DESIGN OPTIMIZATION OF THE HIP NAIL-PLATE-SCREWS IMPLANT

This paper is aimed at determining the number of screws and the dimens ions of the appliance components of a hip nail-plate screw implant suc h that their sizes are a minimum (i.e. the implant weight is minimum), without the stresses in the appliance components as well as in the bo ne shaft exceeding the allowable limits. The implant-bone structure is idealized as a hybrid frame, composed of appliance components and the supporting femur bone elements, which act integrally to support the m aximal load (during the gait cycle) at the hip joint. For various conf igurations of the implant, entailing different numbers of screws, the optimization technique is employed to determine the size of the member s (nail, plate and screws) for a lightweight design. Implant configura tions are analyzed for two, three, four and five nail configurations. The analysis of the hybrid (implant femur) frame, during each integrat ive object function and design parameter evaluation, is done by the Fi nite Element Method. The optimization problem is solved by the sequent ial unconstrained minimization technique (with the introduction of int erior penalty function terms). It is found, from the results of analys es of the four hybrid frames, that the appliance frame, with two screw s, yields the minimum-weight structurally safe design.