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Temporal bone dissections are a critical learning tool for otologic/neurotologic surgery. The “Temporal Bone Dissection Manual” from the House Institute has long served as a ‘gold standard’ for the stepwise demonstration of this process. In this video, a progressive step-by-step dissection of the temporal bone is performed. Key anatomical structures and landmarks and outlined, and their physiological importance in the context of different otologic pathologies is explained. The procedure begins with the identification of soft tissue landmarks and surface anatomy before delving into cortical mastoidectomy and facial nerve identification. The mastoid tip region is discussed, before moving on to describe the tegmen and endolymphatic sac. A facial recess dissection is performed and middle ear anatomy is explained. A labyrinthectomy and exposure of the internal auditory canal conclude the dissection. In addition to going over the anatomy of the temporal bone dissection, a discussion of how to execute these procedures safely and efficiently is conducted.
By having a thorough understanding of the anatomy of the temporal bone, medical students, residents, and fellows are better able to understand the reasoning behind different otologic procedures and how they can be used to treat patients. This demonstration was created to inform and teach residents and medical students about temporal bone anatomy.
Surgery; otology; neurotology; anatomy; mastoid; skull base surgery; otolaryngology; temporal bone
Temporal bone dissections are an essential learning tool prior to performing surgeries of the mastoid and lateral skull base. Accurately knowing anatomical locations of all landmarks is critical to operating in the area safely and efficiently. In addition, performing temporal bone dissections allow surgeons the opportunity to study middle ear anatomy, the otic capsule, relationships of the skull base, and other temporal bone anatomy in way that is not possible during the normal surgical approaches, as removal/injury in the laboratory does not result in any significant consequences. From a teaching perspective, learning different surgical procedures through temporal bone dissection is a significant learning aid for students; in fact, temporal bone dissections are useful learning tools for different surgical procedures such as tympanotomy, atticotomy, mastoidectomy, cochlear implant, endolymphatic sac decompression, and facial nerve decompression .2
Herein we provide a step-by-step dissection of the temporal bone and the important anatomy and surgical techniques to follow when dealing with the facial anatomy are reviewed. We hope that this serves as a primary resource for temporal bone dissection or as a useful adjunct to printed temporal bone dissection manuals.
The malar eminence, that connects posteriorly to the zygomatic arch and ultimately the root of the zygoma, is first identified. The temporal line (linea temporalis) is the posterior extension of the zygomatic root. Its relationship to underlying anatomy cannot be understated, as it is the starting point for the majority of mastoid drilling dissection. It typically approximates the junction of the mastoid cavity (inferior) and the temporal lobe of the brain (superior).
After identifying these soft tissue landmarks, a radius of 5 cm from the external auditory canal (EAC) is measured. This distance is what is used for lateral skull base approaches such as a translabyrinthine approach to vestibular schwannoma. Typical incisions for standard mastoidectomy or cochlear implant surgery are either 5 mm or 1 cm posterior to the postauricular sulcus. The importance of getting enough of the posterior fossa dura exposed behind the sigmoid sinus is emphasized. This specific approach is useful for neurosurgeons in the removal of larger tumors so that they can compress the posterior fossa dura out of the way to create greater access to the temporal area.
The location of the transverse and sigmoid junction in males and females is discussed in relation to the EAC (4 cm for females, 4.5 cm for males). This is an important landmark for the neurosurgical approach for a retrosigmoid craniotomy.
After soft tissue landmarks are described, the mastoid emissary veins are identified. These veins come out to supply the periosteum and undersurface of the muscles coming from the sigmoid sinus. The Spine of Henle is then identified, which demarcates the anterior boundary of the mastoid cortex right before it dips medially to the posterior bony ear canal. The Spine of Henle serves as a landmark for the antrum medially. The cribiform area is identified next, which lies within the Macewan’s triangle, which serves as the true approximation of the location of the mastoid antrum medially. Clinically, this landmark is significant in someone with acute otitis media with a frank mastoiditis or a purulent mastoiditis because they can develop subperiosteal abscesses, which will cause the ear to become proptotic and push the air forward.
This section concludes with a demonstration of surgical technique with the specific Stryker drill used in the dissection. This specific drill has 5 different length settings and a setting of 3 was used for this procedure. The importance of testing the drill pedal is underscored to ensure it’s going in the right direction, starting along the temporal line, and gripping the drill with two fingers (pointer and thumb) to make sure not to obstruct the line of sight.
