DENTAL CERAMICS: THE STATE OF THE SCIENCE C.W. FAIRHURST
School of Dentistry Medical College of Georgia Augusta, 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 were directed toward the understanding, controlling, and developing of new ceramic processes. New and deeper understanding of the structure of non-cry stalline solids, structural imperfections, sintering physics, and other physical phenomena related to the melting and solidification processes has brought ceramics from the near-total art form process of the mid-century to the status of ahighly sophisticated science it enjoyed in the 1980's.
O
f the three basic materials—ceramics, metals, and polymers—ceramics were the last to move into the high-technology phase of development. As compared with metals and polymers, ceramics have some important differences. They are brittle, exhibiting no ductility at environmental temperatures, whereas some metals and polymers can be formed at such temperatures. They have low thermal expansion, high strength, and stiffness as compared with polymers. Dental porcelain belongs to one class of ceramics which are primarily glass. Dental porcelain that requires controlled thermal expansion contains a crystalline component and thus becomes a glass-crystal composite. Industrially, this class of ceramics is large. Other ceramics are composed entirely of crystalline oxides that are sintered together, sometimes under high pressure. Changes in crystal structure when these ceramics are heated and cooled can result either in useful properties, such as the partially stabilized zirconia, or in effects such as cracking and spalling. An understanding of the physical processes occurring can permit these changes in crystalline phases to be utilized beneficially. This review of dental ceramics covers research that deals with nearly every aspect of these basic characteristics of ceramic science.
PROPERTIES OF CERAMICS
This manuscript is published as part of the proceedings of the NIH Technology Assessment Conference on Effects and Sideeffects of Dental Restorative Materials, August 26-28,1991, National Institutes of Health, Bethesda, Maryland, and did not undergo the customary journal peer-review process.
78
Yamada and Grenoble (1977) edited a compendium of manuscripts from a symposium entitled Dental Porcelain: The State of the Art—1977. This compendium covered the properties of porcelain and metals used for porcelain-fused-tometal (PFM) restorations, the technique of their use, and the esthetics of the restoration systems. McLean (1983a) edited the proceedings of an International Symposium on Ceramics, held in 1983. Although this symposium bore the title of "ceramics", the subject matter was concerned primarily with feldspathic-based porcelains and aluminous porcelain (McLean, 1983b). The symposium dealt with porcelain-fused-to-metal restorations in some detail, since this system was the one most generally used for crowns and bridges. Although this review will cover mostly the last seven years, such a review would be seriously flawed if it did not mention the contributions of John McLean to the art and science of dental ceramics. McLean and Hughes (1965) published the first of a series of papers describing the development of aluminous core and veneer porcelains. Mixtures containing 60% powdered A12O3, with a balance of silica, were capable of being fired at temperatures compatible with existing ovens. The material was used to form a core upon which specially formulated veneer porcelain could be fired to gain the esthetic appearance of tooth structure. McLean may have been the first dental materials scientist to use the Timoshenko equation and
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6
79
DENTAL CERAMICS
biaxial flexure test on dental porcelain. The strength value obtained from the aluminous core material was ten times that of feldspathic porcelain. He found that there was limited application for the use of these materials for bridges, since the occlusal tables could not be adequately reinforced (McLean and Kedge, 1988). In 1985, the Fourth International Symposium on Ceramics was held, and the proceedings were edited by Preston and published in 1988 (McLean and Kedge, 1988). In this conference, the feldspathic-based porcelains and the PFM restorations shared the materials emphasis with the newer aluminous core porcelains, non-shrink alumina ceramic materials, and glass composites, as well as with the applications of these systems in dentistry. The Cerestore material depended upon the conversion of alumina and magnesium oxide to a magnesium aluminate spinel with a net volume increase to overcome sintering shrinkage. Morena et al. (1986) reported on the fatigue and strength characteristics of this fine-grained ceramic. The fatigue parameter, n, was five times greater than that for feldspathic porcelain, and the strength was nearly twice that of feldspathic porcelain. They also included aluminous porcelain in this study and showed that, although the dry strength was the same as for the fine-grained ceramic, the fatigue parameter was only slightly higher. This demonstrated that, unlike the glass-based porcelain, the aluminous ceramic material was unaffected by the moist environment. Mante et al. (1987) further characterized the fine-grained ceramic by examining the microstructure and measuring the fracture toughness.
