Ankylosing spondylitis is a form of arthritis that mainly affects the lower back. It causes inflammation and damage at the joints, and first affects the sacroiliac joints between the spine and the pelvis. It also can affect other areas of the spine and other joints, such as the knee. Eventually, inflamed spinal joints can become fused, or joined together so they can't move independently. The word spondylitis refers to inflammation of the spine; ankylosis means fusion or the melding of two bones into one.
Ankylosing spondylitis is relatively rare. It affects about 1 in 1,000 people. It may run in families, although its cause is not understood. It most commonly strikes otherwise healthy young men. Men get this condition 10 times more often than women. The disorder most often appears between the ages of 20 and 40, but can develop in children.