A study of durability of hip implants

Total hip replacement (henceforth abbreviated THR) has become a routine surgical procedure over the past few decades. This has led to advances in understanding both the biological and the mechanistic aspects of hip joints, which in turn has fueled the quest for the ideal implant that performs as close as possible to the real hip joint. As a result, the design of hip implants is still very much evolving, which naturally requires evaluation of new ideas and designs from a durability point of view. The work presented here can be placed within this broader context. The mechanical response of both two-dimensional and three-dimensional (henceforth abbreviated 2D and 3D, respectively) finite element (henceforth abbreviated FE) models of the hip implant, including the femoral ball, have been studied for a single stance phase of gait (defined later). The influences of (i) variations in the femoral ball diameter, (ii) the nature of the contact interactions between the femoral ball and the stem, (iii) the boundary conditions on the stem, and (iv) the loading conditions on the femoral ball, on the stress distribution in the implant stem have been analyzed. In particular, the effects of the size of the femoral ball on the stress/deformation state in the implant was investigated, in order to assess whether a larger femoral ball size has an adverse effect on its durability. The accuracy of the simplified 2D model is evaluated by comparing the results with those obtained from the 3D model. The reliability of the FE results is ensured by performing mesh convergence studies. In addition, an analytical model is developed and its predictions are compared with the 2D FE results. The study suggests that stresses in the neck region decrease, while those in the contact (between the stem and the femoral ball) region increase with increasing femoral ball size. A simple fatigue life analysis based on a set of standard empirical models suggests that the change in the femoral ball size could potentially affect the design life of the implant.