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Causes
By Mayo Clinic staff
Knowing more about the anatomy of your spine makes it easier to understand how spinal stenosis develops and how it can lead to various problems. The main parts of your spine include:
- Vertebrae. Your spine is made up of 24 bones stacked on top of one another, plus the sacrum and tailbone (coccyx). Most adults have seven vertebrae in the neck (cervical vertebrae), 12 at the back wall of the chest (thoracic vertebrae) and five vertebrae at the inward curve of the lower back (lumbar vertebrae). The sacrum consists of five fused vertebrae between the hip bones. The tailbone is composed of three to five fused bones at the very end of the spine.
- Ligaments. These tough, elastic bands of tissue help keep the vertebrae in place when you move.
- Intervertebral disks. These elastic pads of cartilage separate the vertebrae. They keep your spine flexible and act as shock absorbers to cushion the vertebrae when you move. Each disk consists of a ring of tough fibrous tissue (annulus fibrosis) surrounding a jelly-like center (nucleus pulposus).
- Facet joints. Located on the sides, top and bottom of each vertebra, these joints connect the vertebrae to one another and stabilize the spine while still allowing flexibility. The joints are coated with a lubricant so that they slide smoothly.
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Spinal cord. This long bundle of nerves extends from the brainstem at the base of your skull to the second lumbar vertebra in your lower back. When the spinal cord ends, another group of nerves (cauda equina) continues down the spinal canal.
The nerves within the spinal cord (upper motor neurons) carry messages between your brain and the nerves that go to all the parts of your body below your head. Two spinal nerves — one leading to the right side of your body and one to the left side — extend out from the spinal cord between each vertebra. The nerves exit through openings on each side of the vertebrae (intervertebral foramina).
In all, there are 31 pairs of spinal nerves in your neck and back. Some transmit information from your body to your brain, and others send messages from your brain to your muscles, skin and other organs.
- Spinal canal. The spinal cord passes through this channel in your spine. Normally, the spinal canal is spacious enough to accommodate the spinal cord, but degenerative changes in the spine can narrow the channel.
How spinal stenosis develops
Doctors categorize stenosis as either primary or acquired:
- Primary spinal stenosis, which is relatively uncommon, is present at birth.
- Acquired spinal stenosis, which is the type that most people have, develops later in life. It's usually a result of degenerative changes in the spine that occur with aging.
The main cause of spinal degeneration is osteoarthritis, an arthritic condition that affects the cartilage that cushions the ends of bones in your joints. With time, the cartilage begins to deteriorate and its smooth surface becomes rough. If it wears down completely, bone may rub painfully on bone. In an attempt to repair the damage, your body may produce bony growths called bone spurs. When these form on the facet joints in the spine, they narrow the spinal canal.
Other factors that can cause a narrowing of the spinal canal include:
- Herniated disk. By the time you're 30, your disks may start to show signs of deterioration. They begin to lose their water content, becoming flatter and more brittle. Eventually, the tough, fibrous outer covering of the disk may develop tiny tears, causing the jelly-like substance in the disk's center to seep out (herniation or rupture). The herniated disk presses on the surrounding nerves, causing pain in your back, leg or both. Sometimes you may also have numbness, tingling or weakness in the buttock, leg or foot on the affected side.
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Ligament changes. Ligaments in your back can undergo degenerative changes, becoming stiff and thick over time. This loss of elasticity may shorten your spine, narrowing the spinal canal and compressing the nerve roots.
Sometimes wear and tear on the disks and ligaments cause one lumbar vertebra to slip over another — a condition called spondylolisthesis. This often compresses the spinal nerves, leading to numbness, tingling and weakness in your legs, especially when you stand for long periods or when you walk.
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Spinal tumors. In the spine, abnormal growths can form inside your spinal cord, within the membranes (meninges) that cover your spinal cord, or in the space between your spinal cord and the vertebrae — the most common site.
Tumors may also spread (metastasize) to your spine or your spinal cord from other parts of your body. Primary or metastastic tumors can occur anywhere along your spine, including the sacrum and thoracic spine, where osteoarthritis is rare.
Growing tumors may compress your spinal cord and nerve roots. This can cause severe back pain that may extend to your hips, legs or feet; muscle weakness and a loss of sensation — especially in your legs; difficulty walking or even paralysis; and sometimes loss of bladder or bowel function.
- Injury. Car accidents and other trauma can profoundly affect your spine and spinal cord. Sometimes your spine or spinal canal may be dislocated, putting pressure on the cord and lower motor neurons. In other cases, fragments of bone from a spinal fracture may penetrate your spinal canal. Swelling of tissue after back surgery also can put pressure on your spinal cord or nerves.
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Paget disease of bone. Bone is living tissue engaged in a continuous process of renewal. During this remodeling process, old bone is removed and replaced by new bone. In Paget disease, your body generates new bone at a faster than normal rate. This produces soft, weak bones that are prone to fractures. It can also create bones that are deformed or abnormally large.
When these unusually large bones develop in your spine, they compress the spinal cord or the nerves exiting your brain and spinal cord. The resulting pain is often severe and may radiate from your lower back into your legs. You may also experience numbness, tingling or weakness in the legs or, in some cases, double vision.
- Achondroplasia. This genetic disorder slows the rate at which bone forms during fetal development and in early childhood. As a result, people with achondroplasia are of short stature — often no more than 4 feet tall when fully grown (dwarfism). They often have small hands and fingers and unusually short upper arms and thighs. They also have a narrow spinal canal, which puts pressure on the spinal cord.
