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

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. 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. The overall flexion-extension ROM improved significantly with a median from 0° (0 ~ 120°) to 100° (30 ~ 130°) after THA (P