How to: Ultrasound Guided Popliteal Sciatic Nerve Block

Author: SonoSite

- The Popliteal Sciatic Nerve Block is a good block to do for the sciatic nerve because this is where the sciatic nerve is found most shallow. This block can be performed for post year knee pain after knee surgery, as well as, calf, foot and ankle surgery. This is the main nerve that supplies the lower leg. I find a lateral position gives you good control of the patients airways if you sedate them as well as good ergo-dynamics to do the block.

Here we have our patient positioned laterally with a pillow between the legs for comfort. To do the Popliteal Sciatic Nerve Block we use a linear probe. The linear probe allows us to see structures well in the shallow plane, but gives us a wide field of view to see our needle approaching the nerve. To perform the Politeal Sciatic Nerve Block we put the probe directly in the back of the knee. Placement of the probe in the back of the knee reveals a structure of the nerve, the vein, the artery. I call this structure, the snowman in the back of the knee. You have three circles on top of each other.

The upper most circle represented by the tibial component of the sciatic nerve. The middle structure represented by the vein and the deep structure represented by the popliteal artery. If we put color Doppler on the screen we are able to see the pulsating artery in this area. Sometimes it's difficult to see the popliteal vein because this is a low-flow state, such as we see here. To visualize the popliteal vein better we can squeeze the back of the calf, increasing venous flow through the popliteal vein, such as this. There we can see increased venous flow through the popliteal vein represented better by color Doppler. Now, as I mentioned, this is only the tibial component of the sciatic nerve.

We wanna get the nerve where it comes together and has both components, the peroneal and the tibial component. As we scan up the leg, here we now see the peroneal component laterally, the tibial component medially, the popliteal vein and popliteal artery. On the medial side we see the semitendinosus and semimembranosus muscles. On the lateral side we see the biceps femoris, long and short heads. As we keep going up the leg the tibial and peroneal components join into one nerve. Sometimes we block the nerve at this level where the two components are joining. This allows us to get both components with a single injection at the nerve.

How to: Ultrasound Guided Popliteal Sciatic Nerve Block

My needle approach for these blocks are gonna be from lateral to medial. What I typically do is measure the depth of the sciatic nerve in this setting. Here the sciatic nerve is approximately one-and-a-half to two centimeters deep to the probe.

So if I measure one-and-a-half to two centimeters deep, my needle is gonna come in completely perpendicular and have a very bright view of the needle and needle shaft as it advances to the nerve. Now, I typically like to inject deep to the nerve as well as superficial to the nerve in order to get complete surrounding of the popliteal sciatic nerve for the most rapid onset for this block. Typical volumes used are 20 to 30 milliliters of local anesthetic.

This block takes some time to onset and studies have shown even with complete surrounding of the nerve, it takes up to 30 minutes for the nerve block to onset in most patients. Here we see the popliteal sciatic nerve surrounded by the biceps femoris laterally and the semitendinosis/semimembranosis medially. Our needle is advanced directly above the nerve and the local anesthetic is injected, pushing the nerve deeper. We then withdraw the needle and readvance the needle again close to the nerve so we can get local anesthetic spreading to the medial side of the nerve, which is more of the tibial component. Now you can visualize both the peroneal and tibial component with a local anesthetic completely surrounding the nerves.

How to: Ultrasound Guided Popliteal Sciatic Nerve Block

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