The Anatomy of Spinal Stenosis
Vertebrae - The Bones of the Spine
The spine is formed by a column of bones called vertebrae. The vertebrae are organized into 5 regions. For each region, the vertebrae are numbered from the top down, beginning with 1. The top region of the spine is called the cervical spine and includes the 7 vertebrae in our neck. The middle region is called the thoracic spine which consists of 12 vertebrae that bear our ribs. The lower region is called the lumbar spine and includes 5 vertebrae in our low back. All of these vertebrae are able to move, and this allows us to twist and bend our spine. The two lowest regions of the spine are the sacrum and coccyx. In these regions, the vertebrae blend together to form the back wall of our pelvis. Spinal stenosis is most often found in the neck, called cervical spinal stenosis, or in the low back, called lumbar spinal stenosis.
The vertebrae consist of two parts. The front part is a strong cylinder of bone called the vertebral body that is responsible for bearing our weight. The back of the vertebrae is an arc of bone called the neural arch. This arrangement of bone produces a space within the vertebrae that is called the spinal canal, which is essentially a bony passage or tube designed to protect the spinal cord and spinal nerve roots as they travel from the base of the brain to the tailbone.
When spines are compared between healthy people, differences in the size of the spinal canal are often noted. Some people have big spinal canals with lots of extra room for the nerve structures. Others have spinal canals that are small, with little extra space for the nerve structures. This is called congenital spinal stenosis, and people with this finding has less extra space to accommodate the effects of aging on the spinal canal, and are more likely to develop spinal stenosis.
Another feature of the vertebrae that is worth noting are the notches on the upper and lower surfaces of the segments of bone that attaches the neural arch to the vertebral body. Between every two vertebrae, these notches line up to form openings for the spinal nerves to pass through as they leave the spine. These openings are called neural foramina. Degenerative problems can affect the positioning of the vertebrae and cause misalignment of these notches and contribute to nerve opening stenosis call neural foraminal stenosis.
The Joints of the Spine
Movement between the vertebrae is allowed by 2 types of joints. Discs are structures that allow motion between the flat surfaces of the vertebral bodies by slightly change shape with movements. The second type of joints are called facet joints. There are right and left facet joints between the neural arches of every pair of vertebrae. Facet joints act as mechanical stops that limit movements between vertebrae and thereby stop movements that would severely reduce the size of the spinal canal. Because of their location, the discs and facet joints form part of the borders of the spinal canal, and therefore changes in these joints associated with aging can influence the size of the spinal canal and contribute to spinal stenosis.
The Ligaments of the Spine
Ligaments are strong bands of tissue that connect bones together. The spine has many strong ligaments. The ligaments of most importance for spinal stenosis are the ligamentum flavum. These special ligaments connect the neural arches of every two vertebrae and therefore forms part of the back border of the spinal canal. The ligamentum flavum are elastic like. This allows them to stretch and relax with movements of the spine with very little change in thickness, and therefore little effect on the size of the spinal canal. Any changes in the elastic characteristics of the ligamentum flavum result in thickening that can lessen the dimensions of the spinal canal.
The Spinal Cord and Nerve Roots
The spinal cord is the most important structure in the spine as it contains the nerve fibers and nerve cells that connect our bodies with our brain. Correct function of the spinal cord allows us to move and feel our trunk, arms and legs. Pairs of spinal nerve roots extend from the right and left side of the spinal cord throughout its length and exit the spine through the neural foramina. The spinal cord is not as long as the spinal canal, and usually ends in the upper lumbar region. Because of this, the lumbar and sacral nerve roots travel a long distance within the spinal canal before reaching their neural foramina. This bundle of nerve roots is called the cauda equina.
Aging of the Spine
If you have had MRI or CT imaging of your spine, and were told that you have spinal stenosis, it is likely that you were also told that you have a several degenerative problems in your spine that have caused that stenosis. Degeneration of the spine is not actually from wear and tear from using our spines, but instead results from a gradual loss of cell function within the spine tissues that is a normal part of aging. At some point, cell function becomes inadequate and the spine tissues slowly deteriorate. The age at which degeneration of the spine begins and the severity of spinal degeneration is largely determined by genetics.
Disc degeneration, dehydration, bulge, protrusion, herniation, rupture, narrowing and collapse are common terms that are used to describe degenerative findings of our discs. These degenerative changes of the discs can cause the back wall of the disc to extend into the spinal canal and neural foramina, reducing the room for the spinal cord, cauda equina and spinal nerves.
The ligamentum flavum also deteriorate as part of aging, changing from a thin elastic structure to a thick, stiff tissue. Because of its location, changes in the thickness of the ligamentum flavum associated with aging can influence the size of the spinal canal and contribute to spinal stenosis.
Similarly, as part of aging, facet joint arthritis develops with loss of joint cartilage, fluid build-up within the joints, thickening of the outer fibers of the joint called the joint capsule, and overgrowth of the bone surrounding the joints. As the facet joints form part of the back wall of the spinal canal and neural foramina, these changes can also lessen the space for the spinal cord and nerve roots.
Severe facet joint arthritis can disturb the ability of facet joints to act as mechanical stops that help to maintain the size of the spinal canal, allowing the vertebrae to slip forward. This is called degenerative spondylolisthesis, and when severe, this misalignment of the vertebrae can narrow the spinal canal and neural foramina, and worsen spinal stenosis.
Movements of the Spine
As we twist, bend, sit, stand and walking, the vertebrae of the spine are always moving and therefore the shape and size of spinal canal is always slightly changing. Sitting or bending the spine forward causes the spinal canal and neural foramina to become slightly larger. Standing, walking, bending backwards have the opposite effect and cause the spinal canal to become slightly smaller. These changes in spinal canal size are not a problem for most people as the average size of the spinal canal is normally twice as large as it needs to be to accommodate the spinal cord and nerve roots. However, for people with symptomatic lumbar spinal stenosis, these slight changes in the size of the spinal canal caused by changes in postures are often noted to bring on or relieve symptoms.