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Accuracy of cast restorations produced by a refractory die-investing technique
Accuracy of cast restorations produced by a refractory die-investing technique Werner Finger, Dr.Med.Dent.,* and Masahiro Ohsawa, D.D.S., Ph.D.** Royal Dental College, Copenhagen, Denmark
N',w materials and methods have been developed to facilitate the production of accurately fitting inlays and crowns. The reliability of each step in the fabrication of gold alloy restorations is readily controllable except for the wax technique. In routine laboratory procedures, it is not possible to avoid distortion of the wax pattern. However, theoretically, this type of inaccuracy could be excluded by the use of a refractory die-investing technique. When this technique is used, an additional die of investment material is also produced. The wax pattern made on the master die is then transferred to and invested on the refractory die. Consistent accuracy of inlays and crowns is obtainable when the following requirements are fulfilled: (1) accurate impressions, t (2) reliable master dies, 2 (3) uniform die spacing that is approximately 30 #m, s (4) accurate wax pattern or copy in investment material of master die, and (5) investing and casting technique that secures slide fit of the restoration on the die-spacer corrected master die. 4 The purpose of this study was t o evaluate the accuracy of cast gold alloy restorations by controlling the accuracy of the individual laboratory steps.
MATERIAL AND METHODS Conical ring-shaped gold alloy castings were produced by the four different techniques A, B, C, and D described in Table I. Accuracy of each laboratory step was evaluated. The preparations were made from truncated chi'omiurn-steel cones with a base diameter and height of 8 mm with a 10-degree taper. Impressions of the standard steel dies for techniques C and D were made with an addition-type silicone (President, regular body, batch 01028264, Coltene AG, Alstatten, Switzerland). The impression trays were circular, 16 mm diameter, 13 mm high, and fabricated from 0.75 mm thick perforated 18/8 stainless steel plate. The steel die and tray were mounted in a stand to secure a *Professor, Department of Technology, Dental School, Aachen, West Germany. **Lecturer, Department of Operative Dentistry, Niigata, Japan.
800
repi'oducible position with 4 mm of impression material between the base of the die and the wall of the tray, while the distance to the tray was 2 mm in an occlusal direction. The impression material was mixed for 30 seconds, and after an additional minute the assembly was immediately transferred to a water bath (37 ° -+ 1 ° C). The impressions were removed 15 minutes from the start of mixing, and stored at room temperature for 23½ hours and at 37 ° _+ 1 ° C for 30 minutes before the dies were cast with a 37 ° _+ 1 ° C stone slurry? A Type IV dental die stone (Duroc, batch 02036 A, Ransom and Randolph, Toledo, Ohio, water/powder ratio 0.23) with the setting expansion after 2 hours 0.06% _+ 0.02% was Used. The dies were removed from the impressions after 2 hours storage at 37 ° C. Accuracy was measured at room temperature as the axial discrepancy of a steel ring fitted to the master steel cone and expressed as the deviation in micrometers of the base diameter of the steel die) A die spacer (Dedeco, 3M Go., Glostrfip, Denmark ) was precipitated on the stone dies at 23 ° + 1 ° C. The dipping time in the monomer solution was 60 seconds, because a spacer thickness of approximately 30 #m was desired) Control measurements of the spacer thickness on axial sections of 10 spacer-corrected stone dies were recorded. When the die-investing techniques B and D were used, impressions were made at room temperature in air from the steel dies and the die spacer-corrected stone dies. Mixing of the addition silicone, impression making, and curing time before removal were identical with the procedure described above. After 5 to 20 minutes storage of the impressions, refractory dies were made from an experimental phosphate-bonded investment (NPI); the setting expansion was 0.03% _+ 0.01% when measured with a standard dial gauge. 6 The investment powder was mixed with a 20% aqueous glycerol solution at a liquid/powder ratio 0.18 for 15 seconds by hand and for 45 seconds by vacuum (Vacuvest0r, Type B, Whip-Mix Corp., Louisville, Ky.). The investment dies were removed from the impressions after setting at 23 ° _+ 1 ° G for 1 hour. The accuracy of the refractory dies produced for technique B was measured by the method above. DECEMBER 1984
VOLUME 52
NUMBER 6
ACCURACY OF CAST RESTORATIONS
Table II. Ring transfer t e c h n i q u e
Table I. Descriptions of the four different t e c h n i q u e s used Technique Procedure
A
B
C
D
Preparation (steel die) I m p r e s s i o n (37 ° _+ 1 ° C) from steel die; stone (tie p r o d u c e d at 37 ° + 1 ° C; die spacer precipitation 23 + 1 ° C I m p r e s s i o n (23 _+ 1 ° C) from steel die from spacer corrected stone die Refractory die Wax ring made on steel die spacer-.corrected stone die Wax ring e m b e d d e d free on refractory die Casting technique
X
X
X
X
X
X
Wax r i n g o n steel die
Wax r i n g o n spacer corrected s t o n e die
I
A
B
D
X X X
X
X
X X
X
the steel dies. The deviation of the inside diameter of the castings in micrometers and percent from the 8 mm base diameter of the die was determined?
