Restorative Dentistry

Glass-ionomer cements in restorative dentistry

John W. Nicholson. PhD*/Theodore P Croll. DDS**

Abstract This anide reviews ihe eurreni sUiHis aiitijiirure prospeclsfor glass-ionomermalerials. Tliese muieriuls are of ¡wo eheuiicai lypes: ihe older, selfhardeiiingcemeiils. which sei by an uviil-base iieiiii-alLalioii reaeHon to give relalivelybri/rle luaierials: and ¡he newer, re.sin-modified cemeiU.s. which se¡partly hypolymerizaiion and panly by neinraUzaiioii. Compared with ¡he self-hardeningcements, ¡he lauer materials have improved esthetics, improved resistance tomoisture, and greater toughness. Both types qfgla.s.s-ionomer cement bond well ¡oenatnel and dentin and release a clinically useful amount of fluoride. Thev havebeen used in a variety of applications: as liners or bases, for luting of stainless steeleroiins. for Class V restorations in permanent teeth, and for Class 11 and Class HIrestorations in primar}- teeth. The resin-modißcd glass-ionomers are particularlypromising for these latter uses, ahhough it is too early to be sure whether ¡heirlong-term dnrabilHy is sufficient. Self-hardening glass-ionomer materials arelikely to retain specific niches of clinical applicaHon. including in their metal-reinforced and cermet-containing fortns. (Quintessence Int }997;2S:705-7¡4.)

Ë^linical relevance

IKS review of the glass-ionomer genre updates theatist about glass-ionomer and polyacid-modified

sin materials, describes their chemistry' and poten-\. and advises the dinician about methods andjionale for their use in restorative and prosthetic

islry.

ad. Departmem of Dental Biomaterials Science. King's DenialitLlute, Universily of London. London. England

pvate Practice, Pédiatrie Dentjstrj', Doylesloivn. Pennsylvania;lical Professor. Departmem ol Pédiatrie Dentistry. University ofnsylvania. School ofDental Medieine; Clinical Professor, Cranio-

fial Growth and Development (Pcdiatrie Dentiitry), University ofas, Health Science Center at Hojston (Dental Branch); Adjunc-! Assistant Professor, Department of Pédiatrie Dentistry. Univer-( of Iowa. Coliege of Dentistry.

|lrequEsts; Dr J. W. Nicholson. Dental Biomaterials Department,•Dental Institute. Caldecot Road, London SES9RW, United

. E-mail: j . nie hols on@kd.ac.Lk.

Introduction

In recent years, there has been considerable confusionabout what type of material should be called aglass-ionomer cement. Strictly, the term should beapplied only to a material that involves a significantacid-base reaction as part of its setting reaction, wherethe acid is a water-soluble polymer and the base is aspecial glass.' Other materials, for example those thatsome manufacturers have marketed as "light-curedglass-ionomers." are essentially resin composites,although they do contain the fluoroaluminosilicateglass of a conventional glass ionomer material. How-ever, these materials lack the characteristic goodadhesion of glass ionomer cements, tend to releaselittle fiuoride. and undergo polymerization contrac-tion on setting.

A further source of confusion has been the develop-ment of materials that set by polymerization but arebased on resins modified to include acid functionalgroups on them and also contain basic glasses. Thesematerials, such as Dyract (Dentsply), Compoglass(Ivoclar), and Hytac (ESPE), show interesting proper-ties, and are promising as restoratives, but are ceriainlynot glass-ionomer materials. The term compomerh^.^been applied to them by the manufacturers, but this

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term has already been incorrectly applied by cliniciansto glass-ionomer-resin hybrid materials that sel sub-stantially by an acid-base reaction.--' The term poly-acid-iiioüißfd resin coiiiposiie has been recommendedlor these materials, ' although Ihe word aimpoiner is auseftil everyday name. Those glass-ionomer malerialsthat are modified by Che inclusion of resin, generally lomake them partly photocurable, are recommended tobe called resin-inodified glass-ionomer materials, theterm used here.

