Ankylosing spondylitis (AS) is a type of arthritis that primarily affects the spine. The disease is characterized by inflammation and resulting stiffness and pain in the joints along the spine. The knee and shoulder joints may also be affected. With ankylosing spondylitis (pronounced ankle-low-zing spond-ill-eye-tis), the inflammation in the joints of the spine will typically result in parts of the vertebrae and joints in the spine fusing together. As parts of the vertebrae grow (or "fuse") together, the spine becomes more rigid and inflexible. The fusion takes place as a reaction to inflammation of ligaments or tendons at the site of attachment to bone. The inflammation causes bone to erode at the site of the attachment, and then as the inflammation subsides, the body's natural healing process causes new bone growths in its place.