The sacroiliac joint or SI joint is the joint in the bony pelvis between the sacrum and the ilium of the pelvis, which are joined by strong ligaments. The sacrum supports the spine and is supported in turn by an ilium on each side. The joint is a strong, weight bearing synovial joint with irregular elevations and depressions that produces interlocking of the two bones. The human body has two sacroiliac joints, one on the left and one on the right, that often match each other but are highly variable from person to person.
IN SHORT: the SI joint is an interlocking joint stabilized by strong ligaments, designed to support the upper body, connecting torso to legs, and not designed to be a freely moving joint.
When the ligaments of the SI joint are overstretched or injured, the sacral and iliac bones may disengage from their interlocked position, destabilizing the joint and causing pain. This may be called SI joint instability, dysfunction, or injury. The pain may be localized to one hip area, or radiate through the low back or down the legs.
SI joint injury frequently intersects with other causes of low back pain, such as bulging discs, arthritis, and muscular strains.