SURFACE FITTING IN CLINICAL MEASUREMENTS

There is tremendous pressure in Dentistry to develop an aesthetic replacement for the amalgam restoration. One of the requirements of a new posterior restorative material is that it must maintain the vertical dimension of the occlusal scheme. The ADA has stated that any posterior restorative material must not lose more than 50 microns of vertical height in one year. This led to the development of several digitizing techniques for the measurement of the changes in surface contour. All of these methods compare sequential images of the surface of the restoration and calculate the change in the surface, e.g the change in vertical height or the total volume lost. Surface fitting is a fundamental problem for all of these systems. It is so important, that methods with inherent accuracies of 1 or 2 microns can be reduced to 40 or 50 micron accuracy because of the inability to fit two sequential images. This inability to fit surfaces is due to the distortions which occur in the formation of replicas of the surfaces and the inability to perfectly align two replica surfaces for digitizing. Epoxy models of a standard model will be made using normal clinical techniques. These surfaces will be digitized using the digitizing system at the University of Minnesota Dental School, however, the experimental design is such that the results are independent of the digitizing system. The surface fitting program THEWORKS will be used to align the surfaces so that any distortions which occur in the replicas can be identified by comparing profiles of the epoxy models with those of the standard model. Finally, what effect these distortion have on the parameters necessary to describe the changes in surface contour will be determined. The study is designed so that the results are independent of the system used to digitize, therefore, the results of this study will apply to any digitizing system. It is hoped that this project will lead to a protocol which will allow studies to be done at remote clinical site in a shorter period of time.