Ceramic Materials in Dentistry

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Australian Dental Journal 2011; 56:(1 Suppl): 8496

doi: 10.1111/j.1834-7819.2010.01299.x

Ceramic materials in dentistry: historical evolution andcurrent practice

JR Kelly,* P Benetti*University of Connecticut Health Center, Department of Reconstructive Sciences, Farmington, Connecticut, USA.Sa o Paulo State University, Department of Dental Materials and Prosthodontics, Sa o Jose dos Campos, Sa o Paulo, Brazil.

ABSTRACT

Dental ceramics are presented within a simplifying framework allowing for facile understanding of their development,composition and indications. Engineering assessments of clinical function are dealt with and literature is reviewed on theclinical behaviour of all-ceramic systems. Practical aspects are presented regarding the choice and use of dental ceramics tomaximize aesthetics and durability, emphasizing what we know and how we know it.

Keywords: Ceramics, particle-filled glasses, polycrystalline ceramics, CAD CAM, all-ceramic restorations.

Abbreviation: LTD = low temperature degradation.

CERAMICS IN DENTISTRY WHERE DID THIS STUFFCOME FROM?

It is quite useful reviewing how and why ceramics cameto be used in dentistry. This account serves threepurposes: (1) to alert practitioners to the fact that theuse of ceramics, since the very beginning, alwaysrepresented the adoption of high technology versuscraft art; (2) to reinforce the concept that ceramics andimproved ceramics were introduced in order to solvespecic problems or to increase restorative versatility;and (3) to provide a gentle background into the natureand science of ceramics. Astute readers are alsoprovided with clues as to where to watch for theemergence of new ceramic technologies.

Since a distinction was drawn between high tech-nology and craft art it is useful to provide some basicdening characteristics of each. Many would agree thathigh technology should include: (1) dentistry borrow-ing materials processes shortly after their being devel-oped by an unrelated industry; (2) incorporation of newlearning from recent scientic literature outside of dental medicine; and (3) the spread of outright newinventions within dentistry. Craft art, on the otherhand, brings to mind materials and techniques bor-rowed from those involved in jewellery making, the artsand the manufacture of everyday goods. All audiencesthe senior author has spoken to before have chosen

craft art as the likely source of ceramics introducedinto dentistry at any stage of development.

In the early 1700s many European rulers were

spending enormous sums importing porcelain fromChina and Japan. Figure 1, from Schloss Charlotten-burg in Berlin, is representative of just small portions of one of these collections. The collection of Augustus IIIof Saxony was perhaps the largest and is now ondisplay at the Zwinger Museum in Dresden, his formerpalace. Such activity led China to be characterized asbeing the bleeding bowl of Europe. Between 1604 and1657 alone, over three million pieces of Chineseporcelain reached Europe. 1 In 1700, East Indiamenships unloaded 146 748 pieces in a European port injust one day as the market for porcelain grew insatia-ble. 1

One response to this situation involved state spon-sored research into porcelain discovery. NotableEuropean leaders including Augustus (III) the Strong,King of Poland and Elector of Saxony along with theMedici family of Florence, Italy were independentlysponsoring research into the development of a Euro-pean porcelain to match the hard, translucent andsonorous material developed in eastern Asia nearly1100 years earlier. Europeans strived at porcelaindiscovery without much success for about 200 yearsand this activity is credited with being largely respon-sible for the growth of modern analytical chemistry

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from its roots in alchemy. An historical timeline of porcelain discovery appears as Fig 2.

During the late 1600s and early 1700s the rstexamples of state sponsored research were beinginitiated in France and the Germanic state of Saxony.One example that becomes important for dentistry

involves the efforts of Count Walther Von Tschirnhausin developing the mineral resources of Saxony on behalf of Augustus III. One method Von Tschirnhaus usedinvolved subjecting minerals to extensively high tem-peratures (easily in excess of 1436 C)2 produced in thefocal spot of a series of large burning lenses (magni-fying glasses up to one metre in diameter), creating asolar furnace. About this time in Berlin (Germanic stateof Prussia), Johann Friedrich Bo ttger was reaching thelevel of journeyman apothecary. Bo ttger had a cuteparlour trick involving melting base metals such assilver coins to which he added a dose of the Arcanum of

the philosophers stone.3

When poured into moulds and

cooled, the resulting product was analysed to be puregold! Bo ttger inadvisably performed this transmuta-tion demonstration at his employers house to impresssome important guests that resulted in a summons byKing Frederich I (Prussian king) for a commandperformance. Placing discretion ahead of valour ledBo ttger to ee south to Saxony where he attempted tostudy medicine at Wittenberg. Wanted posters hadappeared in Berlin and a price was on the head of Bo ttger. The arrival of a contingent of a dozen troopsfrom King Frederich seemed out of proportion, to thelocal Wittenberg representative of Agustus III, for thecapture and return of a supposedly common criminal.Bo ttger was placed under house arrest for months whileAugustus was alerted and the situation explored. Withthe presence of foreign troops confounding the situa-tion, Bo ttger was nally spirited away by coach in thedead of night using back roads to avoid Prussian troopsand delivered to Augustus in Dresden. The Prussianswere additionally deceived as the Saxons continued tobring food to the room of Bo ttger. This base-metals-to-

gold thing was simply far too important to any stateneeding to support armies, and Bo ttger was put as aprisoner under the wing of Von Tschirnhaus to perfectgold production.

Serendipity and a clever intuition prevailed to saveBo ttger from certain execution following over threeyears of unsuccessful gold making, a project costingAugustus a small fortune. Experimenting with hisburning lenses, Von Tschirnhaus discovered that whileneither sand nor lime (calcium oxide) would fuseindividually, they would do so when combined, andpresented this at a meeting of the Academy of Sciences

of Paris in 1799.2

In fact, the resulting white product

Fig 1. Schloss Charlottenburg in Berlin is representative of just small portions of one of the extensive porcelain collections popular with Europeanroyalty.

Fig 2. Timeline of porcelain development from ancient China tomodern formulations.