Se If-h arden ing glass-ionomer materials

Glass-ioQomer materials in their original, self-hardeningform, became available in the last quarter of the 20thcentury. They belong to the class of material known asacid-base cemeiiis,* and their setting involves neutrali-zation of acid groups on a waler-soluble polymer whha powdered, solid base. The base is a special calciumaluminosilicate glass that also contains tluoride, animportant feature, because it causes the cement torelease clinically useful amounts of this ion and therebyto prevent the development of secondary caries aroundrestorations.- The glasses act as bases in the sense thatthey are proton-acceptors, even though they are notsoluble in water.

As the cements set, water becomes incorporatedinto the material, and there is no phase separation. Infact, water has been identified as having a number ofroles: (!) it is the solvent for the setting reaction,because, without it. the polymeric acid would beunable to exhibit its full properties as an acid. (2) ilis one of the reaction products. (3) it acts as bothcoordinating species to the metal ions released fromthe glass and as hydrating species at well-defined sitesaround the polyanion, and finally (4) it may act as aplasticizer and reduce the rigidity of the bulk poly-meric structure.*'

A number of factors are known to influence thespeed of the setting reaction and the Tmal strength ofthe cement. These include the molar mass of thepolyacid concentration of the acid solution, thepowder-liquid ratio, and the presence of chelatingagents, such as (+) - tartaric acid,' which reduces thesetting time and increases the compressive strength ofthe cement once set.

Glass-ionomer cements undergo gradual maturationprocesses that are pooriy understood. For example, incements prepared from polyiacrylic acid), compressivestrength gradually rises over the first 3 months or so ofthe cement"s life to a maximum value some magnitude

greater than the value at 24 hours. The ratio of boundto unbound water increases, this being defined as theratio of water that may be removed by chemicaldesiccation ( eg. by storage for 24 hours over silica gelat elevated temperature) to water that is retained in thecement during this treatment. Finally, translucencyalso changes, gradually becoming greater and morelike natural tooth material as the cement ages.^

Tlie seiting reactions in glass-ionomer materials areas follows:

1. Decomposition of the glass under the influence ofthe aqueous polyacid. leading to the release of Câ *and Al'* ions. The latter are probably released inthe form of complex oxyanions containing severalaluminum atoms.'* a structure that reflects the formthey have occupied within the glass prior to acidattack.^

2. Rapid reaction of the Cn'* ions with ihe polyacidchains, followed by slower reaction of Al̂ "* speciesgradually released from the anionic complex. Thisreaction displaces water from some of the hydrationsites'" and leads to some ionic crosslinkitig of thepolyacid chains; both effects lead to insolubiliza-tion of the polymer and stiffening of the material.

3. Gradual hydration of the inorganic fragmentsreleased in step I, to yield a matrix of increasingstrength, greater resistance to desiccation, andimproved translucency."'-

Improvements in the strength and durability ofglass-ionomer cements have been sought by suchmeans as the inclusion of finely divided silver alloy orof a silver-cermet formed from the glass plus silver inafusion process. Fibers have also been used lo reinforceexperimental cements.'-*

A disadvantage of glass-ionomers is that they aresensitive to moisture in the early stages following ,placement.^ This may result in either the washing outof reacting ions from the immature cement by saliva or,in patients who tend to breathe through the mouth, in ,desiccation and arrest of the setting reaction. Both,effects are undesirable, and. to overcome the problems,._dentists are advised to cover freshly placed cemenlwith an impervious layer of varnish, petroleum jelly, .or liquid resin bonding agent. .̂

Glass-ionomers are able to form true adhesivebonds to dentin and enamel^ and for this reason have,found a wider range of applications than other dentaicements. These uses are considered in more detail later ;,in this article. .

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Resin-modified glass-ionomer materials

These materials, the majority of which are cured byvisible light, are hybrids that involve the incorporationof polymerizable components into an acid-base glass-ionomer cement. They were first described in the late1980s.'"* The use of visible light to cure these materials,at least as far as the initial development of structure isconcerned, limits the depth of individual layers ofcement that can be used, because of limitations in theextent to which light can penetrate these materials.Typically, this depth is of the order of 2 to 3 mm. afeature which restricts the use of these materials tocertain areas, such as cavity lining or incisai edges.

