J. Dent 1990;18: 227-235

227

Review

Dental ceramics-an

update

V. Piddock and A. J. E. Qualtrough Department

of Restorative

Dentistry,

Turner Dental School, University

of Manchester

Dental Hospital,

UK

ABSTRACT application of certain industrial ceramics and processing techniques has facilitated the introduction of a wide range of new dental restorative products including castable glass-ceramics, shrink-free materials and an ion-strengthening paste. However, these recent advances must be evaluated against the well-established materials and techniques developed more than 20 years ago. This article outlines interesting developments in the evolution of dental ceramics over the past 30 years and considers the current state of the art. The

KEY WORDS: Ceramics, Review .I. Dent. 1990:

18: 227-235

(Received 4 May 1990;

reviewed 14 June 1990;

accepted 25 June 1990)

Correspondence should be addressed too:Dr V. Piddock, Department of Restorative Dentistn/, Turner Dental School, University of Manchester, Dental Hospital, Manchester Ml 5 6FH. UK.

INTRODUCTION in ceramics has held the attention of the dental profession for over two hundred years. In spite of the intrinsic hard and brittle nature of these materials, their unsurpassed aesthetic and biocompatible qualities have provided the stimulus to overcome their limitations. Fluctuations in the popularity of porcelain and ceramic restorations throughout the decades have been influenced by developments in other fields, for example the introduction of acrylic resins, elastomeric impression materials and the air turbine handpiece. Similarly the development of porcelain furnaces and vacuum firing have had major influences on the final product. As the all-porcelain crown is said to meet the exacting aesthetic requirements demanded by both patient and dentist, much of the materials research since the mid1960s has been directed towards producing stronger, reinforced restorations with improved marginal accuracy. Furthermore, the ‘aesthetic revolution’ in dentistry has created an upsurge in popularity of both porcelain and ceramic facings and aesthetic inlays/onlays. Although a new wave of ceramic products appeared in the 1980s most of the materials currently available are developments stemming from ideas established more than 20 years ago. The purpose of this article is to outline some interesting developments in the science of dental ceramics over the past 30 years and to consider the present state of the art with respect to both materials and techniques.

Interest

0 1990 Butterworth-Heinemann 0300-57 12/90/050227-09

Ltd.

DEVELOPMENT

OF DENTAL

Air and vacuum

fired

PORCELAINS

Prior to the early 1960s porcelain powders were required to have a relatively large particle size if excessive opacity was to be avoided. However, the porosity present in these materials, resulting from entrapped air bubbles, was utilized to decrease light reflection from the cement lute. Despite these limitations high standards of aesthetics could be achieved (McLean, 1979). The introduction of vacuum-fired porcelains led to improvements in aesthetics due to the reduction in internal porosity. Wilson and Whitehead (1967) compared the particle size distributions of air-fired and vacuumtired porcelains and observed that the size ranges for low fusing air-fired and vacuum-fired porcelains were similar although high fusing porcelain powders were coarser. The vacuum-fired porcelain exhibited much less porosity than either air-fired material when observed by optical microscopy and was stronger than low fusing porcelain. Jones et al. (1972) compared the strength of several porcelains tired in air and vacuum and found no statistically significant differences despite a marked reduction in porosity in the vacuum-fired samples. Aluminous

porcelains

Great improvements in strength accompanied the development of alumina-reinforced feldspathic porcelain

