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The Anterior Approach

to the

Sciatic Nerve Block

 

Because the anatomical position of the sciatic nerve is more posterior than anterior the most commonly used ultrasound approach to the nerve is from the posterior.  At our institution the posterior approach has produced good results for reliable, easy reproducible and effective sciatic blocks but since the time allowed for performance of the femoral and sciatic blocks for knee surgeries seems to keep shrinking, a faster technique for sciatic blocks was desirable. 

The disadvantage of the posterior approach is that the patient must be re-positioned in the lateral decubitus position for the block.  By using an anterior approach for the sciatic, one prep could be used for the femoral and sciatic block procedures and they could be performed one after the other, with no repositioning necessary. 

There have been several published techniques for performing blocks of the sciatic by the anterior approach but these have been, for the most part, limited in their ultimate usefulness by their dependence on nerve stimulation for placement of the needle.  While these methods work on many individuals they suffer the same shortcomings of all nerve-stimulator dependent techniques.  Specifically, not all sciatics will twitch everywhere you touch them with a stimulator, and some sciatics, seemingly, will not twitch at all.

In addition, every pass you make with a stimulator needle that doesn't produce a twitch represents a possible pass through the sciatic nerve AND since there is no reliable relationship between the current producing the a twitch and the proximity of the needle to the nerve, any needle position which does produce a twitch could be within the body of the nerve (intraneural) for injection purposes.  So while intraneural injection does not always produce a clinically demonstrable nerve injury, it is a situation one should avoid if possible.  (It does however produce one heck of a block!)

For some time we have been working on different ultrasound techniques for the anterior approach seeking a safe, reliable, and technically simple procedure that would save significant block time.  The first 2 techniques created are based on visually identifying and following common anatomical landmarks to find the sciatic nerve.  The 3rd technique introduces a different and more unique method of locating the space in which the sciatic is confined.

In our institution the anesthesiologist sees the next scheduled patient after delivering the previous patient to the recovery room.  The anesthesiologist must meet the next patient, review the chart, interview and evaluate the patient while answering any patient questions and concerns, and formulate a plan, explain it and gain consent then perform the femoral and sciatic blocks in as short a time as possible, generally 10 – 15 minutes.  The room circulator generally arrives to meet the patient just a few minutes after the anesthesiologist, and reviews the chart while the blocks takes place.  The 2 blocks  generally are allotted only about 5-10 minutes total.  Because of this, having a technique that facilitates fast, reliable blocks is an absolute necessity in a successful regional anesthesia/pain control program.

In order to arrive at an acceptable anterior sciatic approach technique the method must have the following characteristics;

  • It must have minimum needs for prep and positioning
  • It should be usable regardless of patient size or condition as long as the patient can be supine and have their upper thigh exposed.
  • There must be at least 2 back-up survey techniques that can be used in case the first method(s) don't produce a usable picture or the individual patient situation demands a different tack.
  • The different survey methods should be able to be performed in any order as quickly as possible.
  • The technique must end with a block.  In other words, there should never be a reason to “bail out” on this method and re-position for another approach.  This method should never be a waste of time.

 

The Different Pathways of the Anterior Approach

This series of techniques is designed to give you a positive target for your injection, you may use it's pathways in any order, alone or together to find or confirm your target.  For the most part the technique you use will probably use will depend on what you see the moment you place the deep probe on the upper thigh.  If you can see the detail of boundaries and layers that make up the muscles groups of the thigh you will likely be able to pinpoint the sciatic visually with a little practice.  If you are unable to see detail in layers below the level of the top of the femur then you may well find yourself needing the unique features of the “Flashing the Adductor” technique.

Prep the upper thigh widely, from above the inguinal crease to about mid-thigh, covering a vertical line laterally extending distally from the anterior superior iliac spine (ASIS) and medially along the inner thigh.  The prep should cover the femoral nerve block as well as the sciatic.

Perform the femoral nerve block using the standard ultrasound method. (Femoral Nerve Block)

After finishing the femoral block, change the ultrasound probe to a deeper (lower frequency) probe.  This is usually a larger curved array probe with a range of between 2 and 5 mHz. 

Additional sterile gel is applied to the thigh 10 cms distal and 5 cms medial to the position of the femoral nerve at the inguinal ligament. 

After establishing right/left orientation of the ultrasound probe, place the deep probe transversely across the upper medial thigh.  Adjust the scan depth to about 13-25 cms.  Adjust the probe's position so that you can visualize the femoral vessels in the upper part of your scan screen and the femur toward the side of the screen about half way down the screen.

 

    The femoral vessels and the femur are 2 points of an anatomical triangle with the third point being the sciatic nerve which brings us to;.

