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The shrink-free ceramic crown
FIXED PROSTHODONTICS
l
OPERATIVE DENTISTRY
SECTION EDITORS
GORDON WILLIAM
J. CHRISTENSEN SAMUEL E. GUYER LEFKOWITZ WILLIAM F. I’. MALONE
The shrink-free
ROBERT C. SPROULL
ceramic crown
Ralph B. Sozio, D.M.D.,* and Edwin J. Riley, D.M.D.** Harvard School of Dental Medicine, Boston, Mass.
H
istorically, the dental profession has strived to perfect an all-ceramic restoration. The pursuit began with Land,’ who described the technique for jacket crown fabrication using a platinum foil matrix. The use of the jacket crown has been limited, however, due to its lack of precision and the inherent weakness of dental porcelain. Realizing the esthetic advantages of all-ceramic restorations, numerous investigators have attempted to improve the fabrication technique and strength. Most notably McLean and Hughs2 described the process of dispersion strengthening, in which alumina was incorporated in the dental porcelain. Despite the increased strength afforded by the dispersed oxide, many of the problems with porcelain restorations still persist. These problems are related to one property of dental porcelain, namely, the shrinkage that occurs from the buildup (green state) to the fired state. Although a clinically adequate fit can be produced, the shrinkage can lead to marginal inaccuracy, distortion of the platinum matrix, and decreased strength. The potential of the ceramic restoration needs additional study. Dental porcelain has been successfully used in the ceramometal composite. However, the esthetics of the ceramometal restoration is altered by the lack of light transmission through the metal casting. In addition to esthetic problems, drawbacks with the ceramometal restoration arise from the numerous techniques used in its fabrication. The thermal coefficient between ceramic and metal, the meticulous surface finishing, the bond strength, and the accuracy of casting are the sources of potential problems. In addition, the increasing number of available metal-ceramic combinations complicates the fabrication of the restoration. The metal casting is fabricated by the
Presented at the Greater New York Academy of Prosthodontics, New York, N.Y. *Associate Clinical Professor of Prosthetic Dentistry and Director, Postdoctoral Fixed Prosthetics. **Assistant Clinical Professor of Prosthetic Dentistry and Director, Predoctoral Fixed Prosthetics.
182
FEBRUARY
1983
VOLUME
49
NUMBER
2
regular lost wax technique. This procedure is technically demanding and replete with potential inaccuracies. This article describes a direct technique for the construction of an all-ceramic crown that uses a nonshrink ceramic as a substrate and an aluminous porcelain veneer. This technique represents a unique alternative to both the ceramometal and the traditional porcelain jacket restorations.
NONSHRINK
CERAMIC
The alumina ceramic used in this technique (Cerestore Non-Shrink Alumina Ceramic, Coors Biomedical Co., Lakewood, Colo.) is a shrink-free composition unlike conventional ceramic bodies, which undergo considerable shrinkage when fired. Its formulation is such that on firing, chemical and crystalline transformations occur to compensate for the decreased shrinkage volume ordinarily experienced with traditional dental ceramics. By controlling the time and temperature of the firing cycle, zero shrinkage of the ceramic from the unfired (green) state to the fired state can be obtained. In fact, by varying the firing schedule the ceramic can be expanded if so desired.
FORMATION
DIRECT TECHNIQUE
The substrate is formed by the technique of transfer molding. The substrate ceramic is supplied as a dense pellet of the compacted shrink-free formulation. The pellet is heated until it is flowable (160” C) and then transferred by pressure into a suitable mold directly on the master die. In addition to being thermoplastic, the ceramic is thermosetting. Therefore, after the flowable ceramic has been forced into the mold and around the die, it automatically sets. This forms the green (unfired) substructure. With the formation done on the master die, the die material must withstand the temperature of the molding process without breaking or distorting. A special epoxy die material (Coors Biomedical Co.) was developed for this process. Unlike most conventional dental epoxies, this product is heat stable and undergoes
OOZZ-3913/83/020182
+ 06X00.60/0 0 1983 The C. V. Mosby Co.
SHRINK-FREE
CERAMIC
Fig. 1. Cured base.
epoxy
CROWN
resin with
Fig. 2. Conventional
conventional
stone
wax-up.