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(McLean and Hughes, 1965). In addition to increased flexural strength (Wilson and Whitehead, 1967; Sherril and O’Brien, 1974; Hussain et al., 1981; Piddock et al., 1984a), enhanced fracture toughness was also achieved by the addition of approximately 50 per cent by weight of alumina crystals (Morena et al., 1986a). However, the presence of a second phase in the glassy matrix substantially reduced the translucency of the porcelain (McLean and Hughes, 1965; Southan, 1987a), limiting its usefulness to providing a refractory framework capable of supporting weaker, more translucent dentine and enamel porcelains. Since ceramics are relatively weak in tension and tensile stresses are generated at the fit surface of porcelain crowns by compressive biting forces, factors affecting the quality of this surface are likely to influence the survival of such restorations. Correctly tired porcelain takes up the smooth texture of the platinum foil matrix and the resultant body is much stronger than porcelain that is poorly adapted (Piddock et al., 1984b). The first work on pre-fritting alumina-glass composites was described by McLean (1966). The recent introduction of a high alumina content porcelain (Hi-Ceram, Vita Zahnfabrik, Bad Sackingen, FRG) was based on this work and improvements in strength, resulting from its higher crystalline content, have been demonstrated (Oilo, 1988; Piddock, 1989). The fracture toughness of this material has also been found to be significantly greater than that of conventional core porcelain (Kvam, 1989).The Hi-Ceram technique, described by Claus (1987) utilizes a refractory die to shape the reinforced core. The alumina-reinforced porcelains described above are produced by prefritting the crystalline component with the glassy matrix. The amount of alumina that can usefully be incorporated in this way is limited by the occurrence of porosity in the tired porcelain. A new technique termed Inceram (Vita Zahnfabrik, Bad Sackingen, FRG) has been developed which permits the inclusion of a higher porportion of crystalline material. Ceramic cores are formed from slurries of fine alumina powder which are applied onto an absorbent refractory matrix, dried and lightly sintered. The residual pores are then filled by capillary action with molten glass to yield a dense composite structure. The manufacturers claim that the strength of this material is three times that of conventional aluminous core porcelain. Magnesia

core

A high thermal expansion core porcelain has been described by O’Brien (1985). Magnesia crystals were used to reinforce a high expansion coefficient glass, resulting in a core porcelain with a modulus of rupture similar to that of conventional alumina-reinforced porcelain (13 1 MPa). Because of its relatively high expansion coefficient, the magnesia-reinforced material is thermally compatible with body porcelains normally used to veneer metalloceramic restorations, offering the possibility of improved

shade matching with such restorations. However, one possible result from using a high expansion magnesia core is that the porcelain will be more liable to thermal shock on cooling. By treating the surface of the magnesia core porcelain with a suitable glass, a doubling in flexural strength was observed. Two mechanisms were suggested for the observed improvement in strength. First the glaze was thought to penetrate open pores, effectively reducing the number of surface flaws, and secondly the glaze may have placed the surface layer in compression. Diametral strength tests on crown forms (Hondrum and O’Brien, 1988)also showed significantly higher values for internally glazed magnesium core samples compared with aluminareinforced and unglazed magnesia-reinforced core porcelains, although the increase was only 50 per cent compared with the 100 per cent improvement in modulus of rupture. The effect on fit of applying a glaze to the inner surface of a restoration is unclear. Inlay and veneer

porcelains

Recent interest in these materials has developed as a result of increased demand for aesthetic restorations. Ceramic inlays may be manufactured directly on refractory dies from porcelains said to have a reduced firing shrinkage, although such claims have not yet been substantiated. Alternatively, castable glass-ceramic inlays may be prepared from wax patterns as described below. Castable apatite ceramic inlays and veneers have also been described (Hobo and Iwata, 1985c, d). The introduction of computer-controlled machined inlays has utilized both preformed porcelain and Dicer blocks (Mbrmann et al., 1989; Smith, 1989). Ceramic veneers may be produced from the same materials and, as an alternative to using refractory dies, platinum foil matrices may be utilized (Plant and Thomas, 1987). A common feature of all these ceramic restorations is the need to provide a link between the etched inlay fitting surface and the preparation via a silane-coupling agent. Several workers have demonstrated enhanced bond strength by the application of silane bonding agents to dental porcelains (Calamia and Simonsen, 1984; Hsu et al., 1985; Lacy et al., 1988; Nicholls, 1988). Tjan and Nemetz (1988) compared the bond strengths of dental porcelain and Dicer samples fixed to resin-based cements with and without a silane bonding agent. They observed an increase in bond strength for both ceramics when the silane treatment was utilized, although the bond strength of the Dicer samples was inferior to that of the porcelain. There is general agreement that etching porcelain improves the toothrestoration bond strength (Simonsen and Calamia, 1983; Calamia et al., 1985; Hsu et al., 1985; Lacy et al., 1988). Calamia et al. studied the effect of etch time and concluded that the optimum etchant/treatment duration was dependent on the type of porcelain. Generally they found that a 2.5 min etch in 5 per cent HF gave better results than 20 min exposure to the same etchant.