X X
X
X
X
X X
X
Wax rings with different wall thickness were fabricated on steel dies (techniques A and B) and on spacer-corrected stone dies (techniques C and D) by dipping technique (Blue inlay casting wax, Type I, Kerr Mfg. Co., Romulus, Mich.). When the rings were sufficiently thick, they were cooled to ambient room temperature, cut flush with the superior surface of the die, and trimmed on a lathe to wali thicknesses of 0.5, 1, 2, and 3 ram, respectively. The completed rings for the techniques A and C were removed from the dies and stored approximately 30 minutes in a water bath at room temperature. The rings were replaced on the die for a final superior surface flush carving. The rings in techniques B and D were transferred to the respective refractory dies and fully seated by a slight pressure from a glass plate (Table II). The wax rings were sprued perpendicular to the conical surface 4 mm from the top with a cylindric 1 mm thick steel pin. The sprue length was adjusted to 7 ram, and the rings were invested either free or mounted on the refractory dies in soft silicone rubber mold filled with freshly mixed NPI (powder/liquid ratio 0.18). The investment mold was removed from the silicone rubber rings after 2 hours and placed in a furnace at 200 ° C for 1 hour and then in a burnout furnace preheated to '700 ° to 750 ° C for 30 minutes. The thermal expansion of the investment material at the given preheating temperature is approximately 1.6%. 6 The alloy Degulor C (a Type III gold alloy, Degussa, Pforzheim, W. Germany) was vacuum cast exhibiting a free contraction during cooling from solidus point to room temperature of approximately 1.7%. v,s After pickling in hot 10% sulphuric acid and removal of nodules from the internal surfaces, the castings were seated on THE JOURNAL OF PROSTHETIC DENTISTRY
Wax pattern freely invested Wax pattern invested on refractory die from impressions
RESULTS Fig. 1 illustrates the accuracy of castings performed by the direct technique A (open circles) and the direct refractory die-investing technique B (closed circles). The diagram illustrates the relation between the wall thickness of the castings in millimeters and the accuracy of the rings expressed as the deviation in microns and percent from the 8 mm base diameter of the steel die. Each circle represents the mean value of five castings; the variation in length of the vertical lines gives the standard deviation. The two straight lines are linear regression lines each calculated from 20 paired coordinate values. The coefficients of correlation given in the diagram differ significantly from zero, and the slopes of the regression lines differ significantly from each other (p <.01). In both series the diameter of the castings decreased with increasing thickness of the patterns. The horizontal, dashed reference line indicates slide-fit, that is, harmony between preparation and internal surface of the casting. The mean base diameter deviation of the refractory dies used in technique B from the base diameter of the steel die was -0.2 + 6.8 #m. Fig. 2 illustrates the accuracy of castings with different thickness made by the indirect technique C (open circles) and by the indirect refractory die-investing technique D (closed circles). Each circle and connected vertical line represents the mean diameter of five castings and the standard deviation. The statistics designated significant differences between the slope of the regression lines (p <.01). The horizontal dashed line describes the mean base diameter increase of the space-corrected stone master dies compared with the original steel die. This base line value (73 #m) contains both the mean diameter increase of the stone dies produced by the reheating technique (11 #m) and twice the mean value found for the die spacer thickness (31 #m). The standard deviation of the reference line is 9 #m. 801
FINGERAND OHSAWA Ad %
/~d prn 32
I
I
I
I
I
24 16
0.4
I
o
r = - 0.84
•
r = - (165
0.3
(
O.2
8
03
+0
+0
8
0.1
16
O.2 I
I
0.5
1.0
24
I
I
I
I
2.0
0.3
3.0
mm
Fig. 1. Effect o f t h i c k n e s s o f r e s t o r a t i o n s o n d e v i a t i o n o f i n s i d e base d i a m e t e r o f castings
in microns and percent from 8 mm base diameter of steel master die. Open circles give mean values from measurements on five castings performed by technique A, closed circles for 5 castings performed by technique B. Vertical lines give twice the standard deviation. straight lines were determined by linear regression analysis. Horizontal dashed line represents congruency between preparation surface and fitting surface of restoration.
