Popliteal Nerve Block

The “popliteal nerve” is the extension of the mid- sciatic nerve. But this term is a bit of a misnomer. Although the name does describe its anatomic placement in the popliteal fossa, the sciatic nerve has already bifurcated into its respective distal branches relative to the popliteal artery and vein.  According to Frank Netter MD, the sciatic nerve, keeps its name deep within the muscles of the posterior leg giving rise to the common peroneal nerve laterally, and the tibial nerve medially. 

So the argument can be made that is no “popliteal nerve”.  In adults, this bifurcation generally occurs about 7 cm above the popliteal crease. This block will cause in-sensation to the entire lower leg, just as a sciatic block would. The only region not affected by this block is a small strip of skin from the saphenous distribution. It’s important to keep this fact in mind when considering the proper block for tourniquet cases. In adults, the sciatic bifurcation generally occurs about 7 cm above the popliteal crease. The nerve stimulator guided popliteal block is generally described by the posterior approach. The  nerve stimulator technique is relatively simple, and can be used to block one or both distal branches of the sciatic nerve.  A limitation of the nerve stimulator technique for posterior approach popliteal blocks is that it will generally only stimulate one portion of the sciatic nerve. For example if the common peroneal distribution is desired for blockade, the needle may only find the tibial nerve to stimulate. If this stimulation is selected and injected, the desired distribution may not become insensate. There are a few different techniques described regarding popliteal block placement. The most popular is the posterior approach. This is done by first identifying the popliteal fossa posteriorly. The posterior knee is bordered by the two large tendons known as the biceps femoris (laterally) and semitendinosus (medially). These two structures create a frame around the popliteal artery, vein and nerves. The artery usually lies deep to the nerve, which follows most regional anatomic relationships. The vein is usually located adjacent to the artery. The procedure begins with identifying the popliteal pulse on the posterior leg. This is sought about 5-7 cm above the popliteal crease. After skin prep infiltrate about 5 mLs generously in the skin. This will allow for some (likely) repositioning of the needle. Begin by inserting the needle perpendicular to the skin and adjacent to the artery. Stimulation will be found for the common peroneal nerve with foot dorsi-flexion, or tibial distribution with plantar flexion. When the desired nerve is stimulated, reduce the stimulator output to 0.2-0.5 mA and maintain the desired stimulation. Reposition the needle if strong stimulation is elicited below 0.2mA. Deposit the local anesthetic. It is advised to reposition the needle to obtain the desired distribution because it is not guaranteed that the other nearby branch will have contact with the deposited local solution. 

Ultrasound guidance for the popliteal block

This block is generally described by the posterior approach, but there are different techniques described regarding popliteal block placement under ultrasound guidance. The patient can be scanned in any position (lateral, supine or prone), making it ideal for unique cases. Begin by preparing all equipment and select a high frequency linear probe set to about 3-5 cm. Begin scanning and image the popliteal artery at the popliteal crease. This should be done in short axis. Locate the nearby nerve (usually the tibial). Maintain the nerve in the center of the screen and continue to scan cephalad. Scan until the bifurcation of the common peroneal and tibial nerves can be visualized. The technique can be cumbersome due to the large tendons comprising the popliteal fossa naturally pushing the probe away from the targets. These two large tendons are the biceps femoris (laterally) and semitendinosus (medially).  Once the proper view has been located, introduce a generous dose of local solution at the skin and underlying tissue. Finally introduce the long block needle in-plane and manipulate the needle tip into the bifurcation of the sciatic nerve. Here small amounts of local solution can be deposited to create a complete circle of local anesthetic surrounding the entire complex. The patient will likely report some sensory changes. A peri-neural catheter can then be placed if desired. A catheter should be placed with the tip extending just beyond the deep surface of the nerves bifurcation to allow gravity to draw the nerve into close proximity to the catheter’s fenestrations.The catheter should then be anchored by skin glue, looping or tunneling, and covered by an clear occlusive dressing. Visit the galleries and movies on the following pages for details.

This video shows the sonoanatomy of the popliteal structures. It also demonstrated the importance of checking the immediate vasculature with compression. The failure to visualize flow through these vessels, or easy compression of the vein, should tip the provider off that pathology should be considered prior to performing the block, and more importantly, taking the patient to surgery. 

 

This image show the preferred patient position (prone) for placement of the popliteal block performed under ultrasound. The needle enters from the lateral side in an in-plane fashion. 

 

On occasion, a large cavernous sac can be appreciated behind the knee during a routine ultrasound scan for the popliteal block. Apply the Doppler mode to be sure of vascularity. If no flow is appreciated through the cavern, it may likely be a Baker’s cyst. These cysts can form in the popliteal region and become quite large. The orthopedist should be notified of your findings although the presence of a Bakers cyst rarely affects the procedure the patient has arrived for. Maneuver your techniques around the cyst and leave the cyst intact. Should it become too problematic, you may consider aborting the procedure. It should be noted that the presence of a Bakers cyst should not eliminate the patient as a popliteal block candidate. The initial scan should be able to determine if it’s presence will be problematic regarding block placement. The following image shows a large Baker’s cyst found incidentally during a scan for a popliteal block.

 

 

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