Piddock and Qualtrough:

The effect of saliva contamination in silane-treated porcelain has been investigated by Nicholls (1988) who demonstrated that a 15 s etch with 37 per cent phosphoric acid restored tensile bond strength. The long-term hydrolytic stability of the silane bonding agent is questionable. The effect of water storage on the stability of silane-treated resin/porcelain bonds utilized in porcelain repair kits has indicated that certain bonding systems are more resistant to hydrolytic attack (Stokes et al., 1988; Bailey, 1989). Metal

ceramic

restorations

Porcelain-fused-to-metal (PFM) restorations were developed to overcome the problems of brittle fracture associated with all-ceramic crowns. In 1956, Brecker described the manufacture of crowns and bridges by fusing dental porcelains to gold alloys. Later, Weinstein et al., (1962) addressed the problem of thermal expansion coefficient mismatch between porcelain and metal substrate, which continues to be a matter of concern. It is now generally thought to be advantageous if the porcelain is placed slightly in compression by the cooling metal (Mackert, 1988). It has been shown that the expansion coefficient of dental porcelains is altered by multiple firings (Fairhurst et al., 1980) which can raise the crystalline leucite content of the ceramic. Increasing the volume fraction of leucite produces an increase in the thermal expansion of the porcelain. The rate of cooling of the tired porcelain has also been found to affect the amount of leucite formed (Mackert and Evans, 1990). The leucite phase itself undergoes a displacive transformation reaction above 400°C from a tetragonal to a cubic form with a resultant 1.2 per cent volumetric expansion and a change in expansion coefficient (Mackert et al., 1986). To prevent exfoliation of the ceramic layer, it is imperative that a strong bond exists between porcelain and metal. The bond must be capable of withstanding the interfacial shear forces generated during fabrication, by differences in expansion coefficient and sintering shrinkage of the porcelain. Linear shrinkage values of 27 per cent to 35.6 per cent have been measured for a range of metalloceramic porcelains, the magnitude depending on the chosen firing temperature (Rosenstiel, 1987). Bonding is also necessary for effective transfer of stresses from the brittle porcelain to the metal framework. Vickery and Badinelli (1968) considered the bond between gold alloys and dental porcelain to be a combination of compressive, chemical, mechanical and Van der Waals forces. A range of test methods have been utilized to determine the magnitude of the bond between porcelain and metal. Jones (1988) has correlated results from a number of studies and has highlighted the wide variation in bond strength data that exists, depending on the test procedures adopted. In fact there still remains the need for an appropriate standard test method for assessing porcelainalloy compatibility.

Dental ceramics

229

A number of studies have looked at the factors affecting colour stability of bonding porcelains. Choice of modelling fluid, variation in firing temperature over a 60°C range and differing condensation techniques produced no statistical differences in the colour parameters measured (Rosenstiel and Johnston, 1988). However, significant variations were observed between different brands of porcelain with the same nominal shade. Jorgensen and Goodkind (1979) reported differences due to porcelain shade but porcelain thickness and multiple firings produced no significant changes in hue. However, porcelain thickness was found to significantly alter colour value. Shaffner and Jones (1988) investigated a technique of shade control by blending porcelain powders to a formula. They found that proportioning the powders by volume resulted in colour variability. Clinically, the masking of metal surfaces with opaque porcelains can result in less than optimal aesthetics and this may be emphasized by inadequate tooth reduction. In order to minimize the thickness of the opaque layer at the cervical region, opaque porcelain powders may be overlaid with opacious dentines which are generally regarded as being high chroma powders with quite reasonable translucency. Shrink-free

