Piriformis muscle

Structure

The piriformis is a flat muscle, and is pyramidal in shape.

Origin

The piriformis muscle originates from the anterior (front) surface of the sacrum by three fleshy digitations attached to the second, third, and fourth sacral vertebrae.

It also arises from the superior margin of the greater sciatic notch, the gluteal surface of the ilium (near the posterior inferior iliac spine), the sacroiliac joint capsule, and (sometimes) the sacrotuberous ligament (more specifically, the superior part of the pelvic surface of this ligament).

Insertion

The muscle inserts onto the greater trochanter of the femur (its tendon unites with the tendons of the superior gemellus, inferior gemellus, and obturator internus muscles prior to insertion).

Innervation

The piriformis muscle is innervated by the piriformis nerve.

Relations

The posterior aspect of the muscle lies against the sacrum. The anterior surface of the muscle is related to the rectum (especially on the left side of the body), and the sacral plexus.

The muscle lies almost parallel with the posterior margin of the gluteus medius. It is situated partly within the pelvis against its posterior wall, and partly at the back of the hip joint.

It exits the pelvis through the greater sciatic foramen superior to the sacrospinous ligament.

Variation

In around 80% of the population, the sciatic nerve travels below the piriformis muscle. In 17% of people, the piriformis muscle is pierced by parts or all of the sciatic nerve. Several variations occur, one of which is the rarely found Beaton's type-b in which the sciatic nerve divides between and below the piriformis.

It may be united with the gluteus medius, send fibers to the gluteus minimus, or receive fibers from the superior gemellus.

It may have one or two sacral attachments; or it may be inserted into the capsule of the hip joint.

Function

The piriformis muscle is one 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. 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 being 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. This causes the femur to rotate and point the knee laterally. The lateral rotators also oppose medial rotation by the gluteus medius and gluteus minimus. When the hip is flexed to 90 degrees, piriformis abducts the femur at the hip and reverses primary function, internally rotating the hip when the hip is flexed at 90 degrees or more.

Clinical significance

Piriformis syndrome

Main article: Piriformis syndrome

Piriformis syndrome occurs when the piriformis irritates the sciatic nerve, which enters the gluteal region beneath the muscle, causing pain in the buttocks and referred pain along the sciatic nerve. This referred pain is known as sciatica. The sciatic nerve runs through the piriformis muscle in seventeen percent of the population; this subgroup of the population is predisposed to developing sciatica. Sciatica is characterized by pain, tingling, or numbness deep in the buttocks and along the sciatic nerve. Sitting down, stretching, climbing stairs, and performing squats usually increases the pain. Diagnosis of the syndrome is usually based on symptoms and on a physical exam. More testing, including MRIs, X-rays, and nerve conduction tests can be administered to exclude other possibilities pathologies. A more invasive but sometimes necessary treatment involves surgical exploration; however, the side effects of such surgery can be much worse than those of alternative treatments such as physical therapy. Surgery should always be a last resort.

Landmark

The piriformis is a very important landmark in the gluteal region. As it travels through the greater sciatic foramen, it effectively divides the region into inferior and superior portions.

This division determines the names of certain vessels and nerves in the region, i.e., the nerve and vessels that emerge superior to the piriformis are known as the superior gluteal nerve and the superior gluteal vessels, while those that emerge below the piriformis are the inferior nerve and vessels.

History

The piriformis muscle was first named by Adriaan van den Spiegel, a professor from the University of Padua in the 16th century.

Additional images

File:Piriformis muscle.PNG|Muscles of the gluteal and posterior femoral regions File:Sobo 1909 296.png File:Sobo 1909 300.png

References

References

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2. Standring, Susan. (2021). "Gray's Anatomy: The Anatomical Basis of Clinical Practice".
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6. (2020). "Composite Anatomical Variations between the Sciatic Nerve and the Piriformis Muscle: A Nepalese Cadaveric Study". Case Rep Neurol Med.
7. Hansen, John T.. (2009). "Netter's Clinical Anatomy". Saunders/Elsevier.
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10. "Muscles of the Gluteal Region". TeachMeAnatomy.
11. Smoll NR. (January 2010). "Variations of the piriformis and sciatic nerve with clinical consequence: a review". Clinical Anatomy.