A sacroiliac (SI) joint injection – also called a sacroiliac joint block – is primarily used either to diagnose or treat low back pain and/or sciatica symptoms associated with sacroiliac joint dysfunction. The sacroiliac joints lie next to the spine and connect the sacrum with the hip on both sides. There are two sacroiliac joints, one on the right and one on the left. Joint inflammation and/or dysfunction in this area can cause pain.