Regional Anesthesia for the Foot and Ankle

Sensory Innervation

Sensory innervation of the foot and ankle is shared by branches of the femoral and sciatic nerves. The five branches providing sensation are: saphenous nerve, superficial peroneal nerve, deep peroneal nerve (DPN), tibial nerve, and the sural nerve.

The saphenous nerve is the only distal branch of the femoral nerve that provides innervation to the foot. The saphenous nerve is widely considered responsible for innervation of the medial lower leg to the medial ankle. However, there is debate regarding the extent of contribution of the saphenous nerve to foot innervation, with some investigators suggesting that blocking it is necessary in only 3% of patients undergoing foot and ankle surgery, whereas others found deep branches of the saphenous nerve innervating the talocrural capsule ( Fig. 6-1 ).

The remaining nerves are branches of the sciatic nerve ( Fig. 6-2 ). At or above the level of the popliteal fossa, the sciatic nerve terminally divides into the tibial and common peroneal nerves. The common peroneal nerve further divides into the superficial and DPNs. The superficial peroneal nerve provides innervation to the lateral ankle and the majority of the dorsal surface of the foot. The DPN provides innervation to the dorsal first webspace ( Fig. 6-3 ). The tibial nerve courses distally and after traversing along the posterior medial malleolus provides innervation to the majority of the plantar surface of the foot via the medial and lateral plantar nerves ( Fig. 6-4 ). The sural nerve generally derives from branches of both the tibial and common peroneal, but there is variation in its composition. The sural nerve innervates the lateral aspect of the foot and the fifth toe. Of note, the deep peroneal, posterior tibial, and the sural nerve all course in close proximity to blood vessels, which can be useful landmarks during nerve blocks ( Fig. 6-5 ).

Ultrasound-Guided Peripheral Nerve Blocks

For decades, electrical nerve stimulation (NS) was the “gold standard” technique for performing nerve blocks. Despite an absence of data showing a consistent relationship between the stimulating needle current and the distance to the nerve, NS remained popular due to the lack of an alternative method to guide needle placement. Advances in portable US and compelling evidence by several meta-analyses and large studies have led to ultrasound being the preferred technique for PNBs. When compared with NS techniques, blocks performed using US have higher success rates, less procedure-related pain, fewer vascular punctures, fewer needle passes, lower local anesthetic requirements, and shorter performance times.

Types of Nerve Blocks

PNBs can be categorized by various criteria—anatomical location, duration of action, density of the block, nerve type being blocked, and presence of a catheter. Furthermore, various medications may be used for the block depending on the required duration and volume required ( Table 6-1 ). The addition of epinephrine to the block causes local vasoconstriction, which may increase the duration of the block and allows for a greater volume of injection. However, caution is required when used with digital blocks due to the potential morbidity of prolonged vasoconstriction to the digit.

For the purposes of this text, the authors will focus on the following types of nerve blocks:

• Single-site single-injection—one target nerve or fascial plane injected in one setting, for example, sciatic nerve block, adductor canal block

• Multiple-site single-injection, that is, combined PNB—more than one target nerve or fascial plane injected in one setting, for example, combined popliteal + saphenous nerve blocks, femoral + sciatic, popliteal + ankle, continuous popliteal + single-shot saphenous, and many other such combinations

• CPNB—single-injection nerve block (SINB) is performed followed by placement of indwelling catheter to be left in place for 2 to 3 days postoperatively, for example, adductor canal saphenous nerve block with catheter placement, popliteal catheter

Continuous Peripheral Nerve Block

Although SINBs can provide analgesia for up to 24 hours, after orthopaedic surgery severe pain may persist for 2 to 3 days. CPNB via indwelling perineural catheters can be used to provide prolonged postoperative analgesia. CPNBs have been associated with several positive outcomes :

• Significantly better pain control on postoperative days 1 to 3

• Lower doses of postoperative opioids for up to 12 weeks

• Higher patient satisfaction scores

• Expedited hospital discharge and lower healthcare costs

Furthermore, with use of portable infusion pumps that patients can manage themselves, CPNBs can facilitate outpatient management for procedures that historically have required hospital admission for pain control. Of note, there remains considerable variability between institutions regarding placement and management of continuous peripheral catheters. Many institutions experienced in the use of CPNB have adopted the use of simple elastomeric pumps for both economic and patient satisfaction reasons. Two common techniques that have been described for CPNBs are summarized in Table 6-2 .

Nerve Blocks Above the Knee

Sciatic Nerve Block at the Popliteal Fossa

The most utilized nerve block for foot and ankle surgery is the sciatic nerve block at the level of the popliteal fossa ( Video 6-1 ). Success with popliteal sciatic nerve blocks has been documented for numerous foot and ankle surgeries—including talar and calcaneal fractures, end-stage arthritis related surgery, and hallux valgus surgery.

With US guidance, the sciatic nerve, popliteal artery, and popliteal vein are easy to visualize within the popliteal fossa. As seen in Fig. 6-6 , the popliteal fossa is bordered medially by the semimembranosus/semitendinosus muscles and laterally by the biceps femoris muscle. The nerves can be located by imaging superficial and lateral to the popliteal artery and vein.

Several techniques and approaches to the popliteal sciatic nerve block have been proposed, with one of the most common being the lateral approach with the patient in the supine position. The key points of this approach are summarized in Table 6-3 followed by images (see Figs. 6-7 and 6-8 ) to allow visualization of approach and US anatomy. The popliteal sciatic nerve block is often combined with a saphenous nerve block to provide complete anesthesia to the foot and ankle.

