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The distortion of cast cobalt-chromium alloy partial denture frameworks fitted to a working cast
The distortion of cast cobalt-chromium alloy partial denture frameworks fitted to a working cast M. Ali, BDS, MSc, PhD, a R. I. Nairn, BDS, MSc, b M. Sherriff, BSc, PhD, c and N. E. Waters, MSc, PhD d U M D S , Guy's Dental School, London, England
P u r p o s e . This in vitro study examined the distortion of 10 identically designed cast cobalt-chromium alloy frameworks, individually constructed, and adjusted by 10 technicians to fit one of 10 replicated stone working casts. M a t e r i a l a n d m e t h o d s . The distortion was measured with a specially developed strain gauge apparatus when the frameworks were seated on the maxillary metal analog, the stone working cast, or a stone duplicate cast. Results. A statistically significant difference between the distortion of each of the frameworks was observed. A significant difference in the distortion was also observed for the frameworks when placed on each of the three casts. The distortion was the least when the frameworks were seated on the stone working cast and the greatest when seated on the metal analog. Conclusion. This study demonstrated that the distortion of a framework is not related to its closeness of fit as observed by the presence or absence of gaps between the framework components and the teeth. (J Prosthet Dent 1997;78:419-24.)
Clinical experience with cast cobalt-chromium (Co-Cr) alloy partial dentures tells us that a framework seldom fits the m o u t h accurately without the need for some adjustment, despite the fact that some adjustment has already been p e r f o r m e d in the laboratory to fit the framework to the master cast. This misfit reflects the dimcnsional inaccuracies that occur during various stages (clinical and laboratory) o f framework construction. Several research studies 1~6that have investigated the fit o f the castings to the stone cast or to the teeth in the m o u t h have reported a lack o f contact between various This paper was presented at the annual meeting of British Society for the Study of Prosthetic Dentistry at Cambridge, U.K., April 1993; and as a table clinic at the annual meeting of American Prosthodontic Society, Chicago, Ill., February 1994. aResearchAssistant, Department of Prosthetic Dentistry. bSenior Lecturer, Department of Prosthetic Dentistry. %enior Lecturer, Department of Dental Materials Science. dEmeritus Professor University of London, Formerly Head of the Department of Dental Materials Science.
OCTOBER 1997
components of the frameworks and the teeth, even though the frameworks concerned were considered to have a clinically acceptable fit and some o f these frameworks were in service for approximately 6 years in these studies. These studies were concerned with the misfit o f the framework by observation o f the spaces between the frameworks and the teeth, either on the cast or in the mouth. These spaces could have occurred as a result of either abrasion o f the stone casts, the loss o f metal during finishing and polishing procedures, 7 or the distortion o f the frameworks. However, a framework that appeared to have a g o o d fit may have been distorted when seated on the cast or in the mouth. Such distortion is possible because o f the flexibility o f the framework, including the major connector. This distortion o f the framework has never been measured in the past. This study presents an in vitro experiment that was designed to measure and compare the distortion o f 10 Kennedy Class I I I frameworks o f one design made by 10 experienced dental technicians, w h o were asked to THE JOURNAL OF PROSTHETICDENTISTRY 419
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Fig 3. Strain gauge assembly with framework seated on table.
Fig 1. Mouth analog. Guiding planes on distal of premolar and mesial of molar crowns.
Fig 2. Reference points (dimples) for measurement. Arrows
indicate four measurement sites.
construct and adjust the frameworks to what they judged was a good fit to 1 o f 10 identical working casts. MATERIAL
AND METHODS
An analog o f the maxillary partially dentate arch was constructcd that had the tooth portion built in metal and the palatal section and base in dental stonc. The missing teeth were the second premolar and first molar on each side. Cobalt-chromium alloy crowns were fitted on the first premolar and second molar abutment preparations. These crowns were constructed to have occlusal rest preparations and parallel guiding planes on the mesial and distal surfaces o f the abutments adjacent to the solid denture bases (Fig. 1). This m o u t h analog was surveyed and wax ledges were formed to aid in waxing circumferential clasp arms, engaging a 0.25 m m undercut depth on the four abutments. The mouth analog was then duplicated with re420
versible hydrocolloid 50 rimes to produce 10 refractory casts and 40 stone casts. A refractory cast was given to each o f the i 0 technicians with the framework design drawn on it. Each technician was also given one o f the stone casts to serve as the working casts onto which the frameworks were fitted. The remaining 30 stone casts (reserve casts) were used while measuring the distortion o f the frameworks. The distortion o f the framework was measured when placed on the working cast, reserve cast, and m o u t h analog. It must be appreciated that the duplication procedure carries an inherent error and is considered in the discussion of the results. The technicians were provided with 0.5 mm thick sheet casting wax and preformed wax clasp patterns and were instructed to wax, cast in Co-Cr alloy (Croform Excel, Davis Schottlander and Davis Ltd., Herts, England), finish, and electropolish the frameworks. The technicians were asked to adjust the framework until it fitted the working cast provided and then return the framework with the cast. The technicians were not aware o f the purpose o f the study, and the casts were distributed and collected by the chief technical instructor. It was hoped that this procedure might prevent tmdue care being taken by the technicians during the production o f these frameworks and overcome any feelings that their individual efforts might be criticized. Having received the frameworks from the technicians, a small round steel bur of 0.5 mm diameter was used to drill small dimples or depressions in the frameworks at various locations. These dimples served as the reference points for measuring the distortion between the specific parts of the framework and were located on the exposed surfaces o f the minor connectors next to the denture base on both left and right sides and on the lingual plates of the first premolar and second molar teeth on both sides (Fig. 2). Strain gauge assembly Three strain gauge jigs were used to measure distortion o f the frameworks. Each o f these consisted o f a VOLUME 78 NUMBER 4
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Fig 4. Calibration procedure for strain gauges with micrometer.
