• RickJ. Deimonte, DPM
• Paul M. Greenberg, DPM

        INTRODUCTION

Foot anesthesia is readily accomplished by blocking the five peripheral nerves that innervate the area by means of local anesthetic deposition either slightly proximal or distal to the malleoli.^1–5 This technique is easily learned and simple to perform, using straightforward visual and palpable anatomic landmarks. It does not require special equipment, paresthesia elicitation, nerve stimulation, special positioning, or patient cooperation.^1–5

        Ankle block can be used for all types of foot surgery and is safe and reliable, with success rates of 89–100%.²’^3,5–9 Because it does not cause motor blockade of the leg, patients are able to ambulate with crutches immediately after surgery and can be discharged home without recovery.⁴ With the use of long-acting local anesthetics such as bupivacaine or ropivacaine, prolonged postoperative analgesia of up to 17 hours or longer may be accomplished.^6,9

Indications & Contraindications

All types of foot surgery can be carried out with the patient under ankle block, including hallux valgus repair, forefoot reconstruction, arthroplasty, osteotomy, and amputation.^1–10 Ankle block can also provide analgesia for fracture and soft tissue injuries¹¹ and gouty arthritis.¹² Moreover, it can be used for diagnostic and therapeutic purposes with spastic talipes equinovarus¹³ and sympathetically mediated pain.¹⁴ Because motor block of the leg is avoided, ankle block may be preferable to sciatic/femoral (saphenous) nerve block for outpatient forefoot surgery.¹⁵

        Ankle block should be avoided whenever there is infection, edema, burn, soft tissue trauma, or distorted anatomy with scarring in the area of block placement. Ankle block should also be avoided in a patient with vascular compromise due to compartment syndrome. In patients with severe coagulopathy, the risk of hematoma is increased, and if ankle block is performed, a more distal approach such as the midtarsal approach, in which blood vessels are more superficial and compressible, may be preferable.

Clinical Pearls

Pertinent Anatomy

The foot is supplied by five nerves (Figures 39–1 and 39–2). The medial aspect is innervated by the saphenous nerve, a terminal branch of the femoral nerve (Figure 39–3). The rest of the foot is innervated by branches of the sciatic nerve:

Figure 39–1. Sensory innervation of the foot.

Figure 39–2. Sensory innervation of the sole of the foot.

Figure 39–3. Saphenous nerve at the level of the ankle (white arrow).

Figure 39–4. Sural nerve at the level of the ankle.

Figure 39–5. Posterior tibial nerve at the level of the medial malleolus.

        At the level of the malleoli, the saphenous, superficial peroneal, and sural nerves are relatively superficial and subcutaneous. The posterior tibial and deep peroneal nerves are deep to the flexor and extensor retinaculi, respectively, and are more difficult to locate.

        The posterior tibial nerve passes with the artery posterior to the medial malleolus deep to the flexor retinaculum, giving off a medial calcaneal branch to supply the lower and posterior surface of the heel.¹⁸ The nerve and artery then become superficial and more accessible as they curve behind and underneath the sustentaculum tali, a bony ridge on the calcaneus about 2 to 3 cm below the medial malleolus. The nerve then divides into medial and lateral plantar nerves.

        The deep peroneal nerve passes lateral to the anterior tibial artery, extensor hallucis longus, and tibialis anterior tendons, and medial to the extensor digitorum longus tendon deep to the extensor retinaculum. It becomes more superficial to travel with the dorsalis pedis artery on the dorsum of the foot, where it is easily accessible.

Figure 39–6. Superficial peroneal nerve. Shown is the emergence of the superficial nerve and its distribution on the dorsum of the foot. 1, Superficial peroneal nerve; 2, sural nerve.