Resin-modified glass-ionomer materials consist ofacomplex mixnire of components,'-' including poly( acrylicacid) or a graft-copolymer of poiy(acrylic) in which aphotocurabie side chain has been added; photocurablemonomers, such as hydroxyethyl methacrylate (HEMA);calcium aluminosilicate glass: and water. These materialsset by a number of competing reactions and havecomplex structures. There is evidence of slight swellingofthe cured cement in aqueous media,"^ but. to date,there is only one account of this leading to acatastrophic clinical failure, when a tooth filled with aninappropriately formulated material thai underwentphase separation prior to use was split by the osmoticpressure in a HEMA-rich cement. '̂ In general, how-ever, clinical indications for their use has beenpromising, and good adaptation"* and adhesion,'''acceptable fluoride release.-" and excellent esthetics,"'-"have been reported. The data available to date onlong-term durability and esthetics will be discussedlater in this article.

fiiocompatibillty

„•Ji¡J Biocompaiibi/Hyis defined as the ability ofa material to-jjftis perform with an appropriate host response in aijiJiS specific application.-^ It is thus distinct from inertness.^0 which would imply no response from the host. More-0'$ over, biocompatibility is not a single phenomenon, butir&t' ^'^^'' ^̂ ̂ collection of processes involving different,li,fi but interdependent mechanisms of interaction be-^^•, Iween a material and the tissue. It is also specific to a

' ;j particular application and location in the body,' Glass-ionomer cements are generally biocompatible

¿with oral tissues and. as restorative materials, result in",.

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Fig 1 Sliver amalgam (mesial fossa) and silver-cermetresloralions oí a primary second molar, 8.5 years afterplacement.

Fjg 2 "interim" silver-cermet restoration of maxillary firsimolar, 9 years after placement.

Glass-ionomcr luting cements, however, were moresuccessful. They are used for cementing stainless steelcrowns for primary teeth, precision casi crowns andfixed prostheses for permanent teeth, space main-tainers. and single orlhodontic bands. Dentists treat-ing caries-prone patients are particularly pleased witha luting cement that has leachable fluoride ions andassociated preventive dentistry implications. Glass-ionomer luting cements of all types have become quitepopular, and their use continues to increase.

Introduction of the glass-ionomer-silver-cermet,Ketac-Silver (ESPE), in 19S4 gave dentists an attrac-tive alternative to silver amalgam for Class I restorationof primary' teeth.•"'"• '̂' Although fracture strengthsremained too low for the material to replace cusps ormarginal ridges. Ketac-Silvcr made a large impact onrestorative dentistry for children (Fig 1 ). A surprisingdevelopment was that Class I silver-cermet restora-tions, originally placed for "interim" use in permanentteeth,""'have routinely lasted for 10 years or more (Fig2). Ketiic-Silver has also been used for "lunneP res-torations."""'^ other restorations using unconventionalpreparations conserving of tooth structure,"'''"'"' as anendodontic filling materiai. and to serve as a corebuildup material prior to complete-crown preparation.

More recently ihe profession has seen the develop-ment of resin-modified glass-ionomer materials. Thesecombine advantages of glass-ionomer systems andvisible light-polymerized resin technology and are asignificant development in restorative dentistry. Theset cement has the main advantages of conventionalglass-ionomers, ie. fluoride release and adhesion todentin and enamel, but also improved fracture re-sistance and better wear characteristics. Introduced

originally as liner/base materials feg. Baseline VLC,Dcntsply/Caulk; Vitrebond, 3M Dental), they are nowavailable as restorative cemenis. Current commercialmaterials for this latter application include Fuji II LC( G O , Photac-Fil (ESPE), and Vitremer Tri-Cure( 3M Dental ), all of which have been useful for Class IIandClassV restorations in primary (Figs 3a to 3c) andpermanent (Figs 4a and 4b) teeth. For Class !restorations of primarj' molars intended to last morethan 3 years, initial clinical observation is that VitrenierTri-Cure appears to have the besi durability. Oneauthor (TPC) has now had more than 5 years'experience using these materials for the repair ofprimary teeth and confidently states that the resiti-modified glass-ionomer cements will become a main-stay restorative material for pédiatrie dentistry'*'"^"(Fig 5).

