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Mark Schuenke, PhD is a Professor of Anatomy and Associate Chair of Biomedical Sciences at the University of New England College of Osteopathic Medicine. Mark has served as the anatomy and publications liaison for the Association of Extremity Nerve Surgeons since 2009, lending his expertise to their surgical courses, delivering lectures at their annual symposia, and contributing to original research. His other research interests include the anatomical considerations of low back pain and lumbopelvic stability. Mark lives on the coast of Southern Maine with his wife and four children.

Dr. Barrett 00:00

Introduction

Dr. Mark Schuenke 02:17

I was always fascinated with, you know, a physics actually, growing up, you know, I started out my academic career thinking I wanted to go into engineering. Actually, I did a lot of math, a lot of physics. But as I got further along in the coursework, I don't know, it just wasn't connecting with me. I did okay, grade wise, not stellar, but I wasn't sailing either. In it all the while that I was kind of, you know, kind of slogging along in it, I was a gym rat. And I love to be in the gym focused on, you know, how my body works, you know, how nutrition plays in with exercise, and all those different things. And so, I started to kind of explore ways that I could pair those two things pair, the kind of physics with the human body. And that led me into some kind of biomechanics and Exercise Physiology coursework that I did for my master's program. And while I was there, I got a taste for higher education, you know, for academia. And I really enjoyed teaching, I have a little bit of a backstory, we feel free to get it out if it's not good for your audience. But my parents are both teachers. And I came from a long line of teachers and my family and I always swore up and down, they'd never be a teacher, because they saw how much effort they put into things. And you know how little reward they got back from it from a financial standpoint. And so I swore up and down, I'd never be a teacher. And then I got into my master's program and started to teach just to make a little bit of money on the side. And I discovered, wow, this is really in my blood. I really love this. And so that led me to go on for PhD because I wanted to teach at a higher ed level, but I didn't have enough education to do that. Right. So, I needed a PhD. So, I went for a PhD in Biology and that's actually where I was introduced to anatomy on a deeper level, like, I knew anatomy enough to, you know how it worked with health and biomechanics, but I had never taken a gross anatomy course. And that was one of the first things I took when I was there. And I discovered that it's a really intimate way to get to know the human body and it's something that most people really shy away from. I mean, I understand that there is a grossness factor, you know, not in the gross anatomy sense, but in the disgust sense, and I can appreciate how some people shy away from it, but if you can get beyond that you can just appreciate the beauty of the body in a whole new way. You as a surgeon would appreciate that, you know, you'd need to do dissection to be able to appreciate the spatial relationships that are inside the body. And, and so that's how I first got introduced to the anatomy. And I realized that a lot of my classmates were struggling with the material, and I just had this natural affinity for it, and a natural kind of aptitude for it. So, I pursued that direction. I discovered that from a teaching standpoint, it's a lot of fun, because it's more interactive, it's more hands on, you know, you're working with just a few students at a time as opposed to lecturing in front of them all the time. I like to lecture too, but it's nice to switch it up. And that led me to seeking out opportunities to teach anatomy. And I knew I wanted to teach at an osteopathic school, because osteopathic schools just focus on anatomy a little bit more than the other allopathic schools tend to do. And that's how I ended up here in Maine at the University of New England College of Osteopathic Medicine.

Dr. Barrett 06:12

Now, it's great. Now, everything that's changed over the last 15 to 20 years, almost every industry, every subset of academia because of advancements, that type thing, a lot of people think about anatomy is this. Well, it hasn't changed much has it, but it really has, and a lot of these platforms now that are being used for teaching these 3d animation things. What's your thought process on that as far as education? Because some people really liked that some people just have great affinity for anatomy and other people who are very bright struggle like hell with it.

Dr. Mark Schuenke 06:51

I mean, I can appreciate those different modalities of teaching. But I feel like they are more kind of supplementary, I don't feel that they are adequate for learning the anatomy, especially at the the level that you need for like going into surgery or something like that. Or I would argue even other types of medicine, anything that's going to require a higher-level understanding of how the body is put together, and the anomalies and things like that. Because those programs just don't appreciate the variability there is from one person to the next. I mean, you need to appreciate the anomalies. And even if they're not true anomalies, there's just a variation in courses of nerves. I mean, you know, this from, you know, your surgeries, you know, the nerves, not always where you expect it to be. Or sometimes there are three branches where you'd expect there to be one and things like that. And you appreciate that in a way that you wouldn't get with, you know, computer simulation.

Dr. Barrett 07:57

No, I didn't mean to imply that I was looking at these platforms more as an adjunct to the real time spent in the cadaver lab, because you know, when you see the variability,

Dr. Mark Schuenke 08:09

Absolutely, yeah. And you know, and pathology and, you know, results of surgery, sometimes good, sometimes not so good. But you're right, they're fantastic for it adjunct, you know, that we have students, oftentimes with an iPad or some other tablet in the lab. Like looking at the simulation, while they're also working on their cadavers, and the ability to, let's say, just turn on and turn off layers. So you could just see the nerve course, and then add in, you know, the bones or the muscles, really is helpful for the students. You know, it's just, again, I would caution against using it as your sole method for learning the anatomy.

