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Retention and resistance of preparations for cast restorations
CLASSIC ARTICLE Retention and resistance of preparations for cast restorations Roger G. Potts, DDS,a Herbert T. Shillingburg Jr, DDS,b and Manville G. Duncanson Jr, DDS, PhDc The University of Oklahoma, College of Dentistry, Oklahoma City, Okla
T
he problem of dislodged cast restorations is familiar. Crown displacement often occurs because the features of the tooth preparation do not counteract the forces directed against the restorations. Therefore the design of the tooth preparation is an important consideration in tooth reconstruction. The use of certain geometric features in preparations for cast restorations has been based largely on experience and individual preference. Early studies of designs of preparations were based on theoretical considerations of the way in which preparation features could be expected to geometrically resist tipping and removal forces.1-3 Experimental studies of the problem have focused on mechanical analysis of the relationship between degree of taper, surface area, preparation length, and the force necessary to remove cemented castings from machined dies.4,5 The retentive capabilities of various complete and partial veneer designs have also been assessed.6,7 The objective of this study was to evaluate the effect of preparation designs on retention and resistance. Retention prevents removal of a cast restoration along the path of insertion or long axis of the tooth preparation. Resistance prevents dislodgement of the restoration by forces directed in an apical or oblique direction and prevents any movement of the restoration under occlusal forces.8
MATERIALS AND METHODS Test dies were made for each of five preparation designs: three-quarter partial veneer crown without axial grooves (Fig. 1), three-quarter partial veneer crown with axial grooves (Fig. 2), seven-eighths partial veneer crown without axial grooves (Fig. 3), seven-eighths partial veneer crown with axial grooves (Fig. 4), and complete veneer crown without grooves (Fig. 5). The preparations had axial walls 6 mm in length with a
6-degree taper. Axial grooves, when present, were approximately 5.5 mm long and 1 mm in diameter.
Die fabrication A master die was made with an ivorine tooth set in a tapered, grooved plastic base. A three-quarter partial veneer crown preparation without grooves was prepared in the ivorine tooth. The buccal surface was recontoured to eliminate all undercuts. A mold was made of the die in a plastic tube with a silicone putty and wash (Xantopren and Optosil, Unitek Corp, Monrovia, Calif) The mold was poured with an acrylic resin (Duralay, Reliance Dental Mfg Co, Chicago, Ill) of fluid consistency. The plastic die was hollowed-out and filled with baseplate wax, since early attempts at investing solid plastic patterns invariably resulted in a severely cracked mold on burnout. A 0.25-inch diameter bolt, 1.5 inches in length, was imbedded in the wax in the base of the die pattern. This bolt was later used for attaching the die to the testing apparatus. A phosphate-bonded investment (Ceramigold, Whip Mix Corp, Louisville, Ky) was used to invest the pattern, and the dies were cast in a nickelchrome alloy (Gemini II, Kerr Mfg Co, Romulus, Mich). The alloy’s modulus of elasticity, 25.53106 psi, and its Vickers hardness No., 340, were such that it could withstand the forces to which the dies would be subjected during loading of the cast restorations.9 When a grooveless three-quarter preparation die had been successfully cast, the ivorine three-quarter preparation was modified by the addition of grooves. The entire process was repeated to produce a nickel-chrome die of that preparation. To produce the grooveless seven-eighths preparation, the grooves from the previous preparations were filled with inlay wax. The process was repeated until a nickel-chrome die had been produced for each of the five preparations.
Pattern fabrication Presented at the 56th General Session of the International Association for Dental Research, Washington, D.C. This study was supported in part by a grant from the Kerr Mfg. Co. a Assistant Professor, Department of Fixed Prosthodontics. b Professor and Chairman, Department of Fixed Prosthodontics. c Associate Professor and Chairman, Department of Dental Materials. Reprinted with permission from J Prosthet Dent 1980;43:303-8. J Prosthet Dent 2004;92:207-12.
SEPTEMBER 2004
Split molds have been described for producing castings of uniform thickness and contour.10 A twopiece mold was made of nickel-chrome alloy to accurately reproduce the external contours of the castings (Fig. 6). To produce a pattern, the two halves of the mold were assembled and secured with a U-bolt. A thin mix of Duralay resin was poured into the mold, THE JOURNAL OF PROSTHETIC DENTISTRY 207
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Fig. 1. Three-quarter partial veneer crown preparation without grooves.
