Sacroilitis. This 32 year old female patient has psoriatic arthritis, which is one of the potential causes of bilateral sacroiliitis. The radiographic features, shown here, are irregularity and blurring of the cortex adjacent to the joint because of erosions, and sclerosis of the subchondral bone (arrows). Although symmetrical in this example, sacroiliitis in patients with psoriatic arthritis is often quite asymmetric, whereas with ankylosing spondylitis it is usually symmetric. Other potential causes of erosions at the sacroiliac joints include Reiter’s (reactive) arthritis, rheumatoid arthritis and infection (septic arthritis).