Factors Associated with Radiographic Spinal Involvement and Hip Involvement in Ankylosing Spondylitis

Objectives: To determine the factors associated with radiographic spinal involvement and hip involvement in ankylosing spondylitis (AS) and assess the influence of the damage seen in the radiographs on functional outcome in patients with AS. Methods: We included 531 consecutive patients and recorded the clinical, laboratory, and radiographic data. Based on the spinal radiographs, patients were classified into 3 categories: (1) no spinal involvement; (2) spinal involvement without fusion; and (3) spinal involvement with fusion. Hip involvement was assessed by the Bath Ankylosing Spondylitis Radiology Hip Index and defined by a score of at least 2. Logistic regression analyses were used to investigate the factors associated with the radiographic spine and hip involvements. Results: Ninety-eight (18.5%) patients had radiographic evidence of spinal fusion and 48 (9.0%) had radiographic evidence of hip involvement. Patients who had longer disease duration, elevated C-reactive protein levels, advanced sacroiliitis, and radiographic hip involvement were significantly more likely to have spinal fusion (P < 0.05). Elevated C-reactive protein levels and advanced sacroiliitis were also significantly associated with the presence of spinal involvement without fusion (P < 0.05). Early disease onset and more radiographic severity in the spine and sacroiliac joints were the predictors of radiographic hip involvement (P < 0.05). Patients with either spine or hip involvement had significantly higher Bath Ankylosing Spondylitis Functional Index scores (P < 0.001). Conclusion: There is a relationship between radiographic sacroiliitis, spinal fusion, and hip involvement in patients with AS. Damage to the spine and hip seen radiographically can contribute to functional impairment.