Stiffness optimisation of cement and stem materials in total hip replacement.

It is acknowledged that bone resorption and fatigue fracture of cement in total hip replacement may cause feature problems. The solution is frequently sought associated with the stiffness of cement and stem. The purpose of this paper is firstly to describe the effect of changes in modulus of elasticity of the cement material for the implanted prosthesis on the fatigue notch factor (Kf). The paper further describes a method of numerical optimisation to determine the optimal stiffness characteristics of cement and stem materials, which minimises the probability of fatigue fracture of cement at all interfaces with the stem and the bone, while limiting the amount of bone resorbed. The parameters describing the elastic moduli of cement and stem were considered as design variables. The method was applied to an equivalent 2D finite element model of femoral hip replacement in combination with an optimisation procedure using the ANSYS program. The results of the first study suggest that lower modulus of elasticity of cement material decreases Kf in the cement at all interfaces and proximal bone while higher values increase Kf. For the second aim, Young's moduli of about 0.6 and 22 GPa are optimal for cement and stem materials, respectively. These characteristics decreased the probability of fatigue fracture of cement at all interfaces with the stem and the bone as a result of decreasing Kf in cement at all interfaces, while limiting the amount of bone resorbed as a result of increasing Kf in the proximal bone.