/~d pm
~d %
10/.
1.3
96
o
r : - 0.85
•
r :-
!.2
0.48
ee
~.1
80
1.0
72
....
0.9
64
0.8
56
05
48
t
0.5
0.6
1.0
2.0
3,0
mm
Fig. 2. Effect of thickness of restorations on deviation of inside base diameter of castings in microns and percent from 8 mm base diameter of steel master die. Open circles represent mean values from measurements on five castings made by technique C, closed circles for five castings made by technique D. Vertical lines give twice the standard deviation, straight lines were determined by linear regression analysis. Dashed line represents congruency between spacer-corrected die surface and fitting surface of restorations, that is, adequate loose fit on original preparation.
DISCUSSION The present study has demonstrated that complete cast gold alloy restorations with an acceptable degree of adaptation can be produced with appropriate material and techniques. 802
The impression material used for this technique is the addition-type silicone, which has a negligible shrinkage during curing and storage. The thermal contraction of the impression material from intraorai to room temperature is reversible and is essentially compensated by the DECEMBER 1984
VOLUME 52
NUMBER 6
ACCURACY OF CAST RESTORATIONS
reheating technique. 2 If the die stone and the investment materials have a low setting expansion of less than 0.1%, as in the present study, the distortion of the resulting dies is minimal. The rationale and the advantage of the use of die spacer with uniform thickness to provide sufficient space for the largest grains of the luting cement between preparation and restoration has been reported. 3 Precipitation of a uniform layer of polymer on the master die with adequate thickness has been routinely obtained. Distortion of the wax pattern caused by stress release during removal from the master die is unavoidable. The wax rings used in techniques A and C had to be recarved occlusally when stored in a water bath at room temperature for 30 minutes after removal from the steel dies. Reduction of wax pattern distortion explains the increased accuracy of the ring castings produced by techniques A and C. The difference in accuracy noted in varying thickness of the castings confirms previously published results? In all eases disto:'tion was moderate and considered clinically acceptable. An important requirement for both the refractory die-investing technique and for the conventional indirect and direct techniques is an investment material with minimal setting expansion but suffÉcient thermal expansion to compensate for the solid thermal shrinkage of the casting alloy. A craze-proof investment should be used without a steel casting ring to exclude (1) uncontrollable hygroscopic expansion caused by the liquid content of the liner in the casting ring, and (2) reduction of the thermal expansion of the investment. Refractory die investment materials have been available commercially for several years. 1°-t5 These commercial products exhibited approximately 1% setting expansion with only 0.6% to 0.7% thermal expansion, which renders them inadequate [or routine laboratory work. SUMMARY The present study has demonstrated a simple, controlled technique to produce consistently accurate gold alloy restorations. Distortion of the wax patterns was minimized with an adequate refractory die-investing technique.