Fig. 3. A, Wax-up sprued. B, Wax-up wi(-b epoxy resin die invested.
permanent expansion during curing. The degree of expansion can be controlled to compare to the dimensional expansion of dental die stones. The transfer molding technique contributes to accurate substrate fits by formation directly on the master die. Once the formed substrate has thermally set, the green state substrate is removed from the die and fired to its final mature state. FIRING
PROCESS
The green state must undergo certain chemical and crystalline transformations to create the mature ceramic with the desirable physical properties. This process must also occur without dimensional change (zero shrinkage). To achieve zero shrinkage, the material is fired with precise control of temperature and time. This precision firing is accomplished by a microprocessor (Coors Biomedical Co.) that automatically controls the firing schedule. TECHNIQUE Fabrication of the die 1. The tooth is prepared in the conventional fashion and an elastomeric impression of the preparation is non-water-soluble impression secured (all the
THE JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 4. Wax is eliminated
by boiling
water.
1.83
SOZIO
AND
RILEY
Fig. 5. A, Flask assembly and the heating element. B, Heating oven with flask assemblies. treatment. The heat treatment of the epoxy hardens the die material, makes it heat stable, and provides the desired amount of permanent expansion. 4. The dowel pin placement, construction of the base (Fig. l), separation, and marginal trimming of the die are done in the conventional manner.
Die preparation 1. Because the ceramic material is molded directly against the master die, it is absolutely essential that all undercuts on the die be blocked out. This is accomplished with a fast-setting, heat-stable epoxy resin (Coors Biomedical Co.). 2. A teflon-based die sealant (Coors Biomedical Co.) is painted over the die and a cement space provided by painting the die with a heat-stable die spacer (Coors Biomedical Co.).
Formation procedure
Fig. 6. A, Ceramic pellet. B, Air press with assembly.
flask
materials can be used). A shoulder or chamfer preparation is recommended. 2. The liquid-powder epoxy material is thoroughly mixed. The mixture is then deaerated under vacuum. The epoxy is then painted into the impression to avoid bubbles and is allowed to bench set. 3. After the initial set, the epoxy is removed from the impression and cured. Curing is achieved by heat
184
1. A wax-up of the desired substrate is done in the conventional manner (Fig. 2). 2. The wax pattern is sprued (Fig. 3, A) and, together with the epoxy resin die, is invested with a dental laboratory plaster in a suitable flask (Coors Biomedical Co.) (Fig. 3, B). 3. Once the plaster has set, the wax is eliminated with boiling water (Fig. 4). 4. The flask, together with its external sleeve, is placed into a heating element (Coors Biomedical Co.) (Fig. 5). 5. After the flask reaches the molding temperature (160” C), it is removed from the oven. A pellet of the ceramic material with the selected color value is inserted into the recessof the outer sleeve (Fig. 6, A). A
FEBRUARY
1983
VOLUME
49
NUMBER
2
SHRINK-FREE
CERAMJC
CROWN
Fig. 7. A, Plaster mold released from flask. 8, Removal of plaster from ceramic substr; tte and die.
Fig. 8. A, Green state ceramic lifted from die. B, Refinement carbide finishing bur.
THE JOURNAL
OF PROSTHETIC
DENTISTRY
of green state ceramic w
185
SO210
Fig. 9. Microprocessor controlled oven.
Fig. 10. A, Fired ceramic substrate with veneer. B, Complete veneer crown.
RILEY
appearance crown, maxillary
precoat
plunger is then positioned on top of the ceramic pellet and the entire assembly is placed in an air press (Coors Biomedical Co.) (Fig. 6, B). The press is then activated, allowing the ceramic to flow into the mold and thermally set. 6. The flask is then quenched. The plaster containing the die and the molded ceramic is released from the flask (Fig. 7, A) and the plaster is easily removed (Fig. 7, B). 7. The green state ceramic substrate is lifted from the die (Fig. 8, A). The sprue is removed and the shape refined by use of grinding stones or carbide finishing burs (Fig. 8, B). 8. Once the ceramic substrate is adjusted to its final
186
Fig. 11. Radiographic first molar.
