D e t e r m i n a t i o n of the a c c u r a c y of t h r e e die s y s t e m s T. J. B l o e m , D . D . S . , M.S.,* B. C z e r n i a w s k i ,
D.D.S.,** J. L u k e , D.D.S.,** a n d
B. R. L a n g , D.D.S., M.S.*** University of Michigan, School of Dentistry, Ann Arbor, Mich. This article reports the findings of two i n t e r r e l a t e d projects that m e a s u r e d the accuracy of electroplated, e p o x y resin, and p o l y u r e t h a n e die m a t e r i a l s a g a i n s t a m a s t e r tooth, u s i n g the Michigan C o m p u t e r Graphics Coordinate M e a s u r i n g S y s t e m (MCGCMS). In the first phase, on occlusal die morphology, replications of a m a s t e r tooth w e r e u s e d to v a l i d a t e the MCGCMS reliability. E i g h t e e n total data sets d e r i v e d from three s i l v e r - p l a t e d dies and three e p o x y resin dies w e r e then c o m p a r e d w i t h a r e p r e s e n t a t i v e data set o f the m a s t e r tooth. The m e a n differences for all s i l v e r - p l a t e d die and e p o x y die m e a s u r e m e n t s a g a i n s t the m a s t e r ranged from - 4 2 to - 4 9 and - 4 6 to - 2 3 0 u m / m m 2, r e s p e c t i v e l y . In the second phase, t w o e p o x y die m a t e r i a l s and a p o l y u r e t h a n e s y s t e m w e r e c o m p a r e d w i t h a m a s t e r tooth in an identical manner. In paired c o m p a r i s o n s to the m a s t e r tooth, m e a n differences for E p o x y d e n t material, e p o x y resin, and p o l y u r e t h a n e d i e s r a n g e d from - 1 4 7 to - 1 5 3 , - 1 4 9 to - 2 3 0 , and - 5 6 to - 1 6 8 u m / m m 2, r e s p e c t i v e l y . A Student t test using a CLR ANOVA w a s p e r f o r m e d b e t w e e n the m a s t e r tooth and the five die s y s t e m s . A significant difference w a s r e v e a l e d b e t w e e n the e n a m e l - s i l v e r difference and the e n a m e l - E p o x y d e n t difference data at the 95% confidence level. A significant difference also occurred b e t w e e n e n a m e l - s i l v e r and e n a m e l - e p o x y resin. It w a s concluded that s i l v e r - p l a t e d d i e s are a c c e p t a b l e replicas o f a m a s t e r tooth. E p o x y d e n t and e p o x y r e s i n d i e s did not provide reliable replications w h e n compared w i t h the s i l v e r - p l a t e d dies. E p o x y and p o l y u r e t h a n e replications, a l t h o u g h reported as accurate, w e r e inconclusive. (J PROSTHET DENT 1991;65:758-62.)
A c c u r a t e replication of a prepared tooth surface is essential for the indirect method of die fabrication and eventual restoration of a tooth. Although numerous studies have reported the accuracy of impression materials, TM literature on the comparative accuracy of die systems is more limited.5, 6 Although there is no ideal die material in terms of all desirable qualities and compatibility with all impression materials, 7 the desirable qualities of a die system should include accuracy, dimensional stability, reproduction of fine detail, durability, and ease in fabrication. Studies of electroplated dies have revealed various results d e p e n d e n t on the types of impression materials and the methods of die measurement used. 81° Investigations on the accuracy of epoxy resin dies have used subjective as well as objective measurement techniques. 1M3 The results of these studies invite varying interpretations because of the subjective nature of examiner assessment or the lack of specific detail in reporting quantitative measurement technique.