ceramics

The fit of conventionally produced aluminous porcelain jacket crowns was said to be limited by the use of platinum foil matrices and by firing shrinkage of the porcelains (Southan and Jorgensen, 1972). In an attempt to overcome these problems, Sozio and Riley (1983) described the use of a shrink-free ceramic coping which is formed on an epoxy die by a transfer moulding process. The relatively fragile, unfired ceramic (Hullah and Williams, 1987) contains a mixture of alumina, magnesia, aluminosilicate glass frit, wax and silicone resin plasticizer. The moulded core is subjected to a lengthy heat treatment during which time some of the alumina reacts with the magnesium oxide to form magnesium aluminate spine1 crystals. This reaction is accompanied by an increase in volume which offsets the sintering shrinkage. The so-called Cerestore coping (Johnson & Johnson, East Windsor, NJ, USA) is finally veneered with aesthetic porcelains. Investigation of the microstructure of the Cerestore coping material by back-scattered electron microscopy revealed residual porosity amounting to approximately 20 per cent (Piddock et al., 1987). Oilo (1988) recorded an even higher figure of 32.5 per cent, with a mean pore size of 10 urn. The strength of the shrink-free ceramic coping material has been compared with other dental ceramics using bar (Oilo, 1988) and disc (Piddock et al., 1987) samples and also crown forms (Philp and Brukl, 1984;Josephson et al., 1985; Dickinson et al., 1989). Oilo reported a flexural strength of 145 MPa for Cerestore core compared with a value of 116 MPa for aluminous core porcelain. The disc

230

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strength of Cerestore coping ceramic was found to be dependent on thickness, tensile strength values ranging from 151 MPa to 190 MPa (Piddock et al., 1987). In this study higher strengths were reported for aluminareinforced porcelain samples. Compressive strength measurements on Cerestore crowns were not found to be significantly different from values obtained for conventional aluminous porcelain crowns, although the mean level was in fact higher (Philp and Brukl, 1984). The results of a similar study by Josephson et al. indicated that a significantly greater load was required to fracture Cerestore crowns. However, this was less than 50 per cent of the force required to break a porcelain-fused-to-metal crown. Dickinson et al. reported that in 70 per cent of the Cerestore crowns they tested, failure occurred in two stages, crack initiation followed by catastrophic failure. No significant difference in strength was found between aluminous porcelain crowns and the initial cracking stage of the Cerestore crowns. However, complete fracture occurred at a significantly higher compressive load. Measurements ofK,c for Cerestore coping material and two aluminous core porcelains indicated that the fracture toughness of the shrink-free material is similar to that of the conventional alumina-reinforced materials (Kvam, 1989). On the other hand, dynamic fatigue testing suggested that a fatigue stress of 95.2 MPa would be necessary to fracture Cerestore crowns within 5 years compared with only 42.1 MPa for aluminous porcelain (Morena et al., 1986b). A number of investigators have examined the lit of Cerestore restorations. Marginal openings ranging from 11 urn to 3 13 urn with a mean value of 84 urn have been determined for crowns cemented onto extracted teeth (Chan et al., 1985). Davis (1988) measured the mean cement thickness at various points around cross-sections of Cerestore copings and crowns. No significant difference was found between the tit of the copings and the completed crowns. The mean thickness of cement at the margins of Cerestore crowns was 37 urn. At the incisal tip this increased to 142 urn. Dickinson et al. also noted poor adaptation at the incisal edge. A mean cement film thickness of 19.5 urn has been reported by Scharer et al. (1988) at the outer margin. A number of factors have been shown to influence marginal tit (Sato et al., 1986b). The accuracy of the epoxy die was found to be affected by the amount of catalyst used. That recommended by the manufacturers resulted in an oversized die. Some deformation of the core during firing was also reported. It was thought to be due to burnout of the organic constituents. Improvements in marginal fit were obtained by adopting a double-sprue technique. The marginal fit of Cerestore copings and crowns produced using optimized procedures was measured both externally and internally (Sato et al., 1986a). The mean marginal discrepancy for unveneered copings was 21.4 urn externally and 32 urn at the midshoulder position. After porcelain addition, the recorded values of 23.2 urn and 35.8 urn respectively, were not

