Prone Approach
Have patient lie prone (see image below); place ultrasound probe in the popliteal fossa in the crease. Look for the pulsation of the popliteal artery; adding color Doppler may help. Superficial and lateral to the artery is the tibial nerve. Angling the probe in different positions (toward and away from the clinician, as well as clockwise and counterclockwise) may help with getting the ideal view of the nerve. Once the tibial nerve is identified, move the probe slowly proximally, keeping the same rotation and angle of the probe looking for the common peroneal nerve.
Supine Approach
Supine is similar to lateral. The landmarks are best visualized with the knee bent; this can be accomplished by placing the leg on a stand and a pillow at the calf or just having the patient bend their leg and prop the foot up with blankets or pillows. Straightening the leg can help identify the nerves and make the block easier. [6]
Above, PN and TN together. Below, TN and PN separate. PA=Popliteal artery, PN=Common Peroneal nerve, TN=Tibial nerve. Reproduced with permission.
The technique is the same in all 3 patient positions. Visualize the tibial and common peroneal nerve coming together as one moves proximally. The popliteal artery is deeper and soon can no longer be visualized. The idea of injecting the local anesthetic in the area around the nerves where the bifurcation occurs comes from the days of using the nerve stimulator. With the use of the ultrasound, which allows direct visualization of the nerves, one does not need to find the bifurcation, which can occur anywhere from the peroneal crease to the pelvis. With the ultrasound, the common peroneal nerve and the tibial nerve can be injected separately, making sure to get the local anesthetic circumferentially around each nerve. A single injection can cover both nerves if they are close enough together.
The preferred approach is to have the needle in plane so it can be visualized during advancement. Know which way the probe is oriented so you can see which way you are coming into the skin. Once the needle is in the correct location, slowly inject the local anesthetic (20–40 mL), aspirating every 3–5 mL to rule out intravascular injection. Watch for the spread of the local anesthetic around the nerve. Relocate, at least once, to ensure a good spread occurs.
Most of the popliteal blocks today are performed with the use of an ultrasound probe. Some facilities do not have the use of an ultrasound. Also, some providers that learned the blocks with a nerve stimulator still like using it as an adjunct to the ultrasound. Block needles are available that allow an attachment for the use of the nerve stimulator. Studies show no improvement in block completion is obtained with using both, even when the nerve stimulator is used to confirm that the nerve is what you are looking at by stimulating that area. [7, 8]
Nerve stimulation
Landmarks for the prone approach are the popliteal fossa, biceps femoris, and semitendinosus. Insert the needle 7–10 cm above the popliteal crease, 1 cm lateral to the point midway between the tendons of the semitendinosus and the biceps femoris.
For the lateral approach, the vastus lateralis and biceps femoris tendons are landmarks. The patient should be lateral, and the knee is flexed. The upper border of the patella is marked, and a line is extended laterally. The groove between the tendon of biceps femoris and vastus lateralis is marked. Having the patient flex the knee against the resistance on the calf muscle accentuates the groove. Place the needle at the junction of the groove and patellar line at about 7 cm proximal of knee crease. Advance the needle until it hits the femur, then withdraw and redirect 20–30° posteriorly while looking for foot twitch.
Basically, look for a foot or toe twitch at 0.3–0.5 mA. Start at about 1–1.5 mA and go down in intensity when a twitch develops. Look for inversion, eversion, plantar, or dorsiflexion. A plantarflexion response is more predictive of complete sensory blockade than a dorsiflexion response. A bit more local anesthetic is injected if it is not 100% clear if you are in the right location.
Approach Considerations
The tibial nerve after bifrucation from the sciatic moves down the back of the leg and wraps around the posterior medial malleolos. The common peroneal nerve passes posterior to the head of the fibula and winds anterior.
When performing the block make sure the nerves you are targeting are in the center of the screen.If the needle is difficult to visualize, make sure you are in plane and rock the probe until you get a better image.
Most practitioners use the ultrasound technique without nerve stimulation. Using ultrasound with neurostimulation has shown to be a better blockade than neurostimulation alone. [6, 7, 8]
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Sciatic nerve anatomy.
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Supine position
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Bent leg-supine approach.
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Leg on stand-supine approach.
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Above, PN and TN together. Below, TN and PN separate. PA=Popliteal artery, PN=Common Peroneal nerve, TN=Tibial nerve. Reproduced with permission.
