The sacroiliac (SI) joint is unlike most other joints in the body in that it often demonstrates early manifestations of arthropathy before other joints. The SI joint is composed of hyaline and fibrocartilage on both sides of the joint; typically, sacroiliitis will affect the iliac side of the joint first where the cartilage is the thinnest, and then progress to the sacral side.1 In addition, synovial inflammation typically occurs first at the anterior-inferior aspect of the SI joint. The posterior-superior aspect of the SI articulation is a syndesmosis without cartilage, synovium, or a joint capsule.
Sacroiliitis is a nonspecific term used to describe any inflammatory condition involving one or both of the SI joints. Typical radiographic features of sacroiliitis include apparent widening of the joint space, osseous erosive change, and reactive subchondral sclerosis. As inflammation progresses, the radiologist may see joint space loss and eventual ankylosis of the joint.2 Sacroiliitis can be seen in the seronegative spondyloarthropathies such as ankylosing spondylitis, psoriatic arthritis, reactive arthritis, or enteropathic arthropathies; in addition, sacroiliitis may be seen in rheumatoid arthritis, in osteoarthritis, or in septic arthritis.
Sacroiliitis has been classified to occur in 3 major patterns: bilateral symmetric, bilateral asymmetric, and unilateral. Each pattern, as discussed in this CME activity in conjunction with additional radiographic findings, can aid the radiologist in forming an accurate differential diagnosis.