Sacroiliitis is the inflammation of the sacroiliac joint, which is the meeting point of the lower spine and the pelvic bone. Symptoms include pain in the lower back and buttocks that can radiate down to the legs. It may be caused by inflammatory arthritis of the spine. Pain is usually exacerbated by prolonged standing or climbing stairs. Sacroiliitis can usually be treated with medication, physical therapy, or joint injections. In rare cases, surgical joint fusion may be necessary to relieve painful symptoms.