Reconstruction of the hip. A mathematical approach to determine optimum geometric relationships.

The normal mechanical function of the hip is substantially altered by a variety of disorders. The surgical treatment of such conditions, particularly total hip replacement, offers the opportunity not only to replace the articular surfaces of the joint, but also to improve long-term mechanical function by reducing the loads on the joint. A mathematical model of the hip was developed to evaluate the effects of such surgically achievable mechanical alterations as acetabular placement, femoral shaft-prosthetic neck angle, neck length of the femoral prosthesis, and transfer of the greater trochanter. The loads on the hip were lowered significantly by placing the center of the acetabulum as far medially, inferiorly, and anteriorly as was anatomically feasible. Minimum joint contact forces occurred when the femoral shaft-prosthetic neck angles were small, while the minimum moments about the prosthesis stem-neck junction were found when the angles were 130 to 140 degrees. A neck length of the femoral prosthesis of thirty-five millimeters resulted in moments that were lower than those for a neck length of forty-five millimeters. Lateral transfer of the greater trochanter reduced hip-joint forces and moments but distal transfer had little mechanical effect.