Femoral neck resorption following hybrid metal-on-metal hip resurfacing arthroplasty: a radiological and biomechanical analysis

Introduction: With the resurgence of resurfacing hip arthroplasty complications such as femoral neck fracture and thinning have been identified. We therefore conducted a radiological and biomechanical evaluation of factors affecting femoral neck resorption following resurfacing hip arthroplasty (RHA). Methods: We retrospectively reviewed 61 resurfacing hip arthroplasties in 53 patients with a minimum of a 2-year fol-low- up. Data regarding age, gender, body mass index, indication for surgery, and component size was obtained from case records. Radiographic measurements were made from standardised digital AP pelvic radiographs. The neck shaft angle, stem shaft angle, and the varus-valgus femoral stem alignment were calculated. Changes in abductor/body moment arm, hip ratio, and cup-to-neck ratio were calculated from the pre-op, immediate post-op and 2 year post-operative radiographs. Results: Femoral neck thinning was identified in 98% of cases (60/61) and was greater than 10% in 59% (39/61). The mean change in component-to-neck ratio was 0.12 (0-0.44). No significant relationship was found between the amount of femoral neck resorption and patient age, BMI, gender, diagnosis, component size or orientation. A significant positive correlation was found between a change in abductor moment arm and femoral neck resorption ( R = 0.575; p < 0.01). We also calculated that approximately one-third of the change in CNR could be explained by a change in abductor moment arm. From this we formulated the Pillai-Joseph equation to calculate projected thinning at 2 years from the initial post-operative radiograph (CNR difference = 0.094 x AMA difference + 0.129). Conclusions: RHA significantly alters hip biomechanics and this may result in altered loading patterns with adaptive remodelling causing neck thinning. In order to minimise neck thinning care must be taken not to increase the abductor moment arm.