Obturator Internus

The obturator internus is a fan-shaped muscle that originates within the pelvis, passes through the lesser sciatic foramen, and enters the deep gluteal region before inserting on the medial surface of the greater trochanter of the femur. It is one of the short lateral rotators of the hip and has important anatomical relationships with the pelvic wall, sciatic foramina, gemelli, pudendal neurovascular structures, and hip joint.^[1][2]

The muscle has a distinctive course. Its broad muscular fibers arise from the internal surface of the obturator membrane and surrounding pelvic bone and converge toward the lesser sciatic foramen. The tendon then makes a sharp turn around the lesser sciatic notch before passing laterally toward the proximal femur.

In the gluteal region, the obturator internus tendon is positioned between the superior and inferior gemelli. Together, these three structures form a closely associated functional unit that laterally rotates the extended thigh, abducts the flexed thigh, and helps stabilize the femoral head within the acetabulum.

Location

The obturator internus occupies both the pelvis and deep gluteal region. Its muscular portion lies primarily within the pelvis, where it covers much of the internal surface of the obturator membrane and adjacent bony surfaces.

The fibers converge posterolaterally toward the lesser sciatic foramen. After leaving the pelvis, the tendon passes between the superior and inferior gemelli and travels toward the greater trochanter.

This arrangement makes the obturator internus unusual because its muscle belly and terminal tendon occupy different anatomical regions connected through the lesser sciatic foramen.

General Anatomy

Feature	Anatomy
Muscle	Obturator internus
Location	Lateral pelvic wall and deep gluteal region
Origin	Internal surface of obturator membrane and surrounding pubic and ischial bone
Insertion	Medial surface of greater trochanter of femur
Innervation	Nerve to obturator internus
Root values	L5, S1, S2
Principal actions	Lateral rotation of extended thigh, abduction of flexed thigh and stabilization of femoral head
Exit from pelvis	Lesser sciatic foramen

Origin

The obturator internus has a broad origin from the internal surface of the obturator membrane and from the surrounding internal surfaces of the pubis and ischium.^[1][3]

The obturator membrane closes most of the obturator foramen. Obturator internus covers much of its pelvic surface, while obturator externus arises from the opposite, external surface of the membrane.

The broad origin gives the muscle a fan-shaped appearance. Its fibers converge posterolaterally toward the lesser sciatic foramen, where they become tendinous.

Insertion

The tendon of obturator internus inserts on the medial surface of the greater trochanter of the femur.

Before reaching the femur, the tendon becomes closely associated with the superior and inferior gemelli. These muscles attach to the superior and inferior aspects of the tendon and effectively accompany it toward the greater trochanter.

The insertion lies proximal to and distinct from the trochanteric fossa, which receives the tendon of obturator externus.

Course

The course of the obturator internus can be divided into pelvic and gluteal portions.

Pelvic Course

Within the pelvis, the muscle fibers pass posterolaterally from their broad origin around the obturator foramen. They converge toward the lesser sciatic notch.

The muscle forms part of the lateral wall of the pelvis. Its medial surface is covered by the obturator fascia, which separates it from structures within the pelvic cavity.

Turn at the Lesser Sciatic Notch

Near the lesser sciatic notch, the fibers converge into a tendon. The tendon makes a marked change in direction as it passes through the lesser sciatic foramen.

The bony margin of the lesser sciatic notch acts as a pulley around which the tendon changes direction. A bursa is associated with this region and reduces friction as the tendon moves across the underlying structures.

Gluteal Course

After entering the gluteal region, the tendon passes laterally between the superior gemellus above and inferior gemellus below. The combined complex then approaches the medial surface of the greater trochanter.

Lesser Sciatic Foramen

The lesser sciatic foramen is formed from the lesser sciatic notch by the sacrospinous and sacrotuberous ligaments. It provides communication between the pelvis, gluteal region, and perineum.

The obturator internus tendon passes through this foramen as it leaves the pelvis. Several important neurovascular structures also use the lesser sciatic foramen, including the pudendal nerve and internal pudendal vessels as they enter the perineum.

The foramen is therefore an important anatomical transition point for both the obturator internus and structures associated with the pudendal neurovascular pathway.