Dr. Barrett 08:51

No, I agree. I but I think there's you know, people have different methods of learning. And you know, some people are very auditory, other people are much more visual. But anatomy really is a very hands on thing and that I think what gets your appreciation as a surgeon is that yeah, you went through all of the you know, the gross and lower extremity anatomy and all of these things, but then you start seeing all of these things in vivo rather than in a cadaver. And then what you do get back to a cadaveric lab, which is a blessing because it just kind of brings everything together, conceptually, so I encourage people to, you know, continue trying to get to the cadaveric labs as much as they can, especially surgeons, because it just, you know, reinforces what you've observed clinically and intraoperatively for a period of time. Is there any like I know you're, you're fantastic at lower extremity anatomy. Of course, he's been hanging around with us at the Association of extremely nerve surgeons for longer than most people could tolerate. And I give me all the accolades for that. But is there any what's like one of your favorite areas of anatomy? Besides lower extremity?

Dr. Mark Schuenke 10:11

I love teaching the entire body at the University of New England, we do a full dissection course and there's not one part that I don't enjoy doing, there are some that are a bit of more of a struggle than others, particularly like the perineal region is challenging for the students to understand and so that that's a unique challenge as a professor to get them to appreciate the different spatial relationships in that tight little bowl of the pelvis. It's because there's so much, you know, pathology and atrophy, and surgeries that are that are occurring from down there before donors would come into our lab, because most of them are, you know, 60, 70, 80, 90 years old, so a lot of menopause and pathologies and things. But I think that the area that I probably enjoy teaching the most is in the head and neck, doing the cranial nerves and following them through the different foramina, and the skull, and then layering in the functions and helping students to appreciate how, you know, this one nerve, let's just say the facial nerve, for instance, has so many different components to it. And depending on where there might be a lesion, you need to, you know, play detective and figure out exactly, you know, what functions would be lost, you know, how is your patient going to present if the lesion is here versus, you know, if it's over here, is just really a lot of fun. And I think I would say that's probably my favorite.

Dr. Barrett 11:51

Well that brings a question up, you know, could we talk about neuro anatomy and the variability of neuroanatomy in lower extremity--it's all over the place? What about what about in the head and neck? Did the cranial nerves have a real high level of variability in your cadaveric experience? Or are they more?

Dr. Mark Schuenke 12:15

That's a good question. Yeah, I don't know that I've ever really thought that through. But now that you're, you're bringing it up, I would say that I think there's less variability, at least within the skull, you know, the foramina are pretty well set. Now there might be a shape difference in the foramina.  Or there might be a slight jog that you might not expect. But in terms of branching patterns, within the skull, it seems to be much more conserved than if we say, down by the knee and below, in the area that you'd like to play in. Yeah, it seems more consistent.

Dr. Barrett 12:55

We're going to talk about what I think is one of the biggest conundrums for people that deal in the lower extremity, regardless of specialty. And that is what's coming from the low back versus what's coming from the lower extremity. And you gave this presentation back at the annual meeting for the Association of extremity nerve surgeons in November of 2023. And, when I saw that lecture, it was all your lectures are great, by the way, except some of your animations with my head on it. But other than that, I think your presentation is fantastic. But when I saw that presentation said oh, we need to we need to get this out to more people than that were that rather than just the group that was there at the meeting. So we'll do a little screen share and you want to take us through anatomy of the low back versus lower extremity and I think it'd be great.

Dr. Mark Schuenke 14:02

Okay, so first things first, what are you seeing? Are you seeing full screen? Are you seeing the notes view?

Dr. Barrett 14:07

Now? I've got the full screen.

Dr. Mark Schuenke 14:09

Perfect. That's what I was hoping for.

Dr. Barrett 14:11

I have to I have to stop and commend you right now. You're the first guest that I've had that actually just popped it up and it worked magically so happy you obviously pay homage to the IT gods.