AND
Fig. 12. A, Full crown preparation of lower right first molar. B, Completed restoration. shape, it is then fired to achieve its ultimate physical properties. The substrate is placed on a tray, inserted into the muffle, and fired (Fig. 9). Precise control of time and temperature achieved by the microprocessor results in zero shrinkage. Once the firing cycle has been completed, the ceramic is ready to be veneered. VENEER The veneer porcelain (Coors Biomedical Co.) has similar properties to those of a conventional dental aluminous porcelain. In order to eliminate any surface
FEBRUARY
1983
VOLUME
49
NUMBER
2
SHRINK-FREE
CERAMIC
CROWN
contamination that may be present on the substrate and to guard against air entrapment, a thin, clear layer of porcelain is applied to the surface of the substrate and fired at a temperature exceeding that used for the subsequent veneer applications. Color modifiers can also be applied at this time (Fig. 10, A). The body and incisal porcelains are then applied and fired in the conventional manner (Fig. 10, B). RADIOGRAPHIC
APPEARANCE
The radiodensity of the shrink-free ceramic is very similar to that of enamel. This affords radiographic examination of fit and, similar to the enamel of natural tooth structure, permits radiographic visualization of what is under the crown, that is, pulp, bases, pins, posts, or caries (Fig. 11). INDICATIONS AND CONTRAINDICATIONS Crown restorations fabricated with the shrink-free ceramic are indicated in situations where the ceramometal restoration with porcelain occlusal or the traditional porcelain crown are the treatment selection. They can be effectively used as both anterior and posterior restorations (Fig. 12). The restoration is contraindicated when there is inadequate thickness, that is, less than 1.3 mm.
DISCUSSION It is quite apparent, that the technique described offers many new and different concepts” The first departure is the application of a shrink-free ceramic that is used for the substrate of the crown restoration. The formation is a direct technique and more accurate when the substrate is formed directly on the master die as compared to an indirect casting procedure. This direct approach produces precise fit of the substrate. The epoxy resin die material also offers some unique properties. It is Iheat stable and can undergo permanent expansion to dimensions comparable to those of type III dental stoners. With all its new concepts, the process is similar to a technique used in dentistry for many years: the processing and curing of an acrylic resin denture base. Although new materials and products are necessary, the shrink-free ceramic restoration is not diflicult to fabricate. SUMMARY A process for the fabrication of a full crown that uses a nonshrink alumina ceramic substrate and reinforced aluminous porcelain veneer has been described. The technique used is direct and is capable of producing precise fitting ceramic restorations.
ADVANTAGES 1. Excellent fit generated by direct molding 2. Excellent esthetics aided by light transmission and lack of metal 3. Absence of distortion with the veneer application 4. Ease of formation 5. Ease of adjustment in green state 6. Low thermal conductivity 7. Radiodensity similar to that of natural enamel
THE JOURNAL
OF PROSTHETIC
DENTISTRY
187
l
OPERATIVE DENTISTRY
SECTION EDITORS
GORDON WILLIAM
J. CHRISTENSEN SAMUEL E. GUYER LEFKOWITZ WILLIAM F. I’. MALONE
The shrink-free
ROBERT C. SPROULL
ceramic crown
Ralph B. Sozio, D.M.D.,* and Edwin J. Riley, D.M.D.** Harvard School of Dental Medicine, Boston, Mass.
H
istorically, the dental profession has strived to perfect an all-ceramic restoration. The pursuit began with Land,’ who described the technique for jacket crown fabrication using a platinum foil matrix. The use of the jacket crown has been limited, however, due to its lack of precision and the inherent weakness of dental porcelain. Realizing the esthetic advantages of all-ceramic restorations, numerous investigators have attempted to improve the fabrication technique and strength. Most notably McLean and Hughs2 described the process of dispersion strengthening, in which alumina was incorporated in the dental porcelain. Despite the increased strength afforded by the dispersed oxide, many of the problems with porcelain restorations still persist. These problems are related to one property of dental porcelain, namely, the shrinkage that occurs from the buildup (green state) to the fired state. Although a clinically adequate fit can be produced, the shrinkage can lead to marginal inaccuracy, distortion of the platinum matrix, and decreased strength. The potential of the ceramic restoration needs additional study. Dental porcelain has been successfully used in the ceramometal composite. However, the esthetics of the ceramometal restoration is altered by the lack of light transmission through the metal casting. In addition to esthetic problems, drawbacks with the ceramometal restoration arise from the numerous techniques used in its fabrication. The thermal coefficient between ceramic and metal, the meticulous surface finishing, the bond strength, and the accuracy of casting are the sources of potential problems. In addition, the increasing number of available metal-ceramic combinations complicates the fabrication of the restoration. The metal casting is fabricated by the
Presented at the Greater New York Academy of Prosthodontics, New York, N.Y. *Associate Clinical Professor of Prosthetic Dentistry and Director, Postdoctoral Fixed Prosthetics. **Assistant Clinical Professor of Prosthetic Dentistry and Director, Predoctoral Fixed Prosthetics.
