Popliteal Fossa Potential: How and Why To Set It Up

by | Mar 7, 2013 | Neuromonitoring Training

The Popliteal Fossa Response

I got an email the other day for someone looking for some help with obtaining a popliteal fossa potential…

Joe,

Thanks again for the help!  Had a question… with the Pop Fossa, what montage do you use for it?  I’ve never done them, nor been trained/taught how to get them and looked through 3 references and cannot find info on obtaining them.
Thank you!
I decided that instead of skimming over it in a quick email response, I’d do a more detailed reply. To start off, here’s what a popliteal fossa response looks like…
Popliteal Fossa Potential

Recording of the pop fossa

A Little Background Info

The popliteal fossa potential is commonly abbreviated PF and is the peripheral response of choice for most in the operating room. You could also use a lumbar potential (active electrode over the T-12 spinous process and reference over the iliac crest), but that’s either inconvenient or in the field for most surgical cases. You can obtain a response by either a referential or bipolar response, though I choose to do a bipolar recording setup.
The SSEP waveform of the pop fossa potentials is a triphasic wave, with your amplitude marker over the peak (a negative wave). This is a non-stationary response, so the latency will change depending on factors like leg length, conduction velocity, limb temperature, etc. As a general estimate, look for the response to come in somewhere around 9 ms. The afferent volley created by the electrical stimulation of the posterior tibial nerve travels through fast conducting, large diameter fibers. The recording is over the same nerve, so there are no synapses along this very short distance. That should tell you as to the reason why there is a very high signal-to-noise ratio in this response, and why very few averages are necessary to provide a very stable wave.

Set-Up The Popliteal Fossa Potential

Like other somatosensory evoked potentials for the lower extremity, you’ll set it up by stimulating over the posterior tibial nerve at the medial malleolus. The recording electrodes will be over the tibial nerve in the back of the knee (AKA popliteal fossa). The more distal electrode (or the one closer to the stimulating electrode at the ankle) will be your active electrode and the more proximal (or distant from the stimulating electrode) will be your reference. This distance between the two recording electrodes should be similar to the distance between your stimulating electrodes (a couple of centimeters).

Here’s a quick video on the anatomy of the popliteal fossa. Notice the surrounding muscles and how poor placement of the electrodes could cause recording problems.

As far as your filter settings, there’s a pretty wide range that you can use, but I try to eliminate as much noise from the operating room from the get go. My settings are the following:
  1. Gain: 10 uv/div
  2. HiCut: 300
  3. LowCut: 30

Troubleshooting Popliteal Fossa Response

Since this is a response that is directly over the nerve before any synapses, you should get a response in most of your cases. Here’re some things to try if you’re not getting a reliable response.

  1. Check to make sure you are plugged in correctly. Switching a left for a right is going to totally eliminate your response.
  2. Make sure your placement is over the tibial nerve. By the time it’s traveled up the medial aspect of your lower leg, it becomes pretty midline as it joins the common peroneal nerve to make up the sciatic nerve. That’s where your best window of recording over the nerve is located. The heads of the gastrocnemius, semimembranosus, semitendinosus, biceps femoris and popliteus muscles make up the border of the popliteal fossa, but also serve as a pretty good barrier. Lower your resistance by avoiding being over these muscles.
  3. Change from a surface to needle electrodes. Sometimes people have swelling or fat deposits that can hamper your recordings. Going subcutaneous can help eliminate some of that.

Precautions

Besides the universal precautions, you need to work with the nurse in the room with the placement of the bovie pad. The electric cautery will shunt current, and it will travel the path of least resistance. If it happens to travel to your electrodes (especially the subcutaneous needles), this could cause a burn injury to the patient. While this is possible at any electrode you place on the body, I’ve noticed it more here than anywhere else. You’ll want the nurse to place the pad up high in the back of the thigh/buttock (yet still out of the field) and not over a hairy patch of skin. They may need to shave them. The placement of the pad is their job, so only make them informed of the possibility of the burns and what seems to work best.

Clinical Use

Just like Erb’s point, the popliteal fossa potential is useful as a peripheral response to verifying that there are no technical problems from the machine to the nerve. If you can record a pop fossa response, then you know you have good electrical output.

It can also be used to determine the amount of stimulation to use. Some use the “twice motor threshold” (or about 6-10 mA X2) to set their stimulation while others might just shock the hell out of them (still within the guidelines, of course).  Instead of just cranking up the juice, you can tell how high you turn up your simulator by watching your pop fossa wave. When its amplitude stops increasing, you have met the maximal threshold.

Practice, Practice, Practice

When I was doing electrodiagnostic studies, I had the opportunity to zap a bunch of different nerves on a bunch of different people with NCV testing. I was able to get immediate feedback if I was on the nerve or not. It is a little bit harder to get that direct feedback in the OR, and I think most of us take the lazy route of increasing the stimulation intensity or pulse duration for our SSEPs. For those that have had to train without this type of feedback, there’s still a place to learn for free. It’s called youtube. Here’s a video showing stimulation sites in multiple areas. It might help you better understand where to stimulate to get something like a popliteal fossa response.

Closing Thoughts

The popliteal fossa potential is an obligate point, and you should be running it. Besides the information it gives you and proof that there is no technical error in stimulation, if you get called to court for the case (or use it on the DABNM exam), you will wish you had it there.

In the long run, could you run your cases without a pop fossa response and still have the same overall outcomes? Probably. Especially since most machines will tell you if there is a stimulation problem or not. But it’s not worth the risk, so CYA.

What do you think? Are you using them? If not, did I change your mind?

 

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