Dysfunction in the sacroiliac joint, or SI joint, is a well-known cause of low back pain. It can also cause leg pain. The leg pain can be particularly difficult to diagnose, and may feel similar to sciatica or pain caused by a lumbar disc herniation.

The sacroiliac joint lies next to the bottom of the spine, below the lumbar spine and above the tailbone (coccyx). It connects the sacrum (the triangular bone at the bottom of the spine) with the pelvis (iliac crest).

While it is not clear how the pain is caused, it is thought that an alteration in the normal joint motion may be the culprit that causes sacroiliac pain. The source of pain can be caused by either:

• Too much movement (hypermobility or instability): The pain is typically felt in the lower back and/or hip and may radiate into groin area.
• Too little movement (hypomobility or fixation): The pain is typically felt on one side of the low back or buttocks, and can radiate down the leg. 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.