Pyriformis Syndrome Anatomy
To better understand piriformis syndrome, it helps to understand the anatomy of the lower back and pelvis.
The piriformis muscle is a flat, pyramid shaped muscle that is attached to the sacrum, passes through the pelvis, and inserts on the femur. The muscle follows the same course as the sciatic nerve through the pelvis.
The sciatic nerve starts in the lower back. It begins as nerve roots in the lumbar area of the spinal cord, which join together to form the large sciatic nerve.
Nerves of the low back:
- Nerve roots as they exit the lumbar spine
- The sciatic nerve deep in the low back
- The sciatic nerve in the leg
The spine is an upright row of stacked bones, called the vertebral column. Individual bones of the spine are called vertebrae. The vertebral column starts under the skull and continues to the buttocks.
Bones of the spine:
The front of each vertebra is a round, solid cylinder of bone. Between each pair of vertebrae, a disk attaches to the bottom of the vertebra above it, and to the top of the vertebrae below it. The discs act as rubber cushions between the vertebrae. In addition, strong ligaments and muscles hold the vertebral column together. All of these structures support, surround, and protect the spinal cord.
Anatomy of the vertebrae, disks and muscles:
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- Lang AM. Botulinum toxin type B in piriformis syndrome. Am J Phys Med Rehabil. 2004 Mar;83(3):198-202. 
- Papadopoulos EC, Khan SN. Piriformis syndrome and low back pain: a new classification and review of the literature. Orthop Clin North Am. 2004 Jan;35(1):65-71.