The sacroiliac joint is the large joint between the sacrum at the base of the spine and the ilium of the pelvis. There are 2 (right and left) and when one becomes painful, it can cause pain in its immediate region or it can refer pain into your groin, abdomen, hip, buttock or leg. A sacroiliac joint injection may reduce inflammation and provide long term relief. It can also help determine the source of pain, if pain decreases after injection.