Sacroiliitis is an inflammation of the sacroiliac joint, which is located on either side of the low back where the spine joins the hips (pelvic bones). Sacroiliitis can occur on one or both sides. Symptoms include pain in the buttocks or low back that can spread down the leg. Other symptoms can include stiffness in the hips or low back and worsening pain when sitting for a long time or when rolling over in bed. Causes of sacroiliitis can include any type of arthritis, infection, trauma, or pregnancy and childbirth.
Jameson, J. Larry, et al. Harrison's Principles of Internal Medicine, 20th Ed. New York: McGraw-Hill Education, 2018.