Piriformis muscle

The piriformis is a flat, pear-shaped muscle located in the deep gluteal region. It extends from the anterior surface of the sacrum to the greater trochanter of the femur, passing through the greater sciatic foramen.

Origin

Anterior (pelvic) surface of the sacrum, between the anterior sacral foramina, and from the margin of the greater sciatic notch and sacrotuberous ligament.

Insertion

Fibres pass laterally through the greater sciatic foramen and insert onto the superior border of the greater trochanter of the femur.

Innervation

Nerve to piriformis (containing nerve fibres from the L5 and S1 nerve roots)— branch of the sacral plexus.

Relations

• Anteriorly: Sacral plexus, branches of internal iliac vessels, and pelvic fascia.

• Posteriorly: Gluteus maximus.

• Superiorly: Gluteus medius.

• Inferiorly: Superior and inferior gemelli, obturator internus, quadratus femoris.

The piriformis muscle divides the greater sciatic foramen into suprapiriform and infrapiriform compartments, through which major gluteal vessels and nerves exit the pelvis.

• Above piriformis: Superior gluteal nerve and vessels.

• Below piriformis: Sciatic nerve, inferior gluteal vessels and nerve, and pudendal structures.

Functions

It acts as a lateral rotator of the extended thigh. Also acts as an abductor of the thigh and stabilizes the hip joint by holding the head of the femur into the acetabulum.

Piriformis Syndrome

Compression or irritation of the sciatic nerve by the piriformis muscle can present with deep buttock pain that radiates down the leg. It is common in runners, prolonged sitters, or after trauma, and can be confused with the sciatic pain. It can also present with sensory deficits, such as numbness, or tingling along the distribution of the sciatic nerve.