Stress-based Design of a Femoral Hip Prosthesis

INTRODUCTION In the normal hip, stress is distributed over the entire crossection of the femur. Bending and axial compression are the major modes of loading. Following THA, the stress in the bone is significantly different mainly due to the manner in which the load is transferred to the femur (Huiskes, 1990, Huiskes, 1995). In this case, the load is primarily transferred through shear across the bone-cementprostheses interfaces. In addition, the bending displacement in the bone surrounding the stem are reduced because of the relatively high flexural stiffness of the prosthesis. This reduced bending unloads the outer fibers of femur leading to a state of stress shielding. The medial calcar, which experiences a significant reduction in stresses (stress shielding) has been commonly seen to atrophy in in-vivo radiological studies. An apparent solution to this problem would be a prosthesis which loads the proximal end of the femur in a manner similar to the natural state. Recently, a stemless prosthesis design using proximal fixation was developed by Munting and Verhelpen, (1995) and has shown promising initial results. This design is fixed to the lateral side of the proximal femur by means of a bolt. Their invitro experiments showed minimal micromotion and the short-term clinical studies have shown low initial failure rates. The motivating hypothesis for the current study, is that it is possible to develop a new femoral prosthesis component designed specifically to reduce the levels of stress shielding and interfacial shear stress in the femur following total hip arthroplasty.