Total hip replacement: increasing femoral offset improves functional outcome

INTRODUCTION: The aim of this study was to assess the independent effect of radiographic measures of implant position, relative to pre-operative anatomical assessment, on the functional outcome of total hip arthroplasty according to change in the Oxford hip score (OHS) 1 year post surgery. METHODS: A prospective cohort study was preformed to assess whether improvement in functional outcome (change in OHS at 1 year) and the relationship with femoral offset and length, and acetabular offset and height. After a power calculation 359 patients were recruited to the study and radiographic measures were performed by blinded observers. Regression analysis was used to assess the independent effect of the four radiographic measurements after adjusting for confounding variables. RESULTS: There was a significant (p < 0.001) decrease in acetabular offset [5.3 mm, 95 % confidence interval (CI) 4.4–6.2] and increase in femoral offset (6.1 mm, 95 % CI 5.4–6.8). Hence there was no significant change in overall offset. Femoral offset was the only radiographic measure to be achieved statistical significance (r = 0.198, 95 % CI 0.063–0.333, p = 0.004) in relation to clinical outcome, with increasing offset being associated with a greater improvement in the OHS. On combining femoral and acetabular offset increasing offset was associated with a greater improvement in the OHS (r = 0.10, 95 % CI 0.01–0.19, p = 0.04). CONCLUSION: This study supports the long-held biomechanical theory of medialisation of the acetabular component with compensatory increased femoral offset results in improved functional outcome.