Injury to the ligaments that hold the sacroiliac joints in proper support is thought to be caused by a torsion or high impact injury (such as an automobile accident) or a hard fall, resulting in the hypermobility. As many as 58% of people diagnosed with sacroiliac joint pain had some inciting traumatic injury based on clinical examination findings. The joint that was once stabilized by strong ligaments, now overly stretched, sprained, or torn, will move beyond its normal range. This is thought to result in the ilium and sacral surfaces "locking" in an incongruent or asymmetrical fashion (one innominate bone is tilted anteriorly; the other innominate bone is tilted posteriorly) causing pain that can be debilitating.Hormone imbalances, particularly those associated with pregnancy and the hormone relaxin, can also cause a ligamentous laxity resulting in the weakening of the sacroiliac structure. During pregnancy, relaxin serves as natures way of allowing the female pelvis to achieve distention of the birthing canal. Pelvic joint pain in post pregnancy women is thought to be derived from the inability of the stretched out ligaments to return to normal tautness. Women who have delivered large babies or who have had extended labors also are prone to developing chronic sacroiliac joint pain and instability.In some people, the sacroiliac joints reverse the normal concave-convex locking relationship, which can lead to rotational misalignment.
Sacroiliac joint dysfunction generally refers to pain in the sacroiliac joint region that is caused by abnormal motion in the sacroiliac joint, either too much motion or too little motion. Signs and symptoms
Common symptoms include lower back pain, buttocks pain, sciatic leg pain, groin pain, hip pain (for explanation of leg, groin, and hip pain, see referred pain), urinary frequency, and "transient numbness, prickling, or tingling." Pain can range from dull aching to sharp and stabbing and increases with physical activity. Symptoms also worsen with prolonged or sustained positions (i.e., sitting, standing, lying). Bending forward, stair climbing, hill climbing, and rising from a seated position can also provoke pain. Pain can increase during menstruation in women. People with severe and disabling sacroiliac joint dysfunction can develop insomnia and depression. Sacral torsion that is untreated over a long period of time can cause severe Achilles tendinosis. Causes
Hypermobility
Sacroiliac joint dysfunction is an outcome of either extra-articular dysfunction or from intraarticular dysfunction. SI joint dysfunction is sometimes referred to as "sacroiliac joint instability" or "sacroiliac joint insufficiency"  due to the support the once strong and taut ligaments can no longer sustain. When the joint is hypermobile or loose, it is classified as an extra-articular dysfunction because abnormal joint movement and alignment is a consequence of weakened, injured, or sprained ligaments, while the joint itself is structurally normal and healthy. The sacroiliac joint itself often will not show degenerative changes, such as arthritis, until many years of the dysfunction being allowed to continue.