THE JOURNAL OF PROSTHETIC DENTISTRY
REFERENCES 1. Ohsawa, M., and J~rgensen, K. D.: Curing contraction of addition-type silicone impression materials. Scand J Dent Res 91:51, 1983. 2. J~rgensen, K. D.: Thermal expansion of addition silicone impression materials. Austr Dent J 27:377, 1982. 3. J~rgensen, K. D., and Finger, W.: Die spacing technique by diffusion precipitation. Stand J Dent Res 87:73, 1979. 4. J~rgensen, K. D., and Finger, W.: A new concept in dental precision casting. Microfilm No. 417, IADR Meeting, New Orleans, 1979. 5. J~rgensen, K. D.: Indlaeg og Kroner. Copenhagen, 1978, Odontologisk Boghandels Forlag. 6. Finger, W., and Kota, K.: A modified phosphate-bonded casting investment. Scand J Dent Res 90:243, 1982. 7. J~rgensen, K. D.: A new vacuum casting furnace. Quintessence Int 3:47, 1973. 8. Finger, W., and J~rgensen, K. D.: An improved dental casting investment. Scand J Dent Res 88:278, 1980. 9. Finger, W.: Effect of thickness of peridental restorations on the casting precision. Scand J Dent Res 88:455, 1980. 10. Hosoda, H., and Mijazu, H.: A study of the refractory dieinvesting material. J Jpn Res Soc Dent Mater Appl 21:60, 1970. 1I. Matono, H., Katagiri, Y., Hanari, M., and Hosoda, H.: Adaptation of full cast crown made by using the refractory die investing technique. J Jpn Res Soc Dent Mater Appl 23:66, 1970. 12. Ohmura, K., Katagiri, Y., Matsuno, H., and Hosoda, H.:
13.
14.
Comparison of adaptation of full crowns made by using various technics. J Jpn Res Soc Dent Mater Appl 26:96, 1972. Hosoda, H., Matsuno, H_, and Katagiri, Y.: Adaptation of cast inlays made by various techniques. Jpn j Conservative Dent
16:133, 1973. Hosoda, H., Mikami, K., Tsurugai, T., and Kanda, S.: New assessment of adaptation of the bridges made by various technics.
J Jpn Res Soe Dent Mater Appl 34:233, 1977. 15. Nishiyama, M., and Ohwa, M.: Refractory die-investing techniques: The tendencies of setting expansion on die-investments in the process of constructing working models. J Nihon Univ Sch Dent 22:245, 1980. Reprint requests to: DR. W~RNF~RF1NCER BAYERAG PHARMA-SECTORDENTAL D-4047 DORMAGEN,BAYERWERK WEST GERMANY
803
N',w materials and methods have been developed to facilitate the production of accurately fitting inlays and crowns. The reliability of each step in the fabrication of gold alloy restorations is readily controllable except for the wax technique. In routine laboratory procedures, it is not possible to avoid distortion of the wax pattern. However, theoretically, this type of inaccuracy could be excluded by the use of a refractory die-investing technique. When this technique is used, an additional die of investment material is also produced. The wax pattern made on the master die is then transferred to and invested on the refractory die. Consistent accuracy of inlays and crowns is obtainable when the following requirements are fulfilled: (1) accurate impressions, t (2) reliable master dies, 2 (3) uniform die spacing that is approximately 30 #m, s (4) accurate wax pattern or copy in investment material of master die, and (5) investing and casting technique that secures slide fit of the restoration on the die-spacer corrected master die. 4 The purpose of this study was t o evaluate the accuracy of cast gold alloy restorations by controlling the accuracy of the individual laboratory steps.
MATERIAL AND METHODS Conical ring-shaped gold alloy castings were produced by the four different techniques A, B, C, and D described in Table I. Accuracy of each laboratory step was evaluated. The preparations were made from truncated chi'omiurn-steel cones with a base diameter and height of 8 mm with a 10-degree taper. Impressions of the standard steel dies for techniques C and D were made with an addition-type silicone (President, regular body, batch 01028264, Coltene AG, Alstatten, Switzerland). The impression trays were circular, 16 mm diameter, 13 mm high, and fabricated from 0.75 mm thick perforated 18/8 stainless steel plate. The steel die and tray were mounted in a stand to secure a *Professor, Department of Technology, Dental School, Aachen, West Germany. **Lecturer, Department of Operative Dentistry, Niigata, Japan.
