EP8.33 Evaluating Calcar Engagement and Post-Implantation Remodeling in Two Collared, Triple Taper Stems

Abstract Background Femoral stem collars are intended to improve axial stability and load transfer following total hip arthroplasty (THA). However, substantial variability exists in collar geometry across implant systems, and how collar size and collar–calcar interaction influences femoral remodeling and the risk of mechanical complications remains incompletely understood. The purpose of this study was to determine radiographic calcar remodeling over time and to compare reoperation rates between femoral stems with constant-length collars (CLC) and size-specific collars (SSC), with the goal of better defining an optimal collar geometry. Methods A retrospective cohort study was performed using an institutional total joint registry to identify patients undergoing primary THA with either a CLC stem or an SSC stem. Radiographic measurements of collar overhang and collar-calcar gap were obtained at immediate postoperative, 3-month, and ≥2-year follow-up. Logistic regression models evaluated predictors of collar-calcar gap formation and closure. Receiver operating characteristic analysis identified a threshold for predictive gap resolution. A numerical optimization model was used to estimate the medial-to-lateral collar length that minimized absolute collar-calcar mismatch over time. Results Collar-calcar gaps were present in 12% of cases postoperatively, with resolution in 87% at final follow-up. Initial gap magnitude was the primary predictor of gap closure (OR 0.30 per mm, p < 0.001), with ROC analysis identifying an optimal cutoff of 1.54 mm for gap resolution. Progressive medial collar overhang increased over time. Optimization modeling demonstrated that the collar length minimizing absolute collar-calcar mismatch decreased from 7.5 mm at implantation to approximately 5.3 mm at final follow-up. Reoperation rates did not differ between implant designs, however occurred disproportionately in smaller CLC femoral stem sizes. Conclusion Proximal femoral remodeling following THA results in predictable thinning of the femoral calcar and progressive medial collar overhang over time. These findings suggest that collar geometry, particularly in smaller femora, should be considered relative to patient anatomy rather than implant design philosophy alone.