Sacroiliac Joint

The sacroiliac joint is a true diathrodial synovial joint, formed at the junction of the bilateral iliac wings with the sacrum. It provides pelvic stabilisation through the transmission and dissipation of forces from the trunk down to he lower extremeties.

The sacral surface of the joint is lined by hyaline cartilage, which is two to three times thicker than the iliac fibrocartilage counterpart. The anterior aspect of the joint is a synovial joint, while the posterior aspect is more of a syndesmosis with multiple myoligamentous attachments. Joint stability is provided by multiple ligaments and muscles. The gluteal muscles form some of the connections between the pelvis and hip, and can be painful in those with sacroiliac joint dysfunction.

The unique joint shape and irregularities of the joint surface causes high frictional resistance limiting joint movement, which is 1–3° in all three axes.

Injury can be caused by axial loading followed by abrupt axial rotation. Repetitive torsional forces can cause inflammation. Pathology can be present in any combination of the sacroiliac joint synovium, capsule, or ligaments, which may lead to hypo- or hypermobility.

The sacroiliac joint is around 1-2mm wide and is formed within S1, S2, and S3. The concave sacral surface joins with the convex ilium surface. The joint has an interlocking mechanism due to an irregular sacral surface, which gives the joint the highest friction coefficient in the whole body. The joint space decreases with age, and becomes stiffer and less effective at shock absorption.

Several ligaments help maintain stability and reduce the mechanical stress applied through the joint. The capsule is often indistinguishable from the surrounding ligaments.

The strongest ligament is the interosseous ligament. Other ligaments are the anterior (a simple thickening of the anterior joint capsule, providing little stability) and posterior sacroiliac ligaments, the sacrotuberous ligament which attaches to the coccygeal vertebrae and the ischial tuberosity (often blended with the posterior sacroiliac ligament, biceps femoris tendon, piriformis, and deep multifidus), and the sacrospinous ligament which joins the ischial spine to the sacrum and coccyx. The anterior joint is thought to be innervated by the ventral rami of L4 and L5, and the posterior joint by the lateral branches of the posterior rami of L5-S4. The superior gluteal nerve contributes.