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The dimensionsal accuracy of improved dental stone, silverplated, and epoxy resin die materials
The dimensionsal accuracy of improved dental stone, silverplated, and epoxy resin die materials J. H. Bailey,
D.D.S.,*
T. E. Donovan
D.D.S.,**
The Ohio State University, College of Dentistry, Dentistry, Los Angeles, Calif.
and J. D. Preston, D.D.S.***
Columbus,
1 mproved dental stone is the predominant material in dentistry for making dies used in the lost-wax process. Although improved dental stones have served the profession well, the main disadvantages of their use as dies are (1) susceptibility to dimensional change due to abrasion, (2) limited reproduction of fine detail,’ and (3) lack of strength. The lack of strength can result in breakage of dies, especially those duplicating long, narrow preparations. In the fabrication of all-porcelain labial margins or some of the new generation cast porcelain restorations, dies that exhibit greater strength and abrasion resistance than gypsum dies are desirable. Alternate die materials that may yield dimensionally accurate dies, are more resistant to abrasion, and are stronger than the improved dental stone dies presently in use are available. The literature concerning dimensional accuracy of various die materials is somewhat equivocal. Gypsum die materials generally exhibit an average setting expansion of O.l%.* Such minimal expansion is thought to be beneficial in terms of aiding compensation for metal shrinkage, wax pattern dimensional change, and other unavoida.ble inaccuracies in the casting process. Electroplated dies have been found to be less accurate dimensionally than improved stone dies when formed with polysulfide rubber or condensation-reaction silicone impression m.aterials.3 One study demonstrated that silver-plated dies tend to be undersized by as much as 0.2%; whereas another found that they tend to be oversized.5 It has been shown that vinyl polysiloxane impression ma.terials are the most accurate for making silver-plated dies.+ Castings made on silver-plated dies have been shown to have smaller marginal openings than those made on improved stone dies.6 Epoxy resin dies have been found to be generally undersized, pri.marily because of polymerization shrinkage.’ The manufacturer of the new generation epoxy resin die systern claims to have controlled this shrinkage and that dimensional accuracy can be precisely con$Crispin B. Persoxal communication, 1984. *Assistant Professor, Department of Restorative Dentistry. **Chairman, Department of Restorative Dentistry. ***Director. Advanced Prosthodontic Education. THE JOURNAL OF PROSTHETIC DENTISTRY
Ohio, and University
of Southern
California,
School of
-3Smm
radius
9 9.0mm
P
-
1 4T
2
E E EE 90 04 -F
+ /I3 k3.0mm4P
E Fig. 1. Master die.
trolled by using calibrated base/catalyst ratios and specific heat-treatment cycles. This study evaluates and compares the dimensional accuracy of an improved dental stone die material, silver-plated dies, and a new epoxy resin die material. METHODS
AND
MATERIAL
A master die was made in the shape of the frustum of a cone on a Southbend lathe (Southbend Manufacturing, Southbend, Ind.), model CL 3702D, with a 10 inch X 22 inch swing between centers. A l/2 inch diameter medicalgrade stainless steel rod was machined to a vertical dimension of 12 mm from the cavosurface line angle to the occlusoaxial line angle. The diameter of the die at the cavosurface line angle was 13 mm. The shoulder width was 1.5 mm wide. The convergence angle was 5 degrees, providing a diameter of 9 mm at the occlusal end of the die (Fig. 1). 307
BAILEY,
Table II. Measurement silver-plated dies
Fig. 2. Two lines perpendicular to long axis of die scribed circumferentially into surface of axial wall of die. One line scribed along vertical axis of die. AB = Dimension I; CD = dimension II; EF = dimension III.
