The sacroiliac joint (SI) is the largest axial joint in the body averaging 17.5 cm2. The SI joint is surrounded by connections of ligamentous structures between the sacrum and the ilium. The main function of this ligamentous structure is to limit motion in all planes of movement. Muscles also surround the SI joint, which deliver regional muscular forces to the pelvic bones. The gluteus maximus, piriformis and biceps femoris are functionally connect to SI joint ligaments, so their actions can affect joint mobility. 

With age, the SI joint changes. During puberty, the iliac surface becomes more rough, more dull, and in some areas is coated with plaque. These changes are accelerated throughout the third and fourth decade of life, in addition to crevice formation, fibrillation and the clumping of chondrocytes. There is about a 10-20 year lag of degenerative changes that happen to the sacral side when compared to the iliac surface. Restriction of the joint is apparent during the sixth decade of life as the capsule becomes collagenous and fibrous ankylosis occurs. Erosions and plaque formations are inevitable by the eighth decade of life.  


The primary function of the SI joint is stability. It transmits loads to the lower extremity by movement in all three axes. A study by Cohen, 2005, found that the main motion in the SI joint of males are translation, where females is mostly rotation. 

Mechanism of Injury

The mechanism of injury of the SI joint has been described as a combination of axial loading, and abrupt rotation. Capsular or synovial disruption, capsular and ligamentous tension, hypomobility or hypermobility, extraneous compression of shearing forces, abnormal joint mechanics, microfractures or macrofractures, chondromalacia, soft tissue injury, and inflammation are all known reasons for SI joint pain. 

There are several risk factors that predispose someone to develop SI joint pain. This includes true leg length discrepency, gait abnormalities, prolonged vigorous exercise, scoliosis, and spinal fusion to the sacrum. Pregnancy predisposes women to SI joint pain because of the increase in weight gain, exaggerated lordotic posture, the mechanical trauma of parturition, and hormone-induced ligamental laxity. Laxity of the ligaments is due, in part, to the increased levels of estrogen and relaxin, and it predisposes parturients to sprains of the SI joint ligaments. 

Sacroiliac pain is most often associated with a physical event that initiates pain, such as stepping off a curb, a fall onto the hip or buttock, or lifting a heavy object. The most common systemic diseases that refers pain to the sacrum and sacroililac joint include endocarditis, prostate cancer or other neoplasm, gynecologic disorders, rheumatic diseases that target the sacroiliac area, and Paget's disease.

Cohen, S. P. (2005). Sacroiliac joint pain: a comprehensive review of anatomy, diagnosis, and treatment. Anesth Analg, 101(5), 1440-1453. doi: 10.1213/01.ane.0000180831.60169.ea