What Is Piriformis? – Origin, Nerve Supply, Function

The piriformis muscle originates from the anterior sacrum? and sacroiliac joint, passes transversely through the greater sciatic foramen via the sciatic notch, and inserts on the greater trochanter. The muscle is innervated by the ventral rami of S1 and S2 (and L5 to a lesser extent), which join to form the nerve to the piriformis. The muscle receives its vascular supply predominantly from a branch of the inferior gluteal artery?. When the hip is in extension, the piriformis muscle externally rotates the femur?.

The piriformis is a triangle-shaped gluteal (buttock) muscle. The base of the triangle attaches to the sacrum (tailbone?), while the opposite end attaches to the side of the femur (upper thighbone). The muscle also passes through an area of the pelvic girdle called the greater sciatic foramen. The piriformis muscle is important in turning the hip out and moving the leg away from the center of the body.

Origin of Piriformis Muscle

• The piriformis muscle originates from the anterior part of the sacrum by three fleshy digitations (middle three pieces of its own half and adjoining lateral mass, and also extending medially between the anterior sacral foramina) 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?.

Insertion

• The piriformis exits the pelvis? through the greater sciatic foramen to insert on the apex 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.

Blood Supply

• The arterial supply is from the inferior gluteal, superior gluteal and internal pudendal arteries?, all branches of the internal iliac artery.

Nerve Supply

The structures that leave the pelvis through the infrapiriform foramen can be easily remembered with the following mnemonic;

The superior gluteal artery and nerve (L4-S1) leave the pelvis through the suprapiriform foramen. The sciatic nerve?, inferior gluteal nerve (L5-S2) and artery, posterior femoral cutaneous nerve (S1-S3) and the nerve to quadratus femoris (L4-S1) leaves the pelvis through the infrapiriform foramen. The pudendal nerve (S2-4) also leaves the pelvis through the infrapiriform foramen, wraps around the sacrospinous ligament, and re-enters the pelvis by passing back into the lesser sciatic foramen.

PIN & PINS

• The posterior  cutaneous nerve of thigh
• Inferior gluteal vessels and nerves
• Nerve to quadratus femoris
• Pudendal nerve
• Internal pudendal vessels
• Nerve to obturator internus
• Sciatic nerve

Function

• Hip external rotation.
• Abductor at 90 degrees of hip flexion.
• In weight-bearing, the PM restrains the femoral internal rotation during the stance phase of walking and running.
• Assists the short hip rotators in compressing the hip joint and stabilizing the joint.
• As it can exert an oblique force on the sacrum, it may produce a strong rotary shearing force on the sacroiliac joint (SIJ). This would displace the ipsilateral base of the sacrum anteriorly (forward) and the apex of the sacrum posteriorly.

The piriformis muscle is part of the lateral rotators of the hip, along with the quadratus femoris, gemellus inferior, gemellus superior, obturator externus, and obturator internus. The piriformis laterally rotates the femur with hip extension and abducts the femur with hip flexion. The abduction of the flexed thigh is important in the action of walking because it shifts the body weight to the opposite side of the foot is lifted, which prevents falling. The action of the lateral rotators can be understood by crossing the legs to rest an ankle on the knee of the other leg.

Reference