Chapter 39. Ankle Block

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.

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:

• The lateral aspect is innervated by the sural nerve arising from the tibial and communicating superficial peroneal branches (Figure 39–4).
• The deep plantar structures, muscles and sole of the foot are innervated by the posterior tibial nerve, arising from the tibial branch (Figure 39–5).
• The dorsum of the foot is innervated by the superficial peroneal nerve, arising from the common peroneal branch (Figure 39–6).
• The deep dorsal structures and web space between the first and second toes are innervated by the deep peroneal nerve (see Figure 39–2).16,17

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.2

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.

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

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).

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, 39–9, and 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).

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

Sterile disposable tourniquets are available if the surgery requires a more proximal operative area.

Alternative Techniques

There are several techniques for performing ankle block; they can be classified as perimalleolar or inframalleolar (midtarsal) block. The location of the block determines the procedures that can be done. With midtarsal block, forefoot surgery is easily accomplished. For midfoot and more proximal foot surgery, perimalleolar block is required. Because success rates are higher with inframalleolar technique, in which the deep peroneal and posterior tibial nerves are more superficial, this technique is preferable for forefoot surgery.2,3

For all approaches, the patient can be supine, with a pillow under the calf of the leg to be blocked to facilitate access. The anesthesiologist can sit on a stool at the foot of the bed, if desired.

Saphenous, Superficial Peroneal, and Sural Nerve Blocks

The saphenous, superficial peroneal, and sural nerves are already subcutaneous just proximal to the malleoli, and all can be blocked by a subcutaneous ring of local anesthetic at this location from just anterior to the Achilles tendon medially to laterally (see Figures 39-8, 39-9, and 39-10). The advantage of blocking these nerves here is that the area under an ankle tourniquet will be anesthetized and tourniquet pain is less likely. By injecting slowly and continuously advancing a 1½-in., 25-gauge needle into the previously injected area, the number of injections and discomfort from them can be minimized. This subcutaneous ring of local anesthetic can also be performed distal to the malleoli for a midtarsal block.

Deep Peroneal Nerve Blocks

For the perimalleolar approach, the patient is asked to extend the great toe, which will tense and identify the extensor hallucis tendon (see Figure 39–7). A 1½-in., 25-gauge needle is inserted immediately lateral to the tendon, perpendicular to the tibia, and is advanced until it contacts bone (see Figure 39–11). The needle is then withdrawn a few millimeters, and after negative aspiration, 8 mL of local anesthetic are injected. For the inframalleolar or midtarsal approach, the extensor hallucis tendon is identified as previously mentioned above, but more distally, and the pulse of the dorsalis pedis artery is identified on the top of the foot as well. A 1½–in., 25-gauge needle is inserted immediately lateral to the tendon and medial to the artery, and after negative aspiration, 5 mL of local anesthetic are injected.

Posterior Tibial Nerve Block

For the retrotibial approach, a 1½-in., 25-gauge needle is inserted just posterior to the pulse of the posterior tibial artery behind the medial malleolus, or if it cannot be palpated, midway between the Achilles tendon and the posterior aspect of the medial malleolus (see Figure 39–12). The needle is directed toward the tibia at a 45-degree angle to contact bone. The needle is then withdrawn a few millimeters, and after negative aspiration, 8 mL of local anesthetic are injected.

For the midtarsal, there are two approaches. Either the posterior tibial artery is identified below and distal to the medial malleolus on the calcaneus, or the sustentaculum tali is identified. The needle is directed toward the calcaneus, slightly under the bony shelf of the sustentaculum tali, or on either side of the tibial artery. After contact with bone, the needle is withdrawn 2 mm, and 8 mL of local anesthetic are injected.

Block of the saphenous, sural, and superficial peroneal nerves by circumferential subcutaneous injection can always be performed proximal to the malleoli even when the deep peroneal and posterior tibial nerve blocks are done more distally. This may help with tourniquet tolerance.

Anatomic Facts

The Mayo block is a combination of the nerve block and a field block that involves the infiltration of local anesthesia through the tissues proximal to a surgical site in a ring shape around the first metatarsal (most commonly) or a lesser metatarsal base.29 When the Mayo block is used around the first metatarsal base, the nerves that are anesthetized include the medial dorsal cutaneous nerve and the deep peroneal nerve on the dorsal aspect. The first and second branches of the common plantar digital nerves, which are superficial branches of the medial plantar nerve, provide sensation to the plantar aspect of the first metatarsal.