Supported in part by the National Institute of Dental Research grant DE06720. Presented at the American Association for Dental Research meeting, San Francisco, Calif. *Assistant Professor, Department of Prosthodontics. **Former prosthodontic resident. ***Professor and Chair, Department of Prosthodontics. 10/1/27548
758
This article reports the findings of two interrelated studies t h a t measured the accuracy of replication with the use of electroplated silver, epoxy resin, and polyurethane die materials. A computer graphics coordinated measuring system was used to measure differences. This system provided accurate three-dimensional contour and positional measurements at the micron levelJ 4,15 The specific test objectives were as follows. 1. To determine the differences in the occlusal surface contours of silver-plated, epoxy resin, and polyurethane dies created from a master tooth and to compare the dies to the master tooth 2. To compare data obtained from the die and master tooth comparisons and to determine whether significant differences exist among the different die systems studied
MATERIAL A N D M E T H O D S First, a well-formed natural molar was modified to provide the master tooth (Fig. 1). Four recesses were placed on the cusp tips to aid in establishing the orientation plane both for accuracy and for replication of the measurements at successive digitizing sessions (Fig. 2). The master tooth was retained on the digitizing stage and was digitized three times. The digitized d a t a for the three measurements of the master tooth were compared by use of a paired-comparison matrix as a measure of the accuracy and reliability of the Michigan Computer Graphics Coordinate Measuring System (MCGCMS). The range of differences between the measurements of the master tooth was 2 to 6 u m / m m 2. This was a small difference over an area of approximately 13
JUNE 1991
VOLUME 65
NUMBER 6
A C C U R A C Y OF T H R E E D I E S Y S T E M S
Fig. 1. Enamel master tooth.
Fig. 3. Representative resin replica.
Fig. 2. Master tooth digitization.
mm 2 representing approximately 14,000 data points. After the analysis was done, one of the data sets from the master tooth recordings was selected as the baseline data for all subsequent comparisons between the enamel master tooth and the die systems being investigated. Second, a standardized impression technique was used to create the dies. A custom autopolymerized acrylic resin tray was made with adequate relief and repeatable positioning with respect to the natural master tooth. A lightbodied polyvinyl siloxane addition-type silicone (President, Coltene AG, Switzerland) was used to produce three impressions for each of the die materials used. Setting time and time to start of die fabrication was standardized. Dies of the master tooth were made by use of a routine silver electroplating technique (Barnett Products, North Miami Beach, Fla.); Ortho-Bond resin (Vernon-Benshoff, Albany, N.Y.); Epoxydent epoxy resin (Oxy Dental Products, Newark, New Jersey); George Taub epoxy die material (George Taub Products and Fusion Co., Inc., Jersey City, New Jersey); and Alpha Die polyurethane die material (Schutz-Dental, Munich, Germany). Resin dies were made with a pour technique, except for the Epoxydent material,
T H E J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
Fig. 4. Graphic plot of replica in phase one.
which was centrifuged. All of the products were handled according to manufacturers' specifications. The maximum time lapse from die fabrication to the initiation of the digitization was 24 hours. All of the mounted dies were handled according to manufacturers' specifications. The maximum time lapse from die fabrication to the initiation of the digitization was 24 hours. The dies were mounted in the milled framing device on the digitizing table for measurement and stored in a humidifier when not in use (Fig. 3). Measurement of the various dies followed the procedure used with the enamel master tooth and previously reportedj4,15 The orientation plane was recorded by use of four reference recesses that had been previously placed. The probe for the MX203 was carefully positioned into each recess and the X, Y, and Z coordinate values were recorded through the computer program to establish the parameters for the plane mathematically. Repeated measure-
759
BLOEM
7 • •
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F i g . 5. Graphic plot of replica in phase two.
F i g . 7. X and Y alignment of paired scan lines before "best, fit."
F i g . 6. Graphic plot of common surface between phase one and phase two samples.
