Why Inferior Alveolar Nerve Blocks Fail: Common Causes and Clinical Fixes

One of the reasons why a local anesthetic doesn't work is because we've slammed it all into the vein and we didn't aspirate and didn't realize what we were doing. So obviously if you put the local anesthetic into a vein, it's getting carried away from where the nerve is and you can't get any anesthesia in that type of a situation. Welcome to Austin, Texas and welcome to the Phil Klein Dental Podcast. In this episode, we go back to the fundamentals. We explore why the inferior alveolar nerve block fails so often, what's really happening anatomically when patients aren't getting numb, and how a deeper understanding of these patterns can help clinicians improve both consistency and patient comfort in everyday practice. We'll also take a closer look at anesthetic selection. specifically the advantages of articaine over lidocaine, and discuss the clinical situations where articaine can make a meaningful difference and why many clinicians consider it an essential part of their anesthetic armamentarium. Joining us today is Dr. David Isen. He practices in Toronto, Canada, where his anesthesia-based practice focuses on patients who require all levels of sedation, general anesthesia. and advanced local anesthetic techniques, often individuals with special medical needs, dental phobia, or unique challenges with anesthesia. Before we bring in our guest, I do want to say that if you're enjoying these episodes and want to support the show, please follow us on Apple Podcasts or Spotify. You'll be the first to know about our new releases, and our entire production team will really appreciate it. Dr. Isen, welcome to the show. Thank you. It's a pleasure to be with you again. I'm very excited to have a discussion over the next little while. Yeah, and this is an important topic because, you know, if we can't get the patient numb or we have challenges with that, that changes the whole trajectory of the office visit. And it could be quite negative. And we want to certainly minimize those kinds of things if we can. And that's one of the reasons why we're doing this episode. So most of our conversation is going to focus on the mandibular block. going after that inferior alveolar nerve. So let's start the conversation with the mandible. Given the variations in mandibular shape from patient to patient, how should that change the way we approach our injection technique? So with respect to mandibular shape, there are a few considerations. First of all, the ramus flares differently in different people. And as such, The endpoint of an inferior alveolar nerve block isn't always where we are expecting it's going to be. You know, you have one of those patients that has a very widely flaring ramus. It might feel like forever to find the bone while you're doing your injection. You may never actually get in there and have your needle tip touch the bone. And in a case like that, often... we inject too medially from where the inferior alveolar nerve happens to be. You're looking typically, what, 25 millimeters to hit bone in most cases? On the average adult. Exactly. On the average adult. But as we all know, there are people who are outside the average. And so we have to expect on a larger person to go deeper than that 25 millimeters. to find the remus if that's what we're trying to touch. Yeah. And when I practiced endodontics, I encouraged every dentist that I talked to about local anesthetic techniques is to use a longer needle and use a 27 gauge. Some of these dentists were using the short needles, which at that time were blue. They were 30 gauge. And for a number of reasons, you know, aspiration, it's tough with a 30 gauge. Plus you need to go to the hub. on these short needles and who wants to go to the hub when you're looking for uh to hit bone because if you don't hit bone you got nothing left right you're exactly right with these short 30 gauge needles which a lot of clinicians are still using um maybe in order to save costs you know stock one type of needle and not have longer needles um again they're going to the hub at the very least on the average size adult in order to get the final depth. Right. And there's a couple of reasons why, and I'll let you go on in a second, but I do want to throw this in there. There's a couple of reasons why I didn't like those short needles for mandibular block injections. One is what we're just talking about. If you need more needle length, you don't have it because we have those mandibles that you said are flared. Two, you know, it's very rare, but it could separate from the actual hub because if that patient... jumps and twists that needle, it could break. And God knows how to get it out. I don't even know how you'd retrieve it. You'd have to send someone to the ER and it would have to be surgically removed. Is that correct? Yeah, exactly. Yeah. And so, you know, as you say, it's rare, but there are cases almost every year, adverse event reports that go to the FDA where in the dental office and that you're exactly right. The scenario is. The dentist intends to do an inferior alveolar nerve block. They're using a short needle. They don't warn the patient about the impending discomfort, especially a child. And then the head