Dr. Mark Schuenke 14:26

Well, you know, we for over a year had to teach this way. You know, during the pandemic, we had to figure out how to teach anatomy exclusively via zoom, which I hope we never have to go back to but a lot of my Zoom experience has come from that as actually I'm sure most people have, but anyways, so you've got the image you can now see the laser pointer.  So, you know, over the past several years, as you've said, you know, spending time around, you know, people from the Association of extremity nerve surgeons as well as my med students and the medical community. You know, I frequently get asked how you differentiate between radiculopathy and peripheral neuropathy, you know, as I'm sure you know, radiculopathies and peripheral neuropathies can look and act very similar, but they have very different etiologies, and very different treatments. I mean, Steve, for you, and most of your colleagues, peripheral neuropathies are in your wheelhouse and radiculopathies are completely out of your scope of practice. So, we need to be able to differentiate between the two. And so, I put together this little overview and you know, share it with you down at Fort Worth last year. And I'm happy to do so again with you today. So now, I need you to recognize, of course that I am an anatomist by training so my patients are 100% compliant, they never complain. And so, I'm at liberty to take a different view of diagnostics, right. It's a low level, low cost kind of in-house anatomical approach. I'm sure any interventional radiologists that my listen to your show would have an extremely different approach from this. For me on the face of it, the difference between radiculopathy and peripheral neuropathy is simple. It begins with anatomy you know structure equals function. Besides as an anatomist you know if we don't start with the anatomy and I don't get to show off my cool dissection. So here we see a donor in the prone position. Obviously the skin and superficial fascia have been removed the trapezius and latissimus dorsi have been removed as well. You can see the ilealcostalis from the erector spinae, a muscle group kind of radiating out onto the ribs more medially there. The other para- spinal muscles have all been removed. And we've laminectomized the entire vertebral column. So, you can see this kind of tan midline structure right through here is the dural sac. And if we zoom in on that, here, you can see the spinal cord. It's still encased in dura. But if you look closely, you can make out different components that will contribute to the spinal nerves and I will highlight those as we move along. But if we first take a look at the microscopic level, you know, here we see a cross section through the spinal cord and the spinal nerve over to the side there. And if we zoom in on this little region indicated by the box, you can see a ventral horn, right that's home to those motor neuron cell bodies. And we can see a ventral horn down here which is going to be containing the axons from sensory neurons. You can also see, you know, the dura mater here which is going to come off the cord and protect the spinal nerve root as it becomes continuous further laterally with the epineureum. In the cadaver now, I've highlighted in green with the help of Adobe Illustrator, the ventral roots.  So, again, these are going to contain axons of alpha motor neurons that we just saw residing in the ventral horn headed down to you know, who knows what, flexor hallucis brevis or something like that. By contrast, here in yellow, now we see the dorsal roots. These are containing axons of pseudo unipolar neurons bringing sensory innervation from the periphery. So at the bottom of the screen, you can see the dorsal root. It's carrying axons of sensory neurons from their cell bodies and the dorsal root ganglion into the dorsal horn, right, if we magnify this region here, now, the histologic image can actually show us the dorsal root ganglia. So, this is the location of those cell bodies from the sensory neurons, right? So, pain receptors or touch receptors all the way down to the foot are going to come up through nerves, such as the sciatic nerve and bring those signals to the neuron cell bodies in the dorsal root ganglion here and from there, through the dorsal root and into the dorsal horn. Note also here that the ventral route is passing adjacent to that dorsal root ganglion without any interaction, there's a distinct separation there. On the cadaveric image, you can see the dorsal root ganglion right there highlighted in red, the axons of those ventral routes are passing just anterior to it. So again, this is a prone specimen. So, the posterior is closer to the screen and a little deeper into the screen would be the ventral side. If we continue a little further laterally, here now we see the spinal nerve, right so this is where those sensory nerve fibers from the dorsal root and those motor nerve fibers from the venture route are finally going to flow together into the same epineurium. So, in other words, we have a mixed nerve for the first time, the spinal nerves are going to pass out through the protection of the vertebrae through the intervertebral foramen. And that's where they might get entrapped by osteophytes or zygoapophyseal joint capsule hypertrophy, it might get entrapped, might get I'm sorry, might get entrapped, it might get irritated. They're the ones outside the vertebrae. Those spinal nerves will then split into dorsal and ventral primary Rami dorsal we see here in purple, ventral in orange, respectively, those dorsal primary Rami going to innervate the muscles and skin that develop posterior to the transverse processes. So, things like those paraspinal muscles, the ventral primary Rami will innervate the muscles and skin that develop anterior to the transverse processes. So, all the things in the lower extremity the things that we really care about, are going to be integrated by those ventral primary Rami. So finally, then here, we see all of those components from the roots all the way out through the Rami if any of these components seen here are compromised due to anything disc herniation osteophytes Z joint hypertrophy, really any cause is considered a radiculopathy. Okay, cool, all of these components contain neural elements from only one spinal cord root level and that is the crux of radiculopathy. Any lesion of the dorsal or ventral root, the spinal nerve, or the dorsal or ventral primary ramus is a radiculopathy and therefore radiculopathies might be sensory only, they might be motor only, or they might be mixed, but they're only going to have deficits from one spinal level. Now, in some places these ventral primary Rami will merge with ventral primary Rami of other chord levels to form a Plexus, that just means braid in Latin so you can imagine kind of the interweaving of nerve fibers from multiple cord levels. And unless I'm mistaken, much of your listening audience is going to be interested in structures downstream of the lumbo sacral plexus. So that's the one that we've highlighted here. So, on the right, we are seeing two dissections that collectively mimic the picture that's on the left there. The top right is a partial dissection of the lumbo sacral plexus on a hemisected specimen. Now you can see the bisected sacrum is right here. You can see some of those ventral primary rami of the sacral spinal nerves merge are emerging from the sacrum and converging on a plexus. They are unnaturally colored here using Adobe Illustrator again, and they are basically mimicking what we see in this image on the left here we're seeing the convergence of those sacral ventral primary rami into a Plexus, and then from there, we'll see it diverging into some different nerves. On the bottom right, you see a posterior view, and we're seeing things like the sacral tuberous ligament right here, the piriformis muscle