The Big Triangle

This is the first (and it may be the only one you'll need) pathway we will discuss for identifying your target sciatic.

As it says above, the femoral vessels and the femur make up 2/3 of a large anatomical triangle which uses the sciatic as the 3rd point. 

illus_triang

Illustration 1. -- Position of the Big Triangle on the Survey

In general, it is not necessary to externally rotate the leg in order to see the sciatic in the upper thigh.  Having said that, there is normally some amount of external rotation that occurs in a relaxed supine patient, this is enough. 

As you have placed the probe over the medial thigh as described above,  the femoral vessels should be at the upper portion of the screen with the femur (mostly shown as artifact, but more on that in a minute) showing about halfway down the lateral side of the screen.  The position of the sciatic will be medial of and below the femur, straight down from the vessels.  Of course the exact location of the big fellow will not always be in the same place in everybody.  It will vary with muscle size and tone, habitus, yada, yada.

The vertical space between the femoral vessels at the top of the picture and the presumptive location of the sciatic nerve at the bottom is filled by adductor muscles, the pectineus, adductor brevis and addductor magnus muscles, with the largest of these being the adductor magnus (no surprise there, huh?).  These muscles will play a larger role in a technique to be discussed later (The Flashing Stop Sign).  This group of muscles which is bounded at the bottom by the adductor magnus form the roof over the plane containing the sciatic.   Meaning if you can see the bottom of this group, you know where to look for the sciatic.

illus_adducts

Illustration 2. -- Position of Adductors on Survey

 

Identifying the sciatic in the big triangle

In some patients you will look at the ultrasound picture and the sciatic will pop out at you as a bright, round, hyperechoic arrangement of small round fascicles, you will be able to visually guide your needle to a position alongside the nerve mass, inject and re-position the needle if necessary to obtain the best possible distribution of local anesthetic.  This is the ideal and with a little practice it will occur in about 1/3 of the legs that you survey.

In most of the rest of the patients you will only be able to see a few of the more obvious structures and the sciatic will remain hidden, and in some patients you will not be able to see much of anything below 3 or 4 cms. 

In order to find the sciatic you first need to know where to look.  The Big Triangle helps you to limit the area of your search.  The Big Triangle is made up the femur, the femoral vessels, and the sciatic.  The first 2 are easy to find and they point to where you will find the 3rd -- the sciatic.

First find the position of the femur and the femoral vessels then focus your attention on a point just below and medial to the level of the femur and on a line dropping down from the position of the femoral vessels.

illus_triang2

Illustration 3. -- Locating the 3rd Point of the Triangle

The geometry of the triangle can vary quite a bit among individuals, but in general the sciatic is located in a plane just deep to the bottom limit of the femur, transversely, just past the posterior layer of the adductor magnus.  This means you should not look very far beneath where the back wall of the femur would be if you could see it. 

If you can see the posterior border of the adductor muscles you will know the depth of the sciatic since it lies in the plane just below this adductor border.  This is seen in Illustration 4. below.

illus_triang3

Illustration 4. -- Using Adductor's Posterior Border to Find Level of Sciatic

At this point, if you can see the posterior border of the adductors, you have enough information to place the needle for the block. 

Pick an entry point for your needle that avoids the femoral vessels.  An easy method is to pick an entry point lateral of the femoral vessels and medial of the femur then plot course for your needle that directs it medially as it is inserted. 

Illustration 5. -- Proposed Needle Pathway

Visually guide the needle to a point just beneath the posterior border of the adductors and straight down from the femoral vessels.  When you think the needle tip is in the correct position, aspirate to insure that you are not in a vascular structure and then inject a small amount of the local anesthetic and watch for it's location on the survey screen.  If you can see the injection on the screen and it looks to be in the right place continue with your injection, aspiration sequence.  Generally speaking objects and layers become more clear on ultrasound after injection of fluid so the structures on the screen may come out the fog, as it were and the picture may make more sense after a bit of local has been placed.

During the injection it is a good idea to reposition the needle at least once during the injection.   A little deeper, a little more superficial, maybe withdraw the needle and re-direct it more medially or laterally and cross the posterior border of the adductors again and inject some more local.  This re-positioning is possible as long as you have visualization at least as good as at the beginning of the injections.  Don't hesitate to change the position of the ultrasound probe to see if you can gain a better view of the target, just be sure to hold the needle still while the probe is in motion. (either the needle or the probe can move but not simultaneously)

In the survey pictures above the sciatic is actually just below the yellow adductor border line centered at about 8 cms in depth.  Pass your mouse over the picture below to reveal the position of the sciatic. 