Fig 5. Measurement of distortion with framework seated on mouth analog.
Table I. Distortion (in millimeters) of 10 frameworks Working cast Max.
Mean
Min.
0.004
0.103
0.030
0.014
0.006
0.116
0.037
0.019
Pm-Pm
-0.160
0.115
0.008
M-M
-0.243
0.108
-0.014
Site
Min.
Left Right
Reserve cast Max.
Mean
Min.
Mouth analog Max.
0.116
0.048
0.028
0.124
0.147
0.068
0.023
0.156
0.080
-0.167
0.168
0.032
-0.192
0.175
0.036
-0.276
0.144
0.006
-0.296
0.173
0.012
Mean 0.063
The mean values reported represent the average of three repeat measurements for each of the 10 frameworks (i.e., n = 30). Site identification: The terms left and right indicate that measurements were made between the dimples on the left and right sides of the framework; Pro-Pro and M-M that the measurements were made between the dimples in the premolar and molar regions of the framework (see Fig. 2). + value: Framework extended when seated on cast. - value: Framework compressed when seated on cast.
strip o f spring steel 0.1 mm thick obtained by cutting a razor blade. One strain gauge ( T M L type FLA-2-IL11, Tokyo Sokki Kenkyujo Co. Ltd., Tokyo, Japan) was b o n d e d to either side o f each strip to produce a half Wheatstone bridge set-up to minimize the effect o f temperature changes on stress values. A strip was held between L-shaped side arms, each o f which carried a pointed probe near the end o f the longer arm (Fig. 3). These probes were used to engage the dimples on the framework. Any distortion o f the framework, when seated on different casts and m o u t h analog, would lead to displacement o f the probes and hence, distortion in the strain gauge assembly. The strain gauge jigs were designed in this way to produce the required strain sensitivity to displacement while ensuring that their stiffness in the direction o f measurement was negligible in comparison with stiffness o f the frameworks on which they were to be used. A strain gauge assembly was suspended with threads from a retort stand. A total o f three strain gauge jigs were constructed by this m e t h o d to measure the distortion at the four sites (Fig. 2). Each strain gauge was calibrated in 20 equal increments over the range 0 to 0.5 m m distortion, with the help o f a m i c r o m e t e r ( M o o r e and Wright, Sheffield, England) (Fig. 4). The strain-displacement plots were linear t h r o u g h o u t this range. OCTOBER 1997
Experimental
procedure
A framework was placed on a table, where the height could be adjusted, so that it could be raised or lowered to engage the probes of the jig in a pair o f dimples on the framework. The length o f these probes was adjusted so that, when the probes engaged the dimples, the strain gauges were lightly stressed. The value of strain recorded when the framework was placed on the table (Fig. 3) was tal¢en as the zero strain reading and hence denoted zero distortion in the framework. The framework was then seated on the worldng cast and the height o f the table adjusted so that the probes reengaged the same set o f dimples. Any change in the strain value in the jig was recorded. This procedure was repeated three times for any given site. Similar measurements at the same site (or sites in the case o f the saddle measurements) when the framework was seated on the reserve cast and then on the m o u t h analog (Fig. 5). An identical procedure was followed with the o t h e r two jigs on additional sessions, one jig for each session. All measurements were carried out at 20 ° -+ 2 ° C. T h e distortion in millimeters was then calculated for various parts o f the framework, according to the sensitivity o f the particular jig used. A new reserve cast was used for each o f the three replicate measurements, as each cast was abraded while seating the framework. 421
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Table II. Analysis of variance for the distortion data on 10 frameworks
Table IV. Difference in the distortion (mm) between casts and mouth analog for four measurement sites: Bonferroni (Dunn) t test
DF
ANOVA SS
Mean sq.
F value
Pr • F
9 2
0.564105 0.022775
0.062678 0.011387
80.82 14.68
0.0001 0.0002
A
0.047917
40
Mouth analog
F*C Site (S)
18 3
0.013959 0.062326
0.000775 0.020775
133.44
0.0001
B C
0.038583 0.015158
40 40
Reserve cast Working cast
F~S C~S
27 6
0.342177 0.001178
0.012673 0.000196
81.40 1.26
0.0001 0.2904
F*C*S
54
0.008407
0.0001557
Source
Bonferroni grouping
Frame (F) Cast (C)
N.B. Main effects F and C tested against "whole plot" error (F'C); S tested against "split plot" error (F*C*S).
Framework
Sites
F
Cast
p
F
p
1 2 3 4
5.7 32.6 6.1 6.7
0.034 0.001 0.030 0.024
1 7.4 23.6 24.3 44.7
0.003 0.001 0.001 0.001
5 6 7
5.9 5.3 90.7
0.033 0.041 0.001
60.8 34.4 0.1
0.001 0.001 0.910
8 9 10
27.8 5.1 221.8
0.001 0.044 0.001
45.0 43.9 0.06
0.001 0.001 0.950
F is the value of the F statistic, and p the associated probability.
