Ankylosing Spondylitis (AS) is the most common form of spondyloarthritis, a type of arthritis that can cause pain and inflammation in and around joints in the spine, hips, chest wall, and anywhere ligaments and tendons join to the bone (such as the heel). AS mainly affects the spine, although other joints can be affected. One of the most common symptoms is lower back pain.

When AS is severe, it can cause some vertebrae in the spine to join together, or fuse. When these bones fuse, it can make the spine inflexible and create a hunched-over posture and severely limit mobility.