April 13, 2007
The Popliteal Sciatic Block
The popliteal sciatic block is a useful block for pain relief of the distal leg, specifically the distal tibia and fibula, ankle and foot. It is easily performed with ultrasound guidance and can be put in place with a minimum of patient manipulation. For this technique you must have access to the popliteal space and the area just proximal to it. So when performing this on a fresh fracture or dislocation, the cast or splint must be just at or below the popliteal space.
Note that the medial portion of the lower leg receives innervation from the femoral nerve via the saphenous nerve and that this can extend around to the midline of the posterior side of the leg. Consider performing a saphenous block along with a sciatic component block if the need for pain relief extends into its area of innervation.
Position of the Patient
The patient must be positioned in such a manner that you can reach the popliteal space with the probe. When there is no fracture or the patient is comfortable assuming different positions, having the patient lie prone will give you full access to the target area. As shown in the illustration below.
Illustration 1. - Patient positioned prone
However many times, due to trauma or other considerations, it's not feasible to have the patient lie prone. In this case placing the patient in a lateral position with the operative leg uppermost will allow access to the popliteal space the target area. It is helpful new situations choose pillows, folded blankets, towels, etc. to bolster the patient's operative leg into a stable lateral position. As shown in illustration below.
Illustration 2. - Patient positioned laterally with pillow bolster between knees.
You'll need an ultrasound machine with a probe going to a depth of approximately 3 to 4 cm (an ultrasound probe in the 10 to 12 Mhz range is adequate for this). Your nerve block needle of choice with a length of approximately 2 inches. Local anesthetic of choice. A skin wheal. And if you intend to place a perineural catheter you should have that and the introducer needle ready.
Physiological monitor should be applied, and sedation and pain relief as per your protocols. Prep the skin according to your standard procedures.
With the ultrasound probe oriented correctly as to right and left, place the probe transversely across the target popliteal fossa, just at the crease. The depth of scan should be approximately 4 cm. Focus your attention towards the bottom of the ultrasound image and hold the probe very still. Watch for the appearance of the popliteal artery. If you are having trouble finding the artery use your power or color doppler.
Once you have located the popliteal artery in focus your attention just superficial of the artery and lateral to it. This is the position of the tibial nerve. If you're a fan of baseball, a good way to remember its position is “high and outside”. If you're not a baseball fan, you're still free to remember it using that phrase, or to become a baseball and then remember it using that phrase.
If you are having trouble finding the density which represents the tibial nerve “high and outside” of the popliteal artery, try angling your probe slightly towards you then slightly away from your, while focusing your attention on the target position of the tibial nerve. This angling is shown the illustration below. You may also want to rotate the probe slightly clockwise slightly counterclockwise in hopes of creating the ideal angle for ultrasound reflection of the structures below.
Illustration 3. - Angling the probe to gain a better view of the artery and nerve
When you finally have the correct angle you will see a picture to somewhat like the one shown below.
Illustration 4. - Scan of popliteal space showing popliteal artery and tibial nerve
The Common Peroneal Nerve
Once you have found the tibial nerve you should move the probe proximally, slowly. As you move the probe is try to maintain the same angle and rotation that got you the best picture of the tibial nerve. Focus your attention superficial of the tibial nerve. The common peroneal nerve is usually slightly medial or lateral to the tibial nerve largely dependent, I think, on the state of development of the “hamstring” muscles of the posterior leg. Look for a density that is slightly smaller (most of the time) than the tibial nerve, this will be the common peroneal nerve.
The common peroneal nerve splits from the sciatic nerve, usually about eight to 10 cm proximal of the popliteal space. The word “usually” and in the previous sentence should be interpreted as, “sometimes”, or even “once in a while”, because it will many times leave the sciatic as high as the pelvis, and the bifurcation could occur any time in between.
The illustration below shows the appearance of both the tibial and common peroneal nerve and their relationship to the popliteal artery. Please note that as you move proximally the relative position of the popliteal artery becomes deeper and will soon be beyond the reach of your scan as you follow the nerve proximally.
Illustration 5. - Image showing popliteal artery, tibial and common peroneal (fibular) nerve
Ideally (at least in some people’s minds) you will want to try to find the bifurcation of the sciatic into the tibial and common peroneal nerve and use that location to inject your local anesthetic. In reality, (in my opinion) this represents more of a “quest for truth” more than a method for a good nerve block (remember this step came from the nerve stimulator technique). Feel free to follow the nerves proximally for 8-10 cms and if you find the bifurcation then, absolutely put your needle there and inject the local. On the other hand, if you can’t find the bifurcation quickly, just inject the 2 individual nerve separately. Just make sure you pick a spot proximal enough to likely cover the area of the lower leg that you interested in rendering anesthetic.
What I’m trying to say here that in the ultrasound guided world you should question the steps of the nerve block as you learned it. Some of those steps may relate solely to adapting to the limitations of a technique which had different attributes (for instance not being able to see the target). In the ultrasound guided method, it would seem more prudent to focus on identifying both component nerves and then dealing with them either together or separately.
The illustration below shows surveying proximally of the politeal artery to find the common peroneal nerve or the bifurcation (if it’s handy).
Illustration 6. - Surveying north of the popliteal artery for the common peroneal and/or bifurcation
As you follow the tibial and common peroneal nerve proximally you may see them began to get closer together. Again, they will begin to get closer if you are nearing the bifurcation. Remember that the bifurcation could be some distance up the leg and even inside the pelvis. It should not be of great concern to you because even as you don't find the bifurcation you can still block the to nerves separately, or if they travel closely together, a single injection will probably surround both of them.
