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A porcelain-faced crown
A porcelain-faced crown Claude R. Baker, B.A., M.S., D.D.S.*
University o[ Missouri at Kansas City, Kansas City, Mo.
W e n the clinical circumstance indicates that the facial surface of a metal crown should be tooth colored, the porcelain-faced crown is often a convenient choice. It can be engineered to apply to a vital tooth, or it can be designed with a dowel and applied to a pulpless t o o t h ?
CLASSIFICATION
Type I. T h e facial surface is excavated from the wax crown pattern. T h e metal crown is cast. A fused porcelain inlay is made indirectly to fit the metal crown recess. T h e inlay is cemented into the recess. Type 1I. T h e facial surface is excavated from the wax crown pattern. T h e metal crown is cast in a special alloy, usually gold. Special porcelain is fused directly to the metal. Type III. Fused porcelain is selected from manufactured stock. It is ground to dimensional requirements, then inlaid in position in the wax crown pattern, then withdrawn for the casting of the metal crown. T h e inlay is cemented into the crown recess. This article describes a technique for making a Type I I I porcelain-faced crown. Dr. George Hollenback used porcelain denture teeth modified to meet different crown or bridge requirements over 50 years ago.t I usually use it because it has best stood the test of time, and the materials are available in the dental office. If an acrylic resin surface is desired, excellent acrylic resin veneer facings are manufactured, and the same general procedure can be followed.* FUSED PORCELAIN SUPPLY Fused porcelain is used in manufacturing artificial teeth which are consistently dependable for color and form. T h e wide varieties of color and molds provide a Presented before the American Academy of Crown and Bridge Prosthodontics in Chicago, February 4, 1967. ~Professor of Dentistry arid Head, Department of Pre-Clinical Technics. "~Personal Communication, September 26, 1966_ :~Myerson Manufacturing Company, Cambridge, Mass.
353
354
Baker
J, Pros. Dent.
October, 1967
reservoir capable of meeting clinical requirements for most patients. Denture teeth manufactured for complete dentures, and pin or slot facings manufactured for bridge pontic construction, are convenient sources of supply.
MATERIAL CONSIDERATIONS The three materials used in constructing this type of crown have physical property differences : 1. The cast metal crown has a degree of elasti~ty under load. If, for any reason, a deformation of the cast metal occurs, the porcelain facing will fracture. Therefore, a high tensile specification Type I I I or I V dental gold alloy should be used. It should be put to service in the heat-treated state. 2. Fused porcelain is incapable of cold flow. It is a fragile material, so the form of the facing or inlay should be as thick as clinically possible. 3. The dental cement used to connect the porcelain facing to the cast metal crown cannot be considered as a factor contributing to the total strength of the crown. It serves as an adhesive space filler.
CROWN DESIGN The evolution of the design for a porcelain-faced crown for a vital tooth is determined by the volume of dentin that must be retained for pulp health and patient comfort, the amount of free space available, and the physical properties of the selected prosthetic materials. It is possible to engineer a general crown form that can give clinical satisfaction if attention is given to resisting major forces so that compressive rather than tensile stresses are the rule in the part that includes the porcelain. 2
PREPARATION The preparation must allow for a volume of space that can be occupied by the finished crown without interference within the functional occlusal range, a, 4 As a clinical guide, the preparation can be made first to satisfy the minimal reduction requirements for a complete metal veneer crown. This fundamental form is then further reduced on the labial or buccal surface to include the general dimensions of the porcelain facing planned within the crown contour (Figs. 1 to 3). The detail of the preparation around the labiogingival margin is exacting. The surface reduction at the shoulder of the preparation should be fully 2 to 9.5 ram. deep. The shoulder should be just subgingival to, and parallel with, the crest of the free gingiva. The gingival shoulder is drawn laterally at full depth around into the mesial and distal surfaces before it is allowed to fade out. The gingival extent of the preparation terminates beyond the shoulder at approximately one half the depth of the gingival sulcus (Fig. 2).
