Piriformis muscle

The piriformis muscle is a muscle in the gluteal region of the lower limb.

Summary

• origin: sacrum
• course: exits the pelvis through the greater sciatic foramen
• insertion: greater trochanter of the femur
• action: abduction and lateral rotation of the thigh
• arterial supply
  • superior gluteal artery
  • inferior gluteal artery
  • lateral sacral artery
• innervation: nerve to piriformis (L5, S1and S2 nerve roots)

Gross anatomy

The piriformis is a flat muscle, pyramidal in shape, lying almost parallel with the posterior margin of the gluteus medius and deep to gluteus maximus. It is situated partly within the pelvis against its posterior wall, and partly at the back of the hip joint. 

Origin

The piriformis originates from the anterior part of the sacrum by three fleshy digitations as well as from the superior margin of the greater sciatic notch, and to a lesser extent the sacroiliac joint capsule and the sacrotuberous ligament.

Course

The muscle passes out of the pelvis through, and fills the upper part of, the greater sciatic foramen into the gluteal region. 

Insertion

The piriformis exits the pelvis through the greater sciatic foramen to insert onto the upper part of the greater trochanter of the femur.

Its tendon often joins with the tendons of the superior gemellus, inferior gemellus and obturator internus muscles prior to insertion.

Relations

Structures also passing through the greater sciatic foramen pass either above of below piriformis:

• passing superiorly: superior gluteal nerve and vessels
• passing inferiorly: inferior gluteal nerve and vessels; sciatic nerve; internal pudendal artery; pudendal nerve; posterior femoral cutaneous nerve; nerve to obturator internus; nerve to quadratus femoris

Variant anatomy

• may be fused or partially fused with gluteus medius, gluteus minimus or superior gemellus
• may have only one or two sacral attachments at its origin
• may be inserted into the capsule of the hip joint
• when the sciatic nerve bifurcates prior to exiting the greater sciatic foramen, the piriformis is frequently pierced by the common peroneal (fibular) nerve

Radiographic appearancefeatures

US

• appears as a hyperechoic line between the greater trochanter and the lateral sacrum with the gluteus maximus overlying it and the hypoechoic sciatic nerve lying deep ⁴

History and etymology

From the Latinpiriformis "piriformis", meaning pear-shaped. 

Related pathology

• piriformis syndrome