Shape and Location
The piriformis is a flat muscle, pyramidal in shape, lying almost parallel with the posterior margin of the gluteus medius.
It is situated partly within the pelvis against its posterior wall, and partly at the back of the hip-joint.
It arises from the front of the sacrum by three fleshy digitations, attached to the portions of bone between the first, second, third, and fourth anterior sacral foramina, and to the grooves leading from the foramina: a few fibers also arise from the margin of the greater sciatic foramen, and from the anterior surface of the sacrotuberous ligament.
The muscle passes out of the pelvis through the greater sciatic foramen, the upper part of which it fills, and is inserted by a rounded tendon into the upper border of the greater trochanter behind, but often partly blended with, the common tendon of the obturator internus and superior and inferior gemellus muscles.
The piriformis is a very important landmark in the gluteal region. As it travels through the greater sciatic foramen, it effectively divides it into an inferior and superior part.
This determines the name of the vessels and nerves in this region – the nerve and vessels that emerge superior to the piriformis are the superior gluteal nerve and superior gluteal vessels. Inferiorly, it is the same, and the sciatic nerve also travels inferiorly to the piriformis.
Read more about this topic: Piriformis Muscle
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