What Causes Sciatica?

Sciatica is a form of peripheral neuropathy and is a relatively common form of back pain.. It occurs when there is damage to the sciatic nerve, positioned in the back of the leg. The sciatic nerve is the largest nerve in the body and controls the muscles of the back of the knee and lower leg and provides sensation to the back of the thigh, part of the lower leg and the sole of the foot. Incomplete damage to the sciatic nerve may appear identical to damage to one of the branches of the sciatic nerve (tibial nerve dysfunction or common peroneal nerve dysfunction).
The usual causes are direct trauma (often due to an injection into the buttocks), prolonged external pressure on the nerve, and pressure on the nerve from nearby body structures. It can also be caused by entrapment -- pressure on the nerve where it passes through a narrow structure. The damage slows or prevents conduction of impulses through the nerve.
The sciatic nerve is commonly injured by fractures of the pelvis, gunshot wounds, or other trauma to the buttocks or thigh. Prolonged sitting or lying with pressure on the buttocks may also injure it. Systemic diseases, such as diabetes, can typically damage many different nerves, including the sciatic nerve. The sciatic nerve may also be harmed by pressure from masses such as a tumor or abscess, or by bleeding in the pelvis.

Common disorders that can cause sciatica, or pain along the sciatic nerve include

Lumbar herniated disc
Lumbar spinal stenosis
Isthmic spondylolisthesis
Degenerative disc disease
Piriformis syndrome
Sacroiliac joint dysfunction