Ankylosing Spondylitis (AS) is a rare form of arthritis that affects 0.3% to 1.1% of the population. If left untreated, AS can cause the bones of the spine to fuse together. In fact, ankylosing means just this: to stiffen or to bend.

Likewise, patients with AS will also experience inflammation of the spine, known as spondylitis. Early warning signs of AS, for example, often include pain that radiates into the buttocks from the sacrum, pelvis, or SI joint.

Inside the body, the story is a bit more complicated. Inflammation attacks the vertebrae, spinal discs, and connective tissues of the spine: your tendons and ligaments. (Tendons connect muscles to bones; and ligaments connect vertebrae to their bony neighbors.)

Swelling in these areas causes pain and makes moving the spine very difficult. As the swelling goes down, your body tries to repair the damage as if you had suffered from an injury. Scar tissue begins to form over the enthesis (any zone where a ligament or tendon links up to a bone).

This process of scar formation, called fibrosis, can go on for quite some time. However, eventually, your body will replace the scar tissue with bone, or osseous tissue.

If enough vertebrae in your spine fuse together, then a patient can experience spinal rigidity, or bamboo spine. On an X-ray, your spine will appear as a singular fused column, much like a stalk of bamboo. Your doctor will no longer be able to see your spinal discs or the spaces that should exist between your bones.