Illustration 6. --Pass your mouse over the picture above to see sciatic

Shown below is the same patient surveyed after injection of the local for the block.  Move your mouse over the picture to see the labels.  Notice the sciatic is showing as a brighter target just below the dark band of local solution. 

Illustration 7. -- Pass your mouse over the picture above to see structures

The Little Triangle

Identifying structures is the name of the game, so here we'll present an alternative way of localizing the sciatic which approaches from a different perspective.  The Little Triangle is the same anatomic structure discussed under the Neuraxiom page dealing with the posterior approach to the sciatic as “Plan B”.  The little triangle is the semitendinosus, semimembranosus, and the long head of the biceps femoris.  In the upper thigh these are arranged in a kind of triangular shape with the sciatic at its lateral tip.  Locating the little triangle is useful to either confirm the identity of the structure as the sciatic in cases where you already have tentatively spotted the target, or as a starting place during surveys that have yet not yielded a clue as to the location of our little friend.

The Little Triangle approach here involves moving the ultrasound probe back and forth, distally and proximally, along the long axis of the sciatic nerve and changing the incident angle of the probe to the skin slightly to try to hi-light this muscle group.

The Little Triangle will appear below the level of the posterior adductor border (therefore below the presumed level of the posterior side of the femur).  The base of the triangle (opposite the sciatic point) will begin medially (directly distal from the ischial tuberosity, which is the origin of these muscles).  The triangle is only roughly a triangle, it may appear with more bulging sides.

Below are some pictures with labels pointing out the Little Triangle structures.  Move your mouse over them to reveal the labels.

Illustration 7. -- Pass your mouse over the image above to see an example of the Little Triangle

 

Illustration 8. -- Another example of the Little Triangle

Flashing the Adductor

While many times your survey of the thigh will show you exactly what you are looking for, there will be times when your survey results will, let’s just say they’ll lack definition.    Don’t give up, we’ll get this block done yet.

Since we know that the sciatic lies just posterior to the adductor magnus muscle, but we can’t see the adductor magnus well, we need a way to make that adductor magnus stand up and wave its hands in the air to get our attention 

Enter Flashing the Adductor.

Flashing the Adductor uses a slightly different and unique method of positioning the needle.  In a nutshell -- we will use a nerve stimulating needle to directly stimulate the adductor magnus muscle and watch it twitch on the ultrasound image in a color doppler window.  We pass the stimulator needle slowly toward the area almost directly below the femoral vessels while watching the flashing on the color doppler window.  When the flashing stops, we know the needle tip has just passed beyond the posterior border of the adductor magnus and now lies in the plane of the sciatic nerve.  Smugly secure in the knowledge that the needle tip is in the right place, we inject the local. 

Now for the details.

Firstly, we can use this technique any time we are doing the anterior approach or we can switch to it when one of the other methods of listed above fails to show you the sciatic.  But as mentioned above, the best time to use Flashing the Adductor is when the lower half of the ultrasound image lacks enough detail to make out the posterior border of the adductor magnus.

Seen below is an example of an ultrasound survey of a right upper thigh which reveals little in the way of the detail needed to pinpoint the sciatic

Illustration 9. -- Example of Feature-Poor Survey Image

Look closely at the image and see if you can identify the femoral edge reflection and the femoral vessels then pass your mouse cursor over the image to see an interpretation of the features.

This type of image occurs occasionally when you doing ultrasound surveys.  It is hard to predict which patients they will occur in.  As far as I can tell it has nothing to do with patient size, habitus, or age.  It seems to be more closely related to general condition.  I have always thought that it occurred in patients who are not adequately hydrated, but I haven’t found a way to confirm that thought.  I do know that, in these cases, it has nothing to do inadequate gel, wrong probe, bad probe handling or wrong settings on the machine (even though these can cause a similar problem).  At least as far as I can tell. 

When confronted with an image such as this, resist the urge to begin plunging a stimulator needle about looking for the sciatic, instead let’s try slightly different approach with the nerve stimulator.

It’s Time to Flash

Began by plotting an entry point for the needle.  As was stated above, we should find a pathway for the needle that will miss the femoral vessels and any lower vessels that you may see on the ultrasound survey of the thigh.  The needle path must also, of course, miss the femur, although you will want to pass relatively close to it, say 1 cm.  You will be aiming for a point in depth roughly equal to the bottom of the femur and horizontally one to 3 cm medial of the medial border of the femur.

needlepathb
Illustration 10. -- Proposed Needle Paths Through the Adductor

For this technique,  a standard 21 or 22 gauge insulated, short bevel,  nerve stimulating needle is used. be sure and pick one that is of adequate length for the job at hand. As a matter of expedience, if you are expecting to perform a femoral nerve block at the same time, you may wish to make one large print area and then use the same needle for both of the blocks changing only the ultrasound probe between the two procedures.

