The sacroiliac (SI) joint lies next to the sacrum (tailbone) and the iliac portion of the pelvic bone. The top portion of each ilium can be felt through the lower abdominal wall just below and lateral to the umbilicus. The pelvic bone connects to the hip joint so that the upper portion of the leg bone (femur) can attach to the pelvis.
The joint is small and strong, has strong ligaments attached, does not move independently, transmits the forces of the upper body to the hips and legs, and acts as a shock absorber. Disorders of the SI joint can cause low back pain. Inflammatory conditions such as sacroiliitis may be the result of arthritic conditions, SI joint dysfunction, degeneration of the SI joint, or misalignment of the joint.
The source or cause of SI joint pain can be due to too much movement called hypermobility that leads to instability. Pregnancy hormones and the pressure of the growing baby may cause this hypermobility. This may explain why it is more common in young women. In other cases, not moving enough or hypomobility can lead to fixation. Both cause pain and stiffness. With fixation, the pain is often in the buttocks, usually one-sided. It can radiate to the knees or ankles. Fractures to the pelvis may result in fixation symptoms.
Pain in the thigh or buttock that may radiate down the back of the leg like sciatica is one possible symptom. Most commonly just on one side, SI joint dysfunction is more common in young and middle-aged women. The pain is a result of excessive, too little or abnormal movement of the joint. Pain may be worse with sitting or worse when rolling over in bed. Stiffness of the hips and lower back after waking is common.