Causes / typical injury mechanism: Sacroiliitis is inflammation of the sacroiliac (SI) joint(s). Chronic inflammation of the SI joint can lead to fibrosis and ossification. This chronic inflammation can arise from rheumatic diseases, pregnancy, infection, trauma, or overuse.

Bilateral sacroiliitis is usually associated with ankylosing spondylitis, prevalent in 90% of cases. Unilateral cases are more associated with reactive arthritis, psoriatic arthrosis, and infection.

Classic history and presentation: A classic history involves a young man experiencing atraumatic lower back pain (below L5), pain in the buttocks, and/or pain radiating down the thighs. Pain may be associated with numbness, tingling, or weakness and may be worse with sitting, lying down, or climbing stairs.

Risk factors:

• HLA-B27 positivity (ankylosing spondylitis)
• Reactive arthritis
• Pregnancy
• Usually seen in adolescent to middle-aged individuals
• More predominant in male individuals

Pathophysiology: The SI joints connect the iliac crests to the sacrum and help absorb shock from the upper and lower body. Because the joint capsule is relatively thin, fluid such as a joint effusion or pus can leak into surrounding areas to cause pain. The distribution of pain can vary if nerve roots are affected, from L2-S3.