This disorder usually involves the narrowing of one or more of four areas of the spine:

• The canal in the center of the column of bones (vertebral or spinal column) through which the spinal cord and nerve roots run!
• The canals at the base or roots of nerves branching out from the spinal cord
• The openings between vertebrae (bones of the spine) through which nerves leave the spine and go to other parts of the body.
• Pressure on the upper part of the spinal cord (that is, the neck area) may produce similar symptoms in the shoulders, or even the legs.
• The narrowing may involve a small or large area of the spine.
• Pressure on the lower part of the spinal cord or on nerve roots branching out from that area may give rise to pain or numbness in the legs.

Who Gets Spinal Stenosis?

This disorder is most common in men and women over 50 years of age. However, it may occur in younger people who are born with a narrowing of the spinal canal or who suffer an injury to the spine. The spine is a column of 26 bones that extend in a line from the base of the skull to the pelvis (see fig. 1). Twenty-four of the bones are called vertebrae. The bones of the spine include 7 cervical vertebrae in the neck; 12 thoracic vertebrae at the back wall of the chest; 5 lumbar vertebrae at the inward curve (small) of the lower back; the sacrum, composed of 5 fused vertebrae between the hip bones; and the coccyx, composed of 3 to 5 fused bones at the lower tip of the vertebral column. The vertebrae link to each other and are cushioned by shock-absorbing disks that lie between them. The vertebral column provides the main support for the upper body, allowing humans to stand upright or bend and twist, and it protects the spinal cord from injury. Following are structures of the spine most involved in spinal stenosis.

Intervertebral disks—pads of cartilage filled with a gel-like substance which lie between vertebrae and act as shock absorbers.

Facet joints—joints located on the back of the main part of the vertebra. They are formed by a portion of one vertebra and the vertebra above it. They connect the vertebrae to each other and permit back motion.

Intervertebral foramen (also called neural foramen)—an opening between vertebrae through which nerves leave the spine and extend to other parts of the body.

Lamina—part of the vertebra at the back portion of the vertebral arch that forms the roof of the canal through which the spinal cord and nerve roots pass.

Ligaments—elastic bands of tissue that support the spine by preventing the vertebrae from slipping out of line as the spine moves. A large ligament often involved in spinal stenosis is the ligamentum flavum, which runs as a continuous band from lamina to lamina in the spine.

Pedicles—narrow stem-like structures on the vertebrae that form the walls of the front part of the vertebral arch.

Spinal cord/nerve roots—a major part of the central nervous system that extends from the base of the brain down to the lower back and that is encased by the vertebral column. It consists of nerve cells and bundles of nerves. The cord connects the brain to all parts of the body via 31 pairs of nerves that branch out from the cord and leave the spine between vertebrae.

Synovium—a thin membrane that produces fluid to lubricate the facet joints, allowing them to move easily.

Vertebral arch—a circle of bone around the canal through which the spinal cord passes. It is composed of a floor at the back of the vertebra, walls (the pedicles), and a ceiling where two laminae join.

Cauda equina—a sack of nerve roots that continues from the lumbar region, where the spinal cord ends, and continues down to provide neurologic function to the lower part of the body. It resembles a “horse’s tail” (cauda equina in Latin).

Degenerative Conditions

Spinal stenosis most often results from a gradual, degenerative aging process. Either structural changes or inflammation can begin the process. As people age, the ligaments of the spine may thicken and calcify (harden from deposits of calcium salts). Bones and joints may also enlarge: when surfaces of the bone begin to project out from the body, these projections are called osteophytes (bone spurs). When the health of one part of the spine fails, it usually places increased stress on other parts of the spine. For example, a herniated (bulging) disk may place pressure on the spinal cord or nerve root (see fig. 6). When a segment of the spine becomes too mobile, the capsules (enclosing membranes) of the facet joints thicken in an effort to stabilize the segment, and bone spurs may occur. This decreases the space (neural foramen) available for nerve roots leaving the spinal cord. Spondylolisthesis, a condition in which one vertebra slips forward on another, may result from a degenerative condition or an accident, or, very rarely, may be acquired at birth. Poor alignment of the spinal column when a vertebra slips forward onto the one below it can place pressure on the spinal cord or nerve roots at that place. Aging with secondary changes is the most common cause of spinal stenosis. Two forms of arthritis that may affect the spine are osteoarthritis and rheumatoid arthritis.

Osteoarthritis—Osteoarthritis is the most common form of arthritis and is more likely to occur in middle-aged and older people. It is a chronic, degenerative process that may involve multiple joints of the body. It wears away the surface cartilage layer of joints, and is often accompanied by overgrowth of bone, formation of bone spurs, and impaired function. If the degenerative process of osteoarthritis affects the facet joint(s) and the disk, the condition is sometimes referred to as spondylosis. This condition may be accompanied by disk degeneration, and an enlargement or overgrowth of bone that narrows the central and nerve root canals.

Rheumatoid Arthritis—Rheumatoid arthritis usually affects people at an earlier age than osteoarthritis does and is associated with inflammation and enlargement of the soft tissues (the synovium) of the joints. Although not a common cause of spinal stenosis, damage to ligaments, bones, and joints that begins as synovitis (inflammation of the synovial membrane which lines the inside of the joint) has a severe and disrupting effect on joint function. The portions of the vertebral column with the greatest mobility (for example, the neck area) are often the ones most affected in people with rheumatoid arthritis.

Other Acquired Conditions

The following conditions that are not related to degenerative disease are causes of acquired spinal stenosis:

Tumors of the spine are abnormal growths of soft tissue that may affect the spinal canal directly by inflammation or by growth of tissue into the canal. Tissue growth may lead to bone resorption (bone loss due to overactivity of certain bone cells) or displacement of bone.

Trauma (accidents) may either dislocate the spine and the spinal canal or cause burst fractures that produce fragments of bone that penetrate the canal.

Paget’s disease of bone is a chronic (long-term) disorder that typically results in enlarged and abnormal bones. Excessive bone breakdown and formation cause thick and fragile bone. As a result, bone pain, arthritis, noticeable bone structure changes, and fractures can occur. The disease can affect any bone of the body, but is often found in the spine. The blood supply that feeds healthy nerve tissue may be diverted to the area of involved bone. Also, structural problems of the involved vertebrae can cause narrowing of the spinal canal, producing a variety of neurological symptoms. Other developmental conditions may also result in spinal stenosis.

Fluorosis is an excessive level of fluoride in the body. It may result from chronic inhalation of industrial dusts or gases contaminated with fluorides, prolonged ingestion of water containing large amounts of fluorides, or accidental ingestion of fluoride-containing insecticides. The condition may lead to calcified spinal ligaments or softened bones and to degenerative conditions like spinal stenosis.

Ossification of the posterior longitudinal ligament occurs when calcium deposits form on the ligament that runs up and down behind the spine and inside the spinal canal. These deposits turn the fibrous tissue of the ligament into bone. (Ossification means “forming bone.”) These deposits may press on the nerves in the spinal canal.