swings quickly to one side and bingo, the needle separates from the hub. It is rare, but it is something that is seen. It happens. Right. And that's scary enough for me. And that's enough risk never to do that. And also, you know, the thought is if you use a 30 gauge needle, you won't hurt the patient as much. But I think you even covered this in one of your webinars, Dr. Isen, where you made it clear that the gauge of the needle is not what causes the discomfort. So there's really no research or no evidence to show that a 30 gauge hurts less than a 27 gauge. And you're also losing the... ability to aspirate if you're in a vein because a 30 gauge the orifice of the needle is so much smaller it's kind of tough to to draw blood if you are in a vein yeah exactly all studies show that if a patient closes their eyes they don't see what they're being injected by they can't tell the difference with different gauge needles you know it's injecting fast um and and touching a nerve or or touching an artery those are the things that cause injection pain not needle size right so getting back to the mandibular shape you talked about the flaring so how do we know that the patient has that flared mandible where they would need to really go deeper to hit bone, you also have that external oblique ridge. And that's why it's really important to kind of put your fingers there to actually identify where that external oblique ridge is. Talk about that to us a little bit, because that goes along with that flaring of the mandible. Yes. So sometimes you can just look at somebody and see their ears flaring. You know, you stare at their face head on and you can see their ears sideways. That's a sign of a flaring ramus. And that goes along with the wide, thick distance from the external to the internal oblique ridge, where you almost get this wall of bone that you hit after two millimeters. And so it's a similar problem in the same shape mandible, where with the flare of the ramus and the thick internal, external... bridge you just number one can't get around that blockade and number two you can't get your needle around that location and so you know sometimes putting the barrel of your syringe more distally on the contralateral side to get a more straight line approach to where the neurovascular bundle is or or injecting a little bit higher sometimes those are ways to find your way around a thick external oblique ridge or widely flaring ramus. Yeah. Now, let's talk about the foramina. Now, in your lectures, you often emphasize foramina location. So talk to us about that and how that directly relates to the success rate of our inferior alveolar block. So maybe this is one of the most unheralded reasons for local anesthetic failure. It's that when we were taught to do an inferior alveolar nerve block, the classic technique was to stick our finger in the mouth, feel for the greatest notch on the anterior border of the ramus, and then allow our finger to go in a bit where the internal oblique ridge would be sitting. The reason we were taught that is because it was thought... that that was a landmark for where the mandibular foramen would be from a superior to inferior perspective. So that landmark would tell us where the foramen is, at least we were taught that, if we find the internal bleak ridge at the greatest notch of the anterior border of the ramus. However, all kinds of studies have shown that the location of the mandibular foramen is very variable. And from an inferior to superior perspective, it could be from 0 to 19 millimeters above the occlusal plane. So, you know, we could be sticking our finger in and landmarking for where we think it is, and then injecting quite inferiorly to where it happens to be. And therefore, the molecules that are local anesthetic won't be bathing a neurovascular bundle. they'll just be being placed beside the ramus, well inferior to where the nerve actually tucked into the foramen. Right. What is it typically, 10 millimeters above the occlusal plane is kind of average? Yeah, it's maybe closer to five. And so on average, actually, it's suggested to use five millimeters. That would be like the average inferior to superior location of the foramen. And then if you talk about anterior to posterior, that's another variable that's dependent variably, of course. It's dependent on class of occlusion. So, you know, class three occlusion, you're going to have to go deeper to find the foramen. Of course, a class two occlusion where the mandible is more anterior, not as deep. But that's another variable, the anterior to posterior location. as well as the inferior to superior now is it usually higher up when it is a variation then lower than what you mentioned was the average of five millimeters so if you aim a little bit higher you're kind of playing it a little bit safer because if that's the case yeah so one study did you know hundreds of ct scans of where the foramen is and they calculated an average And the average they recommended was five millimeters above the occlusal plane and 16 millimeters posterior to the anterior border of the ramus. And I do want to mention that the inferior alveolar nerve block has the highest failure rate of any nerve block in the body. And that's, you know, our luck, right? I didn't