Spinal stenosis is narrowing of any of the vertebral openings that nerves go through. The narrowing compresses the nerve, and symptoms appear due to impaired nerve function. The spine is composed of approximately 33 interlocking bones, called vertebrae. It has two major functions: structural and functional. Structurally, the spine allows us to stand upright, provides a place for muscles, tendons and other bones to attach to, and gives us the flexibility to turn, bend and twist. Functionally, the spine provides protection for the spinal cord and distribution of spinal nerves.
The spinal cord carries all the messages between our brains and our bodies. Some of the fibers carry motor (movement) instructions to the body, and some carry sensation information to the brain. A pair of spinal nerves off of the cord and exit the spine between each pair of vertebrae, carrying messages to and from the body in that area. Nerves from the cervical spine go to the face, neck and diaphragm. Thoracic nerves go to the arms and chest. Lumbar nerves go to the hips, legs, rectum, urethra and sexual organs.
Each vertebra has a total of five openings that nerves pass through: the spinal canal, which contains the spinal cord, two neural foramina, or openings in the bone for the nerve root, and two intervertebral spaces. Spinal stenosis occurs when any of these openings is narrowed enough to compress a nerve.
Some people are born with spinal stenosis, or a congenital narrowing of the spinal canal or other openings. Spinal stenosis can be secondary to other congenital abnormalities, such as scoliosis or achondroplasia.
Most of the time, people acquire spinal stenosis as they age; it's rare in people younger than 50. Calcium deposits develop in the ligaments between vertebrae, we grow bone spurs, the vertebra do not fit together like they once did and they slip out of alignment – these are normal changes that occur as we get older, but they can cause spinal stenosis. Osteoarthritis and rheumatoid arthritis in the discs and facet joints contribute to spinal stenosis, too.
There are a few conditions that cause spinal stenosis that are either congenital nor the result of aging. Tumors can invade any of the spatial spaces and compress nerves. Paget's disease is a bone disorder that causes the vertebrae to thicken, obstructive the openings. Fluorosis, or excess exposure to fluoride, causing calcification of the ligaments around the spinal events. The posterior longitudinal ligament, which runs down the back behind the spinal cord, may turn to bone and put pressure on nerves.
Although there are several causes of spinal stenosis, the mechanism is always the same: a narrowed opening compresses a nerve, and that causes sensory and functional symptoms.