The piriformis muscle () is a flat, pyramidally-shaped

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of mus ...

in the

gluteal The gluteal muscles, often called glutes are a group of three muscles which make up the gluteal region commonly known as the buttocks: the gluteus maximus, gluteus medius and gluteus minimus. The three muscles originate from the ilium and sacr ...

region of the

lower limbs The human leg, in the general word sense, is the entire lower limb of the human body, including the foot, thigh or sometimes even the hip or gluteal region. However, the definition in human anatomy refers only to the section of the lower limb e ...

. It is one of the six muscles in the lateral rotator group. The piriformis muscle has its origin upon the front surface of the

sacrum The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part o ...

, and inserts onto the

greater trochanter The greater trochanter of the femur is a large, irregular, quadrilateral eminence and a part of the skeletal system. It is directed lateral and medially and slightly posterior. In the adult it is about 2–4 cm lower than the femoral head.Stan ...

of the

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates ...

. Depending upon the given position of the leg, it acts either as external (lateral) rotator of the thigh or as abductor of the thigh. It is innervated by the piriformis nerve.

Structure

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

Origin

The piriformis muscle originates from the anterior (front) surface of the

by three fleshy digitations attached to the second, third, and fourth sacral vertebra. 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 posterior inferior iliac spine (Sweeney's Tubercle) is an anatomical landmark that describes a bony "spine", or projection, at the posterior and inferior surface of the iliac bone. It is one of two such spines on the posterior surface, the o ...

), the

sacroiliac joint The sacroiliac joint or SI joint (SIJ) is the joint between the sacrum and the ilium bones of the pelvis, which are connected by strong ligaments. In humans, the sacrum supports the spine and is supported in turn by an ilium on each side. T ...

capsule, and (sometimes) the

sacrotuberous ligament The sacrotuberous ligament (great or posterior sacrosciatic ligament) is situated at the lower and back part of the pelvis. It is flat, and triangular in form; narrower in the middle than at the ends. Structure It runs from the sacrum (the lowe ...

(more specifically, the superior part of the pelvic surface of this ligament).

Insertion

The muscle inserts onto the

of the

(its

tendon A tendon or sinew is a tough, high-tensile-strength band of dense fibrous connective tissue that connects muscle to bone. It is able to transmit the mechanical forces of muscle contraction to the skeletal system without sacrificing its ability ...

often unites with the tendons of the superior gemellus, inferior gemellus, and obturator internus muscles prior to insertion).

Innervation

The piriformis muscle is

innervated A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

by the piriformis nerve.

Relations

The posterior aspect of the muscle lies againt the sacrum. The anterior surface of the muscle is related to the

rectum The rectum is the final straight portion of the large intestine in humans and some other mammals, and the gut in others. The adult human rectum is about long, and begins at the rectosigmoid junction (the end of the sigmoid colon) at the l ...

(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 The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

against its posterior wall, and partly at the back of the

hip joint In vertebrate anatomy, hip (or "coxa"Latin ''coxa'' was used by Celsus in the sense "hip", but by Pliny the Elder in the sense "hip bone" (Diab, p 77) in medical terminology) refers to either an anatomical region or a joint. The hip region is ...

. It exits the

through the

greater sciatic foramen The greater sciatic foramen is an opening (foramen) in the posterior human pelvis. It is formed by the sacrotuberous and sacrospinous ligaments. The piriformis muscle passes through the foramen and occupies most of its volume. The greater sci ...

superior to the sacrospinous ligament.

Variation

In around 80% of the population, the

sciatic nerve The sciatic nerve, also called the ischiadic nerve, is a large nerve in humans and other vertebrate animals which is the largest branch of the sacral plexus and runs alongside the hip joint and down the lower limb. It is the longest and widest ...

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, but the most common type of anomaly (81% of anomalies) is the Beaton's type B which is when the common peroneal nerve pierces the piriformis muscle. 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 part of the lateral rotators of the hip, along with the

quadratus femoris The quadratus femoris is a flat, quadrilateral skeletal muscle. Located on the posterior side of the hip joint, it is a strong external rotator and adductor of the thigh, but also acts to stabilize the femoral head in the acetabulum. Quadratu ...

, gemellus inferior, gemellus superior,

obturator externus The external obturator muscle, obturator externus muscle (; OE) is a flat, triangular muscle, which covers the outer surface of the anterior wall of the pelvis. It is sometimes considered part of the medial compartment of thigh, and sometimes c ...

, 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. (Netter's Clinical Anatomy, 2010)

Clinical significance

Piriformis syndrome occurs when the piriformis irritates the

, which comes into 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 Sciatica is pain going down the leg from the lower back. This pain may go down the back, outside, or front of the leg. Onset is often sudden following activities like heavy lifting, though gradual onset may also occur. The pain is often described ...

''. Seventeen percent of the population has their sciatic nerve coursing through the piriformis muscle. This subgroup of the population is predisposed to developing sciatica. Sciatica can be described by pain, tingling, or numbness deep in the buttocks and along the sciatic nerve. Sitting down, stretching, climbing stairs, and performing squats usually increases pain. Diagnosing the syndrome is usually based on symptoms and on the physical exam. More testing, including MRIs, X-rays, and nerve conduction tests can be administered to exclude other possible diseases. If diagnosed with piriformis syndrome, the first treatment involves progressive stretching exercises, massage therapy (including neuromuscular therapy) and physical treatment. Corticosteroids can be injected into the piriformis muscle if pain continues. Findings suggest the possibility that

Botulinum toxin Botulinum toxin, or botulinum neurotoxin (BoNT), is a neurotoxic protein produced by the bacterium ''Clostridium botulinum'' and related species. It prevents the release of the neurotransmitter acetylcholine from axon endings at the neurom ...

type B may be of potential benefit in the treatment of pain attributed to piriformis syndrome. A more invasive, but sometimes necessary treatment involves surgical exploration; however, the side effects of the surgery could be much worse than 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 it into an inferior and superior part. This determines the name of the vessels and nerves in this region – the nerve and vessels that emerge superior to the piriformis are the superior gluteal nerve and superior gluteal vessels. Inferiorly, it is the same, and the sciatic nerve also travels inferiorly to the piriformis.

History

The piriformis muscle was first named by Adriaan van den Spiegel, a professor from the

University of Padua The University of Padua ( it, Università degli Studi di Padova, UNIPD) is an Italian university located in the city of Padua, region of Veneto, northern Italy. The University of Padua was founded in 1222 by a group of students and teachers from ...

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 297.png File:Sobo 1909 300.png File:Sobo 1909 301.png File:Sobo 1909 302.png File:Slide2ERVA.JPG, Pelvic contents: male.Superior view.Deep dissection.

References

External links

"Piriformis"

University of Washington The University of Washington (UW, simply Washington, or informally U-Dub) is a public research university in Seattle, Washington. Founded in 1861, Washington is one of the oldest universities on the West Coast; it was established in Seatt ...

* - "Gluteal Region: Muscles" * - "The Female Pelvis: The Posterolateral Pelvic Wall" {{Authority control Hip muscles Hip lateral rotators Deep lateral rotators of the hip Muscles of the lower limb