Table I. Measurement
values (mm) for
gypsum dies Samples
Dimension I
Dimension II
Dimension III
l-l
10.42
l-2 l-3 l-4 l-5 l-6 l-7 l-8 1-9
10.40 10.49 10.34 10.44 10.45 10.36 10.45 10.42 10.44
6.45 6.39 6.49 6.28 6.48 6.45 6.36 6.41 6.45 6.46
6.46 6.41 6.48 6.34 6.50 6.49 6.35 6.47 6.49 6.50
l-10
Two mutually perpendicular lines intersecting at the center of the occlusal surface and terminating at their intersections with the occlusoaxial line angle were scribed into the occlusal surface. With the intersection of the two scribed occlusal surface lines as its center, a 3.5 mm circle was scribed into the occlusal surface. The four points at which the circle intersected the scribed lines were used to record dimensions in the plane of the occlusal surface by measuring the distance between the two points along each scribed line. Two lines, each in a plane perpendicular to the long axis of the die, were scribed circumferentially into the surface of the axial wall of the die, one line located 1 mm gingivally from the occlusoaxial line angle and the other 1 mm occlusally from the axiogingival line angle. A line was scribed along the vertical axis of the die. This line intersected one of the perpendicular lines crossing the occlusal surface and the two circumferential lines previously scribed into the axial wall. The measurement between the circumferential lines along the vertical axis line is referred to as dimension I. The occlusal line intersecting the vertical axis line is referred to as 308
DONOVAN,
AND
PRESTON
values (mm) for
Sample
Dimension I
Dimension II
Dimension III
2-1 2-2 2-3 2-4 2-5 2-6 2-7 2-8 2-9 2-10
10.43 10.41 10.42 10.39 10.42 10.41 10.32 10.43 10.40 10.44
6.37 6.33 6.44 6.36 6.43 6.37 6.44 6.36 6.44 6.44
6.44 6.48 6.48 6.46 6.45 6.47 6.48 6.48 6.49 6.48
Table III. Measurement
values (mm) for
epoxy resin dies Sample
Dimension I
Dimension II
Dimension III
3-l
10.49 10.34 10.45 10.40 10.45 10.42 10.29 10.41 10.41 10.43 10.38 10.41 10.43 10.39 10.43
6.56 6.46 6.42 6.45 6.47 6.42 6.49 6.41 6.45 6.40 6.46 6.50 6.44 6.50 6.49
6.47 6.45 6.47 6.50 6.45 6.51 6.50 6.36 6.46 6.53 6.46 6.51 6.44 6.50 6.54
3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-9 3-10 3-11 3-12 3-13 3-14 3-15
dimension II, and the occlusal line perpendicular to dimension II is referred to as dimension III. Fig. 2 represents these dimensions and their orientation to one another. The measurements reported for the master die are the mean of five measurements taken for each of the three dimensions. Three groups of dies were made as follows: Group 1. Ten dies were made from impressions of the machined master die. These impressions were made in an acrylic resin custom tray (Fastray, H.J. Bosworth, Skokie, Ill.) relieved with 2 mm of Kerr No. 8 baseplate wax (Kerr/Sybron, Romulus, Mich.) by using a vinyl polysiloxane impression material (Reprosil, L.D. Caulk Co., Milford, Del.). Retention for the impression material in the resin trays was provided by random placement of mechanical retention with No. 6 steel carbide bur. The impressions were allowed to stand at room temperature for 7 hours to provide for hydrogen gas release. Improved dental stone (Die Keen, Columbus Dental Products, St. Louis, MO.) was mixed under 30 lb vacuum with a 120~second spatulation time and a MARCH
1988
VOLUME
59
NUMBER
3
ACCURACY
OF ALTERNATIVE
DIE MATERIALS
Table IV. Master die measurements
and mean values (mm) for three die systems
SD
x
0.044 0.034 0.048
6.42 6.40 6.46
57 Master Group Group Group
die I. Gypsum dies II. Silver-plated dies III. Epoxy resin dies
Table V. Coefficient groups I through
Group I Group II Group III
of variance
10.39 10.42 10.41 10.41
standardized
Dimension I
Dimension II
Dimension III
Mean
0.424 0.322 0.462
0.992 0.708 0.666
0.932 0.241 0.684
0.78 0.42 0.60
DENTISTRY
x
0.064 0.045 0.043
6.45 6.47 6.48
SD 6.50 0.060 0.016 0.044
Table VI. Diameter measurements (mm> and
(%) for
III
OF PROSTHETIC
SD 6.43