CASTABLE CERAMICS The Dicor product is a castable glass ceramic. Grossman (1988) has traced the history and chemistry of glass ceramics technology to the original work of S.D. Stookey at Corning Glass Works in 1952, and he also cites MacCulloch (1968) as the first to recognize the potential for the use of glass ceramics in dental applications. Glass ceramics are produced by precipitation of one or more finely-divided phases when cooled from a melted stage. These fine particles and their distribution increase the fracture toughness and strength of the glass. The Dicor product is a complex mixture of potassium oxide, magnesium oxide, silicon dioxide, fluorides, and zirconium dioxide. Dicor can be melted and cast by means of lost-wax casting techniques. The properties are optimized by the heating of the cast piece in a furnace according to a prescribed program. The color of the restoration comes from a shaded porcelain coat. The properties of the Dicor material were compatible when 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 were acceptable. Moffa et al. (1986, 1988) reported clinical evaluations on the castable glass crowns and concluded that the material performed well if adequate thickness was maintained on the occlusal surface.
BIOACTIVE GLASS
Hench et al. (1971) have been working with bioacti ve glass and ceramics for 20 years. They define bioactivity as the characteristic of a material that allows it to form a bond with living tissue. This property makes the material suitable for implants (Hench, 1988) and implantable products, such as endodontic pulp-capping materials (Oguntebi et al., 1991). In the proceedings of the Fourth International Symposium on Ceramics, Tamura (1988) reported on the fabrication of hydroxyapatite crowns. This material was also a castable complex glass system of oxides of calcium, phosphorus, and silicon. The general similarity to the apatite structure in the body, bone, and teeth was claimed as its primary asset. Also, Morena et al. (1986) reported on the use of partially stabilized zirconia, PSZ, to toughen dental feldspathic porcelain. This technology was well-known in ceramic science. Toughened alumina and zirconia are commercial products in the ceramics industry. Fifteen percent overall additions of PSZ resulted in modest toughening and a significant increase in fracture stress. The esthetic qualities of the toughened porcelain were significantly reduced.
PFM SYSTEMS Although the interest in greater strength and increased fracture toughness in ceramics remains high, the principal system for crown and bridge restorations today is porcelain fused to metal. Alloy development for PFM use was a major topic of research in the '80's. NiCr- and palladium-based alloys have been studied extensively (Bagby et al., 1985; Jochen et al, 1985; van der Zel and Vrijhoef, 1985; Baran, 1985; Pask and Tomsia, 1985). Later alloy research had turned to titanium as a metal for PFM applications (Hautaniemi and HerR, 1991; Volpe and Caddoff, 1991; Tamaki et al, 1991; Nonaka and Baez, 1991; Baez et al, 1991; Almquist et al, 1991). The biocompatibility as seen in the titanium implant research may be influencing researchers to examine titanium's suitability for PFM applications. During the last several years, research attention has been directed toward a better understanding of the nature of the porcelain-metal bond. Studies have been conducted on 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 porcelain viscosity (Twiggs et al, 1990). Other systems designed for improved properties have been introduced or developed. O'Brien^ al (1990) have developed a magnesium core crown, and a commercial product, OPTECHSP (Vaidyanathan et al, 1988), has been introduced. This latter material is a high-strength porcelain. There have been renewed attempts at further development of high-strength aluminous systems (Kelly et al, 1990). Groh et al (1989), Peregrina et al (1989), and O'Brien et al (1988) have concerned themselves with the very important aspect of color and esthetics of porcelain whether used for PFM or as a final veneering for core material systems. Jones et al (1988) have taken an additional step in considering the esthetics of translucent restorations by assessing the effects of the underlying cement.
FAIRHURST
Seghi et al (1988) studied the relationship between visual and colorimetric analyses of porcelain. A frequently researched subject has been the effect of acidulated gel on the surface characteristic of porcelain (Boudrias, 1988).
