FRICTIONAL INTERFACE MICROMOTIONS AND ANISOTROPIC STRESS-DISTRIBUTIONIN A FEMORAL TOTAL HIP COMPONENT

A numerical model of a femoral total hip component based on the finite element method is developed to evaluate the relative micromotions at the bone-implant interface and the stress distribution in the femoral bone. The interface is modelled as unilateral contact involving Coulom b's dry friction between the bone and the implant. In addition, the mo del includes inhomogeneity, anisotropy as well as plasticity of both c ortical and spongious bones. An automatic data processor coupled to a three-dimensional mesh generator is designed to extract cortical bone geometry and inhomogeneous distribution of trabecular bone density fro m data obtained with quantitative computed tomography (QCT). A prelimi nary application is conducted to evaluate the mechanical behaviour of an existing bone-prosthesis structure for two typical loadings: a load simulating the single leg stance and a load simulating the stair clim bing stance. The obtained results are subdivided in two parts. Firstly , the characterization of stress transfer and micromotions at the bone -stem interface. The peak value of the shear micromotions reaches 600 mum in the proximal medial region with a friction coefficient equal to 0.6. An analysis of the influence of the friction coefficient reveals that the shear and distractive micromotions as well as the shear and normal stresses depend strongly on this coefficient. Secondly, the rep resentation of stresses in the femoral bone. Determination of compleme ntary invariants such as the hydrostatic pressure, the deviatoric stre ss and anisotropic stresses brings additional insights in the evaluati on of the stress field in the femoral bone.