Obturator Fascia

The pelvic surface of obturator internus is covered by the obturator fascia. This fascia forms part of the fascial lining of the pelvic wall.

A thickening of the obturator fascia contributes to the tendinous arch of the levator ani, which provides attachment for part of the pelvic diaphragm.

More inferiorly, the obturator fascia also contributes to the formation of the lateral wall of the pudendal canal, also called Alcock's canal.

Relationship to the Pelvic Wall

The obturator internus forms an important muscular component of the lateral pelvic wall. Its broad internal surface follows the contour of the obturator membrane and surrounding pelvic bones.

The pelvic organs lie medial to the fascial layers covering the muscle, while the obturator foramen and medial thigh lie on its opposite side.

This position explains why obturator internus is relevant to both musculoskeletal anatomy and the organization of the pelvic cavity.

Relationship to the Pelvic Floor

The obturator internus has an important fascial relationship with the levator ani. The tendinous arch of levator ani extends along the obturator fascia and provides origin for part of the pelvic diaphragm.

The muscle itself is not a component of the pelvic floor, but its fascia provides a lateral attachment for pelvic floor musculature.

This relationship is important when studying the support structures of the pelvic viscera and the organization of the lateral pelvic wall.

Relationship to Superior Gemellus

The superior gemellus arises from the ischial spine and attaches to the superior aspect of the obturator internus tendon after the tendon emerges through the lesser sciatic foramen.

The two muscles share the same nerve supply, the nerve to obturator internus, and perform similar actions at the hip.

The superior gemellus can therefore be regarded functionally as an accessory muscle associated with the upper surface of the obturator internus tendon.

Relationship to Inferior Gemellus

The inferior gemellus arises from the ischial tuberosity and joins the inferior aspect of the obturator internus tendon.

Although it functions closely with obturator internus, its innervation differs. The inferior gemellus is supplied by the nerve to quadratus femoris.

The obturator internus tendon is therefore characteristically positioned between two small muscles with similar actions but different neural supplies.

Triceps Coxae

The superior gemellus, obturator internus, and inferior gemellus are sometimes collectively called the triceps coxae.

Obturator internus forms the central and largest component. The superior and inferior gemelli attach to opposite sides of its tendon after it enters the gluteal region.

Muscle	Origin	Innervation
Superior gemellus	Ischial spine	Nerve to obturator internus
Obturator internus	Internal obturator membrane and surrounding bone	Nerve to obturator internus
Inferior gemellus	Ischial tuberosity	Nerve to quadratus femoris

Relationship to Piriformis

The piriformis lies superior to the obturator internus-gemelli complex in the gluteal region. It exits the pelvis through the greater sciatic foramen rather than the lesser sciatic foramen.

Piriformis is an important landmark for structures leaving the greater sciatic foramen. Many major nerves and vessels pass inferior to it before crossing superficial to the obturator internus and gemelli.

The relative positions of piriformis and obturator internus are therefore useful for orienting the deep gluteal anatomy.

Relationship to Quadratus Femoris

The quadratus femoris lies inferior to the inferior gemellus. It extends from the ischial tuberosity toward the intertrochanteric crest of the femur.

Both quadratus femoris and the obturator internus-gemelli complex contribute to lateral rotation of the hip, but they have different insertions and nerve supplies.

The layered sequence of obturator internus, inferior gemellus, and quadratus femoris provides important landmarks in posterior hip anatomy.

Relationship to the Sciatic Nerve

The sciatic nerve usually exits the pelvis inferior to piriformis and descends through the deep gluteal region. It passes superficial to the obturator internus tendon and gemelli.

The obturator internus-gemelli complex therefore forms part of the muscular bed beneath the sciatic nerve.

This relationship is clinically important because the sciatic nerve may be affected by abnormalities within the deep gluteal space, including fibrous bands, muscular abnormalities, trauma, or other space-occupying processes.

Relationship to the Pudendal Nerve

The pudendal nerve has a particularly important relationship with the obturator internus. It exits the pelvis through the greater sciatic foramen, curves around the region of the ischial spine and sacrospinous ligament, and then enters the perineum through the lesser sciatic foramen.

After entering the perineum, the pudendal nerve travels within the pudendal canal formed in the fascia covering the obturator internus.