know that. That's pretty interesting. Yes. It was invented by two medical doctors. who were playing on each other using different they were using uh cocaine as a local aesthetic this was back in the 1880s two surgeons who you know they were doing all nerve blocks and and blocks in the knee and and and they happened to try an inferior alveolar nerve block um and you know maybe that's why it fails all the time because it was invented by medical doctors there you go Before I ask the next question about accessory innervation, which is important, you know, anesthesia may not work, not only because of an anatomical variation, but also physiological factors like infection and inflammation. And to some extent, Dr. Isen, psychological factors, because, you know, we know that there are some dental phobic patients where if you just touch their arm, they jump out of the chair. So, you know, to get those patients numb. is almost impossible. But let me ask you about accessory innervation. It's one of those frustrating clinical realities we've all encountered. You know, you've got a patient whose lip is completely numb, yet they still feel pain in the tooth you're trying to treat. So what's often the culprit here? And which teeth are most commonly affected? And most importantly, the last part of this question is, what can we do as clinicians to reliably get those teeth fully anesthetized? So the nerve that wreaks havoc on the inferior alveolar nerve block is myelohyoid nerve. The myelohyoid nerve in more recent CBCT scanning studies has been shown to innervate all of the teeth in the mandible from the second molar to the central incisor. The departure from the inferior alveolar nerve is 15 millimeters above where we're doing an inferior alveolar nerve block. And that goes lingual. When you say it innervates the teeth and the mandible, it's going lingual. That's right. The nerve travels on the lingual side. And, you know, these fascinating new studies, one in particular that was from about a year ago, showed that. And the number that they coded was 99.5. I don't know why that's not 100%, but 99.5% of the population have these foramen on the lingual side of the mandible. And again, it could be from the second molar to the central incisor. And that's where the myelohioid nerve goes in to specific teeth. Now, the reason we don't notice it all the time when we're practicing in the mandible... is because, you know, let's say the accessory innervation is to the second molar and you're operating on the premolar. Well, you don't know that that accessory innervation is there. And so even though it's there all the time, it's bad luck that you run into it. And that's why, you know, there is some success with the inferior alveolar nerve block, but it's very low because of the myelohyad nerve. So they can actually be present anywhere on the lingual side, of the body of the mandible? So there are foramina on the lingual side of the alveolar ridge, sort of located on the lower half of the ridge. So if you were to see soft tissue on the mandible, you know, there's a mucogingival line that goes across where you would aim to anesthetize that one foramen if you were trying to do that, is just a little bit below the mucogingival line. So it's kind of random which teeth it innervates. Yeah, for some people it'll go, it'll innervate second molar. Some people, it'll innervate a premolar. It's very variable. It's not like the inferior alveolar nerve where it goes down and goes across the mandolin. So does it innervate more than one tooth when it does enter the foramen? It can. I would say it's probably more likely to be in a posterior tooth than an anterior tooth. And, you know, from a dental perspective, they're telling us that the lip is numb and the tongue is numb and everything else is numb except for this one little hot area. And that's sort of the typical scenario of what the myelohyoid nerve would do. So in that case where the patient says my lip is numb, my tongue is numb because you got part of the lingual nerve and all this typical numbness is going on, which is indicative of a successful inferior alveolar injection. When you have that patient that's sensitive to number 30 on a distal box, for instance, or making access for a root canal on a vital tooth, the most prevalent reason for that is accessory innervation from the myelohyoid nerve. Entering on the lingual side through one of those foramen that 99.5% of the population have somewhere on that side of the ridge. Okay, so the way to challenge that is obviously an infiltration on the lingual? Well, there's two. That's one. One way to manage that problem is to put about half a cc of local anesthetic on the alveolar ridge on the lingual side, making sure that we're not in the floor of the mouth where the submandibular salivary glands are. And the patient won't feel it because they're numb everywhere else. So it's a painless injection. wait a minute or so, and that will go into the foramen or foramina and anesthetize that accessory connection. So if you gave two cartridges as a block, Dr. Isen, with vasoconstrictor, should you give the infiltration without the vasoconstrictor or does it matter? In this case, it wouldn't matter because we're going to a completely different location. So we're