right here. And emerging inferior to that piriformis muscle, we are seeing a couple of nerves that have just come out of the greater sciatic foramen. The one on the right, the larger one is the sciatic nerve. The one on the left, is the posterior femoral cutaneous nerve. Both of these, of course, are carrying multiple cord levels, the sciatic nerve, L4 to S3, posterior femoral, cutaneous nerve, S1 to S3. And you know, of course, virtually all of the nerves reaching below the knee or branches of the sciatic nerve. So, what we're going to do is think about how to tell if we're looking at a radiculopathy, somewhere in the L4 to S3 region, any one of those spinal nerves or nerve roots. Or if we're looking at a peripheral neuropathy of either the sciatic nerve or one of its branches, right could be anywhere from the piriformis to the distal foot. There are places where those different nerve branches might get entrapped. And so, this is where we get into diagnostics, which again means I need to remind you that I am an anatomist. So, I do things a little bit differently. Right, here comes your favorite part, Steve, when you are reaching for your little pinwheel, you know, I'm getting out my scalpel. Right when you are grabbing your EMG machine. That's what I'm getting out my bandsaw and by the time you're referring for a PET scan, I'm just reaching for the bourbon because you're waving I can't compete with that kind of technology. But if you're looking at a low will cost in office step one diagnostic, you begin with the anatomy. And conventional anatomic wisdom suggests you begin with a dermatome map. So, this is the area of skin innervated by The Sensory axons of a single spinal nerve, it theoretically should help us identify the radiculopathies. The problem is there's a lot of disagreement amongst dermatomal maps. As you can see, in those three classic maps on the screen there, you'll notice that there is a lot of variation there. But you'll find one of these three maps in most anatomical texts, most surgical texts. But there's just so much disagreement there. And we don't really have time to investigate those differences. But this chart here highlights some of the basic differences. So, I'll leave that for your reference. In 2008, I believe it was an evidence-based map was developed representing the most consistent tactile dermatomal areas, and leaving blank regions with where there's either considerable overlap or just too much variability. And this was followed up in 2011. By the international standards being set for specific sensory points for testing along those dermatomes. So, of interest to your listeners would be things like L4 is typically best tested at the medial malleolus. The L5 dermatome is most accurate on the dorsum of the third metatarsal phalangeal joint, S1 is most accurately tested at the lateral calcaneus. I should be noted that this same group also came out in about 2017, I think it was and conceded that that medial malleolus can be variable for L4. So that one is still a little up in the air, but those others are very consistent. Along with this, it's important to realize that adjacent dermatomes are going to overlap to a variable extent. So, there may be like no patch in the center of a dermatome with burning around the edges of the root distribution. And that tells us that, you know, dermatomes might be helpful to an extent but they aren't enough, right. So, we need to add something else. And so from an anatomical standpoint, what you would add on top of a dermatomal map would be Myotomes, those might be helpful. So here we see kind of a rudimentary list of Myotomal territories this was provided by the National Institutes of Health and not surprising, you know, the ankle dorsiflexors are attributed to L4, the L5 is mostly e-version and hallux extension, S1 is getting your plantar flexors. But much like the dermatomal overlap, very few muscles in the body receive motor axons from only one chord level, you know, most receive at least two certainly that's true of all of the muscles below the knee, as demonstrated by this chart here. And of course, those are the ones that you're interested in. Similarly, here we see a study in which patients were undergoing lumbar surgeries in which the nerve roots were exposed. And so, they took this as an opportunity to place EMG electrodes in the muscle groups that we're generally accepted to be innervated by those nerve roots on neural stimulator was then placed directly on the nerve root sleeves and EMG activity was recorded, and you're seeing the results on this slide here. Again, you can appreciate that even the tiny little abductor hallucis muscles are receiving motor axons from multiple segments. So, dermatomes and myotomes. Both have probably too much overlap to use as diagnostics for radiculopathies by themselves. So what else can we use to identify radiculopathies? Starting with a big picture of symptoms, you know, basically because dermatomal and myotomal overlap so much radiculopathies don't have focal symptoms. Typically, it tends to be more subtle manifestations down a dermatomal stripe, whereas peripheral neuropathy is will be more likely causing pain or weakness or possibly atrophy specifically in a distribution area of a single peripheral nerve like the deep fibular nerve or the medial plantar nerve. A chief complaint for patients with lumbar radiculopathy typically is going to be pain, it might be sharp, it might be throbbing, it might be burning. Generally, there is a lumbar pain component coupled with symptoms in the limb, such as those that we see on the slide here. Sensory nerves are more sensitive to compression and therefore in a radiculopathy those sensory symptoms are generally detected first and you can see on this slide, you know paresthesia and pain are amongst the most common symptoms if the pain is from a herniation, the radiculopathy as a herniation, you could perhaps provoke it further by asking your patient to bend forward or to increase their intra-abdominal pressure by coughing. If the pain is from a spinal stenosis, you might be able to provoke it by having your patient take a walk. So those are just kind of some generalities. So now, if we look at each specific level, L4 is where the lordosis is going to start to become most pronounced that lumbar lordosis. And so, it's usually here or below that radiculopathy is more likely to be due to herniation. And it's going to present with sensory symptoms on the medial leg kind of in the distribution of the saphenous nerve. Right? So, how do we differentiate between L4 radiculopathies or just a saphenous neuropathy? Well, first off, you know, if there is a motor component, a deep tendon reflex of L4 maybe either depressed or absent. But even better way to test it than that the test proven to be most diagnostic for an L4 radiculopathy is the single leg sit to stand test, which is what you're seeing in that gif on the lower left there. Because the quadriceps receives a large motor contribution from L4, this test is going to demonstrate weakness in the muscle group as well as an ilialpsoas. So as that wouldn't be there if it was a saphenous neuropathy. Now, obviously, it goes without saying that if the L4 radiculopathy is purely sensory syndrome is DTR. And the sit to stand test aren't going to show any deficits. So, in that case, your best bet is really determining whether it's a radiculopathy by asking them to lie down, which may lessen the symptoms, asking them to bend over which might exacerbate the symptoms. And beyond that, you can also look on the plantar foot, especially on the medial side, right, the saphenous nerve is going to just barely reach the plantar foot, if at all. But the medial plantar nerve also is going to have contributions from L4, So an L4 radiculopathy should impact the plantar foot to a larger degree than just a simple Saphenous neuropathy would.