The obturator internus and its fascia therefore provide important landmarks throughout a substantial portion of the pudendal nerve's course.

Pudendal Canal

The pudendal canal, also known as Alcock's canal, is a fascial tunnel formed within the obturator fascia on the medial surface of obturator internus.

It contains the pudendal nerve and internal pudendal vessels as they travel anteriorly through the lateral wall of the ischioanal fossa.

The canal demonstrates the close anatomical relationship between obturator internus and the neurovascular supply of the perineum.

Relationship to the Internal Pudendal Vessels

The internal pudendal artery and vein follow the pudendal nerve around the ischial spine, through the lesser sciatic foramen, and into the pudendal canal.

These vessels lie within the fascial compartment associated with the medial surface of obturator internus as they travel toward the anterior perineum.

The muscle is therefore an important landmark for locating both the pudendal nerve and internal pudendal vessels.

Relationship to the Hip Joint

In the gluteal region, the obturator internus tendon passes posterior to the hip joint before reaching the greater trochanter.

Its close relationship to the joint allows it to contribute to dynamic stabilization of the femoral head. The short distance between the muscle's effective line of pull and the joint favors controlled rotational and stabilizing actions.

The tendon and gemelli collectively form a muscular layer superficial to portions of the posterior hip capsule.

Innervation

The obturator internus is innervated by the nerve to obturator internus, a branch of the sacral plexus containing fibers primarily from L5, S1, and S2.

The nerve leaves the pelvis through the greater sciatic foramen inferior to piriformis. It then passes around the region of the ischial spine and enters the lesser sciatic foramen to reach the pelvic surface of obturator internus.

Along its course, the nerve also supplies the superior gemellus.

Nerve to Obturator Internus

The nerve to obturator internus should be distinguished from the obturator nerve. Despite their similar names, they arise from different plexuses and supply different muscular groups.

Feature	Nerve to Obturator Internus	Obturator Nerve
Plexus	Sacral plexus	Lumbar plexus
Typical roots	L5, S1, S2	L2, L3, L4
Major muscular targets	Obturator internus and superior gemellus	Medial thigh muscles and obturator externus
Pelvic exit	Greater sciatic foramen, then enters lesser sciatic foramen	Obturator canal

Blood Supply

The obturator internus receives blood from branches of arteries supplying the pelvic wall and deep gluteal region. Important contributions can arise from the internal pudendal artery, inferior gluteal artery, and obturator artery.

The relative contribution of individual vessels varies along the muscle's pelvic and gluteal portions.

Its position near major pelvic and gluteal vascular pathways provides a rich regional blood supply despite the muscle's relatively small size.

Actions

Lateral Rotation of the Extended Thigh

The obturator internus produces lateral rotation of the extended thigh. After turning around the lesser sciatic notch, its tendon approaches the proximal femur from posteriorly.

Contraction therefore pulls the greater trochanter in a direction that rotates the anterior surface of the femur laterally.

The muscle works with the gemelli, piriformis, quadratus femoris, obturator externus, and other lateral rotators.

Abduction of the Flexed Thigh

When the hip is flexed, the mechanical relationship between the obturator internus tendon and the femur changes. In this position, the muscle contributes to abduction of the thigh.

The superior and inferior gemelli assist this action because their forces are transmitted through the same insertional complex.

Stabilization of the Hip

The obturator internus helps stabilize the femoral head within the acetabulum. Its short effective course around the posterior hip allows it to generate a compressive component of force during movement.

This stabilizing action works together with the other short lateral rotators and the passive restraints of the hip joint.

Role During Movement

The function of obturator internus changes with the position of the femur. In an extended hip, its line of pull favors lateral rotation. As the hip becomes flexed, its rotational relationship changes and an abduction component becomes more prominent.

This position-dependent action is important for understanding why the deep hip rotators cannot be assigned a single unchanging function throughout the complete range of hip motion.

During walking, running, and changes in direction, obturator internus participates in coordinated control of femoral rotation and hip stability.

