Distinguishing Between Sacroiliitis, Mimics

Background: MRI of the sacroiliac joints (SIJs) is often used for the diagnosis of early axial spondyloarthropathy (axSpA), with hallmarks of sacroiliitis being bone marrow edema (BME) and structural changes, such as erosions. Many noninflammatory conditions exist that can mimic sacroiliitis and, if unrecognized, can lead to overdiagnosis and unnecessary management. Objective: To review the most frequent differential diagnoses that may mimic inflammatory sacroiliitis in clinical practice. Discussion: Mechanical strain and osteoarthritis (OA) can mimic sacroiliitis by manifesting as BME, sclerosis, and subchondral fatty deposition on MRI. However, these characteristically manifest along the anterior and middle-third of the SIJ, as this is the primary weight-bearing region. The appearance of true sacroiliitis from axSpA is more diffuse and occurs along the posterior of the SIJ. Axial images along the long axis of the sacrum can confirm the location of imaging findings, with the imaging more...