What causes sciatica?

Sciatica is generally caused by the compression of a lumbar spine nerve root L4 or L5 or sacral nerve roots S1, S2 or S3, or far less commonly, by compression of the sciatic nerve itself.

When sciatica is caused by compression of a lumbar nerve root it is considered a lumbar radiculopathy (or radiculitis when accompanied with an inflammatory response) from a spinal disc herniation (a herniated intervertebral disc in the spine), or from roughening, enlarging, and/or misaligning of the vertebrae (spondylolisthesis), or degenerated discs.

"Pseudo-sciatica", which causes symptoms similar to spinal nerve root compression, is caused by the compression of peripheral sections of the nerve, usually from soft tissue tension in the piriformis or related muscles (see piriformis syndrome and see below).

Spinal disc herniation

One cause of sciatica is a spinal disc herniation, pressing on one of the sciatic nerve roots. The spinal discs are composed of a spongiform cartilage with a liquid center. The discs separate the vertebrae, thereby allowing room for the nerve roots to properly exit through the spaces between the L4, L5, and sacral vertebrae.

The discs cushion the spine from compressive forces, but are weak to pressure applied during rotational movements. That is why a person who bends to one side, at a bad angle, to pick up a piece of paper may more likely herniate a spinal disc than a person falling from a ladder and landing on his or her back. Herniation of a disc occurs when the liquid center of the disc bulges outwards, tearing the external ring of fibers, and compresses a nerve root against the lamina or pedicle of a vertebra, thus causing sciatica.

Spinal stenosis

Other compressive spinal causes include Spinal Canal Stenosis, a condition wherein the spinal canal (the spaces through which the spinal cord runs) narrows and compresses the spinal cord. This narrowing can be caused by bone spurs, vertebral dislocation, inflammation, or herniated disc which decreases available space for the spinal cord, thus pinching nerves in the spinal cord that travel to the sciatic nerve and irritating them with friction.

Piriformis syndrome

The sciatic nerve runs through the piriformis muscle in the buttocks region beneath the gluteal muscles. When the muscle shortens or spasms due to trauma, it can compress or strangle the sciatic nerve beneath the muscle. This cause of sciatic symptoms is piriformis syndrome. This may be the major cause of sciatica when the nerve root is normal.

The risk of self-inflicted sciatica has increased in recent years with the fashion trend of lower-hanging trousers as well as lower-positioning of the pockets. For instance, sitting on a wallet for prolonged hours every day can cause self-inflicted sciatica. Symptoms of numbness and/or pain behind the knee cap are associated with this form of sciatica.
Treatment
The corrective action of removing any objects from rear pockets and not sitting on uneven surfaces can reverse the symptoms of sciatica if the damage has not been too severe.
Manual muscle stretching, massage, and mobilization techniques should be used when the sciatica is piriformis muscle-related. General therapeutic goals include helping the muscles loosen, thereby lessening pain, and to minimize inflammation.

Sacroiliac joint dysfunction

Another cause of sciatic symptoms is sacroiliac joint dysfunction[citation needed]. Unhealthy postural habits, such as excessive time sitting in chairs, and sleeping in the fetal position, along with insufficient stretching and exercise of the relevant myofascial areas, can lead to both the vertebral and soft tissue problems associated with sciatica .

Trigger points

Another source of sciatic symptoms is active trigger points of the lower back and the gluteus muscles. In this case, the referred pain is not consequent to compression of the sciatic nerve, though the pain distribution down the buttocks and leg is similar. Trigger points occur when muscles become ischemic (low blood flow) due to injury or chronic muscular contraction. The most commonly associated muscles with trigger points triggering sciatic symptoms are: the quadratus lumborum, the gluteus medius, the gluteus minimus, and the deep hip rotators.

Pregnancy

Sciatica may also be experienced in late pregnancy, primarily resulting from the uterus pressing on the sciatic nerve, and, secondarily, from the muscular tension and / or vertebral compression consequent to carrying the extra weight of the fetus, and the postural changes inherent to pregnancy.