tablished by the parameters of 0.01 mm by 0.1 mm. Each block was divided into two triangular planes and an algorithm was used to calculate the volume within the blocks. Comparing the volume differences in each block provided the basis for the differences between each sample. The total volume difference between the two samples was calculated by adding up the differences in the individual blocks. The transformation of the differences between the two samples under study, from volume to microns per unit surface, was determined by dividing the volume difference by the total area of the samples being compared. Comparing the information on die contour from the two portions of the study required the analysis of the same surface of the master tooth and die replicas. A three-dimensional plot of the master tooth shows the part used for the first phase of the study, which compared the master tooth to the silver die and an epoxy die (Fig. 4). As seen in another plot, a different surface of the master tooth in relation to the center of the sample, indicated by a "spike," was evaluated for the second phase (Fig. 5). In t h a t study, the master tooth was compared with Epoxydent, epoxy resin, and polyurethane. The "shift" represented approximately 1 mm. To evaluate the different die samples from each phase of the study, it was necessary to establish a common area of d a t a measured in each of the two phases. This is represented in a third plot (Fig. 6). The common area selected was a 3 m m 2 surface t h a t provided 3000 d a t a points for each sample. The raw d a t a collected for each die within this newly defined surface were resampled to provide the d a t a base for this investigation. This surface was somewhat complex, in t h a t it included portions of the central groove
ments and replication of the plane in the M C G C M S have been established to within __5 tim. X and Z values of the dies' surfaces, over an area common to all dies, were recorded every 0.01 m m for a distance of 4 m m along a series of Y axes 0.1 m m apart. Control in the recording of d a t a at each 0.01 m m was provided by the computer program and its interface with a motion controller. A total of 36 scans were made for each die. The d a t a collected at each digitizing session were integrated with a plotting routine. Individual scan plots for three-dimensional graphic representation of each sample were made. The common surfaces of any two samples were compared by dividing the surfaces surveyed into a series of blocks es-
760
JUNE
1991
VOLUME
65
NUMBER
6
I
ACCURACY OF THREE DIE SYSTEMS
Table I. Mean difference values (from paired comparisons) for each replica group compared with master tooth Material
Difference
Enamel NB-silverl
-49
Enamel NB-silver2
-42
Enamel NB-silver3
-4'7
Enamel NB-epoxyl
-46
Enamel NB-epoxy2
-2;30
Enamel NB-epoxy3
-141
Material
Enamel NBEpoxydentl Enamel NBEpoxydent2 Enamel NBEpoxydent3
• •
. .
. •
i i
. .
. .
. .
. .
i w
. .
. .
. .
. .
i i
. .
. .
. .
. .
i i
. .
. .
. .
vs. . .
i J
X . .
. .
. .
. .
i i
. .
. .
. .
. .
L r
. •
Difference
-153 -149 Z
-147
Enamel NBepoxy resinl Enamel NBepoxy resin2 Enamel NBepoxy resin3
-149
Enamel NBpolyurethane 1 Enamel NBpolyurethane2 Enamel NBpolyurethane3
-56
vs.
Scan
line
-230 -197
F i g . 8. X and Y alignment of paired scan lines after best fit.
-168 -168
Epoxy (George Taub); epoxy resin (Ortho Bond); polyurethane {Alpha Die).
of the master tooth as well as supplemental fissures and contours. Although this surface was more complex, the M C G C M S was able to measure it repeatedly. To enhance the d a t a analysis, individual scan line contours were subjected to a "best fit" second-order binomial regression formula, thus eliminating minor discrepancies in the common matrix (Figs. 7 and 8). All comparisons represent differences between the master tooth contours and the specific die systems. Visual comparison of Z-point values was made by observing the graphic representation of specific Y-axis scan lines, b u t numerical comparisons were made by noting the Z values at matching X - Y coordinates from the p r i n t e d hard data. Once a common point within the scanned surfaces of each die was determined, a masking program was used to examine a 1 m m (on the X axis) by 3 m m (on the Y axis) area about t h a t point. The volume of structure difference over this surface was provided by the computer, expressed as micron difference per square millimeter. RESULTS Paired comparisons between the master tooth and each sample of each die material were made. The mean difference values (from the paired comparisons) for each die group, are presented in Table I. The enamel master tooth when compared with the silver die showed a mean difference of - 4 2 to - 4 9 p m / m m 2. T h e enamel master tooth versus George T a u b epoxy die showed
THE JOURNAL OF PROSTHETIC DENTISTRY
Z
Table II. Mean of mean difference values for each die material Material
Mean
SD
Enamel-silver Enamel-epoxy Enamel-Epoxydent Enamel-epoxy resin Enamel-polyurethane
-46 - 139 - 150 -192 - 131
3.6 92.0 3.1 40.7 64.7
See footnote in Table I.