FUSED PORCELAIN FACING The clinical requirements for the individual fused porcelain facing are estimated. These include color distribution, and the size and shape of the vitreous inlay form to be engineered to best resist displacement and fracture. For example, a slightly oversized porcelain denture tooth is selected for color
Volume 18 Number 4
Fig. I
A
porcelain-faced
crown
355
Fig. 2
Fig. 1 A proximal view of a mid-section through a porcelain-faced crown. (a) The fused porcelain face. (b) The tooth preparation. (c) The metal crown. (d) The subgingival crown extent. (e) The estimated lingual extent of the shoulder preparation. (g) The gingival crest. (w) The hollowed out lingual porcelain.
Fig. 2 A proximal view of a mid-section through the gingival region of a porcelain-faced crown. (a) The fused porcelain face. (b) The tooth preparation. (c) The metal crown. (d) Subgingival crown extent. (g) The gingival crest. (w) The hollowed out lingual surface of the porcelain. (x) The gingival porcelain bevel.
Fig. 3 Incisal view of a horizontal section through the middle of a porcelain-faced crown. (a) The fused porcelain face. (b) The tooth preparation. (c) The metal crown. (w) The hollowed out lingual surface of the porcelain. (z) The mesial and distal porcelain bevels.
356
Baker
J- P . . . . Dent. October, 1967
and mold. It is surface-ground to shape. There is some advantage in making direct gross porcelain surface modifications while the natural tooth is being prepared. Final modifications, such as the bevels, are made indirectly on a laboratory cast. STEP BY STEP PROCEDURE A procedure for grinding and shaping a porcelain denture tooth as a facing for a crown is as follows : 1. Hollow out the central lingual bulk of the porcelain tooth to conform in general to the labial contour of the porcelain facing. The form provided should be oversize for the labial or buccal contour of the tooth preparation. It should be to allow a thin wall of metal between the porcelain inlay and the tooth preparation. The fused porcelain inlay should retain at least 2 mm. thickness throughout (Figs. 1 to4). 2. Grind the gingival bevel to butt the shoulder of the tooth preparation (Figs. 2, x and 4, x). The lateral festoon of this bevel should parallel the prepared shoulder radius. T h e amount of gingival porcelain reduced will control the gingival color distribution. 3. Establish a long bevel from the incisal edge to the lingual gingival margin (Fig. 4, y). Ordinarily, 45 degrees from the long axis of the tooth is not sufficient,
Fig. 4 A proximal view showing the adaptation of the fused porcelain facing to the tooth preparation. (a) The fused porcelain facing. (b) The tooth preparation. (d) The subgingival crown extent. (e) The estimated lingual .extent of the shoulder preparation. (x) The gingival porcelain bevel. (y) The incisal porcelain bevel. (z) The mesial and distal porcelain bevels.
Votume IS Number 4
A porcelain-/aced crown
357
30 degrees is better. It is essential to provide an incisal bevel of adequate metallic bulk for support to protect the porcelain from fracture through the functional ranges of occlusion. The amount of incisal porcelain reduced controls the incisal color distribution. 4. Grind the near parallel narrow lateral retention bevels (Figs. 3, z and 4, z). These bevels should converge toward the lingual surface only slightly in order that the porcelain piece can be withdrawn from the wax crown pattern. The near parallelism of these bevels provides the displacement resistance for the porcelain face when pins or other mechanical means cannot be used. Various lateral form parallelisms are possible. Sometimes it is best to use a manufactured porcelain pontic pin-facing. One or the other of the pins will retain the facing when it is placed in the proximal part of a preparation that has previously withstood wide excavation for a filling, or that requires deep excavation for existing caries (Fig. 5). When making a porcelainfaced dowel crown, a pin facing is also convenient because it can retain the face by having the pins straddling the dowel. A fused porcelain inlay retained by reverse-pin retention procedures seldom applies in this type of crown construction for vital teeth. In general the allowable thickness for the porcelain is insufficient to provide the necessary bulk for a pin long enough for retention. The use of this technique in certain instances can demand too great a reduction on the labial or buccal surface of the tooth in making these preparations and jeopardize the health of the pulp.
