Piriformis is a muscle of the gluteal region which lies deep to the visible muscle, i.e. gluteus maximus. Piriformis belongs to a group of six short external rotators of the hip , i.e. gemellus superior, obturator internus, gemellus inferior, quadratus femoris, obturator externus. In this article we will discuss the anatomy of piriformis, as well as its neurovascular supply and function. In addition we discuss the clinical relevance of the muscle.
Attachments and course
This is a flat pyramid-shaped muscle that arises from the anterior surface of the sacrum, between the sacral foramina. It passes laterally to exit the bony pelvis through the greater sciatic foramen. It inserts onto the greater trochanter, posterosuperior to the insertion site of the conjoined tendon of gemellus superior, obturator internus and gemellus inferior.
The muscle divides the greater sciatic foramen into two foramina (suprapiriform and infrapiriform). 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) leave 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. After re-entering the pelvis, it is joined by the internal pudendal artery and vein. The gluteus medius and minimus are medial rotators, and hence oppose the action of the lateral rotators.
In combination with the other posterior hip muscles, the piriformis stabilizes the hip joint by steadying the head of the femur inside the acetabulum of the hip bone. It also contributes to the retroversion and outward rotation of the hip joint, together with abduction of the thigh.
The nerve supply to piriformis is from a specific nerve of the lumbosacral plexus (nerve to piriformis, L5-S2).
The arterial supply is from the inferior gluteal, superior gluteal and internal pudendal arteries, all branches of the internal iliac artery.