Sciatica is pain that starts in the back and radiates down one of the legs. It is quite a common complaint. It is caused by pressure on the sciatic nerve that runs down into the leg. The cause of pressure can be varied from a slipped disk to muscle tension or something less common such as tumors, bony growths and infections. Depending on where the sciatic nerve is pinched will determine where you feel the pain which can radiate to the front of the knee or right down the back of the leg to the foot.