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Obturator Externus – Functional Anatomy

Overview

The obturator externus is a flat, triangular muscle that extends from the lower pelvis to the superior femur. The muscle has a superior belly and the main belly. Its name comes from its attachment to the external surface of the obturator foramen. Its counterpart, obturator internus, originates from the internal surface.

It is usually considered to be part of the external rotators of the thigh. Because of the anomalies that attach a bit lower on the thigh, it is sometimes considered to be part of the medial thigh group.

Origin

  • medial lip of the obturator foramen
  • rami of the ischium and pubis
  • anterior/medial two-thirds of the external surface of the obturator membrane

Insertion

  • trocanteric fossa near the superior aspect of the femoral neck

Function

  • stabilization of the femoral head
  • lateral rotation of the hip
  • abduction of the hip, when flexed

Innervation

  • posterior brach of the obturator nerve (L3-4)

Functional Considerations

This muscle is positioned so that it assists in lateral rotation of the hip joint when standing. As the hip rotates into flexion, and the insertion rotates posteriorly, it can assist in abduction of the hip joint.

When this muscle is tight, it entraps nerves that pass through the obturator and impacts bladder function.

This muscle is the most anterior layer of the lateral rotators. Its origin is more accessible when the patient is supine than prone. Posteriorly, its insertion is more easily palpated after the superficial glutes and the quadratus femoris are released.

Anomalies, Etc.

This muscle is highly variable.

In about one-third of people, there is a muscle that splits off of the obturator externus in utero. This muscle originates on the inferior pubic bone. Its insertion is variable but usually attaches to the aponeurosis of the adductor minimus. However, it may also attach along the femur along the pectineal line or lesser trochanter.

The main belly, without the superior belly, occurs in about twenty percent of cases.

Occasionally, there is a bursa that separates the obturator externus from the femur.

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