Cuts begin along the temporal line and then inferiorly towards the mastoid tip along the posterior ear canal. The deepest part of the dissection should always be anterior and superior into the mastoid; this technique will help you work your way into the antrum while avoiding the brain, sigmoid sinus, and posterior fossa dura. Most sclerotic mastoids will still contain an air cell just lateral to the lateral semicircular canal (LSCC). The temporal line approximates the floor of the cranial fossa and is a good starting point for the dissection. One can observe a layer of vessels (venous plexus) within the bone overlying the temporal lobe dura. The LSCC can be found medial to the trabecular bone of the mastoid air cells. After identifying the lateral canal, the epitympanic bone can be opened between dura and superior EAC to identify the short process of the incus. The surgeon may use light refraction through water/irrigation to see the incus prior to the drill uncovering the bone immediately lateral/posterior to it.
The goal by the end of this section is to get an en face view of the posterior ear canal so that you can get a sharp and thin posterior EAC. This will also allow for the visualization of the disappearance of air cells along the posterior EAC as well as a color change when thinning the bone near the EAC skin.
Identification of the facial nerve begins by having the appropriate landmarks exposed. These include the LSCC and the short process of the incus. The EAC should be adequately thinned. Bone is removed from superior to inferior in the direction of the expected location of the facial nerve. A cutting burr may be used close to the facial nerve, and the timeframe of switching to a diamond burr is variable and surgeon-dependent.
At the end of this section, the chorda tympani branch of the facial nerve should be visible. The branch comes off after the second genu and courses superiorly before moving anteriorly into the middle ear and over the long process of the incus.
When neurosurgeons/ENT oncologists expose the temporal bone externally on the mastoid cortex or behind that, they use the digastric groove as landmark identification for where the muscle is attached into the bone. This is in contrast to the otologist, who identifies the digastric ridge within the temporal bone. The digastric muscle is always lateral to the facial nerve, and it can be traced posterior to anterior to safely remove the mastoid tip while protecting the facial nerve in the stylomastoid foramen.
This procedure is usually done with a diamond burr and involves thinning down the bone overlying the temporal lobe and deepening the sinodural angle. One point of emphasis in this section is that as you work more medially, you are confined by your dissection laterally; therefore, saucerization is critical for a surgery like a mastoidectomy so that the surgeon has a greater degree of freedom when moving their instruments, even if using them in a small area.
The lateral semicircular canal is one of the first landmarks during mastoidectomy surgery, and serves as an important component for locating the endolymphatic sac (ELS). Donaldson’s line is an imaginary line drawn along the lateral semicircular canal from anterior to posterior, bisecting the posterior semicircular canal, and then extending onto the posterior fossa dura anterior to the sigmoid sinus. In this area, the bone can be thinned, and depending on individual patient anatomy, decompression of the sigmoid sinus may be necessary to allow adequate access.
As the bone is thinned, an area of more thickened dura with fibers/blood vessels running in the same direction helps to better localize the ELS. The endolymphatic/vestibular aqueduct may be seen coursing toward the otic capsule, and the entrance of the duct into the petrous bone referred to as the “operculum”. The clinical correlation for the identification of the endolymphatic sac is that decompression of this structure may be used in the treatment of Meniere’s disease.
At this point, a facial recess dissection has been completed and access to the middle ear is through a posterior tympanotomy. The pyramidal process, containing the stapedius muscle, is located just medial to the facial nerve. The stapedius muscle gives rise to the stapedial tendon that is attached to the capitulum of the stapes. The stapes superstructure and footplate are identified.
After identification of the pyramidal process and stapedius muscle, the next step is to remove the incus to be able to visualize the tympanic segment of the facial nerve. After removing the incus and incus buttress, the cochleariform process and tensor tympani tendon should be visible. The head of the malleus may need to be removed to facilitate this as well.
