The external obturator muscle or obturator externus muscle (; OE) is a flat, triangular muscle, which covers the outer surface of the anterior wall of the

pelvis The pelvis (: pelves or pelvises) is the lower part of an Anatomy, anatomical Trunk (anatomy), trunk, between the human abdomen, abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also c ...

. It is sometimes considered part of the medial compartment of thigh, and sometimes considered part of the gluteal region. It is also considered to be part of the short external rotators of the hip, along with the gemellus superior and inferior, piriformis, and quadratus femoris.

Structure

It arises from the margin of bone immediately around the medial side of the

obturator membrane The obturator membrane is a thin fibrous sheet, which almost completely closes the obturator foramen. Its fibers are arranged in interlacing bundles mainly transverse in direction; the uppermost bundle is attached to the obturator tubercles and ...

and surrounding bone, viz., from the inferior pubic ramus, and the ramus of the

ischium The ischium (; : is ...

; it also arises from the medial two-thirds of the outer surface of the

, and from the tendinous arch which completes the canal for the passage of the obturator vessels and nerves. The fibers springing from the pubic arch extend on to the inner surface of the bone, where they obtain a narrow origin between the margin of the foramen and the attachment of the obturator membrane. The fibers converge and pass posterolateral and upward, and end in a tendon which runs across the back of the neck of the femur and lower part of the capsule of the hip joint and is inserted into the trochanteric fossa of the femur.

Relations

The obturator vessels lie between the muscle and the obturator membrane; the anterior branch of the obturator nerve reaches the thigh by passing in front of the muscle, and the posterior branch by piercing it.

Variation

In 33% of people, a supernumerary muscle is found between the

adductor brevis The adductor brevis is a muscle in the thigh situated immediately deep to the pectineus and adductor longus. It belongs to the adductor muscle group. The main function of the adductor brevis is to pull the thigh medially. The adductor brevi ...

and

minimus The Minimus books are a series of school textbooks, written by Barbara Bell, illustrated by Helen Forte, and published by the Cambridge University Press, designed to help children of primary school age to learn Latin. The books espouse some of th ...

. While this muscle, when present, is similar to its neighbouring adductors, it is formed by separation from the superficial layer of the external obturator, and is thus not ontogenetically related to the

adductor muscles of the hip The adductor muscles of the hip are a group of muscles in the medial compartment of the thigh mostly used for bringing the thighs together (called adduction). Structure The adductor group is made up of: * Adductor brevis * Adductor longus * A ...

. This muscle originates from the upper part of the inferior pubic ramus from where it runs downwards and laterally. In half of cases, it inserts into the anterior surface of the insertion

aponeurosis An aponeurosis (; : aponeuroses) is a flattened tendon by which muscle attaches to bone or fascia. Aponeuroses exhibit an ordered arrangement of collagen fibres, thus attaining high tensile strength in a particular direction while being vulnerable ...

of the adductor minimus. In the remaining cases, it is either inserted into the upper part of the pectineal line or the posterior part of the

lesser trochanter In human anatomy, the lesser trochanter is a conical, posteromedial, bony projection from the shaft of the femur. It serves as the principal insertion site of the iliopsoas muscle. Structure The lesser trochanter is a conical posteromedial p ...

. It has been demonstrated by the course of the

posterior branch of obturator nerve The posterior branch of the obturator nerve pierces the anterior part of the obturator externus, and supplies this muscle; it then passes behind the adductor brevis on the front of the adductor magnus, where it divides into numerous muscular branch ...

that the obturator externus is divided into a superior

muscle fascicle A muscle fascicle is a bundle of skeletal muscle fibers surrounded by perimysium, a type of connective tissue. Structure Muscle cells are grouped into muscle fascicles by enveloping perimysium connective tissue. Fascicles are bundled toge ...

and a main belly. The supernumerary muscle described above originates from the superior fascicle, while an anomalous fascicle — also derived from the external obturator — originates from the main belly. The "original" external obturator, i.e. without these supernumerary muscular parts, actually occurs in only 20% of cases, and apparently the external obturator readily undergoes

ontogenetic Ontogeny (also ontogenesis) is the origination and development of an organism (both physical and psychological, e.g., moral development), usually from the time of fertilization of the egg to adult. The term can also be used to refer to the stu ...

variations.

Function

The external obturator muscle acts as a lateral rotator of the hip joint. As a short muscle around the hip joint, it stabilizes the hip joint as a postural muscle. It also helps to abduct the hip joint when in flexion. And it helps to stabilise the head of the femur in the socket during flexion and internal rotation as it's posterior fibers reinforce the posterior capsule of hip joint.

Additional images

File:Obturator externus.png, Muscles of the back of thigh, with insertion of obturator externus muscle labeled in purple File:Gray235.png, Right hip bone. External surface File:Gray344.png, Structures surrounding right hip-joint (viewed from below) File:Gray433.png, Deep muscles of the medial femoral region File:Slide2gen.JPG, Obturator externus muscle. Deep dissection. Anterior view. File:Slide8A.JPG, Obturator externus muscle File:Slide1BIBI.JPG, Muscles of thigh. Anterior views.

References

External links

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PTCentral
{{Authority control Hip muscles Deep lateral rotators of the hip Hip lateral rotators Medial compartment of thigh Muscles of the lower limb