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looked suspiciously like porcelain. What had beendiscovered was the use of a ux to create lowermelting intermediate compounds, allowing the fusionof the high-silica sand. Since it was known that highquality clay was a major ingredient in Chinese porce-lain, Saxony was secretly scoured for sources of purestclay. Bo ttger, whose expertise in chemistry was by thenextensive, realized that porcelain had to have a glassycomponent resulting from very high temperature reac-tions. Building on the discovery of Von Tschirnhaus, hereasoned that lime added to clay was worth exploring.

Research was conducted under extreme secrecybeginning in the Albrechtsburg Castle in Meissen(modern photograph in Fig 3) and then in the dungeonbasement of the feared Jungfernbastei (Maidens Bas-

tion) in Dresden between 1704 and 1708, utilizing anapproach known today as Edisonian research, where awide variety of formulations were systematically tried.Pages from his lab notebook memorializing the suc-cessful mixture of clay and lime (calcined alabaster;pulverized and heated to drive-off water and sulfurleaving fine calcium oxide powder) appear in Fig 4.Manufacturing operations were established back in theAlbrechtsburg Castle in Meissen and by 1708 the firstpieces were being demonstrated at the Leipzig EasterFair with production for sale beginning in 1710. Inabout 1710 Bo ttger substituted feldspar for lime as theflux, a move that cleanly put the Meissen formulationwithin that of the Chinese triaxial porcelains (Fig 5)and introduced feldspathic glass which will later

Fig 4. Page from laboratory notebook of Bo ttger memorializing the successful mixture of clay and calcined alabaster in January 1708. Earlydepiction of the feared Jungfernbastei (Maidens Bastion) in Dresden, site of secret porcelain discovery research.

Fig 3. Albrechtburg Castle in Meissen, Germany, site of an early porcelain discovery laboratory and the rst manufacturing site of Europeanporcelain rivalling that from China.


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become the main ingredient in aesthetic porcelainformulations for dentistry. This high temperaturereaction of the kaolin-type clay (essentially heavilyweathered granite or feldspathic rock) and feldsparyielded a high silica glass containing needle-like crystalsof mullite. 2

Although Saxony tried to maintain a monopoly onporcelain making, the secret escaped via a combinationof its role in state prestige, industrial espionage andgreed within the Meissen porcelain works. By 1776,porcelain making was the topic of a review paper givenat the Academy of Sciences in Paris. In 1770 anotherapothecary, Alexis Duchateau, tired of his stained andmalodorous dentures, sought assistance from Parisiandentist Nicholas Dubois de Che mant. Working withporcelain formulations and (then) high technology kilnsof the Guehard Porcelain Factory, they succeeded by1774 in fabricating a complete denture for Duchateau.Porcelain dentures represented a huge step forward inpersonal hygiene, leading public honours for de Che -mant from the likes of Edward Jenner (smallpoxvaccine), the Academy of Sciences and the Academyof Medicine of Paris University. Since porcelain was anew invention in Europe and only available in collab-oration with a high technology company from the

very beginning its use in dentistry was certainly notcraft art.

de Che mant ed to England ahead of the FrenchRevolution where he rened formulations in collabo-ration with Josiah Wedgewood at the beginning of hisfamous manufacturing company. de Che mant presum-ably worked to improve translucency moving from thecentre of the ternary (three-part) phase diagramtowards a feldspar-rich formulation, characteristic of todays feldspathic materials (Fig 5).

In 1808 another Parisian dentist, GiuseppangeloFonzi, signicantly improved the versatility of ceramics

by ring individual denture teeth, each containing a

platinum pin. This invention allowed teeth to be xedto metal frameworks enabling: (1) partial denturefabrication; (2) repairability; and (3) increased aesthet-ics. Platinum had only been known to Europeans sincearound 1741 and given its extremely high melting point(1769 C) was generally only worked into small wiresand crucibles by hammering individual red-hot nuggets.Platinum was not used in jewellery until 1915. 4 In 1808platinum was used by alchemists in early chemistryexperimentation, so it is likely that Fonzi obtainedplatinum wire from a local university or an earlyscientic supply house. It was also the only metal thatwould not crack the denture tooth on cooling given itsclosely matched coefcient of thermal contraction.Again, this next major improvement in our abilityto use ceramics in dentistry clearly stands as hightechnology.

Leucite ller crystals in porcelain metal-ceramicsystems and strengthened pressable ceramics

Many other advances contributed to both the use of ceramics and the discipline of xed prosthodontics,including: (1) the electric porcelain furnace; (2)elastomeric impression materials; and (3) the high-speed handpiece. One major advance in porcelainitself came in 1962 with the development of aformulation that could be red on common dentalcasting alloys. This invention built on a paperpublished in the Journal of the American CeramicsSociety (JACS) demonstrating an oddity in the thermalexpansion of a certain feldspar rock (with a potassiumcontent over 11%) when melted and cooled quickly,forming a glass. 5 When reheated, this glass had anextremely high thermal expansion due to the forma-tion of a new crystalline component not in the originalrock, called leucite (formed as the rock melted by aprocess known as incongruent melting). Weinsteinet al ., again using the Edisonian research technique(and hiring co-author Koenig of the JACS paper as aconsultant), finally arrived at what they called com-ponent number 1 a porcelain frit that had a thermalexpansion coefficient of nearly 20 10 ) 6 C, allowing

it to be mixed in any ratio with the normal expansionporcelain frit (thermal expansion of 8 10 ) 6 C) todial in an expansion contraction to match any dentalalloy. 6 So this represents an invention built on a paperpublished in a scientic journal outside of dentistry,and therefore again, high technology.

Most dental alloys having expansions in the range of 12 to 14 ( 10) 6 C); their matching porcelains haveleucite contents around 17 to 25 mass%. Manufactur-ers have found that having the porcelain with a slightlyhigher expansion contraction than the metal makes formore durable restorations, presumably by leaving theporcelain in a state of slight tangential compression.

Fig 5. Tertiary phase diagram of quartz (sand), clay and feldspar.Dental formulations began in middle of diagram and evolved towardsfeldspar-rich compositions to improve aesthetics. In Chinese formu-

lations feldspar was the ux.




