Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease that affects the spine and the sacroiliac joints (SI). It makes part of the “seronegative spondyloarthropathies” or spondyloarthritides (SpA). SpA describes a group of inflammatory arthritis diseases that are clinically and genetically related, but have distinct features from one another. In the most advanced cases (but not in all cases), this inflammation can lead to new bone formation in the spine, causing the spine to fuse in a fixed, immobile position, sometimes creating a forward-stooped posture.