considered to have changed significantly. The effect of sintering temperature on dimensional stability has also been studied (Campbell, 1986). A firing temperature of 1280°C resulted in no volumetric change but varying the temperature by 3.7 “C produced a 0.1 per cent dimensional modification. The microleakage of Cerestore crowns cemented to extracted teeth has been assessed by silver staining (Shortall et al., 1989). A glass ionomer cement resulted in more marginal leakage than the two composite resin cements tested, although this finding has not been confirmed by other test methods. Limitations of the Cerestore system led to its withdrawal from the UK market. The product has undergone further development and the new shrink-free ceramic coping, Alceram (Innotek Dental Corp., Lakewood, CO, USA), is claimed by its manufacturers to have superior mechanical properties.

CASTABLE GLASS-CERAMICS In 1968 MacCulloch suggested that glass-ceramics, offering a combination of superior strength and translucency, might provide an alternative to dental porcelains. Industrial development of glass-ceramics was aimed at producing refractory crystalline or partially crystalline bodies at relatively low temperatures. The ceramic material, initially cast as a glass, is subsequently converted to a mechanically stronger, crystalline body. A product of this type strengthened by interlocking mica crystals which also contribute to its machinability, has been developed as a dental restorative material. The properties of this tetrasilicic micaglass ceramic marketed under the name of Dicer (Dentsply International, York, PA, USA) have been well documented (Adair and Grossman, 1984). It is supplied in the form of silica-based glass ingots containing magnesium fluoride which acts as a nucleating agent for devitritication. The laboratory technique is similar to that required to manufacture gold crowns. A wax pattern of the completed crown is invested in a phosphate-bonded refractory and molten glass at 1358°C is cast into the mould. The subsequent devitrilication requires 6 h at a temperature of 1075“C with the casting re-embedded in a refractory matrix. The cerammed restoration is cloudy white in appearance with a high degree of translucency and is coloured by the application of thin layers of tinted porcelain. Shade matching may also be modified by the use of coloured luting cements. The lit surface is acid etched to enhance bonding to the underlying tooth. A detailed description of the laboratory procedures has been reported (Malament and Grossman, 1987). The microstructure of Dicer glass-ceramic consists of interlocking mica-type crystals in a glassy matrix. The refractive index of the crystals is close to that of the surrounding glass, helping to maintain translucency in the devitrified body (Grossman, 1985) which is essentially pore free (Oilo, 1988).

Piddock and Qualtrough: Dental ceramics

Typical of ceramic materials, the modulus of rupture of Dicer is inferior to its compressive strength. Adair and Grossman (1984) reported values of 152 MPa and 828 MPa respectively which compare favourably with strengths of dental porcelains. In addition these workers indicated a hardness value similar to that of natural enamel. More recently, flexural strength measurements on a number of dental porcelains and Dicer glass-ceramic yielded values of 240 MPa for Dicer compared with only 116 MPa for alumina-reinforced core porcelain (Oilo, 1988). However, a comparison of the force required to break Dicer crowns and aluminous porcelain restorations revealed no significant difference in strength (Dickinson et al., 1989). Jones et al. (1988) have studied the fracture toughness of Dicer. A significant increase in Krc was observed following the ceramming heat treatment. However, these workers found that the fracture toughness of the crystalline Dicer did not differ significantly from the toughness of conventional feldspathic porcelains. Indeed& values reported by Kvam (1989) indicated that the fracture toughness of cerammed Dicer is less than that of core porcelains, and he suggested that the crystals formed during the heat treatment process do not function as crack stoppers. Conflicting results have been obtained comparing the lit of Dicer restorations with other ceramic systems. Malament and Grossman (1987) suggested a casting accuracy of 25 nm prior to heat treatment. Facial and lingual marginal openings varying between 10 nm and 25 nm have been observed by Dickinson et al. (1989) although these workers noted the presence of nodules on the tit surface which were removed by grinding before the crowns were seated. In a comparative study with Cerestore and Renaissance crowns, mean marginal discrepancies of 63 nm were determined ranging from 47 nm to 88 nm for Dicer crowns (Scharer et aI., 1988). The inferior fit of the glass-ceramic crowns was attributed to several factors including shrinkage of the glass during the ceramming process coupled with inadequate expansion of the phosphate investment, roughening of the surface during heat treatment, the production of a white layer on the casting surface and damage to the surface during removal of investments. Marquis (1988) has shown that this ‘white layer’ on the surface is due to excessive crystalline growth. The variation in tit around sectioned Dicer and Cerestore crowns cemented to plastic teeth has been studied by Davis (1988). Average marginal and incisal tip cement thicknesses of 35 nm and 100 ym respectively were determined for Dicer crowns. In this study the overall tit of Dicer crowns was found to be superior to that of Cerestore restorations. The effect of etchants and silane coupling agents on the shear bond strength of Dicer samples fixed to resin-based cements has been studied (Bailey and Bennett, 1988). Improved adhesion was obtained using a range of etchants, a 2 min treatment with ammonium bifluoride being the most effective. A further increase was achieved