Indications

The Mayo block is commonly used in podiatric office surgery to anesthetize the area before performing bunion or hallux surgery. The injection can be used with or without epinephrine because it is not being injected into the toe.

Technique

The Mayo block consists of three or four separate injections. The block is performed by raising a wheal of local anesthesia proximally and dorsally in the first intermetatarsal space and advancing the needle in the plantar direction while injecting 3–5 mL of local anesthetic (Figure 39–13). The needle is then withdrawn partially and redirected medially, raising a subcutaneous wheal along its course (3–5 mL) (Figure 39–14). The needle is then removed and re-entered and directed laterally to raise a subcutaneous wheal along the course (3–5 mL) (Figure 39–15). Finally, the needle is removed and directed plantarmedially to the metatarsal and injected from medial to lateral underneath the metatarsal bone (3–5 mL) (Figure 39–16). The block encircles the entire metatarsal bone.

Choice of Local Anesthetic

The decision regarding which local anesthetic solution to use depends on the anticipated duration of surgery and the degree of postoperative pain. The most commonly used local anesthetics are lidocaine 0.5–2%,1–3 mepivacaine 1%,8,30 bupivacaine 0.5–0.75%,3,4 and ropivacaine 0.75%.7 For example, removal of a superficial foreign body would require only a short-acting local anesthetic such as lidocaine, which provides a minimum of 1 hour of surgical anesthesia. However, long-acting anesthetics such as ropivacaine 0.75% and bupivacaine 0.75%, which give a mean duration of 14 or 17 hours of analgesia, respectively, after ankle block would be more beneficial after a longer surgery such as a complex forefoot reconstruction with severe postoperative pain.6,9 When 20–25 mL of 0.5% bupivacaine are used, onset time is 15–20 minutes with a duration of anesthesia from 10–25 hours.5 Mepivacaine 1% is a good choice for intermediate-length procedures.

Combinations of short- and long-acting local anesthetics have been used to speed onset but provide longer postoperative analgesia. A combination of 2% lidocaine and 0.5% bupivacaine in equal parts provides a minimum of 3 hours of anesthesia with an onset time of 10± 3 minutes.32

Mixtures of 1.5% lidocaine and 0.75% ropivacaine in equal parts have been reported to have an intermediate mean duration of analgesia of 8 hours.6

Various additives to local anesthetic solutions have been used to prolong the duration of postoperative analgesia. Adding clonidine 1 mcg/kg to 0.75% ropivacaine increases the duration of analgesia from 14 to 17 hours.6 Another study found a duration of 15.9 hours.7 Similarly, the addition of clonidine 10 mcg/mL to lidocaine improves the quality and duration of postoperative analgesia.36

Blood levels of plain local anesthetic are well below toxic levels, even when large amounts are used. Bilateral midtarsal blocks performed with up to 30 mL of plain 0.75% bupivacaine resulted in peak venous blood levels of 0.5 mcg/mL, whereas 13 mL of 2% lidocaine for unilateral block resulted in 1.1 mcg/mL.9 No adverse local anesthetic effects were reported in a series of 66 patients receiving bilateral ankle blocks with mixtures of plain lidocaine and ropivacaine 0.75%, ropivacaine 0.75% or ropivacaine 0.75% with clonidine 1 mcg/kg.6 The addition of epinephrine with ankle block remains controversial. The preponderance of the literature suggests that epinephrine should not be used in distal extremity local anesthesia.33,34 However, low concentrations of epinephrine in local anesthetic solutions have been used with remarkable safety.35 The overall incidence of severe vascular complications after injection of epinephrine-containing local anesthetics has been estimated to be 1 per 132,000 injections. Note that the use of 1:000,000 solutions of epinephrine has a 2.5 greater risk of complications compared with 1:200,000, suggesting that when epinephrine is indicated, it should be used only as dilute concentrations (ie, 1:300,000 or less). Regardless, epinephrine is probably best avoided altogether in patients with peripheral vascular disease or compromised circulation.

