stable
glass ceramics
Steven A. Lang, DDS,a and Clifford Patrick U.S. Air Force Hospital, Patrick Center, Wright-Patterson AFB, Ohio
for veneer B. Starr,
restorations
DMDb
AFB, Fla., and Wright-Patterson
U.S. Air Force Medical
Porcelain veneer restorations are the usual technique chosen for the placement of esthetic veneer restorations. Cast glass ceramic veneers exhibit properties that make them extremely useful for this dental application. They have a hardness, abrasion resistance, coefficient of thermal expansion, and translucency similar to that of enamel. The use of the lost wax casting technique to provide precise anatomic form and margins allows for increased accuracy during veneer fabrication. Chairside time requirements may be decreased as a result of the improved accuracy inherent in the lost wax casting technique. The dentist’s ability to maximize esthetics and decrease chairside time make this a technique likely to see increased use in the future. (J PROSTHET DENT 1992;67:590-4.)
sthetic veneers made with castable glass ceramics (Dicer, Corning Glass Works, Corning, N.Y.; Dentsply International, York, Pa.) are increasingly popular because of the unique characteristics of the material. Glass ceramics were invented in 1957l and were first applied to dentistry in 1968, when denture teeth of various opacities were fabricated.2 Initially, glass ceramics were found to be too weak to withstand the oral environment. The use of air pressure and a vacuum combined with a lost wax casting machine technique was found to produce a stronger material with properties similar to those of human enamel.3 For years porcelain has been the material of choice for dentists requiring veneers with the ultimate in esthetics. Now a veneer fabrication and shading system that provides durability, accuracy, and improved esthetics has been developed.seg MATERIAL
CONSIDERATIONS
Glass ceramic is a nonporous, homogeneous, microstructure whose uniform size is derived from the controlled growth of crystals within an amorphous matrix of glass. A nucleating agent acts as a starting point for the controlled crystal growth Ereated by a regulated heat process called “ceramming.” The type of crystals grown and the extent of their growth determine the properties of the glass ceramic. The opinions expressed herein are those of the authors and do not necessarily reflect the opinions of the Department of Defense, the USAF, or of other federal agencies. “Captain, USAF Dental Corps, General Clinic Dentist, USAF Hospital, Patrick A.FB, Fla. bLientenant Colonel, lJSAF Dental Corps, Chairman, Department of General Dentistry, Assistant Director, General Practice Residency, Wright-Patterson USAF Medical Center, WrightPatterson AFB, Ohio. IO/X/35555
590
Many different types of glass ceramics are available. One is the @-spodumenecrystal that is used in cooking utensils. Other types of glass ceramics are used in commercial materials, ranging from electromagnetic windows for guided missiles to machinable ceramics in end milling operations. Mica crystals are used in the fabrication of cast ceramic veneers. They are interlocked and randomly oriented, which adds strength and reinforcement to the material and results in a flexural strength twice that of porcelain. The monolithic structure of glass ceramic permits its intrinsic strength to be distributed throughout the body of the castable ceramic restoration.3-5 Castable glass ceramics have a high compressive strength and are similar in density and hardness to natural ename1.5Therefore they exhibit wear characteristics similar to those of enamel, virtually eliminating the abrasion seen with porcelain restorations. 4-6Thermal conductivity is low, impeding
the transfer
of heat
or cold
to the underlying
tooth structure. The coefficient of thermal expansion is similar to that of enamel. Therefore the expansion and contraction that occur with temperature fluctuations is compatible with tooth structure, so marginal integrity is preserved.4, lo Glass ceramics appear radiopaque on x-ray film but have sufhcient radiolucency to allow visualization of underlying tooth structure, restorations, and dental earies.3 Biocompatibility has been extensively investigated and no signs of tissue toxicity have been discovered.6 Chemical durability has been examined in a broad range of temperatures, media, and acidity (PM), and compares favorably to that of porcelain.6 Glass ceramics have been shown to be less plaque-retentive than enamel or porcelain.4, l1 Perhaps the most important property of glass ceramics is the esthetic nature of the material. Transhrcency is very similar to that of enamel.6 Diffracted light from the elon-
MAY
1992
VOLUME
67
NUMBER
5
CAST
GLASS
CERAMIC
VENEERS
Fig. Fig. Fig. teeth Fig.