231

by application of a silane. No significant deterioration in bond strength was measured after storage in water for 1 year. The shear bond strengths using three glass ionomer cements were found to be much weaker than Dicer samples fixed with a resin-based cement (McInnesLedoux et al., 1989). Clinically the restorations are said to offer excellent aesthetics. However, many operators consider the appearance of Dicer crowns to be very variable and there is the additional disadvantage that any modification to the final glazed surface results in removal of the superficial porcelain colourants.

CASTABLE APATITE CERAMIC Synthetic hydroxyapatite would seem to be the ideal replacement for lost tooth tissue, as natural enamel is composed largely of hydroxyapatite. The direct fabrication of ceramic crowns from crystalline hydroxyapatite is not yet possible. However, an indirect technique has been developed which involves conversion of a calcium phosphate glass to a partially crystalline apatitic glassceramic (Hobo and Iwata, 1985b). The technique for producing CeraPearl restorations is very similar in concept to the Dicer glass-ceramic system. In common with the tetrasilicic micaglass ceramic, the form of the apatite crown is produced by casting. The calcium phosphate-based glass is transformed to a partially crystalline body by a controlled heat treatment and is then tinted by the application of coloured glazes. The initial crystalline phase is believed to be composed of oxyapatite which is unstable in the presence of water and is converted to hydroxyapatite (Hobo and Iwata, 1985a). Formation of the crystalline phase was said to produce a three-fold increase in tensile strength from 50 MPa to 150 MPa (Hobo and Iwata, 1985a). Light refractive index, density, hardness, thermal expansion and thermal conductivity were all found to be similar to natural enamel. Exposure of the hydroxyapatite crystals by dissolution of the glassy matrix assisted bonding to glass ionomer cements. It was suggested that the nature of the bond was similar to that which is believed to exist between glass ionomer and natural enamel. Cement thicknesses, measured at the margins of sectioned crowns fixed to epoxy dies, were 30 nm.

REFRACTORY DIE TECHNIQUE Problems resulting from poor adaptation of dental porcelain to platinum foil formers have been discussed (Southan and Jorgensen, 1973) and the use of refractory die systems has been suggested to overcome difficulties associated with the use of foil (Vickery et al., 1969; Southan and Jorgensen, 1972). However, undercut preparations, when duplicated in a refractory die, can result in fracture of the core during seating on the master die. A

