Contact force analysis under femoral head micro-separation for ceramic-on-ceramic hip implants: finite element analysis and experimental validation

When a prosthetic hip fails to perform like an ideal ball-and-socket joint but instead permits small subluxation and edge loading between the joint members, the bearing surfaces are subjected to increased contact stresses and wear that could ultimately cause failure and require costly and painful revision of the prosthesis. The objectives of the present study were (1) to model and analyze one such adverse motion: rapid femoral head reduction; (2) to quantify the ensuing dynamic contact force and elevated contact stresses; and (3) to improve the relevance of hip joint wear tests by providing validated contact force values representing worst case conditions in ceramic-on-ceramic hip bearings. A dynamic model of head micro-separation occurring during normal human gait was examined via a combined approach of laboratory testing and finite element analysis (FEA). The testing validated the FEA (though with some explainable error) against measured values for normal velocity at a point on the femoral head and strain on the femur and femoral neck. Then, the FE model was used to analyze the contact forces and the stresses during the edge loading. This approach contradicted a key hypothesis of the study, specifically, that the duration of edge loading contact would be close to the period of vibrations in the femur. It was revealed that the peak contact stresses are strongly influenced by the model's input conditions rather than the femur's natural vibration characteristics. The results showed an increase in the peak femoral head velocity and the peak femoral strains during the reduction event as the micro-separation increased. Validation of the FEA against experimental results showed large errors in velocity and relatively small errors in strain. The submodel analysis showed much higher contact force and contact stress than the global model analysis, which was attributed to boundary conditions (BCs) and limitations of the software used. A better understanding of the cause of the errors in the approach undertaken demands an improvement in the submodeling procedure, which involves a more detailed analysis beyond the scope of this study.