Obturator Internus Muscle Anatomy

Location & Overview

The obturator internus muscle is primarily situated inside the pelvis, where it plays a key role in supporting the lateral pelvic wall. It originates within the lesser pelvis and acts as a protective pad over much of the lateral pelvic wall. As the muscle fibers progress, they converge posteriorly and laterally and become tendinous. This tendinous segment leaves the pelvis through an opening known as the lesser sciatic foramen. Once outside the pelvis, the muscle attaches to the greater trochanter of the femur.^[1,2]

The obturator internus muscle is mainly positioned medial to the gemelli muscles. These gemelli muscles consist of two parts: the superior gemellus and the inferior gemellus. As the obturator internus extends laterally toward the femur, its tendon passes between and merges with the tendons of the gemelli muscles. Collectively, the tendons of the obturator internus, superior gemellus, and inferior gemellus join to form a single tendon. This common tendon then attaches to the greater trochanter of the femur.^[1,2]

Aside from its anatomical structure, the obturator internus does not function in isolation; it works with the superior gemellus and inferior gemellus to form a muscle group called the ‘triceps coxae’. This muscle group is located between two other muscles: the piriformis and the quadratus femoris. The triceps coxae play a role in hip stabilization and movement, especially in lateral rotation of the thigh.^[1,2]

Origin & Insertion

The obturator internus originates from the pelvic surface of the obturator membrane, which is a thin, fibrous sheet that covers the obturator foramen, a large opening in the pelvic bone. Additionally, it originates from the surrounding bones, particularly the pelvic surfaces of the ilium, which is the broad, fan-shaped upper part of the hip bone, and the ischium, which is the lower and posterior part of the hip bone. The muscle then inserts onto the medial surface of the greater trochanter, a prominent bony projection located at the upper and outer part of the femur (thigh bone). Specifically, it attaches to the trochanteric fossa, which is a shallow depression on the medial aspect of the greater trochanter of the femur.^[1,2]

Action & Function

The obturator internus primarily functions to laterally (or externally) rotate the hip joint. This means it helps turn the thigh outward, away from the midline of the body. This action is particularly emphasized when the hip is in an extended position. Additionally, when the hip is flexed, the obturator internus weakly assists in abduction, which is the movement of the thigh away from the body’s midline.^[1,2]

Beyond these movements, an important function of the obturator internus muscle is stabilization of the hip joint. Furthermore, the obturator internus helps hold the head of the femur within the acetabulum, which is the cup-shaped socket of the hip bone where the ball-shaped head of the femur fits. This helps maintain the stability and integrity of the hip joint, especially during movement.^[1,2]

Innervation

The obturator internus muscle is innervated by the nerve to the obturator internus. This nerve arises from the anterior divisions of the anterior rami of the spinal nerves, specifically from the levels L5 and S1.^[1,2]

The nerve to the obturator internus exits the pelvis via the greater sciatic foramen, which is a large opening in the posterior pelvis formed by the sacrum and the ilium. This foramen is positioned inferior to the piriformis muscle.^[1,2]

Subsequently, the nerve descends behind the sacrospinous ligament, a thin ligament that extends from the sacrum to the ischial spine, helping to convert the greater and lesser sciatic notches into foramina.^[1,2]

During its journey, the nerve supplies the superior gemellus muscle as it passes around the base of the ischial spine, which is a pointed bony projection from the posterior part of the ischium and serves as an attachment point for several ligaments and muscles.^[1,2]

After passing around the ischial spine and sacrospinous ligament region, the nerve to the obturator internus enters the lesser sciatic foramen to reach the obturator internus muscle. The lesser sciatic foramen is a smaller opening formed by the pelvic ligaments and adjacent pelvic bones, located inferior to the greater sciatic foramen.^[1,2]

The nerve to the obturator internus also innervates the superior gemellus muscle. However, there is some variability in this innervation. In a study conducted by Htar Htar Aung et al. (2001), it was shown that the superior gemellus muscle received dual innervation from both the nerve to the obturator internus and the nerve to the quadratus femoris approximately 60.4% of the time. Htar Htar Aung et al. (2001) also confirmed that these two nerves are distinct, as they follow different pathways and do not communicate with each other.^[3]

Blood Supply

The obturator internus receives arterial supply from branches of the obturator, inferior gluteal, and internal pudendal arteries. The internal pudendal contribution includes gemellar and inferior rectal branches that supply the lateral section of the muscle.^[4]