Explicit Non-linear Finite Element Analysis for Prediction of Primary Stability in Uncemented Total Hip Arthroplasty

Primary stability in uncemented total hip arthroplasty is often assessed by insertion force, relative micromotions at the interface or subsidence of the femoral stem. This study aimed to simulate implantation process and evaluate the press-fit effect imposed at this stage on the primary stability outcome. The femur was modeled as an elasto-plastic material with element deletion based on cumulation of plastic strain represented as damage. The implantation phasewas simulated using a displacement constraint that led to implant self-positioning by frictional contact. A subsequent force-controlled, loading-unloading cycle corresponding to the peak of a gait cycle was applied. Implantation force and work, implant subsidence and relative micromotions during the loading cycle were quantified. The modeling strategy was performed on two levels of hypothetical initial press-fit to capture the extreme influence of press-fit on the subsequent outcomes. The implantation force was 21.5% higher for the high press-fit approach. The implant remained more stable during the loading cycle for low press-fit model based on subsidence values. Micromotions showed also a significant dependency on press-fit. The median values of micromotions were below 150 mu m for both press-fit models, which is in good agreement with the literature. This study demonstrates feasibility of simulating the implantation stage and investigating the effect of press-fit on primary stability of an individual total hip arthroplasty planning.