Ankylosing spondylitis (AS) is a form of spinal arthritis. It is a rheumatic disease that is associated with inflammation of the tendons and ligaments around the bones and joints of the spine. AS primarily affects the sacroiliac joints and the spine, but can also cause inflammation in the hips, shoulders and knees. In advanced cases of AS the inflammatory process promotes the development of new bone causing bone to bridge between adjacent vertebrae. In severe cases this can result in biological spinal fusion. Widespread spinal involvement can result in fixation of the spine in an immobile stooped-forward posture. The excessive forward posture is referred to as hyperkyphosis.
AS usually begins in late adolescence or early adulthood. The condition is more common in men although the incidence in women is increasing. The pattern of disease progression is no different between men and women. Unlike other forms of arthritis and rheumatic disease AS tends to develop in younger individuals, usually between the ages of 17 and 35.