While there are multiple ways to perform a labyrinthectomy, choosing a consistent and methodical technique will ensure safe and efficient surgery. I prefer starting with anterior to posterior cuts along the LSCC, while leaving the inferior bone to protect the second genu of the facial nerve. Much like tracing Donaldson’s line, this can then transition into removal of the posterior semicircular canal (PSCC). I start inferiorly away from the crus comminura at the ampullated end of the PSCC. Ensure on preoperative imaging that the jugular bulb does not approximate this area, which is possible in some instances. The lumen can be tracked towards the common crus and then the vestibule is opened. The superior canal lumen can then be traced, which may or may not require tegmen decompression and elevation of the temporal lobe with gentle retraction. This depends on the aeration of the temporal bone. Of note, the saccule and posterior canal are innervated by the inferior vestibular nerve, while the utricle, superior, and lateral semicircular canals are innervated by the superior vestibular nerve.
The final step in this section is to open the vestibule by thinning the bone. Once you visualize the vestibule, you can identify the spherical and elliptical recesses, which represent locations of the utricle and saccule respectively.
This section begins by showing that the internal auditory canal (IAC) is completely encased within the temporal portion of the petrous bone. The ampullated end of the superior semicircular canal serves as the superior border of the IAC, while the ampullated end of the posterior semicircular canal serves as the inferior border. The IAC angle is often comparable to that of the EAC, but the dissection surface moves from medial to lateral as goes from posterior to anterior.
Finally, the demonstration is concluded by describing the pathway of the facial nerve: the intrameatal/intracanalicular nerve exits through the fundus to become the labyrinthine segment of the facial nerve; this branch makes its first genu at the geniculate ganglion and becomes the tympanic segment before making its second genu into the descending/mastoid segment and exits the temporal bone through the stylomastoid foramen.
Temporal bone dissections aid in the learning of critical anatomic landmarks and their variations when performing otologic procedures. A hands-on approach when studying anatomy is crucial for medical students and residents to understand the temporal bone anatomy. In order to overcome the challenges of temporal bone surgery, it is essential for trainees to develop hand-eye coordination and fine hand movements during the surgery and to attain anatomical knowledge under microscopic vision.3
In addition to learning temporal bone anatomy, performance of different procedures on temporal bone models can be used to assess fundamental surgical skills. Specifically, the CanadaWest scale delineated performance by experience level with strong interrater reliability and was successful in distinguishing junior from senior trainees.4 These objective scales are an important indicator of progress being made in a trainee with respect to their surgical abilities.
While using a cadaveric model for temporal bone dissection is useful, these cadavers are often hard to obtain in practice; therefore, simulators have become increasingly common to use in training residents and medical students for different otologic procedures. Despite not being as effective as learning from cadavers, simulators and the development of new teaching tools should be encouraged to continuously improve surgeon trainees.5
Ultimately, temporal bone dissections using a cadaver serve as an essential learning tool for otologic surgical trainees. This dissection can be used to teach foundational anatomy to medical students learning temporal anatomy for the first time, or to assess the surgical skills of otologic surgeons throughout their training. While cadavers have been the gold standard in performing these dissections, simulators and 3D models are also increasing in their prevalence.
The only specialized equipment used for this dissection was a Stryker drill. Remaining instruments are standard for a typical “ear tray.”
C. Scott Brown serves as editor for the Otolaryngology section of the Journal of Medical Insight
- Anatomical Variations of Facial Nerve in Cadaveric Temporal Bone Dissection. Indian Journal of Otolaryngology and Head and Neck Surgery, 73(3), 271–275. https://doi.org/10.1007/S12070-020-01969-9
- Prasad, K. C., K., P., Maruvala, S., T. R., H., Gopi, I. V., & K. R., S. (2018). Impact of temporal bone dissection on the understanding anatomy of the ear among medical students. International Journal of Otorhinolaryngology and Head and Neck Surgery, 4(6), 1489. https://doi.org/10.18203/ISSN.2454-5929.IJOHNS20184365
- Irugu, D. V. K., Singh, A. C., Sikka, K., Bhinyaram, J., & Sharma, S. C. (2016). Establishing a Temporal Bone Laboratory in Teaching Institutes to Train Future Otorhinolaryngologists and Fundamentals of Temporal Bone Laboratory: Considerations and Requirements. Indian Journal of Otolaryngology and Head and Neck Surgery, 68(4), 451–455. https://doi.org/10.1007/S12070-015-0962-0
