Chapter 2: SI Joint Anatomy, Biomechanics & Prevalence

The sacroiliac (SI) joint is the connection between the spine and the pelvis. The SI joint is a true diarthrodial joint. The articular surfaces are ear shaped, containing irregular ridges and depressions. Its concave sacral surface is covered with thick hyaline cartilage and its convex iliac surface lined with thin fibrocartilage.

Biomechanically, the SI joint's movement is induced by motion occurring at other locations in the body. This movement is very small, with less than 4 degrees of rotation and less than 1.6 mm of translation.

While the SI joint is vulnerable to shear during rotation or translation, compression of the joint allows it to resist shear.

Those structures that produce joint compression include the interosseous ligaments and the joint capsule including strong posterior ligaments protecting the network of adjacent nerves. The muscles that help stabilize the SI joint include the piriformis, the psoas, the illiacus, the glutes, and the hamstrings. The hamstring muscles do not cross the SI joint but may be associated with SI joint disorders.


Dr. Reckling is an Employee of SI-BONE Inc.

The iFuse Implant System is intended for sacroiliac fusion for conditions including sacroiliac joint dysfunction that is a direct result of sacroiliac joint disruption and degenerative sacroiliitis. This includes conditions whose symptoms began during pregnancy or in the peripartum period and have persisted postpartum for more than 6 months. There are potential risks associated with the iFuse Implant System. It may not be appropriate for all patients and all patients may not benefit. For information about the risks, visit: www.si-bone.com/risks

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