Ankylosing spondylitis (AS) is an inflammatory disease that causes arthritis (swelling and stiffness of the joints) in your spine. When you have it, your vertebrae slowly fuse and your spine becomes less flexible.
The joint between your hip and spine (called your sacroiliac joint) may also wear away over time. The word "ankylosing" means stiff or rigid. "Spondyl" means spine, and "itis" refers to inflammation.
The inflammation of AS can often cause "hot spots" in your body. These can happen where your ligaments and tendons attach to your bone, or the entheses.
Commonly, this causes inflammation in your heels (Achilles tendinitis), the bottoms of your feet (plantar fasciitis), the outside of your hips (trochanteric bursitis), and along your breastbone (costochondritis). A less common form, called peripheral spondyloarthritis, causes pain and swelling in the large joints of your arms and legs along with inflammation of your spine.
AS is progressive, which means it gets worse over time. This happens because continued inflammation in your spine damages it, and bone fuses together between your vertebrae.
As more damage happens, more bone fuses together, and you get less flexible and mobile.