Ankle block can be used as the primary regional anesthesia for foot surgery, however, because of the usual requirement for thigh tourniquet analgesia, ankle block is more commonly used for postoperative analgesia in combination with light general anesthesia. "Ankle" block is really a misnomer as block placement is too distal to be effective for ankle surgery: "ankle" refers to the level of block placement. The main advantage of the ankle block over SS sciatic block is the relative avoidance of ankle/foot motor block. Therefore, it may be preferable to a sciatic block when only a postoperative shoe (cf. plaster cast) is to be used, particularly bilateral procedures. Bilateral sciatic block is poorly tolerated in the ambulatory setting. The main disadvantage of the ankle block over sciatic block is its limited duration (generally < 12 hrs); therefore, a sciatic block is generally preferred for moderately painful foot/ankle procedures.
Skill level: Simple.
Procedure time: 1-3 mins per nerve (1 mins for experienced practitioners)
Less common: primary anesthesia for foot surgery.
Sedation: e.g. midazolam 2 mg + alfentanil 250-500 mcg or more commonly after GA.
Anatomy: Of the five peripheral nerves innervating the foot, four travel in proximity to vessels: the deep peroneal nerve (lateral to anterior tibial artery), posterior tibial nerve (posterior to posterior tibial artery), saphenous nerve (adjacent long saphenous vein) and the sural nerve (posterior to short saphenous vein). These vessels are readily visualised with ultrasound and provide useful sonographic landmarks for the administration of local anaesthetic.
Surface landmarks: see block procedure.
Needle: 22G blunt needle.
1. High resolution but narrow footprint linear array probe e.g. SonoSite hockey stick or alternatively curvilinear probe e.g Sonosite C11 (C11 curvilinear probe facilitates probe-skin contact adjacent to bony prominences e.g. lean subjects with prominent metatarsals/extensor tendons and concavities e.g. immediately anterior to the achilles tendon).
2. 22G blunt needle.
3. 30 mL LA filled syringe
Procedure (Video 1)
The procedural objective is to visualise the adjacent vasculature and not the nerves themselves. Therefore, the advantage provided by means of the C11 probe shape is considered to outweigh the disadvantage consequent on its lower frequency (lower resolution). Gentle probe-skin pressure should be applied to avoid vascular compression. Aim to administer LA in the presumed positions of those nerves having a consistent relationship to adjacent vascular structures.
1. Deep peroneal nerve block (Fig. 1, 2, Video 2). 5 mL LA is injected at the level of the malleoli, with an out-of-plane technique deep to the anterior tibial artery. 14 LA will spread to both sides of the artery and ensure blockade of the nerve.
Fig 1. Deep peroneal nerve block.
Fig. 2. Deep peroneal nerve block ultrasound image. Red arrow=anterior tibial or dorsalis pedis artery. Blue arrow=tibia/talus/cuneiform (depending on how distal the block is performed)
2. Posterior tibial nerve block (Fig. 3, 4, Video 3). 5 mL LA is injected at the level of the medial malleolus, with an out-of-plane technique immediately posterior to the posterior tibial artery. Posterior to the nerve is flexor hallucis longus tendon, which may be imaged by flexing /extending the ankle.
Fig. 3. Posterior tibial nerve block.
Fig. 4. Posteror tibial nerve block ultrasound image. Nerve = approx. position of the posterior tibial nerve.
3. Sural nerve block (Fig. 5, 6, Video 4). 5 mL LA is injected just cephalad of the lateral malleolus, with an in-plane technique (anterior to posterior) around the short saphenous vein, which is located just anterior to the achilles tendon . 15 The needle is advanced just posterior to the vein then LA is injected on needle withdrawal.
Fig. 5. Sural nerve block
Fig. 6. Sural nerve block ultrasound image. Blue arrow=short saphenous vein. Red arrow=sural nerve just posterior to vein. Green arrow=needle shaft, which will advance superficial/lateral to the vein.
4. Saphenous nerve block (Fig. 7). The long saphenous vein is usually visible or palpable, therefore, just cephalad of the medial malleolus, 7.5 mL LA is subcutaneously injected 2 cm anterior and posterior to the vein. Ultrasound is only used for this nerve on the rare occasion that the vein cannot be visualised or palpated.
Fig. 7. Saphenous nerve block.
5. Superficial peroneal nerve block (Fig. 8). 7.5 mL LA is subcutaneously infiltrated just cephalad of the lateral malleolus, between the levels of the anterior tibial ridge and posterior border of the lateral malleolus.
Fig. 8. Superficial peroneal nerve block
Local anesthetic regimen: 30 mL ropivacaine 0.5-75%.
Specific Complications: None of note.
1. If performing ultrasound guided ankle block immediately after the induction of GA, a small dose of vasopressor (e.g. ephedrine 6-12 mg, metaraminol 0.5-1 mg) can facilitate arterial visualisation.
2. A standard venepuncture tourniquet applied to the calf can facilitate visualisation of the short and long saphenous veins (sural and saphenous nerves respectively).
Video 1. Ankle block. Note the absorbent drape under the ipsilateral foot, but over the contralateral foot. This provides a further safeguard against a wrong side block, which is more common for lower limb blocks. Order of blocks is deep peroneal, posterior tibial, sural, saphenous and superficial peroneal nerves. The posterior tibial and sural nerve blocks illustrate the value of using a curved probe to facilitate probe-skin contact – a wide linear probe would be almost useless for these blocks in this patient. Because the block involves multiple skin punctures, the probe is at risk of blood contamination; therefore, consider using a protective probe sheath.
Video 2. Deep peroneal nerve block (US).
Video 3: Posterior tibial nerve block (US)
Video 4. Sural nerve block (US).