182
FEBRUARY
1983
VOLUME
49
NUMBER
2
regular lost wax technique. This procedure is technically demanding and replete with potential inaccuracies. This article describes a direct technique for the construction of an all-ceramic crown that uses a nonshrink ceramic as a substrate and an aluminous porcelain veneer. This technique represents a unique alternative to both the ceramometal and the traditional porcelain jacket restorations.
NONSHRINK
CERAMIC
The alumina ceramic used in this technique (Cerestore Non-Shrink Alumina Ceramic, Coors Biomedical Co., Lakewood, Colo.) is a shrink-free composition unlike conventional ceramic bodies, which undergo considerable shrinkage when fired. Its formulation is such that on firing, chemical and crystalline transformations occur to compensate for the decreased shrinkage volume ordinarily experienced with traditional dental ceramics. By controlling the time and temperature of the firing cycle, zero shrinkage of the ceramic from the unfired (green) state to the fired state can be obtained. In fact, by varying the firing schedule the ceramic can be expanded if so desired.
FORMATION
DIRECT TECHNIQUE
The substrate is formed by the technique of transfer molding. The substrate ceramic is supplied as a dense pellet of the compacted shrink-free formulation. The pellet is heated until it is flowable (160” C) and then transferred by pressure into a suitable mold directly on the master die. In addition to being thermoplastic, the ceramic is thermosetting. Therefore, after the flowable ceramic has been forced into the mold and around the die, it automatically sets. This forms the green (unfired) substructure. With the formation done on the master die, the die material must withstand the temperature of the molding process without breaking or distorting. A special epoxy die material (Coors Biomedical Co.) was developed for this process. Unlike most conventional dental epoxies, this product is heat stable and undergoes
OOZZ-3913/83/020182
+ 06X00.60/0 0 1983 The C. V. Mosby Co.
SHRINK-FREE
CERAMIC
Fig. 1. Cured base.
epoxy
CROWN
resin with
Fig. 2. Conventional
conventional
stone
wax-up.
Fig. 3. A, Wax-up sprued. B, Wax-up wi(-b epoxy resin die invested.
permanent expansion during curing. The degree of expansion can be controlled to compare to the dimensional expansion of dental die stones. The transfer molding technique contributes to accurate substrate fits by formation directly on the master die. Once the formed substrate has thermally set, the green state substrate is removed from the die and fired to its final mature state. FIRING
PROCESS
The green state must undergo certain chemical and crystalline transformations to create the mature ceramic with the desirable physical properties. This process must also occur without dimensional change (zero shrinkage). To achieve zero shrinkage, the material is fired with precise control of temperature and time. This precision firing is accomplished by a microprocessor (Coors Biomedical Co.) that automatically controls the firing schedule. TECHNIQUE Fabrication of the die 1. The tooth is prepared in the conventional fashion and an elastomeric impression of the preparation is non-water-soluble impression secured (all the
THE JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 4. Wax is eliminated
by boiling
water.
1.83
SOZIO
AND
RILEY
Fig. 5. A, Flask assembly and the heating element. B, Heating oven with flask assemblies. treatment. The heat treatment of the epoxy hardens the die material, makes it heat stable, and provides the desired amount of permanent expansion. 4. The dowel pin placement, construction of the base (Fig. l), separation, and marginal trimming of the die are done in the conventional manner.
Die preparation 1. Because the ceramic material is molded directly against the master die, it is absolutely essential that all undercuts on the die be blocked out. This is accomplished with a fast-setting, heat-stable epoxy resin (Coors Biomedical Co.). 2. A teflon-based die sealant (Coors Biomedical Co.) is painted over the die and a cement space provided by painting the die with a heat-stable die spacer (Coors Biomedical Co.).
Formation procedure
Fig. 6. A, Ceramic pellet. B, Air press with assembly.
flask
materials can be used). A shoulder or chamfer preparation is recommended. 2. The liquid-powder epoxy material is thoroughly mixed. The mixture is then deaerated under vacuum. The epoxy is then painted into the impression to avoid bubbles and is allowed to bench set. 3. After the initial set, the epoxy is removed from the impression and cured. Curing is achieved by heat
184
1. A wax-up of the desired substrate is done in the conventional manner (Fig. 2). 2. The wax pattern is sprued (Fig. 3, A) and, together with the epoxy resin die, is invested with a dental laboratory plaster in a suitable flask (Coors Biomedical Co.) (Fig. 3, B). 3. Once the plaster has set, the wax is eliminated with boiling water (Fig. 4). 4. The flask, together with its external sleeve, is placed into a heating element (Coors Biomedical Co.) (Fig. 5). 5. After the flask reaches the molding temperature (160” C), it is removed from the oven. A pellet of the ceramic material with the selected color value is inserted into the recessof the outer sleeve (Fig. 6, A). A
FEBRUARY
1983
VOLUME
49
NUMBER
2
SHRINK-FREE
CERAMJC
CROWN
Fig. 7. A, Plaster mold released from flask. 8, Removal of plaster from ceramic substr; tte and die.
