Effects of hip joint center location and femoral off set on abductor muscle strength after total hip arthroplasty

Objectives. The purposes of this study were 1) to examine the changes in the hip joint center (HJC) position and the femoral off set (FO) after total hip arthroplasty (THA) and 2) to investigate the effects of the HJC and FO on isometric abductor muscle strength. Methods. We evaluated 51 patients who underwent unilateral primary THA. The FO, and horizontal and vertical distances from the HJC to the tip of the teardrop were measured and isometric hip abductor muscle strength was measured. Results. The HJC of the affected side moved medially postoperatively compared with that of the unaffected side (p < 0.05), and the FO was reconstructed similarly to the unaffected side. There were significant negative correlations between the changes in the horizontal distance from the HJC and FO to the tip of the teardrop. An increase in the FO and infero-medial cup position optimized hip abductor muscle strength. Conclusion. The HJC was reconstructed medially and superiorly, and the change in the FO after THA was influenced by the change in the horizontal distance of the HJC. Multiple regression analysis revealed that the medial and inferior HJC and increase in the FO constitute an effective procedure for restoring abductor strength.