Effect of direct retainer design on dynamics of RPD

Objectives: Designs of removable partial denture (RPD) were suggested to affect the mobility of both abutment teeth and RPD during oral functions. The aim of this study was to examine the effect of direct retainer designs on dynamics of RPDs under a simulated loading conditions. Methods: Fifteen Kennedy class II maxillary RPDs, using 3 different direct retainers designs (n=5) were fabricated. These 3 designs included wrought wire clasp, RPA clasp, and conical crown telescopic retainer. Three dimensional displacements of the RPDs were measured on an experimental model with simulated periodontal ligaments and mucosa made of silicone impression material (Exafine regular, Fit checker; GC) under a simulated 50N loading condition by using displacement transducer type M-3 (Miura, 1998) . The linearity error of the transducer was less than 2% within a range of +/- 0.20 mm. The effects of the direct retainer design on the three dimensional displacements of the RPDs were tested by Kruskal Wallis H-test at a significant level of 5%. Results: The finding revealed the RPDs to be depressed with substantial buccal displacements by the simulated loadings and this tendency was found consistently independent upon the type of direct retainers. The medians of buccal and vertical displacements were 0.15(0.13-0.29) mm and 0.32(0.27-0.34) mm for the wrought wire clasp RPD, 0.14(0.12-0.18) mm and 0.20(0.19-0.39) mm for the RPA clasp RPD, and 0.10(0.01-0.28) mm and 0.14 (0.12-0.26) mm for the conical crown telescopic retainer RPD, respectively. The effect of the direct retainer design on buccal displacements of the RPD resulted to be statistically significant (p=0.023). Conclusion: The mobility of RPD was affected by the design of direct retainers, and that, an increase in connecting rigidity of the retainer was associated with decreased denture mobility.