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The Sciatic, an enigma wrapped in a riddle, smothered in chocolate

An opinion piece formerly posted on the site’s front page concerning difficulties surrounding the sciatic nerve block.  In it I explore the problems with using a nerve stimulator to locate the nerve bundle as well as some other points about using ultrasound instead for the block..

Find it Here

April 8, 2007

Revision April 9, 2007

 

The Complete and Final Neuraxiom Sciatic Nerve Block Manifesto

(at least at this point in time)

 

The sciatic nerve block represents the hardest block in the ultrasound guided nerve block compendium.  While it is the largest peripheral nerve that one will ever encounter while performing nerve blocks, it also seems to be the most obscure nerve in terms of localization.  The sciatic nerve can range from 1 to almost 2 cm in width, and at times be less than 3 cm below the skin surface, and even then still remain, at times, phantom-like in its ability to appear only vaguely present.  Indeed, the sciatic nerve, in at least the mind of this writer, has come to take on an almost mythical quality in its ability to rise from the murky depths, appearing from beyond the misty veil of ultrasound reflection on an everyday basis.  Like the Loch Ness monster, or the giant squid, or yes, even the great white whale of Melville -- “It tasks me”.

All of the other nerve blocks have yielded their secrets save this one... until now.  At least that is what I hope.

I hope here to describe a final, simple, easily understood, and nearly universally adaptable methodology for identifying, localizing, and driving a non-stimulating needle to the sciatic nerve for the purposes of surrounding it with your favorite anesthetic or perineural catheter.  There have been many ultrasound approaches described but in this work I would focus on only one, the posterior approach, at the level of the “Greater Trochanter to Ischial Tuberosity Line” (Plan A), starting sub-gluteal with a slightly more distal approach for backup, or Plan B  for those instances when Plan A. fails.

A brief but worthwhile diversion here -> I want to make it clear here that the techniques that I am describing were developed and practiced on a series of patients from the real world.  As the author of a website dealing exclusively with ultrasound guided regional anesthesia I know better than most, and that in general you choose pictures to post on a website that are specially selected to be clear in their content.  What I'm saying here is you don't show anybody the bad pictures, it would be very frustrating to look at ultrasound images in which it was very difficult to see what the author is describing.  I'm talking about the sciatic block here now.  While this makes for a very readable and enjoyable, and reassuring website, it is not what one encounters in the real world.  Depending on where in the world you practice the ultrasound images you see will be clear and well delineated only between 50 and 75% of the time.  The rest of the time you will have to struggle to produce an ultrasound image that is convincing enough to persuade you that you have localized the sciatic nerve.  This is one of the most frustrating parts of learning to use ultrasound guidance for nerve blocks, you think there's something wrong with you or your equipment that you cannot see what was so clearly demonstrated on a website.  If you know what I'm talking about here, let me assure you that there is probably nothing wrong with you or your equipment.  (That's a very comforting sentence.  Isn't it.)  The techniques described here are developed on patients from middle-of-the-road America.  Meaning, half of them weigh more than 100 kg, and probably a third of them weigh more than 130 kg.  So when I tell you that I have seen some ultrasound images that were difficult to interpret, you should believe me.  I have held off rewriting this portion of my website until I found techniques which I believe can be used in any patient.  At least that is my goal.

So let us begin.

Equipment

Whatever ultrasound machine you're using you must have a probe that can reach the depth of the sciatic.  In most cases this is between four and 6 cm below the skin.  Your ultrasound machine will have some method of allowing you to choose between the upper and lower frequency limits of the probe you are using.  For the most part you'll be wanting to choose a lower frequency as this will make viewing deeper structures easier.  A probe which can cover the frequencies between 2 and 6 Mhz, or something in that range, will probably work well for the sciatic.

The ultrasound machine should be positioned alongside the patient's bed or cart on the non-operative side about even with the patient side or knee.  The screen should be tilted so as to be clearly seen by you the operator.  The lights in the place that you are working should be dimmed to reduce glare and make visualization of subtle structures easier.

