Purpose: The aim of this in vitro study was to measure and evaluate the retention of four different flexible denture base materials.

Materials and Method: Forty Kennedy class I partially edentulous models with the first premolar bilaterally as the last standing abutments were used in this study. Models were divided into four equal groups. Group I: partial denture with PEEK clasp assembly and denture base, Group II: partial denture with Bre-flex clasp assemble and denture base, Group III: partial denture with Polyoxymethylene clasp assemble and denture base (Acetal) and Group IV: partial denture with polyamide clasp assembly and denture base (Valplast). All the RPD were constructed in two steps: the major connector and the meshwork were casted first with chrome –cobalt alloy and the rest of the partial denture was then constructed with different flexible denture base materials according to the distribution of the previously mentioned groups.
Retention of each partial denture in each group was measured after applying repeated insertion and removal cycles using the chewing simulator integrated with thermo-cyclic protocol operated on servo-motor. The data of the retentive force magnitudes at different intervals for the four different materials were collected, tabulated and statistically analyzed. Results: The results of this study showed that PEEK has statistically significant higher retention values when compared to the other three types of thermoplastic materials. Conclusion: PEEK is one of the promising esthetic materials that provides superior retention when used as a framework for a removable partial denture.