RESTORATIONS

FOR POSTERIOR

PULPLESS

TEETH

3. Shillingburg HT, Kessler JC. Restoration of the endodontically treated tooth. Chicago: Quintessence Publishing, 1982. 4. Sorensen JA, Martinoff, JT. Intracoronal reinforcement and coronal coverage: a study of endodontically treated teeth. J PROSTHET DENT 1984;51:760-4.

5. Lovdahl PE, Nicholls JI. Pin retained amalgam cores vs. cast-gold dowel-cores. J PROSTHET DENT 1977;38:507-14. 6. Nayyar A, Walton RE, Leonard LA. Amalgam coronal-radicular dowel and core technique for endodontically treated posterior teeth. J PROSTHET DENT 1980;43:511-5.

7. Brown BR, Barkmeier WW, Anderson tally treated posterior teeth with

RW. Restoration of endodontiamalgam. J PROSTHET DENT

1979;41:40-4.

8. Lieberman R, Judes H, Cohen E, Eli I. Restoration of posterior pulpless teeth: amalgam overlay versus cast gold onlay restoration. J PROSTHET DENT 1987;57:540-3.

9. Summitt JB, Robbins JW. Longevity of complex amalgam restorations [Abstract]. J Dent Res 1987;66:329. 10. Gordon M, Judes H, Laufer BZ, et al. An immediate dual purpose restoration of posterior root-filled teeth (the amalgam crown). Dent Med 1984;2:22-6. 11. Markley MR. Pin reinforcement and retention of amalgam foundations and restorations. J Am Dent Assoc 1958:56:675-9. 12. Going RE. Pin-retained amalgam. J Am Dent Assoc 1966;73:619-24. 13. Dilts WE, Welk DA, Laswell HR, George L. Crazing of tooth structure associated with placement of pins for amalgam restorations. J Am Dent Assoc 1970;81:387-91. 14. Galindo Y. Stress-induced effects of retentive pins. A review of the literature. J PROSTHET DENT 1980;44:183-6. 15. Shave11 HM. The amalgapin technique for complex amalgam restorations. Calif Dent Assoc J 1980,8:48-55. 16. Shave11 HM. Updating the amalgapin technique for complex amalgam restorations. Int J Perio Res 1986;5:23-35. 17. Davis SP, Summitt JB, Mayhew RB, et al. Self-threading pins and amalgapins compared in resistance form for complex amalgam restorations. Oper Dent 1983;8:88-93. 18. Outhwaite WC, Garman TA, Pashley DH. Pin vs slot retention in extensive amalgam restorations. J PROSTHET DENT 1979;41;396-400. 19. Plasmans PJJM, Kusters ST, de Jange BA, van ‘t Hof MA, Vrijhoef

Conservative review Rill Fort

G. Banks, Walton

Beach,

posterior D.D.S.,

ceramic

MMA. In vitro resistance of extensive amalgam restorations using various retention methods. J PROSTHETDm 1987;57:16-20. 20. Birtcil RF, Venton EA. Extracoronal amalgam restorations utilizing available tooth structure for retention. J PROSTHET DENT 1976;35:1718. 21. Kane JJ, Burgess JO, Summitt JB. Fracture resistance of amalgam coronal-radicular restorations [Abstract]. J Dent Res 1988;67:344. 22. Miller AW. Post and core systems: which one is beat? J PROSTHET DENT 1982;48:27-38.

23. Caputo AA, Standlee JP. Pins and posts-wby, when and how. Dent Clin North Am 1976;20:299-311. 24. Deutsch AS, Musikant BL, Cavallari J, Lepley JB. Prefabricated dowels: a literature review. J PR~~THET DENT 1983;49:498-503. 25. Shillingburg HT, Fisher DW, Dewhurst RB. Restoration of endodontitally treated posterior teeth. J PROSTHET DENT 1970;24:401-9. 26. Hudis SI, Goldstein GR. Restoration of endodontically treated teeth: a review of the literature. J PROSTHET DENT 1986;55:33-8. 27. Goerig AC, Mueninghoff LA. Management of the endodontically treated tooth. Part I: concept for restorative designs. J PRO~THET DENT 1983;49:340-5.