The high-efficacy, prolonged postoperative analgesia and safety of plain bupivacaine and ropivacaine suggest that these drugs should be the choice for surgery in which postoperative pain is expected. However, blocks should be performed 30 minutes before surgery (minimum of 20 minutes) when using bupivacaine or ropivacaine to maximize the success rate. In a prospective analysis of 1000 patients, the failure rate was significantly lower after waiting 20 minutes after the injection, with the lowest failure rates occurring after 50 minutes.7

Before block placement, consideration should be given to loading the patient preoperatively with nonopioid analgesics such as acetaminophen, celecoxib, or a nonsteroidal anti-inflammatory drug (NSAID), and gabapentin.

Because performing an ankle block requires more than one injection and requires subcutaneous infiltration, it can cause more discomfort than single-injection blocks. Apart from gentle, slow injection and infiltration, patients usually benefit from anxiolysis and analgesia with midazolam 1–4 mg and fentanyl 25–100 mcg. Before starting surgery, the block should be checked in all five nerve distributions, and supplemental local anesthetic can be injected if necessary.

Many patients benefit from continuing sedation during the procedure. Tourniquets should have a soft lining or padding and should be placed just above the malleoli. Sterile tourniquets are available for use if the surgeon requires a larger exposed field. Midcalf or thigh tourniquets should be avoided because they cause more discomfort. Discomfort from an ankle tourniquet may occur rarely, and more sedation may be required. Supplementation by the surgeon intraoperatively may rescue an incomplete block.

Postoperatively, acetaminophen and an NSAID can be continued routinely. Depending on the extent and type of surgery, small doses of a long-acting opioid such as controlled-release oxycodone may provide a smooth transition from block to postoperative analgesia and may facilitate rehabilitation. Ambulation is possible right after surgery. Elevation of the leg, when not ambulating, may decrease pain.

Complications may occur from injection or from application of the tourniquet. Because most surgery is done under tourniquet, it is difficult to differentiate the cause of neurologic complications. In a retrospective study of 3027 patients with pneumatic ankle tourniquet at relatively high pressures of 325 mm Hg, there were three cases (0.1%) of posttourniquet syndrome.28 Ankle tourniquets have been used routinely with as little as 200 mm Hg pressure,5 although a bloodless surgical field may require 218.6 ± 34.6 mm Hg, with younger normotensive patients requiring only 203.9 ± 22.3 mm Hg.25 Thus, no more than 250 mm Hg pressure is necessary, and more pressure may be harmful.

The incidence of complications after ankle block is low and is usually from transient paresthesias, which almost always resolve. The incidence is usually less than 1%, although it ranges from 0 to 10%, depending on the source of the data. In a prospective survey of 284 patients with posterior tibial, sural, and saphenous blocks at the ankle and common peroneal block at the knee, no patient developed postanesthetic neuralgia or other complications.31 In three other studies with a total of 120 patients who received ankle blocks, no patients developed complications.2,5,10 After midtarsal ankle block in 71 out of 100 patients available for follow-up, one patient developed transient posterior tibial paresthesias, which resolved in 4 weeks.3 In another study of 40 patients, one developed paresthesias lasting 6 weeks, which resolved.1 In a retrospective study of 1373 patients who received ankle block followed by a posterior tibial nerve catheter for postoperative analgesia, five patients had transient paresthesias, with one patient developing neurolysis (probably related to the catheter insertion) but with complete recovery.21 In a prospective randomized trial of 32 patients (40 total feet) undergoing forefoot surgery under ankle tourniquet (inflation pressure 100 mm Hg over systolic) under complete or selective ankle block of which 26 patients (33 total feet) were available for follow-up between 16 and 80 weeks, one patient had paresthesias related to injection or tourniquet, one had ankle pain, and one had cold toes.8

Local anesthetic toxicity would be expected to be rare, given the low blood levels after injection. In the retrospective series of 1373 patients previously mentioned, one patient had a convulsion, thought to be secondary to an intravascular injection.21 In another series of 1295 patients who received standard and modified ankle blocks as well as digital nerve blocks, three patients had vasovagal reactions and one had an episode of hypotension and supraventricular tachycardia, thought by the authors to be from lidocaine toxicity. No other complications were seen in this series.4

There are single case reports of injection-related complications such as an Achilles tendon avulsion from tibial nerve block in a patient with spastic talipes equinovarus,13 and acute compartment syndrome from ankle block in a patient with previous scarring from forefoot arthroplasty.37 Both of these patients had an altered anatomy, which may have predisposed them to the complication.