Injury or pressure affecting the sciatic nerve is called sciatica. It is a symptom of another medical problem which must be treated in order to relieve the sciatica.
The sciatic nerve is the longest nerve of the body, running from the lower spine down the back of the leg. It controls the activity of muscles in the lower leg and the back of the knee. It's the origin of sensation in the back part of the thigh, some of the lower leg, and the bottom of the foot.
Sciatica can be the result of a slipped, or a herniated, disk in the spine, tumors, or pelvic injury. It may be caused by a pain disorder in the buttocks called piriformis syndrome.
Piriformis syndrome occurs when the piriformis muscle constricts or spasms. This can put unpleasant pressure on the sciatic nerve.
Sciatica can be caused by lumbar spinal stenosis or spondylolisthesis.