Sacroiliac (SI) joint anatomy & function
The sacroiliac (SI) joint’s primary responsibility is to transfer the weight of the upper body to the lower extremities. 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.
In the upper portion of the joint, the sacrum and the ilium are not in contact but rather connected with powerful posterior, inter-osseous, and anterior ligaments. The anterior and the lower half of the joint is a typical synovial joint with hyaline cartilage on the joint surfaces. The SI joint is an axial joint with an approximate surface of 17.5 square cm. The joint surface is smooth in juveniles and becomes irregular over time. Motion (primarily rotation) decreases with aging, and increased motion is associated with pregnancy.
The sacroiliac (SI) joint is stabilized by a network of ligaments and muscles, which also limit motion in all planes of movement. The normal SI joint has a small amount of normal motion of approximately 2-4 mm of movement in any direction. The sacroiliac ligaments in women are less stiff than men’s, allowing the mobility necessary for childbirth.
Video - SI Joint Anatomy, Biomechanics & Prevalence
Sacroiliac (SI) Joint Pathology
Mechanical strain and injury to the SI joint are produced by either a combination of vertical compression and rapid rotation (i.e. carrying a heavy object and twisting), or by falls on the backside. Injuries of this type can produce ligamentous laxity and allow painful abnormal motion. Instability can also arise from lumbar spine surgery in which a large portion of the ilio-lumbar ligament is injured.
SI joint pain can also be caused by leg length discrepancy, gait abnormalities, prolonged, vigorous exercise, trauma, traumatic birth, and long scoliosis fusions to the sacrum. Painful sacroiliac joint arthritis can also arise from autoimmune disorders, such as ankylosing spondylitis, juvenile rheumatoid arthritis, Reiter’s Syndrome, psoriatic arthritis, and infections including staphylococcus, gonorrhea and TB.