The Piriformis muscle is situated between the anterior surface of the sacrum (the upside down triangular bone at the base of the spine)and the top lip of the femur (greater trochanter).
Piriformis plays a large role in sciatica and
derivatives of this painful disorder. This is to do with the fact that
the sciatic nerve travels in very close proximity to this muscle.
Several variations occur, but the most common type of anomaly (81% of
anomalies) is the Beaton's type B which is when the common peroneal
nerve pierces the piriformis muscle.
Piriformis Syndrome occurs
when the piriformis muscle irritates the sciatic nerve, causing pain in
the buttocks and referred pain along the sciatic nerve (posterior thigh,
leg and foot).
This referred pain is known as sciatica.In 15%
of the population the sciatic nerve courses through the piriformis
muscle making this subgroup of the population predisposed to developing
sciatica. Sciatica can be described by pain, tingling, or numbness deep
in the buttocks and along the sciatic nerve. Sitting down, stretching,
climbing stairs usually increases pain.