Magnetic resonance imaging (MRI) scan of an axial section through the pelvis of a 26 year old male patient with sacroiliitis. This is an inflammation of the sacroiliac joint, the joint between the sacrum (at the base of the spine, bottom) and the ilium bones of the hip (right and left). In this cases it is a complication of ankylosing spondylitis, a form of inflammatory arthritis that mainly affects the spine. Extra bones forms in the inflamed joint as part of the healing process, eventually fusing them. This causes chronic pain and immobility. Treatment is with anti-inflammatory drugs and physical therapy.