A clinical investigation of the fit of removable partial dental prosthesis clasp assemblies

Statement of problem: Removable partial denture frameworks are complex castings fabricated from high-shrinkage alloys. This may result in difficulty in achieving accurate fit. Purpose: The purpose of this study was to evaluate which component(s) of the clasp assembly contact the abutment tooth clinically. Material and methods: New and existing frameworks with at least 1 conventional prepared occlusal rest were evaluated for fit. The spacing between the bottom of each of the rests of the 50 clasp assemblies and the corresponding rest seat were recorded with vinyl polysiloxane. The fit of each rest was determined with the use of a dial caliper by measuring, in micrometers, the thickness of the record between the rest and the bottom of the rest seat. The Kennedy classification was also recorded. A 2-sample t test was used to evaluate the difference in fit between tooth-tissue supported and tooth-supported designs (α=.05). Results: The average space between the rest and prepared rest seat was 193 ± 203 μm, with a range of 0 to 828 μm. Twenty tooth-tissue frameworks had an average space of 136 ± 160 μm and 30 tooth-supported frameworks had an average space of 230 ± 222 μm. The 2-sample t test showed no significant difference for fit between tooth-tissue supported and tooth-supported frameworks (P=.1081). Twenty-four percent of rests had contact in the prepared rest seat. Conclusion: The majority of rests evaluated did not contact the intended surfaces.