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OBJECTIVES To assess the impact of joint shape variations on inflammatory lesions on sacroiliac (SI) joint magnetic resonance imaging (MRI) in patients with axial spondyloarthritis (axSpA). METHODS A total of 1194 patients from four different prospective cohorts were evaluated with 684 (57.3%) having sufficient imaging data for inclusion available (379 axSpA, 305 controls). All images were evaluated for joint form, erosion, sclerosis, fat metaplasia and bone marrow oedema (BMO) by two independent readers. Logistic regression analyses were used to assess the association of joint form and lesions on imaging, separately for axSpA patients and controls. RESULTS Atypical joint forms were common in both axSpA (43.5%; 154/354) and control patients (44.2%; 134/303); both intra-articular variants and a crescent joint shape were significantly more common in axSpA patients (18.4% vs. 11.6%, and 11.0% vs. 5.3.% respectively; p< 0.001). AxSpA patients with intraarticular joint form variants had two-fold odds to exhibit erosions (OR 2.09; 95%CI 1.18-3.69) and BME (OR 1.79; 95%CI 1.13-2.82); this association was not observed in controls. Accessory joints increased the odds for sclerosis in axSpA patients (OR 2.54; 95%CI 1.10-5.84) and for sclerosis (OR 17.91; 95%CI 6.92-46.37) and BMO (2.05; 95%CI 1.03-4.07) in controls. CONCLUSIONS Joint form variations are associated with the presence of inflammatory lesions on SI joint MRI of axSpA patients. This should be taken into consideration in future research on the interplay of mechanical strain and inflammation in axSpA.