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Obturator internus tendon

The obturator internus tendon is the strong fibrous continuation of the obturator internus muscle, one of the short external rotators of the hip. It plays an important role in external rotation and stabilization of the hip joint. The tendon is notable for its sharp change in direction, as the muscle belly arises within the pelvis, passes through the lesser sciatic foramen, and then turns laterally to insert onto the femur. This complex course makes it a key structure in pelvic and perineal anatomy, with significant relevance in orthopedic and radiologic evaluation.

Synonyms

  • Tendon of obturator internus

  • Obturator internus insertion tendon

Origin, Course, and Insertion

  • Origin (muscle belly): Arises from the internal surface of the obturator membrane and surrounding pelvic bones

  • Course (tendon):

    • The muscle fibers converge posteriorly and exit the pelvis through the lesser sciatic foramen

    • The tendon bends sharply around the lesser sciatic notch, cushioned by a bursa

    • It joins with the tendons of the superior and inferior gemelli, forming the triceps coxae

  • Insertion:

    • Inserts into the medial surface of the greater trochanter of the femur, within the trochanteric fossa

Relations

  • Anteriorly: Pelvic sidewall, obturator membrane

  • Posteriorly: Gluteus maximus and sciatic nerve (lateral to tendon)

  • Superiorly: Superior gemellus muscle

  • Inferiorly: Inferior gemellus muscle

  • Medially (within pelvis): Levator ani muscle and pelvic viscera

Function

  • External rotation of the thigh at the hip joint

  • Assists in abduction when hip is flexed

  • Stabilizes the femoral head in the acetabulum

  • Works synergistically with other short hip rotators to control fine movements

Clinical Significance

  • Important landmark in pelvic and hip surgery (including approaches through the sciatic foramen)

  • May be a site of tendinopathy, tears, or calcification

  • Can be involved in deep gluteal pain syndromes

  • Relation to the sciatic nerve makes it relevant in entrapment and piriformis-like syndromes

  • Often evaluated in MR hip arthrography and pelvic MRI

MRI Appearance

T1-weighted images:

  • Normal tendon shows uniform low signal intensity

  • Surrounding fat appears bright, outlining the tendon path

  • Chronic degeneration may show focal bright areas from fatty infiltration

T2-weighted images:

  • Normal tendon remains dark (low signal intensity)

  • Tendinopathy or partial tears appear as focal bright signal within or around tendon

STIR (Short Tau Inversion Recovery):

  • Tendon appears dark

  • Fluid, edema, or inflammation around tendon appears bright

T1 Fat-Sat Post-Contrast:

  • Normal tendon shows no or minimal enhancement

  • Pathology (inflammation, abscess, tumor infiltration) shows focal or rim enhancement

CT Appearance

Non-Contrast CT:

  • Tendon appears as a soft tissue density band at the medial aspect of the greater trochanter

  • Calcification or ossification may be seen in chronic tendinopathy

Post-Contrast CT:

  • Normal tendon does not enhance significantly

  • Surrounding tissues may enhance if inflamed or infiltrated

  • Abscess or neoplasm shows rim or heterogeneous enhancement near tendon

MRI images

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

MRI images

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

CT image

Obturator internus tendon CT axial anatomy image-img-00000-00000

MRI image

Obturator internus tendon MRI sag anatomy image-img-00000-00000