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The
normal vertebral canal (see fig. 3) 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 (see fig. 4) 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 (see fig. 5). 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.¹ 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 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.
Nonarthritic 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.
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