Obturator internus muscle

Origin and insertion

Obturator internus originates within the pelvic region where it forms part of the anterolateral wall of the true pelvis. It arises from the bony boundaries of the obturator foramen, including the inferior ramus of the pubis, the ischial ramus, the pelvic surface of the hip bone and the upper part of the greater sciatic foramen. Some muscle fibers arise from the anteromedial inner surface of the obturator membrane, a fibrous sheet which closes the obturator foramen of the hip bone.

From this origin the muscle fibers converge posteriorly, making a sharp lateral turn to pass through the lesser sciatic foramen and over the hip joint before inserting onto the medial side of the greater trochanter of the femur.  In its later course, the obturator internus muscle is commonly joined by the superior and inferior gemelli muscles, forming the common tendon of triceps coxae.

A bursa is typically found between the tendon and the ischium, allowing for free movement of the tendon over the bone without friction. This bursa is referred to as the bursa of obturator internus. Another long, narrow bursa is usually interposed between the common tendon and the capsule of the hip joint, sometimes communicating with the aforementioned bursa of obturator internus.

Relations

The obturator internus muscle occupies the true pelvis. Here, the medial surface of the obturator muscle is covered by obturator fascia, whose central thickening provides the attachment point for the muscles of the pelvic diaphragm (levator ani).

The fascia of the obturator internus muscle is medially related to the obturator artery and nerve, as they run anteroinferiorly from the anterior trunk on the lateral pelvic wall to the upper part of the obturator foramen. The gluteus maximus muscle and the ischial nerve sit superficial (posterior) to the obturator internus muscle.

Learn about the relations and anatomy of obturator internus and other pelvic structures with Kenhub videos and quizzes.

Innervation

Obturator internus is innervated by the nerve to obturator internus, derived from spinal roots L5 and S1.

Blood supply

Blood supply of obturator internus is mainly provided by branches of the obturator artery. However the extrapelvic portion of the muscle can also receive arterial blood from the gemellar branches of the internal pudendal artery.

Function

Due to their attachment on the greater trochanter of the femur, obturator internus and the gemelli muscles act as external (lateral) rotators of the extended thigh. They also abduct the flexed thigh.

In addition to this prime mover role, the obturator internus muscle, along with the other short muscles of the hip (piriformis, superior and inferior gemelli, pectineus, quadratus femoris and obturator externus), acts as an important postural muscle, providing stability to the hip joint, particularly when the thigh is flexed.

To go even further, you can test your knowledge on the obturator internus and other gluteal muscles with the following quiz!