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major advantage of the platinum foil technique is that this problem is eliminated. The disc strength of alumina-reinforced core porcelain fired on a refractory die cast from a silicone impression material has been shown to be comparable with porcelain formed on 220 mesh silicon carbide paper (Southan, 1987b). However, the use of an alginate impression gave rise to significantly weaker samples. The advantages of the refractory die technique compared with the conventional platinum foil method have been summarized by Southan (1987~). ION STRENGTHENING The tensile strength of glasses may be increased by introducing compressive stresses into the surface layer. Since failure usually occurs by crack growth from surface flaws, protection of the surface from tensile stresses will greatly increase the strength and toughness of the glass. Compression may be introduced into the surface of alkali metal-containing glasses by replacing relatively small ions from the surface with larger species (Kistler, 1962). Southan (1970) described a method of strengthening quartz-bearing porcelains by immersion in molten IWO, for 19 h. Ion exchange led to improvements in strength of 68 per cent and 122 per cent depending on specimen geometry. The kinetics of ion exchange by immersion in molten KNO, were studied by Dunn et al. (1977). Increase in strength was found to be dependent on the temperature of the molten salt bath and the time of immersion. Since the exchange process is controlled by ionic diffusion, higher temperatures induced a more rapid increase in strength. However, as the glass transition temperature was approached, a competing process of stress relaxation occurred, attenuating the effect. An improvement in strength of approximately 120 per cent was obtained by immersion in molten KNO, at 400 “C for 4 h. In addition, Dunn and co-workers reported no visible change in the optical properties of the porcelain. More recently, Southan (1987a) showed that the translucency of ion exchanged porcelains is reduced by only a small amount compared with the reduction due to dispersed alumina crystals. The routine use of ion exchange strengthening of dental porcelains has been restricted by the need to operate potentially hazardous molten salt baths. However, the recent introduction of a commercial ion strengthening paste offers the possibility of enhancing the strength of porcelains in safety, using conventional porcelain furnaces. Preliminary studies (Anusavice et al., 1990; Piddock and Qualtrough, 1990)have shown improvements in tensile strength using this product. However, the degree of improvement was found to be dependent on the type of porcelain treated. THE PLATINUM-BONDED

CROWN

Flaws present in the lit surface of porcelain jacket crowns, resulting from the method of construction (Southan and

Jorgensen, 1973), may lead to failure in service. Development of porcelain-fused-to-metal crowns attempted to overcome this problem by introducing a relatively ductile metal layer at the fit surface capable of withstanding tensile stresses. However, this was achieved at the expense of aesthetics due to reflection of light from the opaque porcelain which is necessary to mask the metal substructure. The platinum-bonded crown was developed to provide a ductile skin at the fit surface, whilst minimizing the thickness of metal and thus eliminating the need to overbuild porcelain on the labial surface (McLean and Seed, 1976). This technique, described in detail by McLeanetal. (1978) uses twin platinum foils. The first foil is adapted to the die as in the construction of a conventional aluminous porcelain jacket crown. A second foil is burnished over the initial matrix and is cut short at the axiogingival line angle. This foil receives a thin layer of electrodeposited tin which is subsequently oxidized in the porcelain furnace. A specially formulated opaque core, dentine and enamel porcelains are built up in the conventional manner and the tin oxide-coated foil remains an integral part of the restoration. Donovan et al. (1984) have described a simplified technique for the construction of the platinum-bonded crown using only a single foil. A mixture of porcelain and wax is used to form the margin eliminating the need for an inner foil. Since no tinman’s joint is present over the margin, improved marginal tit was anticipated. A single foil technique has also been described by McLean and Kedge (1988). Results of disc rupture tests indicated that tin oxide coating gave an 83 per cent improvement in tensile strength (Seed et al., 1977). However, the increase in strength obtained was found to be dependent on the thickness of porcelain tested (Piddock et al., 1984b). This suggested that the strengthening mechanism may have been due in part to residual compressive stresses induced in the porcelain surface on cooling by the contracting foil. Retention of the foil rather than the presence of a tin oxide layer appeared to be more important in strengthening thick laminated samples. Sarkar and Jeansonne (1981) studied the platinum/tin oxide/porcelain interfacial region and observed better wetting of the coated foil by the porcelain compare with untreated foil. Dissolution of the tin oxide in the glassy porcelain matrix was also seen and it was suggested that these two factors affected the strength of platinum-bonded crowns. However, three-point bend test results showed no significant difference between specimens fired against tin oxide-coated and unplated foils (Edwards et al., 1983). Unfortunately, no fractographic analysis was carried out to account for the different findings. Platinum-bonded crowns prepared by the twin foil technique and conventional aluminous PJCs were loaded to failure along their incisal edge in a study described by Munoz and coworkers (1982). Conventional crowns were found to be significantly stronger. This was probably due to the high level of porosity observed in the platinum-bonded crowns. Lower compressive strength