I. Intraoral view of a dentition affected by severe tetracycline staining. 2. Depth cuts are used to guide uniform enamel reduction. 3. Veneer preparation for tooth No. 8 after completion of depth cut placement in Nos. 5 through 12. 4. Veneer preparations for teeth Nos. 5 through 12 complete.
gated mica crystals gives depth to the translucency in much the same way as enamel rods diffract light in enamel. The level of crystallinity provides many scattering events that allow light to constantly change direction, enhancing translucency. A “chameleon” effect is also produced as the veneer acquires color from adjacent teeth as a result of this translucency.4 Final shading is accomplished using various shades of light-activated resin cement. The proper hue, chroma, and value can.be obtained without loss of translucency. The only limits placed on shading are the dentist’s and technician’s color perception and artistry.
TOOTH
PREPARATION
Maximum esthetic results require 0.5 to 0.75 mm of tooth reduction.lO, l2 This amount provides sufficient thickness to maintain the strength of the cast glass ceramic veneer and adequate material to develop the proper shade and anatomic form without overcontouring the tooth. However,
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
in some teeth the enamel at the cervical margin is not thick enough to permit 0.5 mm reduction. In these instances dentinal exposure can be expected, although every effort is made to leave sufficient enamel to provide a successful bond at the cement-enamel interface. The preparation can be achieved using a 1.0 mm round diamond bur. Fig. 1 shows a preoperative view of a dentition severely affected by tetracycline staining. Depth cuts are placed across the facial surface to produce a visual depth guide (Figs. 2 and 3). The facial surface is then prepared with a medium grit tapered diamond bur (Fig. 4). The preparation should extend interproximally to ensure sufficient masking of the stain with the veneer, although interproximal contacts should be maintained in enamel if possible.12*I3 If no incisal reduction is planned, the dentist must determine that sufficient thickness remains after preparation of the facial surface to prevent weakening of the incisal edge. If the incisal edge is compromised, it should be reduced 1.0 to 1.5
591
LANG
AND
STARR
Fig. 5. Die spacer applied after working dies are trimmed. Fig. 6. Etching tooth No. 8 before veneer cementation.
mm to provids sufficient compressive strength and horizontal resistance for the veneer restoration. The finish line should extend lingually, be continuous with the labial outline, and result in a uniform reduction without sharp line angles.lo: 12,l3 Provisional restorations are generally not required, but can be provided using light-activated composite resin restorative material. To facilitate removal of the temporary material, only 1 mm2 of midfacial enamel should be etched. Gel etch should be used to localize and control the etching process.
LABORATORY
PHASE
The laboratory process begins by fabrication of a wax pattern on a die made from an accurate impression (Fig. 5). Waxing anatomic contours is a significant advantage over the porcelain technique, which requires overbuilding to allow for shrinkage during firing. This procedure improves marginal fit and esthetics and can eliminate chairside contour adjustments. The wax pattern is invested in a phosphate-bonded investment material that has two layers of Kaoliner (Dentsply, York, Pa.) casting liner and is bench set for 1 hour. The investment is then heat-soaked for 30 minutes at 250’ C and is then burned out at 950’ C for 30 minutes.14 Casting involves the use of an electrically heated mufhe centrifugally spun by an electric motor. A casting temperature of 1370” C is required and is held for 6 minutes, after which the melted glass is forced into the mold. The centrifuge is allowed to spin 4 minutes to maintain continuous pressure on the casting during the slow setting process. The casting is then bench-cooled and divested.3 The casting is more accurate than traditional gold castings or metal alloys because of the gradual transition of liquid to solid and the greater wettability of glass oxides to the investment.6 After divestment, the casting, a transparent glass, can be inspected for any defects. The casting is invested again and heat-treated (cerammed) for 6 hours at 1075’ C. During this phase the mica crystals grow and
592
elongate until they contact each other. Once this cycle is completed, the casting assumes an opaque-translucent appearance similar to that of enamel. The casting is fit to the die and sprues are removed with diamond or carborundum disks and finished with conventional finishing stones. The outer surface is cleaned with a finishing stone, grit blasted, and cleaned ultrasonically with water in preparation for shading.3 Colorants and porcelain glazing materials are fired on the external surface to provide the proper shade. Castable glass ceramics can be fired repeatedly if necessary without adversely affecting marginal integrity, hardness, or translucency.6 The silane coupling agent is then heat-cured to the internal surface to provide mechanical retention. This procedure has been found to be 40 % more effective than the use of silane agents applied chairside.lOsl5
TRY-IN
AND CEMENTATIO
The initial try-in should be performed to ensure proper marginal integrity of the veneer and to verify shading. The silane coupling agent protects the veneers from most contaminants, but excessive handling of the etched surface is not recommended.rO,I2 Cast ceramic veneers are as fragile as porcelain veneers, and care in handling is necessary to prevent inadvertant fracture during seating. Pressure should be applied at the margins and not midfacially to prevent fracture. Shaded water soluble try-in pastes enable the dentist to verify the final shade. These shades may be mixed to alter value, chroma, hue, and translucency, giving complete esthetic control to the dentist. Once the shade is verified, the cementing surface of the veneer is decontaminated. It is first scrubbed with alcohol, dried, and then placed in an ultrasonic cleaner with distilled water to remove any contaminants that may inhibit bonding. The tooth surface of enamel should be pumiced and etched before cementation (Fig. 6). The tooth surface and cementing surface should both be dried completely. If a bonding
-MAY
1992
VOLUME
67
NUMBER
5
CAST
GLASS
CERAMIC
VENEERS
Fig. Fig.