Have your medications drawn up already and clearly labeled.  Depending on your routine you may want to have your medication syringe attached to an extension tubing already with a three to 4 inch (80 to 100 mm) short bevel 20-22 gauge needle attached.  Have your antiseptic prep solution ready as well as some sterile ultrasound gel.

Place physiological monitors and administer sedation and narcotics according to your protocols.

Also, bring a towel.  This may will get messy.

Positioning the patient

Have the patient turn on their side, non-operative side down, operative side up.  Instruct the patient to straighten their lower, non-operative leg and to flex their upper, operative leg as much as they can at the hip and to a lesser extent the knee. Many patients at this point will tell you that they cannot bend their knee much, reassure them that you really don't care how much they bend their knee, you want them to bend their leg at the hip, as if they are sitting in a chair.  They all sit in chairs, and don't let them try to tell you otherwise.

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Illustration 1: Position of Patient for Right Sided Sciatic Block

Excusing the crudity of the above illustration, this is approximately what the patient position would look like for a right-sided sciatic block.

 

Plan A.

Finding the Sub-Gluteal Space

The first step in finding the sciatic nerve will be to establish the landmarks that delineate its depth, boundaries and position within the established plane.  The plane that we are establishing here is popularly called “the sub-gluteal space”.  It lies between the deep fascial border of the gluteus maximus muscle and the superficial border of the quadratus femoris muscle.  The sub-gluteal space contains a few small vessels and the sciatic nerve.

The sciatic nerve in the sub-gluteal space generally has a flattened look.  This may be because it is flattened or it may appear flattened because there are actually two nerves here, meaning that the common peroneal nerve may already be separated from the tibial nerve at this level.  The two nerves lying in close proximity may look like a single elongated or flattened structure.

The lateral and medial borders of the sub-gluteal space are formed by the greater trochanter and the ischial tuberosity.  These two structures are seemingly easy to find but I'd encourage you to do some homework on their locations.  By this I mean while you yourself are in bed, lie on your side, straight your lower leg, flex your upper leg at the hip as much as you can, then reach behind yourself with your hand place your middle finger (if, like I suspect, your right side is up) on your ischial tuberosity and your thumb on your greater trochanter.  Note the distance between the two fingers.  This distance does not change much amongst the various sizes of adult humans.  For instance, a 150 kg man is likely to have the same trochanter to tuberosity distance as a 70 kg man.  The same goes for women.

But no matter what the size of person and no matter how hard it is to palpate these structures beneath a thick layer of subcutaneous tissue, we have a secret weapon for finding our landmarks, the ultrasound machine.

The greater trochanter

First, place a goodly amount of ultrasound gel over the patient's, well, buttock. Next place your fingers over the approximate position of the greater trochanter and attempt to palpate this bony prominence as shown below.

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Illustration 2: Palpating for the Greater Trochanter

Next, position the deep ultrasound probe over the approximate position of the greater trochanter and observe the ultrasound image as illustrated below.

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Illustration 3: Surveying for the Greater Trochanter

As you know bone is very dense, and on ultrasound it generally shows as a totally echo reflective boundary.  This means that all of the ultrasound energy that you are directing toward bone will be reflected back.  This hard reflection will show on the ultrasound image as a very bright line with nothing showing on the far side of the line but darkness.  The ultrasound image below shows a greater trochanter as it appears on ultrasound.

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Illustration 4: Greater Trochanter on Ultrasound

In case it's not clear the trochanter is the bright, slightly curved line seen in the center of the picture at a depth of about 2.5 cm.  Note that since all of the ultrasound energy has been reflected there are no returns deeper than the front edge of the bone and so only darkness is seen below it.  In this image lateral is to the left, medial is to the right. 

