Microdentistry on the marginal adaptation of porcelain-fused-to-metal crowns
Date
1982
DOI
Authors
Chou, Tsau-Mau
Version
OA Version
Citation
Abstract
One of the critical factors associated with crown construction is the dimensional accuracy of the casting which must fit the prepared tooth exactly. Laboratory technicians admit that the fit of porcelain fused-to-metal crowns is still a problem. The purpose of this study was to determine whether the use of a microscope in fixed prosthodontics is a helpful tool to improve the accuracy of cast metal restorations with porcelain when used during every step of the required technical procedure.
A deep chamfer preparation without a bevel prepared on an ivorine tooth served as a standard die. It was used to evaluate and compare the marginal integrity of different stages in construction of PFM crowns. Two main groups (no-microscope and microscope groups) and the subdivision of each group into 5 subgroups (die trimming, wax-up, casting and finishing the margins, applying the porcelain, and finishing and polishing the margins) were prepared for the experiment.
Thirty dies for each group were prepared for trimming the margin under the microscope and by means of the conventional method with the naked eye. The microscope group showed significantly smaller marginal discrepancy as compared with the group without a microscope (p [less than] 0. 0001).
The microscope provided an efficient method to correct the margins in the wax-up stage, and significantly reduced the marginal discrepancies in the wax-up stage. It also proved to be very helpful when finishing the internal surface after casting (i.e., removal of small nodules).
In this study, the dual wax technique, as compared to the single wax technique, resulted in ambiguous findings. Both the so-called "stress relief" wax and the "margin" wax did improve the marginal adaptation of the castings when compared to the single wax technique. The level of significance, however, was only at the 93% level. No significant difference was found between either dual marginal wax technique.
In finishing the polishing the margins, the microscope groups were not significantly different from the control group from a practical point of view because too small an improvement (approximately 5 [mu].m) was gained. Perhaps for a beginning dental student or technician, it is advisable to use a microscope for finishing and polishing the margins.
During the course of this study, valuable information was also found with respect to techniques and variables affecting marginal fit. For example, a red pencil commonly used for drawing the finish line to mark the extension of the preparation is not recommended since the width of the line by itself introduces an error of [plus or minus] 40-80 [mu]m.
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Thesis (M.Sc.D.)--Boston University, Henry M. Goldman School of Graduate Dentistry (Prosthodontics), 1982.
Bibliography: leaves 89-94.
Thesis (M.Sc.D.)--Boston University, Henry M. Goldman School of Graduate Dentistry (Prosthodontics), 1982.
Bibliography: leaves 89-94.
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This work is protected by copyright. Downloading is restricted to the BU community. If you are the author of this work and would like to make it publicly available, please contact open-help@bu.edu.