7. Cast ceramic veneers cemented on teeth Nos. 5 through 12. 8. Completed treatment.
agent is used, it should maintain minimal film thickness and pooling of the material should be avoided. Glass ceramic veneers are compatible with most cementing agents, but use of a light-activated resin cement system that is dual-cured is recommended (Figs. 7 and 8). The cement is immediately activated with visible light but will also chemically cure without light. This ensures complete curing of the luting agent, even in areas inaccessible to light.lOs l2 DISCUSSION The indications and contraindications for laboratory processed veneers are well established.12, 13,rszo Teeth discolored by tetracycline stains, fluorosis, or devitalization discoloration can be masked with esthetic veneers. Tooth malpositions, minor malocclusions, diastemata, and enamel defects can also be improved with veneer restorations. Enamel bonding is used to lute esthetic veneers into place and to seal the margins. Therefore veneers are contraindicated when insufficient enamel is available for etching. Patients with tooth-tooth or tooth-foreign object habits such as bruxing, pipe smoking, or pencil chewing may not be good candidates because of the increased shearing stress placed on the veneer.lr Mouth breathers and patients with inadequate oral hygiene are poor candidates because veneers can often aggravate chronically inflammed gingiva. Extreme labioversion is a contraindication because teeth often appear overbulked and may cause lip incompetency. There are many advantages to cast glass ceramics over porcelain. The use of the lost wax technique, as with metal alloys, eliminates the need to compensate for the 20% shrinkage seen with traditional porcelain firing. The waxing of precise anatomy provides more accurate contours, improved margins, better marginal adaption, and decreased chair time needed to seat the veneers.12,21,22The hardness and density are similar to those of enamel, which eliminates the abrasion to opposing natural teeth seen with
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
porcelain. Esthetic, life-like restorations can be fabricated because of the great translucency of cast ceramic veneers. They are also less plaque-retentive than porceiain and enameL4>l1 Some disadvantages to cast ceramic veneers do exist. The laboratory process required is slower than the process used to fabricate porcelain veneers. The need for chairside contouring should be eliminated, but if required, the shade and porcelain glaze will be compromised and restaining will be required. Severely discolored teeth are not as easily masked as with porcelain because of the greater translucency of cast ceramic veneers.23 Shaded opaque cements can be used to provide additional masking and increased opacity if necessary. Despite the initial high cost to equip a laboratory, the use of cast ceramics by commercial dental laboratories is increasing to keep pace with the demand. SUMMARY Cast glass ceramic materials exhibit properties that make them extremely useful for certain dental applications. They have a hardness, abrasion resistance, coefficient of thermal expansion, and a translucency similar to those of enamel. The use of the lost wax casting technique to provide precise anatomic form and margins allows for increased accuracy during veneer fabrication. Laboratory procedures take more time than traditional porcelain veneers, but when properly handled, provide superior results. Clinical aspects such as preparation, shading, tryin, and cementation can be routinely performed once the clinician becomes familiar with the technique. The dentist’s ability to maximize esthetics and decrease chairside time make this a technique likely to see increased future use. REFERENCES 1. Anonymous. Corning Bull 1957;36:279-80.
develops new ceramic material.
Am Ceram Sac
593
LANG AND STARR
2. MacCuIlochWT.Advancesindentalceramics.BrDentJ1968;124:361-5. 3. Grossman DC. Processing a dental ceramic by casting methods. Ceramic engineering and science proceeding. Am Ceram Sot 1985;6:1940. 4. Grossman DG. Cast glass ceramics. Dent Clin North Am 1985;29:72537. 5. Pameijer CH, Grossman DG, Adair PJ. Physical properties of a castable ceramic dental restorative material [abstract]. J Dent Res 1980;59:474. 6. Adair PJ, Grossman DG. The castable ceramic crown. Int J Periodont Restor Dent 1984;4:32-46. 7. Cave1WT, Kelsey WT III, 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. 8. Grossman DG, Walters HV. The chemical durability of dental ceramics [abstract]. J Dent Res 1984;63:234. 9. Mueninghoff LA, Neal SJ, Ramus DL. Six months’ evaluation of clinical esthetic veneers (Dicer). J Dent Res 1988;67:305. 10. Anonymous. Dicer clinical instructions for Dicer restorations and use of the Dicer light-activated cementation kit. York, Pa: Dentsply International Inc, 198&l-13. 11. Savitt E, Socransky S, Melter A, Malament K, Backman H. Effects of oral microbiota by Dicer glass ceramic. Boston: Forsythe Institute, 1986. 12. Garber DA, Goldstein RE, Feinman RA. Porcelain laminate veneers. 1st ed. Chicago: Quintessence Publishing, 198814-23. 13. Jordan RE. Esthetic composite bonding. 1st ed Revised. Toronto: BC Decker, 1988122-39.
