The saphenous nerve is the only nerve innervating the ankle and foot that is not a sciatic nerve branch; it is a purely sensory nerve

The saphenous nerve branches off the femoral nerve in the subinguinal region and descends subsartorially first in the femoral triangle and then into the adductor canal, where it pierces the vastoadductor membrane and emerges between the sartorius and gracilis and pierces the deep fascia on the medial side of the knee

A sensory infrapatellar branch supplies sensory branches to the medial part of the knee

The saphenous nerve supplies sensory branches to the medial side of the leg, ankle and foot including the medial part of the capsules of the ankle joint and the subtalar joints

On the medial side of the leg and ankle the saphenous nerve is joined by the long saphenous vein

The superficial peroneal nerve is most suitably visualized and blocked approx. 10 cm proximal to the lateral malleolus where the target nerve emerges between the anterior and lateral compartments

The superficial peroneal nerve (SPN) branches off the common peroneal nerve in the proximal lateral compartment of the leg

The SPN descends in the lateral compartment between the peroneal muscles and the fibula. It innervates the peroneal muscles

Approx. 10 cm above the lateral malleolus the SPN emerges between the lateral compartment (peroneal muscles) and the anterior compartment (the extensor muscles of the foot) and pierces the deep fascia of the leg

Above the deep fascia the SPN splits into the intermediate and medial dorsal cutaneous nerves that supply cutaneous innervation to the antero-lateral part of the distal leg, ankle and dorsum of foot – including the dorsal digital nerves that supply innervation of dorsal part of the toes and the interdigital spaces – except the first interdigital space

The deep peroneal nerve (DPN) branches off the common peroneal nerve in the proximal part of the lateral compartment

The DPN pierces the anterior intermuscular septum – that separates the lateral and the anterior/extensor compartments of the leg – and descends sandwiched between the interosseous membrane and the extensor muscles of the anterior compartment

The DPN innervates the extensor muscles of the anterior compartment (tibialis anterior, extensor digitorum longus and brevis, extensor hallucis longus & brevis, and fibularis tertius)

The terminal cutaneous branch of the DPN runs deep the extensor retinacula anterior to the ankle together with the doral pedal artery and pierces the deep fascia on the distal part of the dorsum of the foot and supplies the lateral cutaneous nerve of the big toe and the medial cutaneous nerve of the second toe

The sural nerve is a sensory nerve made of the fusion of the medial sural cutaneous nerve from the tibial nerve and of a communicating branch from the lateral sural cutaneous nerve from the common fibular nerve

The two sural cutaneous nerves pierce the deep fascia – together with the short saphenous vein – before the fusion that makes the sural nerve

The sural nerve descends posterior to the lateral malleolus together with the short saphenous vein

The sural nerve innervates the postero-lateral part of the skin cover of the ankle and supply calcaneal branches that innervate the lateral heel and lateral dorsal cutaneous nerves that innervate the lateral margin of the foot

The tibial nerve branches off the sciatic nerve in the popliteal fossa and descends in the midline in the deep part of the posterior compartment together with the posterior tibial artery and vein, continues distally through the tarsal tunnel (below the flexor retinaculum between the calcaneus and the medial malleolus) behind the medial malleolus and supplies cutaneous and muscular innervation via the medial and lateral plantar nerves to the plantar side of the foot and the sensory medial calcaneal branch to the medial side of the heel

It also supplies cutaneous innervation via the sural nerve.

The tibial nerve innervates the muscles of the posterior compartment of the leg: triceps surae, plantaris, popliteus, tibialis posterior, flexor digitorum longus, and flexor hallucis longus

In a non-obese patient the posterior femoral cutaneous nerve (PFCN) can be visualized with a linear probe. It has a spindle shaped cross-sectional profile and is sandwiched between the deep fascia and the long head of the biceps femoris

The epidermis is a narrow hyperechoic layer

The dermis has a superficial hypoechoic and heterogenous layer and a deep hyperechoic and homogenous layer

The superficial fascia is the subcutaneous layer which attaches the skin to the underlying deep fascia. It is a fibroareolar layer composed of hypoechoic fatty tissue with intermingled hyperechoic linear bands which are fibrous septae

The deep fascia is a hyperechoic dense fibrous sheath that envelopes muscles and gives off intermuscular compartmental septae that attaches to the periosteum

The picture on the right shows the image of the previous page without colour coding

The indications of an ultrasound guided ankle block is forefoot surgery as an alternative to a combined popliteal sciatic and midthigh saphenous nerve blockade

It is not indicated for surgical anaesthesia, if a tourniquet is applied proximal to the ankle level

All five nerves innervating the foot have to be anaesthetized which can be painful in the awake patient

Fritz J, Bizzell C, Kathuria S, Flammang AJ, Williams EH, Belzberg AJ, Carrino JA, Chhabra A: High-resolution magnetic resonance-guided posterior femoral cutaneous nerve blocks. Skeletal Radiol 42: 579-86 (2013)