I think that this is the first nerve block that I have introduced on this website where I will not say “this is an easy block that you should start on early in your course of learning ultrasound nerve blocks”. Again, it will certainly come to you, but it is not the easiest to perform ‘right out of the gate’. This block requires, at times, several additional skills and a little more understanding to become proficient…and not pull out your hair and just quit. It is one of the very few that I will commonly use nerve-stimulation to confirm my needle position. At the same time, there are some fortunate anatomical circumstances and ultrasound cues that can actually ‘make up’ for some of the difficulties of this nerve block approach. Once you get this one down, it makes the set up for the combined femoral/sciatic block easier since the patient can remain supine, and you can do a single prep for both blocks. Read on, and I will try to give you some tips and tricks that will hopefully help you to continue after this sometimes very elusive prize.
As many people are getting into continuous blocks with the femoral nerve block for TKA’s, I will make several recommendations with this circumstance in mind. If that is the case, I will say that you will be looking to combine one of the easiest blocks with one of the toughest blocks. The bad news is that if you focus on this as your second block to master just because it is available, the progress will likely be slow and results will be variable for a more extanded period of time. The good news is that you likely will have many cases available to get more and more familiar with it (high repetition in a short period of time is one of the best teachers!), and once you get it, you are going to expand to other blocks like a house on fire. Further, there is low pressure to be ‘excellent’ at this block immediately since it covers such a relatively small portion of the pain for the TKA (if you nail it, kudos to you for the ‘bonus’, but if you fail, the femoral block was still the work horse…and this would’ve just been a ‘bonus’ not realized -not a ‘failure’ as such). Your basic choice here is to either (1) suck it up and work hard to incorporate this more difficult approach because ultimately it will be more convenient to set up or (2) incorporate an easier approach to the sciatic ‘in the meantime’ while you more slowly gain expertise in the anterior approach. See my Tip of the Week under the tab ‘THE ‘RIGHT’ WAY’ and the tabs above for the POPLITEAL and SUBGLUTEAL approaches to read more about this strategy.
There are a few reasons why this is such a tough block to master with ultrasound. First, the obvious one is that it is a ‘deep’ block. You really need a lower frequency curved probe to do this one well (but, yes, there are a few tricks that will help you to do it with a high frequency probe -or at least begin to become familiar with it – if that is all that you have right now). A deep block generally requires a steeper angle with your needle relative to the probe, limiting your view of the needle and requiring some tactile cues and sometimes a few tricks. Being deep (‘very deep’ with some of our patients), this also can make visualizing the nerve itself very difficult. Further, depending on exactly how proximal or distal you look, the nerve is surrounded by different muscles which can confound you if you are expecting one view and get one that is much different. The relative small size of the femoral vessels seen with the lower frequency probe set to 10 cm or so always bothered me as I started doing this block as I was always fearful that I would pierce them (or the femoral nerve for that matter) in my descent toward the sciatic nerve.
Beyond being deep, it can be very difficult to make the sciatic nerve respond to electrical stimulation, especially with a more blunt Tuohy needle which doesn’t penetrate the thick connective tissue covering and epineurium as easily as a sharper needle. So, though I said above that I utilize nerve-stimulation to confirm my needle position (stimulating the nerve AND directly stimulating muscles), I know not to be thrown off by a lack of confirmatory stimulation. I will utilize other cues and experience to confirm my location when the stimulation evidence is lacking.
Remember the concept of
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See Also: Ultrasound Tip #2, The ‘Right’ Way