Comparison with Obturator Externus

Feature	Obturator Internus	Obturator Externus
Origin relative to obturator membrane	Internal surface	External surface
Primary location	Pelvic wall and deep gluteal region	Deep proximal medial thigh
Course	Through lesser sciatic foramen	Posterior to femoral neck
Insertion	Medial surface of greater trochanter	Trochanteric fossa
Innervation	Nerve to obturator internus	Obturator nerve
Major action	Lateral rotation and abduction of flexed thigh	Lateral rotation and stabilization

Anatomical Variations

The obturator internus can vary in the extent of its origin, arrangement of muscular fascicles, and configuration of its tendon. The degree of blending between the tendon and the gemelli may also vary.

Variations may occur in the branching pattern of the nerve to obturator internus and in the relationships of the pudendal neurovascular structures around the ischial spine and lesser sciatic foramen.

The muscle's basic pattern, a broad pelvic origin followed by a tendon passing through the lesser sciatic foramen, remains a characteristic anatomical feature.

Clinical Significance

Deep Gluteal Syndrome

The obturator internus-gemelli complex forms part of the floor of the deep gluteal space, with the sciatic nerve passing superficial to it.

Deep gluteal syndrome refers to non-discogenic sciatic nerve entrapment or irritation within this region. Potential causes include fibrous bands, muscular abnormalities, vascular structures, trauma, and altered relationships between the nerve and surrounding tissues.

The obturator internus complex is therefore anatomically relevant when evaluating sciatic symptoms arising outside the lumbar spine.

Obturator Internus Syndrome

Irritation or altered tension involving the obturator internus has been discussed as a possible contributor to deep gluteal pain and sciatic nerve symptoms because of the nerve's close relationship to the muscle and gemelli.

Clinical localization is difficult because the region contains numerous muscles, nerves, tendons, bursae, and the hip joint itself. Symptoms should therefore be interpreted in the context of the broader deep gluteal anatomy.

Pudendal Nerve Entrapment

The pudendal nerve's course around the ischial spine and through the pudendal canal in the obturator fascia makes the obturator internus region relevant to pudendal nerve entrapment.

The nerve may be affected at several points along its course, including near the sacrospinous and sacrotuberous ligaments or within the pudendal canal.

Knowledge of the obturator internus and its fascia is therefore essential for understanding the anatomical basis of pudendal nerve procedures and entrapment syndromes.

Muscle Strain

Injury to obturator internus may occur during forceful rotational movements of the hip. Because the muscle lies deeply and its symptoms overlap with those of other deep hip structures, isolated injury can be difficult to diagnose clinically.

MRI can help identify edema or structural injury involving the muscle belly or tendon.

Posterior Hip Surgery

The obturator internus tendon and gemelli are important landmarks during posterior approaches to the hip. They lie deep to gluteus maximus and near the sciatic nerve and posterior joint capsule.

The characteristic arrangement of superior gemellus, obturator internus tendon, and inferior gemellus helps surgeons orient the deep posterior anatomy.

The proximity of the sciatic nerve requires careful identification and protection of neural structures during procedures in this region.

Pudendal Nerve Block

The ischial spine and surrounding structures provide landmarks for procedures involving the pudendal nerve. Because the nerve passes around the ischial spine and subsequently travels within the obturator fascia, the obturator internus is an important deeper anatomical reference.

Understanding the relationship among the ischial spine, sacrospinous ligament, obturator internus, and pudendal canal is essential when interpreting the regional anatomy.

Imaging

The obturator internus can be visualized with CT and MRI. Within the pelvis, its muscular belly can be identified along the lateral pelvic wall adjacent to the obturator membrane.

On more inferior and posterior sections, the tendon can be followed as it turns through the lesser sciatic foramen and passes between the gemelli toward the greater trochanter.

MRI can demonstrate muscular injury, edema, atrophy, masses, and abnormalities involving the surrounding deep gluteal structures.

Surface Anatomy

The obturator internus cannot normally be palpated directly from the external surface because most of its muscle belly lies within the pelvis and its tendon is covered by gluteus maximus and other deep structures.

Its gluteal course can be projected from the region of the lesser sciatic foramen laterally toward the medial surface of the greater trochanter.

The obturator internus is best understood through its distinctive anatomical pathway: a broad origin from the internal obturator membrane, convergence toward the lesser sciatic foramen, a sharp tendinous turn around the lesser sciatic notch, passage between the two gemelli, and insertion on the greater trochanter.