Finite element analysis on preferable I-bar clasp shape Y. SATO, K. TSUGA, Y. ABE, S. ASAHARA* & Y. AKAGAWA Department of Removable Prosthodontics, Hiroshima University, Faculty of Dentistry, Hiroshima, Japan and *Private clinic, Nakamachi, Noumi-cho, Saeki-gun, Hiroshima, Japan SUMMARY SUMMARY An I-bar clasp is one of the most popular direct retainers for distal-extension removable par- tial dentures. However, no adequate information is available on preferable shape as determined by biomechanics. This study aimed (1) to investigate, by ®nite element analysis (FEA), the dimensions and stress of I-bar clasps having the same stiffness, and (2) to estimate a mechanically preferable clasp design. Three-dimensional FEA models of I-bar clasps were created with vertical and horizontal straight sections connected by a curved section characterized by six parameters: thickness of the clasp tip, width of the clasp tip, radius of the curvature, horizontal distance between the base and the vertical axis, vertical dimension between the tip and the horizontal axis, and taper (change of width per unit length along the axis). Stress was calculated with a concentrated load of 5 N applied 2 mm from the tip of the clasp in the buccal direction. A thinner and wider clasp having an taper of 0 á 020±0 á 023 and radius of curvature of 2 á 75±3 á 00 showed less stress. The results suggest that such a shape might be the preferable I-bar clasp shape as biomechanical viewpoint. KEYWORDS KEYWORDS: FEM, RPDS, I±bar clasp, design biome- chanics, RPI 2 Introduction An I-bar clasp is one of the most popular direct retainers for distal-extension removable partial dentures. Stress and retention in clasps are the keys to the long-term success of removable partial dentures (RPDs) without deformation or fracture. Clasp retention is affected by the stiffness of the clasp arm (shapes and Young's modulus) and the condition of the abutment (shape, friction, and degree of undercut) (Sato et al., 1997). Although the relation between shape and stiffness has been evaluated (Naik et al., 1997; Sato et al., 2000a), no adequate information is available on the biomechanic- ally preferable shape. Because changes in clasp shape affect both stiffness and stress, preferable shape must be evaluated under constant stiffness, which is propor- tional to the retention force (Sato et al., 1995). The present study aimed to investigate, by ®nite element analysis (FEA), the dimensions and stress of I-bar clasps having the same stiffness and to estimate the mechanically preferable clasp design. Materials and methods Finite element models of full body I-bar clasps (Fig. 1) were constructed with vertical, horizontal and curved sections varying in six parameters: thick- ness of the clasp tip (T), width of the clasp tip (W), radius of the curvature (R), horizontal distance between the base and the vertical axis (H), vertical dimension between the tip and the horizontal axis (V), and taper (change of width per unit length along the axis) (Tp). The Tp is not indicated in Fig. 1b. The ®nite element models were subdivided into 624 elements and 945 nodes (Sato et al., 2000a). All nodes at the base of the clasps were restrained in all directions, and a concentrated load of 5 N (Sato et al., 1995) was applied 2 mm from the tip of the clasp (Krol, 1973) in the buccal direction. The material properties of the clasp were set to be equivalent to those of a cobalt±chromium alloy (Vitallium, which has a Young's modulus (E) of 218 GPa (Morris & Asgar, 1975), and a Poisson's ratio of 0á33). The ã 2001 Blackwell Science Ltd 413 Journal of Oral Rehabilitation 2001 28; 413±417