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Leucite was later used for dispersion strengthening at35 to 50 mass% in both powdered ceramics and later inthe rst pressed ceramics. Leucite was a good choice asa strengthening ller since its index of refraction is closeto that of the feldspathic glass, so moderate strength-ening could be achieved without severely increasingopacity. These compositions are also easily etched tocreate micromechanical features for resin bonding. All-ceramic systems based on leucite-lled feldspathicglasses remain some of the most aesthetic and populardental ceramics, and have been extensively studiedclinically and found to be highly reliable. 7 The use of afeldspathic glass dates back to Bo ttger in 1710. Itbecame the major component of dental formulationswith the work of de Che mant in England in the early1800s and a special property of high potassiumfeldspars was taken advantage of in 1962, leading tothe vast majority of aesthetic dental ceramics in use

today, either metal-ceramic or all-ceramic.

Non-shrinking ceramics

Except for pressing ingots, by the mid 1980s all dentalceramic parts started as powders or mixtures of clayand power particles. Shrinkage is inherent to themaking of parts from such starting materials sincethe volume fraction of porosity is over 30% in thestarting greenware and nearly 0% in the nishedproduct. Seven different approaches were developedbeginning in the mid 1980s through to the late 1990s todeal with or avoid shrinkage to provide prostheses thatwere, what an engineer would call, being made to a netshape: (1) a pressed ceramic powder polymeric binderthat expanded and crystallized during ring to ll alost-wax mould (Cerestore; Johnson & Johnson, NewBrunswick, NJ, USA); 8,9 (2) casting of a special glassinto a lost-wax mould, embedding the clear-glasscasting in an investment and heat-treating to formcrystals within the glass (termed a glass ceramic, tradenamed DICOR; Dentsply International, York, PA,USA); (3) lightly sintering aluminum oxide (and latermagnesium aluminate spinel and zirconia alumina) toform necks between touching particles and then inl-

trating this porous ceramic with glass (In-Ceram; VitaZahnfabrik, Bad Sa ckingen, Germany); 10 (4) pressingsolid ingots of lled-glass (leucite or lithium disilicate)into a lost-wax mould (Empress; Ivoclar Vivadent,Schaan, Liechtenstein); 11 (5) computer-aided machin-ing of net-shape parts from solid, full-dense blocks(CEREC; Sirona, Bensheim, Germany); 12 (6) computer-aided fabrication of an oversized die, pressing of alumina powder onto the die creating an oversizedpart and sintering to nal size (Procera; Nobel Biocare,Zu rich, Switzerland); 13 and (7) computer-aidedmachining of oversized parts from lightly sinteredblocks of zirconia and alumina which are then sintered

to nal size (Cercon, Lava, Vita YZ, Ivoclar e.maxzirCAD). 14

Approach number (1) provided the rst introductionof advanced ceramics processing equipment into thedental laboratory and approach (2) introduced glassceramics into dentistry; where strengthening llerparticles are grown inside of the glass from thechemistry of the glass (during a special heat treatmentcalled ceramming) as opposed to being added asseparate powder particles. Both of these involvedcollaborations with specialty ceramics rms, CoorsCeramics for (1) and Corning Glass Works for (2).

Chairside CAD CAM

While all seven approaches stand clearly as being hightechnology, numbers (5) and (7) can be viewed as beingrevolutionary. In 1987, Mo rmann and Brandestini 12

introduced a prototype machine that would capture a3-D image of a prepared tooth. They used 3-D designsoftware to iteratively develop a proposed restorationand then directed the computer-aided milling of inlaysand onlays from solid blocks of aesthetic, lled-glass(see next section) ceramics (CEREC I, then SiemensDental now Sirona, Bensheim, Germany). Machining of aesthetic glass-based ceramics is relatively straight-forward and special formulations were quickly devel-oped that were much higher quality than what wasavailable from dental laboratory processing based oneither strengthened and ne grained feldspathic ceram-ics (Mark II, Vita) or the rst glass-ceramic introducedfor dental use (containing interlocking tetrasilisicuoromica akes, DICOR-MGC, Dentsply Interna-tional).

Green machining of oversized parts

Machining of tougher structural ceramics such asalumina and especially transformation toughened zirco-nia (see following section) was much more difcult,requiring heavier machinery, longer milling times andquite often involved limited tool life. Further advances inthe manipulation of 3-D data sets along with the fruits of

a decade of research into ceramics processing providedthe underpinning for an innovative solution proposed byFilser and Gauckler at the University of Zu rich. Thisinvolved machining of an oversized part from a ceramicblock only lightly sintered to what is termed the initialsintering stage. 1517 With very careful control over boththe ceramicpowder particle size distribution andparticlepacking density, it became possible to predict theoversized shape needed that would then shrink to thedesired net shape. This technique has been variouslytermed green machining or soft machining in dentalliterature. This technique allowed the individually cus-tomized and high tolerance parts dentistry requires to be




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manufactured from polycrystalline ceramics such asalumina and zirconia.

As of today, the last major advance in dental ceramicscomes with the introduction of transformation tough-ened zirconia. 1719 This ceramic is arguably the mostcomplex material ever introduced for dental use and, aswill be discussed later, its introduction has not beenwithout a learningcurve that we are still climbing. Twoother major changes currently underway involve: (1) theestablishment of dedicated industrial-quality manufac-turing centres for fabrication of prostheses; and (2) theapplication of engineering design research into clinicaland laboratory practices to optimize durability andaesthetics. Fruits of this second activity will be discussedin the last section of this paper.

BACKGROUND CONCEPTS IN CERAMICS SCIENCE

There are two quite useful concepts that help demystifydental ceramics by providing a structure within whichto organize thinking. First, there are only three mainclasses of dental ceramics: (1) predominantly glassymaterials; (2) particle-lled glasses; and (3) polycrys-talline ceramics. 6,20,21 Dening characteristics will beprovided for each of these ceramic types as they arerepresented in Fig 6. Second, virtually any ceramicwithin this spectrum can be considered as being acomposite, meaning a composition of two or moredistinct entities. Quite a number of seemingly differentdental ceramics can be shown to be quite similar orclosely related to each other when reviewed within theframework these two simplifying concepts provide.Additionally, the rationale behind the development of ceramics of both historic and recent interest can bemore easily understood. Two examples of the utility of these concepts include these basic statements: (1) highlyaesthetic dental ceramics are predominantly glassy andhigher strength substructure ceramics are generallycrystalline; and (2) the history of development of

substructure ceramics simply involves an increase incrystalline content to fully polycrystalline. Figures 7aand 7b provide basic composition details and commer-cial examples of many aesthetic and substructure dentalceramics organized by these three main divisions usingthe composite concept (based on matrix and ller).

