Piriformis muscle
Encyclopedia
The piriformis is a muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

 in the gluteal region of the lower limb
Lower limb
The lower limb is a limb of the body.According to Terminologia Anatomica, it includes the pelvic girdle, buttocks, hip, and thigh, as well as the components distal to the knee.-References:...

. It was first named by Spigelius, a professor from the University of Padua in the 16th century.

Origin and insertion

It originates from the anterior (front) part
Pelvic surface of sacrum
The pelvic surface of sacrum is concave from above downward, and slightly so from side to side.Its middle part is crossed by four transverse ridges, the positions of which correspond with the original planes of separation between the five segments of the bone.The portions of bone intervening...

 of the sacrum
Sacrum
In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...

, the part of the spine
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...

 in the gluteal region, and from the superior margin of the greater sciatic notch
Greater sciatic notch
Above the ischial spine is a large notch, the greater sciatic notch, converted into a foramen by the sacrospinous ligament.It transmits the Piriformis, the superior and inferior gluteal vessels and nerves, the sciatic and posterior femoral cutaneous nerves, the internal pudendal vessels, and the...

 (as well as the sacroiliac joint
Sacroiliac joint
The sacroiliac joint or SI joint is the joint in the bony pelvis between the sacrum and the ilium of the pelvis, which are joined together by strong ligaments. In humans, the sacrum supports the spine and is supported in turn by an ilium on each side...

 capsule and the sacrotuberous ligament
Sacrotuberous ligament
The sacrotuberous 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....

). It exits the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...

 through the greater sciatic foramen
Greater sciatic foramen
-Boundaries:It is bounded as follows:* anterolaterally by the greater sciatic notch of the illium* posteromedially by the sacrotuberous ligament* inferiorly by the sacrospinous ligament and the ischial spine* superiorly by the anterior sacroilliac ligament...

 to insert on the greater trochanter
Greater trochanter
The greater trochanter of the femur is a large, irregular, quadrilateral eminence and a part of the skeletal system.It is directed a little lateralward and backward, and, in the adult, is about 1 cm lower than the head...

 of the femur
Femur
The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

. Its tendon
Tendon
A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments and fasciae as they are all made of collagen except that ligaments join one bone to another bone, and fasciae connect muscles to other...

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

Shape and location

The piriformis is a flat muscle, pyramidal in shape, lying almost parallel with the posterior margin of the gluteus medius.

It is situated partly within the pelvis
Pelvis
In human anatomy, the pelvis is the lower part of the trunk, between the abdomen and the lower limbs .The pelvis includes several structures:...

 against its posterior wall, and partly at the back of the hip-joint.

It arises from the front of the sacrum by three fleshy digitations, attached to the portions of bone between the first, second, third, and fourth anterior sacral foramina
Anterior sacral foramina
At the ends of the transverse ridges of the pelvic surface of the sacrum are seen the anterior sacral foramina , four in number on either side, somewhat rounded in form, diminishing in size from above downward, and directed lateralward and forward.They give exit to the anterior divisions of the...

, and to the grooves leading from the foramina: a few fibers also arise from the margin of the greater sciatic foramen
Greater sciatic foramen
-Boundaries:It is bounded as follows:* anterolaterally by the greater sciatic notch of the illium* posteromedially by the sacrotuberous ligament* inferiorly by the sacrospinous ligament and the ischial spine* superiorly by the anterior sacroilliac ligament...

, and from the anterior surface of the sacrotuberous ligament
Sacrotuberous ligament
The sacrotuberous 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....

.

The muscle passes out of the pelvis through the greater sciatic foramen
Greater sciatic foramen
-Boundaries:It is bounded as follows:* anterolaterally by the greater sciatic notch of the illium* posteromedially by the sacrotuberous ligament* inferiorly by the sacrospinous ligament and the ischial spine* superiorly by the anterior sacroilliac ligament...

, the upper part of which it fills, and is inserted by a rounded tendon into the upper border of the greater trochanter
Greater trochanter
The greater trochanter of the femur is a large, irregular, quadrilateral eminence and a part of the skeletal system.It is directed a little lateralward and backward, and, in the adult, is about 1 cm lower than the head...

 behind, but often partly blended with, the common tendon of the obturator internus and superior
Superior gemellus muscle
The superior gemellus muscle is a muscle of the human body.The Gemelli are two small muscular fasciculi, accessories to the tendon of the Obturator internus which is received into a groove between them....

 and inferior gemellus muscle
Inferior gemellus muscle
The inferior gemellus muscle is a muscle of the human body.The Gemelli are two small muscular fasciculi, accessories to the tendon of the Obturator internus which is received into a groove between them....

s.

Action

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 extended thigh and abducts the flexed thigh. 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 keeps us from falling. The action of the lateral rotators can be understood by crossing your 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. 'At a range of more than 90 degrees the actions of the piriformis are reversed to adduct and internally rotate (medial).'

Variations

In 17% of limbs, 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 in to the capsule of the hip joint.

Clinicals

This syndrome occurs when the piriformis irritates the sciatic nerve
Sciatic nerve
The sciatic nerve is a large nerve fiber in humans and other animals. It begins in the lower back and runs through the buttock and down the lower limb...

, 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
Sciatica is a set of symptoms including pain that may be caused by general compression or irritation of one of five spinal nerve roots that give rise to each sciatic nerve, or by compression or irritation of the left or right or both sciatic nerves. The pain is felt in the lower back, buttock, or...

. 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 and physical treatment. Corticosteroids can be injected into the piriformis muscle if pain continues. A more invasive, but sometimes necessary treatment involves surgical exploration as a last resort.

External links

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