CAD/CAM The interest in CAD/CAM has grown in the last few years (Rekow et al, 1990). The introduction of a commercial product 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 is feasible for crowns produced with a CAD/CAM system to fit at 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 link between the patient-dentist operational field and the dental laboratory worker. Other benefits and costs may become evident with additional study. The growing trend toward consideration of the solution of esthetic concerns as atreatment goal brings together the orthodontist and the restorative dentist. Although orthodontic materials are not restorative materials, the restorative materials scientist then becomes concerned with these materials.
CERAMICS IN ORTHODONTICS New orthodontic applications of materials have occurred rapidly since the introduction of acid-etch bonding. Alumina brackets in both polycrystalline and single-crystal form are in common use, although stainless steel brackets are still the major type in use. The original concerns with the use of non-metal brackets were whether the composite bonding agent offered strength sufficient to hold the bracket under the high stresses developed with steel wires (Stich et al, 1985). As it has turned out, the chemical attachment is too strong (Tremblay^r al, 1990). The brackets sometimes break during their removal, leaving the bracket base and composite on the tooth to be removed by means of diamond rotary instruments. Thi§ can lead to damaged facial surfaces of the tooth, requiring restorative treatment. Occasionally, enamel will be broken from the tooth, creating significant damage (Gunn and Powers, 1990)
REFERENCES Almquist P, Chan DCN, Blackman R, Kaiser DA (1991). The effects of sprue number and position on titanium crown margins (abstract). J Dent Res 70 (Spec Iss):486. Anusavice KJ, Hojjatie B, DeHoff PH (1985). Influence of metal thickness on stress distribution in metal-ceramic crowns (abstract). J Dent Res 64 (Spec Iss):246. Baez RJ, Nonajca T, Blackman R (1991). Ti castability and surface characteristics with three phosphate bonded investments (abstract). J Dent Res 70 (Spec Iss):486. Bagby M, Marshall SJ, Marshall GW Jr, Noyan IC (1985). Residual surface stress jn Pd-based metals (abstract). / Dent Res 64 (Spec Iss):227. Baran G (1985). Oxide heterogeneity and composition on NiCr alloys (abstract). J Dent Res 64 (Spec Iss):227. Boudrias P (1988). Counteracting the effect of acidulated
ADV DENT RES SEPTEMBER 1992
fluoride gel on cast glass ceramic (abstract). J Dent Res 67 (Spec Iss): 119. Groh CL, O'Brien WJ, Stone ME (1989). Solubility of dental porcelains and color changes of stain glazes (abstract). / Dent Res 68 (Spec Iss):235. Grossman DG (1988). The science of castable glass ceramics. In: Preston JD, editor. Perspectives in dental ceramics. Proceedings of the Fourth International Symposium on Ceramics. Chicago (IL): Quintessence Publishing Co., Inc., 117. Gunn S, Powers JM (1990). Strength of ceramics brackets in shear and torsion (abstract). J Dent Res 69 (Spec Iss):313. Hautaniemi ^ f Her0 H (1991). Interface structure and bond strength between Ti and porcelain (abstract). J Dent Res 70 (Spec Is$):485. HenchLL(1988). Part II. Bioactive ceramics. In:Ducheyne P, Lemons J, editors. Bio-ceramics: materials characteristics vs. in vivo behavior. Ann NY Acad Sci 523:54-71. Hench LL, Splinter RJM, Allen WC, Greenlee TK (1971). Bonding mechanisms at the interface of ceramic prosthetic materials. J Biomed Mater Res 5:117-141. Isenberg BP, Essig ME, Leinfelder KF, Mueninghoff LA (1991). Clinical evaluation of Cerec® CAD/CAM restorations emphasizing marginal integrity (abstract). / Dent Res 70 (Spec Iss):434. Jochen DG, Caputo AA, Matyas J (1985). Re-use of silver palladium ceramic metal (abstract). / Dent Res 64 (Spec Iss):227. Jones DW, Rizkalla AS, Johnson JA, Sutow EJ (1989). Effect of composition on some physical properties of SiO2-KiONa2O glasses (abstract). J Dent Res 68 (Spec Iss):323. Jones DW, Sutow EJ, Rizkalla AS, Black D (1988). Opacity and colour of a castable glass-ceramic and cement system (abstract). J Dent Res 67 (Spec Iss):44. Kelly JR, Pober RL, Cima MJ (1990). Dissolution of alumina during sintering of aluminous porcelain (abstract). JDent Kleier CW,LoscheGM,RouletJF(1991). Margin fit of CAD/ CAM produced crowns (abstract). / Dent Res 70 (Spec Iss):434. MacCulloch WT (1968). Advances in dental ceramics. Br DentJ 124:361-365. Malament KA (1988). The cast glass-ceramic crown. In: Preston JD, editor. Perspectives in dental ceramics. Proceedings of the Fourth International Symposium on Ceramics. Chicago (IL): Quintessence Publishing Co., Inc., 331. Mante F, Brantley WA, Dhuru VB, Ziebert GJ (1987). Characterization and fracture toughness of the high alumina shrink-free core ceramics (abstract). J Dent Res 66 (Spec Iss):270. McLean JW( 1983). Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago (IL): Quintessence Publishing Co., Inc. McLean JW (1983). The future for dental porcelain. In: Dental ceramics. Proceedings of the First International Symposium on Ceramics. Chicago (IL): Quintessence Publishing Co., Inc., 13.