Predominantly glassy ceramics

Dental ceramics that best mimic the optical properties of enamel and dentine are predominantly glassy materials.Glasses are 3-D networks of atoms having no regularpattern to the spacing (distance and angle) betweennearest or next nearest neighbours, thus their structure isamorphous or without form. Glasses in dental ceram-ics derive principally from a group of mined mineralscalled feldspar and are based on silica (silicon oxide) andalumina (aluminum oxide), hence feldspathic porcelains

belong to a family called aluminosilicate glasses.21

Glasses based on feldspar are resistant to crystallization(devitrication) during ring, have long ring ranges(resist slumping if temperatures rise above optimal) andare extremely biocompatible. In feldspathic glasses, the3-D network of bridges formed by silicon-oxygen-silicon bonds is broken up occasionally by modifyingcations such as sodium and potassium that providecharge balance to non-bridging oxygen atoms (Fig 8).Modifying cations alter important properties of theglass, e.g. by lowering ring temperatures or increasingthermal expansion contraction behaviour.

Particle-lled glasses

Filler particles are added to the base glass compositionin order to improve mechanical properties and tocontrol optical effects such as opalescence, colour andopacity. These llers are usually crystalline but can alsobe particles of a higher melting glass. Such composi-tions based on two or more distinct entities (phases) areformally known as composites, a term often reservedin dentistry to mean resin-based composites. Thinkingabout dental ceramics as being composites is a helpfuland valid simplifying concept. Much confusion is

cleared up in organizing ceramics by the ller particlesthey contain (and how much), why the particles wereadded, and how they got into the glass.

The rst llers to be used in dental ceramicscontained particles of a crystalline mineral calledleucite. 20,21 As mentioned in the history section above,this ller was added to create porcelains that could besuccessfully red onto metal substructures. 22,23 Leucitehas a very high thermal expansion contraction coef-cient (around 20 10 ) 6 C) compared to feldspathicglasses (around 8 10 ) 6 C). Dental alloys haveexpansion contraction coefcients around 12 to 14( 10 ) 6 C). Adding about 17 to 25 mass% leucite ller

Fig 6. Based on their microstructure, dental ceramics fall within threebasic classes: (1) predominantly glass; (2) particle-lled glass; and (3)fully polycrystalline. Each of these is discussed in detail in the text.




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Fig 8. In feldspathic glasses, the 3-D network of bridges formed by silicon-oxygen-silicon bonds is broken up occasionally by modifying cations suchas sodium and potassium that provide charge balance to non-bridging oxygen atoms.

(a)

(b)

Fig 7. Composition of dental ceramics based on their being composites consisting of a matrix and llers; (a) veneering ceramics; (b) structural andCAD CAM ceramics.




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to the base dental glass create porcelains that arethermally compatible during ring with dental alloys.Metal-ceramic systems, rst developed in 1962, areused to fabricate 70 to 80% of xed prostheses.

Moderate strength increases can also be achieved withappropriate llers added and uniformly dispersedthroughout the glass, a phenomenon termed dispersionstrengthening. The rst successful strengthened sub-structureceramicwasmadeof feldspathicglasslled withparticles of aluminum oxide (app. 55 mass%). 23 Leuciteisalsousedfordispersionstrengtheningatconcentrationsof around 40 to 55 mass%, much higher than needed formetal-ceramics. Commercial ceramics incorporating leu-cite llers for strengthening include a group that arepressed into moulds at high temperature (OPC, Pentron;Empress Esthetic, Ivoclar Vivadent and Finesse All-Ceramic, Dentsply Prosthetics) and a group provided asa powder for traditional porcelain build-up (OPC Plus,

Pentron; Fortress, Mirage Dental Systems).Beyond thermal expansion contraction behaviour,there are two major benets to leucite as a ller choicefor dental ceramics; the rst intended and the secondprobably serendipitous. First, leucitewas chosenbecauseits index of refraction is very close to that of feldspathicglasses; an important match for maintaining sometranslucency. Second, leucite etches at a much faster ratethan the base glass and it is this selective etching thatcreates a myriad of tiny features for resin cements toenter, creating a good micromechanical bond.

Glass-ceramics (special subset of particle-lled glasses)Crystalline ller particles can be added mechanically tothe glass, e.g. by simply mixing together crystalline andglass powders prior to ring. In a more recent approach,the ller particles are grown inside the glass object(prosthesis or pellet for pressing into a mould) after theobject has been formed. After forming, the glass object isgiven a special heat treatment, causing the precipitationand growth of crystallites within the glass. Since thesellers are derived chemically from atoms of the glassitself, it stands to reason that the composition of theremaining glass is altered as well during this process

(termed ceraming). Such particle-lled composites arecalled glass-ceramics. The material Dicor (Dentsply), therst commercial glass-ceramic available for xed pros-theses, contained ller particles of a type of crystallinemica (at app. 55 vol%). 24 More recently, a glass-ceramiccontaining70 vol% crystalline lithium disilicateller hasbeen commercialized for dental use (Empress 2, nowe.maxPress and e.maxCAD, Ivoclar-Vivadent).

