Anatomy of Ankylosing Spondylitis
In ankylosing spondylitis (AS), the whole spine can be affected, but symptoms usually begin in the low back. To understand how ankylosing spondylitis can cause your spinal bones to fuse, you should have a basic understanding of how your spine works.
As you can see from the image below, your back, or spine, is made up of many parts. First, we're going to look at the bone structures. Your backbone, also called your vertebral column, helps support a lot of your body weight, and it protects your spinal cord. You have 33 vertebrae (bones) that make up the vertebral column. In the image, they're labeled as "Vertebral Body."
Your spine is divided into regions: There's your neck (cervical spine), mid-back (thoracic spine), and low back (lumbar spine). At the bottom of your spine, you also have the sacrum and the coccyx, which is commonly called your tailbone. Again, AS generally starts in the lumbar spine and works its way up to the cervical spine.
The vertebrae in your neck are labeled C1-C7, meaning that you have 7 vertebrae in that region.
Most adults have 12 vertebrae in the thoracic spine (T1-T12), which goes from your shoulders to your waist.
Then there are 5 vertebrae in your low back (L1-L5).
Below your lumbar region, your sacrum is made up of 5 vertebrae between the hipbones. By the time you're an adult, these 5 bones have fused into one bone. The coccyx is made of small fused bones at the very tail of your spine (hence the tailbone).
In between your vertebrae, you have intervertebral discs (also labeled on the image). These act as pads or shock absorbers for your spine as it moves. Each disc is made up of a tire-like outer band called the annulus fibrosus and a gel-like inner substance called the nucleus pulposus.
Together, the vertebrae and the discs provide a protective tunnel (the spinal canal) to house the spinal cord and spinal nerves. These nerves run down the center of the vertebrae and exit to various parts of the body, where they help you feel and move. With ankylosing spondylitis, your spinal nerves can be pinched (also known as impinged or compressed) by the extra bone that develops as a result of AS.
Your spine also has facet joints, which are on the posterior side (back) of your vertebrae. These joints (like all joints in your body) help facilitate movement and are very important to your flexibility. The joints are covered by cartilage that protects your bones as you move. In ankylosing spondylitis, the cartilage can be destroyed—inflammation and chemicals released by the inflammation can destroy it. The cartilage can then be replaced by scar tissue.
Your back also has muscles, ligaments, tendons, and blood vessels. Muscles are strands of tissues that act as the source of power for movement. Ligaments are the strong, flexible bands of fibrous tissue that link the bones together, and tendons connect muscles to bones and discs. Blood vessels provide nourishment. These parts all work together to help you move.