Influence of stem undersizing during direct anterior approach for total hip arthroplasty on periprosthetic bone biomechanics: a finite element analysis

Aims Stem undersizing during cementless total hip arthroplasty (THA) may occur more frequently when the procedure is performed through the direct anterior approach (DAA) because of limited femoral exposure. Insufficient stem–bone contact may compromise primary stability and alter load transfer in the proximal femur. The aim was to investigate the biomechanical effects of femoral stem undersizing on periprosthetic bone using finite element analysis. A secondary objective was to evaluate whether the presence of a femoral stem collar modifies these biomechanical effects. Methods Patient-specific finite element models of the full femur were generated from CT data. Eight finite element models were analyzed, combining four implantation configurations of the AMIStem-P designs (Adequate Fit, Adequate Fit–Collared, Undersized, and Undersized–Collared) with two cancellous bone density conditions, representing 100% and 90% cancellous bone density. A hip joint force representing the peak loading phase during normal walking/single-leg support was applied. Strain intensity in periprosthetic trabecular bone was assessed according to Frost’s mechanostat theory, and von Mises stress distribution in the femoral stem was evaluated. Results Stem undersizing markedly increased strain intensity in the periprosthetic trabecular bone and enlarged regions exceeding the mechanostat overload threshold (> 3,000 µε). Adequate stem–bone fit with cortical support substantially reduced the volume of overloaded bone tissue. The presence of a collar reduced strain levels, particularly in undersized configurations, indicating improved proximal load transfer. Reduced cancellous bone density resulted in a moderate additional increase in strain. Simulated load reduction markedly decreased periprosthetic strain levels. Implant stresses remained well below material limits. Conclusion Femoral stem undersizing may substantially increase strain within the periprosthetic trabecular bone and enlarge regions exposed to overload conditions. Achieving adequate stem–bone fit with cortical support was associated with a marked reduction in periprosthetic bone strain. The presence of a collar appeared to partially mitigate the adverse mechanical effects of undersizing by improving proximal load transfer. Cite this article: Bone Joint Res 2026;15(11):1354–1363.