a clinical study was unable to distinguishany difference in failure rate between incisalpreparation designs after two and a half years ofservice.27 If the operator intends to eitherimprove the incisal edge aesthetics or to increasethe length of a tooth then either an overlap orbevel design would be the preparation of choice.If it were not necessary to extend the incisaledges, then it may be possible to use a feather-edge design, however the operator has less con-trol of incisal edge aesthetics with this approach.Nordbo et al.14 report no failures but 5% incisalchipping at 3-years for veneers placed using afeather-edge design and 0.3 to 0.5 mm buccaltooth reduction.

The authors would not recommend the buccalwindow, as it is very difficult to mask the incisalfinish line of the restoration. As this style ofrestoration is used to improve the appearance ofteeth, the introduction of an aesthetic defectwould be inappropriate. If the incisal edge is to bemodified then the length should be reduced bysome 0.5–0.75 mm28 to allow adequate strengthwithin the porcelain incisal edge without elongat-ing the tooth. Depth grooves can be used to moni-tor accurately incisal edge reduction (Fig. 1); wewould strongly recommend this approach.

Axial tooth reductionAxial tooth reduction is best undertaken usingdiamond burs in either an airotor or a speed

accelerating handpiece with a conventionalmotor. It is easier to achieve predictable toothreduction using either a parallel sided or taperedbur with straight sides rather than a flame-shaped bur (Fig. 3). Some clinicians advocatepreparing the gingival finish line as the first stepusing a round diamond bur of appropriate diam-eter, which will automatically produce a cham-fered finish line. Alternatively a torpedo shapedbur can be used to produce both the axial reduc-tion and the gingival finish line (Fig. 4), which isthe method we prefer.

Conventional diamond burs leave a macro-scopically roughened surface on enamel. Furtherpreparation of the tooth using either a small par-ticle size diamond bur or a multi-fluted tungstencarbide finishing bur will smooth the surface ofthe preparation and can be used to refine the fin-ishing margin. At this stage the gingival tissues

a) b)

c) d)

Fig. 2 Four incisal preparations are possible for veneers: a) window , b) feather , c) bevel or d) incisal overlap

Fig. 3 Straight sided torpedo-shapeddiamond bur provides morepredictable axial reduction inassociation with depth marks than aflame-shaped bur

PRACTICE


can be protected from damage using a flat plas-tic instrument (Fig. 5) or gingival retraction cordcan be packed for the same purpose, which willin turn facilitate the impression. It is oftenimpractical to provide provisional restorationsfor porcelain veneers (see later) but somepatients are conscious of the roughened toothsurface in their mouths, which should besmoothed.

Proximal finish linesIt is best if the proximal finishing margins forthe preparation do not extend beyond the con-tact point in the incisal third of the tooth — inother words, the contact point with adjacentteeth should be maintained. If it proves neces-sary to prepare through the contact area thensome form of provisional restoration would berequired to prevent inadvertent tooth movementbetween tooth preparation and fitting of theveneers. Cervically it may be necessary toextend the preparation into the gingival embra-sure to mask discoloured tooth substance in theproximal zone immediately above the interden-tal papilla. Care must be used not to create anundercut preparation in this area to the proposedpath of insertion of the restoration.

It is usually necessary to trim the proximalfinish line with a chisel to avoid the sharp lip ofenamel that often results from being unable totake the bur to the very edge of the preparation.In this respect it is better to change to a smallerdiameter bur to prepare the proximal margins ofa single tooth being veneered to permit limitedtooth reduction without damaging the adjacenttooth.

Cervical finish linesThe cervical finish lines for a veneer should be achamfer with about a 0.4 mm maximum depth.

The rounded internal line angle will help toreduce stresses in the margin of the veneer thatmay otherwise develop during firing. Also,porcelain will adapt more readily to this shapeduring manufacture. The finish line should liejust at the crest of the free gingival margin,unless the veneers are being used to mask severestaining when greater sub-gingival extensionmay be required for aesthetic reasons. This posi-tion for the gingival extension of the veneerusually gives the best compromise between aes-thetic control of the finished restoration and theease with which the clinician can control mois-ture during luting.

It is helpful to have a defined cervical finish-ing margin so that the porcelain technician willbe able to identify clearly the desired extent ofthe veneer. However, there is a tendency for thecervical margins of finished veneers to be over-bulked to give greater durability during clinicalhandling. These margins should therefore bethinned as well as finished after luting.

Coping with pre-existing restorationsSome teeth that require veneers will have exist-ing composite resin restorations in place. Thereare two ways to deal with this:

• Bond to a prepared composite resin surface.This is difficult, particularly if the compositerestoration has been in place for any length oftime. Water sorption, exposed un-silanatedsurfaces of filler particles and limited opportu-nities for further polymerisation of the resincomponent of the set material all contribute toa reduced bond strength.

• Replace the restoration. This can be done rela-tively easily, but should be done at the visitwhen the veneer is luted to the tooth so thenew composite has the best chance of bondingto the porcelain veneer as well as the tooth tis-sue. This makes the procedure for bonding theveneer more complex. The old restorationneeds to be removed before the veneer isattached to the tooth. The veneer is then lutedin place using the requisite bonding systemand subsequently the composite resin restora-tion replaced in the same manner as whenplacing a conventional Class III or IV compos-ite filling. It can be difficult to avoid produc-ing overhanging margins using this tech-nique, so care is required to ensure that anysuch overhangs are identified and eliminated.

One of the causes for failure that Dunne andMillar5 identified was that veneers were attachedto pre-existing restorations. It would seem sensi-ble to replace such restorations at the time ofveneer placement to reduce this as a possiblecause of early failure of the veneer. Alternative-ly, if there are extensive restorations present itmay be more sensible to provide a crown.

Recording an impressionImpression technique and soft tissue handlingare dealt with elsewhere in this series, so we willnot go into great detail here. However it is

Fig. 4 The torpedo bur can also beused to create the gingival bevel

Fig. 5 A flat plastic instrumentprotects the gingivae duringfinishing of the chamfer with amulti-fluted tungsten carbide bur. A speed accelerating handpieceprovides greater control than anairotor

PRACTICE


appropriate to use short sections of retractioncord around the margins of the preparations tofacilitate the capture of both the finishing edgeof the preparation and the adjacent area ofunprepared tooth. Electro-surgery is best avoid-ed because of the risk of gingival recessionrevealing the veneer margin.

