Ankylosing spondylitis 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.
Because this new bone is stiff, as opposed to the elastic quality of the tissue or ligaments, the normal range of motion of the spine is diminished.
Ankylosing spondylitis most commonly is diagnosed in young men between the ages of 15 and 30, although it can affect anyone. Almost everyone with ankylosing spondylitis carries a specific gene called HLA-B27.
For more in-depth information, see Ankylosing Spondylitis on Arthritis-health.com