RESULTS Table I presents the minimum, maximum, and mean values o f distortion o f 10 frameworks when seated on the two casts (working and reserve cast) and the m o u t h analog. A split plot analysis o f variance was performed on the mean o f the three repeat readings for each site (Table II), which indicated that there was a statistically significant difference among the frameworks, among the casts and m o u t h analog, and also among the four measurement sites for each o f the 10 frameworks. Breakdown analyses for individual frameworks were therefore carried out. Table III presents the results for the difference in the distortion between sites for the casts (working and reserve cast) and m o u t h analog for individual frameworks. A statistically significant difference for the distortion among the four sites was found for all frameworks. The pattern or order o f distortion for the four sites was different for different frameworks, which is shown by the statistically significant interaction F*S (Table III). However, the least distortion was observed between the minor connectors at the end o f the denture base on the left side o f the framework, and the maximum distortion was observed across the framework in the molar region. 422
N
Table V. Bonferroni (Dunn) t test for the distortion data (mm) and 10 frameworks (N = 12) Bon grouping
Table III. Difference in the distortion (mm) between sites for casts and analog and between casts and analog for sites. Ten separate ANOVA tests, one for each framework
Mean
Mean
Framework
A B
0.13708 0.10458
1 2
Acceptable Acceptable
C D E
0.07481 0.05819 0.05069
6 3 4
Acceptable Borderline Borderline
F G H
0.04506 0.02350 0.01369
8 5 9
Unacceptable Unacceptable Unacceptable
-0.07406 -0.09469
7 10
I J
Fit
Borderline Acceptable
+ value: Framework extended when seated on cast. - value: Framework compressed when seated on cast.
The results for the difference between casts (working and reserve cast) and the m o u t h analog for the distortion at the four measurement sites revealed a significant difference between casts existed for all the frameworks except numbers 7 and 10 (Table III). Bonferroni's multiple comparison o f means grouping for the four measurement sites demons~ated that the greatest distortion occurred when the frame;works were seated on the mouth analog and the least when seated on the working cast (Table IV). The difference between the magnitude o f distortion for the 10 frameworks was statistically significant and, as previously noted, no two frameworks were alike for either the magnitude or the pattern o f the distortion. This is evident from the Bonferroni grouping found for these frameworks (Table V). This table also includes an assessment o f the fit o f each framework on the mouth analog by an experienced clinician. DISCUSSION The objective o f this experiment was the measurement o f distortion o f the frameworks when seated at the time o f fit, hence a rigid replica was required for the measurements. However, this analog did not allow for any movement o f the teeth that occurs intraorally because o f the "tight" fit o f the framework, which would reduce the strain. The m o u t h analog was also constructed in metal to minimize any abrasion during repeated seating o f the framework. In the mouth, depending on the wear :
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characteristics o f the surfaces involved, abrasion could also reduce distortion. The strain gauge assemblies were sensitive to measure displacement to 0.001 mm. This was considered satisfactory and a linear regression analysis confirmed their linear response to load and the reproducibility o f the output. D i f f e r e n c e b e t w e e n m e a s u r e m e n t sites
The magnitude o f distortion for the four sites o f measurement was different in any one framework. The order o f distortion for the four sites remained unchanged f o r a particular framework, irrespective o f the cast or m o u t h analog it was seated on. This indicates that little: dimensional changes occurred during the duplication process. However, the magnitude o f the distortion was different for both the casts and m o u t h analog, indicating that the stone casts were dimensionally different from the analog. This difference could be due to either the abrasion o f the stone casts during seating o f the frame: works or the duplication process. However, the former is more likely the major cause. D i f f e r e n c e b e t w e e n casts and a n a l o g
The finding that the distortion was smallest when the framework was seated on the working cast is probably caused by the abrasion o f the stone cast, which would occur during repeated seating and removal o f the framework while adjusting it to fit. The maximum distortion o f the frameworks was observed when seated on the m o u t h analog. This is to be expected, as any abrasion that occurred during seating would be negligible on the metal analog in comparison with that which would occur with a stone cast. The framework w h e n seated on the reserve cast showed slightly less distortion than when seated on the m o u t h analog. This may be ascribed in part to some abrasion o f the stone reserve cast while seating the framework for talcing the measurement, even though a new cast was used for each recording session. However, as previously mentioned, a part o f this difference in distortion could also be due to the dimensional changes that occurred during the duplication process. As the difference among distortions o f the framework when seated on the working cast and reserve cast was greater than the difference between reserve cast and m o u t h analog, the abrasion o f the stone cast was likely to be the main reason why the distortion was less when framework was seated on the stone casts. Difference between 10 frameworks
Each o f the 10 technicians produced a framework that they considered to have an adequate fit, but these frameworks differed widely in their distortion when seated on the testing Casts and the m o u t h analog. This indicates the complexity o f evaluating the fit o f a framework, and OCTOBER 1997