Dr. Barrett 32:30

That actually is a very important clinical nugget. Would you say that again for everybody? Because I think this will help them clinically, right?

Dr. Mark Schuenke 32:38

Absolutely. Yeah. So, if you're differentiating between a saphenous neuropathy and perhaps an L4 radiculopathy, you need to really consider the distribution patterns, right. And I know I just spent a little bit of time seeing how the, the dermatomal maps are largely useful. You know, they're helpful to guide you, but that's not sufficient. I think in this case, it is particularly helpful to look at the peripheral nerve maps, which is what we're seeing here, this kind of colorful map provided by Dr. Gray, you know, some century ago, you can see in pink is the saphenous nerve distribution, and it comes down that medial leg, but really doesn't touch the foot much at all right? Meanwhile, the medial plantar nerve, which is getting a large portion of the plantar foot here is also going to have an L4 contribution. And so if your patient has some deficits or some pain or paresthesia on that plantar foot, it's likely to be L4 radiculopathy as opposed to being a saphenous neuropathy.

Dr. Barrett 33:50

Okay, that's a good point.  I would add one thing that I've seen clinically regarding the saphenous is that it does definitely go out towards the dorsal medial aspect of the first metatarsal phalangeal joint we've seen it injured a lot by bunion surgery. And you can come up proximally to the saphenous it's pretty easy to find and do a little peripheral nerve block and then all sudden that bunion or repaired bunionectomy patient, now no longer has pain, so that, yeah, I think that's a great point.

Dr. Mark Schuenke 34:24

You said that the dorsal side, right? And so, again, I would not expect it to go to the plantar side. But you're right that there is some variability on how far it reaches out onto the dorsum of the foot. In this map by Dr. Gray, you see that it really doesn't extend out very far onto the dorsum at all. But you're absolutely right that sometimes it will extend out further onto that first digit. So, if we contrast that L4 radiculopathy with an L5, the L5 is going to be the most common radiculopathy in the lower limb. It's quite commonly due to disc herniation. Again, because that's where that lumbar lordosis is the greatest and it will present with paresthesia along the lateral leg and antereomedial foot kind of along the distribution pattern of the common superficial and deep fibular nerves since, or peroneal nerves since I can't break you podiatrists have your old habits there.

Dr. Barrett 37:07

You know, I'm the biggest hypocrite in the world; I'm the biggest hypocrite in the world because I hate the way you guys change common peroneal to common fibular. But at the same time, I cannot convince the profession to get rid of the nomenclature of Morton's neuroma.

Dr. Mark Schuenke  37:26

Those eponyms are. And as you will know, many of your colleagues and many of my colleagues as well aspire to have things named after themselves. So, we can't really get rid of the old eponyms and introduce new ones. Right? It's tricky territory, but I just, I couldn't resist the urge to Jab you on that one.

Dr. Barrett 37:46

I just feel a little bit hypocritical, because I still like common peroneal. And part of that is because that's, you know, it's not just me but it's like an insurance company, or peer review, or, you know, billing and collections matters. And those types of things. It's like, well, they don't have the new nomenclature yet.

Dr. Mark Schuenke 38:10

I will say I was publishing with one of your colleagues last year, and your colleague wrote peroneal in there, and I thought about changing it. And I said, you know what, I'm just gonna let it ride. And then we send it in and the reviewers comments back said change peroneal to fibular, that's the old term.

Dr. Barrett 38:31

You see, that's funny, because I actually had a paper. Oh, I think it was maybe within 2023. And they said, you have to use a peroneal because that's the that's the nomenclature that's understood now rather than fibular. Oh, good. Yeah. It's like, all right. So, you know, just a little bit academic schizophrenia.

Dr. Mark Schuenke 38:57

Anyways, so yeah, the L5 radiculopathy oftentimes will be in the pattern of the common superficial and deep fibula nerves. And so it can be differentiated from the those peripheral neuropathies by additionally having radiating pain from the buttock down the lateral thigh, you know, they will often exhibit weakness, oftentimes in the extensor, hallucis longus and in the lateral compartments. Less commonly, they may also have weakness to inversion and digital extension, as well as ankle dorsiflexion. In chronic cases, you might start to see some atrophy here, especially in the extensor digitorum brevis, and possibly the tibialis anterior, they may even have weakness to thigh abduction. So, if the gluteus minimus and gluteus medius are involved, which both get a decent L5 component, you may actually want to have your patient perform a Trendelenburg tests right where you have your patient, stand on one limb and look to see if the hip drops. If that hip drops, that's indicative of there being some involvement of either the superior gluteal nerve, or L5.  Obviously, if there's no deficits down in the area below the knee, you'd be looking more at the superior gluteal nerve. If there's involvement below the knee, as well as a positive Trendelenburg, then you are thinking more L5 radiculopathy. Furthermore, you may try to elicit a medial hamstring reflex, you know, so tapping on either the semimembranosus or semi tendinosis tendons will elicit a reflex and you can look for asymmetry of those reflexes, an asymmetry would be suggestive of an L5 radiculopathy. And finally, a straight leg test as shown in that gif on the lower left there should elicit pain in about the 30 to 70-degree range if a herniation is present. And to be clear, the straight leg test is not specific for L5. But again, L5 is the most common disc herniation. So, it may be predictive, if you suspect your patient might have a radiculopathy. Moving to S1, that S1 radiculopathy is going to radiate to the posterior thigh and leg in the distribution shown in kind of green and brown and that blue there. And commonly it's gonna have paresthesia to the lateral and plantar foot mostly along the sural and plantar nerve pathways, S1 has motor routes throughout the entire posterior chain. So, you may detect paresis anywhere from gluteus maximus, biceps femoris, gastroc, soleus, flexor hallucis longus, plantar intrinsic foot muscles. That S1 deep tendon reflex measured at the Achilles tendon will be diminished. So that calcaneal tendon reflex that we see in the gif there, something to look for. But this could also easily be confused for a tibial neuropathy, right. So, to differentiate there, it's important to look for paresis proximal to the knee. It's also useful to note that even though dermatomes have a lot of overlap and inconsistency that we covered earlier, that S1 dermatome has been shown to be one of the most reliable. So, watch for paresthesia on the posterolateral leg in order to help rule out a tibial neuropathy. Next, I didn't really make a separate slide for S2 or S3 even though S2 does definitely have involvement all the way down to the foot. But you know an S2 radiculopathy should also have perineal involvement not peroneal, but perineal. So, bladder control or external anal sphincter function may be impaired.