An impression of the opposing arch is indis-pensable if the incisal edges of the veneers areinvolved in guidance.

Laboratory prescription and manufactureAgain, communication with your technician andachieving maximum aesthetics is covered else-where in this series. Of particular importance inrelation to veneers is careful shade selection,especially if you are planning to modify thecolour of the tooth. If you intend to attempt tomodify the shade of the veneer with the lutingagent then it is sensible to ask the technician toprovide space for the luting resin using a propri-etary die-spacing system but bear in mind thatthe porcelain should not be so thin that there is arisk of it being cracked by the thick compositelute.21 In addition, if a diagnostic wax-up hasbeen used to demonstrate a modification inanterior aesthetics then this should be sent to thelaboratory as well. It can also be beneficial tosend a study cast of the teeth prior to prepara-tion if one is available should you want to pre-serve the original tooth form.

There are a variety of methods for manufac-ture of porcelain veneers using either a refracto-ry die material, a platinum matrix laid down ona conventional working model or one of thecastable ceramic materials prepared using thelost wax technique. Sim and Ibbetson29 haveshown that the best quality of marginal fit wasobtained with a platinum foil system, followedby a refractory die and that the worst fit wasassociated with cast glass restorations.

It is best to ask your laboratory for the veneerto be etched with hydrofluoric acid but not toapply the silane-coupling agent. These agentsneed to be applied just prior to luting the veneerin place (whether or not the laboratory hasapplied silane) and are provided in most com-mercially available resin luting kits. Too early anapplication of a coupling agent, or contamina-tion of the coupling agent coated surface prior tobonding can reduce the strength of the attach-ment between resin and veneer. Also, two com-ponent silane systems must not be kept aftermixing as the silane polymerises to an unreac-tive polysiloxane, again with a reduction inbond strength.7

Provisional restorationsIt is difficult and time-consuming to provideprovisional restorations for teeth prepared forporcelain veneers. It is often best simply to leavethe teeth in their prepared state providing thepatient is aware that this is going to happen andthe teeth are not sensitive.

A variety of techniques have been describedfor placement of provisional restorations if they

are required. These include directly placed com-posite resin veneers and producing a transparentmatrix from a thermoplastic material to allowmultiple composite veneers to be made simulta-neously.30 Such provisional restorations need tobe attached to the enamel surface and the onlypractical way to do this is using the acid etchtechnique. Obviously, only a very small area ofenamel in the centre of the preparation shouldbe spot-etched to provide attachment for thecomposite resin, which can then be removedeasily during the next visit without damagingthe periphery of the preparation. It is best toavoid the margins of the preparations whendoing this with spot etching at the centre only.

Provisional restorations should be made withcare, avoiding gingival excess. Any such excesswould cause gingival irritation whilst theveneers are being made and may result in analteration of the position of the gingival marginor cause difficulty with bleeding during luting.

Provisional restorations are useful when youplan to alter the position of the teeth usingveneers. The diagnostic wax-up can be used toprepare a thermoplastic matrix. This matrix isthen used to make composite resin veneersdirectly in the mouth. This will allow the patientto experience the planned changes to their teethat first hand and to approve the change in theirappearance before the definitive restorations aremade, avoiding a potential cause for grievance.

Trial placement. The veneers should bereturned from the laboratory in a foam-linedbox rather than on the working model of thepatient. It is important that neither you nor thelaboratory place the etched veneers back on thestone dies. Any contact between the etchedporcelain surface and dental stone will result inabrasion of the stone model and some stone dustbecoming trapped in the delicate veneer surface.Swift et al.31 have shown that such contamina-tion results in a substantial fall in the bondstrength between veneer and resin. They alsofound that it was very difficult to clean anetched porcelain surface that has been contami-nated with dental stone.

Handling porcelain veneers can be difficult;they are small and delicate. There are commer-cially available devices to help with this, eitherin the form of a tiny suction cup or a small rodwith a tacky resin at one end. Alternatively a lit-tle piece of ribbon wax on the end of an amal-gam plugger makes a useful substitute.

Check the quality of fit and gingival extensionof the veneer against the tooth, which shouldhave been cleaned with pumice in water prior tothe trial. Once you are happy that the quality of fitis acceptable, the next stage is to assess the colourmatch. The colour of a porcelain veneer cannot beassessed if the veneer is simply placed on the sur-face of the tooth. Much of the overall colour forthe final restoration comes from the tooth struc-ture, so a colour-coupling agent is neededbetween the tooth and the veneer (Fig. 6).

In its simplest form water will allow thecolour of the tooth to be expressed through the

PRACTICE


veneer and give a reasonable guide to the overallappearance if a neutral coloured luting resin isto be used.

If it is necessary to try to modify the colour ofthe finished restoration using the luting resinthen the veneer must be tried in with an appro-priate colour of paste. This can either be the lut-ing agent itself or, alternatively, some manufac-turers provide trial pastes that will not set buthave similar optical properties to the luting resin.If the definitive luting resin is used great caremust be taken to ensure that the resin does not setunder the action of the operating light and thatthe paste used for the trial is removed completelyfrom the veneer. Some manufacturer’s trialpastes are water-soluble making their removalrelatively straightforward. However if a resinbased trial paste is used this needs to be removedusing an organic solvent. Swift et al.31 found thatacetone, an agent that has often been suggestedand used for this role, produces markedlyreduced bond strengths. A more acceptable alter-native would be ethanol. Once the quality of fitof the veneer and the shade of the luting pastehave been assessed the etched surface of theporcelain should be coated with a silane couplingagent, following the manufacturer’s instructionsfor the product chosen.

Luting the veneerHaving established that the veneer is of anappropriate shade, or can be modified with a lut-ing resin to be satisfactory, it is ready to be bond-ed to the supporting enamel and dentine. In viewof the reported high prevalence of dentine expo-sure on veneer preparations25 we would advisethe routine use of a dentine bonding agent.

