Accessory Sacroiliac Joint

10% - 30% of the population has an accessory sacroiliac articulation between the posterior superior iliac spine and the lateral sacral crest at the level of the S2 foramen. These most commonly represent fibrocartilagenous connections that develop in response to weight bearing, but may also be present congenitally and represent true joints.

Here we see bilateral accessory sacroiliac joints (arrows) between the posterior superior iliac spine and the lateral sacral crest at the level of the S2 foramen.

References

Ehara S, el-Khoury GY, Bergman RA. The accessory sacroiliac joint: a common anatomic variant. AJR Am J Roentgenol. 1988 Apr;150(4):857-9.