Factors Affecting Early Implant Subsidence after Arthroplasty Using a Cementless Femoral Stem for Displaced Femoral Neck Fracture.

Purpose: Use of cementless femoral stems for treatment of displaced femoral neck fractures is increasing; however, factors influencing early stability remain uncertain. The aim of this study was to identify patient-, morphology-, and surgery-related determinants of early subsidence using a single tapered, proximally porous-coated cementless stem. Materials and Methods: Patients who underwent cementless bipolar hemiarthroplasty (BHA) or total hip arthroplasty (THA) for displaced femoral neck fractures between September 2021 and August 2022 were reviewed retrospectively. Analysis was performed on standardized radiographs taken immediately postoperatively and at 3 and 6 months. Engh's method was used for measurement of stem migration, with ≥5 mm defined as significant. Femoral morphology (Dorr type), bone mineral density (BMD), stem alignment, and canal fill ratio (CFR) were assessed. Pearson correlation and multivariate regression were performed for identification of independent determinants of subsidence. Results: Eighty-six patients met inclusion criteria, and 8.14% showed significant subsidence. Age, BMI, ASA class, fracture pattern, Dorr type, Koval grade, BMD, and operation type showed no association with subsidence. Male sex and larger stem size showed association with greater early subsidence. Varus alignment showed the strongest association, whereas greater metaphyseal fill (high CFR) was protective. Conclusion: Early subsidence of cementless stems in femoral neck fractures is driven primarily by modifiable surgical factors. Achieving neutral alignment and adequate metaphyseal fill markedly reduces early migration, while the impact of demographic and bone-quality variables is limited. Optimizing these technical parameters may enhance initial stability in cementless arthroplasty.