Fig. 3. Seven-eighths partial veneer crown preparation without grooves.
and the die was seated using the grooves in the mouth of the mold to accurately orient the die (Fig. 7). After polymerization the clamp was loosened and the mold was split apart, freeing the die with the acrylic resin pattern. The patterns were designed so that the crowns would have a fossa approximately 4 mm in diameter and 2 mm deep in the center of the buccal incline of the lingual cusp. An 8-gauge round wax form was attached to the marginal ridges to produce a U-shaped sprue (Fig. 8). The fossa accommodated a steel ball bearing to which the forces for resistance testing would be applied. The U-shaped sprue provided the attachment for testing retention. 208
POTTS, SHILLINGBURG, AND DUNCANSON
Fig. 2. Three-quarter partial veneer crown preparation with grooves.
Fig. 4. Seven-eighths partial veneer crown preparation with grooves.
Before each cementation, the castings and dies were pickled in hydrochloric acid, cleaned in an ultrasonic bath for 10 minutes, dipped in acetone, and then dried with a blast of air. Zinc phosphate cement (Fleck’s Cement, Mizzy Inc, Clifton Forge, Va) was mixed for 60 seconds on a 708 F glass slab. Each casting was cemented with 11.5 pounds seating pressure for 10 minutes.11 The cemented castings were stored in a humidor for 24 hours before testing. Each casting was tested first for retention. The dies with cemented crowns were fixed to a machined metal base which was secured to the load cell of an Instron testing machine (Instron Corp, Canton, Mass) (Fig. 9). A self-aligning apparatus attached to the cross-head of VOLUME 92 NUMBER 3
POTTS, SHILLINGBURG, AND DUNCANSON
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Fig. 6. U-clamp and two halves of split mold.
Fig. 5. Complete veneer crown preparation without grooves.
Fig. 8. Test die with cemented seven-eighths crown is ready for testing. Note U-shaped sprue for retention testing and fossa on occlusal surface for resistance testing.
Fig. 7. Die is inserted into assembled split mold.
the Instron was connected to the U-sprue of the crown, so that the long axis of the preparation was coincident with the path of removal. The crowns were separated from the dies at a cross-head rate of 0.05 inches per minute, and the tensile forces required for crown removal were recorded as retention values. The U-sprues were removed from the crowns after retention testing, and the dies and crowns were recleaned and cemented as described previously for retention testing. Testing for resistance of the preparation designs was accomplished by applying a dislodging SEPTEMBER 2004
force in a direction oblique to the path of their insertion. The die with cemented casting was bolted onto a 45degree ramp in a machined stabilizing block which was positioned and secured to the load cell of the Instron machine (Fig. 10). To prevent damage to the die during testing, a machined support was placed under it. A ball bearing 5/32 inch in diameter was placed in the fossa on the lingual cusp of the crown. A tapered steel stylus attached to the cross-head of the Instron was lowered into position until a concave depression in its tip was firmly seated over the ball bearing. Compressive force was applied at a cross-head rate of 0.05 inches per minute until the crown was dislodged. The forces were measured and recorded as resistance values. Retention and resistance tests were conducted with 10 different castings for each of the five preparation designs.
RESULTS Arithmetic means and standard deviations were computed for the retention and resistance values 209
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POTTS, SHILLINGBURG, AND DUNCANSON
Fig. 9. Chain is attached to casting to test retention by tensile removal.
obtained from the respective preparation designs. The means and standard deviations for retention forces are shown in Table I, and those for resistance are in Table II. The data were interpreted by comparing the means for retention values for the five preparation designs by means of Duncan’s New Multiple Range Test at a 5% significance level. The data for resistance forces were analyzed similarly. Although there was an apparent slight increase in retention values with the addition of grooves and/or extension of axial surface coverage in the partial veneer designs, the increase in retention for those preparation designs was not statistically significant. However, there was a significant difference between the retention values for the partial veneer preparations and for the full veneer preparations (Fig. 11). There was significant differences among the resistance values for all of the preparation designs (Fig. 12).
