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. Attaching to the sacrum on one end and the greater trochanter on the other one, this muscle is reponsible for stabilizing the hip joint and moving the thigh in various directions.
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.
|Origins||Anterior surface of the sacrum (between the S2 and S4), Gluteal surface of ilium (near posterior inferior iliac spine), (Sacrotuberous ligament)|
|Insertions||(Apex of) Greater trochanter of the femur|
Nerve to piriformis (S1-S2)
Hip joint: Thigh external rotation, Thigh abduction (from flexed hip); Stabilises head of femur in acetabulum
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, often attaching to the gluteal surface of the ilium, close to the posterior inferior iliac spine. It inserts onto the apex of 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 external rotation of the hip joint, in addition to abduction of the thigh while in the flexed position.
The nerve supply to piriformis comes from the nerve to piriformis, formed from the anterior rami of S1 and S2 spinal nerves.
The arterial supply is from the inferior gluteal, superior gluteal and internal pudendal arteries, all branches of the internal iliac artery.
This is a syndrome that occurs when the sciatic nerve is compressed or irritated (i.e. a sciatica) by the piriformis muscle. This condition is more likely to occur in individuals with anatomical variations of the sciatic nerve and piriformis. Other causes of sciatica include a herniated disc between L5 and S1. This can be diagnosed by a MRI scan, and treated with surgery. The symptoms of piriformis syndrome include:
- buttock pain
- tenderness over the greater sciatic notch
- worsening of pain during sitting and when performing movements which increase tension in the piriformis muscle (Flexion/Adduction/Internal Rotation of the hip, i.e. FAIR test).
There is variation in the location of the insertion footprint of piriformis on the greater trochanter of the femur. The relationship between piriformis and the sciatic nerve can also vary. In the vast majority of individuals, the sciatic nerve leaves the bony pelvis via the greater sciatic foramen, below piriformis and divides into tibial nerve and common peroneal nerve distally to piriformis. In some people, the nerve divides proximally.
Examples of variations include a bifid sciatic nerve passing inferior to a hypertrophied piriformis, or a common fibular nerve passing between the two bellies of a bifid piriformis, the tibial nerve passing inferior to piriformis. Both the tibial and common peroneal nerves can also pass through or superior to piriformis.
Total Hip Replacement
In minimally invasive total hip arthroplasty, such as the direct anterior approach, the hip joint is surgically approached in a way which minimises soft tissue dissection. This method improves the postoperative stability of the joint and reduces the risk of dislocation. However during the detachment of the joint capsule, the short external rotators of the hip, including piriformis, are at risk of damage.