Polycrystalline ceramics

Polycrystalline ceramics have no glassy components; allof the atoms are densely packed into regular arrays that

are much more difcult to drive a crack through thanatoms in the less dense and irregular network found inglasses. Hence, polycrystalline ceramics are generallymuch tougher and stronger than glassy ceramics.Polycrystalline ceramics are more difcult to processinto complex shapes (e.g. a prosthesis) than are glassyceramics. Well-tting prostheses made from polycrys-talline ceramics were not practical prior to the avail-ability of computer-aided manufacturing. In general,these computer-aided systems use a 3-D data setrepresenting either the prepared tooth or a wax modelof the desired substructure. This 3-D data set is used tocreate either an enlarged die upon which ceramicpowder is packed (Procera, NobelBiocare, Zurich,Switzerland) or to machine an oversized part for ringby machining blocks of partially red ceramic powder(Cercon, Dentsply Prosthetics; Lava, 3M-ESPE; Y-Z,Vita Zahnfabrik). Both of these approaches rely upon

well-characterized ceramic powders for which ringshrinkages can be predicted accurately. 13,14

Polycrystalline ceramics tend to be relatively opaquecompared to glassy ceramics, thus these stronger mate-rials cannot be used for the whole wall thickness inaesthetic areas of prostheses. These higher strengthceramics serve as substructure materials upon whichglassy ceramics are veneered to achieve pleasing aesthet-ics. Laboratory measures of the relative translucency of commercial substructure ceramics areavailable, both fora single-layer of materials and for those that areveneered. 25,26 It should be noted, however, that whilelaboratory measures of opacity have equated somepolycrystalline ceramics to cast alloys, all ceramic sub-structures transmit some light and metals simply do not.

Substructure ceramics

The development of higher strength ceramics for ve-neered all-ceramic prostheses can be represented as atransition towards increases in the volume percentage of crystalline material with decreasingly less glass andnally, no glass. In 1965, John McLean reported on thestrengthening of a feldspathic glass via addition of aluminum oxide particles, 23 the same year that General

Electric rst applied that new technology (dispersionstrengthening of glasses) to high-tension power lineinsulators. In the late 1980s, a method was realized tosignicantly increase the aluminum oxide content (fromapp. 55 mass% to 70 vol%) by rst lightly ring packedalumina powder and then inltrating the still porousalumina compact with glass. 11 During the rst lightring, adjacent alumina particles become bonded wherethey touch, forming a 3-D network of linked particles.Inltration involves a low viscosity glass drawn into theporous alumina network by capillary pressure, thusforming an interpenetrating 3-D composite (both thealumina and glass being continuous throughout the




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ceramic, neither representing an isolated ller).Although with only 70 vol% aluminum oxide, thisceramic (In-Ceram Alumina, Vita) has a strength andfracture toughness identical to many 100% polycrystal-line alumina ceramics.

Two key developments allowed fully polycrystallineceramics to become practical for xed prostheses: (1) theavailability of highly controlled starting powders; and(2) the application of computers to ceramics processing.Unlike glassy ceramics, polycrystalline ceramics cannotbe simply pressed as a fully dense material into slightlyoversized moulds (moulds that have expanded justenough to compensate for cooling shrinkage as is donein the casting of metals). Polycrystalline ceramics areformed from powders that can only be packed to around70% of their theoretical density. Hence polycrystallineceramics shrink around 30% by volume (10% linear)when made fully dense during ring. In order for the

nal prostheses to t well, the amount of shrinkageneeds to be accurately predicted and compensated for.Well-characterized starting powders that can be uni-formly packed are a prerequisite for achieving predict-able and reproducible shrinkage. Research in ceramicsprocessing science from the late 1980s through to the1990s led to the commercial availability of powderssuitable for dental use. Almost simultaneously with hightechnology powder renement came the development of computer-aided machining and the ability to captureand manipulate 3-D data sets.

Two different approaches are now being offeredcommercially for fabrication of prostheses from poly-crystalline ceramics, both of which create oversizedgreenware (unred part) using 3-D data sets and thespecic shrinkage characteristics of well-behaved start-ing powders. In the rst approach, an oversized die ismanufactured based on app. 20 000 measurementstaken during the mechanical scanning of a laboratorydie. Either aluminum oxide or zirconium oxide ispressed onto the oversized die and predictably shrunkduring ring to become well-tting single-crown sub-structures (Procera, Nobel Biocare). 13 In the secondapproach, blocks of partially red (app. 10% complete)zirconium oxide are machined into oversized greenware

for ring as single- and multiple-unit prosthesissubstructures (Cercon, Dentsply Prosthetics; Lava,3M-ESPE; Y-Z, Vita; e.max ZirCAD, Ivoclar). In thesesystems, individual blocks are bar coded with the actualdensity of each block (for the ne-tuning of shrinkagecalculations) and the milling machines can keep trackof the number of blocks milled and automaticallychange milling tools to further assure accuracy of t. 14

Transformation toughened zirconium oxide

Potentially the most interesting polycrystalline ceramicnow available for dentistry, transformation toughened

zirconia, needs further explanation since its fracturetoughness (and hence strength) involves an additionalmechanism not found in other polycrystalline ceramics.While fracture toughness and strength are outside thescope of this paper, it is sufcient here to understandtoughness simply as meaning the difculty in driving acrack through a material.

Unlike alumina, zirconium oxide is transformed fromone crystalline state to another during ring. At ringtemperature, zirconia is tetragonal and at room tem-perature monoclinic, with a unit cell of monoclinicoccupying about 4.4% more volume than when tetra-gonal. Unchecked, this transformation was a bit unfor-tunate since it would lead to crumbling of the materialon cooling. In the late 1980s, ceramic engineers learnedto stabilize the tetragonal form at room temperature byadding small amounts (app. 38 mass%) of calciumand later yttrium or cerium. 18 Although stabilized at

room temperature, the tetragonal form is really onlymetastable, meaning that trapped energy still existswithin the material to drive it back to the monoclinicstate. It turned out that the highly localized stress aheadof a propagating crack is sufcient to trigger grains of ceramic to transform in the vicinity of that crack tip. Inthis case, the 4.4% volume increase becomes benecial,essentially altering material conditions around thecrack tip, shielding it from the outside world (moreformally stated, transformation decreases the localstress intensity).

Although most dental zirconia is a bit opaque andcopings need to be veneered for high aesthetics, theseprostheses can be quite lifelike. Zirconia is not asopaque as In-Ceram alumina and can be internallycoloured as can lithium disilicate. Figure 9 is a clinicalcase involving both central incisors where zirconia waschosen over crowns done in three other systems by thesame technician (In-Ceram alumina, Vita; e.maxCAD,Ivoclar; Captek, Precious Chemicals Company, Alta-monte Springs, FL, USA).

