Characteristics of hip involvement in patients with ankylosing spondylitis in Korea

Background/Aims: To evaluate the clinical course of hip arthritis and the risk factors for hip joint replacement in patients with ankylosing spondylitis (AS). Methods: In this retrospective analysis, we evaluated 488 AS patients at a single tertiary hospital. At baseline and the most recent visit to the outpatient clinic from the patients with hip arthritis in AS, radiographic hip arthritis was evaluated using the Bath Ankylosing Spondylitis Radiology Hip Index (BASRI-h). Also the average of the hip joint space width (interbone distance) at three distinct sites between the acetabulum and femoral head was recorded. Results: Among 488 patients with AS, 60 patients (12.3%) had hip arthritis. Erythrocyte sedimentation rate were associated with hip involvement (odds ratio, 1.01; 95% confidence interval [CI], 1.00 to 1.02; p = 0.004). Long disease duration and advanced axial disease were associated with severe hip arthritis ($3{\geq}BASRI-h$) at baseline. BASRI-h and interbone distance did not significantly change in patients with hip involvement during the follow-up period of $81.4{\pm}35.7$ months. Five patients had hip joint replacement surgery during follow-up period. The body mass index (BMI) and BASRI-h at baseline were associated with joint replacement surgery (hazard ratio [HR], 1.30; 95% CI, 1.00 to 1.67; p = 0.049 and HR, 20.64; 95% CI, 2.39 to 178.11; p = 0.006, respectively). Conclusions: Most of the patients with hip arthritis in AS showed no significant radiographic progression during the follow-up period. High BMI and advanced hip arthritis at baseline were associated with hip joint replacement surgery in patients with AS.