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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.
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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.