The success of the block depends on the appropriate spread of local anesthetics in the appropriate fascial planes superficially and deeply to the adductor brevis muscle.
Care should be taken to confirm the spread in the intermuscular fascial planes and not intramuscular.
Change in adductor strength is the best assessment method for the block since the sensory distribution is variable.
With a successful block, some residual adductor strength is secondary to the formal innervation to the pectineus, as well as some sciatic innervation to the adductor magnus.
This block is most often combined with the sciatic, femoral, and lateral femoral cutaneous nerve blocks to allow surgical procedures on the lower extremities. If an operation on the knee using these peripheral blocks is planned, the obturator block is often essential. Another use for this block is in patients who have hip pain. It can be used diagnostically to help identify the cause of pain because obturator nerve block may provide considerable pain relief if the nerve’s articular branch to the hip is involved in pain transmission. The block also may be useful in the evaluation of lower extremity spasticity or chronic pain syndromes.
Patient Selection. As with femoral and lateral femoral cutaneous nerve blocks, elicitation of paresthesia is not essential for obturator block. Any patient able to lie supine is a candidate.
Pharmacologic Choice. Motor blockade is most often not necessary for surgical patients receiving obturator nerve block; thus lower concentrations of local anesthetics are appropriate for obturator block: 0.75% to 1.0% lidocaine or mepivacaine, 0.25% bupivacaine, or 0.2% ropivacaine.
Anatomy. The obturator nerve emerges from the medial border of the psoas muscle at the pelvic brim and travels along the lateral aspect of the pelvis anterior to the obturator internus muscle and posterior to the iliac vessels and ureter. It enters the obturator canal cephalad and anterior to the obturator vessels, which are branches from the internal iliac vessels. In the obturator canal, the obturator nerve divides into anterior and posterior branches ( Fig. 18.1 ). The anterior branch supplies the anterior adductor muscles and sends an articular branch to the hip joint and a cutaneous area on the medial aspect of the thigh. The posterior branch innervates the deep adductor muscles and sends an articular branch to the knee joint. In 10% of patients, an accessory obturator nerve may be found.
Position. The patient should be supine with the legs in a slightly abducted position. The genitalia should be protected from antiseptic solutions.
Needle Puncture. The pubic tubercle should be located and an “X” marked 1.5 cm caudad and 1.5 cm lateral to the tubercle ( Fig. 18.2 ). The needle is inserted at this point, and at a depth of approximately 1.5 to 4 cm it contacts the horizontal ramus of the pubis. The needle is then withdrawn, redirected laterally in a horizontal plane, and inserted 2 to 3 cm deeper than the depth of the initial contact with bone. The needle tip now lies within the obturator canal (see Fig. 18.2 ). With the needle in this position, 10 to 15 mL of local anesthetic solution is injected while the needle is advanced and withdrawn slightly to ensure development of a “wall” of local anesthetic in the canal.