water-to-powder ratio of 17 ml to 50 g. The stone was vibrated into the impression and allowed to set for 1 hour before removal for measurement of the gypsum dies. Group II. Ten dies were made from impressions made as in group I. These impressions were silverplated by using an automatic silver-plating unit (model 112, Yarter-Tek, Inc., Denver, Colo.). The plating solution, Silver-Tek Platabond (Yarter-Tek, Inc.), was prepared to the manufacturer’s specifications. A current of 0.05 amps per impression was applied for 1 hour to flash-coat the impressions followed by 0.10 amps per impression for 12 hours. By the salt and pepper method, methyl methacrylate resin (Duralay Resin, Reliance Mfg. Co.; Worth, Ill.) was placed into the plated impressions to support the silver-plating. Group III. Fifteen epoxy resin dies (Cerestore Epoxy Resin, Johnson & Johnson Co. Inc., East Windsor, N. J.) were made from impressions made as in group I. The impressions were sprayed from a distance of 6 inches wit.h Cerestore Epoxy Die, Separator (Johnson & Johnson Co. Inc.) and allowed to dry until a white residue was evident on the impression surface. The epoxy resin liquid A and powder B were incorporated to a homogenous mixture by using the Cerestore mixing instruments. The predetermined catalyst ratio was introduced into the resin by micropipette (microliter No. 170, Hamilton Co., Reno, Nev.) to the manufacturer’s specification for each lot of resin used. This catalyst and resin mixture was stirred with a metal spatula for 15 seconds to allow an even distribution of the catalyst throughout the resin and the mixture was placed in a plastic cup, under a vacuum of 30 lbs of pressure in a desiccator jar. The resin mixture was allowd to remain in the desiccator jar for an additional 10 seconds after the resin rose, broke, and fell. The resin was then painted on the entire THE JOURNAL
Dimension III
Dimension II
Dimension I
Master Group Group Group
die I II III
optical error Vertical height
Total diameter
10.39 10.42 10.41 10.41
12.93 12.87 12.87 12.94
impression surface with a No. 3 brush, and the remaining resin was poured into the impression to a base thickness of ‘/ inch. The dies were cured at room temperature for 12 hours after which they were removed from the impressions. The dies were then placed in a laboratory oven (model LO-21OC, Grieve Corporation, Round Lake, Ill.) at room temperature and cured for 2 hours at 160” C (320” F). The dies were removed from the oven at the end of the 2-hour cycle and allowed to cool before they were labeled and measured. All of the dies were measured by using a Spencer microscope (model 59-J-D2, A0 Instrument Co., Buffalo, N.Y.) equipped with a metric micrometer vernier scale. A resin reference platform was used to orient all of the dies in the same manner in the microscope to minimize optical error. The measurements were made at X80 magnification with an indirect light producing 6500“ K true daylight. An investigator qualified in the measuring instrumentation, but not involved with the study, did the measuring of all of the specimens.
RESULTS The master die measurement values were 10.39 mm, dimension I, for the vertical height; 6.43 mm, dimension II, for occlusal surface measurement intersecting the vertical height measurement; and 6.50 mm, dimension III, for the occlusal surface measurement perpendicular to dimension II. The measurement values for the three die systems evaluated are reported in Table I for the gypsum die system, Table II for the silver-plated die system, and Table III for the epoxy resin die system. The master die measurements and mean values with their standard deviations for the three die systems studied are reported in Table IV. The differences in dimensions I, II, and III measure309
BAILEY,
ments for the three die systems were not statistically significant. The coefficient of variances computed for the three measurements are reported in Table V.
310
AND
PRESTON
CONCLUSIONS Improved dental stone dies had the greatest variation in measurements of the three die systems examined, epoxy resin dies were next, and silver-plated dies showed the least variation. No statistical difference was found in the accuracy of the three systems evaluated. Silver-plated and epoxy resin die systems are acceptable alternative systems to the improved dental stone dies When used as they were in this study. Additional studies are needed to compare the systems for detail reproduction, hardness, and abrasion resistance.