a mean difference of - 4 6 to - 2 3 0 # m / m m 2. The Epoxydent die showed a mean difference of - 1 4 7 to - 1 5 3 # m / m m 2, whereas the Ortho-Bond epoxy resin die showed - 1 4 9 to - 2 3 0 ~ m / m m 2, and the Alpha Die polyurethane die showed a mean difference of - 5 6 to - 1 6 8 # m / m m 2. The mean of these mean values is reflected in Table II. One should note the large s t a n d a r d deviations for enamelepoxy, enamel-epoxy resin, and enamel-polyurethane. A S t u d e n t t-test using a CLR ANOVA was performed between the master tooth and the five die systems (Table III). The ANOVA reveals a significant difference between the enamel-silver difference and the enamel-Epoxydent difference d a t a at the 95 % confidence level. A significant difference also occurred between enamel-silver and enamel-epoxy resin. DISCUSSION The ANOVA expresses a significant difference between enamel-silver difference values and those for enamelEpoxydent and e n a m e l - O r t h o - B o n d epoxy resin. Epoxy-
761
. ,
BLOEM ET AL
T a b l e III. Differences b e t w e e n five die m a t e r i a l s a n d m a s t e r t o o t h ( S t u d e n t t-test) Variable
Enamel-silver
Enamel-silver Enamel-epoxy Enamel-Epoxydent Enamel-epoxy resin Enamel-polyurethane
Enamel-epoxy
Enamel-Epoxydent
Enamel-epoxy resin
Enamel-polyurethane
0.234
0.000" 0.862
0.029* 0.217 0.230
0.161 0.789 0.674 0.080
See footnote, Table I. *Statistically significant (p _-
D.D.S.,** J. L u k e , D.D.S.,** a n d
B. R. L a n g , D.D.S., M.S.*** University of Michigan, School of Dentistry, Ann Arbor, Mich. This article reports the findings of two i n t e r r e l a t e d projects that m e a s u r e d the accuracy of electroplated, e p o x y resin, and p o l y u r e t h a n e die m a t e r i a l s a g a i n s t a m a s t e r tooth, u s i n g the Michigan C o m p u t e r Graphics Coordinate M e a s u r i n g S y s t e m (MCGCMS). In the first phase, on occlusal die morphology, replications of a m a s t e r tooth w e r e u s e d to v a l i d a t e the MCGCMS reliability. E i g h t e e n total data sets d e r i v e d from three s i l v e r - p l a t e d dies and three e p o x y resin dies w e r e then c o m p a r e d w i t h a r e p r e s e n t a t i v e data set o f the m a s t e r tooth. The m e a n differences for all s i l v e r - p l a t e d die and e p o x y die m e a s u r e m e n t s a g a i n s t the m a s t e r ranged from - 4 2 to - 4 9 and - 4 6 to - 2 3 0 u m / m m 2, r e s p e c t i v e l y . In the second phase, t w o e p o x y die m a t e r i a l s and a p o l y u r e t h a n e s y s t e m w e r e c o m p a r e d w i t h a m a s t e r tooth in an identical manner. In paired c o m p a r i s o n s to the m a s t e r tooth, m e a n differences for E p o x y d e n t material, e p o x y resin, and p o l y u r e t h a n e d i e s r a n g e d from - 1 4 7 to - 1 5 3 , - 1 4 9 to - 2 3 0 , and - 5 6 to - 1 6 8 u m / m m 2, r e s p e c t i v e l y . A Student t test using a CLR ANOVA w a s p e r f o r m e d b e t w e e n the m a s t e r tooth and the five die s y s t e m s . A significant difference w a s r e v e a l e d b e t w e e n the e n a m e l - s i l v e r difference and the e n a m e l - E p o x y d e n t difference data at the 95% confidence level. A significant difference also occurred b e t w e e n e n a m e l - s i l v e r and e n a m e l - e p o x y resin. It w a s concluded that s i l v e r - p l a t e d d i e s are a c c e p t a b l e replicas o f a m a s t e r tooth. E p o x y d e n t and e p o x y r e s i n d i e s did not provide reliable replications w h e n compared w i t h the s i l v e r - p l a t e d dies. E p o x y and p o l y u r e t h a n e replications, a l t h o u g h reported as accurate, w e r e inconclusive. (J PROSTHET DENT 1991;65:758-62.)
