EP8.36 Initial Collar–Calcar Relationship Predicts Calcar Remodeling After Total Hip Arthroplasty

Abstract Background The use of collared femoral stems are becoming increasingly popular in total hip arthroplasty (THA) due to improved axial stability and decreased periprosthetic femoral fractures. However, the effect of the initial collar–calcar relationship on femoral remodeling is poorly understood. Methods A retrospective review of 700 primary THAs using triple-tapered collared stems with a minimum 2-year radiographic follow-up was performed. Radiographic imaging was analyzed to determine the collar–calcar relationship, measured as the distance between the medial edge of the collar and the medial calcar on antero-posterior radiographs at time zero and ≥2-year follow-up. The collar-calcar relationship was classified into three distinct groups: overhang, flush, and underhang. Overhang was defined as the collar extending medial to the calcar, flush as the collar sitting directly on top of the calcar, and underhang as the medial portion of the collar lateral to the medial calcar. The change in the collar–calcar relationship over time was analyzed as calcar remodeling. Furthermore, a linear regression model was used to determine how baseline position predicted 2-year collar position, with the goal to identify an ideal relationship at implantation to achieve a flush collar at two-year follow-up. Results At implantation, 370 (52%) of stems demonstrated a underhang, 56 (8%) were flush, and 274 (39%) showed overhang. For the entire cohort, the mean collar-calcar relationship changed from −0.68 ± 2.5 mm at implantation to 1.9 ± 1.9 mm at final follow-up (p < 0.001). Collared stems with underhang demonstrated the greatest remodeling (3.5 ± 1.9 mm), followed by flush (2.1 ± 1.4 mm) and overhang (1.5 ± 1.4 mm) (p < 0.001). Pairwise comparison demonstrated statistically significant difference between all three groups (p <0.001). Each 1 mm increase in baseline overhang was associated with a decrease of 0.5 mm change in calcar remodeling (95% CI −0.55 to −0.46; p < 0.001). To achieve a flush collar at two years a baseline underhang of −4.52 mm (95% CI −4.97 to −4.15) was required. Reoperation rates were low and did not differ between all three groups. Conclusions The collar–calcar relationship evolves predictably over time, with baseline position impacting final collar prominence. Immediate implant position does not reflect the final construct, however it does plays a role in calcar remodeling.