METAL CROWN A wax crown pattern, with the labial or buccal fused porcelain "inlaid" facing in position, is developed indirectly on a die. The die is related to a laboratory cast
Fig. 5 Incisal view of a horizontal section through the middle of a porcelain-faced crown, showing the adaptation of a pin-facing to the tooth preparation using one of the pins for retention. (a) The fused porcelain face. (b) The tooth preparation. (c) The metal crown. (w) The hollowed out lingual surface of the porcelain.
358
Baker
J. Pros. Dent,
October, 1[]67
which is the counterpart of the clinical area involved. T h e porcelain facing is withdrawn. T h e wax crown pattern is burned out and the crown is cast to dimension in the selected metal alloy.
OBSERVATIONS AND CONCLUSION 1. T h e fusing of porcelain to exacting tooth size and color requirements requires much time and concentration. 2. T h e efforts to popularize "porcelain-fused-to-metal" with dentists have some unfortunate professional shortcomings. Licensee laboratories are committed to commercial rather than professional interests. T h e y are responsible to dentists and not to patients. T h e health value of the appliance in relation to the cost to the patient is frequently out of balance. 3. A fused dental porcelain supply of high quality in wide variety is readily available. Shaping, casting, and connecting tooth size parts from selected dental materials are familiar manipulations in dentistry. Constructing T y p e I I I porcelainfaced crowns, or directing dental laboratory assistants in this procedure, seems reasonable.
References 1. Baker, C. R.: The Dowel Crown, J. A. D. A. 61: 479-483, 1960. 2. Johnston, J. F., Mumford, G., and Dykema, R. W.: Modern Practice in Dental Ceramics, Philadelphia, 1967, W. B. Saunders Company, p. 13. 3. Johnston, J. F., Phillips, R. W., and Dykema, R. W.: Modern Practice in Crown and Bridge Prosthodontics, ed. 2, Philadelphia, 1965, W. B. Saunders Company, pp. 338-350. 4. Tylman, S. D.: Theory and Practice of Crown and Bridge Prosthodontics, ed_ 5, St. Louis, 1965, The C. V. Mosby Company, pp. 751-758. 1108 EAST TENTH ST. KANSAS CITY~Mo. 64106
University o[ Missouri at Kansas City, Kansas City, Mo.
W e n the clinical circumstance indicates that the facial surface of a metal crown should be tooth colored, the porcelain-faced crown is often a convenient choice. It can be engineered to apply to a vital tooth, or it can be designed with a dowel and applied to a pulpless t o o t h ?
CLASSIFICATION
Type I. T h e facial surface is excavated from the wax crown pattern. T h e metal crown is cast. A fused porcelain inlay is made indirectly to fit the metal crown recess. T h e inlay is cemented into the recess. Type 1I. T h e facial surface is excavated from the wax crown pattern. T h e metal crown is cast in a special alloy, usually gold. Special porcelain is fused directly to the metal. Type III. Fused porcelain is selected from manufactured stock. It is ground to dimensional requirements, then inlaid in position in the wax crown pattern, then withdrawn for the casting of the metal crown. T h e inlay is cemented into the crown recess. This article describes a technique for making a Type I I I porcelain-faced crown. Dr. George Hollenback used porcelain denture teeth modified to meet different crown or bridge requirements over 50 years ago.t I usually use it because it has best stood the test of time, and the materials are available in the dental office. If an acrylic resin surface is desired, excellent acrylic resin veneer facings are manufactured, and the same general procedure can be followed.* FUSED PORCELAIN SUPPLY Fused porcelain is used in manufacturing artificial teeth which are consistently dependable for color and form. T h e wide varieties of color and molds provide a Presented before the American Academy of Crown and Bridge Prosthodontics in Chicago, February 4, 1967. ~Professor of Dentistry arid Head, Department of Pre-Clinical Technics. "~Personal Communication, September 26, 1966_ :~Myerson Manufacturing Company, Cambridge, Mass.
353
354
Baker
J, Pros. Dent.
October, 1967
reservoir capable of meeting clinical requirements for most patients. Denture teeth manufactured for complete dentures, and pin or slot facings manufactured for bridge pontic construction, are convenient sources of supply.
