Shape optimization of metal backing for cemented acetabular cup.

Stresses are generated in implant materials and bone, and at their interfaces. These stresses may affect the structural properties of the implant/bone system, or bring it to failure at some time in the postoperative period. Due to these stresses, acetabular cup loosening becomes an important problem for long term survival of total hip arthroplasty. It was found that metal backing would tend to reduce stresses in the underlying acrylic cement and bone. Yet, recent studies of load transfer around acetabular cups have shown that metal backing generates higher stress peaks in cement at the cup edges, while generates lower stress peaks in bone at the central part of acetabulum (dome), thus the bone at the dome becomes more stress shielded. In this study a numerical shape optimization procedure in combination with an axisymmetric finite element model was used in order to optimize the shape of a stainless steel metal backing shell. The design was to minimize fatigue notch factor in cement along cement/bone and cement/metal backing interfaces in order to prevent failure of cement mantel and loosening of acetabular components, at the same time increasing fatigue notch factor in bone at the center of acetabulum to prevent stress shielding. The results of this study indicate that cemented acetabular cup designs can be improved by using metal backing shells of non-uniform thickness, thick at the dome and thin at edges. Fatigue notch factor in cement was reduced by 2.3% at cement/metal backing interface and increased by 1.3% in the central bone of acetabulum. Von Mises stresses in the cement edge were reduced by 17.8% and 19.3% along cement/bone and cement/metal backing interfaces, respectively. Thus the optimal design will reduce the possibility of fatigue fracture of cement and decrease the stress shielding effect and the likely incidence of bone resorption, whereby extend the expected life of the prostheses.