The Ischial Tuberosity

 Next place your fingers over where you believe the ischial tuberosity to be and try to palpate the structure.  As you do this, remember the important lesson that you learned from your homework  (you did do the homework, didn't you) regarding the likely distance from the trochanter to the tuberosity.

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Illustration 5: Palpating the Ischial Tuberosity

Now move the ultrasound probe over the suspected position of the ischial tuberosity.

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Illustration 6: Surveying for the Ischial Tuberosity

Watch the ultrasound image for the telltale sign of the bony prominence.  An example of this image is shown below

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Illustration 7: Ultrasound Image of the Ischial Tuberosity

Illustration 7 above show the ischial tuberosity on the right side of the image is a bright, slightly curved line, once again totally echo reflective therefore stopping the passage of any ultrasound energy and showing only darkness below the white band.  Again medial is to the right lateral is to the left side of the image.

In illustration 7 we can see part of the sub-gluteal space on the left hand side of the image at a depth of about 5 centimeter.  It appears as a dark horizontal line approximately 1 cm thick bordered on the bottom by two bright horizontal lines.  But we'll talk more about that in a few paragraphs.

Centering the Probe over the Sub-Gluteal Space

Now that we have found the position of the trochanter and the ischial tuberosity, we have defined the boundaries, lateral and medial respectively for the sub-gluteal space.  In addition we have begun to define the depth of the plane for the sub-gluteal space

And so now we focus our attention in between the boundaries, directly at the sub-gluteal  space.  Place your fingers midway along a line drawn between the trochanter and the ischial tuberosity as shown below.

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Illustration 8: Locating the midpoint between the 2 bony landmarks

Many times it is possible to feel or even see the longitudinal groove that runs down the back of the side starting at the bottom of the buttocks.  This groove represents the valley between the origin of the semitendinosus from the ischial tuberosity and the biceps femoris long head.

I know, some of you are saying, “If I can see the groove in the first place why do I need to go through all of this rigmarole to find it?  Isn't this a bit of a worthless exercise?”  Okay, fair question. But see how I've lead you into this cunning trap.   The fact is, you can't always see the groove running down the back of the thigh.  And even if you can, you cannot be sure that you will be placing your ultrasound probe at the same level each time.  Why is it important to place the probe at the same place each time?  Ah ha!  So that you create the same ultrasound image each time.  So that you know what structures you will be looking at each time.  It keeps you from encountering an unexpected picture.  And it allows you to build up consistent experience more quickly when you can eliminate variability in the structures that you are encountering.  Besides, only by following the method can one become methodical.

So let's place that ultrasound probe over the midpoint where you have placed your fingers.

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Illustration 9: Surveying directly over the sub-gluteal space

In the majority of patients this will be the final position of the probe, in other words, you can get there from here.  There is one fine exception here, the first time you place the probe in this position, you may not be seeing all that you can see. 

Remember that we are using ultrasound to see with.  Think of it as if you are shining a flashlight at objects in a pitch black room.  Except in this case all of the objects have angular sides and they are placed at random angles to you.  Because of this light striking the object may or may not be reflected directly back toward your eyes.  In order to view the objects more clearly you may need to shift the position of either the flashlight or your eyes.  I think we've taken that analogy far enough.  What I'm getting at here is that, changing the angle of the probe slightly can yield a completely different, sometimes more clear, ultrasound picture of the area that you're looking at.

In order to do this leave the head of the probe exactly where it is and push the handle of the probe slightly away from you and then slightly toward you while observing the image to find the best angle for viewing the target, as seen in the picture below.

Illustration 10: Angling the probe to improve the picture

 

Hopefully, somewhere amongst these maneuvers you will see an ultrasound image that looks something like the one below.

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Illustration 11: Left Ischial Tuberosity & Sub-Gluteal Space

Or this...

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Illustration 12: Right Ischial Tuberosity and Sub-Gluteal Space

Notice on illustration 11 the ischial tuberosity appears as a slightly curved, bright line on the left side of the picture and below and to the right we see two parallel bright lines with a dark and spacing between, the dark space is the sub-gluteal space.