28. Goerig AC, Mueninghoff LA. Management of the endodontically treated tooth. Part II: technique. J PR~~THET DENT 198’&4Sz491-7. 29. Hoag EP, Dwyer TG. A comparison evaluation of three post and core techniques. J PROSY DENT 1982;47:177-81. 30. Chan RW, Bryant RW. Post core foundations for endodonticallytreated posterior teeth. J PR~~THET DENT 1982;48:401-6. 31. Kantor ME, Pines MS. A comparative study of restorative techniques for pulpless teeth. J PROSTHET DENT 1977;38:405-12. 32. Desort KD. The prosthodontic use of endodonticaby treated teeth; theory and biomechanics of post preparation. J PROSTHET DENT 1983;49:203-6.

33. Standlee JP, Caputo AA, Hanson EC. Retention of endodontic dowels: effects of cement, dowel length, diameter, and design. J PROSTHJFT DENT 1978;39:401-5. Reprint requests to: DR. CLIFFORD B. STARR 4062 QUAIL BUSH DR. DAYTON, OH 45424

restorations:

A literature

M.S.

Fla.

Conservative ceramic restorations have much to offer to improve appearance and strengthen posterior teeth. The advent of resin bonding makes possible many designs for inlays, onlays, and partial coverage crowns. This review discusses conventional porcelain, Optec HSP porcelain, Dicer, and Cerapearl with emphasis on strengthening mechanisms, principles-of preparations, accuracy of fit, and indications.(J PROSTRET DENT 1990;63:619-26.)

C

eramic inlays have generated extreme interest in the past few years because of public demand for esthetic restorative materials. Nevertheless, many of the materials

Presented at the Academy pus Christi, Tex.

of Denture

Prosthetics

10/l/18540

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meeting,

Cor-

in use and under development are merely technologic refinements of porcelain systems that originated more than 100 years ago.i In 1339, Land as reported by Marra,2 or& inated the porcelain inlay. Porcelain inlays and crowns were developed by Wain in 1923 as reported by Jones3 in which molten porcelain (glass) was cast into a refractory mold by use of an ordinary gas blowpipe to melt the I;bFcelain. In the 1930s a multishaded, commercially available

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Table

I.

Composition of conventional porcelain6

Table IV. Composition of CerapearlE CaO

Crystalline

4203 K20

p205

Hydroxyapatite @alo . (NM3 . OH21

CaO Na20

SiO2

SiOz

MgO

phase:

B2Oa

Table V. Strength measurements Table II. Composition of Optec HSP porcelain* Crystalline phase: Leucite. [KzO . A1203. 4SiO2]

SiO2 A1203 K20 CaO Na20

Enamel Dentin Porcelain optec

4 gm in size

Cast ceramic [Dicor/Cerapearl]

Jw3 *Personal Communication. Wallingford, Corm. 1989.

Table III.

Day G., Chief Ceramist,

Jeneric/Pentron

SiO2 K2O

MgFz

of (psi)

Compressive strength (psi) 58,000 43,000 25,000 150,000 ‘120,000

1500 7500 11,000 25,000 22,000

Inc.,

Composition of Dicor7

MgO

Crystalline phase: 55% mica Tetrasilicic fluo-mica (KS. Mgs SiB. 020 . Fa)

A1203

ZrO2

porcelain was baked in a refractory matrix for inlays.3 However, the problems of porcelain weakness, microleakage, cement failure, and poor fit were insurmountable and the techniques were abandoned. Dental porcelain is a glasslike solid composed of three naturally occurring minerals: feldspar, quartz, and kaolin. In addition, most porcelains contain various fluxes to modify the way they melt, and fuse together. Potassium alkalis are used as external surface glazes.4 Approximately 30 years ago, it was discovered that certain formulations of glass could be modified with nucleating agents that induced crystallization on heat treatment. The resultant material was stronger, had a higher melting point, and had a variable coefficient of thermal expansion.5 The term glass ceramic was given to this type of material and its use in dentistry has rapidly expanded. Several all-ceramic restorative systems are available or under development today. This review will emphasize information on four materials: conventional porcelain, Optec HSP (Jeneric/Pentron, Inc., Wallingford, Conn.) high-strength porcelain, Dicer (Dentsply/York Division, Dentsply International, Inc., York, Pa.), and Cerapearl (Kyocera Bioceramics Group of Kyocera Corp., Kyoto, Japan). All four systems are essentially types of glasses and are made up of inorganic oxides fused at high temperature.

