Hello!
See two of my posts:
http://www.medhelp.org/posts/Orthopedics/piriforis/show/946940
http://www.medhelp.org/posts/Orthopedics/Help-with-Post-Piriformis-Surgery/show/998596
The Piriformis is one of the small muscles deep in the buttocks that rotates the leg outwards.
Anatomically, the piriformis muscle lies deep to the gluteal muscles. It originates from the sacral spine and attaches to the greater trochanter of the femur bone, which is the big, bony prominence on the outside top of the thigh. The sciatic nerve usually passes underneath the piriformis muscle, but in a fraction of the population, it travels through the muscle. It is thought that acute or chronic injury causes swelling of the muscle and irritates the sciatic nerve, resulting in sciatica. If you have an abnormal course of the nerve through the muscle you are particularly predisposed to this condition. Piriformis syndrome is diagnosed primarily on the basis of symptoms and on the physical examination. There are no tests to confirm the diagnosis, but X-rays, MRI, and nerve conduction tests may be necessary to exclude other diseases.
Take care!