Factors associated with poor hip flexion range of motion after total hip arthroplasty in patients with ankylosing spondylitis: a retrospective study on 538 consecutive hips.

Background: This study aimed to report the mid-term functional outcomes of total hip arthroplasty (THA) for the treatment of advanced hip involvement in ankylosing spondylitis (AS) and identify the factors associated with poor hip flexion range of motion (ROM) after THA in patients with AS.

Methods: We retrospectively investigated the mid-term functional outcomes in 313 AS patients (538 hips) who underwent primary THA from 2012 to 2017, with a mean follow-up of 7 years (range, 4-9 years). Postoperative functional outcomes were assessed by hip flexion ROM, Harris hip score (HHS), and the Western Ontario and McMaster Universities Arthritis Index (WOMAC). The hips were divided into poor (≤ 90°) and good hip flexion ROM (> 90°) groups based on the degree of hip flexion ROM recorded at the most recent follow-up. We grouped factors related to postoperative hip flexion ROM into three categories: preoperative (or patient-related), intraoperative (or surgery-related), and postoperative factors. Multivariate logistic regression was performed to identify the independent factors associated with postoperative poor hip flexion ROM.

Results: The overall flexion-extension ROM improved significantly with a median from 0° (0 ~ 120°) to 100° (30 ~ 130°) after THA (P < 0.001), and the mean HHS increased from 37 to 90 (P < 0.001). There were 102 hips (19%) with a hip flexion ROM of no more than 90°. The poor hip flexion ROM group had significantly lower postoperative HHS and WOMAC than the good hip flexion ROM group (85 ± 6 vs. 91 ± 4, P < 0.001; 63 ± 16 vs. 32 ± 16, P < 0.001). The result of multivariate logistic regression showed that male sex (odds ratio [OR] = 9.42, 95% confidence interval [CI], 1.23 to 72.03), bony ankylosis (OR = 3.02, 95%CI, 1.76 to 5.17), cup anteversion angle (OR = 0.96, 95%CI, 0.93 to 0.98), cup inclination angle (OR = 0.96, 95%CI, 0.93 to 0.99), American Society of Anesthesiologists (ASA) class III (OR = 6.23, 95%CI, 1.83 to 21.70), knee involvement (OR = 7.80, 95%CI, 2.75 to 22.16), and noise (OR = 0.45, 95%CI, 0.25 to 0.81) were independent factors associated with poor hip flexion ROM after THA in patients with AS.

Conclusion: Nearly one out of the five hips in patients with AS have a poor hip flexion ROM after THA. Care has to be taken in acetabular component positioning during THA and its effect on the postoperative hip flexion function should be considered in the patients. The optimum treatment strategy is that THA should be performed before ankylosis in patients with AS.