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Modulus rupture

Material

On cooling, they come a frozen liquid at room temperature.” The Cerapearl ceramic system is currently in a research, development, and test phase and is not yet commercially available. However, it is included in this report because it is a castable glass-ceramic material and has properties that are comparable to other all-ceramic materials. The exact composition and method of manufacture of dental ceramic materials are patented proprietary secrets. Tables I through V give general information on the composition and strength measurements of these ceramic systems.6‘8

STRENGTH MECHANISMS

AND STRENGTHENING OF DENTAL CERAMICS

One of the problems encountered with the use of ceramic materials in dentistry has been their inherent. weakness. Early ceramic materials were amorphous in structure with little stress resistance. Crystalline structures, on the other hand, are generally stronger because their atoms are in a state of maximum packing density. Therefore, much emphasis has been placed on attempting to strengthen dental porcelain by the addition of strengthening oxides or attempts at inducement of crystallization. Modern dental ceramic materials are indeed stronger than the early porcelain, however, much of the reported strength values are clinically meaningless.g It is difficult. to get reliable test data comparing different. ceramic materials because of the problem of standardizing tests and specimens.g Ceramic materials in everyday use are approximately l/ 10 the strength of similar materials prepared in a controlled laboratory proess and l/100 the strength of near perfect crystals.g Most, ceramic materials have a characteristic critical strain limit of 0.1% . Thus, a tiny deformation will imme-

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diately induce a stress crack that progresses rapidly until the material suffers catastrophic rupture. Any increase in strength and toughness can only be achieved by an increase in the elastic modulus or rigidity of the material. However, practical strength values of porcelain are as much as 100 times smaller than theoretical values extrapolated from modulus of elasticity data. Because of a multitude of test variables, average strength values comparing different ceramic materials have little clinical meaning.g, lo In general, the bending test is the most sensitive test of the strength of ceramic materials. This test relates to the modulus of rupture and is a measure of the material’s flexural strength.g-ll The compressive strength test is the least sensitive test of strength and data obtained have little practical value. All dental ceramics are significantly harder than tooth enamel. However, no correlation exists between hardness and other mechanical properties of dental materials.g Surprisingly, there is relatively little effect of internal porosity on strength of dental ceramic materials, with no significant differences between vacuum-fired and air-fired porcelain9 However, strength is significantly affected by surface flaws. Near perfect surfaces give increased strength. The glaze on a ceramic material surface has a slightly increased coefficient of thermal expansion and contracts more than the underlying body of material. The glaze produces increased surface tension and increased strength values.g Dry strength values for test specimens are not reliable. Moisture has an overall negative effect on fracture resistance and is responsible for the phenomenon known as static fatigue. This stress-corrosion process involves a chemical reaction between water vapor and ceramic material and occurs inside microscopic surface faults. Continual low levels of stress initiate propagation of cracks resulting in failure of the ceramic material. Static fatigue is often the causative mechanism where a restoration fails in the mouth for no apparent reason.g

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2. Facial cusp coverage, MODFI, onlay preparation.

A low pH environment in the mouth is extremely corrosive to ceramic material because it induces rapid decomposition of the glass matrix structure. For this reason acidulated topical fluoride solutions are contraindicated.g All dental ceramics reach the level of critical strain at the point of 0.1% flexure.9-11 Thus, an increased margin of safety is required for ceramic material compared with metal restorative material. Increasing the rigidity (modulus of elasticity) and increasing the thickness of the ceramic are two methods of increasing its strength.9-12 However, increased rigidity is not of great significance for ceramic sections of 0.5 to 0.8 mm. These thin sections are flexible and the critical strain level of 0.1% is reached quickly.” The measured flexural strength of enamel (1500 psi) and dentin (7500 psi) in test specimens is far less than that of any restorative material. However, in combination in their natural state, their performance far exceeds that that could be predicted from strength-test data. In vivo, the forces of occlusion applied to the enamel surface create stressesthat are t,ransferred through the dentinoenamel juncture to the supporting dentin where they are effectively distributed and absorbed.‘” The transfer and distribution of stressesin an efficient manner are probably of equal importance to strength and toughness in a restorative system. For this reason, methods of attachment of ceramic materials to tooth structure are being intensely studied. The first all-ceramic restorative materials used in the posterior part of the mouth relied on conventional zinc phosphate cements and, later, on glass ionomer cements for retention.14p l5 Problems with retention and breakage caused researchers to look for better methods of attachment. Progress in porcelain conditioning and enamel and dentin bonding has resulted in light-activated and dualcure cements that have tremendously increased retentive properties.‘“~ “-so The objective in using resin-bonded cement is to intimately attach ceramic restorations to enamel and dentin. This bonding of materials and tooth structure is similar in nature and strength to the natural mechanism of the dentinoenamel juncture.

