Ankle Block
Indications
The ankle block in indicated in patients undergoing foot surgeries, especially those patients who cannot hemodynamically tolerate the depressant effects of general anesthesia or neuraxial blockade. It is also desirable since it facilitates early mobility & is associated with prolonged post operative analgesia.
Drugs
All local anesthetic agents may be used in ankle blockade. However, it should be noted that the use of epinephrine with these local agents is contraindicated due to ischemia of the digits. Maximum doses of the agents may be used, and selection is based upon the length of surgery: short acting – 45 to 90 minutes, intermediate acting – 90 to 120 minutes, and long acting – 120 to 750 minutes.
Anatomy
Five nerves supply sensations to the foot:
a. Superficial Peroneal Nerve – Cutaneous sensation to the anteromedial foot.
b. Deep Peroneal Nerve – Supplies medial half of the dorsal foot.
c. Saphenous Nerve – Superficial sensation to the anteromedial foot.
d. Tibial Nerve – Sensation to the heel, medial, and part of the lateral sole.
e. Sural Nerve – Sensation to the lateral foot.
1. Common Peroneal Nerve
2. Superficial Peroneal Nerve
3. Deep Peroneal Nerve
1. Sural Nerve, which has multiple branches,
(2,3) that innervate the lateral foot.
1. Tibial Nerve
2. Achilles’ Tendon
3. Medial Malleolus
Technique
Gather the necessary supplies, including a 22 gauge, 1.5 inch needle, sterile gloves, chlorhexadine prep, and local anesthetic agents. Position the patient with his/her foot elevated on a pillow to allow access to both sides of the ankle.
1. * Identify the anatomical landmarks (medial & lateral malleoli, dorsalis pedal artery). Inject 5 mL of local anesthetic (LA) 1 cm anterior to the medial malleolous to achieve saphenous nerve blockade.
2. Insert needle posterior to the medial mallelous & posterior tibial artery. Make contact with the bone & withdraw the needle 1 mm. Inject 5 mL of LA to achieve tibial nerve blockade.
3. Insert the needle between the dorsalis pedis pulse and the extensor hallucis longus tendon. Advance 1 cm & inject 5 mL of LA. This will block the deep peroneal nerve.
4. * Withdraw the needle to just below the surface of the skin, re-direct toward the lateral malleolus, and deposit 5 mL of LA into the subcutaneous tissue. This will achieve blockade to the superficial peroneal nerve.
5. Blockade of the sural nerve can be achieved as demonstrated at left. Insert the needle along the line between the lateral mallelous & Achilles’ tendon. Inject the LA with a deep subcutaneous fan infiltration of 3 to 5 mL.
* If one considers depositing LA anterior and posterior to both malleoli, there will be a total of 4 needle insertions, one of which will block 2 nerves. Thus, a total of 5 nerves will be blocked (combine steps #1 and #4).
Assessment of Block Efficacy
The patient should experience profound loss of sensation with preserved motor function. Apply deep pain to the digits to assess sensory blockade.
Complications
Systemic toxicity may occur, as with any other block, if the LA is injected intravascularly. Aggressive injection, especially with large volumes, may result in damage to small nerves secondary to hydrostatic changes. Use care when injecting, especially injection of nerves in closed ligamentous spaces, like the tibial nerve. Rare but self-limiting neuropathy may also develop, but usually resolves in three to four weeks.
References