Good moisture control is critical if adhesivetechniques are to work. Contamination of eitherthe prepared tooth surface or the veneer fitting

surface with saliva, blood or crevicular fluid willresult in reduced bond strengths. Moisture con-trol can be achieved using rubber dam, butwhere this is impractical gingival retraction cordcan be a helpful adjunct to prevention of con-tamination from the gingival crevice. The resid-ual enamel should be cleaned using pumice andwater and then etched, washed and dried. Detailsof technique will not be given here, as they areso material specific. It can be tempting to cutcorners during this stage, but it is vital that youuse the luting resins as the manufacturer sug-gests to achieve optimal results.

Having applied the dentine/enamel bondingsystem the veneer should be loaded with lutingresin and located on the surface of the tooth. Atthis stage the excess of unset resin around theperiphery of the veneer can best be removedusing a metal instrument (Fig. 7) or a brushdipped in unfilled resin. Removal needs to beundertaken with care, as there is a tendency forresin to be pulled out from the periphery of thelute space leaving sub-margination, particularlyif dental floss is used.

Once all gross excess is removed the lutingresin can be cured using a visible light activationunit. It is essential to ensure an adequate exposureto cure fully the luting resin through the porcelainveneer. Most manufacturers’ guidelines suggest30–40s cure times. This is inadequate withresearch suggesting that 60s is more realistic.32–34

Light-activation of a luting agent through anopaque veneer or one of greater than 0.7 mmthickness, is not adequately effective.35 In thesecircumstances a dual-curing resin, which is ini-tiated both by admixing the pastes to give achemical set and by visible-light activation,should be used. Such dual-cure agents shouldnot be used on thinner veneers as they do notpolymerise as effectively as a visible-light acti-vated equivalent alone and may be susceptibleto colour change with time as a product of theresidue of the chemical initiating system.36

Post placement finishingThe final stage for any restoration is finishing themargins of the restoration and any functionalcontacts to give a smooth and harmonious transi-tion from tooth to restoration. It is particularlyimportant to eliminate any occlusal interferences.The finishing process for porcelain veneersinvolves using small particle size diamond burs ormulti-fluted tungsten carbide burs in either anairotor or a speed accelerating handpiece (Fig. 8).Burs are available in a variety of grit sizes to pol-ish the margins progressively and ideally shouldbe followed by the use of 10 mm particle size dia-mond polishing paste to maximise the lustre onthe porcelain and the cement lute (Fig. 9). Finish-ing can also be achieved using rotating abrasivedisks that are available for composite resinrestorations (eg Soflex discs, Super-Snaps etc).

When finishing the gingival extent of theveneer it is sensible to protect the gingival tis-sues using a flat plastic instrument. You willwant to show the finished result to your patient,

Fig. 6 The advantages of using acolour-coupling agent are seen here.The veneer at UL1(21) looksrelatively translucent (try-in pasteused) compared with the opaqueUR1(11) (no try-in paste used)

Fig. 7 Prior to curing the lutingagent, excess material must beremoved from the margin of theveneer and the gingival crevice.Note the small sections ofMelamine® matrix used to preventthe luting agent from bonding teethtogether

PRACTICE


who will not be impressed if the gingival tissuesare lacerated and bleeding! Many operators pre-fer to delay the detailed finishing until a subse-quent appointment at which time any excessmaterial is much easier to identify.

NON-STANDARD VENEERSVeneers are generally prescribed for the buccalaspects of maxillary anterior teeth, but there area number of ‘non-standard’ applications. Theseinclude veneers for:

• The palatal/lingual aspect of teeth which havebeen worn or fractured

• Diastema elimination using slips restricted tothe proximal aspects of teeth

• Lower incisors• Posterior occlusal onlays

All of these applications require some carefulthought to ensure a satisfactory result.

Palatal veneersThere are two main problems with palatalveneers.

Firstly, it is not possible to adjust the occlusalcontacts on the veneer until it is luted in place.This will inevitably result in the need to adjustporcelain in situ. When this is required it isessential that the adjusted porcelain surface bepolished with graded abrasives, culminating indiamond paste, to ensure that the opposing teethare not subject to excessive wear from rough-ened unglazed porcelain.

Secondly, the finish line for such veneers oftenextends onto the buccal surface of the tooth. Itcan be very difficult to disguise that line as theresin luting agent can prove highly visible at thejunction between porcelain and tooth (Fig. 10).One option is to try to hide the finish line as muchas possible. There are three ways to improve this:• Never make the finish line a straight line. The

human eye is very good at identifying straightlines, but is less good at seeing wavy lines. Ifthe finish line is made serpentinous, using thenormal anatomy of the tooth to rise over themamelons and dip between them, it becomesmore difficult to see (Fig. 11).

• Extend the finish line over onto the buccalsurface of the tooth significantly. Then askyour technician to gradually increase thequantity of translucent porcelain in the over-lapping section so that more and more colourfrom the restoration is drawn from the toothand less and less from the veneer. This avoidssudden change in optical properties betweentooth and porcelain restoration. (Figs 10,11)

• Use a luting agent that is colour neutral withthe tooth so that it blends as much as possible.

Lateral porcelain slipsThere are once again two problems with this sortof porcelain addition, commonly used to obliter-ate a diastema between teeth.

• Care must be taken to avoid a bulky gingivalemergence profile. It is not acceptable to pro-

duce artificial overhangs that are not cleans-able and are liable to act as plaque traps.

• The junction between porcelain and toothshould be disguised. This is best hidden withinthe natural anatomy of the tooth by placingthe finish line within the intermamelongroove closest to the addition and by using thesame concepts as above to blend tooth andporcelain. In this circumstance it may be pos-sible to have a straight finish line, at worst itmimics a crack on the crown surface.

An alternative is simply to extend the veneerover the whole buccal surface with an appropri-ate extension into the proximal space.