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that it is a subjective judgment. It depends on the tactile perception o f the technician and h i s / h e r interpretation and response to the fight spots on the framework and the sequence in which h e / s h e chooses to adjust or trim these interferences. When the fit o f the frameworks was examined by eye and touch and compared with the distortion values recorded, it was noted that some o f the frameworks (3 o f 10), which exhibited spaces between their components (such as clasp arms) and the teeth, demonstrated a lower magnitude o f distortion. However, these frameworks had such a poor fit that these would have hot been accepted clinically. On the other hand, other frameworks that had spaces between their components and the teeth showed higher values 0f'distortion. The frameworks that had clinically acceptable or good f i t a n d apparently had no spaces between their components and the teeth exhibited even greater magnitude o f distortion than the frameworks with spaces. Thus the presence or absence o f spaces was not a reliable m e t h o d for evaluating the fit o f the partial denture frameworks. Clinical experience also shows that a framework having a "tight" fit at the time o f insertion and causing slight discomfort to the patient feels comfortable after a few days o f wearing. This indicates the possible movement or shifting o f the teeth within their sockets to relieve the stress applied to them by the distorted framework. Another possible source o f stress relief could be abrasion o f the contacting surfaces. Improved methods or criteria for assessing the fit o f the castings intraorally are clearly needed. SUMMARY
AND CONCLUSION
This study measured and compared the distortion o f 10 frameworks o f one design with the help o f a specially designed strain gauge apparatus. The results revealed that the fit o f the framework was highly subjective and that the closeness o f fit by observation o f spaces between the framework components and the teeth was not related to the stresses present in the framework. The results revealed that the distortion o f the framework, as assessed by eye and touch, was not related to its apparent fit. The distortion o f the framework was dependent on thc cast on which it was seated. Distortion was minimal when the framework was seated on the working cast, which was attributed to the considerable abrasion due to thc repeated seating o f the casting during adjustment. Consequcntly, the framework should be tried on two stone casts at least, if its fit is to be cvaluated in the laboratory, and that two stone duplicates o f the master cast should be made in addition to the refractory duplicate and the blockcd-out duplicate. Statistically significant differences were found in the fit o f the frameworks constructed by 10 experienced dental technicians. This indicated that the judgment o f the fit o f the framework is highly subjective. 423
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We thank the members of the technical staff of the Dental Prosthetics Department, UMDS Guy's Hospital, involved in the study.
REFERENCES 1. BatesJF.The mechanical properties of the cobalt-chromium alloys and their relation to partial denture design. Br Dent J 1965;119:389-96. 2. Shanley JJ, Ancowitz SJ, Fenster RK, Pelleu GB Jr. A comparative study of the centrifugal and vacuum-pressure techniques of casting removable partial denture frameworks. J Prosthet Dent 1981 ;45:18-23. 3. FirteH DN, Muncheryan AM, Green AJ. Laboratory accuracy in casting removable partial denture frameworks. J Prosthet Dent 1985;54:856-62. 4. Stern MA, Brudvik JS, Frank RP. Clinical evaluation of removable partial denture rest seat adaptation. J Prosthet Dent 1985;53:658-62. 5. Calverley M J, Moergeli JR Jr. Effect on the fit of removable partial denture frameworks when master casts are treated with cyanoacrylate resin. J Prosthet Dent 1987;58:327-9.
6. Fenlon MR, Juszcsyk AS, Hughes RJ, Walter JD, Sherriff M. Accuracy of fit of cobalt-chromium removable partial denture frameworks on master casts. Eur J Prosthodont Rest Dent 1993;1:127-30. 7. Brudvik JS, Reimers D. The tooth-removable partial denture interface. J Prosthet Dent 1992;68:924-7. Reprint requests to: PROF. NORMAN E WATERS
43 DIGSWELLRiSE WELWYN GARDENCITY HERTS
AL8 7PP ENGLAND Copyright © 1997 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/97/$5.00 + 0. 10/1/84270
Height o f m a n d i b u l a r basal b o n e in dentate and edentulous subjects Xie Q, Wolf J, Soikkonen K, Ainamo A. Acta Odontol Stand 1996;54:379-83. Purpose. The aim o f this study was to determine whether any differences occurred in the height o f mandibular basal bone between young and old subjects and to assess the basal bone level in elderly edentulous subjects in comparison with the dentate individuals. Material and M e t h o d s . Two groups o f dentate subjects, young group consisting o f 131 individuals (63 men, 68 women) with a mean age o f 27 years and an elderly group consisting o f 91 individuals (46 men, 45 women) with a mean age o f 65 years, were compared by means o f panoramic radiography to the 128 individuals (32 men and 96 women, mean age 80 years) o f elderly edentulous subjects. A selection criteria for the radiographs was used. Selection o f radiographs and measurements were made at four sites (mental foramen, lower most point o f mandibular canal, distance o f lower point o f mandibular canal to inferior border and distance in the molar region between lowest point o f mandibular canal and mandibular border) on each panoramic radiograph by one examiner. Lines and points were drawn on the radiographs with pencil and were scanned, digitized, and measured with a UMAX UC-1200 scanner connected to a IBM-PC computer. Statistical analyses o f the results were made with StatView + graphics package. Unpaired twotailed t test was performed to test differences between the groups. Results. Men exhibited higher mandibular bone than women in youth, old age dentate group an in the elderly edentulous subject group when the distances o f the mental foramen and mandibular canal from the lower border of the mandible were measured (p < 0.05). The height o f the basal bone below the mental foramen was less in elderly edentulous women than in young or old dentate women (p < 0.05). The mental foramen ofedentulous women seemed to move closer to the inferior border of the mandible as a result o f continued reduction o f the residual ridge. In addition, a slight increase in bone deposition along the inferior border o f the mandible seemed to take place with advancing are. 22 references.--RP Renner
424
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P u r p o s e . This in vitro study examined the distortion of 10 identically designed cast cobalt-chromium alloy frameworks, individually constructed, and adjusted by 10 technicians to fit one of 10 replicated stone working casts. M a t e r i a l a n d m e t h o d s . The distortion was measured with a specially developed strain gauge apparatus when the frameworks were seated on the maxillary metal analog, the stone working cast, or a stone duplicate cast. Results. A statistically significant difference between the distortion of each of the frameworks was observed. A significant difference in the distortion was also observed for the frameworks when placed on each of the three casts. The distortion was the least when the frameworks were seated on the stone working cast and the greatest when seated on the metal analog. Conclusion. This study demonstrated that the distortion of a framework is not related to its closeness of fit as observed by the presence or absence of gaps between the framework components and the teeth. (J Prosthet Dent 1997;78:419-24.)