- Pisa, J., Gousseau, M., Mowat, S., Westerberg, B., Unger, B., & Hochman, J. B. (2018). Simplified Summative Temporal Bone Dissection Scale Demonstrates Equivalence to Existing Measures. Annals of Otology, Rhinology and Laryngology, 127(1), 51–58. https://doi.org/10.1177/0003489417745090
- Okada, D. M., de Sousa, A. M. A., Huertas, H. de R., & Suzuki, F. A. (2010). Surgical simulator for temporal bone dissection training. Brazilian Journal of Otorhinolaryngology, 76(5), 575–578. https://doi.org/10.1590/S1808-86942010000500007
Table of Contents
- Thin Posterior EAC
- Remove Posterior Fossa Bone
- Endolymphatic Sac Identification
- Opening the Sac
- Incus Removal
Today we're going to be working our way through Dr. Angeli's and Dr. Chiossone's temporal bone dissection manual, the anatomical temporal bone dissection practical manual presented by the CANES lab. We're going to kind of gloss over some of the setup for the working station, just because that may differ from lab to lab, and really just kind of get right into things. For the most part, you're going to be drilling, you know, temporal bones that are harvested directly from cadaver heads. Today, because we do have a full head, we're going to try to do some of the soft tissue work. Unfortunately, the head is still a bit frozen. But we'll kind of do what we can here. Let's see. Let me get that in view. All right. Hopefully this will show up even on this, I think it will. All right, so unfortunately, you know, I can't really mark out this gentleman's temporal line here because of frozen soft tissue, but just keep in mind that the zygoma extends back to the zygomatic arch root of zygoma, which then extends posteriorly to the temporal line. And the temporal line is an important external landmark, even with this soft tissue, because it's going to be sort of this demarcating site of the temporalis muscle and the separation between the mastoid cortex and the squamous portion of the temporal bone. And more importantly, it's going to help us estimate the roof of our mastoid cavity, or the floor of the middle fossa within the skull. And so, for the purposes of a wide, wide exposure and dissection today, my plan is to kind of do what I would do for a translab approach where I measure sort of a radius of 5 cm from the external auditory canal. And so we're just going to mark that out in a few areas here. So we have about 5 cm to here. And again, we're not going to be picky with the frozen cadaver head today. But in, you know in general for these, the reason that's important is because when you're drilling for a translab approach, you're going to want to make sure that you're getting enough of the posterior fossa dura exposed behind the sigmoid sinus, usually about 2 cm behind the sigmoid sinus, particularly for larger tumors so that the neurosurgeons can compress that out of the way and provide greater access. One of the other important sort of things to think about here is the location of the transverse and sigmoid junction. And so a good estimation in females is 4 cm posterior from the EAC at the temporal line, and 4.5 cm in males. So this is a male cadaver, so we can potentially estimate that that's going to be about the location of our transverse sigmoid junction, okay? All right. So because the cadaver head was so frozen, we elected to just sort of take all the soft tissue off rather than futzing around with that, and so, we're going to point out some of the bony landmarks here. Angela, if you don't mind, can you turn on, just turn my computer on to make sure I point out all of the landmarks that Dr. Chiossone would like.
Hold this because I am going to be a little bit more zoomed in just because of how our - how our microscope works here. So we've lopped off the external ear. So the external ear is missing here, but you can still slightly see, I've kind of pushed it forward as we move the soft tissue here, but this is the external auditory meatus, here. You can see there's still some hair-bearing skin. That hair bearing skin is overlying the cartilage. When the external auditory canal gets to the bony cartilaginous junction, that's where that hair-bearing skin will end. And that's because between the cartilage and the skin there is that sebaceous, or excuse me, not sebaceous, they're soft tissue, or subcutaneous tissue. Whereas the skin on the bony portion of the canal does not have that, and so that's why that skin is much thinner and also not hair bearing. Moving anteriorly here, you can see - see how it looks better on that? You can see the zygomatic root, again as you - if you were to trace that anteriorly, you would trace out to the zygomatic arch and ultimately the zygoma. You can see where we had estimated, you know earlier, just by palpating sort of the cheek and the soft tissue, we were pretty close to getting that right even with the - even with the frozen soft tissue. So, as the root of the zygoma extends back posteriorly, it becomes what we like to call the temporal line, or the linear temporalis. And you can see that as a ridge, here. And that's also something that you can typically palpate quite well. Again, on a - either a person or a cadaver that's not frozen. And above this, you can see that this bone or the squamous portion of the temporal bone here, has a bit of a darker hue