Ankle Block

Figure 1: Needle insertion for saphenous nerve block of the ankle.

Indications: surgery of the foot and toes

Two deep nerves: posterior tibial, deep peroneal

Three superficial nerves: superficial peroneal, sural, saphenous

Local anesthetic: 5-6 mL per nerve

General Considerations

An ankle block is essentially a block of four terminal branches of the sciatic nerve (deep and superficial peroneal, tibial, and sural) and one cutaneous branch of the femoral nerve (saphenous). Ankle block is simple to perform, essentially devoid of systemic complications, and highly effective for a wide variety of procedures on the foot and toes. For this reason, this technique should be in the arma mentarium of every anesthesiologist. At our institution, an ankle block is most commonly used in podiatric surgery and foot and toe debridement or amputation.

Functional Anatomy

It is useful to think of the ankle block as a block of two deep nerves (posterior tibial and deep peroneal) and three superficial nerves (saphenous, sural, and superficial peroneal). This concept is important for success of the block because the two deep nerves are anesthetized by injecting local anesthetic under the fascia, whereas the three superficial nerves are anesthetized by a simple subcutaneous injection of local anesthetic.

Common Peroneal Nerve

The common peroneal nerve separates from the tibial nerve and descends alongside the tendon of the biceps femoris muscle and around the neck of the fibula. Just below the head of the fibula, the common peroneal nerve divides into its terminal branches: the deep peroneal and superficial peroneal nerves. The peroneus longus muscle covers both nerves.

Deep Peroneal Nerve

The deep peroneal nerve runs downward below the layers of the peroneus longus, extensor digitorum longus, and extensor hallucis longus muscles to the front of the leg (Figure 2). At the ankle level, the nerve lies anterior to the tibia and the interosseous membrane and close to the anterior tibial artery. It is usually sandwiched between the tendons of the anterior tibial and extensor digitorum longus muscles. At this point, it divides into two terminal branches for the foot: the medial and the lateral. The medial branch passes over the dorsum of the foot, along the medial side of the dorsalis pedis artery, to the first interosseous space, where it supplies the web space between the first and second toe. The lateral branch of the deep peroneal nerve is directed anterolaterally, penetrates and innervates the extensor digitorum brevis muscle, and terminates as the second, third, and fourth dorsal interosseous nerves. These branches provide innervation to the tarsometatarsal, metatarsophalangeal, and interphalangeal joints of the lesser toes.

Superficial Peroneal Nerve

The superficial peroneal nerve (also called the musculocutaneous nerve of the leg) provides muscular branches to the peroneus longus and brevis muscles. After piercing the deep fascia covering the muscles, the nerve eventually emerges from the anterolateral compartment of the lower part of the leg and surfaces from beneath the fascia 5 to 10 cm above the lateral malleolus (Figure 3). At this point, it divides into terminal cutaneous branches: the medial and the lateral dorsal cutaneous nerves (Figure 4A and B). These branches carry sensory innervation to the dorsum of the foot and communicate with the saphenous nerve medially, as well as the deep peroneal nerve in the first web space and the sural nerve on the lateral aspect of the foot.

Figure 2: Anatomy of the ankle. (1) artery dorsal pedis, (2) deep peroneal nerve.

Figure 3: Anatomy of the ankle. (1) superficial peroneal nerve. (2) sural nerve.

Tibial Nerve

The tibial nerve separates from the common popliteal nerve proximal to the popliteal fossa crease and joins the tibial artery behind the knee joint. The nerve runs distally in the thick neurovascular fascia and emerges at the inferior third of the leg from beneath the soleus and gastrocnemius muscles on the medial border of the Achilles tendon (Figure 5). At the level of the medial malleolus, the tibial nerve is covered by the superficial and deep fasciae of the leg. It is positioned laterally and posteriorly to the posterior tibial artery and midway between the posterior aspect of the medial malleolus and the posterior aspect of the Achilles tendon. Just beneath the malleolus, the nerve divides into lateral and medial plantar nerves. The posterior tibial nerve provides cutaneous, articular, and vascular branches to the ankle joint, medial malleolus, inner aspect of the heel, and Achilles tendon. It also branches to the skin, subcutaneous tissue, muscles, and bones of the sole.