A c c u r a t e replication of a prepared tooth surface is essential for the indirect method of die fabrication and eventual restoration of a tooth. Although numerous studies have reported the accuracy of impression materials, TM literature on the comparative accuracy of die systems is more limited.5, 6 Although there is no ideal die material in terms of all desirable qualities and compatibility with all impression materials, 7 the desirable qualities of a die system should include accuracy, dimensional stability, reproduction of fine detail, durability, and ease in fabrication. Studies of electroplated dies have revealed various results d e p e n d e n t on the types of impression materials and the methods of die measurement used. 81° Investigations on the accuracy of epoxy resin dies have used subjective as well as objective measurement techniques. 1M3 The results of these studies invite varying interpretations because of the subjective nature of examiner assessment or the lack of specific detail in reporting quantitative measurement technique.
Supported in part by the National Institute of Dental Research grant DE06720. Presented at the American Association for Dental Research meeting, San Francisco, Calif. *Assistant Professor, Department of Prosthodontics. **Former prosthodontic resident. ***Professor and Chair, Department of Prosthodontics. 10/1/27548
758
This article reports the findings of two interrelated studies t h a t measured the accuracy of replication with the use of electroplated silver, epoxy resin, and polyurethane die materials. A computer graphics coordinated measuring system was used to measure differences. This system provided accurate three-dimensional contour and positional measurements at the micron levelJ 4,15 The specific test objectives were as follows. 1. To determine the differences in the occlusal surface contours of silver-plated, epoxy resin, and polyurethane dies created from a master tooth and to compare the dies to the master tooth 2. To compare data obtained from the die and master tooth comparisons and to determine whether significant differences exist among the different die systems studied
MATERIAL A N D M E T H O D S First, a well-formed natural molar was modified to provide the master tooth (Fig. 1). Four recesses were placed on the cusp tips to aid in establishing the orientation plane both for accuracy and for replication of the measurements at successive digitizing sessions (Fig. 2). The master tooth was retained on the digitizing stage and was digitized three times. The digitized d a t a for the three measurements of the master tooth were compared by use of a paired-comparison matrix as a measure of the accuracy and reliability of the Michigan Computer Graphics Coordinate Measuring System (MCGCMS). The range of differences between the measurements of the master tooth was 2 to 6 u m / m m 2. This was a small difference over an area of approximately 13
JUNE 1991
VOLUME 65
NUMBER 6
A C C U R A C Y OF T H R E E D I E S Y S T E M S
Fig. 1. Enamel master tooth.
Fig. 3. Representative resin replica.
Fig. 2. Master tooth digitization.
mm 2 representing approximately 14,000 data points. After the analysis was done, one of the data sets from the master tooth recordings was selected as the baseline data for all subsequent comparisons between the enamel master tooth and the die systems being investigated. Second, a standardized impression technique was used to create the dies. A custom autopolymerized acrylic resin tray was made with adequate relief and repeatable positioning with respect to the natural master tooth. A lightbodied polyvinyl siloxane addition-type silicone (President, Coltene AG, Switzerland) was used to produce three impressions for each of the die materials used. Setting time and time to start of die fabrication was standardized. Dies of the master tooth were made by use of a routine silver electroplating technique (Barnett Products, North Miami Beach, Fla.); Ortho-Bond resin (Vernon-Benshoff, Albany, N.Y.); Epoxydent epoxy resin (Oxy Dental Products, Newark, New Jersey); George Taub epoxy die material (George Taub Products and Fusion Co., Inc., Jersey City, New Jersey); and Alpha Die polyurethane die material (Schutz-Dental, Munich, Germany). Resin dies were made with a pour technique, except for the Epoxydent material,
T H E J O U R N A L OF P R O S T H E T I C D E N T I S T R Y
Fig. 4. Graphic plot of replica in phase one.