        Sensory innervation of the foot is highly variable. For example, in a study of 100 patients, 40% had the sural nerve extend medially to involve the fourth toe, and 10% had the saphenous nerve extend distally to involve the first metatarsophalangeal joint and occasionally the great toe.¹⁸

        Because the deep structures of the foot are supplied by the deep peroneal and posterior tibial nerves and because cutaneous innervation is variable, all five nerves should be blocked for any foot surgery, especially if a tourniquet is used.¹⁹ The one exception would be purely cutaneous surgery without tourniquet in the distribution of the sural, saphenous, or superficial peroneal nerves.²⁰ Selective versus complete ankle block for forefoot surgery under ankle tourniquet demonstrated that 43 versus 89% of patients were completely pain-free during surgery, suggesting that complete ankle block is preferable under these conditions.⁸

Clinical Pearls

Landmarks

The landmarks for ankle block are the medial and lateral malleoli, the Achilles tendon, extensor hallucis longus tendon (identified by having the patient extend the great toe) (Figure 39–7), the posterior tibial and dorsalis pedis arteries, and the sustentaculum tali (a bony medial calcaneal ridge 2 to 3 cm below the malleolus).

Figure 39–7. Maneuver to accentuate the landmarks for the deep peroneal nerve block (extensor hallucis longus). 1, Extensor hallucis longus; 2, extensor digitorum longus; 3, medial malleolus; 4, lateral malleolus.

        For blockade at the level of the malleoli, the saphenous, sural, and superficial peroneal nerves are blocked with a circumferential subcutaneous injection of 10–15 mL of local anesthetic along a line just proximal to the malleoli and anterior from the Achilles tendon medially to laterally (Figures 39–8 through 39–10). The deep peroneal nerve is blocked by injection of 5–8 mL of local anesthetic just lateral to the extensor hallucis longus tendon deep to the retinaculum along the same circumferential line (Figure 39–11). The posterior tibial nerve is blocked by injection of the same volume of local anesthetic just posterior to the posterior tibial artery if palpable, or midway between the Achilles tendon and medial malleolus deep to the retinaculum (Figure 39–12).

Figure 39–8. Saphenous nerve block is accomplished by injection of 5–8 mL of local anesthetic subcutaneously at the level of the medial malleolus.

Figure 39–9. Superficial peroneal block.

Figure 39–10. Block of the sural nerve.

Figure 39–11. Block of the deep peroneal nerve.

Figure 39–12. Block of the posterior tibial nerve.

        For block at the midtarsal level, the saphenous, sural, and superficial peroneal nerves are blocked with a circumferential subcutaneous injection of 10–15 mL of local anesthetic along a line distal to the malleoli from the Achilles tendon medially to laterally. The deep peroneal nerve is blocked just lateral to the extensor hallucis longus tendon and medial to the dorsalis pedis artery. The posterior tibial nerve is blocked on the calcaneus on either side of the posterior tibial artery (if palpable) or posterior and inferior to the ridge of the sustentaculum tali.

Equipment

No special equipment other than disinfectant, gauze, and 10-mL syringes with 1½ in., 25-gauge needles is required for ankle block. Although nerve stimulation is not necessary for distal approaches, it has been described for the proximal approach to the posterior tibial nerve.²¹ Although there are no data regarding the use of ultrasound for ankle block, this modality can identify nerves, visualize needles and the spread of local anesthetic around the nerves, and may be useful for proximal approaches to the deep peroneal and posterior tibial nerves.

        If a tourniquet is required for surgery, a pneumatic ankle tourniquet should be used rather than an Esmarch bandage, because pressures with the latter are variable, are unknown, and may be extremely high, up to 380 mm Hg.^22,23 Tourniquet pressures just above the malleoli between 200 and 250 mm Hg should ensure a bloodless field and maximize safety.^24,25 Ankle tourniquets are tolerated better than those placed at the midcalf or thigh, with less pain and no increase in neurologic complications.^26–30 An audit of 1000 cases of ankle block revealed that with proper tourniquet application and the option of sedation, only 3.1% of patients complained of tourniquet pain. Risk factors for tourniquet pain were age over 70 and tourniquet times greater than 30 minutes.³⁰

Clinical Pearl

Related posts:

Regional Anesthesia in the Patient with Preexisting Neurologic Disease. The History of Local Anesthesia. Stimulating Catheters. Ultrasound-Assisted Nerve Blocks in Adults. Cutaneous Blocks for the Upper Extremity. Cervical Plexus Block.