Sural Nerve

The sural nerve is a sensory nerve formed by a union of the medial sural nerve (a branch of the tibial nerve) and the lateral sural nerve (a branch of the common peroneal nerve). The sural nerve courses between the heads of the gastrocnemius muscle, and after piercing the fascia covering the muscles, it emerges on the lateral aspect of the Achilles tendon 10 to 15 cm above the lateral malleolus (Figure 4A and B). After providing lateral calcaneal branches to the heel, the sural nerve descends behind the lateral malleolus, supplying the lateral malleolus, Achilles tendon, and ankle joint. The sural nerve continues on the lateral aspect of the foot, innervating the skin, subcutaneous tissue, fourth interosseous space, and fifth toe.

Figure 4: (A) Anatomy of the ankle. (1) Sural nerve. (2),(3) Superficial peroneal nerve. (B) Anatomy of the ankle. (1) Sural nerve, (2) Superficial peroneal nerve.

Figure 5: Anatomy of the ankle. (1) flexor digitorum longus tendon, (2) posterior tibial artery, (3) tibial nerve, (4) flexor hallucis longus.	Figure 6: Anatomy of the ankle. (1), (2) saphenous nerve, (3) medial malleolus.

Distribution of Blockade

Figure 8: (A) Maneuvers to extenuate the extensor tendons. The deep peroneal nerve is located lateral to the hallucis longus tendon (line). (B) Deep peroneal block. The needle is inserted just lateral to the hallucis longus tendon and slowly advanced to contact the bone. Upon bone contact, the needle is withdrawn 2-3 mm, and 5 mL of local anesthetic is injected.

Figure 9: (A) Landmark for posterior tibial nerve block is found by palpating the pulse of the tibial artery posterior to the medial malleolus. (B) Posterior tibial nerve block is accomplished by inserting the needle next to the pulse of the tibial artery. The needle is advanced until contact with the bone is established. At this point the needle is withdrawn 2-3 mm, and 5 mL of local anesthetic is injected.

Blocks of the Superficial Peroneal, Sural, and Saphenous Nerves

Figure 10: Saphenous nerve block is accomplished by injection of local anesthetic in a circular fashion (line) subcutaneously just above the medial malleolus.

These three nerves are superficial cutaneous extensions of the sciatic and femoral nerves. Because they are positioned superficially to the deep fascia, an injection of local anesthetic in the territory through which they descend to the distal foot is adequate to achieve their blockade. Each nerve is blocked using a simple circumferential injection of local anesthetic subcutaneously (Figures 10, 11, and 12). To block the saphenous nerve, a 1.5 in, 25-gauge needle is inserted at the level of the medial malleolus and a "ring" of local anesthetic is raised from the point of needle entry to the Achilles tendon and anteriorly to the tibial ridge (Figure 10). This can be usually accomplished with one or two needle insertions; 5 mL of local anesthetic suffices. To block the superficial peroneal nerve, the needle is inserted at the tibial ridge and extended laterally toward the lateral malleolus (Figure 11). It is important to raise a subcutaneous "wheal" during injection, which indicates injection in the proper superficial plane. Five milliliters of local anesthetic is adequate. To block the sural nerve, the needle is inserted at the level of the lateral malleolus and the local anesthetic is infiltrated toward the Achilles tendon (Figure 12). Five milliliters of local anesthetic is deposited in a circular fashion to raise a skin "wheal."

Figure 11: Superficial peroneal nerve block is performed by injecting local anesthetic in a circular fashion at the level of the lateral malleolus and extending from anterior to posterior.	Figure 12: Sural nerve block is accomplished by injecting local anesthetic in a fanwise fashion subcutaneously and below the fascia behind the lateral malleolus.