DISCUSSION The optical error in aligning the cross hairs of the microscope is responsible for the offset in the diameter measurements noted between dimensions I and II for all of the systems evaluated. By restricting the measurement process to one investigator, who followed the same measuring procedure for all the dies, this optical error was limited to as consistent an error as possible for the technique used. In the evaluation of dimension I, all three systems reproduced the master die with the same degree of dimensional accuracy. Table VI combines the two diameter measurements for standardization of the optical error. The uniformity with which the three systems reproduced the master die becomes evident in the comparison of these totals. In the correlation of the variance in the three systems, the silver-plated die system recorded the least amount of variation (0.42%) in the three dimensions measured. The epoxy resin system was next with an average of 0.60% and the gypsum die system recorded the greatest variation with an average of 0.78%. The alternative die systems evaluated reproduced the master die with the same degree of dimensional accuracy as the widely used gypsum die system. These data show that the silverplated and epoxy resin systems will provide a dimensional reproduction that is in the same expansion range as the improved dental stone. This finding will permit the use of these alternative systems without having to change wax pattern and casting techniques presently used with the gypsum die systems when they are combined with a vinyl polysiloxane impression material such as the one used in this study. Because both systems have improved fine-detail reproduction, strength, and abrasion resistance compared with improved stone dies,’ increased use of these systems seems logical.
DONOVAN,
We express our appreciation to Ms. Yoshiko Findley and Dr. Robert Nakamura for their technical and statistical assistance.
REFERENCES Gettleman L, Ryge G. Accuracy of stone, metal, and plastic die materials. J Calif Dent Assoc 1970;46:28-31. 2. Hollenback GM, Smith DD. A further investigation of the physical properties of hard gypsum. J Calif Dent Assoc 1976;43:221-7. 3. Phillips RW, Schnell RJ. Electroformed dies from Thiokol and silicone impressions. J PROSTHET DENT 1958;8:992-1002. 4. Toreskog S, Phillips RW, Schnell RJ. Properties of die materials: a comparative study. J PROSTHET DENT 1966;16: 119I.
31.
5. 6. 7. 8.
Cooney JP. A comparison of silver-plated and stone dies from rubber-base impressions. J PROSTHET DENT 1974;32:262-6. Plekavich EJ, Joncas JM. The effects of impression-die systems on crown margins. J PROSTHET DENT 1983;49:772-6. Nomura GT, Reisbick MH, Preston JD. An investigation of epoxy resin dies. J PR~STHET DENT 1980;44:45-50. Craig RG. Restorative dental materials. 6th ed. St Louis: The CV Mosby Co, 1980;221.
Reprint requests to: DR. JOHN H. BAILEY THE OHIO STATE UNIVERXT~ COLLEGE OF DENTISTRY COLUMBUS, OH 43210-1241
MARCH
1988
VOLUME
59
NUMBER
3
D.D.S.,*
T. E. Donovan
D.D.S.,**
The Ohio State University, College of Dentistry, Dentistry, Los Angeles, Calif.
and J. D. Preston, D.D.S.***
Columbus,
1 mproved dental stone is the predominant material in dentistry for making dies used in the lost-wax process. Although improved dental stones have served the profession well, the main disadvantages of their use as dies are (1) susceptibility to dimensional change due to abrasion, (2) limited reproduction of fine detail,’ and (3) lack of strength. The lack of strength can result in breakage of dies, especially those duplicating long, narrow preparations. In the fabrication of all-porcelain labial margins or some of the new generation cast porcelain restorations, dies that exhibit greater strength and abrasion resistance than gypsum dies are desirable. Alternate die materials that may yield dimensionally accurate dies, are more resistant to abrasion, and are stronger than the improved dental stone dies presently in use are available. The literature concerning dimensional accuracy of various die materials is somewhat equivocal. Gypsum die materials generally exhibit an average setting expansion of O.l%.* Such minimal expansion is thought to be beneficial in terms of aiding compensation for metal shrinkage, wax pattern dimensional change, and other unavoida.ble inaccuracies in the casting process. Electroplated dies have been found to be less accurate dimensionally than improved stone dies when formed with polysulfide rubber or condensation-reaction silicone impression m.aterials.3 One study demonstrated that silver-plated dies tend to be undersized by as much as 0.2%; whereas another found that they tend to be oversized.5 It has been shown that vinyl polysiloxane impression ma.terials are the most accurate for making silver-plated dies.+ Castings made on silver-plated dies have been shown to have smaller marginal openings than those made on improved stone dies.6 Epoxy resin dies have been found to be generally undersized, pri.marily because of polymerization shrinkage.’ The manufacturer of the new generation epoxy resin die systern claims to have controlled this shrinkage and that dimensional accuracy can be precisely con$Crispin B. Persoxal communication, 1984. *Assistant Professor, Department of Restorative Dentistry. **Chairman, Department of Restorative Dentistry. ***Director. Advanced Prosthodontic Education. THE JOURNAL OF PROSTHETIC DENTISTRY
Ohio, and University
of Southern
California,
School of
-3Smm
radius
9 9.0mm
P
-
1 4T
2
E E EE 90 04 -F
+ /I3 k3.0mm4P
E Fig. 1. Master die.