Veneers for mandibular incisorsMandibular incisors can be managed withporcelain veneers but the preparation usuallyhas to be extended over the incisal edge of thetooth, particularly if the tooth is in functionalcontact. The incisal coverage of porcelain has tobe sufficiently thick to be durable under contin-uing rubbing contact with the opposing tooth.This would necessitate incisal edge reduction bybetween 0.75 and 1 mm. Obviously if the tooth isnot temporised there is a risk of over-eruption of

Fig. 8 Finishing the gingival marginof the veneer with a small particlesize diamond bur in a speedaccelerating handpiece. Once again,a flat plastic instrument protects thegingival tissues

Fig. 9 Final polishing of the gingivalmargin of the veneer using a rubbercup and diamond polishing paste

Fig. 10 Unsightly, porcelain-toothjunctions at the incisal overlap ofthe palatal surface veneers at UL1,UL2 and UL3 (21, 22 and 23). Thejunctions are clearly visible due tothe abrupt change in opticalcontrast and the straight finish line

PRACTICE


the opposing tooth or the prepared tooth in theperiod between preparation and fit of theveneers. Space maintenance is best provided byadding composite resin to the palatal surface ofthe upper teeth to produce a stable occlusal stopbetween the upper and lower arches, rather thanrisking damage to the prepared enamel surfaceof the lower.

Posterior occlusal onlaysPorcelain occlusal onlays (sometimes termed‘shims’) have the potential to offer an elegantand aesthetic means of reconstructing theocclusal surfaces of worn or broken-down teeth.However, despite the initial enthusiasm of someauthors37 the results with traditional aluminousor feldspathic porcelains have proved disap-pointing. We have experienced both debondingand fracture, especially when restorations areattached mainly to dentine, and subject to therigors of bruxism. It may be that one or more ofthe high strength ceramics would be suitable forthis purpose,38 but it is too early to make anyfirm recommendations.

CONCLUSIONSPorcelain veneers are a useful adjunct to thearmamentarium of the dentist to help in themanagement of aesthetic problems in patients,both young and old. Care needs to be taken dur-ing tooth preparation and particularly duringthe luting phase to ensure maximal results areobtained for the patient.

1. DPB. Dental Data Digest of Statistics. Dental Practice Board,1996.

2. DPB. Dental Data Digests of Statistics for 1997,1998,1999,2000 and 2001. Dental Practice Board, 2001.

3. Pincus C. Building mouth personality. Alpha Omega 1948;42: 163-166.

4. Horn H R. Porcelain veneers. Dental Clinics of North America1983; 27: 671-684.

5. Dunne S M, Millar B J. A longitudinal study of the clinicalperformance of porcelain veneers. Br Dent J 1993; 175: 317-321.

6. Guidelines. Restorative indications for porcelain veneerrestorations. In: Gregg T, editor. Faculty of Dental SurgeryNational Clinical Guidelines. London: Faculty of DentalSurgeons of Royal College of Surgeons of England, 1997.

7. Peumans M, Van Meerbeek B, Lambrechts P, Vanherle G.Porcelain veneers: a review of the literature. J Dent 2000; 28:163-77.

8. Walls A W G, Murray J J, McCabe J F. Composite laminateveneers: a clinical study. J Oral Rehabil 1988; 15: 439-454.

9. Rucker L M R W, MacEntee M, Richardson A,. Porcelain andresin veneers clinically evaluated: 2-year results. J Am DentAssoc 1990; 121: 594-596.

10. Harley K E, Ibbetson R J. Anterior veneers for the adolescent

patient: 1. General indications and composite veneers. DentUpdate 1991; 18: 55-6, 58-9.

11. Welbury R R. A clinical study of a microfilled composite resinfor labial veneers. Int J Paed Dent 1991; 1: 9-15.

12. Clyde J, Gilmoure A. Porcelain veneers: a preliminary review.Br Dent J 1988; 164: 9-14.

13. Strassler H, Nathanson D. Clinical evaluation of etchedporcelain veneers over a period of 18-42 months. J AesthetDent 1989; 1: 21-28.

14. Nordbo H, Rygh-Thoresen N, Henaug T. Clinical performanceof porcelain laminate veneers without incisal overlapping: 3-year results. J Dent 1994; 22: 342-345.

15. Peumans M, Van Meerbeek B, Lambrechts P, Vuylsteke-Wauters M, Vanherle G. Five-year clinical performance ofporcelain veneers. Quintessence Int 1998; 29: 211-221.

16. Kihn P, Barnes D. The clinical evaluation of porcelain veneers:a 48-month clinical evaluation. J Am Dent Assoc 1998; 129:747-752.

17. Fradeani M. Six-year follow-up with Empress veneers. Int JPeriodont Rest Dent 1998; 18: 216-225.

18. Dumfahrt H, Schaffer H. Porcelain laminate veneers. Aretrospective evaluation after 1 to 10 years of service: PartII—Clinical results. Int J Prosthodont 2000; 13: 9-18.

19. Christensen G, Christensen R. Clinical observations ofporcelain veneers. J Aesthet Dent 1991; 3: 174-179.

20. Walls A W. The use of adhesively retained all-porcelainveneers during the management of fractured and wornanterior teeth: Part 2. Clinical results after 5 years of follow-up. Br Dent J 1995; 178: 337-40.

21. Magne P, Kwon K R, Belser U C, Hodges J S, Douglas W H.Crack propensity of porcelain laminate veneers: A simulatedoperatory evaluation. J Prosthet Dent 1999; 81: 327-334.

22. Calamia J R. Materials and techniques for etched porcelainfacial veneers. Alpha Omega 1988; 81: 48-51.

23. Garber D. Traditional tooth preparation for porcelainlaminate veneers. Comp Cont Ed Dent 1991; 12: 316, 318,320, 322.

24. Harley K E, Ibbetson R. Anterior veneers for the adolescentpatient: 2 Porcelain veneers and conclusions. Dent Update1991; 18: 112-116.

25. Nattrass B R, Youngson C C, Patterson C J, Martin D M, RalphJ P. An in vitro assessment of tooth preparation for porcelainveneer restorations. J Dent 1995; 23: 165-170.

26. Hui K, Williams B, Davis E, Holt R. A comparative assessmentof the strengths of porcelain veneers for incisor teethdependent on their design characteristics. Br Dent J 1991;171: 51-55.

27. Meijering A C, Creugers N H, Roeters F J, Mulder J. Survival ofthree types of veneer restorations in a clinical trial: a 2.5-year interim evaluation. J Dent 1998; 26: 563-568.

28. Calamia J. Materials and techniques for etched porcelainfacial veneers. Alpha Omega 1988; 81: 48-51.

29. Sim C, Ibbetson R J. Comparison of fit of porcelain veneersfabricated using different techniques. Int J Prosthodont1993; 6: 36-42.