With fracture toughness twice or more that of alumina ceramics, transformation toughened zirconiarepresents an exciting potential substructure material.Possible problems with these zirconia ceramics may

involve long-term instability in the presence of water,porcelain compatibility issues, and some limitations incase selection due to their opacity. However, as of writing this, three-year clinical data involving manyposterior single-unit and three-unit prostheses (plus 1ve-unit) have revealed no major problems (discussedmore fully below).

Zirconia substructures issues

Two issues are of concern with zirconia, one quite realand one potential. Of real concern are reports of signicant percentages of single-unit and multi-unit




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prostheses having porcelain chipping and cracking (e.g.25 to 50%). 17 Of potential concern is the propensity forpartially-stabilized zirconia to auto-catalytically trans-form at surface grain boundaries due to an interactionwith water. This may create major structural issues inthe future.

Many authorities have offered numerous explana-tions for porcelain chipping that simply do notwithstand critical thinking or analysis, including: (1)non-anatomic substructure design; (2) unsupportedporcelain; (3) weaker porcelain; (4) thermal expan-sion contraction mismatches; and (5) poor porcelain-zirconia bonding. More well-considered hypotheseshave included: (1) residual stresses arising fromthermo-mechanical parameters; 27 (2) auto-catalytictransformation during porcelain ring; 28 and (3)enhanced auto-catalytic transformation of green-machined structures at mouth temperature. 29 As yetunpublished research from three different sourcesstrongly supports residual stresses within the porcelain

developing as a result of too rapid cooling: (1)fractographic evidence for a crack originating 0.5 mmbelow the porcelain surface (Susanne Scherrer, Univer-sity of Geneva); (2) optical interferometer measures of residual stress in fast-cooled versus slow-cooled zirco-nia crowns (Drs Norbert Thiel and Michael Tholey,Vita, with collaborators at the Fraunhofer Institute);and (3) measures of thermal gradients across fast-cooled crowns of up to 160 C (Drs Norbert Thiel andMichael Tholey, Vita). In addition, many clinicalauthorities report that they stopped having porcelainproblems when they enforced a slow-cooling protocol

(including Drs Avi Sadan and Ed McLaren personal

communications). One paper given at the 2010 meetingof the International Association for Dental Research(IADR) on clinical data from four private practicesreported only 2% porcelain chipping in two to threeyears for 702 prostheses (authors D Nathanson, S Chu,H Yamamoto and C Stappert). These cliniciansreported that their laboratories were aware of the needto cool slowly. Numerous dental material companiesnow include a caution to slow cool in written and web-based informational materials. This continues to be anactive area of research.

Allceramics aresusceptible tosubcritical crackgrowthand corrosion effect caused by water, which breaks thebond between atoms at the crack tip, leading to slowgrowth of cracks, resulting in a decrease of materialsstrength. Partially-stabilized zirconia-based materialsare uniquely susceptible to auto-catalytic transformationof the crystals from tetragonal to monoclinic at relatively

low temperatures called low temperature degradation(LTD). 30 While generally studied at autoclave tempera-tures of a few hundred degrees centigrade, signicantpercentages of transformation can be extrapolated asbeing possible at oral temperatures using activationenergy data from ceramics literature (Chevalier). 29

Another striking clinical study presented at the 2010IADR examined partially-stabilized zirconia discsembedded in the anges of mandibular partial dentures,demonstrating a much higherrate of transformation foradental zirconia in two to three years thanpredicted for anengineering zirconia by Chevalier (author Tomaz Kos-mac , Jozef Stepfan Institute).

Water can catalyze the process at surface grainboundaries and the transformation of crystal continuesfrom layer to layer through the entire body, leading tomicrocrack formation, grain pullout and a decrease instrength. 18 This phenomenon was particularly impor-tant in causing the failure of zirconia hip prosthesessubmitted to autoclave sterilization. Although theinformation presented above raises concern, bothpublished clinical trials and those presented at interna-tional research meetings evidenced no bulk fracture indental restorations under the observation time, indicat-ing that LTD has no major inuence on the clinical

behaviour of dental restorations.31

Maximizing durability

Understanding how to maximize durability owsfrom understanding clinical failure and the factorsinuencing the stress state at failure that are withinour clinical control. Efforts to understand clinicalfailure of single crowns has involved studying fracturesurfaces (fractography), nite element modelling anddeveloping a clinically-valid laboratory test. 32 Theseall demonstrate that the main mechanism of failuredoes not involve damage from wear facets but

Fig 9. Both central incisors were restored with veneered zirconiacrowns (Y-Z, Vita). Mirror shot (top) demonstrates vital translucency.




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stresses on the cementation surface due to occlusalloading. Figure 10 is a cartoon representation of niteelement modelling illustrating this stress state. Theseengineering analyses point to four clinical techniquesfor maximizing durability: (1) provide maximumocclusal thickness for the ceramic (strength increaseswith the square of the thickness); (2) use the highestelastic modulus (stiffness) substrate possible; i.e.metal or ceramic versus resin-based composite); (3)bond the restoration, ceramic-cement and cement-tooth (or substructure); and (4) develop broad, notpinpoint, occlusal contacts.

