Impact of stem collar to femoral calcar gap distance on postoperative bone remodeling in a fully hydroxyapatite-coated stem

The collar of a cementless stem in total hip arthroplasty (THA) improves fixation and stability by connecting with the femoral calcar. Proper attachment is essential for reducing micromotion and preventing periprosthetic fractures of the femur. However, gaps between the femoral calcar and stem collar can occur during surgery, and it is unclear what degree of gap can be tolerated. This study evaluated 156 patients who received primary THA with the CORAIL stem. Medial and lateral gap distance were measured by frontal X-ray of the hip joint at 1 week and 1, 3, 6, 12, 24, and 36 months after THA. Clinical outcomes were assessed using Japanese Orthopaedic Association hip scores, and radiographic assessments included femoral morphology and stem subsidence. Gaps were initially observed in 55 patients, with 42 showing complete gap disappearance within 36 months. On logistic regression analysis, lateral gap distance was shown to significantly influence gap disappearance, with an optimal cut-off of 5.5 mm by receiver operating characteristic curve analysis. A smaller lateral gap was correlated with quicker closure. Our findings suggest that a lateral gap ≤ 5.5 mm allows postoperative bone remodeling to close the gap, contributing to long-term implant stability in THA using the CORAIL stem.