DISCUSSION The addition of grooves to partial veneer preparations did not significantly augment retention; neither did extending axial surface coverage unless that extension was total. This was evident for both the threequarter and seven-eighths design series. Extending surface coverage to the complete crown design more than doubled the retention exhibited by any of the four partial veneer preparation designs. A possible explanation for the insignificant gains in retention associated with the addition of grooves may rest with the fact that placement of a groove adds very little to the total surface area of the preparation. This 210
might also explain the insignificant increase in retention as axial surface coverage is increased from a threequarter to a seven-eighths preparation. However, the relatively small increase in surface area covered between a seven-eighths and a complete crown preparation does not explain the significantly greater retention associated with full coverage. Obviously, factors other than surface area alone must come into play when explaining the greater retention associated with full veneer crowns. The primary function served by the traditional proximal groove in the partial veneer crown preparation is resistance. The extension of axial coverage of the restoration onto the surface on the same side from which the forces are being directed enhances resistance. Combining these features, i.e., grooves and coverage of the distal half of the buccal surface, produces a cumulative effect on resistance. Resistance is increased by any preparation feature which opposes dislodgement of the restoration by nonaxial external forces.
SUMMARY Five preparation designs were tested for retention and resistance. Retention values for all partial veneer crowns were significantly lower than those for the complete veneer crown. Resistance values increased significantly with the addition of grooves and/or extension of axial surface coverage. Addition of grooves and/or extension of axial surface coverage produced small increases in retention values but marked increases in resistance values. VOLUME 92 NUMBER 3
POTTS, SHILLINGBURG, AND DUNCANSON
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 10. Stylus is used to test resistance by compressive removal. Machined prop prevented damage to bolt embedded in die.
Fig. 11. Retention values are shown for each of the 5 preparation designs tested.
Fig. 12. Resistance values are shown for each of the 5 preparation designs tested. Table II. Resistance forces (pounds)
Table I. Retention forces (pounds) Preparation design
Mean
SD
Preparation design
Mean
SD
3/4 Partial veneer crown (No grooves) 3/4 Partial veneer crown (Grooves) 7/8 Partial veneer crown (No grooves) 7/8 Partial veneer crown (Grooves) Complete crown (No grooves)
92 106 94 114 243
7.6 8.8 5.9 10.6 36.7
3/4 Partial veneer crown (No grooves) 3/4 Partial veneer crown (Grooves) 7/8 Partial veneer crown (No grooves) 7/8 Partial veneer crown (Grooves) Complete crown (No grooves)
267 1366 943 1828 3119
27.7 41.0 39.7 65.8 245.7
n = 10.
REFERENCES 1. Smyd ES. Advanced thought in indirect inlay and fixed-bridge fabrication, Part I. J Am Dent Assoc 1944;31:759.
SEPTEMBER 2004
n = 10.
2. Smyd ES. Advanced thought in indirect inlay and fixed-bridge fabrication, Part II. J Am Dent Assoc 1944;31:913. 3. Rosenstiel E. The retention of inlays and crowns as a function of geometrical form. Br Dent J, Ser 2. 1957;103:388.
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4. Jørgensen KD. The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Scand 1955;13:35. 5. Kaufman EG, Coelho DH, Laurence C. Factors influencing the retention of cemented gold castings. J Prosthet Dent 1961;11:487. 6. Lorey RE, Myers GE. The retentive qualities of bridge retainers. J Am Dent Assoc 1968;76:568. 7. Reisbick MH, Shillingburg HT. Effect of preparation geometry on retention and resistance of cast gold restorations. Calif Dent Assoc J 1975;3:51. 8. Shillingburg HT, Hobo S, Whitsett LD. Fundamentals of fixed prosthodontics. Berlin: Buch-und Zeitschriften-Verlag ‘‘Die Quintessenz;’’ 1976. p. 67. 9. Moffa JP. Physical and mechanical properties of gold and base alloys. In: Valega, TM, editor. Alternatives to gold alloys in dentistry. DHEW Pub. No. (NIH)77-1227. Washington: U.S. Government Printing Office; 1977. p. 85.
