obturator nerve

**Origin:** Anterior divisions of ventral rami L2, L3, L4 (lumbar plexus). Mnemonic: L2–L4 "split the pelvis" – the obturator nerve emerges from the medial border of psoas major.

**Course:** Descends along the medial border of psoas major → passes posterior to the common iliac vessels and lateral to the internal iliac vessels along the pelvic sidewall → exits the pelvis through the obturator canal (superior part of obturator foramen) with the obturator artery and vein → divides into anterior and posterior branches.

**Motor innervation (medial compartment of thigh):** - Anterior branch: adductor longus, adductor brevis, gracilis (± pectineus, which is mainly femoral nerve; may have dual innervation). - Posterior branch: obturator externus, adductor magnus (adductor part; the hamstring part is tibial nerve). - Mnemonic: Everything in the medial compartment except the hamstring part of adductor magnus (tibial) and pectineus (femoral).

**Sensory:** Cutaneous branch to a small area of the medial/inferomedial thigh. Articular branches to the hip joint (anterior branch) and knee joint (posterior branch).

**Clinical significance:** - **Injury** → weakened thigh adduction (difficulty crossing legs), impaired external rotation, wide-based/circumducted gait, and sensory loss over the medial thigh. Abduction is unopposed. - **Causes:** pelvic or retroperitoneal surgery (gynecologic, urologic, colorectal, lymph node dissection), pelvic fracture, childbirth (prolonged labor/forceps; lumbosacral trunk is more classically injured), obturator hernia, hip arthroplasty, and prolonged lithotomy positioning. - **Obturator hernia:** Typically in elderly, thin women ("little old lady's hernia"). It can compress the nerve, causing pain along the medial thigh to the knee (**Howship–Romberg sign**), which is relieved by thigh flexion and worsened by extension, abduction, or medial rotation. It can also cause small bowel obstruction. - **Obturator sign** (appendicitis, pelvic abscess): pain with internal rotation of the flexed thigh, from irritation of the obturator internus. It is a muscle sign (the obturator internus is innervated by the nerve to obturator internus from L5–S1, not the obturator nerve). - **Ovarian fossa:** The nerve runs along the lateral pelvic wall, so ovarian pathology or surgery in this area can cause medial thigh pain. - **Obturator reflex/jerk:** Stimulation of the nerve during transurethral resection of bladder tumors (lateral bladder wall) can cause sudden thigh adduction and risk bladder perforation. It is prevented with an obturator nerve block or paralytic. - **Differentiating from femoral nerve injury:** Femoral (L2–L4) affects hip flexion and knee extension, with an absent patellar reflex and sensory loss over the anterior thigh/medial leg (saphenous). Obturator affects adduction.

**High-yield pearls:** The femoral and obturator nerves are both L2–L4. The obturator nerve is the only lumbar plexus nerve that traverses the obturator canal. Do not confuse it with the nerve to obturator internus (L5–S1, exits via the greater sciatic foramen).