Ankle Block

Indications and Contraindications

Indications¹

• Analgesia or anesthesia for distal foot and toe surgeries, including:
  • Forefoot procedures
  • Osteotomies
  • Bunionectomies
  • Amputations
• Analgesia for non-operative foot pain, including:
  • Fractures
  • Soft tissue injuries
  • Gout flares

Considerations

• If a tourniquet is necessary, a calf tourniquet is generally better tolerated, often with minimal intravenous sedation, than a thigh tourniquet.
• For very painful procedures where short-term weight-bearing is not expected, a single-injection popliteal sciatic block may provide longer-lasting analgesia.
• Ankle blocks do not provide adequate anesthesia for most true ankle surgeries (e.g., total ankle replacement, open reduction internal fixation, arthrodesis). These surgeries require coverage of structures proximal to the malleoli, which the ankle block does not reach; deeper tibial and peroneal nerve branches at the popliteal level must be blocked for sufficient surgical anesthesia.

Contraindications¹

Absolute

• Patient refusal
• Known allergy or prior severe adverse reaction to a local anesthetic, preservative, or additive
• Active infection at the intended site of needle insertion or local anesthetic deposition

Relative (Case-by-Case Assessment)

• Distorted anatomy at the intended block site, including:
  • Scarring
  • Burns
  • Edema
• Difficulty identifying landmarks with ultrasound, increasing risk of incomplete or failed block
• Pre-existing neuropathy, due to the risk of exacerbation
• Landmark-based approach in patients with coagulopathy or on anticoagulation, due to risk of hematoma formation

Anatomy and Technique

Anatomy

Anatomical Considerations

• As with all procedures, relying on anatomical landmarks can be more difficult when there is a history of prior procedures in the relevant area, injuries to the area, or other abnormalities of habitus.
• When the anatomy appears atypical, it can be helpful to use ultrasound to identify subcutaneous structures and landmarks, or to use nerve stimulation to confirm appropriate proximity to nerves for adequate anesthesia.

Innervation

• Five peripheral nerves innervate the foot. These include the superficial peroneal nerve, the deep peroneal nerve, the SuN, and the tibial nerve, which all originate from the sciatic nerve, as well as the saphenous nerve (SaN), which originates from the femoral nerve.

Technique

• Two primary approaches to foot and ankle blocks exist. One of these is an ultrasound-guided approach, which involves identifying nerves or planes where nerves are known to exist and injecting local anesthetic in those areas.
• The most commonly blocked nerves using ultrasonography are the tibial and deep peroneal nerves. However, with careful ultrasound technique, all 5 nerves can be visualized and blocked.
• Alternatively, some providers perform regional blockade of the nerves to the foot using an anatomical landmark-based field approach. More commonly, a combination of the two techniques is employed to ensure efficient and effective blockade.

Ultrasound-Guided Approach¹

• There are several benefits to using ultrasound guidance for peripheral nerve blocks at the ankle. A primary benefit is that visualization of the nerves allows the clinician to minimize the necessary volume of local anesthetic to approximately 3-5mL per nerve blocked by ensuring that all administered medication is deposited perineurally and in clinically meaningful locations.
• Although such superficial structures should be relatively straightforward to visualize, effective blockade requires a thorough understanding of the local anatomy. For some of the described nerves, application of a tourniquet can aid visualization of the nerve and make visualization of nearby vasculature easier.
• As previously mentioned, innervation to the foot is supplied by five nerves that must each be blocked at discrete locations. All but one (superficial personal nerve) are associated with vascular landmarks that help with identification. From most medial to most lateral, ultrasound blockade of the nerves is described below.

