Spinal stenosis is the narrowing of spaces within the spine, which can compress the nerves or the spinal cord traveling through them. This narrowing most frequently occurs in the lower back (lumbar stenosis) or in the neck (cervical stenosis). While acute injury or trauma may contribute, the vast majority of cases are acquired over many years. Spinal stenosis is a progressive disorder primarily related to the cumulative effects of aging, where spinal structures gradually change.
Degenerative Changes as the Primary Cause
The main cause of spinal stenosis is the natural, long-term wear and tear on the spine, medically termed degeneration or spondylosis. This age-related breakdown affects nearly all adults, but only causes symptomatic stenosis in a smaller percentage, typically those over 50. The spine attempts to stabilize itself in response to this decay, which physically reduces the space available for neural tissues. These cumulative changes build up over decades before symptoms like pain or weakness begin to manifest.
Osteoarthritis of the spine (spondylosis) is the primary driver of this narrowing. This condition involves the breakdown of cartilage in the facet joints, the small joints connecting the vertebrae. As protective cartilage wears away, the body attempts to secure the joint by triggering the formation of new bone and thickening soft tissues. This reaction directly encroaches upon the nerve pathways.
Intervertebral disc deterioration is another powerful contributor to the loss of spinal space. With age, the discs lose water content and elasticity, causing them to flatten and the space between vertebrae to collapse. A bulging or herniated disc may directly intrude into the spinal canal or the exit holes for the nerves, reducing volume and putting pressure on neural elements.
This degenerative process is the single most significant risk factor for acquired spinal stenosis. The condition is most commonly diagnosed in people aged 60 and older, reflecting the slow nature of the underlying structural changes. The progression is gradual, often allowing the body to adapt until the narrowing irritates or compresses the nerves enough to cause noticeable symptoms.
Key Structural Components that Lead to Narrowing
The mechanism of nerve compression involves specific structures physically invading the spinal space. One common narrowing agent is the formation of bone spurs, known as osteophytes, resulting from the body’s reaction to osteoarthritis. These bony growths develop on the edges of the vertebrae and facet joints in response to instability and cartilage loss. Osteophytes project into the central spinal canal or the neural foramen (the small openings where nerve roots exit), causing direct mechanical compression.
Another significant structural factor is the thickening of the ligamentum flavum, a strong ligament running down the back of the spinal canal. Due to chronic stress from disc collapse, this ligament loses elasticity and becomes thickened (hypertrophied) over time. When the spine moves backward, the thickened ligament can buckle inward, occupying limited space, particularly in the lumbar spine.
A secondary effect of advanced degenerative joint disease is spondylolisthesis, which also narrows the canal. This occurs when one vertebra slips forward or backward over the one below it, often caused by facet joint instability. This slippage displaces the bony alignment, reducing the available volume within the central canal and potentially trapping the nerves.
Less Common Contributors and Congenital Factors
While degeneration is the primary cause, spinal stenosis can result from factors distinct from typical age-related wear. Approximately nine percent of cases are congenital, meaning the individual was born with a naturally smaller spinal canal. Individuals with congenital stenosis, often due to short pedicles, are susceptible to becoming symptomatic earlier in life, as minor degenerative changes rapidly lead to nerve compression.
Acute trauma or injury, such as a severe fall, can cause stenosis by immediately compromising the canal space. This may involve vertebral fractures pushing bone fragments into the canal or major soft tissue swelling and dislocation. Additionally, prior spine surgery, such as a fusion, can accelerate degeneration in adjacent segments. This phenomenon, known as adjacent segment disease, develops because the fused segment places increased mechanical stress on neighboring unfused levels.
Systemic diseases can also contribute to spinal narrowing by altering bone structure or occupying space. For instance, Paget’s disease of the bone causes abnormal bone remodeling, resulting in the overgrowth and enlargement of vertebrae that constrict the spinal canal. Other less frequent causes include tumors or cysts that grow near the spinal column, physically occupying space and placing pressure on neural structures.