30. Raigrodski A J, Sadan A, Mendez A J. Use of a customizedrigid clear matrix for fabricating provisional veneers. J EsthetDent 1999; 11: 16-22.

31. Swift B, Walls A W, McCabe J F. Porcelain veneers: the effectsof contaminants and cleaning regimens on the bondstrength of porcelain to composite. Br Dent J 1995; 179:203-208.

32. Strang R, McCrossan J, Muirhead M, Richardson S. Thesetting of visible light cured resins beneath etched porcelainveneers. Br Dent J 1987; 163: 149-151.

33. Warren K. An investigation into the microhardness of a light-cured composite when cured through varying thicknesses ofporcelain. J Oral Rehabil 1990; 17: 327-334.

34. O’Keefe K, Pease P, Herren H. Variables affecting the spectraltransmittance of porcelain through porcelain veneersamples. J Prosthet Dent 1991; 66: 434-438.

35. Linden J J, Swift E J, Jr., Boyer D B, Davis B K. Photo-activationof resin cements through porcelain veneers. J Dent Res 1991;70: 154-157.

36. Berrong J M, Weed R M, Schwartz I S. Color stability ofselected dual-cure composite resin cements. J Prosthodont1993; 2: 24-27.

37. Mabrito C, Roberts M. Porcelain onlays. Curr Opin CosmeticDent 1995: 1-8.

38. Denissen H, Dozic A, van der Zel J, van Waas M. Marginal fitand short-term clinical performance of porcelain-veneeredCICERO, CEREC, and Procera onlays. J Prosthet Dent 2000;84: 506-13.

Fig. 11 Here the incisal overlap hasbeen hidden. The palatal veneers onUR1 (11) and UL1 (21) have hadtheir incisal margins extended some2mm onto the buccal surface of thetooth with a gradual increase intranslucency of the porcelain toimprove colour transmission. Also,the finish line is serpentinous

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BRITISH DENTAL JOURNAL VOLUME 193 NO. 3 AUGUST 10 2002 135

Crowns and other extra-coronal restorations:Resin-bonded metal restorationsA. W. G. Walls1 F. S. A. Nohl2 and R. W. Wassell3

Resin-bonded metal restorations is the final part of the series. Cast metal restorations which rely on adhesion for attachmentto teeth are attractive because of their potential to be much more conservative of tooth structure than conventional crownswhich rely on preparation features providing macromechanical resistance and retention.

1Professor of Restorative Dentistry,3Senior Lecturer in Restorative Dentistry,Department of Restorative Dentistry, TheSchool of Dental Sciences, FramlingtonPlace, Newcastle upon Tyne NE2 4BW2Consultant in Restorative Dentistry, TheDental Hospital, Richardson Road,Newcastle upon Tyne NE2 4SZ*Correspondence to: Prof A. W. G. Walls,Department of Restorative Dentistry, TheDental School, University of Newcastleupon Tyne, Newcastle upon Tyne NE2 4BWE-mail: [email protected]


● The indications for the management of worn teeth, in occlusal management, and followingmolar endodontics

● Design and tooth preparation for anterior and posterior teeth● Clinical procedures, including management of existing restorations and bonding● Problems with aesthetics and temporisation● Maintenance and, where necessary, rebonding

I N B R I E F

The past 25 years have witnessed great stridesin the development of adhesive dentistry.Many would say that our day-to-day practicehas been influenced more fundamentally bythese advances than by any other recent dentalinnovation.

One of the advances is the introduction ofresin-bonded metal restorations (RBMR). Anteri-orly, the RBMR resembles a retentive wing of aresin-bonded minimal preparation bridge. Pos-teriorly, the RBMR (sometimes termed a shim)resembles a conventional metal onlay but usual-ly without box forms or other mechanical meansof providing retention. Sometimes however,where pre-existing restorations have beenreplaced as part of the RBMR, the distinctionbetween onlay and shim becomes blurred; theonly differentiation being the means of cemen-tation which has evolved from the technologydeveloped for resin-bonded bridgework.

The need to remove tooth substance toachieve mechanical resistance and retention ofworn teeth for conventional crowns would seemparticularly counterproductive: RBMR can offerconservative solutions in this situation. A num-ber of case reports and two surveys1,2 haveappeared documenting and supporting the use ofRBMR on the palatal aspects of worn maxillaryanterior teeth (Fig. 1). The use of RBMR on theocclusal surfaces of posteriors has also beendescribed (Fig. 2).2–4 However, there are no well-controlled clinical studies of the long-term suc-cess of RBMR in comparison with conventionalrestorations. Other uses have included produc-tion of rests for the support of partial dentures5

and restorations to alter the morphology ofocclusal holding and guiding surfaces ofcanines.6

The ability to bond cast metal alloys to teeth

was first demonstrated clinically by Rochette in1973.7 His periodontal splint was made of castgold and retained macromechanically by com-posite resin extruded through countersunk per-forations in the metalwork. Tooth surfaces wereetched with acid to provide micromechanicalretention for composite resin cement. The majorbreakthrough was that other than etching nodestructive tooth preparation was required.

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restorations

Fig. 1 Occlusal view of anterior palatal resin-bondedmetal restorations

Fig. 2 Posterior resin-bonded metal restoration

PRACTICE

136 BRITISH DENTAL JOURNAL VOLUME 193 NO. 3 AUGUST 10 2002

As the adhesive minimal preparation bridgebecame commonplace, methods of modifyingbase metal alloys were developed to improveadhesion of the retainers to tooth substance via aresin-based cement. One technique was to incor-porate irregularities into the fitting surface ofthe retainers during pattern formation, whichwere subsequently reproduced in metal; thesetook the form of voids left after the wash out ofsalt crystals, spheres or meshwork, but had thedisadvantage that castings were bulky and thelaboratory technique was exacting. Microscopicetch patterns in the fitting surface of bridgeretainers greatly increase the surface area forcontact with luting agents and can be producedby electrolytic corrosion in an acidic environ-ment. Again this approach was technique sensi-tive but could produce reliable attachmentbetween metals and resin.8 Base metal retainerscan also be air abraded with alumina particlesthat as well as increasing the surface area mayenhance the bond with some cements by chemi-cal interactions.9

