The sacroiliac (SI) joint is a large joint in the lower back. When it becomes painful, surrounding areas like the abdomen, hip, buttock, groin, or leg area can be affected. A sacroiliac joint injection is a treatment that injects anesthetic into the joint and the amount of pain relief you experience as a result will help in diagnosing whether the sacroiliac joint is truly responsible for the pain that you feel. Cortisone will also be injected into the joint and that will help with inflammation which in turn will relieve pressure and can provide long-term pain.