800
repi'oducible position with 4 mm of impression material between the base of the die and the wall of the tray, while the distance to the tray was 2 mm in an occlusal direction. The impression material was mixed for 30 seconds, and after an additional minute the assembly was immediately transferred to a water bath (37 ° -+ 1 ° C). The impressions were removed 15 minutes from the start of mixing, and stored at room temperature for 23½ hours and at 37 ° _+ 1 ° C for 30 minutes before the dies were cast with a 37 ° _+ 1 ° C stone slurry? A Type IV dental die stone (Duroc, batch 02036 A, Ransom and Randolph, Toledo, Ohio, water/powder ratio 0.23) with the setting expansion after 2 hours 0.06% _+ 0.02% was Used. The dies were removed from the impressions after 2 hours storage at 37 ° C. Accuracy was measured at room temperature as the axial discrepancy of a steel ring fitted to the master steel cone and expressed as the deviation in micrometers of the base diameter of the steel die) A die spacer (Dedeco, 3M Go., Glostrfip, Denmark ) was precipitated on the stone dies at 23 ° + 1 ° C. The dipping time in the monomer solution was 60 seconds, because a spacer thickness of approximately 30 #m was desired) Control measurements of the spacer thickness on axial sections of 10 spacer-corrected stone dies were recorded. When the die-investing techniques B and D were used, impressions were made at room temperature in air from the steel dies and the die spacer-corrected stone dies. Mixing of the addition silicone, impression making, and curing time before removal were identical with the procedure described above. After 5 to 20 minutes storage of the impressions, refractory dies were made from an experimental phosphate-bonded investment (NPI); the setting expansion was 0.03% _+ 0.01% when measured with a standard dial gauge. 6 The investment powder was mixed with a 20% aqueous glycerol solution at a liquid/powder ratio 0.18 for 15 seconds by hand and for 45 seconds by vacuum (Vacuvest0r, Type B, Whip-Mix Corp., Louisville, Ky.). The investment dies were removed from the impressions after setting at 23 ° _+ 1 ° G for 1 hour. The accuracy of the refractory dies produced for technique B was measured by the method above. DECEMBER 1984
VOLUME 52
NUMBER 6
ACCURACY OF CAST RESTORATIONS
Table II. Ring transfer t e c h n i q u e
Table I. Descriptions of the four different t e c h n i q u e s used Technique Procedure
A
B
C
D
Preparation (steel die) I m p r e s s i o n (37 ° _+ 1 ° C) from steel die; stone (tie p r o d u c e d at 37 ° + 1 ° C; die spacer precipitation 23 + 1 ° C I m p r e s s i o n (23 _+ 1 ° C) from steel die from spacer corrected stone die Refractory die Wax ring made on steel die spacer-.corrected stone die Wax ring e m b e d d e d free on refractory die Casting technique
X
X
X
X
X
X
Wax r i n g o n steel die
Wax r i n g o n spacer corrected s t o n e die
I
A
B
D
X X X
X
X
X X
X
the steel dies. The deviation of the inside diameter of the castings in micrometers and percent from the 8 mm base diameter of the die was determined?
X X
X
X
X
X X
X
Wax rings with different wall thickness were fabricated on steel dies (techniques A and B) and on spacer-corrected stone dies (techniques C and D) by dipping technique (Blue inlay casting wax, Type I, Kerr Mfg. Co., Romulus, Mich.). When the rings were sufficiently thick, they were cooled to ambient room temperature, cut flush with the superior surface of the die, and trimmed on a lathe to wali thicknesses of 0.5, 1, 2, and 3 ram, respectively. The completed rings for the techniques A and C were removed from the dies and stored approximately 30 minutes in a water bath at room temperature. The rings were replaced on the die for a final superior surface flush carving. The rings in techniques B and D were transferred to the respective refractory dies and fully seated by a slight pressure from a glass plate (Table II). The wax rings were sprued perpendicular to the conical surface 4 mm from the top with a cylindric 1 mm thick steel pin. The sprue length was adjusted to 7 ram, and the rings were invested either free or mounted on the refractory dies in soft silicone rubber mold filled with freshly mixed NPI (powder/liquid ratio 0.18). The investment mold was removed from the silicone rubber rings after 2 hours and placed in a furnace at 200 ° C for 1 hour and then in a burnout furnace preheated to '700 ° to 750 ° C for 30 minutes. The thermal expansion of the investment material at the given preheating temperature is approximately 1.6%. 