Femoral component collar and calcar contact during total hip arthroplasty: gap incidence, subsidence, and biomechanical testing - a scoping review.

Introduction: In recent years collared femoral stems have been increasingly utilized in total hip arthroplasty (THA) for their ability to improve rotational stability, metaphyseal load transfer, and prevent periprosthetic femoral fracture (PFF). While literature has demonstrated their benefit over collarless stems, the clinical and biomechanical implications of a postoperative collar-calcar gap (CCG) after implantation remain incompletely understood. Objective: The purpose of this scoping review was to evaluate existing literature around CCG in primary THA. Methods: A PRISMA Scoping Review literature search was conducted across Ovid MEDLINE, Embase, Cochrane CENTRAL, and Scopus databases. Studies were included if they reported CCG incidence, subsidence, remodeling, or biomechanical performance in primary THA. Two independent reviewers screened studies and extracted data, and clinical study quality was assessed using the Newcastle-Ottawa Scale. Results: A total of 408 studies were identified and 11 met inclusion criteria. The incidence of postoperative CCG varied from 18.7% to 53.9%. Most gaps demonstrated closure over time (66-76%). Biomechanical and finite element studies showed improved stability, reduced micromotion, and decreased periprosthetic fracture risk when direct collar-calcar contact was achieved. In contrast, stems with persistent gaps demonstrated increased subsidence (up to 19% vs 1.3%) and biomechanical behavior comparable to collarless designs. Conclusion: CCG is common following THA. The majority of gaps resolve, however biomechanical evidence demonstrates that the advantages of collared stems depend on calcar contact. Preventing CCG may optimize early implant stability, though surgical technique must balance mechanical engagement with preservation of proximal femoral bone stock.