Marginal
fit of castable
ceramic
14. Judge SF. Dicer castable ceramic veneers. Dent Lab Prac 1987;12:12-5. 15. Bennett RJ, Baily LF. Bonding to Dicer laminate veneers [abstract]. J Dent Res 1986;65:314. 16. Reid JS. Tooth color modification and porcelain veneers. Quintessence Int 1988;19:477-81. 17. Christensen GJ. Veneering of teeth-state of tbe art. Dent Clin North Am 1985;29:373-90. 18. Covey DA, Oliveira FC Jr, Denehy GE. Selecting an esthetic veneering technique. Quintessence Int 1987;18:247-52. 19. Nathanson D. Etched porcelain restorations for improved esthetics. Part I. Anterior veneers. Compend Contin Educ Dent 1986;7:706-12. 20. Goldstein RE. Diagnostic dilemma: to bond, laminate, or crown? Int J Periodont Restor Dent 1987;5:9-29. 21. Farah JW, Powers JM, eds. All ceramic restorations. Dent Advisor 1989;6:1-3. 22. Woolsey G, Weir D, Dootz E, Morris H. Marginal accuracy of Dicer, Cerestore, porcelain-butt, and cast metal margins [abstract]. J Dent Res 1987;66:284. 23. Jones DW, Sutow EJ, Rizkalla AS, Black D. Opacity and colour of a castable glass-ceramic and cement system [abstract]. J Dent Res 1988;67:118. Reprint requests to: DR. CLIFFORD B. STARR 4062 QUAIL BUSH DRIVE DAYTON, OH 45424
crowns
J. Robert Holmes, DDS, MS, MEd,a William D. Sulik, DDS, MS,b Gene A. Holland, DDS, MS,” and Stephen C. Bayne, MS, PhDd Medical University of South Carolina, College of Dental Medicine, Charleston, SC., and University of North Carolina, School of Dentistry, Chapel Hill, N.C. The objective of this study was to measure marginal fit of castable ceramic versus gold crowns. Full veneer gold and ceramic crowns were made on Ivorine dies. Crowns were cemented, embedded, sectioned faciolingually and mesiodistally, and photographed for measurement of absolute marginal discrepancies (cavosurface angle to casting margin) to evaluate fit. Results revealed no statistically significant differences in fit among four locations around the margins of either ceramic or gold crowns. There was no statistically significant difference in the combined absolute marginal discrepancy (fit) between ceramic and gold crowns. The variance of the combined absolute marginal discrepancy (fit) of the ceramic crowns was significantly different than that of the gold crowns (p = 0.01 level). The standard deviation (variability) of the gold crowns was more than twice that of the ceramic crowns, Randomized block ANOVA demonstrated statistically significant differences among individual gold crown specimens, but none among individual ceramic crown specimens. (J PROSTHET DENT 1992;67:594-9.)
Supported in part by NIH-NIDR grant No. 2-S07-RR05333. ‘Assistant Professor, Department of Crown and Bridge Dentistry, Medical University of South Carolina. College of Dental Medicine. %linical Associate Professor (part-time), Department of Prosthodontics, University of North Carolina, School of Dentistry. cProfessor, Department of Prosthodontics, University of North Carolina, School of Dentistry. dAssociate Professor, Department of Operative Dentistry, University of North Carolina, School of Dentistry. 19/l/35864 I
594
S.
mce the introduction of the porcelain jacket crown by Land in the late 188Os,there have been continual efforts to improve all-ceramic restorations. The esthetic advantages inherent in eliminating the metal substructure of ceramometal crowns is well d0cumented.i Castable ceramic (Dicer, Dentsply Int., York, Pa.) crowns, made by the lost wax technique, are a recent development of the all-ceramic crown concept. These benefits, however, must be evaluated against the possible disadvantage of a poorer “fit.”
