Obturator Nerve Block

The obturator nerve is derived primarily from the third and fourth lumbar nerves with an occasional minor contribution from L2. The nerve lies deep in the obturator canal, having descended from the medial border of the psoas muscle. As the nerve leaves the obturator canal, it divides into anterior and posterior branches. The anterior branch supplies an articular branch to the hip and the anterior adductor muscles and a variable cutaneous branch to the lower medial thigh. The posterior branch innervates the deep adductor muscles and may send an articular branch to the knee.

Clinical Applications

The obturator nerve usually is blocked as part of regional anesthesia for knee surgery. Because it is primarily a motor nerve, it is rarely blocked on its own; however, obturator nerve block can be useful in treating or diagnosing the extent of adductor spasm in patients with cerebral palsy and other muscle or neurologic diseases affecting the lower extremities before surgical intervention (e.g., adductor tenotomy).

Technique

The patient is placed in the supine position, and a mark is made 1 to 2 cm lateral and 1 to 2 cm caudad to the pubic tubercle. A skin wheal is raised, and a 22-gauge, 8- to 10-cm needle is advanced perpendicular to the skin entry site with a slight medial direction. The inferior pubic ramus is encountered at a depth of 2 to 4 cm, and the needle is walked in a lateral and caudad direction, until it passes into the obturator canal. The obturator nerve is located 2 to 3 cm past the initial point of contact with the pubic ramus (see Fig. 44-13 ; see Plate 12 in the color atlas of this volume). After negative aspiration, 10 to 15 mL of local anesthetic is injected. A nerve stimulator is helpful in locating the obturator nerve, and correct needle position is evidenced by contraction of the adductor muscles of the medial thigh.

The classic approach to obturator nerve block involves painful periosteal contact and multiple needle redirections. An alternate interadductor approach was described by Wasseff.^[44] In this technique, the needle is inserted behind the adductor tendon, near its pubic insertion, and is directed laterally toward a mark on the skin 1 to 2 cm medial to the femoral artery and immediately below the inguinal ligament representing the obturator canal. The nerve is identified by a motor response to peripheral nerve stimulation in the adductor muscle.

Side Effects and Complications

Complications are rare, but this block is technically more difficult than other lower extremity blocks. The obturator canal contains vascular and neural structures, and there is a theoretical risk of intravascular injection, hematoma, and nerve damage. Absence of anesthesia in the obturator nerve distribution can render an otherwise perfect lower extremity block inadequate for surgical procedures on the knee.