In illustration 12 above, we can see the ischial tuberosity on the right side of the image and we can trace the bright line, which represents the posterior fascial boundary of the gluteus maximus muscle, off the top of the tuberosity then down and to the left to a level of about 5.5 cm.  This is the upper boundary of the sub-gluteal space.  The sub-gluteal space itself is the darkened line below the fascial boundary bordered below by another bright line running parallel to the back of the gluteus maximus muscle.  This is the anterior fascial boundary of the quadratus femoris muscle.

Just to add additional orientation to the picture, along the upper border of the picture the darkened area which deepens to the left (lateral) side is subcutaneous tissue.  The next layer which is brighter and contains bright striations running more or less horizontal to the plane of its boundary is the gluteus maximus muscle.  If you've done this before you've probably noticed that in both of the above illustrations you can see the sciatic nerve, but we'll get to that later.

Exploring the Sub-Gluteal Space

The superficial boundary of the sub-gluteal space is formed by the deep fascial layer of the gluteus maximus muscle.  The deep fascial layer of the gluteus maximus muscle is stretched between these two bony landmarks.  This fascial layer hangs like a hammock pegged at one end on the greater trochanter and on the other end on the ischial tuberosity.  In other words, if you draw a straight line between these two bony landmarks on the ultrasound image, the fascial layer will hang slightly below this straight line.  As shown, in crude manner, in the illustration below.

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Illustration 13: The "Hammock" of the Gluteus

 Also shown in the above illustration are 2 yellow markers.  The light yellow oval on the medial side, next to the ischial tuberosity, represents a common position of the sciatic nerve.   The brighter yellow blob next to the greater trochanter represents the common position of a great distractor in this process.  This bright object which lies on the lateral side of the sub-gluteal space is often mistaken for the sciatic nerve.  It is probably the tendinous attachment of the quadratus femoris muscle and possibly some of the gluteal attachment to the femur as seen end on.  Do not be fooled by its siren song.  It will try to draw you away from the true prize which lies always more medial.

The sciatic nerve may appear in several different forms.  Most of the time it appears as flattened, but it may appear round or even triangular, as in a spear point.  But you will always find it on the medial side of the sub-gluteal space.  It usually appears as if it is dependent from the gluteal fascia layer bordering the top of the space.  I have come to think of the gluteal fascia layer as a hammock suspended on one end from the trochanter and on the other end from the tuberosity of the ischium.  I also find it useful to think that the hammock has a hole in it and that someone lying in the hammock would soon find their bottom hanging through the bottom of the hammock.  Hence the illustration below.

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Illustration 14: The position of the sciatic in relation to the other structures

With the above guidelines in mind, revisit illustrations 11 and 12 and see if you can spot the densities that represent the sciatic nerves.  Remember to visualize the sub-gluteal space and focus on the medial half of it, looking for a density which may appear to be hanging from the upper border of the space.

In keeping with this article's goal of helping you tackle real-world problems related to finding the sciatic nerve in real people, I would like to offer some tips which are useful in almost any scan.  Not every patient will produce scans of the same quality.  There is quite a bit of variability in the ways that tissues of different patients reflect and propagate ultrasound energy.  I can promise you that some of the scans you do will look like pictures from a book, a pleasure to see, making the placement of the block needle a very enjoyable and positive experience.  I can also guarantee that you will see some scans that leave you at a loss as to what you are looking at, as well as doubting your own skill level.  I hope that some the following tips will be tools in your kit that you can pull out and fix seemingly untenable situations.

