Lumbar stenosis is caused by a gradual narrowing of the spinal canal in the lower back, most often from age-related wear and tear on the spine’s discs, joints, and ligaments. About 20% of people over 60 show signs of it on imaging, though 80% of them never develop symptoms. When the canal does narrow enough to press on nerves, the result is pain, numbness, or weakness in the legs that typically worsens with standing and walking.
How the Spinal Canal Narrows Over Time
The spinal canal is a bony tunnel that runs through the center of your vertebrae, housing the spinal cord and the bundle of nerves that branch off from it. In your lower back, there isn’t much extra room. The canal is only about 15 to 20 millimeters across at baseline, so even a few millimeters of encroachment from surrounding structures can start compressing nerves.
Lumbar stenosis isn’t caused by one thing going wrong. It’s the result of several structures deteriorating at the same time, each one contributing a little bit of narrowing until the cumulative effect becomes a problem. The process typically unfolds over years or decades, which is why symptoms rarely appear before middle age and become increasingly common after 60. A Japanese population study found symptomatic stenosis in 5.5% of people aged 60 to 69, rising to nearly 11% in those aged 70 to 79.
Disc Degeneration: Where the Process Starts
The intervertebral discs between your vertebrae act as cushions. Over time, they lose water content, flatten, and bulge outward. This matters because a bulging disc pushes directly into the spinal canal from the front, reducing the available space for nerves. But the damage doesn’t stop there. Disc degeneration may actually be the critical first step in a chain reaction that affects the rest of the spine.
As a disc loses height, the vertebrae above and below it settle closer together. This changes the alignment and loading of the joints and ligaments behind the disc, accelerating their breakdown too. The increased mechanical stress from a collapsing disc transfers to the facet joints and the ligaments connecting the vertebrae, pushing those structures to degenerate in turn. So while disc changes alone can cause some narrowing, their bigger role is triggering problems in the structures around them.
Facet Joint Enlargement
The facet joints are small, paired joints at the back of each vertebra that guide and limit spinal movement. When discs lose height, the facet joints absorb more force than they were designed for. The joint surfaces develop arthritis, and the bone around them thickens and enlarges in response to the extra load. This overgrowth, called facet hypertrophy, pushes into the spinal canal from the sides and back.
Facet joint enlargement is a major contributor to lateral recess stenosis, which is narrowing in the side channels where individual nerve roots pass before exiting the spine. These lateral recesses are already tight corridors, so even modest bone growth can pinch a nerve root. The superior articular process (the upward-facing portion of each facet joint) tends to enlarge the most, and bone spurs called osteophytes often form along the joint margins, adding to the encroachment.
Over time, the combination of extra pressure and reduced movement can cause these joints to stiffen and even fuse, a process called ankylosis. By that point, the spine has lost both its normal spacing and its flexibility.
Ligamentum Flavum Thickening
The ligamentum flavum is a stretchy band of tissue that lines the back wall of the spinal canal. In a healthy spine, it’s thin and elastic, flexing as you bend forward and snapping back as you straighten up. With age and repetitive loading, this ligament undergoes a structural shift: its elastic fibers are gradually replaced by stiffer collagen. The result is a thicker, less flexible band of tissue that buckles into the canal, particularly when you stand upright or arch your back.
Histological studies of stenosis patients show increased collagen content and decreased elastic fibers in this ligament, explaining why it loses its ability to stay taut. The thickening is driven largely by fibrosis, a scarring-like process where the tissue remodels itself in response to chronic mechanical stress. In severe cases, the ligament can even calcify. Because it sits directly behind the nerve bundle, even a few millimeters of thickening can significantly reduce canal space.
Spondylolisthesis and Spinal Instability
Degenerative spondylolisthesis occurs when one vertebra slips forward over the one below it. This happens because the disc and facet joints holding the segment in place have deteriorated enough to allow abnormal movement. The slip itself narrows the canal by misaligning the bony tunnel, and it also accelerates all the other degenerative changes. The loosening of the ligamentum flavum that results from disc space narrowing creates redundant folds of tissue that buckle into the canal. Meanwhile, the facet joints respond to the instability by thickening further.
In some cases, a defect in a part of the vertebra called the pars interarticularis (a thin bridge of bone connecting the front and back of the vertebra) allows slippage even without severe disc degeneration. Fibrocartilage tissue thickens at the defect site, and osteophytes form beneath it, both of which narrow the canal and the nerve exit channels.
Being Born With a Narrow Canal
Some people are born with a spinal canal that’s smaller than average. A canal diameter below about 11.5 millimeters, or a canal area under 142.5 square millimeters, meets the threshold for congenital lumbar stenosis based on CT measurements. People with a congenitally narrow canal may develop symptoms earlier in life because they have less buffer space. Degenerative changes that a person with a wider canal might never notice can produce nerve compression in someone who started with less room.
Achondroplasia, the most common form of dwarfism, is one well-known cause of congenital spinal narrowing. But many people with mildly narrow canals have no identifiable genetic condition. They simply inherited a spine with tighter dimensions, and the normal aging process tips them into symptomatic stenosis sooner than it otherwise would.
Bone Diseases That Cause Overgrowth
Paget’s disease of bone is a metabolic condition that disrupts normal bone remodeling, producing large, deformed bone structures that are mechanically weaker than healthy bone. When Paget’s affects the lumbar spine, it causes vertebral enlargement with cortical thickening and facet joint disease, directly narrowing the spinal canal. The abnormal joint mechanics can also lead to cyst formation that further compresses nearby nerves. While Paget’s disease is far less common than ordinary degenerative stenosis, it’s an important secondary cause, particularly in people who develop stenosis alongside other signs of abnormal bone metabolism.
Previous Surgery and Trauma
Spinal fractures can directly deform the canal if bone fragments displace inward, but a more common scenario involves scar tissue formation after spinal surgery. Postoperative epidural fibrosis, the buildup of dense scar tissue around the spinal cord and nerve roots, is one of the most prevalent complications of spine surgery and the leading cause of persistent pain after back operations.
The scar tissue forms in response to surgical trauma. Inflammation triggers an overproduction of connective tissue that can adhere to the outer covering of the spinal cord and to nerve roots themselves. These adhesions compress the tiny blood vessels feeding the nerves, causing a cycle of reduced blood flow, swelling, and further compression. When scar tissue immobilizes nerve structures, even normal movements like extending your back can become a source of pain because the nerves can no longer glide freely within the canal.
Why Multiple Causes Matter
In most people with lumbar stenosis, no single structure is entirely to blame. A disc bulges a couple of millimeters into the canal from the front. The ligamentum flavum thickens a couple of millimeters from the back. The facet joints enlarge from each side. Individually, none of these changes would cause symptoms. Together, they can reduce the canal to a fraction of its original size. This is why stenosis tends to be a gradual condition. Each small change is tolerable on its own, but the accumulation eventually crosses a threshold where nerves run out of room.
The combination of causes also explains why symptoms often fluctuate. Positions that further narrow the canal, like standing upright or leaning backward, tend to worsen symptoms because the ligamentum flavum buckles inward and the disc bulges more prominently. Leaning forward or sitting opens the canal slightly, which is why people with lumbar stenosis often find relief bending over a shopping cart or sitting down.