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10. Shillingburg HT, Hobo S, Fisher DW. Preparation design and margin distortion in porcelain-fused-to-metal restorations. J Prosthet Dent 1973; 29:276. 11. Jørgensen KD. Factors affecting the film thickness of zinc phosphate cements. Acta Odontol Scand 1960;18:479. 0022-3913/$30.00 Copyright ª 2004 by The Editorial Council of The Journal of Prosthetic Dentistry
doi:10.1016/j.prosdent.2004.03.025
VOLUME 92 NUMBER 3
T
he problem of dislodged cast restorations is familiar. Crown displacement often occurs because the features of the tooth preparation do not counteract the forces directed against the restorations. Therefore the design of the tooth preparation is an important consideration in tooth reconstruction. The use of certain geometric features in preparations for cast restorations has been based largely on experience and individual preference. Early studies of designs of preparations were based on theoretical considerations of the way in which preparation features could be expected to geometrically resist tipping and removal forces.1-3 Experimental studies of the problem have focused on mechanical analysis of the relationship between degree of taper, surface area, preparation length, and the force necessary to remove cemented castings from machined dies.4,5 The retentive capabilities of various complete and partial veneer designs have also been assessed.6,7 The objective of this study was to evaluate the effect of preparation designs on retention and resistance. Retention prevents removal of a cast restoration along the path of insertion or long axis of the tooth preparation. Resistance prevents dislodgement of the restoration by forces directed in an apical or oblique direction and prevents any movement of the restoration under occlusal forces.8
MATERIALS AND METHODS Test dies were made for each of five preparation designs: three-quarter partial veneer crown without axial grooves (Fig. 1), three-quarter partial veneer crown with axial grooves (Fig. 2), seven-eighths partial veneer crown without axial grooves (Fig. 3), seven-eighths partial veneer crown with axial grooves (Fig. 4), and complete veneer crown without grooves (Fig. 5). The preparations had axial walls 6 mm in length with a
6-degree taper. Axial grooves, when present, were approximately 5.5 mm long and 1 mm in diameter.
Die fabrication A master die was made with an ivorine tooth set in a tapered, grooved plastic base. A three-quarter partial veneer crown preparation without grooves was prepared in the ivorine tooth. The buccal surface was recontoured to eliminate all undercuts. A mold was made of the die in a plastic tube with a silicone putty and wash (Xantopren and Optosil, Unitek Corp, Monrovia, Calif) The mold was poured with an acrylic resin (Duralay, Reliance Dental Mfg Co, Chicago, Ill) of fluid consistency. The plastic die was hollowed-out and filled with baseplate wax, since early attempts at investing solid plastic patterns invariably resulted in a severely cracked mold on burnout. A 0.25-inch diameter bolt, 1.5 inches in length, was imbedded in the wax in the base of the die pattern. This bolt was later used for attaching the die to the testing apparatus. A phosphate-bonded investment (Ceramigold, Whip Mix Corp, Louisville, Ky) was used to invest the pattern, and the dies were cast in a nickelchrome alloy (Gemini II, Kerr Mfg Co, Romulus, Mich). The alloy’s modulus of elasticity, 25.53106 psi, and its Vickers hardness No., 340, were such that it could withstand the forces to which the dies would be subjected during loading of the cast restorations.9 When a grooveless three-quarter preparation die had been successfully cast, the ivorine three-quarter preparation was modified by the addition of grooves. The entire process was repeated to produce a nickel-chrome die of that preparation. To produce the grooveless seven-eighths preparation, the grooves from the previous preparations were filled with inlay wax. The process was repeated until a nickel-chrome die had been produced for each of the five preparations.
Pattern fabrication Presented at the 56th General Session of the International Association for Dental Research, Washington, D.C. This study was supported in part by a grant from the Kerr Mfg. Co. a Assistant Professor, Department of Fixed Prosthodontics. b Professor and Chairman, Department of Fixed Prosthodontics. c Associate Professor and Chairman, Department of Dental Materials. Reprinted with permission from J Prosthet Dent 1980;43:303-8. J Prosthet Dent 2004;92:207-12.
SEPTEMBER 2004
Split molds have been described for producing castings of uniform thickness and contour.10 A twopiece mold was made of nickel-chrome alloy to accurately reproduce the external contours of the castings (Fig. 6). To produce a pattern, the two halves of the mold were assembled and secured with a U-bolt. A thin mix of Duralay resin was poured into the mold, THE JOURNAL OF PROSTHETIC DENTISTRY 207
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 1. Three-quarter partial veneer crown preparation without grooves.