Lesser demands on rigidity with single unitrestorations enabled the use of precious metalalloys (type III gold [ADA classification]) ratherthan the nickel based alloys used in adhesivebridgework. This gives advantages in castingaccuracy, ease of adjustment and finishing, thepotential for reduced wear of opposing teeth andperhaps of appearance. Several precious metalsurface treatments have been documented.These include tin plating,9 heat treatment ofhigh copper content gold alloys,10 air abrasionof the cast metal surface,2,10 and the Silicoater.11

Air abraded base metal luted to etched enamelusing two chemically active cements gave high-er bond strengths in-vitro than precious metalalloy/surface treatment combinations.9 Howev-er, tin plating or heat treating air abraded pre-cious metal alloys gave enhanced bondstrengths in-vitro compared with this alloy airabraded alone.10 Clinically, air abraded nickel-chromium anterior RBMRs cemented withPanavia Ex gave a survival probability of 0.74 at56 months,1 and air-abraded gold RBMRs (ante-rior and posterior), also cemented with PanaviaEx, were associated with a survival probabilityof 89% at 60 months.2 However it cannot beassumed that because a metal surface treatmentworks with one cement that it will necessarily beeffective with others.

INDICATIONS

In the management of worn teethRBMR can protect worn and vulnerable toothsurfaces from the effects of further wear byforming a barrier against mechanical and chem-ical insults.

Any technique, which could delay entry intoa restorative spiral necessitating ever enlargingrestorations with endodontic implications, is tobe welcomed. Although RBMR are susceptible todebonding, marginal recurrent caries and mar-ginal lute wear, the fact that little if any tooth

preparation has been carried out prior to place-ment means that cumulative insults to the pulpare likely to be less than when conventionalrestorations have been placed (assuming that thebonding process to dentine is not damaging tothe pulp!).

Central to the provision of RBMR are tech-niques to create occlusal space for the restora-tion; suffice it to say that non-preparation tech-niques, such as the Dahl approach,11 involvingcontrolled axial movement of teeth are attrac-tive. In this approach teeth are built-up to causetheir intrusion and the supra-eruption of otherstaken out of occlusion. This topic is summarisedin Part 3 of this series. However, it is worthemphasising that the build-up must result inaxial loading. Non-axial loading, resulting froma deflective contact or interference on the build-up, can cause problems such as pain and toothmobility.

In occlusal managementRBMRs are made in the laboratory using the lostwax casting technique. In conjunction with thedental technician, the dentist has good controlover form of occlusal surfaces of RBMRs, whichcan be used therefore to create occlusal stopsand guiding surfaces with a high degree of pre-cision. RBMRs are particularly helpful whensuch teeth are unrestored and where the alterna-tive of conventional crowns would be unaccept-ably destructive.

A drawback of the technique is that the newguidance surfaces cannot be tested using provi-sional restorations as with conventional crowns.Guidance surfaces therefore need to be carefullyformed with the use of a semi-adjustable articu-lator and the dentist must accept that someadjustment may be required after the RBMRshave been cemented.

Following molar endodonticsMany posterior teeth which have been roottreated are at risk of fracture and will benefitfrom a protective cusp covering castrestoration.13 A RBMR with occlusal coveragecan provide a conservative restoration for atooth already compromised by the need forendodontic access.

TECHNIQUES

Choice of metalIf facilities do not exist to heat treat or tin plategold after try-in, it may be more sensible to useair abraded nickel-chromium, accepting that itsshade may look less harmonious in the oralenvironment than yellow gold.

Design and tooth preparation: anterior teethVery thin portions of unsupported buccal enam-el remaining on some worn maxillary anteriorteeth are highly vulnerable to damage on a stonemaster cast resulting in a casting which will notfit the tooth. Such enamel should be removedprior to making the impression and defects

PRACTICE


waxed-up on the master cast before building uppatterns for RBMR (Fig. 3). After cementation,composite resin can be packed against the RBMRto replace lost buccal enamel. The latter tech-nique can also be used to restore pre-existingbuccal tooth defects. No other tooth preparationis required for anterior palatal RBMR.

To optimise adhesion, the maximum possiblepalatal tooth surface should be covered by theRBMR. Type III gold should probably be a mini-mum of approximately 0.5 mm thick though itmay be reasonable to use nickel-chromium inthinner section because it is more rigid. It is nec-essary to incorporate features that aid accuratelocation during seating of the RBMR. A layer ofmetal overlying the whole of the incisal edge ofanterior teeth facilitates accurate seating andfollowing careful thinning can often be left inplace without affecting appearance significantly(Fig. 4). In function, the latter feature should alsoreduce the likelihood that opposing tooth con-tacts will act directly on the cement layer tocause shear failure.

Design and tooth preparation – posterior teethThe occlusal part of the restoration must be ableto withstand functional forces and, in theabsence of evidence to the contrary, dimensionsfor gold alloy RBMR should follow those recom-mended for conventional cusp covering crowns(see Part 6). As with anterior RBMRs it may bepossible to reduce these dimensions when usingnickel-chromium because it is more rigid.Occlusal preparation should only be performedwhere the treatment plan indicates that occlusalspace for the restoration is required (see Part 3).

Whereas conventional crowns are designedso that non-axial forces tend to put the cementlayer in compression (non-adhesive cements arebest able to resist compression), RBMR rely sub-stantially on their adhesive luting agent to resisttensile and shearing forces. Axial preparation isuseful in as much as it will facilitate accurateorientation on the tooth during bonding. Such apreparation would have an axial reduction ofapproximately 0.5 mm depth extending downthe axial surfaces by one millimetre or so, termi-nating on a chamfer margin (Fig. 5). Axialpreparation will also give the advantage ofincreasing the surface area for bonding to etchedenamel. To what extent axial preparation willhelp resist peel and shear forces is currentlyunclear.

Managing existing restorations RBMR rely primarily on adhesion to enamel forretention. Existing restorations which accountfor a large proportion of the surface area avail-able for adhesion but also extend beyond theperiphery of the RBMR are not be ideally suitedto restoration using this technique. The criticalfactor is to finish the margin of the restorationon enamel if at all possible.

