Accuracy of Joint Center Reconstruction in Total Hip Arthroplasty and Its Effect on Postoperative Femoral Axial Rotation.

Background: Displacement of the native hip center of rotation (COR) following total hip arthroplasty (THA) alters biomechanical load distribution and may adversely affect outcomes. This analysis quantified alterations in the COR following THA and their relationship with postoperative axial femoral rotation.

Methods: There were 938 patients evaluated following staged bilateral THA. Each underwent a three-dimensional preoperative planning analysis. Automated registration between preoperative and postoperative computed tomography scans enabled the hip COR to be accurately compared preoperatively versus postoperatively. Changes in global anterior-posterior (AP) shift were defined by the combined difference between the anteriorization of the femoral COR and the posteriorization of the pelvic COR. Changes in global AP shift versus femoral internal rotation were assessed using linear regression.

Results: The mean acetabular COR was medialized by 4.6 mm and femoral COR by 5.1 mm relative to the native COR, resulting in a global medial-lateral offset increase of 0.5 mm. Similarly, the achieved acetabular and femoral CORs were shifted proximally by 0.4 and 2.8 mm, respectively, resulting in an average increase in hip length of 2.4 mm. Acetabular and femoral CORs were shifted anteriorly by -1.7 and -0.8 mm, respectively, resulting in a global AP shift decrease of 0.9 mm. Change in global AP shift was moderately correlated with change in femoral internal rotation (r = 0.5), with a regression slope indicating an average increase in femoral internal rotation of 0.6° per millimeter increase in global AP shift (P < 0.0001). Global AP shift increased by > 5 mm in 24% of cases, resulting in an average increase in femoral internal rotation of 10°.

Conclusions: Acetabular COR is medialized by approximately five mm following THA with a compensatory increase in femoral offset, increasing muscle moment arms and increasing torque at the bone-stem interface. On average, global AP shift decreased by 1.0 mm, though there was significant patient-specific variability. When these changes exceed five mm, they can significantly alter axial rotational limb alignment.