  • In general, less gain applied to the image is better than more gain
  • Always try to follow the same survey pattern – find the trochanter, find the ischial tuberosity, place your probe in between, angle fore and aft, establish the target depth
  • If you cannot immediately find the sub-gluteal space using the method described above, try lowering the frequency of the probe you're using
  • If your machine supports focus points, move the focus points down a centimeter or two
  • Always remember to focus your attention more medial than lateral (you know where medial is because you can find the ischial tuberosity with the ultrasound)
  • Make sure your depth of scan is deep enough to cover the target depth
  • Un-focus your eyes, move the ultrasound probe back and forth over the target area, let your eyes pick out the subtle density of the sciatic nerve
  • If possible try repositioning the patient's leg into a more flexed position at the hip
  • You can try making various adjustments in the machine controls, such as; gray maps, edge enhancement, filters, total gain control, frequencies, focus
  • When all else fails reset all controls back to their defaults, start your survey pattern again

Remember that with this method, for all practical purposes you will know where the sciatic is as soon as you have established the lateral the medial borders of the sub-gluteal space and the depth.   You will just be trying to confirm the location with ultrasound before introducing your needle.

    A word about nerve stimulators and ultrasound guidance

    It is understandable to want to confirm your ultrasound guided localizations of the sciatic nerve by using a nerve stimulator before injecting local anesthetic.  This is most common when you are just starting to use ultrasound guidance. It's like having your older, trusted big brother watch over you as you try out this new technique.  It's supposed to help build you confidence.   BUT, I would like to speak about about something that you'll probably notice in the first few times you use ultrasound guidance and try to confirm with nerve stimulation especially in the sciatic. 

    Let me give you a common scenario that will illustrate this.  You have a relatively young person that you are going to perform a sciatic nerve block using this method.  You place the patient in the correct position, you apply a liberal amount of ultrasound gel, you establish the position of the trochanter and then the ischial tuberosity. You then place the probe over the midpoint and easily find the sub-gluteal space.  You  then turn your attention to the medial half of the sub-gluteal space and see, right where it is supposed to be dangling from the top border, the bright flattened oval which is characteristic of the sciatic nerve.  You introduce your nerve stimulator needle which is properly grounded, turned on, and set to 1 milliamps.  You watch your needle on ultrasound, real-time, as it approaches the density.  You feel a slight pop as you pass through the inferior fascial border of the gluteus maximus and you see your needle touch the target.  You glance at the patient's calf and... nothing happens.  You assume you are in the wrong place.  What is the deal here?  You thought ultrasound was supposed to lead you directly to the target. You physically feel your faith in the principles of the ultrasound technique weaken and quiver.  Where did you go wrong?  Should you start over?  Should you bag this ultrasound thing and go back to the nerve stimulator, which stills loves you and is willing to forgive you in spite of your dalliance with the ultrasound? 

    For the most part here the answer is no.  There is nothing wrong with your technique.  That is the sciatic nerve you are looking at.  And that's right, the nerve stimulator needle is touching it and nothing is happening.  What you should feel weakening and quivering is your faith in the technique of nerve stimulation.

    If you redirect your needle several times along the length of the bright target that you thought was the sciatic nerve you will eventually find an exposed motor fiber that will cause the calf muscles to twitch.  So what does this mean.  As far as I can tell what it means is this, you are looking for motor fibers to block sensory nerves.  If you were doing this block with only a nerve stimulator needle and no ultrasound, you would probably have passed the needle through the sciatic nerve in your search for a twitch not once, but several times.  But you would not have been able to tell because it didn't twitch and you weren't watching.

    The point of this is, after you have performed many of these ultrasound guided surveys of the sciatic nerve, your confidence in the technique should lead you to abandon the use of the nerve stimulator and deposit the local anesthetic around what you believe to be the sciatic nerve on ultrasound.  Then sit back and watch what happens, the leg will get numb.  So how would you have known whether you would have gotten a twitch on all of those anyway, the answer is you wouldn't, but who cares, the leg got numb any way.

    If this particular rant has raised your hackles in any direction, there is additional material in this vein currently parked on the front page in an article called,  “The sciatic, an enigma wrapped in a riddle smothered in chocolate”.  Have at it.