Fig. 3. Seven-eighths partial veneer crown preparation without grooves.
and the die was seated using the grooves in the mouth of the mold to accurately orient the die (Fig. 7). After polymerization the clamp was loosened and the mold was split apart, freeing the die with the acrylic resin pattern. The patterns were designed so that the crowns would have a fossa approximately 4 mm in diameter and 2 mm deep in the center of the buccal incline of the lingual cusp. An 8-gauge round wax form was attached to the marginal ridges to produce a U-shaped sprue (Fig. 8). The fossa accommodated a steel ball bearing to which the forces for resistance testing would be applied. The U-shaped sprue provided the attachment for testing retention. 208
POTTS, SHILLINGBURG, AND DUNCANSON
Fig. 2. Three-quarter partial veneer crown preparation with grooves.
Fig. 4. Seven-eighths partial veneer crown preparation with grooves.
Before each cementation, the castings and dies were pickled in hydrochloric acid, cleaned in an ultrasonic bath for 10 minutes, dipped in acetone, and then dried with a blast of air. Zinc phosphate cement (Fleck’s Cement, Mizzy Inc, Clifton Forge, Va) was mixed for 60 seconds on a 708 F glass slab. Each casting was cemented with 11.5 pounds seating pressure for 10 minutes.11 The cemented castings were stored in a humidor for 24 hours before testing. Each casting was tested first for retention. The dies with cemented crowns were fixed to a machined metal base which was secured to the load cell of an Instron testing machine (Instron Corp, Canton, Mass) (Fig. 9). A self-aligning apparatus attached to the cross-head of VOLUME 92 NUMBER 3
POTTS, SHILLINGBURG, AND DUNCANSON
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 6. U-clamp and two halves of split mold.
Fig. 5. Complete veneer crown preparation without grooves.
Fig. 8. Test die with cemented seven-eighths crown is ready for testing. Note U-shaped sprue for retention testing and fossa on occlusal surface for resistance testing.
Fig. 7. Die is inserted into assembled split mold.
the Instron was connected to the U-sprue of the crown, so that the long axis of the preparation was coincident with the path of removal. The crowns were separated from the dies at a cross-head rate of 0.05 inches per minute, and the tensile forces required for crown removal were recorded as retention values. The U-sprues were removed from the crowns after retention testing, and the dies and crowns were recleaned and cemented as described previously for retention testing. Testing for resistance of the preparation designs was accomplished by applying a dislodging SEPTEMBER 2004
force in a direction oblique to the path of their insertion. The die with cemented casting was bolted onto a 45degree ramp in a machined stabilizing block which was positioned and secured to the load cell of the Instron machine (Fig. 10). To prevent damage to the die during testing, a machined support was placed under it. A ball bearing 5/32 inch in diameter was placed in the fossa on the lingual cusp of the crown. A tapered steel stylus attached to the cross-head of the Instron was lowered into position until a concave depression in its tip was firmly seated over the ball bearing. Compressive force was applied at a cross-head rate of 0.05 inches per minute until the crown was dislodged. The forces were measured and recorded as resistance values. Retention and resistance tests were conducted with 10 different castings for each of the five preparation designs.
RESULTS Arithmetic means and standard deviations were computed for the retention and resistance values 209
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POTTS, SHILLINGBURG, AND DUNCANSON
Fig. 9. Chain is attached to casting to test retention by tensile removal.
obtained from the respective preparation designs. The means and standard deviations for retention forces are shown in Table I, and those for resistance are in Table II. The data were interpreted by comparing the means for retention values for the five preparation designs by means of Duncan’s New Multiple Range Test at a 5% significance level. The data for resistance forces were analyzed similarly. Although there was an apparent slight increase in retention values with the addition of grooves and/or extension of axial surface coverage in the partial veneer designs, the increase in retention for those preparation designs was not statistically significant. However, there was a significant difference between the retention values for the partial veneer preparations and for the full veneer preparations (Fig. 11). There was significant differences among the resistance values for all of the preparation designs (Fig. 12).
