FEMORAL NECK RESORPTION AFTER METAL ON METAL HIP RESURFACING

Introduction: Hip resurfacing is a popular alternative to total hip arthroplasty. It aims to provide more physiological loading of the proximal femur, avoiding stress shielding and associated bone resorption. However, finite element analysis of hip resurfacing systems suggests stress shielding occurs in the femoral neck below the prosthesis and theoretically may lead to femoral neck fracture. Nevertheless, recent DEXA studies indicate femoral bone stock preservation with no evidence of stress shielding. It is not yet known what actually occurs in clinical practice. The aim of this study was to assess whether femoral neck resorption occurs in one particular type of hip resurfacing post operatively and quantify the degree of narrowing. Methods: Femoral neck resorption of the Cormet 2000 metal-on-metal resurfacing prosthesis (Corin, UK) was assessed by measuring the prosthesis-neck ratio on postoperative radiographs. 39 prostheses (35 patients) were measured on plain AP pelvis radiographs taken immediately and mean 2 years post-operatively. Subsequent follow up radiographs were measured up to maximum 7 years follow up, mean 4.75 years. Results: There was evidence of neck atrophy in 35/39 hips at an average of 2 years post operation, with the overall average neck narrowing from a ratio of 0.86 to 0.81 at 2 years. Average neck ratios were calculated up to 7 years post resurfacing; 25 hips measured at 3 years average ratio of 0.81, 20 at 4 years ratio 0.8, 25 at 5 years ratio 0.8, 9 at 6 years ratio 0.8, 1 at 7 years ratio 0.81. Discussion: A degree neck resorption occurs with this resurfacing prosthesis by 2 years, however no further resorption appears to occur beyond this, to a maximum follow up of 7 years. This may indicate that early narrowing is more likely to be due to harmless bone adaption then a progressive absorption jeopardising the femoral neck.