The popliteal nerve block is a sciatic nerve block within the popliteal fossa with the patient in the lateral decubitus position or prone. The anatomy of the popliteal fossa allows ultrasound visualization of the sciatic nerve without interference from overlying muscle tissue. It is an ideal block for surgeries of the foot and ankle. It anesthetizes the same dermatomes as the other approachs to the sciatic nerve (Figure 43.3). The popliteal block preserves hamstring function, but all patients should be cautioned to avoid bearing weight on the leg for 24 hours.
The popliteal fossa is bordered laterally by the biceps femoris muscle and medially by the semimembranosus muscle. Within the fossa the sciatic nerve typically splits into tibial and common peroneal nerves (Figure 46.1). The needle should be placed just proximal to the splitting of these 2 nerves to avoid an incomplete block (Figure 46.2). Optimal needle placement for simulation blocks is usually 10 cm from the popliteal crease. A larger volume of local anesthetic (30–40 mL) is used to ensure adequate coverage of both nerves in case the nerve has already diverged. The sciatic nerve supplies motor innervation to the entire lower leg via the posterior tibial nerve and the superficial and deep peroneal nerves. The sural nerve is composed of branches from both the tibial and common peroneal nerves, but is sensory only. The sciatic nerve also supplies sensation to the entire lower leg except for the medial lower extremity, which is supplied by the saphenous nerve from the lumbar plexus.
Teaching Points. Vascular puncture and intravascular injection are rare with this block (the nerve is superficial to the popliteal artery and vein). For a complete sensory blockade of the lower extremity, the saphenous nerve must also be blocked.
The patient should be prone and the operative leg supported below the knee. The knee should be slightly flexed with the foot resting freely above the bed. If body habitus necessitates, the fossa can be accentuated by having the patient bend the knee against resistance. The borders of the popliteal triangle are as follows: the popliteal crease forms the base, the biceps femoris muscle forms the lateral edge, and the semitendinosus and semimembranosus muscles form the medial border (Figure 46.3). The needle should be inserted at a 45° to 60° angle to the skin in a cephalad direction and at least 7 cm superior to the popliteal crease and approximately 1 cm lateral to the apex of the popliteal triangle (Figure 46.4).
• 21-gauge, 10-cm insulated needle.
• 18-gauge, 10-cm insulated Tuohy needle for catheter placement. Catheter placed 3 to 5 cm beyond needle tip.
Set the nerve stimulator to 1.0 to 1.2 mA. Stimulation patterns are as follows:
• plantar flexion → posterior tibial nerve
• dorsiflexion → deep peroneal nerve
• eversion of the foot → superficial peroneal nerve
• inversion of the foot → tibial and deep peroneal nerves
Inversion of the foot leads to the best sensory and motor block, with dorsiflexion yielding the second-best block.
If the biceps femoris muscle twitches after needle insertion, the needle should be directed more medially. Conversely, if the semitendinosus and semimembranosus muscles twitch, the needle should be directed more laterally.
3.4. Local anesthetic
In most adults, 30 mL of local anesthetic is sufficient to block the nerves.
Teaching Points. Failure to elicit a motor response with initial stimulation should be corrected with more lateral redirection rather than medial redirection (increase risk of vascular penetration of the medially and deep positioned artery and vein from the nerve). Try to maneuver the stimulating needle as cephalad in the popliteal fossa as possible to attempt to block the nerve before it divides.
4. Ultrasound-Guided Popliteal Sciatic Nerve Block
4.2. Probe position
The best image of the sciatic nerve can be obtained in a transverse plane (Figure 46.5), yielding either 1 large or 2 small round, hyperechoic structures depending on the location of the sciatic nerve’s divergence. If the popliteal artery is seen, the nerve will be lateral and superficial to the artery (Figure 46.6).
4.3. Approach and injection
The in-plane approach allows for complete visualization of the needle. Place the probe parallel to the popliteal crease at a level proximal to the nerve division. Insert the needle from the lateral side of the probe. Pierce the sciatic sheath and inject 30 to 35 mL of local anesthetic. Needle repositioning may be necessary to surround the nerve structures with local anesthetic.
Teaching Points. Local anesthetic should be injected just proximal to nerve divergence. Scan the leg in the cephalad direction and look for the tibial and peroneal nerves to coalesce to form the sciatic nerve (Figure 46.7.). The popliteal block is performed in the same area as the lateral sciatic block but with the patient prone instead of supine. The common peroneal and tibial nerves can be separately blocked more distally; 2 separate injections are required however.