Material optimisation of the femoral component of a hip prosthesis based on the fatigue notch fatigue approach

Previous studies have optimised the shape of a cemented stainless steel stem in order to minimise the fatigue notch factor K_f in the cement whilst at the same time maximising K_f in the proximal medial bone to prevent bone resorption [1]. The present study firstly describes the effect of changes in the modulus of elasticity of the stem material for both the original Charnley stem and the ciptimised shape on K_f as predicted by a 2D finite element (FE) model of the implanted prosthesis. The paper further describes a method for parametric optimisation to determine the best material properties of a layered composite femoral stem consisting of a core material (stainless steel) and an outer layer of a different material, the elastic modulus of which is used as a design variable. The overall objective of the optimisation was to maximise K_f in the proximal bone whilst at the same time constraining K_f at all cement interfaces to be no greater than its initial value. The results of the first study suggest that Young's moduli of about 145 and 210 GPa are optimal for the monolithic Charnley and optimised stems, respectively. A composite prosthesis with a layer of modulus 31 GPa added to the optimised stainless steel stem in the proximal region only was found to significantly increase the stresses in the proximal bone and reduce K_f in the cement whilst retaining the advantages of an outer stem profile very similar to that of the original Charnley prosthesis.