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McLean JW, Hughes TH (1965). The reinforcement of dental porcelain with ceramic oxides. Br Dent / 119:251. McLean JW, Kedge MI (1988). High-strength ceramics. In: Preston JD, editor. Perspectives in dental ceramics. Proceedings of the Fourth International Symposium on Ceramics. Chicago (IL): Quintessence Publishing Co., Inc. Moffa JP, Lugassy AA, Ellison JA (1986). Clinical evaluation of a castable ceramic material—1 year (abstract). / Dent Res 65 (Spec Iss):343. Moffa JP, Lugassy AA, Ellison JA (1988). Clinical evaluation of a castable ceramic material. Three year study (abstract). J Dent Res 61 (Spec Iss):118. Morena R, Beaudreau GM, Lockwood PE, Evans AL, Fairhurst CW (1986). Fatigue of dental ceramics in a simulated oral environment. / Dent Res 65:993-997'. Morena R, Lockwood PE, Evans AL, Fairhurst CW (1986). Toughening of dental porcelain by tetragonal ZrO2 additions. J Am Ceram Soc 69:C75-C77. Nonaka T, Baez RJ (1991). Strength comparisons of cast titanium using three phosphate type investments (abstract). J Dent Res 70 (Spec Iss):485. O 'Brien WJ, Groh CL, Boenke KM (1988). A one dimensional color order system for shade guides. J Dent Res 67 (Spec Iss):119. O'BrienWJ,McPheeER,YamanP,DootzER(1990). Strength of magnesia core crowns (abstract). / Dent Res 69 (Spec Iss):211. Oguntebi B, Clark A, Wilson J, Hall M (1991). Pulp capping with bioglass and demineralized dentin in miniature swines (abstract). J Dent Res 70 (Spec Iss):384. Pask JA, Tomsia AP (1985). Oxidation and ceramic coatings on Ni80Cr20 alloy (abstract). J Dent Res 64 (Spec Iss):227. Peregrina A, Scott JO, Johnston WM (1989). Significant factors affecting the color of natural teeth and restorations (abstract). J Dent Res 68 (Spec Iss):235. Pines M, Sumithra N, Vaidyanathan TK, Shalita S, Schulman A (1985). Influence of additional firings on PFM bond strength. J Dent Res 64 (Spec Iss):246. Rekow ED, Speidel TM, Pong TC (1990). Data acquisition devices for a CAD/CAM system for automated production of dental restorations (abstract). / Dent Res 69 (Spec Iss):314.