Dr. Barrett 43:00

So, now let's do you notice I didn't even take that soft ball and hit it? It took a lot of restraint on my part Mark to not hit that thing out of the park. But we'll save that for a glass of wine later for sure.

Dr. Mark Schuenke 43:33

That sounds good. So, if we flip this around now and look at the presentation of common neuropathies in your treatment areas, nearly all of them are going to be branches of the sciatic right. And so, we'll address individual branches of the sciatic in a bit. But first, let's start with the entire sciatic. So, that could be entrapped at the piriformis, it could be damaged by the hip dislocation, or it could be damaged by intramuscular injection. Generally speaking, if the damage is from an injection, motor fibers are more likely to be compromised. And those motor fibers of the common fibular nerve are more susceptible than those of the typical tibial division just due to their more lateral position in the nerve. Actually, as a matter of fact, the sciatic nerve damage of any origin is estimated to be almost three fourths of the time I believe I believe it's 75% of the time, it's more likely that the common fibular nerve is going to be involved in his tibial.

Dr. Barrett 44:41

Why do you think that's the case just because of anatomical location. Is there a vascular component to it?

Dr. Mark Schuenke 44:49

I don't think it's vascular. There are a few different factors that I believe play into it. And then one is that if you look at the mapping of the nerve roots. The well, I guess this is kind of let me let me address that secondarily. First, I think there is a common anomaly in which the sciatic nerve comes out partially through the piriformis muscle. So here we see the piriformis muscle. I forget the exact percentage, but I feel like it's maybe in the 15% range, maybe we can check that before this airs. And if I'm dramatically off on that, we can put a little disclaimer in there.

Dr. Barrett 45:38

I think it's, I think you're spot on there, because I was, I was looking at variability of the sciatic while back and there are I mean, it, it can have a high division of the common and the tibial, even before it gets to the piriformis, I forget the percentage of time and then one branch can pierce through the piriformis. And then the other branch can be inferior to the piriformis.

Dr. Mark Schuenke 46:04

And that's exactly where I was going.  It's the common fibular branch that will go through the piriformis. And if that's the case, every time that piriformis muscle contract, it's going to be clamping down on that common fibular nerve, which, you know, certainly will lead to your more deficits being shown in that common fibular division. In addition to that, it's just more laterally located, the common fibular is more on the lateral side of the nerve. And so, it's at greater risk, it's not as well protected. Just, it's more exposed, again, to intramuscular injections to contusion, you know, it's just at greater risk.

Dr. Barrett 46:55

Could some of it be to the fact that distally, it appeared stuck, and correct me if I'm wrong on this but distally, it appears that the tibial nerve is a little bit less tethered than the common fibular nerve at the fibular neck. You know, and it's, it's so if you have a think about a piece of rope, right, that maybe was split into two pieces, distally. But one has a more freedom of excursion and then the other, you could maybe assign some of that to that phenomena, just that mechanical tension issue.

Dr. Mark Schuenke 47:32

I mean, that makes sense. Yeah, definitely is true in my experience of the dissections that it is more tethered. And actually, I should say that the two theories that I was throwing out there were specific to the gluteal region, but definitely when you follow those nerves further down, yeah, that that common fibular nerve is is at much greater risk as it's wrapping around that fibular neck, right? I mean, it's, it's very superficial, you know, any contusion to that side, certainly, any fracture of the fibula is gonna put that nerve at much greater risk, whereas that tibial nerve is running right down between the, you know, the heads of the hamstrings, and then it goes right through that soleal sling. And so, it's well protected through much of the posterior lower leg, you know, really only becoming exposed again, when we get to that tarsal tunnel. So, yeah, you know, if if I were choosing to be either the tibial nerve or the common fibular, I would go tibial every time it's just much safer that way.  So, it's also kind of interesting to note here, that they've begun to map the route contributions. And it appears if you look at these images on the left here, that the L5 roots take a more anterolateral position, whereas S1 takes a more posterior medial location. And so, you know, again, when we're thinking about radiculopathy, versus peripheral neuropathies, and specifically thinking about, you know, this sciatic neuropathy, you know, thinking about the positioning of those nerve roots, can really help make sense out of how the, the neuropathy might be manifesting. So, if the entire sciatic is compromised, you could expect to see paresthesia or paresis really down the entire posterior of the extremity. If just the common fibular portion of the sciatic is damaged, it's going to look very much like a common fibular neuropathy. So, it's going to be you know, a lot of weakness and atrophy. And to differentiate it from a common fibular neuropathy, this more specifically to differentiate a sciatic neuropathy. Yeah, that's kind of more proximal to a common fibula neuropathy that is just at let's say the fibular neck, you know really your  best bet is going to be to look for damage in the posterolateral thigh look for paresis look for paresthesia there. But again, that's less likely than damage at the fibular head or neck region. So, let's go there next. I mean, as I just said that fibular head and neck region is the region where that common fibular nerve is most commonly damaged just because of just how superficial it is there right, so in this image here, we see the tibia here, fibula right here, here it is that that fibularis longus muscle being peeled back. And you can see that common fibular nerve wrapping right around the neck there before it branches into its various branches. And so, it's very susceptible, right lesions of that common fibular, are going to present as you'd expect, right paresthesia, the lateral leg and the dorsal foot paresis of the ankle and the toe dorsiflexors as well as the foot evertors. It is important to note that lesions of this fibular head and neck region commonly only are going to involve the fascicles of the deep fibular nerve. And again, that's just I think, because of the positioning of those fascicles within the common fibular. It's also important to note that when the entire common fibular nerve is affected, it's those deep fibular nerve branches that are going to suffer more profound motor deficits. Specifically, that extensor hallucis longus is preferentially going to be affected. So, you might see hallux drop as opposed to true foot drop there.

