Continuous Popliteal Nerve Block: Lateral Approach

• Indications: Ankle and foot surgery
• Landmarks:
  • Popliteal fossa crease
  • Vastus lateralis
  • Biceps femoris muscles
• Nerve stimulation: Twitch of the foot or toes at 0.2-0.5 mA current
• Local anesthetic: 35-45 mL
• Complexity level: Intermediate

The sciatic nerve consists of two separate nerve trunks, the tibial and common peroneal nerves. A common epineural sheath envelops these two nerves at their outset in the pelvis. As the sciatic nerve descends toward the knee, the two components eventually diverge in the popliteal fossa, giving rise to the tibial and common peroneal nerves. This division of the sciatic nerve usually occurs between 50 and 120 mm proximal to the popliteal fossa crease.

An MRI image of the popliteal fossa at the level of the blockade (8 cm above the popliteal fossa crease). Note the position of the sciatic nerve between the biceps and semitendinosus muscles. During block performance, stimulation of the common peroneal nerve is usually obtained first (65%) because this nerve is positioned lateral and more superficial than the tibial nerve.

Popliteal block results in anesthesia of the entire distal two thirds of the lower extremity, with the exception of the medial aspect of the leg. Cutaneous innervation of the medial leg below the knee is provided by the saphenous nerve. The saphenous nerve is a cutaneous extension of the femoral nerve. Depending on the level of surgery, the addition of a saphenous nerve block may be required for complete surgical anesthesia.

The patient is in the lateral position. The foot on the side to be blocked should be positioned so that even the slightest movements of the foot or toes can be easily observed. This is best achieved by placing the foot on a foot rest. Attention should be paid so that the Achilles tendon is also protruding off the foot rest. This positioning allows easy visualization of any foot movement during nerve stimulation.

A standard regional anesthesia tray is prepared with the following equipment:

• Sterile towels and 4"x4" gauze packs
• Three 20-mL syringes with local anesthetic
• Sterile gloves, marking pen, and surface electrode
• One 1½" 25-gauge needle for skin infiltration
• A 10-cm long, insulated stimulating needle
• Catheter
• Peripheral nerve stimulator

The landmarks for continuous popliteal block through the lateral approach are essentially the same as those in the single-shot technique and include:

1. Popliteal fossa crease
2. Vastus lateralis
3. Biceps femoris

The needle insertion site is marked at 8-cm proximal to the popliteal fossa crease in the groove between vastus lateralis and biceps femoris.

The continuous popliteal block technique is similar to the single-shot technique. With the patient in the prone position, infiltrate the skin with local anesthetic at the injection site 8 cm above the popliteal fossa crease and in the groove between the biceps femoris vastus lateralis muscles using a 25-gauge needle. A 10-cm Tuohy-style tip needle for continuous nerve block is connected to the nerve stimulator (1.5 mA) and inserted to contact the femur.

Once the femur is contacted, the needle is withdrawn to the skin and redirected in a slight cranial and 30º posterior direction to the insertion plane in which the femur is contacted.

The needle is advanced slowly while seeking a plantar flexion or dorsi-flexion of the foot or toes. After obtaining appropriate twitches, continue manipulating the needle until the desired response is seen or felt using a current of 0.5 mA. After negative aspiration for blood, 35-45 mL of the local anesthetic of choice is injected.

• Stimulation at a current intensity of 0.5 mA may not be possible in some patients with long standing diabetes mellitus, peripheral neuropathy, sepsis, or severe peripheral vascular disease. In these cases, stimulating currents up to 1.0 mA should be accepted as long as the motor response is specific and clearly seen or felt.
• Occasionally, a very small (e.g., 1 mm) movement of the needle results in a change of the motor response from that of the popliteal nerve (plantar flexion of the foot) to that of the common peroneal nerve (dorsiflexion of the foot). This indicates an intimate needle-nerve relationship at a level before the divergence of the sciatic nerve.

The catheter should be advanced some 5-7 cm beyond the tip of the needle. The needle is then withdrawn back to the skin level, while simultaneously advancing the catheter to prevent its inadvertent removal. Before activating, the catheter is checked for inadvertent intravascular placement by negative test for blood and injection of an epinephrine-containing local anesthetic.

The needle is then withdrawn while continuously advancing the catheter to prevent accidental withdrawal of the catheter.

The catheter is left 5-10 cm beyond the tip of the needle. The inser-tion site is dressed and the catheter secured to the skin.

TIP: When catheter insertion proves difficult, rotate the needle slightlyand try reinserting again. When these maneuvers do not fascilitate insertion of the catheter, angle the needle in a cephalad direction before reattempting to insert the catheter.

Continuous infusion is initiated after an initial bolus of dilute local anesthetic through the catheter is administered. It should be noted that for initiation of the block, the bolus is administered first through the needle using a higher concentration of local anesthetic. For the initial bolus, 20 mL of 0.5% ropivacaine or 0.5% bupivacaine (l-bupivacaine) would be most appropriate. This is followed by a continuous infusion of dilute concentration of a local anesthetic (e.g. 0.2% ropivacaine). The infusion is maintained at 10 mL/hr or 5 mL/hr when a PCA dose is planned (5 mL).

• Breakthrough pain in patients on continuous infusion is always managed by administering a bolus of local anesthetic. Increasing the rate of infusion only is not adequate. With patients on the ward, a higher concentration of a shorter acting local anesthetic (e.g., 1% mepivacaine or lidocaine) is useful to both manage the pain and test the position of the catheter.
• When the bolus injection through the catheter fails to result in blockade after 30 minutes, the catheter should be considered dislodged and it should be removed.
• Every patient receiving continuous nerve block infusion should be prescribed an alternative pain management protocol because incomplete analgesia and catheter dislodgment can occur. For inpatients, this is probably best done using a back-up IV PCA.

After the initial stimulation of the sciatic nerve is obtained, the stimulating current is gradually decreased until twitches are still seen or felt at 0.2-0.5 mA. This typically occurs at a depth of 5-7 cm. At this point, the needle should be stabilized and after negative aspiration for blood, 35-45 mL of local anesthetic is slowly injected. The hands should be kept as immobile as possible to prevent injection outside the sheath of the sciatic nerve.