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Brown: Atlas of Regional Anesthesia, 3rd ed., Copyright © 2006 Saunders, An Imprint of Elsevier
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PLACEMENT

Anatomy

The peripheral nerves requiring block during ankle block are all derived from the sciatic nerve, with the exception of a terminal branch of the femoral nerve, the saphenous. The saphenous nerve is the only branch of the femoral nerve below the knee; it courses superficially anterior to the medial malleolus, providing cutaneous innervation to an area of the medial ankle and foot. The remaining nerves requiring block at the ankle are terminal branches of the sciatic (i.e., common peroneal and tibial nerves). The tibial nerves divide into posterior tibial and sural nerves, which provide cutaneous innervation, as outlined in Figure 17-1 . The common peroneal nerve divides into its terminal branches in the proximal portion of the lower leg by dividing into the superficial and deep peroneal nerves. Their cutaneous innervation is also illustrated in Figure 17-1 . Figure 17-2 identifies where these nerves are located in a cross-sectional view at the level of the ankle block.

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Figure 17-1  Ankle block: peripheral innervation.


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Figure 17-2  Ankle block: cross-sectional anatomy and technique.


Needle Puncture

It is often helpful (though not necessary) to have the patient in the prone position initially to facilitate block of the posterior tibial and sural nerves. Once these two nerves have been blocked, the patient assumes the supine position so block of the saphenous and peroneal nerves can be carried out. The block can be performed with the patient in the supine position if the lower leg is placed on a padded support; this position facilitates appropriate intravenous sedation.

Posterior Tibial Nerve.

With the patient in the prone position, the ankle to be blocked is supported on a pillow. A 22-gauge, 4-cm needle is directed anteriorly at the cephalad border of the medial malleolus, just medial to the Achilles tendon (see Figure 17-2 ). The needle is inserted near the posterior tibial artery, and when a paresthesia is obtained 3 to 5 mL of local anesthetic is injected. If no paresthesia is obtained, the needle is allowed to contact the medial malleolus, and 5 to 7 mL of local anesthetic is deposited near the posterior tibial artery.

Sural Nerve.

The sural nerve is blocked with the patient positioned as for the posterior tibial nerve block. As illustrated in Figure 17-2 , block of the sural nerve is carried out by inserting a 22-gauge, 4-cm needle anterolaterally immediately lateral to the Achilles tendon at the cephalad border of the lateral malleolus. If no paresthesia is obtained, the needle is allowed to contact the lateral malleolus, and 5 to 7 mL of local anesthetic is injected as the needle is withdrawn.

Deep Peroneal, Superficial Peroneal, and Saphenous Nerves.

After the patient assumes the supine position, the anterior tibial artery pulsation is located at the superior level of the malleoli. A 22-gauge, 4-cm needle is advanced posteriorly and immediately lateral to this point (see Fig. 17-2 ). An alternative is to insert the needle between the tendons of the anterior tibial and the extensor hallucis longus muscles. Approximately 5 mL of local anesthetic is then injected into the area. From this midline skin wheal, a 22-gauge, 8-cm needle is advanced subcutaneously laterally and medially to the malleoli, injecting 3 to 5 mL of local anesthetic in each direction. These lateral and medial approaches block the superficial peroneal and saphenous nerves, respectively.

Potential Problems

The ankle block can be painful if the patient is not adequately sedated, although this should be an infrequent problem because an alert patient is not essential for the block.

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