to that. And that's because this bone is one, much thinner than what we're seeing here at the temporal line. And we're actually seeing the dura of the temporal lobe underneath that. So, we got kind of a wide exposure here. As we were doing so, just something I want to point out, you know, when you're getting a wide exposure of your mastoid cortex - as you're getting a wide exposure, sometimes if you bring this muscle back posteriorly, you'll come across these mastoid emissary veins, and these are veins that are coming out to supply the periosteum and under surface of these muscles here that are coming directly from the sigmoid sinus. And so, you know, when we drill back here, you'll see that again, but these are these can be - you know, they can be a pain. You can see there's actually an artery here that's been injected as well. And so if you're coming back through that, if you come through it, you can apply a little bit of bone wax, you can use a Bovie when you come through those. And there was actually a much larger emissary vein up here, which I wouldn't have expected to come across unless we were doing a big skull base case, but that's actually probably more closer back to where the transverse or sigmoid junction was. So, down here we have our mastoid tip, we have our mastoid cortex here. As we would have normally elevated the soft tissue flap anteriorly, one of the primary landmarks that we look for is right here. And this is the spine of Henle. And you can see that this is a true spine or sort of a ridge of bone that's here. And this is important for a couple of reasons. One, you can see that it's sort of demarcating the anterior boundary of our mastoid cortex right before it dips medially to the posterior bony ear canal. And so if you're dissecting sharply here, it's important to know and define the spine here. And because you don't want to carry your soft tissue dissection more anteriorly. Or, as you can see we've done here, you'll come through that skin and potentially the cartilage and external auditory canal here. The other reason why this is an important landmark is because this is a nice approximation of the location of the antrum medially. And so that's always a good thing to keep in mind as you're drilling and you're working. And you don't see it as well in this cadaver as you do in a live specimen, I'm going to take off a little bit more of sort of this underlying periosteum just to see if we can get that a little bit more, and you can. What you're going to start to see here are these little pockmarks in the bone and sort of a triangular area right behind, or actually right posterior to the spine of Henle here. And I'm going to try to point those out. Zack, let me know if you can see them on the screen. Do you see these little pockmarks here? It's very, very subtle. It's subtle, but you can see these little pockmarks here. So this is the cribriform area, of the mastoid cortex, and the clinical importance of this is that in somebody who has, you know, an acute otitis media and ultimately developed sort of a frank mastoiditis or a purulent mastoiditis, they can develop these subperiosteal abscesses, and that's actually what causes the ear to become proptotic and push the air forward. And so, the clinical relevance of this is that cribriform area is a connection to the underlying mastoid cavity. So if you get purulence there, it can actually come out through that. And that's where that abscess develops. So if you feel like you have a subperiosteal abscess, and you want to explore for that, that's the area where it's likely to be. All right, I don't think I'm missing anything for our posterior cortical anatomy here, and so we're going to proceed to the drilling portion finally. One of the things that is nice here that you'll see is once you get this kind of clicked in, so we've got the drill in the unlocked position here, that's easily changed by this. So when you slide the drill in, you'll feel a little click, like that. And then you see these little ridges here, the Stryker drill has sort of five different length settings that you can apply these to. And so, I'm going to just put it down here at about three - I feel that nice and secure there. I'm going to lock it again and then pull the test to make sure that's there. The other thing that we'll do, is just to test our drill pedal, which is good, making sure that's going the correct direction. All right, so with our cortical mastoidectomy, we're going to start along the temporal line. That's where our first cut is going to go. I guess a couple of other things I do want to note before we get started is how to grip the drill. You're not gripping the drill like this with your fingers together, you're not gripping it with three fingers like this. You're gripping it with two fingers - pointer and thumb, clamped on it here. And the reason that that's important - you can use a third finger to support more on the other side here. But the reason that that's important is because if you're working like this and down to the shaft and you have your fingers pinched together, you're obstructing not only your line of sight but also light from getting further in. And so that's just an important thing to consider or not consider, but just to do.
