Obturator Block

Published on 06/02/2015 by admin

Filed under Anesthesiology

Last modified 22/04/2025

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15 Obturator Block

Perspective

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 paresthesias 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.

Placement

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. 15-1). 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.

Figure 15-1. Obturator nerve: functional anatomy.

Position

The patient should be supine with the legs positioned 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. 15-2). 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. 15-2) 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.

Figure 15-2. Obturator nerve block: technique.

Potential Problems

The obturator canal is a vascular location; thus, the potential exists for intravascular injection or hematoma formation, although these are more theoretical than clinical concerns.

Pearls

This block, even in trained hands, has a variable success rate. My experience suggests that one must rely on volume of anesthetic delivered rather than on absolute accuracy of needle position. Fortunately, use of an obturator block with the other lower extremity peripheral nerve blocks is not an absolute requirement for most surgical procedures. If this block is used diagnostically for patients with chronic pain, it is helpful to use a nerve stimulator to guide needle placement. This will minimize diagnostic confusion when pain relief is produced with a small volume of local anesthetic. Large-volume injections (approximately 15 mL) are performed with this block for many surgical procedures.

Related

Atlas of Regional Anesthesia 4e