Tibial Nerve

• The tibial nerve provides innervation to the heel and sole of the foot, including most of the intrinsic muscles and ligaments of the plantar foot.
• This is the largest of the five nerves at the ankle and the easiest to block using ultrasonography. Given the size and importance of this nerve, a larger amount of local anesthetic (5-8mL) is typically administered.
• To visualize this nerve, a linear array ultrasound probe should be placed transversely just posterior and superior to the medial malleolus.
• Five similar-looking structures are present in this view; from anterior to posterior, they are the tibialis posterior tendon, the flexor digitorum longus tendon, the posterior tibial artery, the posterior tibial nerve, and the flexor hallucis longus tendon. Although the structures are small and may appear similar initially, Doppler ultrasound can help identify the artery, which is typically immediately anterior to the tibial nerve within a triangular fascial compartment under the flexor retinaculum.
• The tibial nerve block is commonly performed using an in-plane approach to deposit local anesthetic into the fascial plane containing the nerve in this view.¹

Tips

• Site of injection: The tibial nerve gives off the medial calcaneal, medial plantar, and lateral plantar nerves. For the most complete block, inject above the medial malleolus, proximal to the takeoff of the medial calcaneal branch.
• Ultrasound identification: Tendons may look like nerves. Slide the probe proximally to make the tendons disappear, leaving the tibial nerve and corresponding artery (Figure 2).
• Anatomical variation: In some cases, the tibial nerve may lie anterior rather than posterior to the posterior tibial artery. Tracing the nerve proximally will usually reveal its normal position relative to the artery.

SaN

• The SaN provides innervation to the medial lower leg, including the medial malleolus and variable portions of the medial foot, typically terminating around the level of the arch of the foot.
• This nerve typically runs along the greater saphenous vein (SaV) and can be visualized by placing a linear array ultrasound probe transverse to the leg and anterior to the medial malleolus. Due to the small size of this nerve, it can be helpful to place a tourniquet below the knee to aid in visualizing the vein, and thus the nerve. It can also be helpful to look for this nerve proximal to the ankle if it is too difficult to visualize as far distally as the ankle. Here, local anesthetic can be deposited peri-vascularly to bathe and block the nerve.¹

Tips

• Useful landmark: The greater SaV and SaN lie between two fascial layers. Because the nerve can be difficult to visualize, injecting into same fascial plane as the SaV helps ensure a successful blockade.
• Performance: Apply light pressure with the probe to avoid compressing the superficial structures and to maintain clear visualization.

Deep Peroneal Nerve

• The deep peroneal nerve is a branch of the common peroneal nerve. It provides sensory innervation to the web space between the dorsal aspect of the first and second toes and a small area over the dorsum of the foot near the ankle. It also supplies motor innervation to the foot via the extensor muscles.
• This nerve descends along the anterior aspect of the tibia and lies adjacent to the anterior tibial artery, which becomes the dorsalis pedis artery in the foot. To visualize the deep peroneal nerve, place a linear array ultrasound probe transversely across the lower end of the tibia, across the proximal dorsal foot. Doppler can be utilized to identify the artery and, thus, help locate the nerve.

Tips

• Identification of the nerve: Use Doppler to locate the artery; the nerve is hypoechoic and may be medial, lateral, or superficial to it (Figure 4).
• Performance: Apply light probe pressure to avoid compressing the artery against the tibia and to maintain clear visualization.¹

Superficial Peroneal Nerve

• The superficial peroneal nerve provides innervation to the dorsal aspect of the foot. The nerve runs down the anterolateral aspect of the lower leg, splitting into a variable number of branches as it courses towards the foot and ankle. This is a small superficial nerve best visualized proximal to the ankle, about 1/3 of the way up the lower leg. Branches of the nerve can be seen superficial to the fascia of the lower leg, and these branches can be traced proximally to ensure that all branches have been seen and blocked with local anesthetic.
• The ultrasound technique involves placing a linear-array probe transversely 1/3 of the way up the lateral leg. Identify the fibula as a hyperechoic arc with dense posterior shadowing (ensure proper depth on the ultrasound). Two muscles will be seen above the fibula: the peroneus brevis muscle and the extensor digitorum longus muscle. The superficial peroneal nerve travels in the superficial border between these two muscles in what appears to be a hyperechoic triangular shape. The fibula often seems to point toward the nerve. Identity of the nerve is further confirmed by tracing it distally and seeing it rise to pierce the crural fascia, where it lies in the subcutaneous location and subsequently divides and branches. Dynamic scanning proximally and distally along the lateral lower leg can make the nerve easier to visualize and block.¹

Tips

• Injection plane: Use caution when injecting if the nerve lies deep to the crural fascia, as depositing local anesthetic within this tight compartment may increase the risk of mechanical nerve injury.
• Nerve appearance: The superficial peroneal nerve may initially appear flattened. As local anesthetic is injected, it gently expands the surrounding space, and the nerve typically regains its typical, more rounded cross-sectional appearance.

