Shape optimisation of a Charnley prosthesis based on the fatigue notch factor.

The femoral component of the artificial hip joint implanted in a patient's femur is subjected to a complex set of forces exerted due to normal life activities. It is thought that high values of stress in the cement in a cemented prosthesis can lead to fractures of the cement mantle and loosening of the stem. The incidence of such problems may be diminished by reduction of the fatigue notch factor in the cement, such that stress concentrations are avoided and the crack initiation time maximised. This study describes a method for numerical shape optimisation to determine an optimal shape for the femoral stem of a hip prosthesis in order to minimise the fatigue notch factor in the cement layer at the interfaces with the bone and stem whilst at the same time maintaining or increasing the stress levels in the proximal medial femoral bone to help prevent stress shielding. The method when used to optimise the shape of a stainless steel Charnley stem was found to be extremely efficient and effective. The resulting optimal shape was heavily waisted in the proximal region below the neck but distally was quite similar to the original design. The fatigue notch factors in the cement were reduced by 16% and 19% for medial and lateral cement/stem interfaces, respectively, and by 8% and 2% for the corresponding cement/bone interfaces. The fatigue notch factor in the proximal medial bone was increased by 57% which indicates that the general stress level in this region is markedly increased. Thus the optimised design should increase the fatigue life of the cement and at the same time reduce stress shielding in the proximal bone. Both of these effects may help prevent loosening of the femoral component and hence reduce the need for early revision operations.