Dr. Barrett  51:44

Let me interrupt you there, because I think that's a really important clinical nugget for people that are watching this because the EHL is by far the most commonly affected and most early affected muscular innervation. If there's a common fibular injury, and they can have almost completely five out of five motor strength for tibialis anterior and extensor digitorum longus. But they may have a one or two out of five EHL. And so, it's a really great muscle to test. Because it's like the canary in a coal mine. Do you think that deep fibular is more affected to the EHL just because it has to course through a couple more septa to get down to it?

Dr. Mark Schuenke  52:36

That would be my guess. Yeah, I mean, it'd be interesting to, to do a dissection of that deep fibular nerve, specifically, keeping an eye on the different motor branches. But yeah, just the additional things that it has to pass through. And oftentimes those septae can be pretty tight would be my guess as to why it's going to be more affected than the tibialis anterior, it's also probably getting its muscle fibers a little bit lower a little bit more inferiorly or distal today.  That's a good point. So, you know, if we break down this common fibular nerve into its terminal branches, I use that word terminal loosely because we know that, you know, the deep and superficial fibular do have their own branches further distally. But you know, we kind of already talked about this in general manifestations of the superficial and deep fibula neuropathies but again, the deep fibular neuropathies at the fibular neck, may lead to weakness and atrophy in the anterior leg, you know, notably those extrinsic dorsiflexors. But neuropathy is of either the fibula or sorry, neuropathies at either the fibula neck or at that extensor retinaculum would mostly impact the dorsal intrinsic muscles. And I should point out that in about 12% of the population, we find an accessory to the fibular nerve, which branches off the superficial fibular and that's going to go to that extensor digitorum brevis. So, in that case, that extensor digitorum brevis wouldn't be spared, and it could actually confuse your diagnosis. So, you need to be aware of this variation. It's possible that you have a neuropathy of the deep fibular but no involvement or at least not discernible involvement. Not that extensor digitorum brevis

Dr. Barrett 54:40

Where does that branching happen?

Dr. Mark Schuenke 54:51

I would have to review the literature. I would guess it's actually after it comes into the subcutaneous space right After, after the extensor retinaculum it would kind of pop off and it gives that cutaneous component. And my guess is that there's the motor branches in there because that extensor digitorum brevis is, you know, muscle bellies, right on the dorsum of the foot. And so, it makes sense to me that those fibers as they're going cutaneous Lee would also just carried some, some fascicles that are going directly into that muscle.

Dr. Barrett 55:33

That was just now that was just clinical observation and some anatomical dissection. So we we may I make that assumption and activity completely erroneous, but it appeared that, that that was the case. So I think it just reinforces the point that, you know, because of the difference in neuroanatomy, it can mess up your clinical diagnosis, sometimes.

Dr. Mark Schuenke 56:45

No question. No question. They're always anomalies. And then that makes this so fun and challenging. But yeah, I mean, most texts will suggest that the sural nerve is exclusively sensory. So if you are able to demonstrate that it's giving motor branches into the EDB, that would be really interesting. So, anyways, I think I was done talking about the deep fibular nerve. So if we go to the superficial fibular nerve, it's most commonly going to be entrapped as it exits the lateral compartment of the leg to become cutaneous, right. So, it's typically going to manifest as paresthesia of the anterolateral leg in the dorsal foot. But it might also getting trapped by the retinaculum at the ankle, in which case, only the foot would be expected to experience the paresthesia not that lateral leg ankle area. I guess I should also point out that rarely, there is a motor branch of this superficial fibular that might be compromised that the fibular neck right, I mean, that's not typically the way that it happens, but it's possible. And so you know, your patient might experience weakness in eversion If that were the case. But again, that's pretty uncommon to get motor branches all the way up with the fibular neck entrapped. If we turn to that tibial nerve. As you're very aware, you know tibial neuropathies commonly occur in either the popliteal fossa, the soleal sling or the tarsal tunnel, and the most common presenting symptoms are going to be pain and changes in sensation on the plantar foot. So, you know, I think the place to start there is to use a Tinel test at the popliteal fossa and the medial malleolus. Obviously, if the entrapments in the tarsal tunnel the Tinel's will be negative in the popliteal fossa if it's positive in the fossa, entrapments more proximal right so a key component for differentiating tibial neuropathy at the sling versus the tarsal tunnel or distally is the sparing of sensation over the heel. It's another important thing to keep in mind those calcaneal nerves branch typically are coming off proximal to the tarsal tunnel. So that heel is typically not going to be affected by tarsal tunnel lesions. If you suspect a more proximal lesion, you know, somewhere in that Soleal sling or in the popliteal fossa, test the large plantar flexor muscles, right? Have them perform one legged calf raises to look for unilateral weakness. Unfortunately, if you suspect that it's in the tarsal, tunnel strength testing, there really isn't going to be reliable because testing intrinsic foot muscles just doesn't work very well, at least, that's what I would assume you as a clinician might have a different opinion on that. But I would imagine it would be difficult to find weakness in such small muscles.