     

Other views

Occasionally, advantage can be gained by looking at the whole scene from a different direction.  Sometimes turning the probe 90 degrees, so that its plane lies longitudinal to the axis of the sciatic will provide a confirmatory view which can even be used to guide the needle to the target. The probe position is roughly as seen below.

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Illustration 15: Probe held longitudinal to the long axis of the sciatic

In the best of situations you will something like the screen shot below.

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Illustration 16: Longitudinal view of the sub-gluteal sciatic nerve

For those of you who are adherents to the “in plane approach” with the needle combined with a longitudinal scan of the nerve, this should offer the ideal opportunity to track the progress of the needle all the way to the target.  If you are having difficulty identifying the target or guiding the needle to it, this approach can offer advantages even if it is not your common method.

To this point we have been following the method described as Plan A.  Plan A is adequate for locating the sciatic nerve in the vast majority of the patients you will encounter.  Occasionally you will encounter patient in which Plan A yields indistinct results at best even after every adjustment that you can think of for both patient position and  the ultrasound machine.  When this happens it's time to engage the secret weapon; Plan B.   Plan B. works where Plan A fails because it uses a slightly different set of landmarks.  The essence of Plan B  is to find the sciatic nerve distal of the “trochanter to tuberosity” line.  This results in locating the nerve in the upper thigh.  It's not necessary to wait until you're completely stymied using the initial method to put Plan B into effect, you may, in fact, try Plan B after you have located the sciatic using Plan A just to practice the technique.

So, if Plan B works where Plan A fails; why not just always use Plan B?  Well, the answer is; you could... but (then Plan B would become Plan A,  a clear semantic complication) and more importantly, Plan B encounters the sciatic nerve further down the thigh and some key nerves may have already been distributed anatomically from the sciatic above the position of the Plan B block site.  In such cases you may have breakthrough pain at the top of the block in other words the mid to distal thigh.  Having said that, it's also just as likely that local solution injected around the nerve in a more distal point such as that found in Plan B, would travel proximal some distance and surround those nerves distributed earlier in the sciatic's course.

So without further ado, I give you...

 

Plan B.

Patient positioning

The patient is positioned the same way for plan B. as in Plan A.

Landmarks

The beginning landmark for the use of plan B. is the ischial tuberosity.  Place your fingers on the ischial tuberosity has shown an illustration below.

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Illustration 17: Locating the ischial tuberosity for Plan B.

Remember that when looking for the ischial tuberosity upon other people we tend to look too far south.  As mentioned above,  much can be learned by placing yourself in this position (in the privacy of your own bedroom) and palpating your own ischial tuberosity.

Once you've located initial tuberosity move your ultrasound probe, set at a depth of approximately 6 or 7 cm, to the place held by your fingers. 

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Illustration 18: Surveying the ischial tuberosity for Plan B.

Observe the screen and make sure you see the telltale sign of the ischial tuberosity on ultrasound.  As seen below.

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Illustration 19: Ischial tuberosity as seen on ultrasound for Plan B.

Keep in mind that when using Plan B. in a real-life situation in which Plan A did not work, the ultrasound image that you encounter will be far less impressive than the one you see in the illustration above.  Quite likely, in that case, you will see only the highly echo reflective slightly curved arc of the ischial tuberosity, while seeing no other discernible structures on your screen.  This is the reason you're using Plan B in the first place, right?  If you can see other structures Plan A would work.

Now with the ultrasound probe showing the ischial tuberosity, push the probe in a straight line down the leg along the medial border of the posterior thigh in a line straight down from the ischial tuberosity.  Your probe should end up in a position something like that seen in the illustration below.

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Illustration 20: Position of the probe distal to the tuberosity for Plan B.

You may notice in the illustration above that the hand has angled the ultrasound probe slightly toward the operator as it came to rest distal of the ischial tuberosity.  This is actually done on purpose as it usually results in the most advantageous image.  No matter, I would encourage you to watch the ultrasound image as you change the angle of the probe to the skin handle slightly away from you and then handle slightly towards you.  As seen in the illustration below.