All right, so our first cut is going to come along the temporal line here. I expect that we'll see some air cells as we do that, which we already are here. Okay, and our second cut is going to come along the posterior ear canal, and keeping our spine of Henle in mind here. It may jump and catch some of the soft tissue that's still on the bone here, so we're just going to kind of gently clean that up. All right, so we're going to kind of level this off now. And the deepest, the deepest part of our dissection should always be anterior and superior because that's going to guide us into the mastoid, or excuse me, into the antrum. I'm going to go ahead and kind of turn through some of these. So I'm going to come back and just go ahead and trim some of this soft tissue first here, just to get us a nice, cleaner-looking mastoid. Come up and saucerize this area here. All right, so again, you come through these air cells. And once you know that you're safe in those air cells, you can bring it posteriorly. There's sort of two ways of going about finding the antrum. One is to, like I was describing before, so use always that anterosuperior as your deepest point. And when you know you're safe, then you can kind of bring it down inferiorly on the posterior ear canal. And posteriorly, here. So we're going to come along our posterior ear canal here, all the way down to the mastoid tip. I'm just going to kind of come up and come quickly through the rest of this cortical bone here, we know that's safe. I am starting to see a little bit of a color change here, interestingly, but I usually don't expect to see a vein like that that lateral. But we'll respect it nonetheless. Just kind of slowly work our way through here. All right, slowly, this is actually one of the parts of the procedure you should be able to move quite rapidly through it. Zack, can you try to turn the water up a little bit? Off? Up. Or just turn that thing to the right now that that one - yeah. More, keep coming. There you go. Perfect. Thank you. All right, now we can open up more of the mastoid cavity here. I came all - a little bit more posteriorly, we can see that air cell there. I'm going to trim some more of the soft tissue off. I'm sorry, before I got - we got sidetracked before. We were talking a little bit about kind of two ways that you can work your way into the antrum. And one of those is to always have this sort of anterior and superior portion be the deepest part of your dissection. So that's one way of doing it. The other is that you can identify the tegmen and the sinodural angle. Follow the sinodural angle more medially, and ultimately, you'll identify Koerner's septum, and then you can drill through Koerner's septum in a posterior-to-anterior direction. And that's a safe way of opening your antrum as well. You know, depending on the type of surgery that we're doing, we may not always identify the tegmen completely. You know, if you're doing a cochlear implant, it's not always necessary to skeletonize the tegmen completely. But at the same time, you know, I had one of my - sorry, my suction's full. Can we clamp this? All right, so we can see this patient has a well aerated mastoid. Interestingly though, you can't see this - they inject these patients ahead of time with latex - specifically blue should delineate a vein, but you know, it's running through these mastoid air cells. So we'll sort of, again, we'll respect for now and just kind of carefully take away some of those septations in the meantime, until we switch over. Okay, so as we're doing this, I'm going to thin the posterior bony ear canal wall. It's going to be particularly important for our facial recess dissection. So I think what I was alluding to before, again before we had to change the irrigation out, was that some people argue that you don't have to find the tegmen in every case. But if you have a standard way of doing a case every time, the tegmen - finding and following the tegmen is one of the safest ways to lead you into the antrum, and so I don't think you'll ever be faulted for kind of having that method of doing things. I'm just going to kind of quickly coming through all this in the mastoid tip. We're lateral to the lateral semicircular canal. We know all of this is safe. And we're actually about to come up on an important landmark here. So I'm just kind of drilling the cortex over where the epitympani is going to be. All right, so we've opened up our master air cells, all of these here, right? Koerner's septum was right here, which we're still kind of seeing part of, and then just medial to Koerner's septum, we get into the antrum, which is this space here. Okay? So, this structure here, you can see all this kind of very trabeculated bone in the mastoid cavity, or a very kind of classic appearance of that honeycomb trabeculated bone. And then we see this much smoother - it's a little bit - it doesn't have as much of a color distinguishing here in the cadaver as it can in a patient where it's almost a more yellowish-looking bone, but this is the lateral semicircular canal of the labyrinth here, okay? So while I still have the 6 cutter on, we are going to finish some of this cortical work here before we then open up the… And would you mind turning that to the right some more, Zack?. So this is great. This is a really nice example of how thin - down, less water, less water. That's good, right there. Here's a great example of just how thin this guy's squamous simple bone was. I'm barely touching drill through that, and you can already see the color change to the dura there. See that? And because we are going to ultimately do a wide dissection