Spinal stenosis is caused by the narrowing of the spaces within your spine, and in most cases, the culprit is wear and tear that accumulates over decades. By age 70, imaging studies show some degree of spinal narrowing in up to 80% of people. While aging drives the vast majority of cases, some people are born with a naturally narrow spinal canal, and others develop stenosis after injuries or certain bone diseases.
Several structures can encroach on the spinal canal at the same time, which is why stenosis tends to worsen gradually rather than appearing all at once. Understanding what’s actually changing inside your spine helps make sense of the symptoms and the treatment options that follow.
How Aging Narrows the Spinal Canal
The process typically starts with the intervertebral discs, the rubbery cushions between each vertebra. As these discs lose water content and flatten with age, the vertebral segments above and below them become less stable. That instability forces the small joints at the back of the spine (called facet joints) to absorb more load than they were designed for. Over time, the extra pressure causes those joints to enlarge, stiffen, and eventually develop arthritis. In some cases, the joints fuse entirely.
This chain reaction doesn’t stop at the joints. As the facet joints enlarge, they push inward toward the spinal canal. The discs, now thinner and weaker, may bulge outward into the canal as well. And the ligaments that run along the back of the canal start to thicken. All of these changes eat into the same limited space where your spinal cord and nerve roots sit.
What makes degenerative stenosis so common is that none of these changes happen in isolation. A flattened disc shifts stress to the joints, which enlarge and crowd the canal, which is already being squeezed by a thickening ligament. Each problem feeds the next, and the narrowing compounds over years.
Bone Spurs and Osteoarthritis
Osteoarthritis is the single most common cause of bone spurs in the spine. As cartilage between the facet joints breaks down, the body attempts to stabilize the area by depositing new bone along the joint edges. These bony growths, called osteophytes, are essentially your body’s misguided repair attempt. The new bone doesn’t replace what was lost. Instead, it grows outward in irregular shapes that can jut directly into the spinal canal or the smaller openings where nerve roots exit.
Bone spurs can narrow the canal from multiple directions. They form along the facet joints at the back of the spine, along the edges of vertebral bodies at the front, and around the small passageways (foramina) where individual nerves branch off. A spur only a few millimeters thick can be enough to compress a nerve root in a space that’s already tight from disc degeneration or ligament thickening.
Ligament Thickening
A band of elastic tissue called the ligamentum flavum runs along the back wall of the spinal canal. In a healthy spine, it’s flexible and thin. But repeated mechanical stress over the years causes tiny injuries in the tissue. Each round of damage and repair gradually replaces the original elastic fibers with stiffer collagen fibers, a process called fibrosis. Researchers define clinical thickening at roughly 4 millimeters or more, at which point it starts to meaningfully reduce canal space.
The thickening is worst on the side of the ligament that faces the spinal cord, which is also the side that absorbs the most mechanical stress during everyday bending and straightening. Because this ligament spans nearly the entire length of the spine, it can contribute to narrowing at multiple levels simultaneously.
Disc Bulges and Herniations
A disc doesn’t have to rupture dramatically to contribute to stenosis. As the tough outer ring of a disc weakens, it can bulge outward like an underinflated tire, pressing into the front of the spinal canal. When the soft inner material actually pushes through a tear in that outer ring, the herniated portion can compress the spinal cord directly or press against a nerve root as it exits the spine.
Disc problems cause narrowing in different zones depending on where the bulge or herniation occurs. A central bulge narrows the main canal where the spinal cord sits. A herniation toward one side can compress a nerve root in the lateral recess, the small gutter where the nerve travels before exiting through the foramen. A far-lateral herniation can pinch the nerve at its exit point. Each location produces a different pattern of pain and symptoms.
Vertebral Slippage
When disc degeneration destabilizes a spinal segment enough, one vertebra can slip forward over the one below it. This condition, called degenerative spondylolisthesis, directly reduces the diameter of the spinal canal at that level. The forward shift misaligns the bony ring of the upper vertebra with the one beneath it, creating a step-off that pinches the canal like a kink in a garden hose.
Spondylolisthesis is particularly common at the L4-L5 level in the lower back and tends to affect women more than men. Even a small amount of slippage, combined with the disc and ligament changes already happening at the same level, can tip a mildly narrowed canal into symptomatic stenosis.
Being Born With a Narrow Canal
Some people are born with a spinal canal that’s narrower than average, a condition called congenital stenosis. In the cervical (neck) spine, this has historically been defined as a canal diameter under 12 to 13 millimeters on X-ray, or under 10 millimeters on MRI. People with congenital narrowing may have no symptoms for decades, but they have far less margin for error. The same age-related changes that a person with a roomy canal might never notice can cause significant nerve compression in someone who started with less space.
Research on cervical congenital stenosis has found that a canal diameter below 8 millimeters is the threshold where even minor trauma can result in spinal cord injury. Between 10 and 13 millimeters, pre-symptomatic changes in the spinal cord are already detectable on imaging. This is why some people develop stenosis symptoms in their 40s or 50s while others with more generous anatomy never do.
Paget’s Disease and Other Bone Disorders
Paget’s disease of bone causes an abnormal cycle of bone breakdown and regrowth. The process unfolds in three stages: an initial phase of excessive bone breakdown, a mixed phase where the body tries to rebuild, and a final phase where new bone is laid down in a disorganized, bulky pattern. In the spine, this produces enlarged, thickened vertebrae that encroach on the spinal canal. The bone itself is structurally weaker despite being larger, so it can also deform under load, compounding the narrowing.
Other conditions that alter bone structure, including certain metabolic disorders and skeletal dysplasias like achondroplasia, can narrow the canal through similar mechanisms of abnormal bone growth or shape.
Trauma and Spinal Fractures
A spinal fracture or dislocation can cause stenosis immediately if bone fragments are pushed backward into the canal. This type of acute narrowing is a medical emergency because the displaced fragments can compress the spinal cord directly. But trauma can also set the stage for stenosis that develops months or years later. A healed fracture may leave behind a slightly misaligned vertebra or accelerate degenerative changes at the injured level, gradually narrowing the canal over time.
Where Narrowing Happens Matters
The spinal canal isn’t a single uniform tube. It has distinct zones, and narrowing in each zone compresses different structures and produces different symptoms. Central stenosis affects the main canal and can compress the spinal cord itself (in the neck) or the bundle of nerve roots at the base of the spine (in the lower back). Lateral recess stenosis squeezes individual nerve roots just before they exit, often causing leg pain on one side. Foraminal stenosis narrows the small windows where nerves leave the spine entirely, which can cause sharp, radiating pain along the path of a single nerve.
Most people with symptomatic stenosis have narrowing in more than one zone and at more than one spinal level. The combination of where and how much the canal is narrowed determines whether you experience back pain, leg pain, numbness, weakness, or the characteristic difficulty walking that worsens with standing upright and improves when you lean forward.