Illustration 21: Angling the probe toward and away from the operator.

So what are we supposed to see

Now we are looking for the free range sciatic.  Well, it's not exactly free range, it's kind of trapped in between a lot of long muscles traveling down the posterior thigh.  It will usually be found, in a cross-sectional view, at the pointy end of a triangle.  The triangle at this level is made up mostly of the semitendinosus muscle, which is triangular in cross-section, having its broader base arising medially (as the muscle arises from the ischial tuberosity) and the pointy end pointing laterally.

As you look at the plan be ultrasound image, it's helpful to de-focus your eyes and look for a small triangle inside of a larger triangle.  The smaller triangle as mentioned above, is made up of the semitendinosus muscle (and possibly some semimembranosus muscle) is.  The larger enveloping triangle is made up of the adductor magnus muscle and the biceps femoris muscle.

At this level of the thigh the sciatic nerve can have several different appearances on cross section.  Of these, two forms are most common-- circular and “spear point”.

The circular form of the sciatic in cross section usually appears as a round hyperechoic (bright) structure with multiple uniform slightly darker spots forming a regular pattern within its area.  With regard to the brightness of the sciatic nerve cross-section, it may be subtly brighter or significantly brighter, depending on the angle of the probe to its reflective cross-section, and to its depth.

The “spear point” form of the sciatic cross section will usually appear as an elongated triangular extension of the triangular cross-section of the semitendinosus muscle.  The effect is of a bright triangular tip to the triangle.

Below are shown several scans illustrating this picture.  Remember that the broad side of the triangle pointing to the sciatic nerve will be medial.  To remember this, keep in mind that the semitendinosus arises from the ischial tuberosity which is of course medial itself.

The images below are rollover images.  This means that if you move your mouse pointer over them you will see another more explanatory interpretation of the scan.  Before doing that, try to see if you can spot the smaller triangle with the sciatic at the end it.

 

MOVE YOUR MOUSE OVER THE IMAGES BELOW TO SEE INTERPRETATIONS.

Illustration 22: Distal Sciatic Survey Practice Image 1

 

Illustration 23: Distal Sciatic Survey Practice Image 2

 

Illustration 24: Distal Sciatic Survey Practice Image 3

 

Illustration 25: Distal Sciatic Survey Practice Image 4

This finishes this section on locating the sciatic by Plan B, we will now discuss some issues that are common regardless of your chosen method of locating the sciatic nerve.

 

About the Injection

For this article I am purposely not spending a lot of time on how you get your needle to the target, what solution your injecting, or blow-by-blow discussion of how you inject.  I will however take a moment to talk about needles and injection technique here.

I think we've talked enough about using nerve stimulator needle with this technique to make it clear that I personally would rather see the use of nerve stimulators go by the wayside as soon as you have established your own comfort level in identifying the sciatic nerve.  As I probably stated above, I will say again here, nothing will increase your confidence in your ability to identify the sciatic nerve as much as taking that “leap of Faith” and just injecting the local anesthetic solution around the target, without stimulating, once you have reasonably satisfied yourself that what you see is indeed the sciatic nerve.  Remember that once you identify the subgluteal space you restrict your survey to the medial side of the sub-gluteal space, and then focus your attention on the upper side of that area, any density you can identify it is roughly the right shape inside WILL be the sciatic nerve.

I do not believe that it matters which method of needle approach you prefer, “in plane” or “out of plane”, as long as you're comfortable with it.

