Inflammation of a SACROILIAC JOINT. These lie between the sacrum and the iliac bones. The condition may be the result of RHEUMATOID ARTHRITIS, ankylosing spondylitis (see under SPINE AND SPINAL CORD, DISEASES AND INJURIES OF), REITER'S SYNDROME, or the arthritis that occurs with PSORIASIS or infection. Sacroileitis causes pain in the lower back, buttocks, thighs, and groin. Stiffness may occur with ankylosing spondylitis. NON-STEROIDAL ANTI-INFLAMMATORY DRUGS (NSAIDS) relieve the symptoms. If the cause is infection, antibiotics are used.