The sciatic nerve stretches from the lower back down through the back of both legs and into the feet. It is made up of five nerve roots (L4, L5, S1, S2, and S3) that branch out from lumbar spine. When one of the nerve roots is pinched or irritated, it can lead to symptoms of pain and possibly numbness and tingling that start in the lower back and radiate through the buttock and down the legs. These symptoms are called sciatica. The following conditions commonly cause sciatica.
Lumbar Disc Degeneration/Disc Herniation
Spinal discs are made up of a gel-like interior, called the nucleus pulposus, surrounded by a tough outer ring called the annulus. Natural, age-related wear-and-tear from degenerative disc disease, and/or pressure or stress placed on the spine may cause the annulus to bulge, crack, or tear. Sciatic nerve irritation can occur if a degenerating disc in the lumbar spine collapses and pushes against a nearby nerve root. Additionally, if a cracked or torn disc herniates, leaking some of its inflammatory interior onto the nearby sciatic nerve root, nerve irritation, and sciatica may result.
Isthmic spondylolisthesis occurs when a small fracture in the bone that connects facet joints (isthmus) causes one of the spinal vertebrae to slip forward onto the vertebrae below it. Disc degeneration from isthmic spondylolisthesis can cause the spinal discs to become flatter, leaving less room for the nerve root to exit the spine and causing nerve root compression. Because isthmic spondylolisthesis occurs most often in the L5-S1 level of the lumbar spine, sciatic nerve root compression that results in sciatica is a common symptom of this condition.
Lumbar Spinal Stenosis
Spinal stenosis refers to pain that is caused by normal, age-related, degenerative changes in the spine that cause the spinal canal and other spaces to narrow. One degenerative change that may cause pain is bony growths (osteophytes). These growths are normal and occur in most people as they age, but when they form in the opening where the nerve root leaves the spinal canal (foramen), they can compress nerve roots. If this condition, called foraminal stenosis, occurs in the lumbar spine and compresses the sciatic nerve roots, sciatica often results.
The piriformis muscle is located deep in the buttock. The muscle, which helps to rotate the hip and turn the leg and foot outward, runs diagonally and connects to the upper surface of each femur. Injury or irritation to the piriformis muscle or other nearby structure like the sacroiliac joint or hip may cause the muscle to spasm, tighten, swell, or bleed. Any one, or combination, of these problems may irritate the adjacent sciatic nerve and cause sciatica.
Sacroiliac Joint Dysfunction
The sacroiliac joint connects the hip bones to the sacrum—the triangular bone between the lumbar spine and the tailbone. Small movements at the joint help to absorb shock between the upper body and the pelvis and legs. Pain often results when the sacroiliac joint either moves too much (hypermobility) or too little (hypomobility). Although sacroiliac joint dysfunction does not typically irritate the sciatic nerve, hypomobility often leads to pain that is felt on one side of the low back or buttocks, and/or pain that radiates down the back of the leg, similar to sciatica.