"Why Does It Get Narrow?" — How Spinal Stenosis Works

Inside your spine, there's a "tunnel" where your nerves travel.

Last week, we talked about how lumbar spinal stenosis affects an estimated 103 million people worldwide, so it is far from rare. This week, I'll explain as simply as I can why the spinal canal narrows.

Understanding the Structure of Your Spine

Your spine is made up of individual bones called vertebrae, stacked on top of each other like building blocks. In the lower back, there are five of these bones, and together they're called the lumbar spine.

Between the vertebrae sit discs, cushions filled with water and gel that absorb shock when you move.

Stacked together, the vertebrae leave a hollow space down the center. This is the spinal canal.

Inside this canal runs a bundle of nerves extending from the brain. In the lower back, these nerves are called the cauda equina (literally "horse's tail"), and they control sensation and movement in your legs, as well as bladder and bowel function.

Lateral view of the lumbar spine showing intervertebral discs, facet joints, vertebral bodies, and nerve roots

Cross-sectional view of a lumbar vertebra showing the intervertebral disc, spinal canal, facet joints, and ligamentum flavum

One way to picture it:

Imagine building blocks (vertebrae) stacked to form a tunnel (the spinal canal), with a precious electrical cable (nerves) running through the middle.

Three Reasons the Tunnel Gets Narrower

As we age, the parts that make up this tunnel slowly wear down and change shape. This is the main cause of spinal stenosis.

1. Bulging Discs

Over time, the discs lose water and elasticity. As they flatten, they can bulge into the spinal canal and take up space.

A new cushion is plump and firm, but after years of use it flattens and spreads outward. Your spinal discs do the same.

A bulging disc compressing the spinal canal

2. Thickening of the Ligaments

The ligaments that hold your spine together (strong bands that connect bone to bone) can thicken with age. In particular, the ligamentum flavum (a yellow-colored ligament at the back of the spinal canal) can become much thicker than normal, squeezing the tunnel from behind.

Cross-section of a stenotic spinal canal showing hypertrophic ligamentum flavum and degenerative facet joints

3. Bone Spurs (Osteophytes)

Small bony growths called osteophytes (bone spurs) can develop along the edges of the vertebrae. They are part of aging and make the walls of the tunnel stick out inward.

Osteophytes (bone spurs) forming along the edges of the vertebrae

These three changes often occur together and slowly narrow the nerve tunnel.

Keep in mind: These changes develop slowly, over many years. And not everyone who has narrowing will experience symptoms. Many people whose MRI scans show a narrow spinal canal have no pain or discomfort at all.

The Connection with Spondylolisthesis

You may also hear about spondylolisthesis, where one vertebra slips forward over the one below it.

A slipped vertebra narrows the spinal canal by itself, and spondylolisthesis and stenosis frequently occur together.

Spondylolisthesis — one vertebra slipping forward over the one below

If spondylolisthesis is present, it may affect the type of surgery recommended. (We'll cover this in detail in Week 9.)

What If Your MRI Shows "Stenosis"?

An MRI image alone does not determine whether surgery is needed.

Many people whose scans show narrowing have only mild symptoms. And some people with relatively normal-looking scans experience significant pain and disability.

What matters most is an evaluation that combines imaging findings with your actual symptoms. A finding of stenosis on MRI does not automatically mean you need surgery.

We'll talk more about diagnostic tests in Week 4.

It's Not Your Fault

Spinal stenosis comes from completely natural changes of aging. It doesn't happen because of bad posture, not exercising enough, or lifting heavy objects.

Of course, some people are born with a naturally narrow spinal canal (congenital stenosis). That is simply a variation in body type, and it's nobody's fault either.

These changes happened in a spine that has supported you for decades. Instead of blaming yourself, learn about the condition and respond to it sensibly.

Key Takeaways

  • The spinal canal is a tunnel inside your spine through which nerves travel
  • With age, discs, ligaments, and bones change, causing the tunnel to narrow
  • These changes happen gradually, and not everyone develops symptoms
  • MRI findings alone don't determine the course of treatment
  • This is a natural part of aging, and it is not your fault