Table 6-3

Popliteal Sciatic Nerve Block

		Authors’ Comments
Patient position	Supine with leg elevated and slight internal rotation ( Fig. 6-7 ) Alternatives: Lateral position, prone position	Advantages of this position include many foot and ankle surgeries performed in supine position, ease of access to lateral leg, US probe not in sterile field, in-plane needle approach, ease of scanning along course of the nerve
Proceduralist position	Seated or standing along the side to be blocked
Transducer	High-frequency (10–15 MHz) linear probe placed in transverse position proximal to the popliteal crease with initial depth set to 4–5 cm	This probe orientation allows easy in-plane needle visualization
Block needle	22–20 gauge (user preference) Length dependent on patient habitus, typically 7–10 cm	Consider using echogenic needles allowing for better needle visualization Use of beveled needle allows for minor redirection without having to fully retract needle
Summarized technique	Short-axis in-plane approach Scan proximally and distally along the sciatic nerve and identify the point of bifurcation Point of needle entry along lateral leg in line with midline of US probe and typically 3–4 cm anterior to probe position Tip of needle directed to connective tissue between tibial nerve (TN) and common peroneal nerve (CPN) ( Fig. 6-8 ) Inject anesthetic mixture and observe for separation of the nerves spread around the nerves—“halo” appearance
Anesthetic agents commonly used	Lidocaine: 20–30 mL volume Bupivacaine 20–30 mL volume Ropivacaine 20–30 mL volume	Choice of local anesthetic (LA) depends largely on desired density of nerve block and duration of anesthesia needed Consider epinephrine containing LA to prolong duration Addition of dexamethasone 4–10 mg has been shown to prolong duration of anesthesia up to 10 hours
Anatomy pearls	TN is often 2x the size of the CPN—block takes longer TN is superficial and lateral to the popliteal artery and vein CPN is superficial and lateral to TN—closest to probe with lateral approach

Continuous Popliteal Sciatic Nerve Block

The continuous popliteal sciatic nerve block is particularly well suited to foot and ankle surgery (see Video 6-1 ). As detailed above, CPNB can provide superior postoperative anesthesia compared to single-shot injections. The catheter can be placed by either of the two methods described in Continuous Peripheral Nerve Block section ( Figs. 6-9 and 6-10 ). The optimal local anesthetic agent, concentration, and dosing regimen remains ill defined. Commonly used concentrations include ropivacaine 0.1% to 0.4%, bupivacaine 0.125% to 0.15%, and levobupivacaine 0.1% to 0.125%.

Saphenous Nerve Block Above the Knee

The saphenous nerve (SN) is a pure sensory branch of the femoral nerve. Although there is much debate about the true contribution of the saphenous nerve to medial foot and ankle sensation, there are enough data recommending that a SN block be included as a component of comprehensive regional anesthesia for the lower leg, foot, and ankle, particularly in cases where a calf tourniquet may be in use or dissection will be on the medial aspect of the foot and ankle. At the level of the superior pole of the patella, the SN exits the adductor canal in a tissue plane between the vastus medialis and sartorius muscles ( Fig. 6-11 ). Fig. 6-12 demonstrates dissection of the tissue plane between the vastus medialis and the sartorius that is readily visible with ultrasound after injection of 15 mL of local anesthetic. See Table 6-4 for nerve block technique details.

Table 6-4

Adductor Canal Saphenous Nerve Block

		Authors’ Comments
Patient position	Supine with operative leg externally rotated at the hip and flexed at the knee Medial thigh exposed	Advantages of this position include many foot and ankle surgeries performed in supine position, ease of access to medial leg, in-plane needle approach, ease of scanning along course of the nerve Mid-portion of the canal is the best site for block
Proceduralist position	Seated or standing along the side to be blocked
Transducer	High-frequency (10–15 MHz) linear probe placed in transverse position on the lower third of the medial thigh with initial depth set to 3–4 cm	This probe orientation allows easy in-plane needle visualization
Block needle	22–20 gauge (user preference) Length dependent on patient habitus—typically 5–10 cm	Consider using echogenic needles allowing for better needle visualization Use of beveled needle allows for minor redirection without having to fully retract needle
Summarized technique	Plan for short-axis in-plane approach Scan cephalad and caudad along the adductor canal to identify relevant anatomy (nerve not always visualized) Point of needle entry along lateral leg lined up with midline of US probe and typically 2–3 cm lateral to probe Tip of needle directed through the sartorius muscle into the fascial plane between the sartorius and vastus medialis Inject anesthetic mixture and observe for dissection of the fascial plane
Anesthetic agents commonly used	Lidocaine: 10–20 mL volume Bupivacaine 10–20 mL volume Ropivacaine 10–20 mL volume	Choice of local anesthetic (LA) depends largely on desired density of nerve block and duration of anesthesia Consider epinephrine containing LA to prolong duration Addition of dexamethasone 4–10 mg has been shown to prolong duration of anesthesia up to 10 hours
Anatomy pearls	Adductor canal may be located by distally observing the medial border of the sartorius muscle converging with the medial border of the adductor longus muscle and then sliding the probe proximally to the mid-thigh keeping the femoral artery in view Saphenous nerve is lateral to femoral artery in proximal thigh and crosses over artery to become medial in distal thigh As many motor nerves have branched off proximally there is lesser concern for postoperative quadriceps weakness

Nerve Blocks Below the Knee

Historically, nerve blocks below the knee have been limited to the ankle ( Video 6-2 ), where surface landmarks can be used to identify injection sites. Advances in ultrasound provide an added degree of versatility because nerves of the foot and ankle can be visualized along most of their length and blocked where most appropriate.