what is si joint

The sacroiliac joint (SI joint) is a joint that links the pelvis and lower spine. It is located in the pelvis and connects the iliac bone to the sacrum, which is the lowest part of the spine above the tailbone. The joint is made up of the bony structure above the tailbone and the top part of the pelvis, with ligaments in each joint that hold the bones together. Sacroiliac joint dysfunction refers to abnormal motion in the sacroiliac joint, either too much motion or too little motion, that causes pain in this region. Symptoms of sacroiliac joint pain include lower back pain, buttocks pain, sciatic leg pain, groin pain, hip pain, urinary frequency, and "transient numbness, prickling, or tingling". Pain can range from dull aching to sharp and stabbing and increases with physical activity. Symptoms also worsen with prolonged or sustained positions (i.e., sitting, standing, lying) . Sacroiliac joint pain can be difficult to diagnose as other conditions can cause similar symptoms. Imaging is often recommended, starting with plain radiographs (X-rays). Sometimes, more advanced imaging, including a CT scan or MRI, is recommended. If the diagnosis is not clear, a diagnostic (anesthetic) injection into the sacroiliac joint can provide valuable diagnostic information.