Spinal stenosis happens when the canal that houses your spinal cord and nerves gets too narrow, squeezing the structures inside. In most people, this narrowing develops gradually over decades as the spine wears down, but it can also be present from birth or triggered by specific diseases. Nearly half of adults in their 60s show some degree of narrowing on imaging, though not all of them experience symptoms.
How the Spinal Canal Narrows With Age
The spinal canal is a bony tunnel running from your neck to your lower back, protecting the spinal cord and the nerves branching off it. In a healthy adult lumbar spine, this canal measures roughly 15 to 20 millimeters front to back. When that diameter shrinks to 12 millimeters or less, it’s considered relative stenosis, meaning it can cause compression if anything else crowds the space. At 10 millimeters or less, it’s absolute stenosis, where the narrowing alone is enough to compress nerves.
Age-related degeneration is by far the most common driver. Several structures deteriorate at the same time, and their combined effects are what ultimately narrow the canal enough to cause problems.
Bone Spurs From Arthritis
Osteoarthritis of the spine causes the same kind of cartilage breakdown that affects knees and hips. As cartilage wears away at the facet joints (the small joints connecting each vertebra), the body tries to stabilize itself by growing extra bone. These bony overgrowths, called bone spurs, project into the spinal canal or into the side channels where nerve roots exit. They’re one of the most common findings on imaging in people with stenosis, and they tend to grow slowly over years.
Disc Degeneration and Height Loss
Your spinal discs act as shock absorbers between each vertebra. Over time, they lose water content and flatten. This dehydration reduces disc height, which has a cascading effect: as the vertebrae settle closer together, the openings where nerves exit (the foramina) shrink. The spine tries to stabilize itself around these flattened discs, and that stabilization process often produces more bone spurs and joint enlargement, compounding the narrowing. In advanced stages, the discs can bulge or collapse entirely, pressing directly against nearby nerves.
Thickening of the Ligaments
A band of elastic tissue called the ligamentum flavum lines the back of the spinal canal. In a healthy spine, it measures about 2 to 3 millimeters thick. In people with stenosis, it commonly reaches 4 to 5 millimeters or more. That might not sound like much, but inside a canal that’s only 15 millimeters wide, an extra 2 to 3 millimeters of tissue on each side can cut the available space dramatically. The thickening happens partly from the ligament losing elasticity and buckling inward, and partly from actual tissue growth in response to years of mechanical stress.
Vertebral Slippage
When the facet joints and discs degenerate enough, one vertebra can slide forward over the one below it. This is called degenerative spondylolisthesis, and it’s especially common at the L4-L5 level in the lower back. The slippage itself misaligns the canal, but it also triggers thickening of the ligamentum flavum and accelerates bone spur formation at the facet joints. Together, these changes produce both central narrowing (where the spinal cord or main nerve bundle sits) and lateral narrowing (where individual nerve roots exit).
Cysts Near the Facet Joints
Fluid-filled sacs called synovial cysts sometimes develop along the facet joints, particularly in the lumbar spine. They form in areas of repetitive motion and instability, essentially as a byproduct of joint degeneration. On imaging, they appear as bubble-like growths attached to a facet joint. These cysts are most common in people over 50 and can press into the spinal canal or nerve root channels, adding another source of compression on top of existing degenerative changes.
Congenital Stenosis
About 9% of spinal stenosis cases are congenital, meaning the person is born with a naturally narrow canal. One common form involves short pedicles, the bony projections that form the sidewalls of the spinal canal. When these pedicles are shorter than normal, the canal has less room from the start. People with congenital stenosis often don’t notice problems until their 30s or 40s, when even mild age-related changes push a canal that was already borderline into symptomatic territory. What would be a trivial disc bulge in someone with a spacious canal can become a significant compression event in someone whose canal was small to begin with.
How Common Stenosis Becomes With Age
Data from the Framingham Study, one of the longest-running health studies in the U.S., shows just how strongly age drives this condition. Among adults under 40, about 20% had relative stenosis and 4% had absolute stenosis on imaging. By ages 60 to 69, those numbers jumped to 47% and 19%, respectively. That means roughly one in five people in their 60s has significant narrowing of the spinal canal, whether or not they feel it.
Many people with radiographic stenosis have no symptoms at all. The mismatch between what imaging shows and what patients actually experience is one of the trickiest aspects of the condition. Narrowing on a scan doesn’t automatically mean pain or disability.
Metabolic Bone Diseases
Paget’s disease, a condition where bone breaks down and rebuilds abnormally, can cause vertebrae to enlarge and deform. When the pedicles and laminae (the bony arch behind the spinal canal) expand, they encroach on the canal from the sides and back. Compression fractures of the affected vertebrae can compound the problem by collapsing the front of the canal. Paget’s disease of the spine is uncommon, but it produces some of the most dramatic stenosis because the bone growth can be extensive and unpredictable.
Trauma and Post-Injury Changes
Spinal fractures, particularly burst fractures where bone fragments scatter into the canal, can cause immediate stenosis. But trauma also sets the stage for delayed narrowing. A fracture that heals with even slight misalignment changes how forces travel through that segment of the spine, accelerating wear on the discs, joints, and ligaments above and below the injury. Years later, the degenerative changes around the old fracture site can produce stenosis in someone who recovered fully from the original injury.
Why Multiple Causes Often Overlap
In practice, spinal stenosis rarely comes from a single source. A typical case in a 65-year-old might involve flattened discs reducing foraminal height, bone spurs from arthritic facet joints projecting into the canal, and a thickened ligamentum flavum taking up space from behind. Each of these changes might be mild on its own, but together they narrow the canal past the threshold where nerves start getting compressed. This is why stenosis tends to develop gradually and why symptoms often appear slowly rather than all at once. The canal can tolerate a surprising amount of narrowing before running out of reserve space.