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Fig. 3. Maximum facial cusp coverage, 314 crown preparation.

Fig. 5. Rounded, smooth flowing internal form, DO inlay preparation. PREPARATION

DESIGN

This article addresses seven aspects of preparation design. These are (1) occlusal reduction, (2) axial reduction, (3) types of margins, (4) internal form and finish, (5) treatment of dentin, (6) taper and extension, and (7) cuspal preparation/reduction.

Occlusal

reduction

Approximately 1.5 to 2 mm of occlusal tooth structure should be removed on posterior teeth for all ceramic systems examined. This thickness of material provides adequate bulk for strengthi I49i5, 25-2g(Figs. 1 and 2).

Axial Fig. 4. Heavy reduction needed for strength, 314 crown preparation.

Presently at least five dentin bonding systems are available. These systems all use dentin primers to assist the adhesion of resin cement to dentin.21, 22A bond is created that is hopefully both stable and durable. Recent research shows evidence that teeth restored with resin-bonded porcelain inlays have cuspal stiffness equal to unrestored teeth. Teeth restored with conservative restorations have been found to be stronger because of the bond created between the tooth structure and ceramic material. These teeth have decreased cuspal flexure, increased fracture resistance, and reduced microleakage.23* 24 Increased physical strength of ceramic materials and increased bond strength make it feasible for the first time to use these materials in the posterior part of the mouth with some degree of certainty that they will survive the destructive forces of occlusion. Because of the brittle nature of ceramic materials, tooth preparation must be accomplished in a way that minimizes stress in these materials.

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reduction

One author suggests that teeth prepared for conventional porcelain should have only 0.8 to 1 mm axial reduction.27 Cerapearl material needs 1.5 mm reduction,2s Dicer restorations need 1.2 to I.5 mm reduction, and Optec restorations require 1 mm or more of axial depth.2g A uniform axial reduction of 1.5 mm should be adequate for all ceramic systems (Fig. 3).

Types

of margins

Most authors recommend cavosurface butt joints for occlusal margins,24-27130,31with the exception of two authors who recommend a 0.5 mm continuous bevel on all margins23*32and another author who prefers chamfered occlusal marginss3 Recommendations for axial surface margins are uniform in that either a heavy chamfer (110 to 135 degrees) with a rounded gingivoaxial line angle, or a rounded shoulder (90 degrees) will provide adequate juncture of ceramic material and enamel.12p14,27,28,31,a4,35 Knife-edge margins are not recommended.31, 35,36One author, however suggests enamel bevels on gingival, facial, and lingual surfaces of box forms.25* 26Bevels might also be permissible on non-stress-

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MA

AR

. ../ ,--

6. Pulpal protection in areas of deep preparation, 314 crown preparation.

Fig.

7. Tapering 718 crown preparation restoring distofacial cusp.

Fig.

bearing facial and lingual surfaces on onlays to enhance color blend.25, 26In general, sharp margins and bevels create thin sections of brittle porcelain (Fig. 4).

Internal

form

and finish

All teeth prepared for ceramic inlays or onlays should exhibit a smooth internal surface with rounded line and point angles.149 2% 27,29,31,34 1 n t ernal stress concentration is avoided by eliminating all rough surfaces and sharp angles. Rounded, smooth-flowing internal box forms are recommended for interproximal preparations (Fig. 5). There is lack of consensus regarding placement of grooves.12,25

Treatment

of dentin

Undercuts should be blocked out with glass ionomer cement. All dentinal surfaces should be covered with a 0.5 mm glass ionomer cement liner, and near pulpal exposures should be covered with calcium hydroxide before placement of the glass ionomer cement.24-27,31,33,37 Glass ionomer cement has several advantages in that it bonds to dentin, possessesan equivalent coefficient of thermal expansion and releases fluoride.25p 26 The glass ionomer base is highly susceptible to acid etchants, and present research questions the value of etching this liner.25*26 Dentinbonding agents should be used to provide additional retention for onlay preparations with significant exposed dentin (Fig. 6).13,22,25,26

