Objective: The purpose of this in vitro study was to compare the retentive force and deformation of acetal resin and cobalt-chromium clasps in simulated 36 months of clinical use. Method: Acetal resin clasps and cobalt-chromium clasps prepared with identical semicircular standard prefabricated clasp patterns (1.2 mm thickness) and acetal resin clasps with 2.0 mm thickness were made on the premolar and molar metal models with 0.25 mm and 0.50 mm undercuts. The retention force of the clasps was measured in distilled water by a special designed insertion-removal testing apparatus with intervals corresponding 0, 6, 12, 18, 24, 30, 36 months use of a removable partial denture. The distance between the clasp tips was measured with a Tools Microscope before and after the insertion-removal testing procedure. Comparison of the results was conducted with ANOVA and Least Significant Difference multiple comparison test. Results: The retention force of acetal resin clasps with 1.2 mm and 2.0 mm thickness (111.62 g and 178.40 g) were found significantly lower than the retention force of cobalt-chromium clasps with 1.2 mm thickness (694.10 gr) (p<.001). After simulated 36 months of clinical use, it was found significant deformation in the cobalt-chromium clasps (p= .022), but no deformation was seen in the acetal resin clasps with 1.2 mm and 2.0 mm thickness (p= .993, and p= .995). The retention force of acetal resin clasps, which did not show deformation, was still significantly lower than cobalt-chromium clasps which showed deformation (p<.001). Conclusion: Within the limitations of this study, the results suggest that the retention force of acetal resin clasps was significantly lower than the cobalt-chromium clasps. There was evidence of deformation in the cobalt-chromium clasps but no deformation was found in the acetal resin clasps after 36 months of simulated clinical use.

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