trolled by using calibrated base/catalyst ratios and specific heat-treatment cycles. This study evaluates and compares the dimensional accuracy of an improved dental stone die material, silver-plated dies, and a new epoxy resin die material. METHODS
AND
MATERIAL
A master die was made in the shape of the frustum of a cone on a Southbend lathe (Southbend Manufacturing, Southbend, Ind.), model CL 3702D, with a 10 inch X 22 inch swing between centers. A l/2 inch diameter medicalgrade stainless steel rod was machined to a vertical dimension of 12 mm from the cavosurface line angle to the occlusoaxial line angle. The diameter of the die at the cavosurface line angle was 13 mm. The shoulder width was 1.5 mm wide. The convergence angle was 5 degrees, providing a diameter of 9 mm at the occlusal end of the die (Fig. 1). 307
BAILEY,
Table II. Measurement silver-plated dies
Fig. 2. Two lines perpendicular to long axis of die scribed circumferentially into surface of axial wall of die. One line scribed along vertical axis of die. AB = Dimension I; CD = dimension II; EF = dimension III.
Table I. Measurement
values (mm) for
gypsum dies Samples
Dimension I
Dimension II
Dimension III
l-l
10.42
l-2 l-3 l-4 l-5 l-6 l-7 l-8 1-9
10.40 10.49 10.34 10.44 10.45 10.36 10.45 10.42 10.44
6.45 6.39 6.49 6.28 6.48 6.45 6.36 6.41 6.45 6.46
6.46 6.41 6.48 6.34 6.50 6.49 6.35 6.47 6.49 6.50
l-10
Two mutually perpendicular lines intersecting at the center of the occlusal surface and terminating at their intersections with the occlusoaxial line angle were scribed into the occlusal surface. With the intersection of the two scribed occlusal surface lines as its center, a 3.5 mm circle was scribed into the occlusal surface. The four points at which the circle intersected the scribed lines were used to record dimensions in the plane of the occlusal surface by measuring the distance between the two points along each scribed line. Two lines, each in a plane perpendicular to the long axis of the die, were scribed circumferentially into the surface of the axial wall of the die, one line located 1 mm gingivally from the occlusoaxial line angle and the other 1 mm occlusally from the axiogingival line angle. A line was scribed along the vertical axis of the die. This line intersected one of the perpendicular lines crossing the occlusal surface and the two circumferential lines previously scribed into the axial wall. The measurement between the circumferential lines along the vertical axis line is referred to as dimension I. The occlusal line intersecting the vertical axis line is referred to as 308
DONOVAN,
AND
PRESTON
values (mm) for
Sample
Dimension I
Dimension II
Dimension III
2-1 2-2 2-3 2-4 2-5 2-6 2-7 2-8 2-9 2-10
10.43 10.41 10.42 10.39 10.42 10.41 10.32 10.43 10.40 10.44
6.37 6.33 6.44 6.36 6.43 6.37 6.44 6.36 6.44 6.44
6.44 6.48 6.48 6.46 6.45 6.47 6.48 6.48 6.49 6.48
Table III. Measurement
values (mm) for
epoxy resin dies Sample
Dimension I
Dimension II
Dimension III
3-l
10.49 10.34 10.45 10.40 10.45 10.42 10.29 10.41 10.41 10.43 10.38 10.41 10.43 10.39 10.43
6.56 6.46 6.42 6.45 6.47 6.42 6.49 6.41 6.45 6.40 6.46 6.50 6.44 6.50 6.49
6.47 6.45 6.47 6.50 6.45 6.51 6.50 6.36 6.46 6.53 6.46 6.51 6.44 6.50 6.54
3-2 3-3 3-4 3-5 3-6 3-7 3-8 3-9 3-10 3-11 3-12 3-13 3-14 3-15
dimension II, and the occlusal line perpendicular to dimension II is referred to as dimension III. Fig. 2 represents these dimensions and their orientation to one another. The measurements reported for the master die are the mean of five measurements taken for each of the three dimensions. Three groups of dies were made as follows: Group 1. Ten dies were made from impressions of the machined master die. These impressions were made in an acrylic resin custom tray (Fastray, H.J. Bosworth, Skokie, Ill.) relieved with 2 mm of Kerr No. 8 baseplate wax (Kerr/Sybron, Romulus, Mich.) by using a vinyl polysiloxane impression material (Reprosil, L.D. Caulk Co., Milford, Del.). Retention for the impression material in the resin trays was provided by random placement of mechanical retention with No. 6 steel carbide bur. The impressions were allowed to stand at room temperature for 7 hours to provide for hydrogen gas release. Improved dental stone (Die Keen, Columbus Dental Products, St. Louis, MO.) was mixed under 30 lb vacuum with a 120~second spatulation time and a MARCH