Clinical experience with cast cobalt-chromium (Co-Cr) alloy partial dentures tells us that a framework seldom fits the m o u t h accurately without the need for some adjustment, despite the fact that some adjustment has already been p e r f o r m e d in the laboratory to fit the framework to the master cast. This misfit reflects the dimcnsional inaccuracies that occur during various stages (clinical and laboratory) o f framework construction. Several research studies 1~6that have investigated the fit o f the castings to the stone cast or to the teeth in the m o u t h have reported a lack o f contact between various This paper was presented at the annual meeting of British Society for the Study of Prosthetic Dentistry at Cambridge, U.K., April 1993; and as a table clinic at the annual meeting of American Prosthodontic Society, Chicago, Ill., February 1994. aResearchAssistant, Department of Prosthetic Dentistry. bSenior Lecturer, Department of Prosthetic Dentistry. %enior Lecturer, Department of Dental Materials Science. dEmeritus Professor University of London, Formerly Head of the Department of Dental Materials Science.
OCTOBER 1997
components of the frameworks and the teeth, even though the frameworks concerned were considered to have a clinically acceptable fit and some o f these frameworks were in service for approximately 6 years in these studies. These studies were concerned with the misfit o f the framework by observation o f the spaces between the frameworks and the teeth, either on the cast or in the mouth. These spaces could have occurred as a result of either abrasion o f the stone casts, the loss o f metal during finishing and polishing procedures, 7 or the distortion o f the frameworks. However, a framework that appeared to have a g o o d fit may have been distorted when seated on the cast or in the mouth. Such distortion is possible because o f the flexibility o f the framework, including the major connector. This distortion o f the framework has never been measured in the past. This study presents an in vitro experiment that was designed to measure and compare the distortion o f 10 Kennedy Class I I I frameworks o f one design made by 10 experienced dental technicians, w h o were asked to THE JOURNAL OF PROSTHETICDENTISTRY 419
THE J O U R N A L OF PROSTHETIC DENTISTRY
ALl ET AL
Fig 3. Strain gauge assembly with framework seated on table.
Fig 1. Mouth analog. Guiding planes on distal of premolar and mesial of molar crowns.
Fig 2. Reference points (dimples) for measurement. Arrows
indicate four measurement sites.
construct and adjust the frameworks to what they judged was a good fit to 1 o f 10 identical working casts. MATERIAL
AND METHODS
An analog o f the maxillary partially dentate arch was constructcd that had the tooth portion built in metal and the palatal section and base in dental stonc. The missing teeth were the second premolar and first molar on each side. Cobalt-chromium alloy crowns were fitted on the first premolar and second molar abutment preparations. These crowns were constructed to have occlusal rest preparations and parallel guiding planes on the mesial and distal surfaces o f the abutments adjacent to the solid denture bases (Fig. 1). This m o u t h analog was surveyed and wax ledges were formed to aid in waxing circumferential clasp arms, engaging a 0.25 m m undercut depth on the four abutments. The mouth analog was then duplicated with re420
versible hydrocolloid 50 rimes to produce 10 refractory casts and 40 stone casts. A refractory cast was given to each o f the i 0 technicians with the framework design drawn on it. Each technician was also given one o f the stone casts to serve as the working casts onto which the frameworks were fitted. The remaining 30 stone casts (reserve casts) were used while measuring the distortion o f the frameworks. The distortion o f the framework was measured when placed on the working cast, reserve cast, and m o u t h analog. It must be appreciated that the duplication procedure carries an inherent error and is considered in the discussion of the results. The technicians were provided with 0.5 mm thick sheet casting wax and preformed wax clasp patterns and were instructed to wax, cast in Co-Cr alloy (Croform Excel, Davis Schottlander and Davis Ltd., Herts, England), finish, and electropolish the frameworks. The technicians were asked to adjust the framework until it fitted the working cast provided and then return the framework with the cast. The technicians were not aware o f the purpose o f the study, and the casts were distributed and collected by the chief technical instructor. It was hoped that this procedure might prevent tmdue care being taken by the technicians during the production o f these frameworks and overcome any feelings that their individual efforts might be criticized. Having received the frameworks from the technicians, a small round steel bur of 0.5 mm diameter was used to drill small dimples or depressions in the frameworks at various locations. These dimples served as the reference points for measuring the distortion between the specific parts of the framework and were located on the exposed surfaces o f the minor connectors next to the denture base on both left and right sides and on the lingual plates of the first premolar and second molar teeth on both sides (Fig. 2). Strain gauge assembly Three strain gauge jigs were used to measure distortion o f the frameworks. Each o f these consisted o f a VOLUME 78 NUMBER 4
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THE JOURNAL OF PROSTHETIC DENTISTRY
Fig 4. Calibration procedure for strain gauges with micrometer.
