Design optimization of the hip nail-plate-screws implant

This paper is aimed at determining the number of screws and the dimensions of the appliance components of a hip nail-plate screw implant such 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 bone 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 maximal load (during the gait cycle) at the hip joint. For various configurations of the implant, entailing different numbers of screws, the optimization technique is employed to determine the size of the members (nail, plate and screws) for a lightweight design. Implant configurations are analyzed for two, three, four and five nail configurations. The analysis of the hybrid (implant femur) frame, during each integrative object function and design parameter evaluation, is done by the Finite Element Method. The optimization problem is solved by the sequential unconstrained minimization technique (with the introduction of interior penalty function terms). It is found, from the results of analyses of the four hybrid frames, that the appliance frame, with two screws, yields the minimum-weight structurally safe design.