Introduction

The ankle block is a safe and effective method for obtaining anaesthesia and analgesia of the foot for surgical procedures on bones and soft tissues. 

Indications

• Surgical anaesthesia of the foot especially when general, epidural or spinal anaesthesia is contra-indicated.
• For post-operative analgesia. 

Anatomy

Five nerve branches supply sensation to the foot. All are branches of the sciatic nerve, except the saphenous nerve, which is the terminal branch of the femoral nerve. The sciatic nerve divides into the tibial nerve and the common peroneal nerve at a variable point between the buttock and the popliteal fossa. The tibial nerve then divides into the posterior tibial and sural nerves, and the common peroneal nerve into the deep and superficial peroneal nerves. The posterior tibial nerve finally divides into the medial and lateral plantar nerves.

Figures 1-4 show the sensory distributions of these nerves. Of particular note;

• The posterior tibial nerve innervates all but one of the intrinsic muscles of the foot, via its terminal branches, the medial and lateral plantar nerves. Blockade of this nerve is important for surgery to deeper structures.
• The deep peroneal nerve innervates the first web-space and so must be blocked for anaesthesia of the great toe.
• Surgery is unusual in the territory of the sural nerve therefore it is not often blocked.

Figures 1-4 show the anatomical relations of these five nerves. Note:

• The posterior tibial nerve lies immediately posterior to the posterior tibial artery, at the medial malleolus.
• The superficial peroneal nerve divides into terminal branches anterior to the ankle, necessitating a wide fan of infiltration for blockade. 

Preparation

1. Check resuscitation equipment and drugs.
2. Perform block in an anaesthetic or operating room.
3. Explain procedure to patient and obtain consent.
4. Establish IV access.
5. Full monitoring is advised where available (ECG, pulse oximetry, NIBP). 

Technique - General

• As performing the block can be painful, remember to inject the local anaesthetic slowly. Heating the local anaesthetic to body temperature may also help to reduce pain. Sedation may be required.
• All five nerves can be blocked with the patient supine and the foot on a padded support. Some prefer to block the posterior tibial and sural nerves with the patient prone. To block the posterior tibial nerve in a supine position, flex the knee and place the ankle on top of the contralateral shin. This allows easy access to the medial and lateral maleolus.
• As four of the nerves are almost entirely sensory an infiltration technique is used. Where available, nerve stimulation can be used to localise the posterior tibial nerve. Stimulation will produce movement of the big toe. A 23G needle, 4cm in length, is appropriate for all injections. It is important always to aspirate prior to injection of local anaesthetic, to exclude intravascular injection.
• The aim is sensory block alone and so low concentrations of local anaesthetic (LA) are sufficient (e.g. 0.25% bupivacaine) in most cases.

The five nerves are blocked by injections that form a ring of infiltration around the ankle at the level of the malleoli.