Flashing the Adductor doesn’t really lend itself to an in-plane approach with the needle, mostly because the distance is great and control of a 21 or 22 gauge needle 6 or more inches long through many layers is fraught with misdirection problems.  You would also have to approach from the medial side most of the time to avoid the femur, we have performed the approach in-plane from the lateral side, over the femur, but it was a bit nip-and-tuck.

 Note that in this example the depth scale of the right side of the screen shows a depth of 15 cm.  This is an unusually thick thigh.  Most of the other survey images you see on the screen have a depth of 10 or 11 cm with the target being at about seven to eight.  In this example we can only infer where the bottom border of the femur will be, but it is likely at about 12 cm.  this means that the needle that you should use to perform this particular block should be at least 12 cm.  In this case 15 cm is probably a more usable distance because you will be approaching it at a and not straight down and also measured is made from the face of the probe which is probably indented in the patient’s sub Q. at least two or 3 cm.

After inserting the nerve stimulating needle through the skin on a path under direct ultrasound visualization until you reach a depth which is just past the top border of the femur.  Now turn on the color or power Doppler feature of your ultrasound machine.  Adjust the Doppler window so that it covers an area medial to the medial border of the femur to a depth just below the lower border of the femur.

Now turn on the nerve stimulator.  If possible adjust your nerve stimulators pulse width to above 0.1 ms.  A suggested setting here is 1.0 ms.  You should adjust the current to a point high enough to create direct stimulation of the adductor muscle as you slowly advance the needle.

Notice that in this technique we are not aiming to stimulate the sciatic nerve, instead we want stimulator settings at a level sufficient to directly simulate skeletal muscle.  A popular brand of nerve stimulator allows settings up to 5 milli-amps in current, pulse width of 0.1, 0.3, and 1.0 milli-seconds, and one or two pulses per second.  You may need the top level of every one of these parameters.

What we are looking for on the color Doppler window is a rhythmic flashing in time with the stimulator settings.  The flashing should take up most of the Doppler window as the machine displays muscle jerking from stimulation.  Make sure that you have the gain turned up high enough so the flashing will show.  If you see the muscle twitching on the ultrasound screen but no color is shown in the color window, the gain is not high enough. 

Continue to advance the needle slowly along the chosen path until the flashing suddenly stops. I want to say that again.

Continue to advance the needle along the chosen path until the flashing suddenly stops.

Illustration 11. -- Stimulating the Adductor Magnus Muscle Directly in a Color Doppler Window

When you have reached the point where the flashing stops, the stimulator needle tip has passed the posterior fascial border of the adductor magnus, your needle tip is in the plane of the sciatic nerve.  Aspirate to assure no blood return and inject the local anesthetic slowly.  If you encounter excessive resistance advance the needle a millimeter or 2.  If the resistance continues withdraw the needle a little with the stimulator still on and watch for the adductor twitch to start up again, then advance the needle  again. 

The important point above is to advance the needle slowly while watching the color doppler flash.  If you are pushing the needle too hard or fast it may move a significant distance between pulses (especially if you have your pulse frequency set to 1/sec).  The lower your pulse frequency, the slower you should advance your needle.

After injecting a few mls of local you should advance your needle another 2 mms or so, then continue your slow injection of local. 

If, for any reason, you feel like you missed the correct position for the needle while advancing it, withdraw the needle and begin the procedure again.  Even if you already injected local anesthetic this technique will continue to work because we are stimulating the skeletal muscle directly and not creating an action potential in a nerve which supplies skeletal muscle.

The Flashing the Adductor technique is applicable in most cases.  However, occasionally you may find that you are unable to make the adductor magnus twitch using your nerve stimulator and nerve stimulator needle.  We are, after all, performing an electromyogram with much lower current and stimulus settings.  Before giving up on this technique, I suggest withdrawing a needle trajectory slightly more lateral or slightly more medial to see if there section of skeletal muscle more amenable to stimulation.

About half of the time, just as you lose the flash on the Doppler, you will immediately see a sciatic twitch.  Because of the level and the direction you are approach the sciatic from, you may see stimulation of the of the hamstring muscle group instead of the calf  muscles, so you may not see the familiar foot movements.  The important thing is that you are in the sciatic space and it is the right place to inject.  If you do see evidence of a sciatic just back the needle off slightly and if you don’t start the adductor flash again you are good to go.  Inject the local.

Once again, remember that our goal in the Flashing the Adductor technique is to find the sciatic space not to find the sciatic, so resist the urge to go sciatic fishing.  Fewer needle passes means less tissue trauma, means less chance of collateral damage AND faster, better blocks.  Sound good?

 

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