OONTINUINC EDUOATION 1DENTAL CERAMICS

Dental Ceramics: A Current ReviewNathaniel C. Lawson, DMD, PhD; and John O. Burgess, DDS, MS

Abstract: Ceramics are used for many dental applications and are characterizedin various ways, including by their hardness, brittleness, thermal and electricalinsulation, and biocompatibility. The ceramics most commonly used in dentistiyare oxides, particularly silicon dioxide (SiOi), or silica; aluminum oxide (AI2O3), oralumina; and zirconium dioxide (ZrO2), or zirconia. This article reviews the micro-structure of current dental ceramic materials and how it relates to their mechanicalproperties, clinical techniques, and optical properties. Typical ceramics currently inuse are described, and their clinically relevant properties such as strength, fracture,polishability, and wear are compared. Cementation methods are also discussed.

LEARNING OBJECTIVES

" categorize all-ceramicrestorative materiaisaccording to glass/crystaicontent

prescribe type, design, and fabrication methodof ail-ceramic restorativemateriais

ciioose surface finisii foraii-ceramic restorationand determine metiiodof cementing

Metals, polymers, ceramics, and composites areused in dentistry to restore teeth. Ceramics areused for many dental applications ranging fromthe glass in clinicians' loupes to the filler in re-storative resins. Ceramics are characterized bytheir hardness, brittleness, thermal and electrical insulation, andbiocompatibility. The ceramics most commonly used in dentistryare oxides, particularly silicon dioxide (SiO,), aluminum oxide(AljOg), and zirconium dioxide (ZrO^). The nomenclature for nam-ing oxide ceramics is achieved by removing the suffix of the metallicatom and replacing it with ~a; for example, silicon dioxide becomessilica. This article reviews the microstructure of current ceramicmaterials and how it relates to their mechanical properties, clinicaltechniques, and optical properties. Typical ceramics currently inuse are described and their clinically relevant properties compared.

Figi.

Fig 1. Microstructure of a giass (A), crystai (B), and partiaily crystalline(C) ceramic. Spiieres represent metaiiic and non-metaiiic eiements;rods represent chemicai bonds.

OverviewDental ceramics can be divided into primarily glass-containing(ie, feldspathic porcelain), reinforced glass (ie, leucite and lithiumdisilicate), and crystalline (ie, zirconia and alumina). The mostfrequent clinical failure of bilayered all-ceramic restorations ischipping of veneering porcelain caused by core-veneer coeffi-cient of thermal expansion (CTE) mismatch, surface grinding,inadequate core design, or overloading. Monolithic crystallineceramic crowns have a smaller incidence of fracture, because theseceramic materials are stronger than reinforced glass ceramics.Crystalline (zirconia and alumina) and reinforced glass ceram-ics (lithium disilicate) produce less opposing enamel wear thanveneered porcelain. Polishing ceramic restorations after occlusaladj ustments typically produces less opposing wear than stainingor glazing the restoration.

Ceramic crowns can either be traditionally cemented or ad-hesively bonded depending upon several factors, including: thestrength of the ceramic used; the retentiveness of the preparation;whether the preparation is in dentin or enamel; and the ability toisolate. Porcelain and reinforced glass ceramics should be etchedwith hydrofluoric acid (HF) and silanated prior to bonding. Zirconiaand alumina crowns should be tribochemically coated, 10-methac-ryloyloxydecyldihydrogen phosphate (MDP)-primed, or both priorto adhesive bonding. Bonding with resin cements produces a higherbond of dentin to porcelain and glass ceramics than traditionalcementing with resin-modified glass ionomer. Cementing untreat-ed zirconia to dentin with resin-modified glass ionomer or many

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Fig 2. Chipping of veneering iayer of aii-ceramic restoration.

resin cements produces a sim-ilar bond strength. Adhesiveresin bonding strengthensfeldspathic porcelain but notnecessarily glass ceramics orpolycrystalline ceramics.

CeramicMicrostructureCeramics are composed of ametal and non-metal element.In a liquid state, these ele-ments are freely moving. Uponsolidification, these elementscan either arrange themselvesin an ordered structured crystal or into an amorphous unstructuredglass. Generally, cooling a ceramic material slowly will allow it timeto solidify into a ciystal, whereas cooling it rapidly forces the atomsinto random orientations as seen in a glass (Figure 1, elements Aand B). The process of heating a crystalline or partially crystallineceramic and then rapidly cooling it, thereby creating a glassy coat-ing, is termed vitrification. A crown can be intentionally vitrified inorder to self-glaze its surface or unintentionally de-vitrified duringthe fabrication process, increasing its opacity.