which was centrifuged. All of the products were handled according to manufacturers' specifications. The maximum time lapse from die fabrication to the initiation of the digitization was 24 hours. All of the mounted dies were handled according to manufacturers' specifications. The maximum time lapse from die fabrication to the initiation of the digitization was 24 hours. The dies were mounted in the milled framing device on the digitizing table for measurement and stored in a humidifier when not in use (Fig. 3). Measurement of the various dies followed the procedure used with the enamel master tooth and previously reportedj4,15 The orientation plane was recorded by use of four reference recesses that had been previously placed. The probe for the MX203 was carefully positioned into each recess and the X, Y, and Z coordinate values were recorded through the computer program to establish the parameters for the plane mathematically. Repeated measure-
759
BLOEM
7 • •
, •
, .
i i
. .
. .
. .
. .
i i
. .
. .
. .
. .
i i
. .
. .
. .
. .
i T
. .
. .
vs. . .
. .
i i
ET
AL
X . .
. .
. .
. .
i i
. .
. .
. .
. .
i i
, •
, •
_
Z .. .. . . . .
~i
.. .. . . . .
,i
vs. .. .. . . . .
Scan J
~
.. .. . . . .
Line ~J
.. .. . . . .
I
I
I
,
,
I
I
I
I
I
F i g . 5. Graphic plot of replica in phase two.
F i g . 7. X and Y alignment of paired scan lines before "best, fit."
F i g . 6. Graphic plot of common surface between phase one and phase two samples.
tablished by the parameters of 0.01 mm by 0.1 mm. Each block was divided into two triangular planes and an algorithm was used to calculate the volume within the blocks. Comparing the volume differences in each block provided the basis for the differences between each sample. The total volume difference between the two samples was calculated by adding up the differences in the individual blocks. The transformation of the differences between the two samples under study, from volume to microns per unit surface, was determined by dividing the volume difference by the total area of the samples being compared. Comparing the information on die contour from the two portions of the study required the analysis of the same surface of the master tooth and die replicas. A three-dimensional plot of the master tooth shows the part used for the first phase of the study, which compared the master tooth to the silver die and an epoxy die (Fig. 4). As seen in another plot, a different surface of the master tooth in relation to the center of the sample, indicated by a "spike," was evaluated for the second phase (Fig. 5). In t h a t study, the master tooth was compared with Epoxydent, epoxy resin, and polyurethane. The "shift" represented approximately 1 mm. To evaluate the different die samples from each phase of the study, it was necessary to establish a common area of d a t a measured in each of the two phases. This is represented in a third plot (Fig. 6). The common area selected was a 3 m m 2 surface t h a t provided 3000 d a t a points for each sample. The raw d a t a collected for each die within this newly defined surface were resampled to provide the d a t a base for this investigation. This surface was somewhat complex, in t h a t it included portions of the central groove
ments and replication of the plane in the M C G C M S have been established to within __5 tim. X and Z values of the dies' surfaces, over an area common to all dies, were recorded every 0.01 m m for a distance of 4 m m along a series of Y axes 0.1 m m apart. Control in the recording of d a t a at each 0.01 m m was provided by the computer program and its interface with a motion controller. A total of 36 scans were made for each die. The d a t a collected at each digitizing session were integrated with a plotting routine. Individual scan plots for three-dimensional graphic representation of each sample were made. The common surfaces of any two samples were compared by dividing the surfaces surveyed into a series of blocks es-
760
JUNE
1991
VOLUME
65
NUMBER
6
I
ACCURACY OF THREE DIE SYSTEMS
Table I. Mean difference values (from paired comparisons) for each replica group compared with master tooth Material
Difference
Enamel NB-silverl
-49
Enamel NB-silver2
-42
Enamel NB-silver3
-4'7
Enamel NB-epoxyl
-46
Enamel NB-epoxy2
-2;30
Enamel NB-epoxy3
-141
Material
Enamel NBEpoxydentl Enamel NBEpoxydent2 Enamel NBEpoxydent3
• •
. .
