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. 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 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 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.
The following conditions, 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 over activity 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.