Background Hip involvement is a characteristic extra-spinal manifestation of ankylosing spondylitis (AS). Inflammation of the hip joints may lead to structural damage. Hip involvement in AS is associated with more severe disease and functional impairment.
Objectives To assess the prevalence of hip involvement in AS patients with active disease. Furthermore, to investigate the mutual associations between the different assessments used to evaluate hip involvement.
Methods Between November 2004 en October 2008, consecutive outpatients from the Groningen Leeuwarden AS (GLAS) cohort before starting with Tumor Necrosis Factor (TNF)-α blocking therapy with radiographs of the pelvis and available Power Doppler Ultrasound (PDUS) examination of the hip joints were included. All patients fulfilled the modified New York criteria for AS. Patients history of hip involvement was assessed (yes /no) and the following assessments were evaluated in bilateral hip joints separately: tender hip joint assessed with physical examination (yes/no), PDUS examination for inflammatory (yes/no) and structural lesions (yes/no) and the Bath Ankylosing Spondylitis Radiology Index (BASRI-hip 0-4) to assess radiographic hip involvement. This was defined as BASRI-hip score ≥2. To investigate the mutual associations between these assessments Chi-square, Phi coefficients and percentage concordance were calculated.
Results Mean age of the 111 included AS patients was 43 years (SD ±10.9), median symptom duration was 15 years (range 2-49), 71% was male and 81% was HLA-B27 positive. History of hip involvement was present in 22 (20%) patients. Twenty-five (23%) patients had ≥1 tender hip joint. PDUS examination showed inflammatory lesions in 19 (17%) patients and structural lesions in 22 (20%) patients. Radiographic damage of at least one hip joint was seen in 11 (10%) patients. A strong correlation was found between history of hip involvement and BASRI-hip total score ≥2 (Table 1). In all 222 hip joints evaluated moderate correlations were found between BASRI-hip score ≥2 and structural PDUS lesions and between structural PDUS lesions and tender hip joints. As to the other assessments of hip involvement no significant correlations were found (Table 1). The concordance rates were relatively high due to overall few hip abnormalities (Table 1).
Conclusions In this cohort study the prevalence of hip involvement in AS patients with active disease ranged between 10% and 23% depending on which assessment used. The strongest correlation was found between radiographic hip joint damage and the history of hip involvement. Surprisingly, no association was found between tender hip joints assessed with physical examination and the inflammatory PDUS lesions.
Disclosure of Interest None declared