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The Sacroiliac Joint



The sacroiliac joint is a complex, imperfectly understood joint in the posterior (back) pelvis.

The SIJ is a shock absorber between the trunk, the legs and the pelvis itself.  In addition, it relaxes during pregnancy to allow the pelvis to expand to accommodate the baby.

The joint moves in an extraordinarily complex fashion, which has added to the confusion about its function and role.  It rotates, tilts, glides and "nods".  It has been difficult to determine exactly what the normal amount of motion the average joint has, although estimates range between 1 and 4 mm.



Anatomy of the SI Joint

The pelvis consists of three bones, the two iliac or wing-like bones that form the flair where you rest your belt and the sacrum or tail bone that forms the bridge in the back between the iliac bones.  The sacrum or tailbone joins the spine to the pelvis.  This is the point where the trunk or upper half of the body joins that lower extremities or bottom half of the body.

Although the sacroiliac joint is described as a synovial joint.  It is in fact, unique in that the joint is composed to two distinct types of cartilages, one of which promotes motion, the other restricts motion.  Furthermore, as we age the apposing surfaces of the joint become progressively more interdigitating, which further restricts motion.  Surrounding the joint is a soft tissue envelope called the capsule (think of the rubber boot around the universal joint in your car) and supporting soft tissue structures called ligaments with further stabilize the joint.

Adding to the confusion surrounding the anatomy of this joint is the fact that, unlike the knee or wrist, no two SIJs are shaped or oriented exactly alike; there is variation from one side to the other on the same individual and wide variation between different people. 




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