81
Rekow ED, Thompson VP, Yang HS (1991). Margin fit of CAD/CAM produced crowns (abstract). / Dent Res 70 (Spec Iss):434. Rizkalla AS, Jones DW, Hall GC, King HW, Sutow EJ (1989). Chemical composition variables of feldspathic glass synthesized by sol-gel (abstract). / Dent Res 68 (Spec Iss):323. Searle JR, Twiggs SW, Ringle RD, Fairhurst CW (1985). Laser dilatometer measurements at rapid heating and cooling rates (abstract). J Dent Res 64 (Spec Iss):245. Seghi RR, Hewlett ER, Kim J (1988). Visual assessment of colorimetric analysis on translucent dental porcelains (abstract). J Dent Res 67 (Spec Iss):51. Stich FJ, Sarkar NK, Schwaninger B (1985). Early bond strengths of five commercial orthodontic direct bonding systems (abstract). J Dent Res 64 (Spec Iss):184. Tamaki Y, Miyazaki T, Lautenschlager EP, Greener EH (1991). Titanium castings with newly developed phosphate bonded investments (abstract). J Dent Res 70 (Spec Iss):475. TamuraK(1988). Fabrication of hydroxyapatite crowns. In: Preston JD, editor. Perspectives in dental ceramics. Proceedings of the Fourth International Symposium on Ceramics. Chicago (IL): Quintessence Publishing Co., Inc., 395. Tremblay R, Nathanson D, Gianelly A (1990). Clinical and laboratory evaluation of a ceramic bracket system (abstract). J Dent Res 69 (Spec Iss):313. Twiggs SW, Hashinger DT, Fairhurst CW (1990). Viscosities of porcelains formulated from the Weinstein patent. J Am Ceram Soc 73:446-449. Vaidyanathan TK, Prasad A, Vaidyanathan J (1988). Influence of heat treatment on a high strength ceramic (abstract). / Dent Res 67 (Spec Iss):118. van der Zel JM, Vrijhoef MMA (1985). Oxide formation of some palladium enriched ceramic alloys (abstract). JDent Res 64 (Spec Iss):227. Volpe C, Caddoff I (1991). Pressure arc casting of titanium, precious and non-precious alloys (abstract). JDent Res 70 (Spec Iss):485. YamadaHN, Grenoble PB (1977). Dental porcelains: the state of the art—1977. Los Angeles (CA): The University of Southern California, School of Dentistry.
School of Dentistry Medical College of Georgia Augusta, 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 were directed toward the understanding, controlling, and developing of new ceramic processes. New and deeper understanding of the structure of non-cry stalline solids, structural imperfections, sintering physics, and other physical phenomena related to the melting and solidification processes has brought ceramics from the near-total art form process of the mid-century to the status of ahighly sophisticated science it enjoyed in the 1980's.
O
f the three basic materials—ceramics, metals, and polymers—ceramics were the last to move into the high-technology phase of development. As compared with metals and polymers, ceramics have some important differences. They are brittle, exhibiting no ductility at environmental temperatures, whereas some metals and polymers can be formed at such temperatures. They have low thermal expansion, high strength, and stiffness as compared with polymers. Dental porcelain belongs to one class of ceramics which are primarily glass. Dental porcelain that requires controlled thermal expansion contains a crystalline component and thus becomes a glass-crystal composite. Industrially, this class of ceramics is large. Other ceramics are composed entirely of crystalline oxides that are sintered together, sometimes under high pressure. Changes in crystal structure when these ceramics are heated and cooled can result either in useful properties, such as the partially stabilized zirconia, or in effects such as cracking and spalling. An understanding of the physical processes occurring can permit these changes in crystalline phases to be utilized beneficially. This review of dental ceramics covers research that deals with nearly every aspect of these basic characteristics of ceramic science.
PROPERTIES OF CERAMICS
This manuscript is published as part of the proceedings of the NIH Technology Assessment Conference on Effects and Sideeffects of Dental Restorative Materials, August 26-28,1991, National Institutes of Health, Bethesda, Maryland, and did not undergo the customary journal peer-review process.
