An inflammatory arthritis affecting the spine and large joints. Ankylosing spondylitis is an inflammatory disease that, over time, can cause some of the small bones in your spine (vertebrae) to fuse. Ankylosing spondylitis (AS) is a rare type of arthritis that causes pain and stiffness in your spine. Ankylosing spondylitis (pronounced ank-kih-low-sing spon-dill-eye-tiss), or AS, is a form of arthritis that primarily affects the spine, although other joints can. Ankylosing spondylitis (AS) (ank-ee-lo-zing spon-dee-li-tus) is a type of arthritis that mainly affects the back, by causing inflammation in the spine. Ankylosing spondylitis is a form of ongoing joint inflammation (chronic inflammatory arthritis) that primarily affects the spine.