DISCUSSION The addition of grooves to partial veneer preparations did not significantly augment retention; neither did extending axial surface coverage unless that extension was total. This was evident for both the threequarter and seven-eighths design series. Extending surface coverage to the complete crown design more than doubled the retention exhibited by any of the four partial veneer preparation designs. A possible explanation for the insignificant gains in retention associated with the addition of grooves may rest with the fact that placement of a groove adds very little to the total surface area of the preparation. This 210
might also explain the insignificant increase in retention as axial surface coverage is increased from a threequarter to a seven-eighths preparation. However, the relatively small increase in surface area covered between a seven-eighths and a complete crown preparation does not explain the significantly greater retention associated with full coverage. Obviously, factors other than surface area alone must come into play when explaining the greater retention associated with full veneer crowns. The primary function served by the traditional proximal groove in the partial veneer crown preparation is resistance. The extension of axial coverage of the restoration onto the surface on the same side from which the forces are being directed enhances resistance. Combining these features, i.e., grooves and coverage of the distal half of the buccal surface, produces a cumulative effect on resistance. Resistance is increased by any preparation feature which opposes dislodgement of the restoration by nonaxial external forces.
SUMMARY Five preparation designs were tested for retention and resistance. Retention values for all partial veneer crowns were significantly lower than those for the complete veneer crown. Resistance values increased significantly with the addition of grooves and/or extension of axial surface coverage. Addition of grooves and/or extension of axial surface coverage produced small increases in retention values but marked increases in resistance values. VOLUME 92 NUMBER 3
POTTS, SHILLINGBURG, AND DUNCANSON
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 10. Stylus is used to test resistance by compressive removal. Machined prop prevented damage to bolt embedded in die.
Fig. 11. Retention values are shown for each of the 5 preparation designs tested.
Fig. 12. Resistance values are shown for each of the 5 preparation designs tested. Table II. Resistance forces (pounds)
Table I. Retention forces (pounds) Preparation design
Mean
SD
Preparation design
Mean
SD
3/4 Partial veneer crown (No grooves) 3/4 Partial veneer crown (Grooves) 7/8 Partial veneer crown (No grooves) 7/8 Partial veneer crown (Grooves) Complete crown (No grooves)
92 106 94 114 243
7.6 8.8 5.9 10.6 36.7
3/4 Partial veneer crown (No grooves) 3/4 Partial veneer crown (Grooves) 7/8 Partial veneer crown (No grooves) 7/8 Partial veneer crown (Grooves) Complete crown (No grooves)
267 1366 943 1828 3119
27.7 41.0 39.7 65.8 245.7
n = 10.
REFERENCES 1. Smyd ES. Advanced thought in indirect inlay and fixed-bridge fabrication, Part I. J Am Dent Assoc 1944;31:759.
SEPTEMBER 2004
n = 10.
2. Smyd ES. Advanced thought in indirect inlay and fixed-bridge fabrication, Part II. J Am Dent Assoc 1944;31:913. 3. Rosenstiel E. The retention of inlays and crowns as a function of geometrical form. Br Dent J, Ser 2. 1957;103:388.
211
THE JOURNAL OF PROSTHETIC DENTISTRY
4. Jørgensen KD. The relationship between retention and convergence angle in cemented veneer crowns. Acta Odontol Scand 1955;13:35. 5. Kaufman EG, Coelho DH, Laurence C. Factors influencing the retention of cemented gold castings. J Prosthet Dent 1961;11:487. 6. Lorey RE, Myers GE. The retentive qualities of bridge retainers. J Am Dent Assoc 1968;76:568. 7. Reisbick MH, Shillingburg HT. Effect of preparation geometry on retention and resistance of cast gold restorations. Calif Dent Assoc J 1975;3:51. 8. Shillingburg HT, Hobo S, Whitsett LD. Fundamentals of fixed prosthodontics. Berlin: Buch-und Zeitschriften-Verlag ‘‘Die Quintessenz;’’ 1976. p. 67. 9. Moffa JP. Physical and mechanical properties of gold and base alloys. In: Valega, TM, editor. Alternatives to gold alloys in dentistry. DHEW Pub. No. (NIH)77-1227. Washington: U.S. Government Printing Office; 1977. p. 85.
212
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10. Shillingburg HT, Hobo S, Fisher DW. Preparation design and margin distortion in porcelain-fused-to-metal restorations. J Prosthet Dent 1973; 29:276. 11. Jørgensen KD. Factors affecting the film thickness of zinc phosphate cements. Acta Odontol Scand 1960;18:479. 0022-3913/$30.00 Copyright ª 2004 by The Editorial Council of The Journal of Prosthetic Dentistry
doi:10.1016/j.prosdent.2004.03.025
VOLUME 92 NUMBER 3