What causes sciatica?

Back problems and the sciatic nerve The sciatic nerve is the largest nerve in the body and is composed of individual nerve roots that combine to form the “sciatic nerve”. It starts in the low back at lumbar segment 3 (L3). The sciatic nerve roots run through the bony canal in the spine, and at each level in the lower back a pair of nerve roots exits from the spine and then comes together to form the large sciatic nerve that runs all the way down the back of each leg. Portions of the sciatic nerve then branch out in each leg to innervate certain parts of the leg (e.g. the calf, the foot, the toes). The nerve roots that originate in the lower back are named for the upper vertebral body that they run between (for example, the nerve that exits at L4-L5 in the spine is named L4). The nerve passing to the next level runs over a weak spot in the disc space, which is the reason discs tend to herniate (extrude) right under the sciatic nerve root and can cause sciatica. The sciatica symptoms (e.g., low back pain, leg pain, numbness, tingling, weakness) are different depending on where the pressure on the nerve occurs. For example, a lumbar segment 5 (L5) nerve impingement can cause weakness in extension of the big toe and potentially in the ankle (foot drop) (See Figure 1). Back problems that cause sciatica pain Common low back problems and other spinal conditions that can cause sciatica - pain along the sciatic nerve - include: * Lumbar herniated disc. A herniated disc occurs when the soft inner core of the disc (nucleus pulposus) extrudes through the fibrous outer core (annulus) of the disc and the bulge places pressure on the contiguous nerve root as it exits the spine. In general, it is thought that a sudden twisting motion or injury can lead to an eventual disc herniation and sciatica. However, most discs weaken due to repetitive stress and the final result is a herniation. A herniated disc is sometimes referred to as a slipped disk, ruptured disk, bulging disc, protruding disc, or a pinched nerve. o See also Lumbar disc herniation * Lumbar spinal stenosis. This condition commonly causes sciatica due to a narrowing of the spinal canal. It is more common in adults over age 60, and typically results from a combination of one or more of the following: enlarged facet joints, overgrowth of soft tissue and a bulging disc placing pressure on the nerve roots as they exit the spine. o See also Lumbar spinal stenosis * Degenerative disc disease. While disc degeneration is a natural process that occurs with aging, in some cases one or more degenerated discs can also irritate a nerve root and cause sciatica. Degenerative disc disease is diagnosed when a weakened disc results in excessive micro-motion at the corresponding vertebral level and inflammatory proteins from inside the disc become exposed and irritate the area (including the nerve roots). The term “degenerative disc disease” is an unfortunate one as it is a process, not a disease. o See also Degenerative disc disease * Isthmic spondylolisthesis. This condition is not that uncommon in adults (approximately 5% to 7% of adults are thought to have it), but it only rarely causes back pain or sciatica pain. Spondylolisthesis occurs when a small stress fracture (most often at the fifth segment) allows the L5 vertebral body to slip forward on the S1 vertebral body. Caused by a combination of disc space collapse, the fracture, and the vertebral body slipping forward, the L5 nerve can get pinched as it exits the spine. o See also Isthmic spondylosthesis * Piriformis syndrome. The sciatic nerve can also get irritated as it runs under the piriformis muscle in the rear. If the piriformis muscle irritates or pinches a root that comprises the sciatic nerve, it can cause sciatica-type pain. This is not a true radiculopathy, but the pain can feel the same as sciatica caused by a nerve irritation. o See also Piriformis syndrome—another irritation to the sciatic nerve * Sacroiliac joint dysfunction. Irritation of the sacroiliac joint at the bottom of the spine can also irritate the L5 nerve, which lies on top of it, and cause sciatica-type pain. This is not a true radiculopathy, but the pain can feel the same as sciatica caused by a nerve irritation. Source: Spine-health.com