The effects of femoral head size on the deformation of ultrahigh molecular weight polyethylene acetabular cups.

Late loosening of cemented acetabular cups is increasingly being recognized as a clinical problem. One of the factors which may contribute to loosening is high localized deformation and stress at the cement-bone interface, the magnitude of which depends on the size of the total hip replacement (THR) femoral head. The effects of varying the femoral head size, from 22 to 32 mm, on strain values measured on the surface of the cup were investigated using experimental stress analysis techniques. The largest absolute strains were recorded when loading with the 22 mm head size. Peak strain values decreased to a minimum with the 26 mm head size and increased steadily with head sizes beyond 26 mm. The selection of an acetabular cup size and corresponding femoral head size in a total hip arthroplasty should not be an arbitrary one, but should be based on scientific studies which indicate minimum states of stress within the cup and cement mantle, as well as clinical evidence that the combination of components shows a reduced incidence of failure. This study experimentally quantifies the states of stress on the surface of the acetabular cup and points to the possible existence of an optimum component size to minimize surface stress.