SuN

• The SuN provides innervation to the lateral aspect of the foot and ankle. This nerve runs along the posterolateral aspect of the calf, beside the small SaV. It can be very small and challenging to visualize. The small SaV serves as a helpful landmark during ultrasound-guided blockade of this nerve, and application of a proximal calf tourniquet can make this visualization easier by enlarging this vein.
• To locate the SuN, a linear array ultrasound transducer is placed transversely on the leg between the posterior aspect of the lateral malleolus and anterior to the Achilles tendon. In this area, just above the lateral malleolus, the SuN typically lies medial to the small SaV, and local anesthetic can be deposited peri-vascularly to block the nerve.¹

Tips

• Ultrasound identification: A consistent fascial “hammock” exists between the Achilles tendon and the peroneus brevis muscle. The SuN and small SaV reliably lie within this space. Sliding the probe proximally will bring both the SuN and the short SaV up onto the Achilles tendon.
• Nerve appearance: The SuN often becomes visible as the local anesthetic begins to surround it.

Anatomical-Based Approach²

• Nerve blocks of the foot and ankle can also be performed in an anatomical, “field-based”, landmark-guided approach, which does not require ultrasound. At the level of the malleoli, the saphenous, superficial peroneal, and SuNs are all superficial and exist in the subcutaneous space. The posterior tibial nerve lies deep to the flexor retinaculum, and the deep peroneal nerve lies beneath the extensor retinaculum, but both remain accessible for reliable deposition of local anesthetic.
• Superficial nerves: The superficial nerves are easily reached with a subcutaneous wheal in an anklet-like distribution. Approximately 10-15mL of local anesthetic can be deposited circumferentially from the medial aspect of the Achilles tendon to the lateral aspect just proximal to the malleoli.
• Deep nerves: The deep peroneal nerve can be blocked by depositing ~5mL of local anesthetic just lateral to the extensor hallicus longus tendon at the same level as the circumferential infiltration. Finally, the posterior tibial nerve can be blocked by palpating the dorsalis pedis artery at the dorsum of the foot and injecting an additional ~5mL aliquot of local anesthetic adjacent to it.¹
• While effective, the landmark-based approach carries several pitfalls, including insufficient blockade from improper local deposition, a higher risk of nerve injury or hematoma, and a greater likelihood of local anesthetic systemic toxicity (LAST) due to the potential for unrecognized intravascular injection.

Complications³

• Complications of peripheral nerve blockade are rare and vary by injection site. Both transient and permanent nerve damage have been reported, potentially resulting in sensory or motor deficits depending on the affected nerve’s function; however, most cases are transient and resolve over time. Caution is advised when using additives such as epinephrine in ankle blocks to minimize the risk of local vasoconstriction and nerve injury.
• Vascular injury is also a rare complication. Significant vascular injury is rare. Most cases involve only small hematomas at the site of needle insertion that resolve without intervention.
• Infection at the site of injection is another rare complication, which is more frequently associated with prolonged catheter-based techniques rather than with single-injection blocks.
• Finally, LAST is a potentially life-threatening complication resulting from excessive absorption of local anesthetic. Presentation can range from mild symptoms such as tinnitus, agitation, and circumoral numbness to severe symptoms such as seizure, coma, respiratory arrest, and cardiovascular collapse. LAST is relatively uncommon in single-shot peripheral nerve blockade of the distal lower extremity, primarily because these blocks are typically performed with a low volume of local anesthetic; however, it remains a potential risk even with ultrasound guidance.³