The needle size and style are likewise up to you and your technique.  You probably will start out using a 22 gauge short bevel nerve stimulator needle, about 4 inches long (100 mm).  I hope once you become comfortable with the ultrasound identification of the sciatic nerve and you stop using the nerve stimulator (at least on a consistent basis) that you will begin to wonder why you're using a nerve stimulator needle to perform these blocks when you are not using a nerve stimulator.  Good enough question when you think of it.  It is also somewhat liberating as you are then free to consider the use of other needles for the technique.  For example, a plain 22 gauge short bevel needle about 4 inches long is a good place to start.  Then maybe consider a 20 gauge Tuohy needle about 3 inches long such as one might use on a single shot epidural.  This can be quite a nice needle to use for sciatic block (and the femoral block as well) it offers a good rigidity for repositioning without bending, a good size cross-section for tissue deformation or spotting on ultrasound during the in plane approach. I'm not quite sure but it is probably even less expensive, even combined with a short segment of an extension tubing than a nerve stimulator needle.  As yet I cannot say if there is an appropriate catheter that can be placed through a 20 gauge Tuoh,y but if you're interested in placing perineural catheters then this may lead you to consider and 18 gauge Tuohy choose to  needle's

Whichever needle you end up using, you will, of course, need to be sensitive to the feel of it in your hands.  You should get used to the feel of tissue planes as you pass through them.

Once again I would like to emphasize that the hand holding the ultrasound probe should be braced against the patient to prevent it from moving.  Many people try to practice a no touch technique with the probe and allow their hand holding the probe to hover over the position.  This seldom works as it allows drift in the probe and subsequently in the image on the ultrasound screen.  This is common when attention is being diverted to the moving hand which holds the needle.  Therefore I strongly encourage you to rest the side of your hand holding the probe on the patient's body surface to prevent the probe from moving, and therefore your target image from changing.

Another key point is to move the needle tip under and into the plane of the ultrasound probe as quickly as possible after inserting it into the skin.  Be aware of the angle formed by the direction of the needle and the plane of the ultrasound probe.  In order to be successful in guiding the needle under ultrasound you must be able to track the passage of the needle.  After a little practice it should become clear that the smaller the angle between the plane of the probe and the needle, the longer the time the needle travel will stay within the plane of the probe.

If you are using the “out of plane” needle approach, keep your needle entry point close to the probe and the needle travel should remain nearly perpendicular to the surface of the skin.

Injection pressures

Much has been written and discussed regarding injection pressures, specifically high injection pressures being a indicator that you are, or maybe intraneural with your needle.  I think that the best it can be said of this is that the discussion is not over.  It is obvious that many things can create high injection pressures most of them having nothing to do with proximity to the substance of the nerve.  For instance, injection pressures measured at the syringe hub are much higher for smaller gauge needles than for larger gauge needles and that hub pressures are much higher before fluid flow begins out of the needle than before fluid actually begins leaving the end of the needle.  Syringe hub pressures are also much higher during rapid or forceful injection then during steady relaxed injection.  As for where the needle tip is; a needle placed into tendonous muscle insertion point, ligament, fascial layer, or dense connective tissue of any kind will present the same resistance to blunt dissection or infiltration by an injected fluid, as well or better than the substance of the nerve.

I worry that studies performed on rat sciatic nerves with high-pressure injection creating nerve disruption may be over-generalized to human nerves, and that pressure data determined by studies such as these may only provide a false sense of security regarding the problems of potential nerve injury as it relates to issues of injection.  I would urge you to keep an open mind on the subject and while working within the confines of this technique, to place more trust in reliance on the positive potential of visualizing the needle and the nerve, and the injected solution, to minimize or prevent intraneural injection and any possible damage that it could cause.

Summary

I hope this article is helpful to you.  It has been distilled from the experience of a couple hundred sciatics blocks, hundreds of hours of study and research, and countless conversations and emails.  I believe it to be, at least for now the most universally  applicable set of techniques I can come up with and I believe that after a little practice, the sciatic block will become one of the most enjoyable procedures you will commonly perform, not because it is easy but because it is difficult and you can perform it consistently well.

I would also encourage you to contact me if you have suggestions for improvements to the methods, and to help to make the forum a place where your worthwhile information and opinions can be shared.

Thanks for visiting Neuraxiom.

Jack Vander Beek

     

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