Fig. 8. A, Green state ceramic lifted from die. B, Refinement carbide finishing bur.
THE JOURNAL
OF PROSTHETIC
DENTISTRY
of green state ceramic w
185
SO210
Fig. 9. Microprocessor controlled oven.
Fig. 10. A, Fired ceramic substrate with veneer. B, Complete veneer crown.
RILEY
appearance crown, maxillary
precoat
plunger is then positioned on top of the ceramic pellet and the entire assembly is placed in an air press (Coors Biomedical Co.) (Fig. 6, B). The press is then activated, allowing the ceramic to flow into the mold and thermally set. 6. The flask is then quenched. The plaster containing the die and the molded ceramic is released from the flask (Fig. 7, A) and the plaster is easily removed (Fig. 7, B). 7. The green state ceramic substrate is lifted from the die (Fig. 8, A). The sprue is removed and the shape refined by use of grinding stones or carbide finishing burs (Fig. 8, B). 8. Once the ceramic substrate is adjusted to its final
186
Fig. 11. Radiographic first molar.
AND
Fig. 12. A, Full crown preparation of lower right first molar. B, Completed restoration. shape, it is then fired to achieve its ultimate physical properties. The substrate is placed on a tray, inserted into the muffle, and fired (Fig. 9). Precise control of time and temperature achieved by the microprocessor results in zero shrinkage. Once the firing cycle has been completed, the ceramic is ready to be veneered. VENEER The veneer porcelain (Coors Biomedical Co.) has similar properties to those of a conventional dental aluminous porcelain. In order to eliminate any surface
FEBRUARY
1983
VOLUME
49
NUMBER
2
SHRINK-FREE
CERAMIC
CROWN
contamination that may be present on the substrate and to guard against air entrapment, a thin, clear layer of porcelain is applied to the surface of the substrate and fired at a temperature exceeding that used for the subsequent veneer applications. Color modifiers can also be applied at this time (Fig. 10, A). The body and incisal porcelains are then applied and fired in the conventional manner (Fig. 10, B). RADIOGRAPHIC
APPEARANCE
The radiodensity of the shrink-free ceramic is very similar to that of enamel. This affords radiographic examination of fit and, similar to the enamel of natural tooth structure, permits radiographic visualization of what is under the crown, that is, pulp, bases, pins, posts, or caries (Fig. 11). INDICATIONS AND CONTRAINDICATIONS Crown restorations fabricated with the shrink-free ceramic are indicated in situations where the ceramometal restoration with porcelain occlusal or the traditional porcelain crown are the treatment selection. They can be effectively used as both anterior and posterior restorations (Fig. 12). The restoration is contraindicated when there is inadequate thickness, that is, less than 1.3 mm.
DISCUSSION It is quite apparent, that the technique described offers many new and different concepts” The first departure is the application of a shrink-free ceramic that is used for the substrate of the crown restoration. The formation is a direct technique and more accurate when the substrate is formed directly on the master die as compared to an indirect casting procedure. This direct approach produces precise fit of the substrate. The epoxy resin die material also offers some unique properties. It is Iheat stable and can undergo permanent expansion to dimensions comparable to those of type III dental stoners. With all its new concepts, the process is similar to a technique used in dentistry for many years: the processing and curing of an acrylic resin denture base. Although new materials and products are necessary, the shrink-free ceramic restoration is not diflicult to fabricate. SUMMARY A process for the fabrication of a full crown that uses a nonshrink alumina ceramic substrate and reinforced aluminous porcelain veneer has been described. The technique used is direct and is capable of producing precise fitting ceramic restorations.
ADVANTAGES 1. Excellent fit generated by direct molding 2. Excellent esthetics aided by light transmission and lack of metal 3. Absence of distortion with the veneer application 4. Ease of formation 5. Ease of adjustment in green state 6. Low thermal conductivity 7. Radiodensity similar to that of natural enamel
THE JOURNAL
OF PROSTHETIC
DENTISTRY
187