DENTAL CERAMICS:THE STATE OF THE SCIENCE

C.W. FAIRHURSTSchool of DentistryMedical College of GeorgiaAugusta, Georgia 30912

Adv Dent Res 6:78-81, September, 1992

Abstract—This review covers the properties of dental ceramics.Castable systems, bioactive glass, PMF systems, CAD/CAM,and ceramic brackets in orthodontics are briefly discussed.Many of the advances made between 1960 and 1975 weredirected toward the understanding, controlling, and developingof new ceramic processes. New and deeper understanding ofthe structure of non-cry stalline solids, structural imperfections,sintering physics, and other physical phenomena related to themelting and solidification processes has brought ceramicsfrom the near-total art form process of the mid-century to thestatus of ahighly sophisticated science it enjoyed in the 1980's.

This manuscript is published as part of the proceedings of theNIH Technology Assessment Conference on Effects and Side-effects of Dental Restorative Materials, August 26-28,1991,National Institutes of Health, Bethesda, Maryland, and did notundergo the customary journal peer-review process.

O f the three basic materials—ceramics, metals, andpolymers—ceramics were the last to move into thehigh-technology phase of development.

As compared with metals and polymers, ceramicshave some important differences. They are brittle, exhibitingno ductility at environmental temperatures, whereas somemetals and polymers can be formed at such temperatures. Theyhave low thermal expansion, high strength, and stiffness ascompared with polymers.

Dental porcelain belongs to one class of ceramics which areprimarily glass. Dental porcelain that requires controlledthermal expansion contains a crystalline component and thusbecomes a glass-crystal composite. Industrially, this class ofceramics is large. Other ceramics are composed entirely ofcrystalline oxides that are sintered together, sometimes underhigh pressure. Changes in crystal structure when these ceramicsare heated and cooled can result either in useful properties,such as the partially stabilized zirconia, or in effects such ascracking and spalling. An understanding of the physicalprocesses occurring can permit these changes in crystallinephases to be utilized beneficially.

This review of dental ceramics covers research that dealswith nearly every aspect of these basic characteristics ofceramic science.

PROPERTIES OF CERAMICSYamada and Grenoble (1977) edited a compendium ofmanuscripts from a symposium entitled Dental Porcelain:The State of the Art—1977. This compendium covered theproperties of porcelain and metals used for porcelain-fused-to-metal (PFM) restorations, the technique of their use, and theesthetics of the restoration systems.

McLean (1983a) edited the proceedings of an InternationalSymposium on Ceramics, held in 1983. Although thissymposium bore the title of "ceramics", the subject matter wasconcerned primarily with feldspathic-based porcelains andaluminous porcelain (McLean, 1983b). The symposium dealtwith porcelain-fused-to-metal restorations in some detail, sincethis system was the one most generally used for crowns andbridges.

Although this review will cover mostly the last seven years,such a review would be seriously flawed if it did not mentionthe contributions of John McLean to the art and science ofdental ceramics. McLean and Hughes (1965) published thefirst of a series of papers describing the development ofaluminous core and veneer porcelains. Mixtures containing60% powdered A12O3, with a balance of silica, were capable ofbeing fired at temperatures compatible with existing ovens.The material was used to form a core upon which speciallyformulated veneer porcelain could be fired to gain the estheticappearance of tooth structure. McLean may have been the firstdental materials scientist to use the Timoshenko equation and

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biaxial flexure test on dental porcelain. The strength valueobtained from the aluminous core material was ten times thatof feldspathic porcelain. He found that there was limitedapplication for the use of these materials for bridges, since theocclusal tables could not be adequately reinforced (McLeanand Kedge, 1988).

In 1985, the Fourth International Symposium on Ceramicswas held, and the proceedings were edited by Preston andpublished in 1988 (McLean and Kedge, 1988). In thisconference, the feldspathic-based porcelains and the PFMrestorations shared the materials emphasis with the neweraluminous core porcelains, non-shrink alumina ceramicmaterials, and glass composites, as well as with the applicationsof these systems in dentistry.

The Cerestore material depended upon the conversion ofalumina and magnesium oxide to a magnesium aluminatespinel with a net volume increase to overcome sinteringshrinkage. Morena et al. (1986) reported on the fatigue andstrength characteristics of this fine-grained ceramic. Thefatigue parameter, n, was five times greater than that forfeldspathic porcelain, and the strength was nearly twice thatof feldspathic porcelain. They also included aluminousporcelain in this study and showed that, although the drystrength was the same as for the fine-grained ceramic, thefatigue parameter was only slightly higher. This demonstratedthat, unlike the glass-based porcelain, the aluminous ceramicmaterial was unaffected by the moist environment. Manteet al. (1987) further characterized the fine-grained ceramicby examining the microstructure and measuring the fracturetoughness.

CASTABLE CERAMICSThe Dicor product is a castable glass ceramic. Grossman(1988) has traced the history and chemistry of glass ceramicstechnology to the original work of S.D. Stookey at CorningGlass Works in 1952, and he also cites MacCulloch (1968) asthe first to recognize the potential for the use of glass ceramicsin dental applications.

Glass ceramics are produced by precipitation of one or morefinely-divided phases when cooled from a melted stage. Thesefine particles and their distribution increase the fracturetoughness and strength of the glass. The Dicor product is acomplex mixture of potassium oxide, magnesium oxide, silicondioxide, fluorides, and zirconium dioxide. Dicor can be meltedand cast by means of lost-wax casting techniques. The propertiesare optimized by the heating of the cast piece in a furnaceaccording to a prescribed program. The color of the restorationcomes from a shaded porcelain coat.