not worried about building up acidity from lots of vasoconstriction or cardiac issues. This is like a brand new location. The practitioner can choose whatever they want and any local anesthetic should do the job. Right. So that's one way to address this issue. And how long would you wait once you give that infiltration locally on the lingual of the mandible? How long do you wait? I'd wait a full minute just as I would for infiltration in the maxilla. So wait about a minute for that to work. Right. Okay. And then... other option I assume would be to give a block higher up. Yeah, exactly. Given either high inferior alveolar nerve blocks would do the exact same approach with your landmarking and palpating in the patient's mouth or a Gau Gates block, which one of the reasons why Gau Gates came up with his mandibular block was because of what he thought. was accessory innervation. He didn't necessarily know it was the myelohyoid nerve, but he thought that there were accessory nerves coming off the inferior alveolar nerve. And his idea to inject higher, that was one reason why. And is that the injection that you typically give in the mandible? I sort of give a hybrid. The reason why I like the Gal Gates block is because it depends on touching the ramus. And, you know, that makes me comfortable. And I know I'm not too medial, which is a common place where we miss. And so, you know, I don't look at the exact landmarks and the side of the patient's face and all of those things. But I sort of do a high hybrid Gau Gates, high inferior alveolar nerve block for everybody because I know everybody has a myelohyoid nerve. And the key thing is really. aspiration, right? You want to aspirate to make sure you're not in a vessel. Right, which is an unthought-of reason for local anesthetic failure. You know, one of the reasons why a local anesthetic doesn't work is because we've slammed it all into the vein and we didn't aspirate and didn't realize, you know, what we were doing. So, obviously, if you put the local anesthetic into a vein, it's getting carried away from where the nerve is and you can't get any anesthesia. in that type of a situation. And also if you miss the first one and then you give a second one, you mentioned about the vasoconstrictor, which is really important because if you give the second one with a vasoconstrictor, you're increasing the acidity of the environment and therefore that the actual anesthetic solution is not going to be as effective. Talk about that for a minute because that's a really important point. Yeah. So the property of a local anesthetic on its own without the vasoconstrictor is such that it doesn't like acidity. The acidity of the environment actually stops the correct. So the local anesthetic will exist in equilibrium in a basic form and an ionic form. And the basic form is the one that can get through the nerve. But with an acidic environment, it's hard for the basic form to exist. The acidic environment favors the ionic form, which doesn't do anything for us. And so too much acid like infection or too much vasoconstrictor local anesthetic will impede the basic form from existing, thereby making it hard for the local anesthetic to work. Yeah, that's a really good point. You know, it's interesting that when you get a bunch of dentists together, you inevitably hear a conversation about artacaine versus lidocaine. So I want to touch on that in this episode. One of the things that comes up in that conversation is how at a clinical level, articaine seems to penetrate cortical bone and diffuse through nerve tissue more effectively than lidocaine. What's the research showing on that? And what are your thoughts, Dr. Isen? So there are a lot of meta-analysis studies that have been done over the past five years comparing articaine in different clinical environments. with other local anesthetics like ladocaine and, you know, mepivacaine or whatever else they're comparing it to. And the meta-analyses have shown, without a doubt, that arcticaine is simply a more effective local anesthetic. So it has a faster onset, it has a longer duration, and it has a higher efficacy. in pretty much every clinical injection that we do. There's some argument about an inferior alveolar nerve block if it's more efficacious, but some meta-analysis studies say that it is. There's a few that say it's equal to, let's say, lidocaine. But for infiltration anesthesia, there's no doubt about it. Articaine is simply a better local anesthetic. The reason for that is because as a molecule, it's completely different than our other amides. So the other amide local anesthetics, for example, lidocaine and the other four have something called a benzene ring in the molecule. And that benzene ring is a larger structure and itself isn't very lipid soluble. Articaine is the only local anesthetic. It has something called a thiophene ring. And thiophene has an ester link on it and is a very lipid-soluble part of the molecule. And not only is it more lipid-soluble than the benzene ring, it's actually smaller as well. So with a smaller size and more lipid solubility, arcticane can penetrate the nerve sheath more effectively and get through those tissue barriers better, like the buccal cortex. or even the soft tissue when an infiltration