Seven S, et al. - Through a prospective cross-sectional study that involved 204 subjects in axial spondyloarthritis (axSpA) (n=41), lumbar disc herniation (n=25), women with (n=46) and without (n=14) postpartum (birth within last 16 months) buttock/pelvic pain, cleaning assistants (n=26), long-distance runners (n=23) and healthy men (n=29) without pain groups, experts assessed whether different types of magnetic resonance imaging (MRI) sacroiliac joint (SIJ) lesions could distinguish axSpA from conditions with buttock/pelvic pain of other reasons, including post-partum women and healthy persons. In axSpA subjects, MRI SIJ ankylosis and backfill were only observed, while bone marrow edema (BME) and fat lesions were seen in all non-axSpA groups. Only in axSpA and women with postpartum pain, erosion was noted. Spondyloarthritis Research Consortium of Canada BME ≥5 was only discovered in axSpA and women with postpartum pain, while fat lesions were ascertained at high cut-off levels, albeit seldom, in almost all groups. Out of 38 women that were randomized in the control groups and who gave birth, 2 had BME, none erosion or fat lesions, and none BME ≥4. Hence, especially women with postpartum pain, BME and fat lesions were most obvious in axSpA, however, it also occurred in other groups. Moreover, for axSpA, erosion above a certain threshold, backfill and ankylosis were extremely specific.