MAY 1992 VOLUME67
NUMBER5
glass ceramics
Steven A. Lang, DDS,a and Clifford Patrick U.S. Air Force Hospital, Patrick Center, Wright-Patterson AFB, Ohio
for veneer B. Starr,
restorations
DMDb
AFB, Fla., and Wright-Patterson
U.S. Air Force Medical
Porcelain veneer restorations are the usual technique chosen for the placement of esthetic veneer restorations. Cast glass ceramic veneers exhibit properties that make them extremely useful for this dental application. They have a hardness, abrasion resistance, coefficient of thermal expansion, and translucency similar to that of enamel. The use of the lost wax casting technique to provide precise anatomic form and margins allows for increased accuracy during veneer fabrication. Chairside time requirements may be decreased as a result of the improved accuracy inherent in the lost wax casting technique. The dentist’s ability to maximize esthetics and decrease chairside time make this a technique likely to see increased use in the future. (J PROSTHET DENT 1992;67:590-4.)
sthetic veneers made with castable glass ceramics (Dicer, Corning Glass Works, Corning, N.Y.; Dentsply International, York, Pa.) are increasingly popular because of the unique characteristics of the material. Glass ceramics were invented in 1957l and were first applied to dentistry in 1968, when denture teeth of various opacities were fabricated.2 Initially, glass ceramics were found to be too weak to withstand the oral environment. The use of air pressure and a vacuum combined with a lost wax casting machine technique was found to produce a stronger material with properties similar to those of human enamel.3 For years porcelain has been the material of choice for dentists requiring veneers with the ultimate in esthetics. Now a veneer fabrication and shading system that provides durability, accuracy, and improved esthetics has been developed.seg MATERIAL
CONSIDERATIONS
Glass ceramic is a nonporous, homogeneous, microstructure whose uniform size is derived from the controlled growth of crystals within an amorphous matrix of glass. A nucleating agent acts as a starting point for the controlled crystal growth Ereated by a regulated heat process called “ceramming.” The type of crystals grown and the extent of their growth determine the properties of the glass ceramic. The opinions expressed herein are those of the authors and do not necessarily reflect the opinions of the Department of Defense, the USAF, or of other federal agencies. “Captain, USAF Dental Corps, General Clinic Dentist, USAF Hospital, Patrick A.FB, Fla. bLientenant Colonel, lJSAF Dental Corps, Chairman, Department of General Dentistry, Assistant Director, General Practice Residency, Wright-Patterson USAF Medical Center, WrightPatterson AFB, Ohio. IO/X/35555
590
Many different types of glass ceramics are available. One is the @-spodumenecrystal that is used in cooking utensils. Other types of glass ceramics are used in commercial materials, ranging from electromagnetic windows for guided missiles to machinable ceramics in end milling operations. Mica crystals are used in the fabrication of cast ceramic veneers. They are interlocked and randomly oriented, which adds strength and reinforcement to the material and results in a flexural strength twice that of porcelain. The monolithic structure of glass ceramic permits its intrinsic strength to be distributed throughout the body of the castable ceramic restoration.3-5 Castable glass ceramics have a high compressive strength and are similar in density and hardness to natural ename1.5Therefore they exhibit wear characteristics similar to those of enamel, virtually eliminating the abrasion seen with porcelain restorations. 4-6Thermal conductivity is low, impeding
the transfer
of heat
or cold
to the underlying
tooth structure. The coefficient of thermal expansion is similar to that of enamel. Therefore the expansion and contraction that occur with temperature fluctuations is compatible with tooth structure, so marginal integrity is preserved.4, lo Glass ceramics appear radiopaque on x-ray film but have sufhcient radiolucency to allow visualization of underlying tooth structure, restorations, and dental earies.3 Biocompatibility has been extensively investigated and no signs of tissue toxicity have been discovered.6 Chemical durability has been examined in a broad range of temperatures, media, and acidity (PM), and compares favorably to that of porcelain.6 Glass ceramics have been shown to be less plaque-retentive than enamel or porcelain.4, l1 Perhaps the most important property of glass ceramics is the esthetic nature of the material. Transhrcency is very similar to that of enamel.6 Diffracted light from the elon-
MAY
1992
VOLUME
67
NUMBER
5
CAST
GLASS
CERAMIC
VENEERS
Fig. Fig. Fig. teeth Fig.