Piddock and Qualtrough: Dental ceramics

measurements have also been determined for twin foil crown forms compared with conventional aluminous crowns (Philp and Brukl, 1984). However, diametral compression testing of cylindrical samples showed an increase in fracture strength in the case of the platinumbonded samples, although this was not statistically significant (Oram et aI., 1984). Faull et al, (1985) have studied the tit of platinumbonded crowns produced by single and twin foil techniques. The average marginal discrepancies of crowns produced by these techniques cemented onto extracted teeth were 41 urn and 74 urn respectively. This difference was not considered to be statistically significant. A qualitative examination of single and twin foil platinumbonded crowns indicated that better adaptation is produced by the single foil technique (Munoz et al., 1982). An analysis of numbers of crowns provided under the NHS has shown a drop in the quantity of platinumbonded crowns fitted from a peak value of 48000 in 1984 to under 32000 in 1988 (Farrell and Dyer, 1989).This decline in popularity probably reflects the variable quality of crowns produced using this technique.

RENAISSANCE CROWN The Renaissance, CeraPlatin or Ceplatec crown is similar to the platinum-bonded type of restoration in that a thin metal substructure is used to protect the tit surface of the crown. The technique, described by Schdssow (1984) utilizes a gold-coated foil matrix which is folded like an umbrella. In fact the foil has been described as consisting of four layers (Scharer et al,, 1987) the outermost being pure gold. The next layer is an alloy of gold, platinum and palladium, the third layer is 100 per cent palladium and the innermost layer also contains gold, platinum and palladium. The umbrella form is burnished onto the die and heated. The gold coating melts and acts as a solder which secures the folds in place, thus creating a stable coping. The crown is then built up with metal bonding porcelains. A twin foil technique has been developed to create an all-porcelain facial margin. Sticky wax mixed with shoulder porcelain adheres to the first platinum foil, and minimizes distortion when the unfired crown is removed from the die (CofIield, 1988). The compressive strength of Renaissance crown forms has been found to be significantly inferior to conventional aluminous porcelain jacket crowns (Brukl and Ocampo,

1987).However, there is little further information conceming the mechanical properties of this type of restoration. Using the conventional method of pn :paration, Scharer of the et al. (1987) found that the sintering shrinkage .f porcelain resulted in considerable distortion 01‘the metal coping. Mean marginal discre :pancies of 100.2 pm and 96.2 pm were measured facially and lingually. This distortion was minimized by scoring the second opaque porcelain layer prior to firing and filling in with an additional application. It was also possible to improve the fit by reswaging the foil to the die without fracturing the

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porcelain. The mean marginal tit of Renaissance crowns using the modified technique was found to be 29 urn (Scharer et al., 1987,1988). However, it was noted that care must be taken to avoid the flow of molten gold onto the fit surface during the soldering operation.

PREFORMED COPINGS In 1987 McLean and Seed described the use of preformed platinum alloy copings to reinforce aluminous porcelain crowns. Preforms were produced in a range of sizes based on a previous survey of the dimensions of porcelain crown preparations, which indicated that a foil thickness of just over 0.1 mm would be sufftcient to prevent fracture (Seed and McLean, 1987). Having selected a coping of approximately the right size to tit the preparation, the metal was swaged onto the die by pressing in a block of an elastomeric material. It was anticipated that better creep resistance during tiring would result from the use of platinum alloy rather than gold alloy. In addition this technique provided a more economic method of producing crown copings than the lost wax process. A shoulder preparation was preferred to that of a chamfer since unstiffened edges of metal tended to increase the risk of peripheral cracking of the porcelain.

FUTURE TRENDS Although predictably stronger materials have been developed, the universal use of all-ceramic intra- and extra-coronal restorations is not yet a reality. However, the clinical evaluation of recent developments may indicate their wider useage. Further development of existing products is likely and may include ion exchange strengthened porcelains, intrinsically coloured glass-ceramics and the introduction of more reproducible laboratory procedures. The demand for ‘aesthetic dentistry’ is expected to continue and will be influential in determining the range of products made rH,~.;l~hl,

LI*LI”U”‘~’

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