What Defines an MRI Considered Indicative of Axial Spondyloarthritis in the Sacroiliac Joints: A Systematic Literature Review.

OBJECTIVE: To systematically review the literature to support guidance defining MRI criteria of sacroiliac joint (SIJ) involvement indicative of axial spondyloarthritis (axSpA) for classification. METHODS: The primary question for the SLR was 'Which MRI lesion, or combination of lesions, in the SIJ is most sensitive and specific for an MRI considered indicative of axSpA'. Three additional questions addressed an endpoint of clinician diagnosis of axSpA and the extent and location of the lesion(s). The SLR included all studies from January 2017 through October 2025, and 5 databases (Scopus, Web of Science Core Collection, Ovid MEDLINE, Ovid Embase, and Cochrane Library (via Wiley)) were searched. Data extraction and risk of bias assessment was done independently by three reviewers. RESULTS: Searches returned 1871 unique results, of which 30 were selected for full review and data extraction. Only 2 reports described 'MRI global indicative of axSpA' as reference criterion and only 5 assessed MRI lesion definitions in an inception cohort study design. The quantitative component for bone marrow edema (BME) of the ASAS 2009/2016 MRI definitions lacked specificity. BME in <4 SIJ quadrants was seen in anterior SIJ slices of peripartum women, and healthy individuals. Erosion and fat lesion were uncommon (<5%) in healthy individuals and disorders that mimic axSpA. BME adjacent to erosion and/or fat lesion was highly specific for axSpA. The quantitative cut-offs of active and structural lesions are derived from a single inception cohort, which would require future studies for external validation. CONCLUSION: These results reinforce the need to revise previously reported definitions of a positive MRI indicative of axSpA towards more stringent cut-offs that detail the extent and location of MRI lesions.