I. Intraoral view of a dentition affected by severe tetracycline staining. 2. Depth cuts are used to guide uniform enamel reduction. 3. Veneer preparation for tooth No. 8 after completion of depth cut placement in Nos. 5 through 12. 4. Veneer preparations for teeth Nos. 5 through 12 complete.
gated mica crystals gives depth to the translucency in much the same way as enamel rods diffract light in enamel. The level of crystallinity provides many scattering events that allow light to constantly change direction, enhancing translucency. A “chameleon” effect is also produced as the veneer acquires color from adjacent teeth as a result of this translucency.4 Final shading is accomplished using various shades of light-activated resin cement. The proper hue, chroma, and value can.be obtained without loss of translucency. The only limits placed on shading are the dentist’s and technician’s color perception and artistry.
TOOTH
PREPARATION
Maximum esthetic results require 0.5 to 0.75 mm of tooth reduction.lO, l2 This amount provides sufficient thickness to maintain the strength of the cast glass ceramic veneer and adequate material to develop the proper shade and anatomic form without overcontouring the tooth. However,
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
in some teeth the enamel at the cervical margin is not thick enough to permit 0.5 mm reduction. In these instances dentinal exposure can be expected, although every effort is made to leave sufficient enamel to provide a successful bond at the cement-enamel interface. The preparation can be achieved using a 1.0 mm round diamond bur. Fig. 1 shows a preoperative view of a dentition severely affected by tetracycline staining. Depth cuts are placed across the facial surface to produce a visual depth guide (Figs. 2 and 3). The facial surface is then prepared with a medium grit tapered diamond bur (Fig. 4). The preparation should extend interproximally to ensure sufficient masking of the stain with the veneer, although interproximal contacts should be maintained in enamel if possible.12*I3 If no incisal reduction is planned, the dentist must determine that sufficient thickness remains after preparation of the facial surface to prevent weakening of the incisal edge. If the incisal edge is compromised, it should be reduced 1.0 to 1.5
591
LANG
AND
STARR
Fig. 5. Die spacer applied after working dies are trimmed. Fig. 6. Etching tooth No. 8 before veneer cementation.
mm to provids sufficient compressive strength and horizontal resistance for the veneer restoration. The finish line should extend lingually, be continuous with the labial outline, and result in a uniform reduction without sharp line angles.lo: 12,l3 Provisional restorations are generally not required, but can be provided using light-activated composite resin restorative material. To facilitate removal of the temporary material, only 1 mm2 of midfacial enamel should be etched. Gel etch should be used to localize and control the etching process.
LABORATORY
PHASE
The laboratory process begins by fabrication of a wax pattern on a die made from an accurate impression (Fig. 5). Waxing anatomic contours is a significant advantage over the porcelain technique, which requires overbuilding to allow for shrinkage during firing. This procedure improves marginal fit and esthetics and can eliminate chairside contour adjustments. The wax pattern is invested in a phosphate-bonded investment material that has two layers of Kaoliner (Dentsply, York, Pa.) casting liner and is bench set for 1 hour. The investment is then heat-soaked for 30 minutes at 250’ C and is then burned out at 950’ C for 30 minutes.14 Casting involves the use of an electrically heated mufhe centrifugally spun by an electric motor. A casting temperature of 1370” C is required and is held for 6 minutes, after which the melted glass is forced into the mold. The centrifuge is allowed to spin 4 minutes to maintain continuous pressure on the casting during the slow setting process. The casting is then bench-cooled and divested.3 The casting is more accurate than traditional gold castings or metal alloys because of the gradual transition of liquid to solid and the greater wettability of glass oxides to the investment.6 After divestment, the casting, a transparent glass, can be inspected for any defects. The casting is invested again and heat-treated (cerammed) for 6 hours at 1075’ C. During this phase the mica crystals grow and
592
elongate until they contact each other. Once this cycle is completed, the casting assumes an opaque-translucent appearance similar to that of enamel. The casting is fit to the die and sprues are removed with diamond or carborundum disks and finished with conventional finishing stones. The outer surface is cleaned with a finishing stone, grit blasted, and cleaned ultrasonically with water in preparation for shading.3 Colorants and porcelain glazing materials are fired on the external surface to provide the proper shade. Castable glass ceramics can be fired repeatedly if necessary without adversely affecting marginal integrity, hardness, or translucency.6 The silane coupling agent is then heat-cured to the internal surface to provide mechanical retention. This procedure has been found to be 40 % more effective than the use of silane agents applied chairside.lOsl5
TRY-IN
AND CEMENTATIO
The initial try-in should be performed to ensure proper marginal integrity of the veneer and to verify shading. The silane coupling agent protects the veneers from most contaminants, but excessive handling of the etched surface is not recommended.rO,I2 Cast ceramic veneers are as fragile as porcelain veneers, and care in handling is necessary to prevent inadvertant fracture during seating. Pressure should be applied at the margins and not midfacially to prevent fracture. Shaded water soluble try-in pastes enable the dentist to verify the final shade. These shades may be mixed to alter value, chroma, hue, and translucency, giving complete esthetic control to the dentist. Once the shade is verified, the cementing surface of the veneer is decontaminated. It is first scrubbed with alcohol, dried, and then placed in an ultrasonic cleaner with distilled water to remove any contaminants that may inhibit bonding. The tooth surface of enamel should be pumiced and etched before cementation (Fig. 6). The tooth surface and cementing surface should both be dried completely. If a bonding
-MAY
1992
VOLUME
67
NUMBER
5
CAST
GLASS
CERAMIC
VENEERS
Fig. Fig.
