Piriformis muscle

At a glance

English

Piriformis muscle

Latin (TA2)

Musculus piriformis

System

Muscle

Region

Lower limb

Side

Paired, left and right

About the piriformis muscle

Piriformis is a flat, pyramid-shaped muscle that lies partly inside the pelvis on its posterior wall, and partly behind the hip joint. Its name is Latin for pear-shaped. It is the highest of the short lateral rotators of the hip, and in the buttock its upper border runs almost parallel to the back edge of gluteus medius.

Inside the pelvis it arises by three fleshy slips from the front of the sacrum, between and around the second to fourth anterior sacral foramina. Further fibres come from the upper edge of the greater sciatic notch, the capsule of the sacroiliac joint and the sacrotuberous ligament. The belly narrows as it passes out by way of the greater sciatic foramen, then ends on a rounded tendon attached to the upper border of the greater trochanter, often blended with the tendons of obturator internus and the gemelli.

As it fills the greater sciatic foramen, piriformis splits it into an upper and a lower gap, and it is the key landmark of the gluteal region. The superior gluteal nerve and vessels leave above it. The inferior gluteal nerve and vessels, the pudendal nerve and the sciatic nerve leave below. In most people the whole sciatic nerve passes beneath the muscle, but in a sizeable minority part of it, usually the common fibular division, pierces the muscle or runs above it.

With the hip extended, the muscle rotates the thigh laterally; with the hip flexed it abducts the thigh, and at about ninety degrees of flexion or more its rotating effect reverses. It also steadies the femoral head in its socket. It is supplied by the nerve to piriformis from the sacral plexus and by the superior and inferior gluteal arteries. Tightness or swelling of the muscle can irritate the sciatic nerve, causing buttock pain spreading down the leg that worsens with sitting, a condition called piriformis syndrome, which is usually managed with stretching and physiotherapy before injections or surgery are considered.

Attachments, action and supply

Origin

Anterior surface of the sacrum (S2 to S4), upper edge of the greater sciatic notch, sacroiliac joint capsule and sacrotuberous ligament.

Insertion

Upper border of the femur's greater trochanter.

Action

Laterally rotates the extended thigh, abducts the flexed thigh and helps hold the femoral head in the acetabulum.

Nerve supply

Nerve to piriformis (S1, S2).

Blood supply

Superior and inferior gluteal arteries, with the lateral sacral arteries inside the pelvis.