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Obturator externus muscle

The obturator externus is a flat, fan-shaped muscle located deep within the medial thigh and pelvis. It arises broadly from the pelvis and courses laterally, lying beneath the pectineus and adductor muscles. Its tendon passes posterior to the femoral neck before inserting into the femur. The muscle is considered one of the short external rotators of the hip, alongside the obturator internus, gemelli, quadratus femoris, and piriformis.

It forms part of the deep muscular layer stabilizing the hip joint and has important relations with the hip capsule, femoral vessels, and obturator neurovascular bundle, making it a structure of clinical importance during hip surgery and in cases of deep pelvic infection or trauma.

Synonyms

  • External obturator

  • Obturator externus of the hip

  • External obturator muscle of medial thigh

Origin, Course, and Insertion

  • Origin:

    • Arises from the external surface of the obturator membrane

    • Extends onto the margins of the obturator foramen (ischium and pubis)

  • Course:

    • Fibers converge posterolaterally beneath the acetabulum and hip joint capsule

    • The tendon passes horizontally and posteriorly, wrapping under the femoral neck

    • The muscle lies deep to the pectineus and adductor brevis, and anterior to the quadratus femoris

  • Insertion:

    • Attaches by a strong tendon into the trochanteric fossa of the femur

Relations

  • Anteriorly: Pectineus, adductor longus and brevis

  • Posteriorly: Hip joint capsule, quadratus femoris

  • Superiorly: Acetabulum and obturator foramen

  • Inferiorly: Upper fibers of the adductor magnus

  • Medially: Obturator nerve and vessels passing through the obturator canal

Nerve Supply

  • Obturator nerve (posterior division, L3–L4)

Arterial Supply

  • Obturator artery (primary supply)

  • Medial circumflex femoral artery

  • Inferior gluteal artery (minor contribution)

Venous Drainage

  • Venous drainage via the obturator vein, medial circumflex femoral vein, and profunda femoris vein, ultimately draining into the femoral vein

Function

  • Primary external rotator of the thigh at the hip joint

  • Contributes to adduction of the thigh

  • Provides anteroposterior stabilization of the femoral head within the acetabulum

  • Acts as a dynamic stabilizer during walking, running, and pivoting movements

Clinical Significance

  • May be involved in deep groin pain syndromes

  • Can be injured in hip dislocations, pelvic fractures, or surgical procedures involving the hip joint

  • Recognized as a landmark in total hip arthroplasty and pelvic surgical approaches

  • Infections or abscesses near the obturator externus may mimic hip joint pathology

MRI Appearance

T1-weighted images:

  • Muscle shows low-to-intermediate signal intensity

  • Chronic fatty change or infiltration may show bright areas

T2-weighted images:

  • Low-to-intermediate baseline signal intensity

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

STIR (Short Tau Inversion Recovery):

  • Normal signal is low-to-intermediate

  • Pathology (strain, inflammation, infection) appears bright

Proton Density Fat-Sat (PD FS):

  • Baseline signal is low-to-intermediate

  • Pathological changes show localized or diffuse bright signal

T1 Fat-Sat Post-Contrast:

  • Normal muscle enhances mildly and uniformly

  • Pathology demonstrates heterogeneous or rim enhancement depending on etiology

CT Appearance

Non-Contrast CT:

  • Muscle appears as homogeneous soft tissue density

  • Hematomas appear as localized hyperdense regions

  • Chronic fatty degeneration shows as low attenuation areas

Post-Contrast CT:

  • Normal muscle enhances mildly and evenly

  • Inflammatory or neoplastic lesions enhance more intensely and irregularly

  • Abscess appears as central low density with peripheral rim enhancement

MRI image

Obturator externus muscle CT axial anatomy image-img-00000-00000

CT image

Obturator externus muscle CT axial image

MRI image

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