Taper

and extension

Ceramic preparations differ significantly from gold restorations in that the taper should be increased from 3 to 5 degrees to 6 to 8 degrees. l4 Because ceramic restorations are extremely brittle before bonding, it is important that they not bind during seating for try-in. For this reason, proximal and occlusal walls must be flared (Figs. 7 and 8).27p32,37 Interproximal finish lines should extend into the facial and lingual embrasures for ease of finish after bonding.25r26 One of the chief advantages of a ceramic restoration is its

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Fig.

8. Brittle margins necessitate a careful try-in.

close color match with tooth structure, which makes the interproximal extension undetectable (Fig. 9). When entire cusps are fractured and require replacement, the facial or lingual finish lines may be carried to within 0.5 mm of the gingival tissue to provide a harmonic blend of tooth color (Fig. 1O).27 Teeth prepared for conservative ceramic restorations must have adequate tooth length for retention. In many instances, a tooth that is reduced 2 mm in height becomes too short to provide the needed retentioni These teeth often require periodontal crown-lengthening procedures. This prerestorative surgery is routinely recommended for short posterior teeth by one author.12

Cuspal

preparation

and reduction

Cuspal preparation and extension is needed when a cusp has fractured or is hopelessly undermined. Cuspal capping must also be considered when the margin of an inlay

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’ / kI/t .

(et y&--

-_” .’

Fig. 9. Flared margins and cuspal protection, MODL onlay preparation.

approaches to within 1.5 mm of a functional cusp that must withstand maximal occlusal loading.27 No standards have been developed for depth of reduction for functional cusps, but the ceramic thickness necessary to avoid critical strain should approach 1.5 mm for premolar5 and 2 mm for molars. Nonfunctional cusps require less protection and 1 to 1.5 mm seems adequate (Fig. 11).s7 Axial reduction of working cusps should extend 2 to 3 mm cervically from the original cusp height. This distance roughly coincides with the color transition from the occlusal shade to the body shade.27s37Non-stress-bearing capped cusps may have a bevel present to provide better esthetic blending of color and increased surface for bonding retention.25l26 However, the danger of chipping is always present before bonding.

TEMPORIZATION Temporization procedures for conservative ceramic restorations are slightly more difficult because of the increased complexity of the preparation design. Most authors recommend using a noneugenol type of cement to preclude interference with the set of the bonding resin at the time of placement.12p 13p24127*35One author, however, downplays the effect of residual eugenol and recommends thorough cleansing of the prepared tooth before bonding.25l 26

FIT Marginal fit of ceramic restorations has been reported to be good to excellent. Conventional porcelain systems tend to be more technique sensitive and fit is directly related to the skill of the ceramist.25* 26 One study reports that the average marginal gap for Dicer ceramic crowns is 28 pm.3a Dicer ceramic castings are found to fit better than gold castings.38 However, in a castability study, gold outperformed Dicer ceramic material. Gold can be cast to diameters of 132 Grn whereas Dicer ceramic can be cast to 724 pm, but no smaller.3g When the marginal fit of Dicer ceramic crowns was compared with other ceramic systems,

624

Fig. 10. Facial margin extension for optimum match, MODF onlay preparation.

color

there were conflicting results. In one study, Dicer ceramic placed first in fit. 4o In another study, Dicer ceramic rated third. Increased thickness and damaged margins were reported as problems with the material.41 With conventional ceramic systems and cast ceramic systems alike, clinically acceptable margins can be routinely produced, providing that accurate impressions are made, appropriate ceramic technique is followed, and the restoration is carefully placed and cemented.

