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

"We will briefly review the characteristics and biomechanics of the SI Joint.

The 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. In addition, a number of muscles contract and co-contract to provide stabilization for the SI Joint during movement.

The sacroiliac joint as a joint, just like any other joint in the body, is subject to both internal and external forces and can be affected by a variety of processes or problems.  One of those problems could be an inflammatory arthritis or autoimmune problem such as rheumatoid arthritis or ankylosing spondylitis.  

The joint can become degenerative because of trauma, either acute trauma such as a motor vehicle accident or a fall or a chronic repetitive trauma.

In some series, up to 50% of the patients with sacroiliac joint dysfunction or pain had reported a precipitating traumatic event. 

The joint could also be subjected to increased stress as a result of being next to a lumbar fusion.  We call that adjacent segment degeneration.

Another common problem with the sacroiliac joint is that it becomes hypermobile or dysfunctional.  This would typically occur in a middle aged female.  Females are somewhat more ligamentously lax than males and it commonly occurs in females who’ve had multiple pregnancies.

The clinical literature would represent that between 15 and 25 percent of patients presenting with low back pain have the sacroiliac joint as the source of their pain."