The ankle block targets five nerves responsible for sensory supply distal to the ankle. The nerves consist of four branches of the sciatic nerve (the superficial peroneal, the deep peroneal, the sural, and the posterior tibial nerve) and one cutaneous branch of the femoral nerve (the saphenous nerve). The sciatic nerve gives off two terminal branches, the common peroneal and the tibial nerve.
The deep peroneal branch (also known as the deep fibular nerve) of the common peroneal nerve (also known as common fibular nerve) runs down at the anterior aspect of the leg in proximity to the anterior tibial artery. At the ankle level, it runs deep to the extensor retinaculum and superficial to the tibia. It is bounded medially by the tendon of the extensor hallucis longus and laterally by the anterior tibial artery. The deep peroneal nerve provides sensory innervation to the first interdigital space tarsal and metatarsal joint of the great toe. It also provides the nerve supply to the tarsometatarsal, metatarsophalangeal, and interphalangeal joints of the lesser toes.
The superficial peroneal nerve travels distally between the peroneus muscles and the extensor digitorum longus giving off branches to both the peroneus longus and brevis and the skin over the lower part of the leg. It then becomes superficial above the lateral malleoli where it terminates into medial and lateral branches. The superficial peroneal nerve provides sensory innervation to the dorsum of the foot and toes except for the interspace between the great and second toes.
The tibial nerve is the larger of the two terminal branches of the sciatic nerve. It divides into the posterior tibial nerve and the sural nerve.
The posterior tibial nerve descends to the leg with the posterior tibial vessels. At the level of the ankle, it is located between the calcaneus and the medial malleolus. It passes deep to the flexor retinaculum and terminates in the medial and lateral plantar nerves. It supplies sensation to the sole of the foot, the heel, and the medial side of the foot below the territory innervated by the saphenous nerve.It also provides motor supply to the flexor digitorum longus and flexor hallucis longus. Along its course it sends vascular twigs to the accompanying blood vessels and articular branches to the ankle joint.
The sural nerve is the second branch of the tibial nerve. It is joined by a communicating tributary of the common peroneal nerve. It descends between the heads of the gastrocnemius muscle and, after piercing the fascia covering the muscles, emerges on the lateral aspect of the Achilles tendon, 10 cm above the lateral malleolus. The sural nerve runs superficially with the small saphenous vein and lies subcutaneously behind the lateral malleolus and between the malleolus and Achilles tendon. It provides sensory supply to the lateral aspect of the foot, fourth interosseous space, and the fifth toe.
The saphenous nerve is a terminal cutaneous branch of the femoral nerve. It runs superficially with the great saphenous vein where it divides into terminal branches. Some branches terminate at the ankle joint, whereas others pass in front of the ankle joint and supply the skin on the medial side of the foot, often as far as the first metatarsophalangeal joint.