The normal vertebral canal provides adequate room for the spinal cord. Narrowing of the canal, which occurs in spinal stenosis, may be inherited or acquired. Some people inherit a small spinal canal or have a curvature of the spine (scoliosis) that produces pressure on nerves and soft tissue and compresses or stretches ligaments.

In an inherited condition called achondroplasia, defective bone formation results in abnormally short and thickened pedicles that reduce the diameter of (distance across) the spinal canal. Acquired conditions that can cause spinal stenosis are explained in more detail in the sections that follow.

Degenerative (Aging) Conditions, Including Osteoarthritis

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, and osteophytes (bone spurs) may form. When the health of one part of the spine fails, it usually places increased stress on other parts of the spine.

For example, a degenerative condition affecting the facet joints may eventually cause secondary changes, such as a herniated (bulging) disk that places pressure on the spinal cord or nerve root. 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.

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

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 change 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 root canals.

Spondylolysthesis

A condition in which one vertebra slips forward on another, may result from a degenerative condition or an accident, or 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.

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 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) 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.

Non-arthritic Acquired Spinal Stenosis

The following conditions that are not related to arthritis or 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 and the eventual collapse of the supporting framework of the vertebral column.

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

Although surgery that involves fusion (union) of vertebrae may be skillfully performed, tissue swelling after surgery may place pressure on the spinal cord.

Paget‘s disease of bone is a chronic (long-term) disorder that typically results in enlarged and deformed bones. Excessive bone breakdown and formation cause thick and fragile bone. As a result, bone pain, arthritis, noticeable deformities, 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 deformities of the involved vertebrae can cause narrowing of the spinal canal, producing a variety of neurological symptoms.

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.