DISPLACEMENT OF CENTRE OF ROTATION MAY IMPACT PATIENT FUNCTIONAL OUTCOMES POST TOTAL HIP ARTHROPLASTY

Restoration of native hip joint mechanics and preservation of normal hip anatomy are critical in hip arthroplasty to reduce post-operative complications and attain full recovery of joint function. An important consideration in restoration of native mechanics involves the transfer of the hip joint center of rotation (COR) into the true acetabulum. Due to the mismatch of geometry between the native acetabulum (subhemispherical) and the prosthetic acetabular component (hemispherical), inadvertent displacement of COR is expected following full implantation of the acetabular component. Significant mediolateral (M-L) or superoinferior (S-I) displacement may lead to increased joint reaction forces, bearing surface wear, limb length discrepancy, abnormal gait patterns, reduction in original muscle function, joint instability, and ultimately, implant loosening. Maintaining S-I and M-L COR displacement within 3 mm and 5mm, respectively, has been suggested to avoid post-operative complications. The purpose of this study was to assess how COR displacement affects functional outcomes in patients who undergo THA through the direct anterior approach. COR was determined by drawing a circle of best fit on the native femoral head (preoperatively) or prosthetic femoral head (postoperatively). The S-I, M-L, and overall COR displacements were calculated based on these measurements. Vertical distance was measured as the distance between the inter-ischial line and the COR. Horizontal distance was measured as the distance between the COR and the line drawn perpendicular to the inter-ischial line and passing through the pubis symphysis. All measurements were calibrated based on a fiducial marker on preoperative radiographs or the known prosthetic femoral head on postoperative radiographs. Timed-up and Go (TUG) scores, hip abductor strength, pelvic tilt, limb length discrepancy, and patient-reported outcome questionnaires were collected. Sixty-five patients were recruited (24 females, age = 65.3 ± 11.9 years, BMI = 28.6 ± 5.1 kg/m2). Means of displacement were 2.1 ± 6.4 mm medially and 0.9 mm ± 7.4 mm superiorly. Overall displacement was 8.8 ± 4.5 mm. Superior displacement alone was weakly negatively correlated with limb length discrepancy (p = 0.04, r = -0.27). Greater medial COR displacement was correlated with worse functional results based on WOMAC (p = 0.01) and Harris Hip (p = 0.01) scores. When considering overall COR displacement, greater displacement correlated well with lower Harris Hip scores (p = 0.02) but was not correlated to any other outcome measure. Comparing high-risk (COR shifts greater than 5 mm medially and/or 3 mm superiorly) participants (n = 32) to their low-risk counterparts (n=33), we found WOMAC (p = 0.01) and Harris Hip (p = 0.03) scores suggesting significantly lower function post-operatively, but no other correlations were observed. Results of this study indicate that COR displacement following total hip arthroplasty through the direct anterior approach may meaningfully influence patient functional outcomes.