Fig 5. Measurement of distortion with framework seated on mouth analog.
Table I. Distortion (in millimeters) of 10 frameworks Working cast Max.
Mean
Min.
0.004
0.103
0.030
0.014
0.006
0.116
0.037
0.019
Pm-Pm
-0.160
0.115
0.008
M-M
-0.243
0.108
-0.014
Site
Min.
Left Right
Reserve cast Max.
Mean
Min.
Mouth analog Max.
0.116
0.048
0.028
0.124
0.147
0.068
0.023
0.156
0.080
-0.167
0.168
0.032
-0.192
0.175
0.036
-0.276
0.144
0.006
-0.296
0.173
0.012
Mean 0.063
The mean values reported represent the average of three repeat measurements for each of the 10 frameworks (i.e., n = 30). Site identification: The terms left and right indicate that measurements were made between the dimples on the left and right sides of the framework; Pro-Pro and M-M that the measurements were made between the dimples in the premolar and molar regions of the framework (see Fig. 2). + value: Framework extended when seated on cast. - value: Framework compressed when seated on cast.
strip o f spring steel 0.1 mm thick obtained by cutting a razor blade. One strain gauge ( T M L type FLA-2-IL11, Tokyo Sokki Kenkyujo Co. Ltd., Tokyo, Japan) was b o n d e d to either side o f each strip to produce a half Wheatstone bridge set-up to minimize the effect o f temperature changes on stress values. A strip was held between L-shaped side arms, each o f which carried a pointed probe near the end o f the longer arm (Fig. 3). These probes were used to engage the dimples on the framework. Any distortion o f the framework, when seated on different casts and m o u t h analog, would lead to displacement o f the probes and hence, distortion in the strain gauge assembly. The strain gauge jigs were designed in this way to produce the required strain sensitivity to displacement while ensuring that their stiffness in the direction o f measurement was negligible in comparison with stiffness o f the frameworks on which they were to be used. A strain gauge assembly was suspended with threads from a retort stand. A total o f three strain gauge jigs were constructed by this m e t h o d to measure the distortion at the four sites (Fig. 2). Each strain gauge was calibrated in 20 equal increments over the range 0 to 0.5 m m distortion, with the help o f a m i c r o m e t e r ( M o o r e and Wright, Sheffield, England) (Fig. 4). The strain-displacement plots were linear t h r o u g h o u t this range. OCTOBER 1997
Experimental
procedure
A framework was placed on a table, where the height could be adjusted, so that it could be raised or lowered to engage the probes of the jig in a pair o f dimples on the framework. The length o f these probes was adjusted so that, when the probes engaged the dimples, the strain gauges were lightly stressed. The value of strain recorded when the framework was placed on the table (Fig. 3) was tal¢en as the zero strain reading and hence denoted zero distortion in the framework. The framework was then seated on the worldng cast and the height o f the table adjusted so that the probes reengaged the same set o f dimples. Any change in the strain value in the jig was recorded. This procedure was repeated three times for any given site. Similar measurements at the same site (or sites in the case o f the saddle measurements) when the framework was seated on the reserve cast and then on the m o u t h analog (Fig. 5). An identical procedure was followed with the o t h e r two jigs on additional sessions, one jig for each session. All measurements were carried out at 20 ° -+ 2 ° C. T h e distortion in millimeters was then calculated for various parts o f the framework, according to the sensitivity o f the particular jig used. A new reserve cast was used for each o f the three replicate measurements, as each cast was abraded while seating the framework. 421
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Table II. Analysis of variance for the distortion data on 10 frameworks
Table IV. Difference in the distortion (mm) between casts and mouth analog for four measurement sites: Bonferroni (Dunn) t test
DF
ANOVA SS
Mean sq.
F value
Pr • F
9 2
0.564105 0.022775
0.062678 0.011387
80.82 14.68
0.0001 0.0002
A
0.047917
40
Mouth analog
F*C Site (S)
18 3
0.013959 0.062326
0.000775 0.020775
133.44
0.0001
B C
0.038583 0.015158
40 40
Reserve cast Working cast
F~S C~S
27 6
0.342177 0.001178
0.012673 0.000196
81.40 1.26
0.0001 0.2904
F*C*S
54
0.008407
0.0001557
Source
Bonferroni grouping
Frame (F) Cast (C)
N.B. Main effects F and C tested against "whole plot" error (F'C); S tested against "split plot" error (F*C*S).
Framework
Sites
F
Cast
p
F
p
1 2 3 4
5.7 32.6 6.1 6.7
0.034 0.001 0.030 0.024
1 7.4 23.6 24.3 44.7
0.003 0.001 0.001 0.001
5 6 7
5.9 5.3 90.7
0.033 0.041 0.001
60.8 34.4 0.1
0.001 0.001 0.910
8 9 10
27.8 5.1 221.8
0.001 0.044 0.001
45.0 43.9 0.06
0.001 0.001 0.950
F is the value of the F statistic, and p the associated probability.
