Anatomically, the Piriformis muscle lies deep in the gluteal muscles. It originates from the sacral spine and attaches to the greater trochanter of the femur, which is the big, bony "bump" on the outside top of the thigh. The sciatic nerve usually passes underneath the Piriformis muscle, but in approximately 15% of the population, it travels through the muscle.
Piriformis syndrome can develop when the Piriformis muscle becomes tight or spasms and places pressure on the sciatic nerve that runs beneath it. When the Piriformis muscle and its tendon are too tight, the sciatic nerve is choked. This may decrease the blood flow to the nerve and irritate the nerve because of pressure. This can cause low back pain and/or pain that radiates to the rear and down the leg (similar to sciatica pain). From a technical standpoint, Piriformis syndrome does not cause true sciatica (as sciatica is usually defined as a radiculopathy, or compression of a nerve root as it exits the spine). However, just like sciatica, Piriformis syndrome can cause pain, numbness and tingling along the sciatic nerve, which runs down the back of the leg and into the foot.