This thickness-squared relationship is illustrated inFig 11 from calculations using the analytical solution of Hsueh. 33 For example, predicted failure loads for alithium disilicate restoration increase from 1400 N toover 2000 N with a ceramic thickness increase from1.6 mm to only 1.8 mm. Higher elastic modulus

(stiffness) substructures decrease the stress for anygiven load. This effect has also been validated fromclinical study of all-ceramic crowns. 34,35 Failure loadsof crowns have been found to be doubled with bondingin laboratory testing (L May, JR Kelly, University of Connecticut, unpublished research) and this effect hasbeen veried in retrospective clinical studies of bothinlays and crowns. 36,37 Figure 10 also predicts that forany given total thickness there is little to no structuralimprovement from making the higher strength ceramicthicker. This counter-intuitive prediction is beingveried in laboratory testing (R Yau, P Rungruanga-nunt, JR Kelly, University of Connecticut, unpublisheddata) and may allow crowns to be maximized foraesthetics without decreasing durability. Failure stressesalso increase with decreased wear facet size, e.g. failureloads as a function of loading piston diameter were:1 mm = 343 N; 2 mm = 382 N; and 3 mm = 522 N. 38

Etching and bonding

Ideal bonding involves having microstructural struc-tures within the ceramic that can be selectively attackedby acids (etched) at a higher rate than surrounding

ceramic leaving micromechanical features for cementinltration. The selective etching of crystalline leucite,leaving behind microscopic glassy crypts, is the mostcommon dental example. A second requirement forgood bond formation relates to the size of thestructure(s) formed by etching and how well they arestill attached to the remaining bulk ceramic. Forexample, some selective etching of In-Ceram aluminais possible but the scale of roughness that develops isinsufcient for good bond formation. Polycrystallineceramics can be etched, revealing the boundarybetween crystalline grains, but these etched grainboundaries provide little micromechanical retention.

Chemical bonding is possible with virtually all dentalceramics, but only with the use of resin cementscontaining special adhesive molecules. The durabilityof chemical bonding between resin cements and sub-structure ceramics has not been denitively addressed.Laboratory data suggests that bonds can decreasesignicantly during water storage. 39

Aesthetic considerations

Achieving a lifelike match requires the clinician technician to choose a base ceramic having an appro-priate translucency (value in the Munsell system) for thepatient. For a single anterior tooth, all ceramics havesufcient longevity based on clinical trial data, so thechoice can be made on aesthetic capability alone. Newresearch on comparative translucencies is in prepara-tion that will update decades-old information.

Three things need to be measured and recorded forcommunication with the dental laboratory: (1) baseshade (gingival one-half to one-third); (2) incisalenamel characterization; and (3) surface gloss. Base

shade is best determined within a rational shade

Fig 11. Calculated loads to reach 360 MPa (breaking strength of lithium disilicate) for three different thicknesses as a function of

varying core thickness (remainder veneer).

Fig 10. Failure stresses are linearly related to the elastic modulusdifferent (ceramic-substrate) and to the square of the ceramic

thickness.




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system such as 3-D Master (Vita). Numerous auto-mated shade taking devices are also available, with themost highly regarded from laboratory research beingthe EasyShade (Vita).

Ceramic systems offer different levels of translu-cency and colour control. These are best learned byclinical practice. In general, an aesthetic match is best

achieved by specifying a ceramic system offering thesame translucency as the case being matched. Cur-rently available ceramics are organized below bytranslucency measurements (L Spink, P Rungruanga-nunt, JR Kelly, University of Connecticut, unpub-lished research).

Many all-ceramic systems are clinically indicated forsingle-unit anterior teeth, including most in Table 1.Thus, clinicians are free to choose a system offering thebest aesthetic match. 7 Good choices (i.e. well-studiedclinically) for posterior single-unit teeth includeIn-Ceram alumina (Vita), Empress Esthetic (Ivoclar),e.max Press or CAD (Ivoclar) and zirconia-based(various). 7 Only the zirconia systems are indicated formulti-unit xed prostheses.

Glazing versus polishing

Auto glazing (ring in air) and polishing are twooptions for nishing the surface of aesthetic porcelains.These two techniques received recent attention in areview of a number of studies comparing preparedsurfaces using visual, microscopic and prolometrymeasures. 40 All studies agree that glazing can produce asmooth porcelain surface. However, polishing can

produce as smooth a surface that can be moreaesthetically similar to natural enamel. Many author-ities favour polishing given that a higher level of controlis possible over nal surface nish and that an addedring can add problems and time to the deliveryappointment.

Repair

Approaches to the repair of porcelains have beenreviewed relatively recently. 41 Repair often offers bothdentist and patient a cost-effective alternative to

replacement. Repair involves the bonding of resin-

based products to remaining porcelain. The porcelain-resin bond is formed by both etching the surface tocreate micromechanical attachment features and by theapplication of silane coupling agents to provide somechemical interaction between the silicon-based ceramicand carbon-based resins. It is reported that porcelainrepair systems form durable bonds to fractured porce-lain and exposed metal surfaces. 41

SUMMARY

Ceramics are widely used in dentistry due to theirability to mimic the optical characteristics of enameland dentine as well as for their biocompatibility andchemical durability. Most highly aesthetic ceramics arelled glass composites based on aluminosilicate glassesderived from mined feldspathic minerals. One commoncrystalline ller is the mineral leucite, used in relatively

low concentrations in porcelains for metal-ceramicsystems and in higher concentrations as a strengtheningller in numerous all-ceramic systems. In general, thehigher the fraction of polycrystalline components,the higher the strength and toughness of a ceramic.The development of substructure ceramics for xedprosthodontics represents a transition towards fullypolycrystalline materials. While the strength rating of adental ceramic can be a meaningful number, it is notreally an inherent property and varies due to testingparameters that are often not well controlled tooptimize clinical relevance. Fracture toughness is a farmore inherent measure of the structural potential of aceramic and represents a more easily compared value.Clinical data for all-ceramic systems is becomingincreasingly available and results exist for manycommercial materials, providing guidance regardingclinical indications.

REFERENCES1. Plumb JH. In the Light of History. New York: Delta, 1971:59

65.2. Kingery WD, Vadiver PB. Ceramics Masterpieces: Art, Structure,

and Technology. New York: Free Press, 1986:322.3. Gleeson J. The Arcanum: The Extraordinary True Story. Warner

Books, 2000:336.4. Ring ME. Dentistry: An Illustrated History. New York: Harry N

Abrams Inc., 1985:108211.5. Orlowski HJ, Koenig CJ. Thermal expansion of silicate uxes in

the crystalline and glassy states. JACS 1941;24:8084.6. Weinstein M, Weinstein LK, Katz S, Weinstein A. Fused Porce-

lain-to-Metal Teeth. US Patent no. 3 052 982. 1962.7. Della Bona A, Kelly JR. The clinical success of all-ceramic res-

torations. J Am Dent Assoc 2008;139:8S13S.8. Starling LB, Stephan JE, Stroud RD. Shrink-free ceramics and

method and raw batch for the manufacturer thereof. US Patentno. 4 265 669. 1981.

