Sacroiliac (SI) joint is the joint in the bony pelvis that connects the pelvic bone (ileum) to the lowest part of the spine (sacrum). It is a small and very strong joint, reinforced by strong ligaments that surround it. SI joint does not have much motion, and it transmits all the forces of the upper body to the pelvis (hips) and legs, acting as a shock-absorbing structure. Whenever an SI joint is irritated or injured, the resultant joint dysfunction may cause pain in the lower back and legs.
The pain is typically felt on one or both sides of the lower back and/or buttocks, and can radiate down the leg(s). The pain usually remains above the knee, but at times pain can extend to the ankle or foot. The pain is similar to sciatica, or pain that radiates down the sciatic nerve and is caused by a radiculopathy. The pain may also radiate into the groin area.
- Different types of arthritis that affect all the joints of the body can also affect the sacroiliac joint causing pain.
- Too much movement (hypermobility or instability)
- Too little movement (hypomobility or fixation)