. •
i i
. .
. .
. .
. .
i w
. .
. .
. .
. .
i i
. .
. .
. .
. .
i i
. .
. .
. .
vs. . .
i J
X . .
. .
. .
. .
i i
. .
. .
. .
. .
L r
. •
Difference
-153 -149 Z
-147
Enamel NBepoxy resinl Enamel NBepoxy resin2 Enamel NBepoxy resin3
-149
Enamel NBpolyurethane 1 Enamel NBpolyurethane2 Enamel NBpolyurethane3
-56
vs.
Scan
line
-230 -197
F i g . 8. X and Y alignment of paired scan lines after best fit.
-168 -168
Epoxy (George Taub); epoxy resin (Ortho Bond); polyurethane {Alpha Die).
of the master tooth as well as supplemental fissures and contours. Although this surface was more complex, the M C G C M S was able to measure it repeatedly. To enhance the d a t a analysis, individual scan line contours were subjected to a "best fit" second-order binomial regression formula, thus eliminating minor discrepancies in the common matrix (Figs. 7 and 8). All comparisons represent differences between the master tooth contours and the specific die systems. Visual comparison of Z-point values was made by observing the graphic representation of specific Y-axis scan lines, b u t numerical comparisons were made by noting the Z values at matching X - Y coordinates from the p r i n t e d hard data. Once a common point within the scanned surfaces of each die was determined, a masking program was used to examine a 1 m m (on the X axis) by 3 m m (on the Y axis) area about t h a t point. The volume of structure difference over this surface was provided by the computer, expressed as micron difference per square millimeter. RESULTS Paired comparisons between the master tooth and each sample of each die material were made. The mean difference values (from the paired comparisons) for each die group, are presented in Table I. The enamel master tooth when compared with the silver die showed a mean difference of - 4 2 to - 4 9 p m / m m 2. T h e enamel master tooth versus George T a u b epoxy die showed
THE JOURNAL OF PROSTHETIC DENTISTRY
Z
Table II. Mean of mean difference values for each die material Material
Mean
SD
Enamel-silver Enamel-epoxy Enamel-Epoxydent Enamel-epoxy resin Enamel-polyurethane
-46 - 139 - 150 -192 - 131
3.6 92.0 3.1 40.7 64.7
See footnote in Table I.
a mean difference of - 4 6 to - 2 3 0 # m / m m 2. The Epoxydent die showed a mean difference of - 1 4 7 to - 1 5 3 # m / m m 2, whereas the Ortho-Bond epoxy resin die showed - 1 4 9 to - 2 3 0 ~ m / m m 2, and the Alpha Die polyurethane die showed a mean difference of - 5 6 to - 1 6 8 # m / m m 2. The mean of these mean values is reflected in Table II. One should note the large s t a n d a r d deviations for enamelepoxy, enamel-epoxy resin, and enamel-polyurethane. A S t u d e n t t-test using a CLR ANOVA was performed between the master tooth and the five die systems (Table III). The ANOVA reveals a significant difference between the enamel-silver difference and the enamel-Epoxydent difference d a t a at the 95 % confidence level. A significant difference also occurred between enamel-silver and enamel-epoxy resin. DISCUSSION The ANOVA expresses a significant difference between enamel-silver difference values and those for enamelEpoxydent and e n a m e l - O r t h o - B o n d epoxy resin. Epoxy-
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T a b l e III. Differences b e t w e e n five die m a t e r i a l s a n d m a s t e r t o o t h ( S t u d e n t t-test) Variable
Enamel-silver
Enamel-silver Enamel-epoxy Enamel-Epoxydent Enamel-epoxy resin Enamel-polyurethane
Enamel-epoxy
Enamel-Epoxydent
Enamel-epoxy resin
Enamel-polyurethane
0.234
0.000" 0.862
0.029* 0.217 0.230
0.161 0.789 0.674 0.080
See footnote, Table I. *Statistically significant (p _-
