Only two of the variants described, dysmorphic SIJ and unfused nuclei, occur at the cartilaginous SIJ compartment and may therefore directly cause MRI changes, such as subchondral edema, simulating sacroiliitis confined to the cartilaginous joint compartment. BME adjacent to accessory SIJ and iliosacral complex will be located to the ligamentous joint compartment where BME areas will not be mistaken for sacroiliitis changes. However, the presence of variants in the ligamentous compartment may elicit strain-related changes in the cartilaginous joint compartment, often located anteriorly in the sacrum, doi:10.1007/s00256-021-03843-3, an area often presenting strain-related edema.

The MRI appearance of the different variations is shown beneath.

Accessory (ACC) SIJ is an additional small joint in the ligamentous joint compartment. It can be uni- or bilateral and is mostly seen at the level of S2, but can also occur at S1 and S3, doi:10.1002/ca.20772. ACC SIJ is best visualized on semi-axial slices, but can be confirmed in the semi-coronal plane by the lack of interposed fat and ligaments between the joint facets.

Iliosacral complex consists of an osseous protrusion of the iliac bone in the ligamentous joint compartment with a corresponding groove in the opposing sacral bone visualized in both the semi-coronal and semi-axial plane.

Bipartite iliac bony plate is a cleft-like appearance posteriorly and inferiorly in the iliac bone best seen in the semi-axial plane. It is formed by a deep iliac insertion of an iliosacral ligament which is often surrounded by vessels, which can give rise to edematous changes, doi:10.1055/s-0034-1375574.

For comparison, a histological section showing the ligament (arrowhead) inserting deep within the iliac bone.

A semicircular defect is a round well-defined defect in the iliac and/or sacral bone superiorly in the ligamentous joint compartment.

Crescent-like iliac bony plate is a concave configuration of the iliac bone posteriorly/superiorly in the ligamentous joint compartment with a congruent bulging of the opposite sacral bone.

Dysmorphic SIJ is a term used for abnormal osseous protrusions in the cartilaginous joint compartment, either of the sacrum or the ilium. It can be accompanied by strain-related subchondral edema and/or erosion-like osseous irregularities and thereby mimic sacroiliitis, doi:10.1007/s00256-021-03843-3; doi:10.1007/s00330-018-5540-x.

Nuclei at the upper sacral corners (sacral wing), which are normal developmental findings in childhood, may occasionally persist in adulthood and can be associated with adjacent BME.