Background and indications: The obturator nerve emerges from the medial border of the iliopsoas muscle, posterior to the psoas muscle, and anterior to the obturator internus muscle. The nerve lies approximately 2 cm lateral and 2 cm distal to the pubic tubercle (Fig. 27.1). It carries a mixed population of sensory and motor fibers from roots L2, L3, and L4. As it enters the obturator canal, it divides into an anterior and posterior branch. The anterior branch supplies the adductor longus, adductor brevis, and gracilis muscles as well as the skin over the medial thigh or posterior knee and a branch to the hip joint. The posterior branch supplies the obturator externus, the quadratus femoris, and the adductor magnus muscles as well as the knee joint. Ten percent of patients have an accessory branch from the lumbar plexus.

Obturator nerve block is used to prevent adductor contraction evoked by electrocautery near the bladder wall and to supplement analgesia for major knee surgery. Obturator nerve block can also be used to treat spasticity of the adductor thigh muscles, obturator neuralgia, or pain in the hip joint.

Anatomy: The femoral neurovascular bundle and pectineus muscle medial to the femoral vein serve as major landmarks. More medially, the three muscle layers of the adductor muscles (from superficial to deep: adductor longus muscle, adductor brevis muscle, and adductor magnus muscle) must also be identified. The anterior branches of the obturator nerve at this level can be found between the adductor longus muscle and the adductor brevis muscle. The posterior branch is found between the adductor brevis muscle and the adductor magnus muscle. The nerves appear as oval hypoechoic structures within the hyperechoic fascia.

Patient position: Supine. The leg is externally rotated with the hip and knee flexed.

Transducer type: 25 mm linear probe oscillating at 10 to 13 MHz. 11 mm curved array oscillating at 6 to 10 MHz (obese patients).

Transducer position: