Most sciatica is caused by problems that affect the L4, L5, or S1 nerve roots. The nerve may be compressed or irritated, usually because it’s being rubbed by a disc, bone, joint, or ligament. The resulting inflammation makes the tissues and the nerves more sensitive and the pain feel worse.
Damage to or pinching of the sciatic nerve, or the nerves that feed into it, can have several causes.
One of the most common causes of sciatica is a herniated disc in the lower part of the spine. It’s also called a slipped disc, though there’s no slipping going on.
Spinal discs are tucked between the vertebrae, where they act as cushions to keep the bones from touching one another. The discs absorb all the forces placed on the spine from walking, running, sitting, twisting, lifting, and every other activity we do. They also absorb forces from falls, collisions, and other accidents.
A herniated disc can happen at any age, but becomes more common in middle age and beyond. It occurs when the jelly-like filling in a spinal disc breaks through the disc's outer shell and bulges through the tear. When this happens, the material may press on a nearby nerve. The pressure on the nerve causes abnormal signals to be sent to the brain, and you get sciatica.
The spinal canal protects the spinal cord and the nerves that run up and down the spine. Spinal stenosis is the narrowing of the spinal canal. When this occurs, nerves can be compressed, causing pain. Because the lumbar vertebrae undergo the most consistent stress and support the most weight, lumbar stenosis is the most common type of spinal stenosis.
Most often, spinal stenosis is caused by a combination of these age-related changes:
- degeneration of the disc
- osteoarthritis of the facet joint
- thickened ligaments.
Spinal stenosis can be present from birth as a result of rare conditions. More often, the spinal canal may be somewhat narrower than normal at birth, a condition called congenital stenosis. Because the canal is not as wide as the average person’s, even mild age-related changes, such as mild disc bulging and facet arthritis, can cause symptoms of stenosis. Some people get these symptoms in their 40s or earlier. Injuries, tumors, and thickened ligaments also can narrow the spinal canal.
The bones of the spine are stacked on top of one another, separated by discs. Spondylolisthesis occurs when one spinal bone slips forward in relation to the bone below it. When the L4 vertebra moves over the L5 vertebra, it can cause a kink in the spinal canal leading to pressure on a nerve root and sciatica.
Spondylolisthesis is a common finding when teenagers experience persistent back pain. In this group, it’s usually caused by a traumatic injury or a congenital defect. In adults, spondylolisthesis is more commonly caused by osteoarthritis.
The sciatic nerve itself is susceptible to damage from any injury or accident that affects the buttock or stretches a hamstring, one of three muscles at the back of the thigh. Falls, car accidents, and sports injuries may lead to sciatica.
During the second and third trimesters, the shifting of weight and loosening of ligaments (not to mention the little person growing inside you!) can put indirect pressure on the sciatic nerve.
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