When you arrive at the bifurcation, if you arrive at the bifurcation the combined sciatic nerve will look larger than either of the two nerves individually. If you're satisfied with the level of the now single sciatic, then this is the place that you would want to introduce your needle in perform the block.
In the Illustration below you will the bifurcation of the common peroneal and the tibial nerves in the sciatic as the probe moves from proximal to distal.
Illustration 8. - Bifurcation of the sciatic into the common peroneal and tibial nerves
Guiding the Needle to the Target
There is not much to be said in this section that hasn't been said before elsewhere, but I will go through a couple things.
Your needle approach may be “in plane” or “out of plane” depending on your personal preference. I would encourage you, however, to aim for the side of the nerve instead of the center. The reason for this becomes somewhat obvious if you have ever directed a needle and a large solid nerve directly in the center, once your needle reaches the nerve it is sometimes difficult to determine whether the tip of the needle is within the substance of the nerve before you inject or, whether there is some intervening plane of tissue between you and the nerve. Also, this assumes that you can see the exact position of the tip of your needle. Many times your main indicator of where the tip of your needle is, is by watching the nerve move as you press against it with your needle.
Placing the needle at the side of the nerve (instead of the head-on approach) allows you to assess the deposition and spread of test injection of local anesthetic without having to worry exactly where the tip of the needle is. Since your injecting around a circular object, at least in cross-section, theoretically, injecting at one point in its circumference has no obvious advantage over any other point along its circumference. Furthermore, injecting your local anesthetic first on one side, then repositioning the needle to the other side and injecting more, seems a reasonable way to ensure that you are surrounding the nerve of local anesthetic.
As you may or may not be aware, I am generally in favor of phasing out the use of nerve stimulators when you are using ultrasound guidance (or really any other time) as I believe they add little in terms of identification of nerves on a consistent basis, and within that, the position of needles in proximity to nerves on a consistent basis. Having said this (many times) I understand that when you are first using ultrasound guidance for location of nerves in his reassuring to have a second opinion in the form of nerve stimulation, on the competency of your opinion in localizing nerves. So for the first few times that you performed the sciatic block in the popliteal approach, by all means, include your trusty nerve stimulator but... remember that not all nerves in all patients respond to nerve stimulation at all times. So after identifying the target nerve, and placing the nerve stimulator needle on the target, and receiving no muscle twitch, at least consider the possibility that you are right and the nerve stimulator is wrong. I know this goes against everything you have been taught, but you are still learning, right?
In any case, after performing this block several times using both ultrasound and nerve stimulation I beg of you, believe in your own competency and just in inject the local where you see the nerve, then sit back and see what happens. Performing this block successfully using only ultrasound guidance will do more to increase your competence and confidence than 100 blocks performed with nerve stimulator backup.
Don't get me started.
Injecting the Local Anesthetic
Once your needle is in position inject slowly and watch the spread of the local it should push the nerve and muscle boundaries apart. Keep your eye fixed firmly on the scene and reposition the needle is necessary or if you just want to improve your position.
I would encourage you to reposition the needle at least one time during the injection on any nerve block that you're doing under ultrasound guidance. As long as you can see the relationship between your target and your needle you can safely maneuver. This will improve the distribution of the local and prevent you from falling into the “All your eggs in one basket” trap. By this I mean, while you can see a lot on ultrasound, there will be times when a thin tissue septum may be present in proximity to the nerve. When you inject the local the septum acts as a divider (like a tarp) and direct the local into an adjacent area, away from the nerve. In the “Modes of Failure ...” section on the “Ultrasound” page there is a further description of this “Adjacent Compartment” mode of failure. Anyway, this problem can usually be avoided if you spread your local solution around just a little bit. I think it's also helpful when re-directing the needle to change you direction slightly so that your needle track is not always along the same parallel.
Of course, you should be sensitive to the amount of pressure it takes to make the injection. I've said enough about injection pressures elsewhere in this site (Sciatic page and Front page), so I'm not going to belabor the point here. Suffice to say that a good view of the needle and target should be enough to keep everything regular.
Slow injection produces a better spread of the local solution than a hurried, high-pressure injection.
The popliteal sciatic block also lends itself to being done without assistance by using a 20 ml control syringe with a needle that has no extension tubing.
The sciatic nerve component block at the level of the politeal fossa or just above is one of the easier blocks to perform under ultrasound guidance because it has all of the perfect conditions for the use of ultrasound; the targets are superficial in almost everyone, there is a great landmark very near the target (the popliteal artery), and the tissues surrounding the landmark and target have moderately different densities and reflective properties.
The block can be performed prone or lateral and with a short leg cast or splint in place. the block has great applicability because of the frequency or fractured ankles and distal tibias and fibulas, as well as orthopedic and podiatric corrective procedures.
After performing the popliteal sciatic component block under ultrasound guidance a couple times it can quickly become one your favorite blocks to perform and one you will look forward to.
Remember that while the majority of the innervation of the lower leg comes from the sciatic, there is some coverage of the medial side of the lower leg by the cutaneous branches of the saphenous nerve. This coverage can reach the midline of the lower leg posteriorly. Consider a saphenous nerve block if the operative site extends near the area of its innervation.
Thanks for visiting the Neuraxiom website,
Jack Vander Beek.