A type of arthritis that affects the sacroiliac joint and spine. Symptoms include pain and stiffness that often starts in the low back. It can progressively affect the entire spine from the neck down. The spine's bones (vertebrae) may fuse together, resulting in a rigid spine. These changes may be mild or severe. Ankylosing spondylitis can also cause pain and stiffness in other joints, such as the hips and knees. The disorder is more common in males in the 20 to 40 year age range.