MATERIAL CONSIDERATIONS The three materials used in constructing this type of crown have physical property differences : 1. The cast metal crown has a degree of elasti~ty under load. If, for any reason, a deformation of the cast metal occurs, the porcelain facing will fracture. Therefore, a high tensile specification Type I I I or I V dental gold alloy should be used. It should be put to service in the heat-treated state. 2. Fused porcelain is incapable of cold flow. It is a fragile material, so the form of the facing or inlay should be as thick as clinically possible. 3. The dental cement used to connect the porcelain facing to the cast metal crown cannot be considered as a factor contributing to the total strength of the crown. It serves as an adhesive space filler.
CROWN DESIGN The evolution of the design for a porcelain-faced crown for a vital tooth is determined by the volume of dentin that must be retained for pulp health and patient comfort, the amount of free space available, and the physical properties of the selected prosthetic materials. It is possible to engineer a general crown form that can give clinical satisfaction if attention is given to resisting major forces so that compressive rather than tensile stresses are the rule in the part that includes the porcelain. 2
PREPARATION The preparation must allow for a volume of space that can be occupied by the finished crown without interference within the functional occlusal range, a, 4 As a clinical guide, the preparation can be made first to satisfy the minimal reduction requirements for a complete metal veneer crown. This fundamental form is then further reduced on the labial or buccal surface to include the general dimensions of the porcelain facing planned within the crown contour (Figs. 1 to 3). The detail of the preparation around the labiogingival margin is exacting. The surface reduction at the shoulder of the preparation should be fully 2 to 9.5 ram. deep. The shoulder should be just subgingival to, and parallel with, the crest of the free gingiva. The gingival shoulder is drawn laterally at full depth around into the mesial and distal surfaces before it is allowed to fade out. The gingival extent of the preparation terminates beyond the shoulder at approximately one half the depth of the gingival sulcus (Fig. 2).
FUSED PORCELAIN FACING The clinical requirements for the individual fused porcelain facing are estimated. These include color distribution, and the size and shape of the vitreous inlay form to be engineered to best resist displacement and fracture. For example, a slightly oversized porcelain denture tooth is selected for color
Volume 18 Number 4
Fig. I
A
porcelain-faced
crown
355
Fig. 2
Fig. 1 A proximal view of a mid-section through a porcelain-faced crown. (a) The fused porcelain face. (b) The tooth preparation. (c) The metal crown. (d) The subgingival crown extent. (e) The estimated lingual extent of the shoulder preparation. (g) The gingival crest. (w) The hollowed out lingual porcelain.
Fig. 2 A proximal view of a mid-section through the gingival region of a porcelain-faced crown. (a) The fused porcelain face. (b) The tooth preparation. (c) The metal crown. (d) Subgingival crown extent. (g) The gingival crest. (w) The hollowed out lingual surface of the porcelain. (x) The gingival porcelain bevel.
Fig. 3 Incisal view of a horizontal section through the middle of a porcelain-faced crown. (a) The fused porcelain face. (b) The tooth preparation. (c) The metal crown. (w) The hollowed out lingual surface of the porcelain. (z) The mesial and distal porcelain bevels.
356
Baker
J- P . . . . Dent. October, 1967
and mold. It is surface-ground to shape. There is some advantage in making direct gross porcelain surface modifications while the natural tooth is being prepared. Final modifications, such as the bevels, are made indirectly on a laboratory cast. STEP BY STEP PROCEDURE A procedure for grinding and shaping a porcelain denture tooth as a facing for a crown is as follows : 1. Hollow out the central lingual bulk of the porcelain tooth to conform in general to the labial contour of the porcelain facing. The form provided should be oversize for the labial or buccal contour of the tooth preparation. It should be to allow a thin wall of metal between the porcelain inlay and the tooth preparation. The fused porcelain inlay should retain at least 2 mm. thickness throughout (Figs. 1 to4). 2. Grind the gingival bevel to butt the shoulder of the tooth preparation (Figs. 2, x and 4, x). The lateral festoon of this bevel should parallel the prepared shoulder radius. T h e amount of gingival porcelain reduced will control the gingival color distribution. 3. Establish a long bevel from the incisal edge to the lingual gingival margin (Fig. 4, y). Ordinarily, 45 degrees from the long axis of the tooth is not sufficient,
Fig. 4 A proximal view showing the adaptation of the fused porcelain facing to the tooth preparation. (a) The fused porcelain facing. (b) The tooth preparation. (d) The subgingival crown extent. (e) The estimated lingual .extent of the shoulder preparation. (x) The gingival porcelain bevel. (y) The incisal porcelain bevel. (z) The mesial and distal porcelain bevels.