PLACEMENT

AND

CEMENTATION

Because of their brittle nature, placement procedures for ceramic inlays and onlays are more demanding and timeconsuming than for conventional restorative materials. A try-in is necessary so that interproximal and internal fit may be verified. Most authors prefer to adjust the occlusion after bonding to prevent fracture of the ceramic material.“* 27~30,37142,43 This procedure makes it difficult to perfect the occlusion for multiple opposing units. One author, who advocates Dicer ceramic restorations, recommends a remount procedure using cerammed castings before shading.12 All articles reviewed agree that strict isolation and maintenance of a dry field is necessary for proper bonding to integrate ceramic restorations with tooth structure. Moisture contamination causes microleakage, discoloration, loss of bonding, and sensitivity. Several authors make specific recommendations regarding the use of the rubber dam.19,24,27,

30,33,43

Conventional zinc phosphate and glass ionomer cements are no longer recommended for cementation of ceramic restorations because resin-bonded cements are much stronger and more retentive. Light-activated resin cements with dual chemical-cure components are now routinely used to ensure uniform bonding, even in deep interproxima1 preparations.i6* 27,31pa5937,43Most ceramic restorations demonstrate enhanced bond strengths when their internal surfaces have been etched and treated with a silane conditioner.12y16 The Optec ceramic system, however, recommends that internal surfaces be grit-blasted with an

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NT OR t lilof‘lr.

OUT

11. Heavy reduction for maximum cuspal protection, MODFL onlay preparation.

Fig.

abrasive followed by silane conditioner without etching. Equivalent bond strengths have been produced by using this method of surface conditioning.18 The conventional technique of acid etching enamel provides a strong bond for conservative ceramic restorations. In addition, the use of dentin-bonding systems increases the retention of ceramic restorations and reduces microleakage.17r25,s6

INDICATIONS Conservative ceramic posterior restorations should be considered when esthetics is a major factor in replacing failing restorations and when cuspal fracture has occurred.26s27Ceramic inlays, onlays, and conservative crown restorations are indicated for endodontically treated teeth where one chooses to use partial coverage restorations.27 The increased strength provided by ceramic bonding allows this deviation from complete coverage, Posterior ceramic restorations work well when there is surrounding porcelain-restored dentition.27 These restorations should also be considered when there is a requirement for nonmetallic restorations based on patient need or desire.26

CONTRAINDICATIONS Conservative ceramic restorations have their limitations and will fail under various unfavorable conditions. Close analysis of selection criteria is essential. Teeth that show evidence of abusive occlusal habits such as bruxism or clenching, with severe wear facets, are poor candidates for ceramic restorations.26, 27136 Short teeth and teeth with large immature pulps should be restored with other materials.25,35s36An inability to maintain a dry field will produce failure. 27Teeth needing small class I or class II esthetic restorations are better restored with direct fill composites. 27Teeth that oppose large resin restorations are problematic in that the ceramic material has the potential to produce rapid wear of the opposing resin material.27

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All four systems have numerous advantages in common that make them appealing to restorative dentists. Posterior conservative ceramic restorations are esthetic to the point of being virtually indistinguishable from natural teeth.12s26,27,30,31,36,43-47Their radiodensity is similar to tooth structure, making it easier to read areas previously shielded by radioopaque metal restorations.i2s I9931,36,45,46 Thermal conductivity is similar to tooth structure, which may reduce sensitivity.r2j lgl 30,31,48 Ceramic restorations are strong and durable after placement and bonding, and they are abrasion resistant.27B43,48Ceramic materials have good color stability, are stain resistant, and are biocompatible. lg,s7*34,36,42*46,48They have excellent marginal integrity with resultant decreased microleakage.lg, 24,27,43,46,48Less clinical time and effort are required in the fabrication of posterior ceramic restorations compared with direct fill resin materials because much of the work is done in the laboratory.12* 26,27 Castable ceramics offer several unique advantages that are important. Conventional lost-wax casting procedures are used that allow restorations to be made with precise anatomy.12pl*, 28,4g This quality is useful when restoring cusp and fossa form. The wear rate of cast ceramics is similar to enamel, which should produce a harmonious rate of attrition in opposing teeth.12r31z46Dicer cast ceramic material has been reported to develop less plaque formation than conventional restorative materials.“l, 34,5o Conventionally fired ceramic systems have an advantage in that they do not require special fabrication systems and use an ordinary porcelain technique.2g* 44 All shading is blended inside the porcelain and no surface colorant is required.44

