Cadaveric study of the obturator nerve: frequency of skin innervation and the optimal site for blocking the cutaneous branch.

Background: Clinical understanding of the obturator nerve's cutaneous and motor branches is crucial for performing effective obturator nerve blocks, particularly for procedures involving the thigh and hip. Literature and anatomical references report highly variable patterns of the frequency and cutaneous distribution of obturator nerve innervation.This study examines the frequency and distribution of the cutaneous branch of the anterior ramus of the obturator nerve (cb-ar-ON) and assesses the most effective anatomical sites for nerve blockade.

Methods: Dissections were conducted on 14 sides from eight cadavers to identify the presence, branching pattern and innervation areas of the cb-ar-ON. Ultrasound-guided injections of low-volume dye were performed proximally between the adductor longus and gracilis muscles and distally near the great saphenous vein to assess optimal targeting.

Results: cb-ar-ON were found in 43% of cadaveric sides, branching off the anterior ramus of the obturator nerve 6-10 cm distal to the inguinal ligament. When present, the cb-ar-ON innervated a 4-9 cm² area in the posteromedial popliteal fossa. Proximal ultrasound-guided blocks targeting the area between the adductor longus and gracilis muscles effectively stained the cb-ar-ON in 100% of cases where the branch was present.

Conclusion: The cb-ar-ON provides variable and limited cutaneous innervation, appearing in 43% of cases, primarily in the posteromedial popliteal fossa. Effective blockade can be achieved with a proximal approach between the adductor longus and gracilis muscles.These findings suggest that clinical evaluation of obturator nerve block based on cutaneous anesthesia is unreliable.