RESULTS Table I presents the minimum, maximum, and mean values o f distortion o f 10 frameworks when seated on the two casts (working and reserve cast) and the m o u t h analog. A split plot analysis o f variance was performed on the mean o f the three repeat readings for each site (Table II), which indicated that there was a statistically significant difference among the frameworks, among the casts and m o u t h analog, and also among the four measurement sites for each o f the 10 frameworks. Breakdown analyses for individual frameworks were therefore carried out. Table III presents the results for the difference in the distortion between sites for the casts (working and reserve cast) and m o u t h analog for individual frameworks. A statistically significant difference for the distortion among the four sites was found for all frameworks. The pattern or order o f distortion for the four sites was different for different frameworks, which is shown by the statistically significant interaction F*S (Table III). However, the least distortion was observed between the minor connectors at the end o f the denture base on the left side o f the framework, and the maximum distortion was observed across the framework in the molar region. 422
N
Table V. Bonferroni (Dunn) t test for the distortion data (mm) and 10 frameworks (N = 12) Bon grouping
Table III. Difference in the distortion (mm) between sites for casts and analog and between casts and analog for sites. Ten separate ANOVA tests, one for each framework
Mean
Mean
Framework
A B
0.13708 0.10458
1 2
Acceptable Acceptable
C D E
0.07481 0.05819 0.05069
6 3 4
Acceptable Borderline Borderline
F G H
0.04506 0.02350 0.01369
8 5 9
Unacceptable Unacceptable Unacceptable
-0.07406 -0.09469
7 10
I J
Fit
Borderline Acceptable
+ value: Framework extended when seated on cast. - value: Framework compressed when seated on cast.
The results for the difference between casts (working and reserve cast) and the m o u t h analog for the distortion at the four measurement sites revealed a significant difference between casts existed for all the frameworks except numbers 7 and 10 (Table III). Bonferroni's multiple comparison o f means grouping for the four measurement sites demons~ated that the greatest distortion occurred when the frame;works were seated on the mouth analog and the least when seated on the working cast (Table IV). The difference between the magnitude o f distortion for the 10 frameworks was statistically significant and, as previously noted, no two frameworks were alike for either the magnitude or the pattern o f the distortion. This is evident from the Bonferroni grouping found for these frameworks (Table V). This table also includes an assessment o f the fit o f each framework on the mouth analog by an experienced clinician. DISCUSSION The objective o f this experiment was the measurement o f distortion o f the frameworks when seated at the time o f fit, hence a rigid replica was required for the measurements. However, this analog did not allow for any movement o f the teeth that occurs intraorally because o f the "tight" fit o f the framework, which would reduce the strain. The m o u t h analog was also constructed in metal to minimize any abrasion during repeated seating o f the framework. In the mouth, depending on the wear :
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characteristics o f the surfaces involved, abrasion could also reduce distortion. The strain gauge assemblies were sensitive to measure displacement to 0.001 mm. This was considered satisfactory and a linear regression analysis confirmed their linear response to load and the reproducibility o f the output. D i f f e r e n c e b e t w e e n m e a s u r e m e n t sites
The magnitude o f distortion for the four sites o f measurement was different in any one framework. The order o f distortion for the four sites remained unchanged f o r a particular framework, irrespective o f the cast or m o u t h analog it was seated on. This indicates that little: dimensional changes occurred during the duplication process. However, the magnitude o f the distortion was different for both the casts and m o u t h analog, indicating that the stone casts were dimensionally different from the analog. This difference could be due to either the abrasion o f the stone casts during seating o f the frame: works or the duplication process. However, the former is more likely the major cause. D i f f e r e n c e b e t w e e n casts and a n a l o g
The finding that the distortion was smallest when the framework was seated on the working cast is probably caused by the abrasion o f the stone cast, which would occur during repeated seating and removal o f the framework while adjusting it to fit. The maximum distortion o f the frameworks was observed when seated on the m o u t h analog. This is to be expected, as any abrasion that occurred during seating would be negligible on the metal analog in comparison with that which would occur with a stone cast. The framework w h e n seated on the reserve cast showed slightly less distortion than when seated on the m o u t h analog. This may be ascribed in part to some abrasion o f the stone reserve cast while seating the framework for talcing the measurement, even though a new cast was used for each recording session. However, as previously mentioned, a part o f this difference in distortion could also be due to the dimensional changes that occurred during the duplication process. As the difference among distortions o f the framework when seated on the working cast and reserve cast was greater than the difference between reserve cast and m o u t h analog, the abrasion o f the stone cast was likely to be the main reason why the distortion was less when framework was seated on the stone casts. Difference between 10 frameworks
Each o f the 10 technicians produced a framework that they considered to have an adequate fit, but these frameworks differed widely in their distortion when seated on the testing Casts and the m o u t h analog. This indicates the complexity o f evaluating the fit o f a framework, and OCTOBER 1997