9. Sozio RB, Riley EJ. The shrink-free ceramic crown. J ProsthetDent 1983;49:182187.

Table 1. Recommendation for use of an all-ceramicrestoration based on translucency (MO denotesmoderate opacity and HO denotes high opacity)

High translucency Moderate translucency Low translucency

Empress Esthetic Vita Y-Z zirconia e.max Press HO

e.max Press e.max Press MO In-Ceram zirconiaEmpress CAD Vita aluminaIn-Ceram Spinell In-Ceram alumina

Lava zirconia




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10. Della Bona A, Mecholsky JJ Jr, Barrett AA, Griggs JA. Charac-terization of glass-inltrated alumina-based ceramics. Dent Mater2008;24:15681574.

11. Hornberger H, Marquis PM. Mechanical properties and micro-structure of In-Ceram, a ceramic-glass composite for dentalcrowns. Glastech Ber Sci Technol 1995;68:188194.

12. Mo rmann WH, Brandestini M. Cerec-System: computerized in-lays,onlays and shellveneers. Zahnarztl Mitt1987;77:24002405.

13. Andersson M, Ode n A. A new all-ceramic crown. A dense-sintered, high-purity alumina coping with porcelain. Acta Od-ontol Scand 1993;51:5964.

14. Raigrodski AJ. Clinical and laboratory considerations for the useof CAD CAM Y-TZP-based restorations. Pract Proced AesthetDent 2003;15:469476.

15. Filser FT. Direct ceramicmachining of dental restorations. Zurich:Swiss Federal Institute of Technology Zurich, 2001. PhD thesis.

16. Filser F, Kocher P, Gauckler LJ. Net-shaping of ceramic compo-nents by direct ceramic machining. Assembly Autom2003;23:382390.

17. Denry I, Kelly JR. State of the art of zirconia for dental appli-cations. Dent Mater 2008;24:299307.

18. Kelly JR, Denry I. Stabilized zirconia as a structural ceramic: anoverview. Dent Mater 2008;24:289298.19. Denry IL. Recent advances in ceramics for dentistry. Crit Rev

Oral Biol Med 1996;7:134143.20. Kelly JR. Ceramics in restorative and prosthetic dentistry. Annu

Rev Mater Sci 1997;27:443468.21. Giordano R. A comparison of all-ceramic restorative systems:

Part 2. Gen Dent 2000;48:3840, 43-45.22. Weinstein M, Weinstein AB. Porcelain covered metal-reinforced

teeth. US Patent no. 3 052 983. 1962.23. McLean JW, Hughes TH. The reinforcement of dental porcelain

with ceramic oxides. Br Dent J 1965;119:251267.24. Grossman DG. Cast glass-ceramics. Dent Clin North Am

1985;29:725739.

25. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, et al. Relativetranslucency of six all-ceramic systems. Part I: core materials. JProsthet Dent 2002;88:49.

26. Heffernan MJ, Aquilino SA, Diaz-Arnold AM, et al. Relativetranslucency of six all-ceramic systems. Part II: core and veneermaterials. J Prosthet Dent 2002;88:1015.

27. Swain MV. Unstable cracking (chipping) of veneering porcelainon all-ceramic dental crowns and xed partial dentures. ActaBiomater 2009;5:16681677.

28. Tholey MJ, Swain MV, Thiel N. SEM observations of porcelainY-TZP interface. Dent Mater 2009;25:857862.

29. Kim JM, Covel NS, Guess PC, Rekow ED, Zhang Y. Concerns of hydrothermal degradation in CAD CAM zirconia. J Dent Res2010;89:9195.

30. Kobayashi K, Kuwajima H, Masaki T. Phase change andmechanical properties of ZrO2-Y2O3 solid electrolyte afteraging. Solid State Ionics 1981;3-4:489493.

31. Kelly JR. Dental ceramics: current thinking and trends. Dent ClinNorth Am 2004;48:513530.

32. Kelly JR, Rungruanganunt P, Hunter B, Vailati F. Developmentof a clinically validated bulk failure test for ceramic crowns. JProsthet Dent 2010;104:228238.

33. Hsueh CH, Miranda P. Modeling of contact-induced radialcracking in ceramic bilayer coatings on compliant substrates. JMater Res 2003;18:12751283.

34. Malament KA, Socransky SS. Survival of Dicor glass-ceramicdental restorations over 14 years. Part II: effect of thickness of Dicor material and design of tooth preparation. J Prosthet Dent1999;81:662667.

35. Malament KA, Socransky SS. Survival of Dicor glass-ceramicdental restorations over 16 years. Part III: effect of luting agentand tooth-substitute core structure. J Prosthet Dent 2001;86:511519.

36. van Dijken JW, Ho glund-Aberg C, Olofsson AL. Fired ceramicinlays: a 6-year follow up. J Dent 1998;26:219225.

37. Malament KA, Socransky SS. Survival of Dicor glass-ceramicdental restorations over 14 years: Part I: survival of Dicor com-plete coverage restorations and effect of internal surface acidetching, tooth position, gender, and age. J Prosthet Dent1999;81:2332.

38. Yi Y, Kelly JR. Effect of occlusal contact size on interfacialstresses and failure of a bonded ceramic: FEA and monotonicloading analyses. Dent Mater 2008;24:403409.

39. Kern M, Thompson VP. Bonding to glass inltrated aluminaceramic: adhesive methods and their durability. J Prosthet Dent1995;73:240249.

40. al-Wahadni A, Martin DM. Glazing and nishing dental porce-lain: a literature review. J Can Dent Assoc 1998;64:580583.

41. Latta M, Barkmeier WW. Approaches for intraoral repair of ceramic restorations. Compend Contin Educ Dent 2000;21:635639.

Address for correspondence:Dr J Robert Kelly

University of Connecticut Health Center263 Farmington AvenueFarmington, CT 06030

USAEmail: [email protected]



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Ceramic Materials in Dentistry