This block is often used for surgical procedures carried out on the foot, especially for those not requiring high lower-leg tourniquet pressure.
Patient Selection. The ankle block is principally an infiltration block and does not require elicitation of paresthesia. Thus patient cooperation is not mandatory. Although the block is most efficient for the anesthesiologist if the patient can assume both the prone and supine positions, this is not essential.
Pharmacologic Choice. Because motor blockade is not often needed for procedures carried out during ankle block, lower concentrations of local anesthetics may be used. Practical choices are 1% lidocaine, 1% mepivacaine, 0.25%–0.5% bupivacaine, and 0.2%–0.5% ropivacaine. Many physicians suggest that epinephrine not be used during ankle block, especially if injection is circumferential.
Traditional block technique
Anatomy. The peripheral nerves requiring block are derived from the sciatic nerve, with the exception of a terminal branch of the femoral nerve—the saphenous nerve. 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 nerve—the common peroneal and tibial nerves. The tibial nerve divides into the posterior tibial and sural nerves, which provide cutaneous innervation as outlined in Figs. 21.1 and 21.2 . The common peroneal nerve divides into its terminal branches—the superficial and deep peroneal nerves—in the proximal portion of the lower leg. Their cutaneous innervation is also illustrated in Fig. 21.2 . Fig. 21.3 identifies the locations of these nerves in a cross-sectional view at the level of ankle block.
Needle Puncture: General. It is often helpful (although 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 that 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, and this position facilitates appropriate intravenous sedation.
Needle Puncture: 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, as shown in Fig. 21.3 . The needle is inserted near the posterior tibial artery, and if paresthesia is obtained, 3–5 mL of local anesthetic is injected. If paresthesia is not obtained, the needle is allowed to contact the medial malleolus, and 5–7 mL of local anesthetic is deposited near the posterior tibial artery.
Needle Puncture: Sural Nerve. The sural nerve is blocked with the patient positioned as for the posterior tibial nerve block. As illustrated in Fig. 21.3 , the sural nerve is blocked by inserting a 22-gauge, 4-cm needle anterolaterally immediately lateral to the Achilles tendon at the cephalad border of the lateral malleolus. The sural nerve (lateral ankle) is found in a more superficial position relative to the malleolus than is the tibial nerve (medial ankle). If paresthesia is not obtained, the needle is allowed to contact the lateral malleolus, and 5–7 mL of local anesthetic is injected as the needle is withdrawn.
Needle Puncture: 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 Figs. 21.3 and 21.4 ). 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 injected into this area to block the deep peroneal nerve. From this midline skin wheal, a 22-gauge, 8-cm needle is advanced subcutaneously laterally and medially to the malleoli, injecting 3–5 mL of local anesthetic in each direction. These lateral and medial approaches block the superficial peroneal and saphenous nerves, respectively.