Having been found to be satisfactory in the primarydentition, resin-modified glass-ionomers are now beingused to restore permanent teeth-'-' (Figs 4, 6, and 7).However, brand selection and material handling areimportant (Fig 8). VitremerTri-Cure restorative cement,mixed at high powder-liquid ratio so that all of thepowder is wetted during mixing, appears to performthe best on the occiusal surfaces of permanent teeth.This may be the result of the high powder-liquid ratio,or it may be that Vitremer Tri Cure has better wearresistance, is less soluble, or both. All three brands ofresin-modified glass-ionomer material perform well inClass III and Class V restorations, in many casesholding up well for more than 4 years in permanentleeth.

The use of self-hardening resin-modified glass-ionomer luting cements is growing rapidly. These

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Fig 3a Sixteen-month-old cfiild with severe lingual caries Fig 3b Caries debnded wilh an inverted cone bur, used inof the maxillary primary incisors. a slow-speed handpiece.

Fig 3c Resin-modified glass-ionomer restorations, 26 Fig 4a Sensitive decalcitication/carious lesion asso-months after placement. ciated with poor oral hygiene during orlhodontic therapy

Fig 4b Resin-modified glass-ionomer cement restoration.; 1 yearpostoperatiuely.

Fig 5 Four-year postoperative view ct primary secondmolar (mesio-occlusohngual) restoration; primary first molar(disto-occlusal) restoration. (Vitremer Tri-Cure ResforativeCement).

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Fig 6a Occlusal and occlusolingual carious lesions ot ihemaxillary permanenl first molar in a caries-prone child.

Fig 6b Initial outline form cut with water-cooled high-speed bur, followed by complete debrjdement ot carioussubstance

Fig 6c VJtremer Tri-Cure Restorative Cement mixed withhigh powder-liquid ratio and syringe injected into the cavitypreparation.

Fig 6d Excess cement purposely left over the cavo-surface margins acts as an adhesive sealant

Fig 6e Enamel and cemenf surfaces, etched, rinsed,dried, and coated with unfilled resin sealant.

Fig 6f Resforation 17 months after placement.

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Fig 7 Class I resJn-moditied glass-ionomer restoration 34months after placement

Fig 8 Failure ot a Class I resin-modified glass-ionomerrestoration as a result of incorporation of air bubbles duringsyringing, degradation of the material, and, perhaps, brandof maferial.

Fig 9 Excess resin-mod i tied glass-ionomer luling cemenlexuding at the margins during placemeni of a stainless steelcrown

Fig 10 Resin-modified glass-ionomer luting cement usedfor a band and soldered wire loop space mainfainer

materials are not light-activated but contain thenecessary monomers to undergo polymerization, to-gether with initiators of the same type as used incold-cure acrylics, eg. bcnzoyi peroxide and amineaccelerator. The commercial materials of this type areAdvance (Dentsply/Caulk), Fuji Pius (GC; originallycalled Fuji Duet) and Vitremer Luting ( 3M Dental ).^'These cements are easily handled, cause no significantpostcementation sensitivity when luted to dentinalsurfaces, and have significant fluoride release and high«impressive and fracture strengths. A report pub-iished in February 1996 confirmed these observalionsand predicted that the resin-modified glass-ionomer'uling cement wiil soon dominate the market forroutine crown and fixed prosthetic ce menta tion. •̂ "' In

pédiatrie dentistry, these luting cements are becomingthe material of choice for stainless steel crowns {Fig9). space maintainers (Fig 10), and individual orth-odontic bands. Band cementation can also be carriedout with light-curable resin-modified glass-ionomercements where the radiant light is transmitted throughthe tooth to bring about the polymerization part of thecuring process.""

The future

What does the future hoid for the clinical use ofglass-ionomer systems? Clearly, the new resin-modifiedmalerials will have a major role to play. Although it is

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too early to be sure ofthe long-term durability andreliability of these materials, some predictions can bemade. If they prove to have adequate wear resistanceand fracture strengths so that a Class II restoration in aprimary molar can survive for 6 to ÍÍ years, they willreplace silver amalgam for the treatment of such teeth.The logic of this becomes even more compellingconsidering that resin composite can be used incosmetically prominent teeth and stainless steel crownrestorations are available for severely involved primarymolars and canine teeth. The history of glass-ionomer-silver-cermet restorations over the past 10 years alsolends credence to predictions of an optimistic futurefor resin-modified glass-ionomer materials, given theirbetter physical properties and handling characteristics.

