Effect of Femoral Head Radial Clearance on Acetabular Cartilage Degradation in Hip Hemiarthroplasty: An In Vitro Anatomical Simulator Study

Over 30,000 hip hemiarthroplasty (HA) operations are performed every year across England and Wales to treat fractured necks of the femur. HA reduces surgical and recovery time with lower complication rates; however, it may cause acetabular deterioration, which can lead to revision surgery and possible conversion to total hip arthroplasty (THA). This study assessed hip hemiarthroplasty under gait-representative loading in an anatomical hip simulator. Paired natural acetabula were tested against a CoCr femoral head with radial clearance (RC) of −0.75 mm (head larger than natural) and positive RCs of <0.6 mm (small), 2 mm–4 mm (large), and >4 mm (extra-large). Cartilage surface deterioration was quantified via photogrammetry. Cartilage surface changes were observed in all hemiarthroplasty groups, while no changes were observed in the control group. No statistically significant between-groups in the affected area were detected (Kruskal–Wallis, p > 0.29). The negative RC group showed statistically significant progressive worsening over time (Friedman: (χ²(2) = 8.00, p = 0.018). Groups differed in damage onset, location, intensity, and presence of delamination. Samples with negative RC (oversized head) produce earlier and progressive cartilage changes. The results highlight the importance of carefully measuring the native head diameter and choosing a femoral head size when performing HA.