Votume IS Number 4
A porcelain-/aced crown
357
30 degrees is better. It is essential to provide an incisal bevel of adequate metallic bulk for support to protect the porcelain from fracture through the functional ranges of occlusion. The amount of incisal porcelain reduced controls the incisal color distribution. 4. Grind the near parallel narrow lateral retention bevels (Figs. 3, z and 4, z). These bevels should converge toward the lingual surface only slightly in order that the porcelain piece can be withdrawn from the wax crown pattern. The near parallelism of these bevels provides the displacement resistance for the porcelain face when pins or other mechanical means cannot be used. Various lateral form parallelisms are possible. Sometimes it is best to use a manufactured porcelain pontic pin-facing. One or the other of the pins will retain the facing when it is placed in the proximal part of a preparation that has previously withstood wide excavation for a filling, or that requires deep excavation for existing caries (Fig. 5). When making a porcelainfaced dowel crown, a pin facing is also convenient because it can retain the face by having the pins straddling the dowel. A fused porcelain inlay retained by reverse-pin retention procedures seldom applies in this type of crown construction for vital teeth. In general the allowable thickness for the porcelain is insufficient to provide the necessary bulk for a pin long enough for retention. The use of this technique in certain instances can demand too great a reduction on the labial or buccal surface of the tooth in making these preparations and jeopardize the health of the pulp.
METAL CROWN A wax crown pattern, with the labial or buccal fused porcelain "inlaid" facing in position, is developed indirectly on a die. The die is related to a laboratory cast
Fig. 5 Incisal view of a horizontal section through the middle of a porcelain-faced crown, showing the adaptation of a pin-facing to the tooth preparation using one of the pins for retention. (a) The fused porcelain face. (b) The tooth preparation. (c) The metal crown. (w) The hollowed out lingual surface of the porcelain.
358
Baker
J. Pros. Dent,
October, 1[]67
which is the counterpart of the clinical area involved. T h e porcelain facing is withdrawn. T h e wax crown pattern is burned out and the crown is cast to dimension in the selected metal alloy.
OBSERVATIONS AND CONCLUSION 1. T h e fusing of porcelain to exacting tooth size and color requirements requires much time and concentration. 2. T h e efforts to popularize "porcelain-fused-to-metal" with dentists have some unfortunate professional shortcomings. Licensee laboratories are committed to commercial rather than professional interests. T h e y are responsible to dentists and not to patients. T h e health value of the appliance in relation to the cost to the patient is frequently out of balance. 3. A fused dental porcelain supply of high quality in wide variety is readily available. Shaping, casting, and connecting tooth size parts from selected dental materials are familiar manipulations in dentistry. Constructing T y p e I I I porcelainfaced crowns, or directing dental laboratory assistants in this procedure, seems reasonable.
References 1. Baker, C. R.: The Dowel Crown, J. A. D. A. 61: 479-483, 1960. 2. Johnston, J. F., Mumford, G., and Dykema, R. W.: Modern Practice in Dental Ceramics, Philadelphia, 1967, W. B. Saunders Company, p. 13. 3. Johnston, J. F., Phillips, R. W., and Dykema, R. W.: Modern Practice in Crown and Bridge Prosthodontics, ed. 2, Philadelphia, 1965, W. B. Saunders Company, pp. 338-350. 4. Tylman, S. D.: Theory and Practice of Crown and Bridge Prosthodontics, ed_ 5, St. Louis, 1965, The C. V. Mosby Company, pp. 751-758. 1108 EAST TENTH ST. KANSAS CITY~Mo. 64106