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Obturator nerve

The obturator nerve is a major branch of the lumbar plexus, formed by the ventral rami of L2, L3, and L4 spinal nerves. It emerges from the medial border of the psoas major muscle, descends along the pelvic brim, and runs toward the obturator canal, which it passes through to reach the medial thigh.

Within the thigh, the nerve divides into anterior and posterior branches, separated by the adductor brevis muscle. These branches supply motor innervation to the medial compartment of the thigh, articular innervation to the hip and knee joints, and sensory fibers to the medial thigh.

Course

  1. Origin: Lumbar plexus (L2–L4), medial to psoas major.

  2. Pelvic course: Travels along lateral wall of pelvis, deep to common iliac vessels.

  3. Obturator canal: Passes with obturator vessels through obturator canal, formed by obturator membrane and surrounding bone.

  4. Thigh division: Splits into anterior and posterior branches around adductor brevis.

Branches

  • Anterior branch: runs anterior to adductor brevis; supplies adductor longus, gracilis, and adductor brevis (partly); gives cutaneous branch to medial thigh; articular branches to hip joint.

  • Posterior branch: runs posterior to adductor brevis; supplies obturator externus, adductor magnus (adductor portion), and adductor brevis (partly); articular branch to knee joint.

  • Accessory obturator nerve (10–30% of cases): when present, arises from L3–L4, supplies pectineus, and contributes to hip joint innervation.

Synonyms

  • Nervus obturatorius

  • Medial thigh nerve (functional description)

  • Lumbar plexus motor-sensory branch

Function

  • Motor: Innervates adductor longus, adductor brevis, adductor magnus (adductor portion), gracilis, and obturator externus.

  • Sensory: Provides cutaneous sensation to medial thigh (variable, often small area).

  • Articular: Supplies hip and knee joints.

  • Critical for adduction of thigh, pelvic stability during gait, and fine control of hip rotation.

MRI Appearance

T1-weighted images:

  • Obturator nerve appears as a linear hypointense structure within bright perineural fat in the pelvis and obturator canal.

  • In the thigh, visualized as a small hypointense band between adductor muscles.

T2-weighted images:

  • Normal nerve: intermediate-to-slightly hyperintense compared to muscle.

  • Pathology (neuropathy, compression, inflammation): bright hyperintense with fascicular distortion.

STIR:

  • Fat suppression enhances conspicuity of nerve edema or entrapment.

  • Shows obturator neuropathy as bright signal in the nerve and surrounding soft tissues.

T1 Fat-Suppressed Post-Gadolinium:

  • Normal obturator nerve: minimal or no enhancement.

  • Pathologic nerve: linear, nodular, or diffuse enhancement, often with thickening.

  • Useful for tumor infiltration, perineural spread, or inflammatory neuropathy.

3D T2-weighted Imaging:

  • Provides isotropic high-resolution datasets for multiplanar reconstructions.

  • Shows the entire course of the nerve: pelvic brim → obturator canal → anterior/posterior thigh.

  • Excellent for nerve mapping, entrapment localization, and presurgical planning.

3D T1 Post-Gadolinium Imaging (with fat suppression):

  • Normal nerve: faint outline, no enhancement.

  • Pathology: enhancing tubular or nodular signal along nerve trajectory.

  • Critical for evaluating tumor spread, perineural invasion, or postsurgical fibrosis.

CT Appearance

Non-contrast CT:

  • Nerve itself not directly visible.

  • Location inferred by bony landmarks: pelvic brim, obturator canal.

  • Detects structural causes: fractures, pelvic exostoses, or masses compressing the nerve.

Post-contrast CT:

  • With high-resolution thin slices with contrast and multiplanar reconstruction, the obturator nerve can sometimes be directly visualized as a small, low-density linear structure relative to adjacent fat planes.

  • Contrast-enhanced CT shows masses, lymphadenopathy, or infection compressing or displacing the nerve.

  • CT neurography (less common than MR neurography) can sometimes depict nerve indirectly by fascial planes.

Clinical Significance

  • Obturator neuropathy: Causes medial thigh pain, weakness of adduction, and difficulty with ambulation.

  • Howship–Romberg sign: Pain radiating to medial thigh from obturator hernia compressing the nerve.

  • Surgical risk: Injured during pelvic surgery, lymph node dissection, or hip arthroplasty.

  • Sports medicine: Stretch or entrapment injuries in athletes, especially dancer

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