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.
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.
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.