Lumbar spinal stenosis is caused primarily by age-related wear and tear that gradually narrows the spinal canal in the lower back. Degenerative spinal changes affect up to 95% of people by age 50, and lumbar stenosis is one of the most common results. By ages 60 to 69, nearly half of people show some degree of canal narrowing on imaging, with about 1 in 5 having significant narrowing. While aging drives most cases, several specific processes contribute to the problem, and a small number of people are born with a naturally narrow canal that makes them vulnerable earlier in life.
How the Spinal Canal Narrows Over Time
Your spinal canal is a bony tunnel running through the center of your vertebrae, housing the spinal cord and the nerves that branch off from it. In the lumbar spine (lower back), this canal doesn’t have much room to spare. When surrounding structures thicken, shift, or grow, they encroach on that limited space and compress the nerves inside.
No single change causes stenosis on its own in most people. Instead, several degenerative processes happen simultaneously over years or decades, each one stealing a little more space from the canal. The combination eventually crosses a threshold where nerves get squeezed enough to cause symptoms.
Disc Degeneration and Bulging
The discs between your vertebrae act as shock absorbers. They have a tough outer shell and a gel-like center that keeps them flexible. With age, discs lose water content and become less spongy. This causes two problems at once: the disc loses height, which changes how the vertebrae sit on top of each other, and the hardened disc can bulge backward into the spinal canal.
A bulging disc doesn’t have to be fully herniated (ruptured) to contribute to stenosis. Even a modest posterior bulge takes up space in an already tight canal. When combined with other changes happening at the same spinal level, disc bulging can be the factor that tips things from “narrowed but fine” to “narrowed and symptomatic.”
Facet Joint Overgrowth
Facet joints are small joints at the back of each vertebra that guide spinal movement. Like any joint, they’re lined with smooth cartilage. Over time, or with repeated stress, that cartilage wears down. When bone starts grinding against bone, your body responds by growing extra bone at the joint, a process similar to what happens in osteoarthritis of the knee or hip.
This overgrowth, sometimes called facet hypertrophy, enlarges the joint. Because the facet joints sit right at the edges of the spinal canal, an enlarged facet joint can push directly into the canal or into the smaller side tunnels (foramina) where individual nerve roots exit. The result is mechanical compression of the nerves.
Thickened Ligaments
A thick band of tissue called the ligamentum flavum runs along the back wall of the spinal canal. Its job is to stabilize the spine during movement. With age, this ligament can thicken and stiffen, folding inward toward the canal. Since it sits directly behind the spinal nerves, even a few millimeters of thickening can meaningfully reduce the available space. This is one of the most consistent findings on imaging in people with symptomatic stenosis.
Bone Spurs
Bone spurs (osteophytes) are bony projections that form along the edges of vertebrae and joints. They develop as the body tries to stabilize a spine that’s become less stable due to disc loss and cartilage breakdown. While bone spurs themselves aren’t inherently painful, they become a problem when they grow into the spinal canal or the nerve exit tunnels. Combined with disc bulging and facet overgrowth, bone spurs further crowd the already compromised canal.
Vertebral Slippage (Spondylolisthesis)
Spondylolisthesis occurs when one vertebra slides forward over the one below it. In adults, this usually happens because the facet joints and discs have degenerated enough that they can no longer hold the vertebra in place. Even a small amount of slippage changes the alignment of the spinal canal at that level, creating a pinch point where nerves get compressed.
Slippage is graded on a scale from I to IV. Most cases are low-grade (Grade I or II), meaning the vertebra has shifted a relatively small distance. Even low-grade slippage can narrow the canal enough to cause leg pain, numbness, or weakness, especially when it occurs alongside other degenerative changes at the same level. High-grade slippage (Grade III or IV) is less common but more likely to require surgical correction.
Being Born With a Narrow Canal
A small percentage of people are born with a spinal canal that’s naturally narrower than average. This is called congenital stenosis, and it’s determined by anatomy you can’t control. Research comparing people with congenital stenosis to those with normal canals found markedly shorter pedicles (the bony bridges connecting the front and back of each vertebra) at every lumbar level. At L4, for instance, pedicle length averaged 6.5 mm in the stenosis group compared to 9.2 mm in controls. Shorter pedicles mean less front-to-back depth in the canal.
People with congenital stenosis may never know it until the normal degenerative changes of middle age push their already-narrow canal past the tipping point. They tend to develop symptoms earlier than people who start with a wider canal, because they have less room to lose before nerves get compressed.
Spinal Injuries
Trauma from car accidents, falls, or sports injuries can contribute to stenosis in two ways. A fracture or dislocation can directly narrow the canal by pushing bone fragments or displaced vertebrae into the space. Even after the acute injury heals, the area may develop accelerated degeneration over the following years, leading to the same disc, joint, and ligament changes described above, just concentrated at the injured level. Swelling after spinal surgery can also temporarily compress the cord or nerves, though this is usually short-lived.
Less Common Causes
Several conditions outside of normal aging can narrow the spinal canal. Paget’s disease of bone causes abnormal bone remodeling, where bone breaks down and rebuilds faster than normal. The new bone is often larger, softer, and disorganized. When Paget’s disease affects the spine, the overgrown bone can encroach on the spinal canal and compress nerves. Spinal tumors, both cancerous and noncancerous, can grow within or next to the canal and take up space. Certain metabolic conditions that cause abnormal calcium deposits in spinal ligaments can also contribute.
Why Multiple Causes Matter
In most people over 50, lumbar stenosis isn’t the result of one problem. It’s the cumulative effect of discs losing height, facet joints enlarging, ligaments thickening, and bone spurs forming, all at the same spinal levels, over many years. This is why stenosis is rarely sudden. Symptoms like leg pain during walking, numbness, or heaviness in the legs typically develop gradually as the canal slowly runs out of room. Understanding that multiple structures are involved also explains why treatments often need to address more than one source of compression to provide relief.