78
Yamada and Grenoble (1977) edited a compendium of manuscripts from a symposium entitled Dental Porcelain: The State of the Art—1977. This compendium covered the properties of porcelain and metals used for porcelain-fused-tometal (PFM) restorations, the technique of their use, and the esthetics of the restoration systems. McLean (1983a) edited the proceedings of an International Symposium on Ceramics, held in 1983. Although this symposium bore the title of "ceramics", the subject matter was concerned primarily with feldspathic-based porcelains and aluminous porcelain (McLean, 1983b). The symposium dealt with porcelain-fused-to-metal restorations in some detail, since this system was the one most generally used for crowns and bridges. Although this review will cover mostly the last seven years, such a review would be seriously flawed if it did not mention the contributions of John McLean to the art and science of dental ceramics. McLean and Hughes (1965) published the first of a series of papers describing the development of aluminous core and veneer porcelains. Mixtures containing 60% powdered A12O3, with a balance of silica, were capable of being fired at temperatures compatible with existing ovens. The material was used to form a core upon which specially formulated veneer porcelain could be fired to gain the esthetic appearance of tooth structure. McLean may have been the first dental materials scientist to use the Timoshenko equation and
VOL.
6
79
DENTAL CERAMICS
biaxial flexure test on dental porcelain. The strength value obtained from the aluminous core material was ten times that of feldspathic porcelain. He found that there was limited application for the use of these materials for bridges, since the occlusal tables could not be adequately reinforced (McLean and Kedge, 1988). In 1985, the Fourth International Symposium on Ceramics was held, and the proceedings were edited by Preston and published in 1988 (McLean and Kedge, 1988). In this conference, the feldspathic-based porcelains and the PFM restorations shared the materials emphasis with the newer aluminous core porcelains, non-shrink alumina ceramic materials, and glass composites, as well as with the applications of these systems in dentistry. The Cerestore material depended upon the conversion of alumina and magnesium oxide to a magnesium aluminate spinel with a net volume increase to overcome sintering shrinkage. Morena et al. (1986) reported on the fatigue and strength characteristics of this fine-grained ceramic. The fatigue parameter, n, was five times greater than that for feldspathic porcelain, and the strength was nearly twice that of feldspathic porcelain. They also included aluminous porcelain in this study and showed that, although the dry strength was the same as for the fine-grained ceramic, the fatigue parameter was only slightly higher. This demonstrated that, unlike the glass-based porcelain, the aluminous ceramic material was unaffected by the moist environment. Mante et al. (1987) further characterized the fine-grained ceramic by examining the microstructure and measuring the fracture toughness.
CASTABLE CERAMICS The Dicor product is a castable glass ceramic. Grossman (1988) has traced the history and chemistry of glass ceramics technology to the original work of S.D. Stookey at Corning Glass Works in 1952, and he also cites MacCulloch (1968) as the first to recognize the potential for the use of glass ceramics in dental applications. Glass ceramics are produced by precipitation of one or more finely-divided phases when cooled from a melted stage. These fine particles and their distribution increase the fracture toughness and strength of the glass. The Dicor product is a complex mixture of potassium oxide, magnesium oxide, silicon dioxide, fluorides, and zirconium dioxide. Dicor can be melted and cast by means of lost-wax casting techniques. The properties are optimized by the heating of the cast piece in a furnace according to a prescribed program. The color of the restoration comes from a shaded porcelain coat. The properties of the Dicor material were compatible when 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 were acceptable. Moffa et al. (1986, 1988) reported clinical evaluations on the castable glass crowns and concluded that the material performed well if adequate thickness was maintained on the occlusal surface.
BIOACTIVE GLASS
Hench et al. (1971) have been working with bioacti ve glass and ceramics for 20 years. They define bioactivity as the characteristic of a material that allows it to form a bond with living tissue. This property makes the material suitable for implants (Hench, 1988) and implantable products, such as endodontic pulp-capping materials (Oguntebi et al., 1991). In the proceedings of the Fourth International Symposium on Ceramics, Tamura (1988) reported on the fabrication of hydroxyapatite crowns. This material was also a castable complex glass system of oxides of calcium, phosphorus, and silicon. The general similarity to the apatite structure in the body, bone, and teeth was claimed as its primary asset. Also, Morena et al. (1986) reported on the use of partially stabilized zirconia, PSZ, to toughen dental feldspathic porcelain. This technology was well-known in ceramic science. Toughened alumina and zirconia are commercial products in the ceramics industry. Fifteen percent overall additions of PSZ resulted in modest toughening and a significant increase in fracture stress. The esthetic qualities of the toughened porcelain were significantly reduced.