Anterior teethRestorations deemed to be in need of replace-ment involving the labial surface of anteriorteeth can be managed in several ways:

1. Replace prior to impressions. However by thetime the RBMR is cemented the surface willbe waterlogged and may only offer sub-opti-mal bonding to chemically active cements.Use of an intra-oral air abrasion device onthe plastic restoration may be beneficial.

2. Remove prior to cementation of RBMR andpack fresh composite against either the trial

Fig. 3 Enamel defects waxed up on master cast prior toconstructing restorations

Fig. 4 Appearance of metal incisal coverage because ofanterior palatal resin-bonded metal restorations

Table I Precautions for intra-oral air abrasion

Alumina particles are hazardous if inhaled, can scratch glass (eg spectacle lenses) and can leave patientsfeeling like they have a mouthful of sand. To avoid these problems:

1. Use rubber dam where possible.

2. Pack-off area around tooth with wrung out wet paper towels (alumina will stick to towel — notrebound).

3. Cover patient’s whole face including spectacles, with wet paper towels. Fold to allow patient tobreathe from beneath towel.

4. Dentist and nurse must wear masks and eye protection.

5. Use high volume aspiration.

Fig. 5 Preparation form for posteriorresin-bonded metal restoration

PRACTICE


seated or cemented RBMR. Placement ofcomposite against the cemented RBMR canmake the job of shade matching easier thanwhen replacement is carried out prior toimpressions because opaquers and appropri-ate shades of composite can be used over themetal.

Posterior teethSome caution is required in relation to existingrestorations that will be completely covered bythe RBMR, as they may not offer as great a bondto chemically active cements as etched enamel.Much will therefore depend on the area of enam-el available for bonding. Strategies to manageexisting restorations, which will be completelycovered by the RBMR, would include:

1. Leaving the restoration undisturbed. In thiscase it may be best to assume that the oldrestoration offers no additional retention.An example for this approach would be asmall sound restoration surrounded by agood periphery of enamel.

2. Air abrading the surface of existing restora-tions with the aim of providing microme-chanical retention for the resin cement(Table 1).

3. Replacement of an existing amalgamrestoration with GIC to facilitate bonding.

4. Removal of whole or part of the restorationwith the aim of providing a retentive intra-coronal feature on the fit surface of theRBMR and exposing tooth structure forbonding. The resulting preparation willresemble that for a conventionally cementedonlay incorporating box forms, bevels andflares.14 However, removal of old restora-tions may be associated with unnecessarydamage to the tooth and where necessaryundercuts should be blocked out with glassionomer cement.

RecordsImpressions for the laboratory fabrication ofRBMR should meet the same quality criteria asfor conventional crowns (see Part 10 of thisseries). Anterior palatal wear often spares a rimof enamel in the proximity of the gingivalcrevice which should be captured by the impres-sion as it may enhance adhesion significantly. Itis helpful to use a gingival retraction techniqueto achieve this.

Fabrication can be carried out by investingand casting a pattern which has been liftedfrom the master cast or by forming the patternfor the restoration on refractory materialwhich is itself incorporated within invest-ment.

BondingAlthough occlusal adjustments are more easilypolished if carried out before the RBMR isattached to the tooth, stabilising the restorationsufficiently to analyse occlusal contacts can bedifficult. A small amount of paraffin jellysmeared onto the fitting surface of the RBMRcan provide some retention but needs to beremoved completely before bonding.

The fitting surface should ideally be airabraded and steam or ultrasonically cleanedbefore cementation. Gold alloy RBMR are heattreated at this stage. A brief cycle in a porce-lain oven is required (400°C for 4 minutes inair). Despite the colour of the oxidised alloy, nofurther polishing should be carried out untilafter the restoration has been cemented as toattempt this risks contaminating the all-important oxide layer developed in the heattreatment (Fig. 6).

In the past there has been concern that thequality of bonding of the chemically activecements advocated for RBMR may be affectedby the presence of eugenol.15 However anotherstudy16 indicates that eugenol containing tem-porary cements have no adverse effect on theshear bond strength of a dual-curing lutingcement to enamel although there may be aneffect if a composite core is used.

Several chemically active cements are avail-able to bond RBMR: the same cement as wouldbe chosen for adhesive bridgework. Manufac-turers instructions for handling the chosenchemically active cement must be followedclosely: it is the responsibility of the dentist toensure that this is so. Rubber dam is mandatory.Floss ligatures can assist retraction of rubber atthe gingival margins of maxillary anteriorteeth (Fig. 7). Soft wax on the end of an instru-ment can be helpful to carry the RBMR to thetooth but great care must be taken not to smearwax onto the fitting surfaces.

After attaching a RBMR to the tooth,removal of excess cement, occlusal adjust-ments and polishing can be achieved with handscalers and a sequence of rotary instruments(Table 2). Care must be taken not to overheatthe restoration or the resin cement will be soft-ened and the RBMR dislodged.

Fig. 6 Appearance of heat-treatedgold just after cementation

Fig. 7 Floss ligatures to facilitateisolation with rubber dam

PRACTICE


PROBLEMS

AppearanceMaxillary anterior teeth, which have beenthinned by wear on their palatal aspects, maytransmit light easily. RBMR luted to the palatalaspects of these teeth may cause a grey coloura-tion that can be unacceptable and is more likelyif non-opaque cement is used. On the otherhand, opaque cements may help disguise metalbut can also cause a lightening in shade. At theinitial assessment it is wise to assess possibleshade change caused by a RBMR and its cement.White modelling clay applied to the palatalaspect of the thin tooth can mimic the effect ofopaque cement. Tin foil burnished onto thepalatal surfaces of teeth to be restored can indi-cate the effect of grey nickel chromium or darkoxidised gold in combination with non-opaquecement.

Showing metal is aesthetically acceptable tosome patients but simply not for others! Yellowgold can look more harmonious in the oral envi-ronment than nickel-chromium. A useful tech-nique is to use an air abrader to reduce thereflectance of the polished RBMR. In our experi-ence the surface produced by air abrasion alsopicks up ink of occlusal marking tape more easi-ly than metal left highly polished.17 A chairsideair abrader for intra-oral use is a ideal for thispurpose but needs to be used with care (Table 1).The advantages of RBMR should be fullyexplained to the patient: the informed patientmay accept this compromise in appearance.