Resin-modified glass-ionomer luling cements giveevery indication of becoming the materials of choicefor cementation of stainless steel crowns, spacemaintainers, and individual orthodontic bands. Thiscould also be true for cementation of precision castcrowns and fixed prostheses to prepared permanentteeth. Although more long-term data are neededconcerning resin-modified glass-ionomers, they showremarkable promise for materials at such an early stagein their development.

With all this development on the resin-modifiedglass-ionomers, it might be tempting to conclude thatthe original self-hardening glass-ionomer cements areobsolete. However, this is far from the case. Thesematerials, too, are undergoing exciting developmentsof their own. For example, new restorative-gradematerials have been launched recently, such as Ketac-Molar (ESPE) and Euji IX( GC), which set only by aconventional neutralization reaction but have proper-ties that rival or exceed those of the resin-modiliedsystems. Setting is rapid, early moisture sensitivity isconsiderably reduced, and solubility in oral fiuids isvery low.-"**" These results have been obtained byaltering the particle size and particle size distributionofthe glass powder, so that setting occurs more rapidlythan in the older formulations. These developmentsseem likely to be of particular importance in ThirdWorld countries, where there is an alarming growth inthe incidence of dental caries but where, becausesupplies of electricity are sparse or nonexistent,sophisticated dental facilities, such as power hand-pieces and dental curing lamps, cannot be relied on.̂ ^

Another subgroup ofthe self-hardening systems isthat involving the inclusion of silver metal particles(as opposed to silver fused with glass as a cermet), astrategy that gives cements of good properties, ie. high

compressive strength, radiopacity, and excellent clini-cal wear.^' Materials ofthis type currently on the marketinclude Miracle Mix (GC) and Hi-Dense (Shofti).

Also wilhin the dental field, se If-hardening glass-ionomers have been used in bone contact applications.In particular, this has involved their use in augmenta-tion ofthe alveolar ridge in edentulous patients.̂ '̂̂ ^Such studies have shown thai glass-ionomer materialsform intimate bioactive bonds with bone cells andbecome fully integrated into the bone. It is an excellentmaterial for this application and performs better than,for example, hydroxyapatite, which has been used todate for this purpose. Self-harden ing glass-ionomershave also been used in maxillofacial and craniofacialreconstruction surgery.''"''' The technique involves thefabrication of custom-made preset implants, curedoutside the body to develop their full mechanicalstrength, and then cemented into place with a self-curing glass-ionomer cement. The material has excel-lent biocompatibility, and early applications of thistechnique have been encouraging.

Conclusion

Overall, glass-ionomer cements, both self-hardenedand resin-modified, are important materials for modemclinical dentistry and will remain so for years to come.The development of rcsin-modifled materials hasopened up new dimensions in restorative dentistry,while the development of metal-reinforced and rapid-setting self-hardening cements has enhanced theproperties and extended the usefulness of the well-established original materials. Glass-ionomers of alltypes continue to combine fiuoride release, adhesion,good marginal seal, and reasonable esthetics.^^ Nomaterial is perfect, but. with the current level ofintensive research on glass-ionomers. the deficienciesthat exist seem certain to be eliminated, or at leastreduced, resulting in an ever-improving range ofmaterials ofthis type.

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61. Geyer G. Hetms J. Reconilruclion of Ihe posterior audilory canalwall and oblileraticn of ihe maslojd cavity using glass-ionomertemenl. Transplants and Implants in Olology I49hl U165-I70. D

Coming in January 1998

Q] CONTINUING EDUCATION

Beginning in 1998, Ql subscribers can earn 4 hours of CE credit per issue—or up to48 hours per year!

It's easy:

1. Read the four articles in each issue designated as CE resources.2. Answer the 16 questions (4 per article) on the answer sheet provided.1^. Return the answer sheet to Quintessence with a $10 processing fee.

A certificate for 4 hours of CE credit will be sent to those who score at least 75%.

Credit is awarded by Baylor College of Deniistry—The Texas A&M University System. Baylor College of Dentistry

is an ADA CERP Recognized Provider, is designated as a nationally approved sponsor by tbe Academy of General

Dentistry, and has received provisional accreditation as a continuing education provider by the Accreditation

Council for Continuing Dental Education.

714 Quintessence International Volume 28, Number 11/199'