The Sciatic nerve is the largest nerve in the human body. It can be a contributing factor to sciatica along from several causes. These causes can include nerve pain from a herniated lumbar disc, piriformis syndrome which involves a muscle as well as direct pressure injuries to the nerve from trauma or cumulative trauma.
Sciatic nerve pain can be felt in the buttock area or travel down the leg to the foot. Sometimes the pain will stop at the knee and this is referred to many times as sciatica, however pain to the knee from the buttock can sometimes be caused by sacro-iliac (or SI joint) pain.
A very common cause of sciatica results from a herniated lumbar disc(2) or several herniated lumbar discs. The nerve irritation is felt down the the leg and can be associated with numbness and tingling. Problems such as spondylolisthesis(3) which is a slipping or misalignment can also apply pressure to the nerve causing leg pain.
Piriformis syndrome is a condition involving a particular muscle. Anatomically, the sciatic nerve exits the sciatic notch right below the piriformis muscle and the muscle itself can apply abnormal pressure to the sciatic nerve and cause leg pain. This can mimic a disc herniation and piriformis syndrome is diagnosed through a process of elimination using MRI's of the lumbar spine as well as physical exam.