Does hip center location affect the recovery of abductor moment after total hip arthroplasty?

Background: In total hip arthroplasty (THA), placing the cup in an anatomic position is not always possible in case of deformities related to developmental dysplasia of the hip (DDH). Thus far, the influence of a hip center on the abductor moment after THA has not been clearly elucidated. Therefore, we performed a retrospective study to assess (1) how abductor muscle moment recovers postoperatively in THA and (2) whether acetabular cup position affects the recovery of abductor moment. Hypothesis: A high hip center affects the recovery of abductor moment of a dysplastic hip after THA. Patients and Methods: We evaluated 100 patients, who underwent unilateral primary THA, at 12 months postoperatively. The study included 86 women and 14 men, with a mean age of 65.5 ± 9.9 years (range, 40 to 86 years). Patients with secondary osteoarthritis due to DDH were included (Crowe 1: 76; Crowe 2: 15, Crowe 3:9, and Crowe 4: none). A cementless straight stem was implanted in all hips. Hip abductor moment was measured using a belt-stabilized hand-held dynamometer. The ratio of moment of the affected side to that of the contralateral side was calculated as moment ratio. The horizontal and vertical centers of rotation (H-COR and V-COR) (with respect to the inter teardrop line) and vertical shift (V-shift) (difference in V-COR between the affected hip and the contralateral normal hip) were determined. Results: At 6 and 12 months postoperatively, the abductor moment ratios were 95.1 and 94.7%, respectively. Significant negative correlation was observed between the postoperative abductor moment ratio and V-COR at 6 months postoperatively (r = −0.2436, p = 0.0238). Significant delay in the recovery of abductor moment ratio was observed in the groups with higher hip center (V-shift > 15 mm) (odds ratio = 12.7; 95% CI: 2.11–232.1, p = 0.0034) at 6 months postoperatively, which was fully recovered at 12 months postoperatively. Discussion: Superior placement of a hip center, more than 15 mm above the true hip center, delayed the recovery of abductor muscle moment after THA. Level of evidence: III, retrospective comparative study.