Obturator nerve blockade is extremely useful as a diagnostic, prognostic, or therapeutic procedure in patients with adductor spasm that interferes with rehabilitation or personal hygiene.
After informed consent is obtained, the patient is placed in the supine position with the leg to be blocked placed in slight abduction. It is not necessary to shave the pubic area. The patient is prepared in a standard aseptic fashion over an area large enough to allow palpation of landmarks, and sterile technique is used throughout the procedure. A 3-in. (8-cm), 22-gauge needle is inserted perpendicular to the skin at a point 1.5 cm lateral and inferior to the pubic tubercle. The needle is advanced until the inferior ramus of the pubis is contacted. The needle depth at which the bone is contacted should be noted. The needle is withdrawn to skin level and redirected in a lateral and slightly superior direction, parallel to the superior ramus of the pubis. The needle is advanced 2 to 3 cm beyond the previously noted depth until a paresthesia is elicited. A nerve stimulator makes it relatively easy to identify the obturator nerve by adductor muscle contraction. After negative aspiration for blood, 10 mL of local anesthetic is injected to block the obturator nerve. This traditional approach was first described by Labat (37) (Fig. 67-14).
FIGURE 67-14. Obturator nerve block. Approach for obturator nerve injection and neural blockade. Using the above techniques, Wassef has described an alternative approach using the femoral artery and adductor longus tendon as landmarks (38). A mark is made on the skin 1 to 2 cm medial to the femoral artery just below the inguinal ligament. This mark is used to indicate the direction of the needle toward the obturator canal. The adductor longus tendon is then identified near its insertion site at the pubis. A 3-in. (8-cm), 22-gauge insulated needle is introduced behind the adductor longus tendon and directed laterally, with a slight posterior and superior inclination toward the skin mark. The needle is advanced until adductor muscle contraction is elicited with a nerve stimulator (39).
The obturator nerve is formed by the union of the ventral branches of the anterior primary rami of L2, L3, and L4 within the substance of the psoas muscle. It emerges from the medial border of the psoas muscle at the brim of the pelvis. The nerve runs caudad and anteriorly along the lateral wall of the pelvis, along the obturator vessels to the obturator foramen. There it enters the thigh, supplying the adductor muscles and providing innervation to the hip and knee joints.
As the nerve passes through the obturator canal, it divides into anterior and posterior branches. The anterior branch supplies the hip joint, the anterior adductor muscles, and cutaneous branches to the medial aspect of the thigh. The cutaneous innervation of the obturator nerve can be extremely variable and can be nonexistent in some people. The posterior branch supplies the deep adductor muscles and frequently sends a branch to the knee joint.
This procedure is often performed on rehabilitation patients with spasticity or contractures that result in positioning difficulty. Confirmation of a successful obturator nerve block is demonstrated by paresis of the adductor muscles because the cutaneous contribution of the obturator nerve is inconsistent. An alternative to this procedure is selective root blockade at levels L2, L3, and L4 using a nerve stimulator to establish muscle innervation.
Hematoma and intravascular injection are possible due to the close proximity of the obturator vessels. If an arterial puncture occurs, prolonged direct pressure is usually adequate to prevent the development of a hematoma.