The microstructure of the ceramic determines its mechanicaland optical properties. Crystalline ceramics have atoms arrangedinto closely packed crystals with a high atomic density; glasses havea lower atomic density. Therefore, a crack propagating through acrystalline ceramic will have to break more atomic bonds per unitarea than a crack travelling through the same unit area of a glassceramic. Thus, crystalline ceramics are generally stronger thanglass ceramics. The lower atomic density of glass also allows light topass through it, which causes it to be translucent. Crystal ceramics,conversely, are typically opaque. There are some exceptions (suchas cubic zirconia or quartz), where the crystalline microstructure ofthe ceramic corresponds to the wavelength of light and the crystal istranslucent. In summary, assuming proper processing techniquesare used, higher crystal content of a ceramic generally contributesto higher strength and decreased translucency.

The crystal content of a ceramic also affects its CTE, and, re-sultantly, crystal content is used to match the CTE of a veneer-ing ceramic to its supporting core material. Crystals have a lower

CTE than glasses. Additionally,ceramics are stronger in com-pression than in tension orshear. Therefore, veneeringceramics need to have a lowerCTE than the core materialthey are covering in order toplace the veneering ceramicin compression. When fabri-cating a bilayered crown, thecore and veneer material faseat the melting temperature ofthe veneer. As the two materi-als cool, the veneering ceramic(with a lower CTE) will shrink

less than the core material, causing the core material to squeeze theveneer with compressive force. The greater shrinkage of the corematerial places the veneer under compression, thus strengthening it.

Although it is helpful to generalize ceramics as either crystals orglasses to explain their physical properties, in reality most ceramicshave both crystal and glass phases (Figure 1, element C). Varioustypes of ceramics are used in current dental prostheses.

Types of CeramicsCurrent dental ceramics can be classified in three categories: porce-lain, which contains mostly glass phase; glass ceramics, which havea high concentration of reinforcing crystal content; and polycrystal-line ceramics, which are composed of mostly crystals. Strength andtoughness values of each category are presented in Table 1.

PorcelainDental porcelain is the most translucent type of ce-ramic and is typically used for esthetic applications such as ve-neers or veneering core materials. Dental porcelain, also calledfeldspathic porcelain, is a specific type of ceramic composed offeldspar, kaolinite, and quartz. The feldspar contributes the glassymatrix of the porcelain, and the kaolinite and quartz contributereinforcing crystals of alimiina and silica. Since porcelain is alsothe weakest ceramic, it is either used as a veneer of a stronger corematerial or chemically bonded to the underlying tooth to increaseits strength. When used to veneer a core material, the crystal con-tent and CTE of the porcelain is adjusted to match the CTE of thematerial it is covering (ie, higher crystal content and lower CTEwhen veneering zirconia than metal).

TABLE 1

Dental Ceramic Materials'

CERAMIC MATERIALFeldspathic porcelainLeucite-reinforced porcelainLithium disilicateGlass-infused aluminaGlass-infused zirconiaZirconia

Strength and Toughness Values

FLEXURAL STRENGTH (MPa)80100 to 120360 to 4004005509 to 1,100

FRACTURE TOUGHNESS (MPam'^^)1.11.22.54.55.57 t o l l

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Glass ceramicsCeramics in this category contain a high con-centration of reinforcing crystals. Crystals embedded within a glassmatrix help to deflect cracks and therefore strengthen the ceramic'Leucite (KAlSi^ Oj^ crystals are used to strengthen feldspathic por-celain while maintaining its translucency and decreasing its CTE.Lithium disilicate (LiS^ O^^ is a needle-shaped crystal composedof 30% amorphous silica and 70% crystalline lithium-disilicatecrystals with increased flexural strength but decreased translu-cency compared to feldspathic porcelain. These materials providea balance between strength and translucency that allows them tobe used as monolithic (single-layered) prostheses for restorationof anterior teeth.