DISADVANTAGES Conventional porcelain, Optec, Dicer, and Cerapearl ceramic materials have several disadvantages, including an inherent fragility before cementation.34, 42Because a deep preparation is required for strength of the material, occasional endodontic treatment is needed.&” Fabrication of temporary restorations is time-consuming and their cementation is difficult.27T 42The dentist must rely on laboratory assistance to obtain accurate restorations and there are variables in levels of laboratory quality.25-27r42Moisture contamination in deep preparations will induce failure.31s42 At present no ceramic restorative materials possess adequate strength to be used for posterior fixed partial dentures. Therefore, these materials are limited to singleunit use.46 Cast ceramic materials have a few unique disadvantages. The initial start-up cost to laboratories for equipment is high, which translates to a high unit cost to the restorative dentist.46 The casting and ceramming process is intricate and time-consuming, requiring approximately 14 hours.51 This disadvantage is minimized by use of a strict

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24hour production schedule. If occlusal adjustments are made after bonding, the outer color layer may be removed, revealing a darker gray shade which may diminish the esthetic result.31* 34*36 Conventional porcelain ceramic systems have two main problems, (1) there is a possibility of wear on opposing natural enamel surfaces2’* 42 and (2) it is difficult to reproduce complex occlusal anatomy after firing because grinding the material tends to induce cracks.47For this reason, grooves and surface characteristics must be made shallow, and precise occlusal contacts are exceedingly difficult to obtain. REFERENCES 1. Hagman HC. History of ceramics. Part II. Dent Lab Rev 1981;55:34-6. 2. Marra LM. An historical review of full coverage of the natural dentition. NY State Dent J 1970;36:147-51. 3. Jones DW. Development of dental ceramics-an historical perspective. Dent Clin North Am 1985;29:621-43. 4. Hagman HC. History of ceramics, Part I. Dental Lab Rev 1980;55:34-6. 5. MacCuIlock WT. Advances in dental ceramics. Br Dent J 1968;124: 361-5. 6. Binns D. The chemical and physical properties of dental porcelain. Proc. 1st Int Symposium On Ceramics. Chicago: Quintessence Pub1 Co, Inc, 198341-5. 7. Adair PJ, Grossman DG. The castable ceramic crown. Int J Perio Rest Dent 1984;2:33-45. 8. Hobo S, Iwata T. Castable apatite ceramics as a new biocompatable restorative material. I. Theoretical considerations. Quintessence Int 1985;16:135-41. 9. Jones DW. The strength and strengthening mechanism of dental ceramics. First Symposium on Ceramics. Chicago: Quintessence Pub1 Co, Inc, 198383-141. 10. McLean JW, Kedge MI. High strength ceramics. Quintessence Int 1987;18:97-106. 11. McLean JW. Ceramics in clinical dentistry. Br Dent J 1988;164:187-94. 12. Malament KA. Considerations in posterior glass-ceramic restoration. Int J Perio Rest Dent 1988;4:33-49. 13. Grossman DG, Nelson JW. Dicer research report. The bonded Dicer crown. York, Pa: Dentsply International, 1987;3(1). 14. Malament KA, Grossman DG. The cast glass-ceramic restoration. J PROSTHET DENT 1987;57:674-83.

15. Malament KA. The cast glass ceramic crown. Proc. 4th Int Symposium on Ceramics. Chicago: Quintessence Pub1 Co, Inc, 1988;331-42. 16. McInnes-Ledoux PM, Ledoux WR, Rappold A. Luting castable ceramic restorations-a bond strength study [Abstract]. J Dent Res 1987;66:207. 17. Waknine S, Gable P, Penugonda B, SchuImann A. Bond strength characterization of an experimental series of dentin adhesives. Academy of Dental MateriaIs paper, Feb 1988, Chicago, l-21. 18. Waknine S, Prasad A, Gable P. Characterization of interfacial adhesion of a high strength porcelain [Abstract]. J Dent Res 1988;67:223. 19. Cave1 WT, Kelsey WP, Barkmeier WW, Blankenau RJ. A pilot study of the clinical evaluation of castable ceramic inlays and a dual cure resin cement. Quintessence Int 1988;19:257-62. 20. Bailey LF, Bennett RJ. Dicer surface treatments for enhanced bonding. J Dent Res 1988;67:925-31. 21. Freedman G. The new bonding methods. Dentistry Today 1988;7:32-3. 22. Christensen GJ, Christensen RP. Tooth bonding system. CRA News 1988,12:1-2.

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