1988
VOLUME
59
NUMBER
3
ACCURACY
OF ALTERNATIVE
DIE MATERIALS
Table IV. Master die measurements
and mean values (mm) for three die systems
SD
x
0.044 0.034 0.048
6.42 6.40 6.46
57 Master Group Group Group
die I. Gypsum dies II. Silver-plated dies III. Epoxy resin dies
Table V. Coefficient groups I through
Group I Group II Group III
of variance
10.39 10.42 10.41 10.41
standardized
Dimension I
Dimension II
Dimension III
Mean
0.424 0.322 0.462
0.992 0.708 0.666
0.932 0.241 0.684
0.78 0.42 0.60
DENTISTRY
x
0.064 0.045 0.043
6.45 6.47 6.48
SD 6.50 0.060 0.016 0.044
Table VI. Diameter measurements (mm> and
(%) for
III
OF PROSTHETIC
SD 6.43
water-to-powder ratio of 17 ml to 50 g. The stone was vibrated into the impression and allowed to set for 1 hour before removal for measurement of the gypsum dies. Group II. Ten dies were made from impressions made as in group I. These impressions were silverplated by using an automatic silver-plating unit (model 112, Yarter-Tek, Inc., Denver, Colo.). The plating solution, Silver-Tek Platabond (Yarter-Tek, Inc.), was prepared to the manufacturer’s specifications. A current of 0.05 amps per impression was applied for 1 hour to flash-coat the impressions followed by 0.10 amps per impression for 12 hours. By the salt and pepper method, methyl methacrylate resin (Duralay Resin, Reliance Mfg. Co.; Worth, Ill.) was placed into the plated impressions to support the silver-plating. Group III. Fifteen epoxy resin dies (Cerestore Epoxy Resin, Johnson & Johnson Co. Inc., East Windsor, N. J.) were made from impressions made as in group I. The impressions were sprayed from a distance of 6 inches wit.h Cerestore Epoxy Die, Separator (Johnson & Johnson Co. Inc.) and allowed to dry until a white residue was evident on the impression surface. The epoxy resin liquid A and powder B were incorporated to a homogenous mixture by using the Cerestore mixing instruments. The predetermined catalyst ratio was introduced into the resin by micropipette (microliter No. 170, Hamilton Co., Reno, Nev.) to the manufacturer’s specification for each lot of resin used. This catalyst and resin mixture was stirred with a metal spatula for 15 seconds to allow an even distribution of the catalyst throughout the resin and the mixture was placed in a plastic cup, under a vacuum of 30 lbs of pressure in a desiccator jar. The resin mixture was allowd to remain in the desiccator jar for an additional 10 seconds after the resin rose, broke, and fell. The resin was then painted on the entire THE JOURNAL
Dimension III
Dimension II
Dimension I
Master Group Group Group
die I II III
optical error Vertical height
Total diameter
10.39 10.42 10.41 10.41
12.93 12.87 12.87 12.94
impression surface with a No. 3 brush, and the remaining resin was poured into the impression to a base thickness of ‘/ inch. The dies were cured at room temperature for 12 hours after which they were removed from the impressions. The dies were then placed in a laboratory oven (model LO-21OC, Grieve Corporation, Round Lake, Ill.) at room temperature and cured for 2 hours at 160” C (320” F). The dies were removed from the oven at the end of the 2-hour cycle and allowed to cool before they were labeled and measured. All of the dies were measured by using a Spencer microscope (model 59-J-D2, A0 Instrument Co., Buffalo, N.Y.) equipped with a metric micrometer vernier scale. A resin reference platform was used to orient all of the dies in the same manner in the microscope to minimize optical error. The measurements were made at X80 magnification with an indirect light producing 6500“ K true daylight. An investigator qualified in the measuring instrumentation, but not involved with the study, did the measuring of all of the specimens.
