Functional Significance of Interprosthetic Joint Motion in Bipolar Hemiarthroplasty for Femoral Neck Fractures

Background: Fracture neck of femur is a common injury in the elderly and is frequently treated with bipolar hip hemiarthroplasty. The proposed advantage of a bipolar prosthesis is dual articulation, with motion occurring both at the prosthesis-acetabulum interface and at the interprosthetic joint (IPJ). However, previous studies suggest that the bipolar prosthesis may progressively behave as a unipolar device due to reduced IPJ mobility. The relationship between IPJ movement and functional outcomes remains controversial. Objective: To radiologically assess IPJ movement in bipolar hip hemiarthroplasty performed for fracture neck of femur and to correlate these findings with functional outcomes at six months follow-up. Methods: This observational retrospective study was conducted at a tertiary care center and included patients who underwent bipolar hip hemiarthroplasty for fracture neck of femur. Radiological assessment of IPJ movement was performed using standard anteroposterior radiographs obtained in the immediate postoperative period and at six months follow-up. A total of 60 patients with displaced femoral neck fractures were included. The proportion of motion occurring at the IPJ was measured relative to total hip movement. Functional outcomes were assessed using the Harris Hip Score (HHS), and correlations between radiological findings and functional outcomes were analyzed. Results: Radiological evaluation demonstrated variable IPJ mobility among patients at six months of follow-up. A decline in IPJ movement was observed in a proportion of cases, indicating a tendency toward unipolar function. Functional assessment revealed predominantly good to excellent outcomes. Patients with preserved IPJ movement demonstrated better HHSs compared to those with reduced or absent IPJ mobility, showing a positive correlation between maintained IPJ motion and improved functional outcomes. Conclusion: IPJ mobility in bipolar hip hemiarthroplasty shows variability at six months following surgery for fracture neck of femur. Preservation of the IPJ movement is associated with superior functional outcomes. Radiological assessment of IPJ movement serves as a useful tool in evaluating prosthesis behavior and predicting functional performance after bipolar hip hemiarthroplasty.