Dr. Barrett 1:00:52

Well, one of the things I think you've mentioned there that is very valuable clinically as if they do a calf raise. And you think that you have a lesion at the level of the Soleal sling, you can have them do up to 10 calf raises, like they're leaning up against the wall, and then on the 10th, have them hold it and then you palpate or provoke in that area where the Soleal sling is and usually if they have a problem. They're not going to like that too much.  That's very useful clinical tests.

Dr. Mark Schuenke 1:01:29

This is something that I'll ask you. So, I'll turn this around, you'll be the expert. I'll be the podcast host for a moment. If the entrapments at the Soleal sling, then motor innervation to the gastrocnemius is still intact, and possibly to the soleus as well. So, are you seeing any deficit or any substandard performance in the single leg?

Dr. Barrett 1:01:57

Oh, yeah, I mean, a lot of times, they can't even come up on it now, whether it's due to muscle atrophy, versus just, you know, the wicked pain of the entrapment itself, and then they are guarding it, I can't answer that. But I will tell you that the tibial nerve, as we've already mentioned, is far more robust and seems to be far more removed from symptomatology in the lower extremity compared to the common fibular nerve. And there's, I think we've kind of laid out why that why that is. But if you see a motor, excuse me, if you see an atrophy, and like the medial head of the gastroc, you really have a very serious clinical condition. Because the Tibial nerve, it takes a lot to mess it up.

Dr. Mark Schuenke 1:03:00

the but again, if you're seeing atrophy of the gastrocnemius at all, I mean, that's suggestive of an entrapment or, you know, radiculopathy or something more proximal than the soleus. Sling, you're talking about maybe in the popliteal fossa, maybe all the way up at the piriformis muscle.

Dr. Barrett 1:03:24

Mark, I'm not sure about that, because clinically, I've had a handful of cases over the years where, you know, patient had some kind of complication post gastrocnemius resession, which is done very distal. And they would, for some reason, in my recollection, Model C to be medial gastroc that was affected. So, are there not more distal motor branches that that are affecting versus just proximal? I don't know.

Dr. Mark Schuenke  1:03:54

Well, yeah. Okay. That's a fair point. Yeah. So many of the motor branches are going to come off prior to the tibial nerve actually entering the soleal sling, they'll come off, you know, proximal to the knee even.

Dr. Barrett  1:04:12

To the femoral condyles.

Dr. Mark Schuenke  1:04:17

I guess your point is, well taken that there further distally there still would be some motor branches coming off the tibial nerve going into the more distal portion of the gastroc. So yeah, I there could be weakness there still or pain.  So, the medial plantar nerve is, is going to be most commonly compressed as by the abductor hallucis it's going to result in paresthesia as to the medial two thirds of the foot, you know, the lateral plantar nerve is commonly going to get entrapped as the nerve is passing kind of between the deep surface of the abductor hallucis and the medial edge of the quadratus Plantae. Or perhaps just anterior to the medial calcaneal tuberosity kind of deep to the flexor digitorum brevis there. And in that area, you know plantar fasciosis or plantar calcaneal enthesophytes might be the cause, or at least an exacerbating factor of of entrapment. And again, you're going to really be looking mostly for paresthesia, or pain right along the route of that nerve there. So, in neither case, would you really expect to detect much muscle weakness. And really, that takes us all the way out of the bottom of the extremity. So, you know, in conclusion, you know, discerning radiculopathy is from neuropathies is difficult and nuanced, right? There's a lot of anomalies there a lot of just very similar things that can both present with numbness or weakness. So, you have to keep in mind that radiculopathy These usually are also going to involve back pain and generally are going to be less concrete, but will have reproducible effects with postural changes, right? Whereas neuropathies often are worse at night with impact on sensory and motor function to a more specific nerve pattern. Either way, you know, diagnosis is going to require an in-depth anatomical knowledge of of nerve branching patterns and anomalies. So, when you're in doubt, consult your friendly neighborhood anatomists, you know, I've consulted with several of the surgeons of the ANS group, and I'm happy to help out with the your broader listening audience as well. So, Steve, I thank you for giving me this opportunity. It's been a lot of fun. And that's where the presentation ends.

Dr. Barrett 1:07:30

The other thing that one thing that I wanted to touch on was the double crush phenomena, because I think that's a confounder for a lot of people. I mean, I can't tell you how many common fibular decompressions that we've done over the last couple of decades where people come in and say, Oh, my little back pain is improved or gone, and you haven't done anything even remotely close. So, you know, Upton and McCall has talked about this. I think it was back in 1973. The double crush. We see this a lot in peripheral nerve cases. So, it could be that they have radiculopathy and a peripheral nerve entrapment. There's no, there's no rule that says a human can't have more than one thing going on at one time. I wish that was the case would have felt like a lot easier.

Dr. Mark Schuenke 1:09:10

Yeah, I always look forward to you and your crew coming along as such a fun group to work with. And, you know, I should say that anyone who's listening to this, if they have questions about this, feel free to distribute my contact information.