6 The alloy Degulor C (a Type III gold alloy, Degussa, Pforzheim, W. Germany) was vacuum cast exhibiting a free contraction during cooling from solidus point to room temperature of approximately 1.7%. v,s After pickling in hot 10% sulphuric acid and removal of nodules from the internal surfaces, the castings were seated on THE JOURNAL OF PROSTHETIC DENTISTRY
Wax pattern freely invested Wax pattern invested on refractory die from impressions
RESULTS Fig. 1 illustrates the accuracy of castings performed by the direct technique A (open circles) and the direct refractory die-investing technique B (closed circles). The diagram illustrates the relation between the wall thickness of the castings in millimeters and the accuracy of the rings expressed as the deviation in microns and percent from the 8 mm base diameter of the steel die. Each circle represents the mean value of five castings; the variation in length of the vertical lines gives the standard deviation. The two straight lines are linear regression lines each calculated from 20 paired coordinate values. The coefficients of correlation given in the diagram differ significantly from zero, and the slopes of the regression lines differ significantly from each other (p <.01). In both series the diameter of the castings decreased with increasing thickness of the patterns. The horizontal, dashed reference line indicates slide-fit, that is, harmony between preparation and internal surface of the casting. The mean base diameter deviation of the refractory dies used in technique B from the base diameter of the steel die was -0.2 + 6.8 #m. Fig. 2 illustrates the accuracy of castings with different thickness made by the indirect technique C (open circles) and by the indirect refractory die-investing technique D (closed circles). Each circle and connected vertical line represents the mean diameter of five castings and the standard deviation. The statistics designated significant differences between the slope of the regression lines (p <.01). The horizontal dashed line describes the mean base diameter increase of the space-corrected stone master dies compared with the original steel die. This base line value (73 #m) contains both the mean diameter increase of the stone dies produced by the reheating technique (11 #m) and twice the mean value found for the die spacer thickness (31 #m). The standard deviation of the reference line is 9 #m. 801
FINGERAND OHSAWA Ad %
/~d prn 32
I
I
I
I
I
24 16
0.4
I
o
r = - 0.84
•
r = - (165
0.3
(
O.2
8
03
+0
+0
8
0.1
16
O.2 I
I
0.5
1.0
24
I
I
I
I
2.0
0.3
3.0
mm
Fig. 1. Effect o f t h i c k n e s s o f r e s t o r a t i o n s o n d e v i a t i o n o f i n s i d e base d i a m e t e r o f castings
in microns and percent from 8 mm base diameter of steel master die. Open circles give mean values from measurements on five castings performed by technique A, closed circles for 5 castings performed by technique B. Vertical lines give twice the standard deviation. straight lines were determined by linear regression analysis. Horizontal dashed line represents congruency between preparation surface and fitting surface of restoration.
/~d pm
~d %
10/.
1.3
96
o
r : - 0.85
•
r :-
!.2
0.48
ee
~.1
80
1.0
72
....
0.9
64
0.8
56
05
48
t
0.5
0.6
1.0
2.0
3,0
mm
Fig. 2. Effect of thickness of restorations on deviation of inside base diameter of castings in microns and percent from 8 mm base diameter of steel master die. Open circles represent mean values from measurements on five castings made by technique C, closed circles for five castings made by technique D. Vertical lines give twice the standard deviation, straight lines were determined by linear regression analysis. Dashed line represents congruency between spacer-corrected die surface and fitting surface of restorations, that is, adequate loose fit on original preparation.
