Some design considerations on the stems of total hip replacement prostheses

Fatigue fracture of the femoral stem of total hip replacement prostheses is now a recognised complication. A simple analytical stress model of the load transfer in femoral prostheses is described. The model has been evaluated experimentally and shown to give a good representation of the expected in vivo behaviour of femoral prostheses. Some design considerations on the femoral stem based on the model and tests are discussed. These features include stress reduction in the middle-third region of the stem, improved load transfer surfaces in the medial-proximal and distal-lateral stem faces, and the importance of better flexural matching of the stem with the surrounding bone and cement. The objective being to provide a stem with improved performance, while at the same time preserving as closely as possible the normal physiological loading on the femur.