Topics

Topic

design image
Obturator internus muscle

The obturator internus is a thick, fan-shaped muscle located deep in the pelvis and gluteal region. It arises within the pelvic cavity and exits through the lesser sciatic foramen to insert on the femur. Functionally, it belongs to the group of short external rotators of the hip along with the obturator externus, gemelli, piriformis, and quadratus femoris.

The muscle has both pelvic and extra-pelvic parts. It forms an important muscular boundary of the lateral pelvic wall and perineum, while in the gluteal region its tendon blends with the superior and inferior gemelli. Clinically, it is relevant in hip pathology, perianal abscesses, and surgical approaches to the pelvis.

Synonyms

  • Internal obturator muscle

  • Obturator internus of the hip

  • Internal obturator of the pelvis

Origin, Course, and Insertion

  • Origin:

    • Arises from the internal surface of the obturator membrane

    • Extends to the margins of the obturator foramen and adjacent surface of the ischium and pubis

  • Course:

    • Fibers converge toward the lesser sciatic foramen

    • The tendon makes a sharp bend around the lesser sciatic notch, cushioned by a bursa

    • It passes laterally into the gluteal region, deep to the gluteus maximus

  • Insertion:

    • Attaches to the medial surface of the greater trochanter of the femur, often blending with the tendons of the superior and inferior gemelli

Relations

  • Anteriorly: Pelvic viscera (rectum, prostate in males; vagina in females)

  • Posteriorly: Sacrotuberous ligament, gluteus maximus

  • Superiorly: Levator ani muscle, pelvic floor structures

  • Inferiorly: Superior and inferior gemelli muscles

  • Medially (pelvis): Obturator vessels and nerve branches

Nerve Supply

  • Nerve to obturator internus (from sacral plexus, L5–S2)

Arterial Supply

  • Obturator artery

  • Internal pudendal artery

  • Superior gluteal artery (minor contribution)

Venous Drainage

  • Venous drainage via the obturator vein, internal pudendal vein, and tributaries to the internal iliac vein

Function

  • Primary external rotator of the hip joint

  • Assists in abduction of the flexed thigh

  • Helps stabilize the femoral head in the acetabulum during movement

  • Acts as a dynamic stabilizer in gait and posture

Clinical Significance

  • May be a source of deep gluteal or perineal pain

  • Infections (e.g., obturator internus abscess) can mimic hip joint disease

  • Important landmark in pelvic and perineal surgeries

  • Frequently evaluated in cases of piriformis-like syndrome and deep gluteal syndrome

MRI Appearance

T1-weighted images:

  • Muscle shows low-to-intermediate signal intensity

  • Chronic fatty infiltration appears as bright signal areas

T2-weighted images:

  • Muscle shows low-to-intermediate baseline signal

  • Acute strain, inflammation, or edema appear as bright signal

STIR (Short Tau Inversion Recovery):

  • Normal muscle appears low-to-intermediate signal

  • Pathology such as strain, inflammation, or abscess shows bright signal

Proton Density Fat-Sat (PD FS):

  • Baseline muscle shows low-to-intermediate signal

  • Pathological changes demonstrate bright signal intensity

T1 Fat-Sat Post-Contrast:

  • Normal muscle shows mild uniform enhancement

  • Pathological lesions show heterogeneous or rim enhancement depending on etiology

CT Appearance

Non-Contrast CT:

  • Appears as homogeneous soft tissue density in the pelvic wall and gluteal region

  • Hematoma or calcification appears as hyperdense focus

  • Fatty change appears as low attenuation areas

Post-Contrast CT:

  • Normal muscle enhances mildly and evenly

  • Inflammatory or neoplastic lesions demonstrate greater or heterogeneous enhancement

  • Abscess shows central low density with rim enhancement

MRI image

Obturator internus muscle MRI axial anatomy image-img-00000-00000

CT image

Obturator internus muscle CT axial image

MRI image

Obturator internus muscle MRI coronal anatomy image-img-00000-00000