Polycrystalline ceramicsThe last category of dental ceramicsis polycrystals. The two common polycrystals (also called metalceramics) are alumina and zirconia. In these materials, zirconiumor aluminum and oxygen atoms arrange in specific crystal patterns.Auniform arrangement of atoms in a given crystal pattern forms acrystalline grain, and a bulk piece of zirconia or alumina containsmany crystalline grains. A unique property of zirconia is that itundergoes transformation toughening. In this process, grains ofthe more compact tetragonal crystal structure of zirconia expandinto the monoclinic phase, induced by a propagating crack. The ex-pansion causes compression and halting of the crack.^ Polycrystalsare the most opaque and strongest class of ceramics and have beentypically used as core materials veneered with porcelain or formonolithic posterior crowns. Current research to produce moretranslucent forms of these materials by manipulating grain sizeand varying doping agents to produce more esthetic monolithicrestorations is underway.^

Resin ceramics represent a new category of ceramics. Thesematerials contain a high content of ceramic particles (more than80% wt) in a resin matrix and are essentially very highly filledresin composites that offer a high surface polish, high elasticity,and shorter milling times.

Clinically Relevant Properties of Dental CeramicsIn this section, clinically relevant properties of dental ceramics areaddressed, and a comparison of these properties between classesof materials is provided.

Strength and Fracture PropertiesCeramics are brittle, meaning that they tend to fracture withoutsignificant deformation (ie, high strength but relatively low tough-ness). Failures are more frequent in the veneering material (Figure2), as reported in a recent systematic review of 3-year clinical trialsof zirconia fixed partial dentures (FPDs), which found a 27% chip-ping occurrence in the veneering porcelain and only a 1% incidenceof fracture of the framework.* The severity of the veneer fracturewill determine whether the defect can be polished, repaired withcomposite, or if the restoration requires replacement. Reasonsfor replacing the restoration include fractures that extend to afunctional area (ie, occlusal contact or supporting a connector) orproduce unacceptable anatomic contours or esthetics."

Veneer ft-acture typically originates either in the veneering ceramicor at the core-veneer interface. An in-vitro study of lithium-disilicate

crowns loaded to fracture discovered that the failure mode was 75%core veneer interfacial, 20% within the veneering material, and 5%core fractures."" Fractographic analysis of 19 failed veneered zirconiacrowns found 10 fractures originating from the surface of the ve-neer and six originating from the core-veneer interface.' Interfacialfractures of bilayered all-ceramic restorations have been attributedto tensile preloads in the veneer created by mismatched CTEs ofthe core and veneer ceramics. Adjusting the CTE of the veneeringceramic can help improve the bond between the veneer and thecore.^ '^ Another source of interfacial failure is the bond betweenthe veneer and core material. Unfortunately, this bond is poorlyunderstood and may develop from mechanical or chemical bond-ing.'" Surface treatments (particle-abrasion and liners) have beendeveloped to improve the bond, however studies" '^ have reportedthat these techniques provide limited improvement. Cohesive failurewithin the veneering material results from inadequate core sup-port of the veneer, overloading of the crown, and surface defectsfrom adjustments.' Cores that are designed to support an even layerof veneering ceramic (particularly at cusps and marginal ridges)perform better than cores of uniform thickness.'*''^ Anatomic coresincrease the fracture strength of the restoration by 30%."^ Basedon this information, clinical recommendations to prevent fractureinclude proper occlusal adjustment and polishing with a heatlessstone and rubber polisher, as well as ensuring that the laboratoryhas adequately designed an anatomical core framework and chosena veneering porcelain that matches the core CTE.

COMPRESSION

Fig 3.

_ COMPRESSION

Fig 4.

Fig 3. Compressive and tensile zones of a crown. Fig 4. Compressiveand tensile zones of a fixed partial denture.

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Recently, monolithic or full-contour restorations have becomepopular because they avoid porcelain veneer chipping. The choiceof material for a monolithic restoration is partially based on thestrength of the ceramic and the amount of tooth reduction thatis possible. Lin reported that the biaxial flexural strength of ce-ramics increases with increased crystal content: 163.95 MPa forleucite, 365.06 MPa for lithium disilicate, and 1,039.71 MPa forzirconia." Based on these differences, manufacturers have recom-mended axial tooth reduction for posterior monolithic crowns of1.5 mm for lithium disilicate and 0.6 mm for zirconia. An in-vitrotest reported an ultimate crown fracture strength of 1,668 MPafor 0.6 mm uniform monolithic zirconia, 2,026 MPa for 1.5 mmuniform monolithic lithium disilicate, and 1,465 MPa for 1.2 mmuniform monolithic lithium disilicate after thermocycling andload cycling.'" When preparing full-contour crowns, it is there-fore important to ensure proper tooth reduction for the selectedrestorative material.