is given. Those are the reasons we're thinking that arcticane just simply is a more effective local anesthetic. Yeah, and that's in the form of Oroblock in the U.S. and Canada. I think it's sold as Oroblock. Yeah, so Oroblock is one of the arcticanes that's available. It's an excellent form of arcticane, and it comes in both a 1 in 100 and 1 in 200,000 solution. And the clinician doesn't really need the one in 100,000 except for vasoconstriction. So the oral block one in 200,000 will give all of the efficacy that the one in 100 will give. The only reason to use the one in 100 would be for more hemostasis. Right. Now, oral block is 4%. in volume, whereas lidocaine is 2%. So tell us how we have to keep that in mind when we are giving multiple corpules. Right. So any arcticine solution in the US or Canada is a 4% solution. And as such, we have to be careful because 4% is twice as much as 2%. Why does that matter? Well, Articane and lidocaine have an equal potency with respect to toxicity. So when you think about that, you're actually given twice as much articane as you are lidocaine. They have the same milligram per kilogram maximum dose. And so you really can only give half as much of that drug. in one sitting for a patient. So essentially, a general dentist who's doing regular restorative work and some endo and whatever they need to get the patient numb, they could go with just artacaine, right, and not even have lidocaine? Or is there a good reason to have both? I think that having lidocaine for some situations is important because any local anesthetic... cause neurotoxicity. They're all neurotoxic. And is one more neurotoxic than the other? I mean, there's lots of argument about that, and there's lots of different studies that will argue one side versus the other. But for people who are smaller, you know, children, for example, I think that using a 2% solution like gladocaine is prudent because, number one, we're keeping our dosages to a minimum for these smaller people. And if there is a very small nerve in the area and there happens to be the slight risk of neurotoxicity, will a 2% solution be less neurotoxic? I don't know. I can't say that for sure, but some people might argue that. Right. So let me ask you about advanced imaging and scanning technologies, how that relates to all this, because We've seen these tools really advance over the last decade, certainly in the last five years. More docs are using CBCT. We're learning more about nerve pathways from these images. How does that affect the clinician regarding local anesthetic techniques and success rate? So these new CBCT scanners have been uncovering nerve pathways that we didn't know existed. So a study just from three or four months ago using a process called micro CT scanning, which I guess means a very small CT scan where they scan the maxilla. This isn't the mandible we're talking about here. And using these micro CT scans, they uncovered new nerve pathways in the maxilla to the teeth. And so now for the first time, we didn't know this before, but just like from six months ago, now we know that the maxillary teeth are actually innervated not only by the posterior, middle and anterior superior alveolar nerves, they're also innervated by the nasal palatine and greater palatine nerves. When I read that study, I was going, wow, that's like mind-blowing. You know, it might not change. what we're doing with a local anesthetic because we still should get our anesthesia despite that new information. But, you know, what we were all taught in dental school, all of us, just up until six months ago is wrong. What is actually showing in these micro-CTs that are showing the nerve pathway? What are we looking at? Just nerve fibers? Yeah. So first they found the pathways, the foramen or the canals. the canals, actually. And once they saw the canals, then they did liquid dye and latex on cadavers, on skulls, where they pushed the liquid dye and latex through these pathways, and they clearly went to the dentition. And we didn't pick up any of that with all the... gross anatomy and dissection we've done over the years of the skull we did not know that yeah it's for i mean for me it's fascinating you know but somebody else might say who cares right these fibers are so small we probably couldn't even detect them they were embedded in muscle and soft tissue and and the foramen the foramen were probably so small that we couldn't even see what was going in and out of there yeah exactly yeah so as we get to the bottom of this podcast and it's been really interesting dr eisen um What are some of the most common injection mistakes that you see clinicians make? Maybe we should have started off the podcast with this question, especially when it comes to areas they should avoid. And how could we improve both safety and success in our technique? So I think that question is a great summary to the important points we've already covered. I think that one of the biggest misses that people make is injecting too medially in the mandible when they're doing an inferior alveolar nerve block. Even for those patients who have an average size mandible, that's still a source of error. And so, you know, touching bone, certainly it hurts. The patients have the periosteum is innervated and