It is important to know the imaging characteristics of disorders which may simulate SpA changes in the sacroiliac joint (SIJ) and the spine. The main differential SIJ differential diagnoses are listed beneath and illustrated under the different headings, and the spinal differential diagnosis are described in the section Spine - differentials.

Sacroiliitis as part of spondyloarthritis (SpA) has to be differentiated from disorders which may present with imaging features somewhat similar to those seen as part of axSpA changes. Differentials are especially important in the interpretation of the SIJ because sacroiliitis is a hallmark for the diagnosis of axSpA changes (doi:10.3390/jcm12031039).

The most frequent differential diagnoses to axSpA sacroiliitis by MRI are:

• Osteitis condensans ilii (OCI) and pregnancy-related changes

• Other strain-related SIJ changes

• Degenerative SIJ changes/osteoarthritis

• Diffuse idiopathic skeletal hyperostosis (DISH)

• Infectious sacroiliitis

• Fractures (stress and insufficiency)

• Inflammatory conditions

• Metabolic disorders

• Tumors