The sacroiliac (SI) joints connect the lower spine, called the sacrum, to the pelvis. Like all joints, the SI joints may become irritated and inflamed. A SI joint injection may be used to diagnose low back pain, confirm SI joint dysfunction, and treat SI joint pain. The injection delivers a local anesthetic and corticosteroid medication to the joint. Many clients find immediate relief from an injection and are able to resume normal activities and participate in a physical therapy program.