we have to go in slowly and touch it gently. But touching bone avoids the pitfall of doing too medially of an injection with respect to the neurovascular bundle that were. always trying to get as close as possible to. The other place I would say that we miss is, as we discussed already, is injecting into a vessel and just not carefully aspirating and using the correct size needle to make sure we're not doing an intravascular injection. That's not going to be a risk, you know, over the central incisors, but certainly in the mandible. In the inferior alveolar area, it's very vascular. What is the typical response, Dr. Isen? when we inject into a vessel? So typically people will say that they have like a little bit of palpitation or short of breath or feel just a little bit unwell and they might want to stand up and walk around a little bit. They just don't feel good and they get those palpitations in their heart from the epinephrine stimulating the cardiovascular system from that very fast push. of the adrenergic response because of the intravascular injection. And how fast does that happen after they give the injection? Almost immediately. So, I mean, at that point, I think we should recognize that that's a possible intravascular injection and make sure the patient feels better. They feel back to themselves before we give them any more local anesthetic. You know, there are people who are very... On the rare side of things, they react very strongly to the toxic effect of a local anesthetic. You know, it's kind of like alcohol where most of us can have two or three glasses of wine and not feel drunk. Some people feel drunk after one or two. Local anesthetics are just like that where, you know, if you get a patient who is very sensitive to toxicity, a second cartridge into a vessel. could certainly elicit an overdose reaction. Right. So it's really important that after you give an injection, you sit with the patient. You said it's almost instantaneous if you go into a vein. So you sit with that patient for, you know, 20 seconds to make sure they feel comfortable because the last thing you want to do is walk out of that operatory and then they experience this by themselves. Exactly. Yeah. So that's... Yeah, go ahead. I was going to say, and with veins, it's very interesting because veins have very... smooth muscle walls, which are not innervated. And so the patient won't feel the needle going into it until they feel the palpitations and the untoward effect. An artery has a very thick, smooth muscle wall, which is very well innervated. And so when our needle inadvertently touches the wall of an artery, The artery, as a protective mechanism, squeezes down. The side of the patient's face often turns white. And the patient feels this deep pain. It really, really hurts. So we can't do an intra-arterial injection. It's pretty hard to do. But the signs that we would notice are deep, severe pain. and blanching on the side of the patient's face. That's very important. Yeah. So that's one type of discomfort. And then the needle tip touching the lingual nerve, for example, where the patient feels that little shock in the side of their tongue or on their lower lip. That's yet again, something different that people sometimes feel. Yeah. I've hit that many times, many times. I think we covered quite a bit here. And I think that's been, it's really been a great conversation. Do you pretty much use Articane in your practice? And then you keep lidocaine for the smaller folks? Yeah, so articane is my go-to. It's my favorite local anesthetic. And I do have lidocaine as well for smaller people. Also, I have a plain solution available, either prilocaine plain or mepivacaine plain, because if I have to use more than one cartridge in the same location in order to minimize the acidity, the plain solutions... much more basic and will help me buffer the tissue if I have to use more than one cartridge in the same area. Yeah, good stuff. I mean, I remember when I first learned how to give an injection in dental school, you know, we gave it to each other. I think it was the third, I guess it was the beginning of the third year of dental school. We went to, I went to this dental office with like eight of my classmates and we just gave injections on each other. And of course the guy that gave me the injection hit my lingual nerve, sparks were flying on my tongue. I didn't know what he did, but I knew that You know, you kind of look at the people that are giving you these injections when you're in third year dental school and you're like, you know who the better dental students are? And you're kind of praying that that guy is going to give you the injection. But of course, that's not what happened to me. I started sweating when I saw who was giving me the learning on me as a guinea pig. I don't know. Is that how they do it in Canada? In our class, it was the exact same thing. And one of our students, our classmates was a registered nurse. before going to dental school and we all wanted her yeah there you go who wouldn't know what to do yeah that's that's funny all right dr eisen a great conversation we've had you on many times in the past you've done some great stuff for us and we really do appreciate it have a great evening thank you very much