Fractures that initiate in the core material arise from radial crackson the internal surface of the crown.''^ Ceramics are subjected to ten-sile forces at the internal surfaces of crowns, making them more sus-ceptible to fracture in this area (Figure 3). Grinding the ceramic corewith a rough diamond has shown to decrease the flexural strengthof the material,^ " so clinicians should adjust the tooth preparationinstead of the intaglio surface of the ceramic crown when fitting anall-ceramic crown. Another source of core fracture is at the FPDconnectors with inadequate connector area. Recommendations forconnector dimensions are 16 mm^ for lithium disilieate and 9 mm^for zirconia. Connector fractures originate from veneering porcelainin sharply contoured gingival embrasures where tensile stressesconcentrate (Figure 4). '^

Polishing and Wear PropertiesCeramic hardness ranges from 481 Hv to 647 Hv for veneeringporcelain to 1,354 Hv to 1,378 Hv for zirconia.-^ ^ '' Since enamelhas a hardness of300Hvto500 Hv,^ * concerns have been raisedthat ceramic restorations will cause destructive wear to oppos-ing teeth. Studies measuring the wear of enamel opposing zir-conia and lithium disilicate, however, have proven that thesehigh-strength ceramics produce less opposing enamel wear thanveneering ceramics or enamel itself.^ ""^ ^ For example, in-vitro

volumetric enamel wear from 400,000 chewing cycles measured0.33 mm^ against zirconia, 0.36 mm^ against lithium disilicate,2.15 mm'' against veneering porcelain, and 0.45 mm^ againstenamel. '^' When veneering porcelains are worn against enamel,the porcelain surface becomes rough from microfractures ofthe material. The rough surface of the porcelain is abrasive tothe enamel and results in opposing enamel wear (Figure 5 andFigure 6).^' High-strength ceramics do not fracture when wornagainst enamel; therefore, their surface remains smooth andwear-friendly to opposing enamel. Additionally, high-strengthceramics experience very little wear on their own surface. Therecent trend in full-contonr monolithic lithium-disilicate andzirconia crowns is partially justified by the wear-compatibilitybetween these ceramics and opposing enamel.

To maintain the smooth surface of ceramics after occlusal adjust-ments, it is important to polish the surface of the zirconia or lithiumdisilicate. There is significantly less opposing enamel wear whenceramics are polished following grinding than with grinding alone.^ "Studies have also compared opposing enamel wear after polishingand glazing zirconia. Glazing zirconia produces a 30-^m-to 50-[im-thick, relatively soft layer of glassy glaze.^ During function, this layerof glaze quickly wears away and the roughened glaze layer causeswear of opposing enamel. '^ Clinically, it is recommended to polishceramic restorations with a heatless alumina stone followed by asilica, silicon carbide, or diamond impregnated rubber polisher.^ '

Cementation MethodsCeramic prostheses can be cemented to a tooth preparation eitherthrough traditional cementing or adhesive bonding. Traditionalcementing relies on micromechanical retention, whereas adhesivebonding utilizes chemical and micromechanical retention (Figure7). Traditional cementation can be accomplished with glass-iono-nier cement (GIC), resin-modified glass-ionomer cement (RMGIC),or zinc-phosphate cement. Adhesive bonding implies that an adhe-sive bonding agent is applied to the tooth surface, a coupling agent(eg, silane) is applied to the ceramic, and the prosthesis is bondedwith a resin cement. The decision to cement or bond a restorationis based on several clinical factors such as: the type of restoringceramic, the substrate (enamel or dentin), the retentiveness of thepreparation, and the ability to isolate the tooth.

Fig 5 and Fig 6. All-ceramic restorations (Fig 5) and opposing dentition wear (Fig 6) produced by ail-ceramic feldspathic restorations.

