Sciatica is not a disease; it is a symptom. This symptom is caused by the nerve being irritated or compressed. The culprit nerve root is in the spine, and it goes through the hips, buttocks, and down to each leg. When the culprit nerve root is compressed because of a problem in the spine, it can be very painful.
Some factors include slipped disc, spinal stenosis, degenerative disc disease, bone spurs, and sometimes injuries and spinal tumours.