The accessory obturator nerve is an inconstant nerve arising from the lumbar plexus (L3-L4) that is present in approximately 10-30% of individuals. When present it passes over the superior pubic ramus (rather than through the obturator canal like the main obturator nerve) to supply the pectineus muscle and give articular branches to the hip joint. It may be the sole supply to the pectineus when the femoral nerve contribution is absent.
The accessory obturator nerve is relevant in inguinal and femoral hernia repair where it passes over the superior pubic ramus and may be injured during Cooper ligament (pectineal ligament) suture placement in McVay repair or during laparoscopic dissection medial to the iliac vessels. Its injury causes weakness of the pectineus and altered hip adduction and flexion. During nerve block procedures for the obturator nerve, supplemental anaesthesia above the pectineal line may be needed to block this accessory pathway.
The accessory obturator nerve crossing the superior pubic ramus is at risk when placing Cooper ligament sutures for femoral hernia repair or during laparoscopic herniorrhaphy, causing pectineus weakness and medial groin numbness that is typically transient.
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