Obturator externus muscle

Origin and insertion

Obturator externus muscle is a triangular muscle, which means it has a much broader attachment area at its base and a small attachment area at its apex. Its broad base arises from the external surface of the obturator membrane, specifically the anteromedial portion, and the surrounding pubic and ischial rami.

The fibers of the muscle converge into a single tendon, which travels in a groove on the inferior aspect of the acetabulum. It then proceeds superolaterally on the posterior aspect of the femoral head and inserts into the trochanteric fossa of the femur.

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Relations

Obturator externus is located in the pelvis on the anterior aspect of the innominate bones. It covers the obturator foramen and is located deep to pectineus and superior parts of the adductors of the thigh. Its tendon lies deep to the quadratus femoris muscle and separates it from the neck of the femur. The obturator vessels (anterior and posterior branches of the obturator artery and vein) are found deep to the obturator externus muscle, on the external surface of the obturator membrane. There are also nerves travelling in close proximity to this muscle. The anterior branch of the obturator nerve passes over the anterior surface of the muscle while the posterior branch pierces the muscle, before both branches descend to innervate the muscles of the thigh.

Sometimes a bursa may be present between the tendon of the obturator externus and the hip joint capsule, known simply as the obturator externus bursa. This bursa communicates with the hip joint to reduce the friction between the joint capsule and the tendon.

Innervation

Obturator externus is innervated by the posterior branch of the obturator nerve (L3 and L4), originating from the lumbar plexus.

Blood supply

Obturator externus is supplied by the anterior branch of the obturator artery and medial circumflex femoral artery. These blood vessels form a variable pattern, meaning the muscle may receive blood supply from both or just one of these vessels.

Functions

Obturator externus has a dual primary function, which depends on the position of the thigh. When the hip is extended (body is in the anatomical position), the contraction of the obturator externus causes lateral, or external rotation of the thigh. This action is especially useful in climbing. It is also believed to play a role in walking, counteracting the medial rotation caused by the anterior adductors of the thigh.

When the hip joint is flexed, i.e. the thigh is closer to the body, obturator externus muscle abducts the thigh. It does this by pulling the superior part of the femur medially, which causes the inferior part to move away from the body.

It also contributes to the stability of the hip joint with other short muscles surrounding it (pectineus, piriformis, obturator internus, quadratus femoris and the gemelli superior and inferior). Although supporting the hip joint is generally described as a secondary function, it has been suggested that it may be more important than what’s considered to be the primary functions.

If you would like to learn about muscles of this region consider taking a look at the useful resources listed below.