Temporisation of RBMRIn many cases temporary restorations areunnecessary but as with porcelain labial veneersretention can be a problem. These aspects areaddressed in the ninth article in this series.

It is a significant disadvantage that RBMRcannot be reliably attached to teeth for a trialperiod using temporary cement. Glass ionomercement (GIC) may afford easy retrieval (orunplanned loss) in some situations but in othersacts as a final cement!

MAINTENANCEErosion can cause loss of tooth tissue at theperiphery of a RBMR (Fig. 8). This problem mayoccur as a result of not identifying or not con-trolling the aetiology of the patient’s presentingtooth wear. Repair with an adhesive filling mate-rial may however be straightforward, althoughconcern has been raised about the ability of therepairing material to bond to the metal casting.

A RBMR whose lute has failed is more likely

to declare itself by debonding than a conven-tionally retained crown which may stay in placelong enough for the consequences of leakage totake effect. Analysis of the cause of failure for aRBMR may indicate that an attempt should bemade to re-attach it after appropriate cleansingand surface treatments. All traces of old cementshould be removed from the RBMR, whichshould then be handled and treated as new. Anair abrasion device, abrasive discs and ultrasonicscalers are useful in removing cement from thetooth surface. A round diamond bur can be usedwithout water in a turbine or speed increasinghandpiece. The powdery white surface of theinstrumented cement can easily be distinguishedfrom the glossy appearance of instrumentedenamel. Occasionally etching tooth surface canhelp to establish whether or not cement remains:areas not appearing frosty are either dentine orresidual cement. It is important to remove theresin-infiltrated layer in both enamel and den-tine and hence facilitate bonding. Cementremoval must be carried out carefully or changesin tooth shape or fit surface of the RBMR willresult in an increase in lute thickness. Inevitably,repeated attempts at reattachment are increas-ingly likely to fail as the lute thickness rises.

RBMRs linked rigidly together to act as apost-orthodontic retainer or periodontal splint,carry the risk that one or more retainers maydebond leaving the restoration as a wholeattached without causing any initial symptoms.If this happens caries can progress uncheckedbeneath decemented elements with disastrousresults (Fig. 9). Adhesive splints need careful fol-low-up: patients must be instructed to seekattention if they think a tooth has becomedebonded. It is often necessary to remove thewhole restoration and attempt to re-bond it. Asharp tap to a straight chisel whose blade is posi-tioned at the lute space is often sufficient to dis-lodge the cemented portions of an adhesive

Table 2 Suggested sequence of instruments forremoval of excess set cement at periphery of resin-bonded metal restorations

Hand scalers

Fine and very fine high speed diamond burs

Rubber points

Polishing cup and prophy pasteFig. 8 Recurrent erosion at theperiphery of resin-bonded metalrestorations in situ

Fig. 9 Partially de-bonded adhesivemetal splint

PRACTICE


splint. Occasionally it is possible to accept thecompromise of removing a decemented retainerif this is at the end of the restoration. LinkingRBMR should be avoided wherever possible.

CONCLUSIONSRBMR rely for their attachment on chemicallyactive cements. The choice is between preciousmetal and base metal alloys with various surfacetreatments to enhance adhesion with thecement. RBMR have the potential to be veryconservative of tooth tissue but are techniquesensitive. To date few clinical studies existexamining their success.

1. Nohl F S, King P A, Harley K E, Ibbetson R J. Retrospectivesurvey of resin-retained cast-metal veneers for thetreatment of anterior palatal tooth wear. Quintessence Int1997; 28: 7-14.

2. Chana H, Kelleher M, Briggs P, Hooper R.J. Clinical evaluationof resin-bonded gold alloy veneers. J Prosthet Dent 2000;83: 294-300.

3. Foreman P C. Resin-bonded acid-etched onlays in two casesof gross attrition. Rest Dent 1988; 15: 150-153.

4. Harley K E, Ibbetson R J. Dental Anomalies- Are adhesivecastings the solution? Br Dent J 1993; 174: 15-22.

5. Lyon H E. Resin-bonded etched-metal rest seats. J ProsthetDent 1985; 53: 366-368.

6. Thayer K E, Doukoudakis A. Acid-etch canine riser occlusaltreatment. J Prosthet Dent 1981; 46: 149-152.

7. Rochette A L. Attachment of a splint to enamel of loweranterior teeth. J Prosthet Dent 1973; 30: 418-423.

8. Livaditis G J, Thompson V P, Etched castings: an improvedmechanism for resin bonded retainers. J Prosthet Dent 1982;47: 52-58.

9. Dixon D L, Breeding L C, Hughie M L, Brown J S. Comparisonof shear bond strengths of two resin luting systems for abase and a high noble metal alloy bonded to enamel. J Prosthet Dent 1994; 72: 457-461.

10. Eder A, Wickens J. Surface treatment of gold alloys for resinadhesion. Quintessence Int 1996; 27: 35-40.

11. Hansson O. The Silicoater technique for resin-bondedprostheses: clinical and laboratory procedures. QuintessenceInt 1989; 20: 85-99.

12. Dahl B L, Krogstad O, Karlsen K. An alternative treatment incases with advanced localised attrition. J Oral Rehabil 1975:2: 209-214.

13. Sorensen J A, Martinoff J T. Intracoronal reinforcement andcoronal coverage: a study of endodontically treated teeth. J Prosthet Dent 1984; 51: 780-784.

14. Shillingburg H T, Hobo S, Whitsett L D, Brackett S E.Fundamentals of fixed prosthodontics. 3rd ed. pp171-180.Chicago: Quintessence, 1997.

15. Paul S J, Scharer P. Effect of provisional cements on the bondstrength of various adhesive systems on dentine. J OralRehabilitation 1997; 24: 8-14.

16. Jung M, Gnass C, Senger S. Effect of eugenol-containingtemporary cements on bond strength of composite toenamel. OperDent 1998; 23: 63-68.

17. Kelleher M G, Setchell D J. An investigation of markingmaterials used in occlusal adjustment. Br Dent J 1984; 156:96-102.

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