Joint anatomy in axial spondyloarthritis: strong associations between sacroiliac joint form variation and symptomatic disease

Objectives The aim of this investigation was to determine the prevalence of variant SI joint forms in symptomatic patients with mechanical joint disease (MJD) and axial SpA (axSpA) compared with control patients. Method A total of 973 patients were included in this cross-sectional study (95 axSpA; 61 MJD; 817 controls); clinical diagnosis, age and gender were noted. An established scoring system was used to classify joint forms on CT scans of the pelvis. Frequencies of joint forms were compared between groups (axSpA; MJD; controls). Results Patients with MJD exhibited the highest proportion of atypical joints (80.3% in MJD vs 44.1% in axSpA and 37.5% in controls; P < 0.001). Overall, females had a significantly higher proportion of atypical joints than men (65.0 vs 17.8%; P < 0.001); proportions of atypical joints were significantly higher in males with symptomatic joint disease than in male controls: 32.2% in axSpA, 55.0% in MJD and 13.9% in controls; P <= 0.001. Two specific joint forms were significantly more prevalent in symptomatic patients than in controls: the iliosacral complex (16.2 vs 4.2%; P < 0.001) and the crescent-shaped ilium (9.1 vs 2.8%; P < 0.05). Conclusions Our data demonstrate a strong association between atypical joint forms and symptomatic joint disease.