7. Cast ceramic veneers cemented on teeth Nos. 5 through 12. 8. Completed treatment.
agent is used, it should maintain minimal film thickness and pooling of the material should be avoided. Glass ceramic veneers are compatible with most cementing agents, but use of a light-activated resin cement system that is dual-cured is recommended (Figs. 7 and 8). The cement is immediately activated with visible light but will also chemically cure without light. This ensures complete curing of the luting agent, even in areas inaccessible to light.lOs l2 DISCUSSION The indications and contraindications for laboratory processed veneers are well established.12, 13,rszo Teeth discolored by tetracycline stains, fluorosis, or devitalization discoloration can be masked with esthetic veneers. Tooth malpositions, minor malocclusions, diastemata, and enamel defects can also be improved with veneer restorations. Enamel bonding is used to lute esthetic veneers into place and to seal the margins. Therefore veneers are contraindicated when insufficient enamel is available for etching. Patients with tooth-tooth or tooth-foreign object habits such as bruxing, pipe smoking, or pencil chewing may not be good candidates because of the increased shearing stress placed on the veneer.lr Mouth breathers and patients with inadequate oral hygiene are poor candidates because veneers can often aggravate chronically inflammed gingiva. Extreme labioversion is a contraindication because teeth often appear overbulked and may cause lip incompetency. There are many advantages to cast glass ceramics over porcelain. The use of the lost wax technique, as with metal alloys, eliminates the need to compensate for the 20% shrinkage seen with traditional porcelain firing. The waxing of precise anatomy provides more accurate contours, improved margins, better marginal adaption, and decreased chair time needed to seat the veneers.12,21,22The hardness and density are similar to those of enamel, which eliminates the abrasion to opposing natural teeth seen with
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
porcelain. Esthetic, life-like restorations can be fabricated because of the great translucency of cast ceramic veneers. They are also less plaque-retentive than porceiain and enameL4>l1 Some disadvantages to cast ceramic veneers do exist. The laboratory process required is slower than the process used to fabricate porcelain veneers. The need for chairside contouring should be eliminated, but if required, the shade and porcelain glaze will be compromised and restaining will be required. Severely discolored teeth are not as easily masked as with porcelain because of the greater translucency of cast ceramic veneers.23 Shaded opaque cements can be used to provide additional masking and increased opacity if necessary. Despite the initial high cost to equip a laboratory, the use of cast ceramics by commercial dental laboratories is increasing to keep pace with the demand. SUMMARY Cast glass ceramic materials exhibit properties that make them extremely useful for certain dental applications. They have a hardness, abrasion resistance, coefficient of thermal expansion, and a translucency similar to those of enamel. The use of the lost wax casting technique to provide precise anatomic form and margins allows for increased accuracy during veneer fabrication. Laboratory procedures take more time than traditional porcelain veneers, but when properly handled, provide superior results. Clinical aspects such as preparation, shading, tryin, and cementation can be routinely performed once the clinician becomes familiar with the technique. The dentist’s ability to maximize esthetics and decrease chairside time make this a technique likely to see increased future use. REFERENCES 1. Anonymous. Corning Bull 1957;36:279-80.
develops new ceramic material.