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that it is a subjective judgment. It depends on the tactile perception o f the technician and h i s / h e r interpretation and response to the fight spots on the framework and the sequence in which h e / s h e chooses to adjust or trim these interferences. When the fit o f the frameworks was examined by eye and touch and compared with the distortion values recorded, it was noted that some o f the frameworks (3 o f 10), which exhibited spaces between their components (such as clasp arms) and the teeth, demonstrated a lower magnitude o f distortion. However, these frameworks had such a poor fit that these would have hot been accepted clinically. On the other hand, other frameworks that had spaces between their components and the teeth showed higher values 0f'distortion. The frameworks that had clinically acceptable or good f i t a n d apparently had no spaces between their components and the teeth exhibited even greater magnitude o f distortion than the frameworks with spaces. Thus the presence or absence o f spaces was not a reliable m e t h o d for evaluating the fit o f the partial denture frameworks. Clinical experience also shows that a framework having a "tight" fit at the time o f insertion and causing slight discomfort to the patient feels comfortable after a few days o f wearing. This indicates the possible movement or shifting o f the teeth within their sockets to relieve the stress applied to them by the distorted framework. Another possible source o f stress relief could be abrasion o f the contacting surfaces. Improved methods or criteria for assessing the fit o f the castings intraorally are clearly needed. SUMMARY
AND CONCLUSION
This study measured and compared the distortion o f 10 frameworks o f one design with the help o f a specially designed strain gauge apparatus. The results revealed that the fit o f the framework was highly subjective and that the closeness o f fit by observation o f spaces between the framework components and the teeth was not related to the stresses present in the framework. The results revealed that the distortion o f the framework, as assessed by eye and touch, was not related to its apparent fit. The distortion o f the framework was dependent on thc cast on which it was seated. Distortion was minimal when the framework was seated on the working cast, which was attributed to the considerable abrasion due to thc repeated seating o f the casting during adjustment. Consequcntly, the framework should be tried on two stone casts at least, if its fit is to be cvaluated in the laboratory, and that two stone duplicates o f the master cast should be made in addition to the refractory duplicate and the blockcd-out duplicate. Statistically significant differences were found in the fit o f the frameworks constructed by 10 experienced dental technicians. This indicated that the judgment o f the fit o f the framework is highly subjective. 423
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We thank the members of the technical staff of the Dental Prosthetics Department, UMDS Guy's Hospital, involved in the study.
REFERENCES 1. BatesJF.The mechanical properties of the cobalt-chromium alloys and their relation to partial denture design. Br Dent J 1965;119:389-96. 2. Shanley JJ, Ancowitz SJ, Fenster RK, Pelleu GB Jr. A comparative study of the centrifugal and vacuum-pressure techniques of casting removable partial denture frameworks. J Prosthet Dent 1981 ;45:18-23. 3. FirteH DN, Muncheryan AM, Green AJ. Laboratory accuracy in casting removable partial denture frameworks. J Prosthet Dent 1985;54:856-62. 4. Stern MA, Brudvik JS, Frank RP. Clinical evaluation of removable partial denture rest seat adaptation. J Prosthet Dent 1985;53:658-62. 5. Calverley M J, Moergeli JR Jr. Effect on the fit of removable partial denture frameworks when master casts are treated with cyanoacrylate resin. J Prosthet Dent 1987;58:327-9.
6. Fenlon MR, Juszcsyk AS, Hughes RJ, Walter JD, Sherriff M. Accuracy of fit of cobalt-chromium removable partial denture frameworks on master casts. Eur J Prosthodont Rest Dent 1993;1:127-30. 7. Brudvik JS, Reimers D. The tooth-removable partial denture interface. J Prosthet Dent 1992;68:924-7. Reprint requests to: PROF. NORMAN E WATERS
43 DIGSWELLRiSE WELWYN GARDENCITY HERTS
AL8 7PP ENGLAND Copyright © 1997 by The Editorial Council of The Journal of Prosthetic Dentistry. 0022-3913/97/$5.00 + 0. 10/1/84270
Height o f m a n d i b u l a r basal b o n e in dentate and edentulous subjects Xie Q, Wolf J, Soikkonen K, Ainamo A. Acta Odontol Stand 1996;54:379-83. Purpose. The aim o f this study was to determine whether any differences occurred in the height o f mandibular basal bone between young and old subjects and to assess the basal bone level in elderly edentulous subjects in comparison with the dentate individuals. Material and M e t h o d s . Two groups o f dentate subjects, young group consisting o f 131 individuals (63 men, 68 women) with a mean age o f 27 years and an elderly group consisting o f 91 individuals (46 men, 45 women) with a mean age o f 65 years, were compared by means o f panoramic radiography to the 128 individuals (32 men and 96 women, mean age 80 years) o f elderly edentulous subjects. A selection criteria for the radiographs was used. Selection o f radiographs and measurements were made at four sites (mental foramen, lower most point o f mandibular canal, distance o f lower point o f mandibular canal to inferior border and distance in the molar region between lowest point o f mandibular canal and mandibular border) on each panoramic radiograph by one examiner. Lines and points were drawn on the radiographs with pencil and were scanned, digitized, and measured with a UMAX UC-1200 scanner connected to a IBM-PC computer. Statistical analyses o f the results were made with StatView + graphics package. Unpaired twotailed t test was performed to test differences between the groups. Results. Men exhibited higher mandibular bone than women in youth, old age dentate group an in the elderly edentulous subject group when the distances o f the mental foramen and mandibular canal from the lower border of the mandible were measured (p < 0.05). The height o f the basal bone below the mental foramen was less in elderly edentulous women than in young or old dentate women (p < 0.05). The mental foramen ofedentulous women seemed to move closer to the inferior border of the mandible as a result o f continued reduction o f the residual ridge. In addition, a slight increase in bone deposition along the inferior border o f the mandible seemed to take place with advancing are. 22 references.--RP Renner
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