Determinants of the primary stability of cementless acetabular cup implants: A 3D finite element study

Primary stability of cementless implants is crucial for the surgical success and long-term stability. However, primary stability is difficult to quantify in vivo and the biomechanical phenomena occurring during the press-fit insertion of an acetabular cup (AC) implant are still poorly understood. The aim of this study is to investigate the influence of the cortical and trabecular bone Young's moduli E_c and E_t, the interference fit IF and the sliding friction coefficient of the bone-implant interface μ on the primary stability of an AC implant. For each parameter combination, the insertion of the AC implant into the hip cavity and consequent pull-out are simulated with a 3D finite element model of a human hemi-pelvis. The primary stability is assessed by determining the polar gap and the maximum pull-out force. The polar gap increases along with all considered parameters. The pull-out force shows a continuous increase with E_c and E_t and a non-linear variation as a function of μ and IF is obtained. For μ > 0.6 and IF > 1.4 mm the primary stability decreases, and a combination of smaller μ and IF lead to a better fixation. Based on the patient's bone stiffness, optimal combinations of μ and IF can be identified. The results are in good qualitative agreement with previous studies and provide a better understanding of the determinants of the AC implant primary stability. They suggest a guideline for the optimal choice of implant surface roughness and IF based on the patient's bone quality.