The properties of the Dicor material were compatiblewhen used in single-crown applications (Malament, 1988).The strength of Dicor was higher than that of the porcelains.However, the elastic modulus, coefficient of expansion,and hardness were lower. The esthetic qualities wereacceptable.

Moffa et al. (1986, 1988) reported clinical evaluations onthe castable glass crowns and concluded that the materialperformed well if adequate thickness was maintained on theocclusal surface.

BIOACTIVE GLASSHench et al. (1971) have been working with bioacti ve glass andceramics for 20 years. They define bioactivity as thecharacteristic of a material that allows it to form a bond withliving tissue. This property makes the material suitable forimplants (Hench, 1988) and implantable products, such asendodontic pulp-capping materials (Oguntebi et al., 1991).

In the proceedings of the Fourth International Symposiumon Ceramics, Tamura (1988) reported on the fabrication ofhydroxyapatite crowns. This material was also a castablecomplex glass system of oxides of calcium, phosphorus, andsilicon. The general similarity to the apatite structure in thebody, bone, and teeth was claimed as its primary asset.

Also, Morena et al. (1986) reported on the use of partiallystabilized zirconia, PSZ, to toughen dental feldspathic porcelain.This technology was well-known in ceramic science.Toughened alumina and zirconia are commercial products inthe ceramics industry. Fifteen percent overall additions of PSZresulted in modest toughening and a significant increase infracture stress. The esthetic qualities of the toughened porcelainwere significantly reduced.

PFM SYSTEMSAlthough the interest in greater strength and increased fracturetoughness in ceramics remains high, the principal system forcrown and bridge restorations today is porcelain fused tometal. Alloy development for PFM use was a major topic ofresearch in the '80's. NiCr- and palladium-based alloys havebeen studied extensively (Bagby et al., 1985; Jochen et al,1985; van der Zel and Vrijhoef, 1985; Baran, 1985; Pask andTomsia, 1985). Later alloy research had turned to titanium asa metal for PFM applications (Hautaniemi and HerR, 1991;Volpe and Caddoff, 1991; Tamaki et al, 1991; Nonaka andBaez, 1991; Baez et al, 1991; Almquist et al, 1991). Thebiocompatibility as seen in the titanium implant research maybe influencing researchers to examine titanium's suitability forPFM applications. During the last several years, researchattention has been directed toward a better understanding of thenature of the porcelain-metal bond. Studies have been conductedon thermal compatibility (Searle et al, 1985; Anusavice et al,1985; Pines et al, 1985), microstructure compositional effects(Rizkalla et al, 1989; Jones et al, 1989), and porcelainviscosity (Twiggs et al, 1990).

Other systems designed for improved properties have beenintroduced or developed. O'Brien^ al (1990) have developeda magnesium core crown, and a commercial product, OPTEC-HSP (Vaidyanathan et al, 1988), has been introduced. Thislatter material is a high-strength porcelain. There have beenrenewed attempts at further development of high-strengthaluminous systems (Kelly et al, 1990).

Groh et al (1989), Peregrina et al (1989), and O'Brien etal (1988) have concerned themselves with the very importantaspect of color and esthetics of porcelain whether used for PFMor as a final veneering for core material systems. Jones et al(1988) have taken an additional step in considering the estheticsof translucent restorations by assessing the effects of theunderlying cement.

FAIRHURST ADV DENT RES SEPTEMBER 1992

Seghi et al (1988) studied the relationship between visualand colorimetric analyses of porcelain. A frequently researchedsubject has been the effect of acidulated gel on the surfacecharacteristic of porcelain (Boudrias, 1988).

CAD/CAMThe interest in CAD/CAM has grown in the last few years(Rekow et al, 1990). The introduction of a commercialproduct has stimulated further interest (Isenberg et al, 1991;Rekow et al, 1991; Kleier et al, 1991). Rekow et al (1991)summed up the state of the art of CAD/CAM thus; "It isfeasible for crowns produced with a CAD/CAM system to fitat least as well as those produced with ideal casting conditions."The benefits of this system are that impressions are not needed,which saves the dentist chair time and removes one asepsis linkbetween the patient-dentist operational field and the dentallaboratory worker. Other benefits and costs may becomeevident with additional study. The growing trend towardconsideration of the solution of esthetic concerns as atreatmentgoal brings together the orthodontist and the restorative dentist.Although orthodontic materials are not restorative materials,the restorative materials scientist then becomes concernedwith these materials.

CERAMICS IN ORTHODONTICSNew orthodontic applications of materials have occurred rapidlysince the introduction of acid-etch bonding. Alumina bracketsin both polycrystalline and single-crystal form are in commonuse, although stainless steel brackets are still the major type inuse. The original concerns with the use of non-metal bracketswere whether the composite bonding agent offered strengthsufficient to hold the bracket under the high stresses developedwith steel wires (Stich et al, 1985). As it has turned out, thechemical attachment is too strong (Tremblay^r al, 1990). Thebrackets sometimes break during their removal, leaving thebracket base and composite on the tooth to be removed bymeans of diamond rotary instruments. Thi§ can lead todamaged facial surfaces of the tooth, requiring restorativetreatment. Occasionally, enamel will be broken from the tooth,creating significant damage (Gunn and Powers, 1990)

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