DISCUSSION The present study has demonstrated that complete cast gold alloy restorations with an acceptable degree of adaptation can be produced with appropriate material and techniques. 802
The impression material used for this technique is the addition-type silicone, which has a negligible shrinkage during curing and storage. The thermal contraction of the impression material from intraorai to room temperature is reversible and is essentially compensated by the DECEMBER 1984
VOLUME 52
NUMBER 6
ACCURACY OF CAST RESTORATIONS
reheating technique. 2 If the die stone and the investment materials have a low setting expansion of less than 0.1%, as in the present study, the distortion of the resulting dies is minimal. The rationale and the advantage of the use of die spacer with uniform thickness to provide sufficient space for the largest grains of the luting cement between preparation and restoration has been reported. 3 Precipitation of a uniform layer of polymer on the master die with adequate thickness has been routinely obtained. Distortion of the wax pattern caused by stress release during removal from the master die is unavoidable. The wax rings used in techniques A and C had to be recarved occlusally when stored in a water bath at room temperature for 30 minutes after removal from the steel dies. Reduction of wax pattern distortion explains the increased accuracy of the ring castings produced by techniques A and C. The difference in accuracy noted in varying thickness of the castings confirms previously published results? In all eases disto:'tion was moderate and considered clinically acceptable. An important requirement for both the refractory die-investing technique and for the conventional indirect and direct techniques is an investment material with minimal setting expansion but suffÉcient thermal expansion to compensate for the solid thermal shrinkage of the casting alloy. A craze-proof investment should be used without a steel casting ring to exclude (1) uncontrollable hygroscopic expansion caused by the liquid content of the liner in the casting ring, and (2) reduction of the thermal expansion of the investment. Refractory die investment materials have been available commercially for several years. 1°-t5 These commercial products exhibited approximately 1% setting expansion with only 0.6% to 0.7% thermal expansion, which renders them inadequate [or routine laboratory work. SUMMARY The present study has demonstrated a simple, controlled technique to produce consistently accurate gold alloy restorations. Distortion of the wax patterns was minimized with an adequate refractory die-investing technique.
THE JOURNAL OF PROSTHETIC DENTISTRY
REFERENCES 1. Ohsawa, M., and J~rgensen, K. D.: Curing contraction of addition-type silicone impression materials. Scand J Dent Res 91:51, 1983. 2. J~rgensen, K. D.: Thermal expansion of addition silicone impression materials. Austr Dent J 27:377, 1982. 3. J~rgensen, K. D., and Finger, W.: Die spacing technique by diffusion precipitation. Stand J Dent Res 87:73, 1979. 4. J~rgensen, K. D., and Finger, W.: A new concept in dental precision casting. Microfilm No. 417, IADR Meeting, New Orleans, 1979. 5. J~rgensen, K. D.: Indlaeg og Kroner. Copenhagen, 1978, Odontologisk Boghandels Forlag. 6. Finger, W., and Kota, K.: A modified phosphate-bonded casting investment. Scand J Dent Res 90:243, 1982. 7. J~rgensen, K. D.: A new vacuum casting furnace. Quintessence Int 3:47, 1973. 8. Finger, W., and J~rgensen, K. D.: An improved dental casting investment. Scand J Dent Res 88:278, 1980. 9. Finger, W.: Effect of thickness of peridental restorations on the casting precision. Scand J Dent Res 88:455, 1980. 10. Hosoda, H., and Mijazu, H.: A study of the refractory dieinvesting material. J Jpn Res Soc Dent Mater Appl 21:60, 1970. 1I. Matono, H., Katagiri, Y., Hanari, M., and Hosoda, H.: Adaptation of full cast crown made by using the refractory die investing technique. J Jpn Res Soc Dent Mater Appl 23:66, 1970. 12. Ohmura, K., Katagiri, Y., Matsuno, H., and Hosoda, H.:
13.
14.
Comparison of adaptation of full crowns made by using various technics. J Jpn Res Soc Dent Mater Appl 26:96, 1972. Hosoda, H., Matsuno, H_, and Katagiri, Y.: Adaptation of cast inlays made by various techniques. Jpn j Conservative Dent
16:133, 1973. Hosoda, H., Mikami, K., Tsurugai, T., and Kanda, S.: New assessment of adaptation of the bridges made by various technics.
J Jpn Res Soe Dent Mater Appl 34:233, 1977. 15. Nishiyama, M., and Ohwa, M.: Refractory die-investing techniques: The tendencies of setting expansion on die-investments in the process of constructing working models. J Nihon Univ Sch Dent 22:245, 1980. Reprint requests to: DR. W~RNF~RF1NCER BAYERAG PHARMA-SECTORDENTAL D-4047 DORMAGEN,BAYERWERK WEST GERMANY
803