2. Foot

forefoot. Five nerves need to be blocked before local anaesthesia is complete. Concentrations may need to be reduced if the patient is frail or if the procedure is bilateral • Saphenous • Posterior tibial nerve: • Deep peroneal nerve • Sural nerve: • Superficial peroneal nerve

Saphenous nerve: this supplies a variable portion of the medial border of the foot and ankle. It is a terminal branch of the femoral nerve and is anaesthetized immediately anterior to the medial malleolus where it is superficial, close to the saphenous vein. It is blocked with subcutaneous local anaesthetic, for example, levobupivacaine 0.5%  5 ml.

this supplies the plantar surface of the foot. It is a branch of the sciatic nerve (which divides into tibial and common peroneal branches in the popliteal fossa) and is blocked behind the medial malleolus where it lies posterior to the posterior tibial artery. The needle is gently directed perpendicular to the skin until it encounters bone, and is then withdrawn 1–2 mm prior to injection of 3–5 ml levobupivacaine 0.5% on either side of the artery. This nerve can also reliably be blocked in the popliteal fossa.

: this supplies only a small area of skin on the dorsum of the foot between the first and second toes. It passes beneath the extensor retinaculum at the front of the ankle joint and is most readily blocked between the tendons of extensor hallucis longus and extensor digitorum longus where it lies lateral to the dorsalis pedis artery. It is blocked with a total of 3–5 ml levobupivacaine 0.5% placed on either side of the artery and deep to the fascia.

Sural nerve: this supplies sensation to the fifth toe and the lateral border of the foot. It is a branch of the tibial nerve; at the level of the ankle it lies superficially behind the lateral malleolus. Subcutaneous infiltration of levobupivacaine 0.5% 5 ml between the lateral malleolus and the tendo Achilles usually provides effective analgesia.

Superficial peroneal nerve: this supplies much of the dorsum of the foot (excepting the small area supplied by the deep peroneal nerve, and the lateral foot which is supplied by the sural nerve). It is a branch of the common peroneal nerve, which divides further into terminal branches at the level of the malleoli. It is blocked with a ring of superficial infiltration of levobupivacaine 0.5%  10 ml between the anterior tibia and the lateral malleolus.

include subarachnoid (spinal) block, lumbar extradural (epidural) block, sacral extradural (caudal) block, sciatic nerve block at the hip, sciatic nerve block in the popliteal fossa, intraosseous nerve block between the metatarsals (for procedures in the distal foot which cannot be performed under digital nerve [ring] block), intravenous regional anaesthesia (Bier’s block, which needs high compression pressures and high volumes to obtain satisfactory analgesia), and local infiltration (this is unlikely to be satisfactory for awake surgery but is included for completeness).

Complications: these are largely generic and include failure and partial failure, local anaesthetic toxicity (you may need to modify the concentrations quoted above to reduce the total dose), nerve and vessel damage, intravascular and intraneural injection, and complications related to the lower limb arterial tourniquet (see under ‘The Arterial Tourniquet’ in Chapter 3)

The popliteal artery (pulsatile) and vein (compressible) are identified and confirmed using Doppler at the level of the knee crease. The TN is seen lying just posterior to these structures. Scanning proximally, the CFN moves from lateral (just deep to the biceps femoris tendon) to medial to join the TN. Using dynamic scanning (proximal/distal movements of the transducer) helps visualise the CFN and TN

The patient can be positioned supine with the leg raised, lateral or prone. Keep the patient’s leg (and therefore the SN) straight to aid visualisation. (ii) Perform the block just distal to the SN bifurcation, where the two branches lie in a common paraneural sheath. This allows injection of local anaesthetic both around and between the CFN and TN, ensuring that local anaesthetic is within the paraneural sheath. (iii) A ‘ComptoneCruveilhier septum’ lies between the two branches; this is pierced to achieve spread of local anaesthetic around both branches.14 Careful ‘hydrodissection’ and observation of spread around the CFN and TN branches ensures successful blockade. iv The needle is inserted laterally and passed medially. Choosing an insertion point slightly anterior to the transducer results in better needle visualisation owing to a more parallel transducer/needle orientation. (v) Deposit up to 20 ml local anaesthetic around and between the branches.

e plantar aspect of the foot. The TN is identified posterior to the medial malleolus adjacent to the posterior tibial artery Position the patient laterally with the side to be blocked underneath or supine with the leg to be blocked crossed in a figure of 4 shape to allow a posterior needle approach. (ii) Classically the TN was blocked at the medial malleolus using a landmark technique. (iii) The ultrasound approach allows more proximal blockade (so expect to cover the heel). An approach 5e10 cm proximal to the medial malleolus helps with good skin to transducer contact and moves the bulk of the Achilles tendon (AT) away from an in-plane needle path. (iv) A posterior to anterior in-plane needle approach avoids TP and FDL tendons. (v) Gentle ‘hydrodissection’ is required to achieve circumferential spread with up to 5 ml local anaesthetic. (vi) This nerve should be blocked first because it is the largest of the ankle nerves and therefore has the longest onset time

The DFN supplies cutaneous innervation to the first webspace on the foot. The nerve is visualised in the distal leg proximal to the ankle joint, usually lying lateral or anterior to the anterior tibial artery (ATA), i) Use a lateral to medial in-plane needle approach, depending on the nerve’s relationship with the ATA. (ii) Use careful ‘hydrodissection’ to deposit 1e3 ml local anaesthetic around the nerve.

The SFN supplies the majority of the cutaneous innervation to the dorsal aspect of the foot. The SFN has no vasculan Position the patient with the leg internally rotated. (ii) Scan proximally from the lateral malleolus. As the fibula moves deeper, visualise the FL and FB overlying the fibula and the EDL anteriorly. Look at the intersection between the muscle groups and for the nerve passing through the muscular fascia overlying the muscles. (iii) Dynamic scanning will help to visualise the nerve as it crosses the fascia. (iv) Aim to block the nerve superficial or deep to the muscular fascia (depending on best view) with 1e3 ml local anaesthetic. (v) The SFN may give rise to branches (that supply the foot) more proximally after emerging through the fascia, so the block should be placed either just after or before it passes through the fascia to both block any early branches.

The sural nerve provides cutaneous innervation to the lateral malleolus and the fifth phalanx in both the plantar and dorsal aspects. The sural nerve enters the foot posteriorly to the lateral malleolus. Scanning the lower leg distally towards the lateral malleolus, the nerve and short saphenous vein (SSV) are seen to ‘roll of i) The patient can be positioned laterally or with the leg internally rotated. (ii) Scan proximally just posterior to the lateral malleolus so that the FB lies anteriorly and the AT posteriorly. (iii) Identify the sural nerve and SSV ‘rolling off’ the AT when scanning distally into the space just anterior to the AT. (iv) Use an anterior to posterior in-plane needle approach to block the nerve with 1e3 ml local anaesthetic. (v) A perivenous injection may be effective if it is difficult to identify the sural nerve. Using ‘hydro-dissection’, adva-nce the needle tip into fluid to avoid nerve contact.

The saphenous nerve provides sensory innervation to structures over the medial aspect of the leg and foot including the medial ankle joint. We approach the saphenous nerve either: (i) deep to sartorius muscle (subsartorius) or (ii) just proximal to the medial malleolus (ankle). The saphenous nerve runs adjacent to the femoral artery deep to sartorius muscle in the medial thigh Subsartorius approach 6 Sonoanatomy of the saphenous nerve at the ankle joint. LSV, long saphenous vein; SN, saphenous nerve; DT, distal tibia; FDL, flexor digitorum longus.