RESULTS The master die measurement values were 10.39 mm, dimension I, for the vertical height; 6.43 mm, dimension II, for occlusal surface measurement intersecting the vertical height measurement; and 6.50 mm, dimension III, for the occlusal surface measurement perpendicular to dimension II. The measurement values for the three die systems evaluated are reported in Table I for the gypsum die system, Table II for the silver-plated die system, and Table III for the epoxy resin die system. The master die measurements and mean values with their standard deviations for the three die systems studied are reported in Table IV. The differences in dimensions I, II, and III measure309
BAILEY,
ments for the three die systems were not statistically significant. The coefficient of variances computed for the three measurements are reported in Table V.
310
AND
PRESTON
CONCLUSIONS Improved dental stone dies had the greatest variation in measurements of the three die systems examined, epoxy resin dies were next, and silver-plated dies showed the least variation. No statistical difference was found in the accuracy of the three systems evaluated. Silver-plated and epoxy resin die systems are acceptable alternative systems to the improved dental stone dies When used as they were in this study. Additional studies are needed to compare the systems for detail reproduction, hardness, and abrasion resistance.
DISCUSSION The optical error in aligning the cross hairs of the microscope is responsible for the offset in the diameter measurements noted between dimensions I and II for all of the systems evaluated. By restricting the measurement process to one investigator, who followed the same measuring procedure for all the dies, this optical error was limited to as consistent an error as possible for the technique used. In the evaluation of dimension I, all three systems reproduced the master die with the same degree of dimensional accuracy. Table VI combines the two diameter measurements for standardization of the optical error. The uniformity with which the three systems reproduced the master die becomes evident in the comparison of these totals. In the correlation of the variance in the three systems, the silver-plated die system recorded the least amount of variation (0.42%) in the three dimensions measured. The epoxy resin system was next with an average of 0.60% and the gypsum die system recorded the greatest variation with an average of 0.78%. The alternative die systems evaluated reproduced the master die with the same degree of dimensional accuracy as the widely used gypsum die system. These data show that the silverplated and epoxy resin systems will provide a dimensional reproduction that is in the same expansion range as the improved dental stone. This finding will permit the use of these alternative systems without having to change wax pattern and casting techniques presently used with the gypsum die systems when they are combined with a vinyl polysiloxane impression material such as the one used in this study. Because both systems have improved fine-detail reproduction, strength, and abrasion resistance compared with improved stone dies,’ increased use of these systems seems logical.
DONOVAN,
We express our appreciation to Ms. Yoshiko Findley and Dr. Robert Nakamura for their technical and statistical assistance.
REFERENCES Gettleman L, Ryge G. Accuracy of stone, metal, and plastic die materials. J Calif Dent Assoc 1970;46:28-31. 2. Hollenback GM, Smith DD. A further investigation of the physical properties of hard gypsum. J Calif Dent Assoc 1976;43:221-7. 3. Phillips RW, Schnell RJ. Electroformed dies from Thiokol and silicone impressions. J PROSTHET DENT 1958;8:992-1002. 4. Toreskog S, Phillips RW, Schnell RJ. Properties of die materials: a comparative study. J PROSTHET DENT 1966;16: 119I.
31.
5. 6. 7. 8.
Cooney JP. A comparison of silver-plated and stone dies from rubber-base impressions. J PROSTHET DENT 1974;32:262-6. Plekavich EJ, Joncas JM. The effects of impression-die systems on crown margins. J PROSTHET DENT 1983;49:772-6. Nomura GT, Reisbick MH, Preston JD. An investigation of epoxy resin dies. J PR~STHET DENT 1980;44:45-50. Craig RG. Restorative dental materials. 6th ed. St Louis: The CV Mosby Co, 1980;221.
Reprint requests to: DR. JOHN H. BAILEY THE OHIO STATE UNIVERXT~ COLLEGE OF DENTISTRY COLUMBUS, OH 43210-1241
MARCH
1988
VOLUME
59
NUMBER
3