Dysfunction in the sacroiliac joint, or SI joint, is thought to cause low back and/or leg pain. The leg pain can be particularly difficult, and may feel similar to sciatica or pain caused by a lumbar disc herniation.
Anatomical Source of Sacroiliac Joint Pain
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).
The joint typically has the following characteristics:
- Small and very strong, reinforced by strong ligaments that surround it
- Does not have much motion
- Transmits all the forces of the upper body to the pelvis (hips) and legs
- Acts as a shock-absorbing structure
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 joint pain. This 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.
This condition is generally more common in young and middle-aged women.