PFM SYSTEMS Although the interest in greater strength and increased fracture toughness in ceramics remains high, the principal system for crown and bridge restorations today is porcelain fused to metal. Alloy development for PFM use was a major topic of research in the '80's. NiCr- and palladium-based alloys have been studied extensively (Bagby et al., 1985; Jochen et al, 1985; van der Zel and Vrijhoef, 1985; Baran, 1985; Pask and Tomsia, 1985). Later alloy research had turned to titanium as a metal for PFM applications (Hautaniemi and HerR, 1991; Volpe and Caddoff, 1991; Tamaki et al, 1991; Nonaka and Baez, 1991; Baez et al, 1991; Almquist et al, 1991). The biocompatibility as seen in the titanium implant research may be influencing researchers to examine titanium's suitability for PFM applications. During the last several years, research attention has been directed toward a better understanding of the nature of the porcelain-metal bond. Studies have been conducted on 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 porcelain viscosity (Twiggs et al, 1990). Other systems designed for improved properties have been introduced or developed. O'Brien^ al (1990) have developed a magnesium core crown, and a commercial product, OPTECHSP (Vaidyanathan et al, 1988), has been introduced. This latter material is a high-strength porcelain. There have been renewed attempts at further development of high-strength aluminous systems (Kelly et al, 1990). Groh et al (1989), Peregrina et al (1989), and O'Brien et al (1988) have concerned themselves with the very important aspect of color and esthetics of porcelain whether used for PFM or as a final veneering for core material systems. Jones et al (1988) have taken an additional step in considering the esthetics of translucent restorations by assessing the effects of the underlying cement.
FAIRHURST
Seghi et al (1988) studied the relationship between visual and colorimetric analyses of porcelain. A frequently researched subject has been the effect of acidulated gel on the surface characteristic of porcelain (Boudrias, 1988).
CAD/CAM The interest in CAD/CAM has grown in the last few years (Rekow et al, 1990). The introduction of a commercial product 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 is feasible for crowns produced with a CAD/CAM system to fit at 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 link between the patient-dentist operational field and the dental laboratory worker. Other benefits and costs may become evident with additional study. The growing trend toward consideration of the solution of esthetic concerns as atreatment goal brings together the orthodontist and the restorative dentist. Although orthodontic materials are not restorative materials, the restorative materials scientist then becomes concerned with these materials.
CERAMICS IN ORTHODONTICS New orthodontic applications of materials have occurred rapidly since the introduction of acid-etch bonding. Alumina brackets in both polycrystalline and single-crystal form are in common use, although stainless steel brackets are still the major type in use. The original concerns with the use of non-metal brackets were whether the composite bonding agent offered strength sufficient to hold the bracket under the high stresses developed with steel wires (Stich et al, 1985). As it has turned out, the chemical attachment is too strong (Tremblay^r al, 1990). The brackets sometimes break during their removal, leaving the bracket base and composite on the tooth to be removed by means of diamond rotary instruments. Thi§ can lead to damaged facial surfaces of the tooth, requiring restorative treatment. Occasionally, enamel will be broken from the tooth, creating significant damage (Gunn and Powers, 1990)
REFERENCES Almquist P, Chan DCN, Blackman R, Kaiser DA (1991). The effects of sprue number and position on titanium crown margins (abstract). J Dent Res 70 (Spec Iss):486. Anusavice KJ, Hojjatie B, DeHoff PH (1985). Influence of metal thickness on stress distribution in metal-ceramic crowns (abstract). J Dent Res 64 (Spec Iss):246. Baez RJ, Nonajca T, Blackman R (1991). Ti castability and surface characteristics with three phosphate bonded investments (abstract). J Dent Res 70 (Spec Iss):486. Bagby M, Marshall SJ, Marshall GW Jr, Noyan IC (1985). Residual surface stress jn Pd-based metals (abstract). / Dent Res 64 (Spec Iss):227. Baran G (1985). Oxide heterogeneity and composition on NiCr alloys (abstract). J Dent Res 64 (Spec Iss):227. Boudrias P (1988). Counteracting the effect of acidulated
ADV DENT RES SEPTEMBER 1992
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