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When choosing to bond an all-ceramic restoration, the protocoldiffers for the type of ceramic material. To chemically bond tofeldspathic porcelain and reinforced glass ceramics, hydrofluoricacid (Figure 8) and a silane coupling agent (Figure 9) are used.Etchant concentration and etch time vary depending on the ce-ramic substrate.^ "- Silane is a coupling molecule that bonds on oneend to silica in glass and bonds to the organic matrix of the resincement on the other end (Figure 10, element B). Silane is appliedto the etched intaglio surface of the ceramic crown and bondsthe porcelain or glass ceramic to the resin cement. Porcelain andglass ceramics are not alumina air-abraded as this will decreasethe strength of the ceramic.^ ^ (The cited abstract is a project com-pleted in the authors' laboratory, which showed a decrease inexural strength of lithium disilicate after air abrasion.) Bondingto zirconia or alumina cannot be accomplished with silane alonebecause there is not enough silica in these materials. Effectivestrategies for chemically bonding to polycrystalline ceramicsinclude tribochemical silica coating and 10-methacryloyloxydec-yl dihydrogen phosphate (MDP) monomer coating.-" The firststrategy is to air-abrade the ceramic surface with silica-coveredalumina particles, which both roughens the ceramic surface anddeposits silica on its surface. The deposited silica can then bondto silane or MDP coupling agents.^ '^ The second strategy is theuse of MDP, a bifunctional molecule that bonds to metal oxides(including zirconia) at one end and the organic matrix of resin onthe other (Figure 10, element A).^ '*'^ '

One factor to consider when luting ceramic crowns is the bondstrength of the cement. A study by Peutzfeldt compared the shearbond strength of dentin to porcelain (etched and silanated), leu-cite glass ceramic (etched and silanated), and zirconia (untreated).Cementing porcelain and leucite ceramic with zinc phosphate, glassionomer, and resin-modified glass ionomer produced lower bondstrengths than adhesively bonding them with most resin cements.Conversely, zirconia showed similar bond strength when cementedwith RMGIC as with many of the resin cements.^ ^ As mentionedpreviously, adhesively bonding zirconia with an MDP primer ortribochemical coating will increase its bond strength.^ '*''' The typeof resin cement, such as total-etch (separate etch and primer step),self-etch (separate primer step), or self-adhesive (no etch or prim-er), can affect the bond strength. Total-etch cements show a higherbond strength than self-etch or self-adhesive resin cements.'''^ Fortooth preparations with less than 3 mm of occlusal height or morethan 5 degrees of taper, adhesively bonding zirconia and lithium-disilicate crowns is recommended to achieve sufiicient retention.'*"

Another consideration when selecting a cement is its ability tostrengthen the ceramic material. Adhesively bonding feldspathicporcelain to tooth structure increases the fracture strength ofthe porcelain.*' Heintze demonstrated that leucite and lithium-disilicate crowns adhesively bonded with resin cement had higherfracture strength than those cemented with GIC.*^ Another studyby Al-Wahadni showed no difference in fracture strength of lithi-um-disilicate and alumina crowns cemented with GIC or bondedwith resin cement.**^ Clinical studies have shown no differencein the 8-year success rate of lithium-disilicate crowns cementedwith RMGIC or bonded with resin cement""*^

CERAMIC

Fig 7.

Fig 10.

Fig 7. Luting a ceramic restoration can be done via adhesive bondingthrough a chemicai bond (A) or with traditionai cementing throughmicromechanicai retention (B). Fig 8 and Fig 9. Etching a iithium-disiiicate crown with 5% HF for 20 seconds (Fig 8), and applying siiane(Fig 9) to the iithium-disiiicate crown. Fig 10. MDP molecuie (A) andsiiane molecuie (B) with bonding groups for organic resin (biue), siiica(green), and metal oxides (tan).

ConclusionThe choice of ceramic selected for a clinical application is de-pendent on the required strength and esthetics of the restoration.Broadly, polycrystalline ceramics are stronger and more opaquethan glass ceramics and porcelain. The type of ceramic selectedwill dictate the design of the restoration and the options for lut-ing the restoration. All-ceramic materials should be polishedfollowing delivery.

ABOUT THE AUTHORS

Nathaniel C. Lawson, DMD, PhDAssisianl Professor, University of Alabama at Birmingham School of Dentistry,Birmingham. Alabama

John O. Burgess, DDS, MSProfessor and Assistant Dean for Clinical Research, University of Alabama atBirmingham School of Dentistry, Birmingham, Alabama

Queries to the author regarding this course may be submitted toauthorqueries@aegiscomm.com.

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