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2. MacCuIlochWT.Advancesindentalceramics.BrDentJ1968;124:361-5. 3. Grossman DC. Processing a dental ceramic by casting methods. Ceramic engineering and science proceeding. Am Ceram Sot 1985;6:1940. 4. Grossman DG. Cast glass ceramics. Dent Clin North Am 1985;29:72537. 5. Pameijer CH, Grossman DG, Adair PJ. Physical properties of a castable ceramic dental restorative material [abstract]. J Dent Res 1980;59:474. 6. Adair PJ, Grossman DG. The castable ceramic crown. Int J Periodont Restor Dent 1984;4:32-46. 7. Cave1WT, Kelsey WT III, 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. 8. Grossman DG, Walters HV. The chemical durability of dental ceramics [abstract]. J Dent Res 1984;63:234. 9. Mueninghoff LA, Neal SJ, Ramus DL. Six months’ evaluation of clinical esthetic veneers (Dicer). J Dent Res 1988;67:305. 10. Anonymous. Dicer clinical instructions for Dicer restorations and use of the Dicer light-activated cementation kit. York, Pa: Dentsply International Inc, 198&l-13. 11. Savitt E, Socransky S, Melter A, Malament K, Backman H. Effects of oral microbiota by Dicer glass ceramic. Boston: Forsythe Institute, 1986. 12. Garber DA, Goldstein RE, Feinman RA. Porcelain laminate veneers. 1st ed. Chicago: Quintessence Publishing, 198814-23. 13. Jordan RE. Esthetic composite bonding. 1st ed Revised. Toronto: BC Decker, 1988122-39.
Marginal
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14. Judge SF. Dicer castable ceramic veneers. Dent Lab Prac 1987;12:12-5. 15. Bennett RJ, Baily LF. Bonding to Dicer laminate veneers [abstract]. J Dent Res 1986;65:314. 16. Reid JS. Tooth color modification and porcelain veneers. Quintessence Int 1988;19:477-81. 17. Christensen GJ. Veneering of teeth-state of tbe art. Dent Clin North Am 1985;29:373-90. 18. Covey DA, Oliveira FC Jr, Denehy GE. Selecting an esthetic veneering technique. Quintessence Int 1987;18:247-52. 19. Nathanson D. Etched porcelain restorations for improved esthetics. Part I. Anterior veneers. Compend Contin Educ Dent 1986;7:706-12. 20. Goldstein RE. Diagnostic dilemma: to bond, laminate, or crown? Int J Periodont Restor Dent 1987;5:9-29. 21. Farah JW, Powers JM, eds. All ceramic restorations. Dent Advisor 1989;6:1-3. 22. Woolsey G, Weir D, Dootz E, Morris H. Marginal accuracy of Dicer, Cerestore, porcelain-butt, and cast metal margins [abstract]. J Dent Res 1987;66:284. 23. Jones DW, Sutow EJ, Rizkalla AS, Black D. Opacity and colour of a castable glass-ceramic and cement system [abstract]. J Dent Res 1988;67:118. Reprint requests to: DR. CLIFFORD B. STARR 4062 QUAIL BUSH DRIVE DAYTON, OH 45424
crowns
J. Robert Holmes, DDS, MS, MEd,a William D. Sulik, DDS, MS,b Gene A. Holland, DDS, MS,” and Stephen C. Bayne, MS, PhDd Medical University of South Carolina, College of Dental Medicine, Charleston, SC., and University of North Carolina, School of Dentistry, Chapel Hill, N.C. The objective of this study was to measure marginal fit of castable ceramic versus gold crowns. Full veneer gold and ceramic crowns were made on Ivorine dies. Crowns were cemented, embedded, sectioned faciolingually and mesiodistally, and photographed for measurement of absolute marginal discrepancies (cavosurface angle to casting margin) to evaluate fit. Results revealed no statistically significant differences in fit among four locations around the margins of either ceramic or gold crowns. There was no statistically significant difference in the combined absolute marginal discrepancy (fit) between ceramic and gold crowns. The variance of the combined absolute marginal discrepancy (fit) of the ceramic crowns was significantly different than that of the gold crowns (p = 0.01 level). The standard deviation (variability) of the gold crowns was more than twice that of the ceramic crowns, Randomized block ANOVA demonstrated statistically significant differences among individual gold crown specimens, but none among individual ceramic crown specimens. (J PROSTHET DENT 1992;67:594-9.)
Supported in part by NIH-NIDR grant No. 2-S07-RR05333. ‘Assistant Professor, Department of Crown and Bridge Dentistry, Medical University of South Carolina. College of Dental Medicine. %linical Associate Professor (part-time), Department of Prosthodontics, University of North Carolina, School of Dentistry. cProfessor, Department of Prosthodontics, University of North Carolina, School of Dentistry. dAssociate Professor, Department of Operative Dentistry, University of North Carolina, School of Dentistry. 19/l/35864 I
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mce the introduction of the porcelain jacket crown by Land in the late 188Os,there have been continual efforts to improve all-ceramic restorations. The esthetic advantages inherent in eliminating the metal substructure of ceramometal crowns is well d0cumented.i Castable ceramic (Dicer, Dentsply Int., York, Pa.) crowns, made by the lost wax technique, are a recent development of the all-ceramic crown concept. These benefits, however, must be evaluated against the possible disadvantage of a poorer “fit.”
MAY 1992 VOLUME67
NUMBER5
