Dependence of the primary stability of cementless acetabular cup implants on the biomechanical environment

Biomechanical phenomena occurring at the bone–implant interface during the press-fit insertion of acetabular cup implants are still poorly understood. This article presents a nonlinear geometrical two-dimensional axisymmetric finite element model aiming at describing the biomechanical behavior of the acetabular cup implant as a function of the bone Young’s modulus E_b, the diametric interference fit (IF), and the friction coefficient µ. The numerical model was compared with experimental results obtained from an in vitro test, which allows to determine a reference configuration with the parameter set: μ^* = 0.3, E b * = 0 . 2 GPa , and IF^* = 1 mm for which the maximal contact pressure t_N = 10.7 MPa was found to be localized at the peri-equatorial rim of the acetabular cavity. Parametric studies were carried out, showing that an optimal value of the pull-out force can be defined as a function of μ, E_b, and IF. For the reference configuration, the optimal pull-out force is obtained for μ = 0.6 (respectively, E_b = 0.35 GPa and IF = 1.4 mm). For relatively low value of µ (µ < 0.2), the optimal value of IF linearly increases as a function of µ independently of E_b, while for µ > 0.2, the optimal value of IF has a nonlinear dependence on µ and decreases as a function of E_b. The results can be used to help surgeons determine the optimal value of IF in a patient specific manner.