here, I am going to come through and expose that. as well as posteriorly, here. Again, we have that mastoid emissary. You don't need to worry about it in a cadaver, but it's a good principle to keep in mind. I'm going to come through this soft tissue over the mastoid tip here as well. And we are lucky that our cadaver here has a beautiful well-aerated mastoid. And what we're starting to see, through some of these air cells, and you can see again, this trabeculated bone, and then usually when you have this change between nice, smooth bone - we're going to have to change that again. Just this bone here, and then suddenly you see a smooth covering - that always has to alert you to the fact that there could be an important underlying structure. And in this area, the thing that we're going to be expecting is the sigmoid sinus, so… Ready? Yeah, go ahead. We'll thin some of the bone on the posterior ear canal. We're going to open up all that's done in the mastoid tip. I'm going to take down more of this… Cortex here as well. We'll make quick work of that. There's a deep sinodural angle here. And these little vessels, superficially. And of course, again, when those trabeculations start to disappear, and the bone starts to become smoother, that's where the sigmoid sinus is going to be. Did you see that color change Zack? And how the bone became much smoother, again, you know, when the bone doesn't start to look like this anymore, and you start to see smooth areas, that's when you have to be alerted to, you know, something being there. That and, again, this works - we're expecting our sigmoid sinus to be there. So, we're going to, here - in a little while, we're going to take more of that soft tissue off right there, so we can have a nice, wide dissection. We know where our sigmoid sinus is now, we know our lateral canal is more medial, so we should be safe and away from our facial nerve. We're going to deepen our sinodural angle, still while we have this 6 cutter here. I am going to come and thin this a little bit more. So Zack, this is something that I'll be doing a lot with you here when we start doing our cochlear implant mastoids. And that's opening up this anthem here, and usually we always push down to a four at this point to do this. But he's pretty - he's nice and open. So, and the way I like to explain this is you have a couple of things. You have your medial structures, but you have your inferior, which is going to be your ear canal, it's - recognize the curvature of that, right? Like this. You see the posterior ear canal? You see the soft tissue, so recognize that and then recognize that you have the tegmen right above you. So when you're doing this, you're basically getting what you can from the tegmen, and then getting what you can from the ear canal. All right? And then kind of getting what you can from the tegmen and anteriorly, and get what you can from the ear canal. Because if you're working here, and you're not keeping an eye on what's going on inferiorly, then you can still catch the ear canal or you can catch your drill burr, so it's kind of always like all right, well, I'm going to take a little bit more from here and then a little more from there. And then you can kind of slowly make your way anteriorly. And what we've done now is expose this structure in the middle ear, which we'll point out here. So this is going to be the short process of the incus. So that's sort of our important landmark for knowing that we've gone anterior enough if we're not planning on doing an epitympanectomy. And so we can palpate that and see that that's mobile, you see that there? And the other thing that's important is the short process of the incus is going to come back posteriorly. And it's going to be supported in something called the incus buttress. So there's a posterior incudal ligament that's going to be attaching to the short process and sitting in sort of an indentation of bone here called the fossa incudis. And that's what's representative of our incus buttress. And so that's one of our landmarks for our facial recess. So, the short process of the incus, as you can imagine, is going to be pointing back right to sort of this area of the expected location of the facial recess and the facial nerve here. And it's a little bit difficult to appreciate. We'll see it a little bit more when we open things up later. But you can see that just medially here, right under the Gimmick is going to be the tympanic segment of the facial nerve, okay? So, we are going to complete taking out some of these air cells here. And we have the large burr on. And then our mastoid tip. A little bit more out of our sinodural angle here. I'll come back and get that in a little bit. And then open up the mastoid tip here a little bit as well. So I'm going to go ahead and come back and take off some more of this bone here. What landmarks do you use for the fallopian? So, one is the lateral canal. So the facial nerve very rarely, if ever, will be lateral to the lateral aspect of the lateral semicircular canal, if that makes sense. So, I know that the second genu is going to be just anterior and inferior to the lateral semicircular canal. And so then I find the incus buttress because our facial recessed is going to be incus buttress, like right here, chorda tympani nerve, and then facial nerve. And so once I do that, I kind of know and I'll take a 4 cutter and do this here in a little bit, and I'll kind of just take some gentle swipes and some gentle passes. Once I get to about this level on it, then I should find the fallopian canal pretty easily there. I know you know what's coming, but what makes you stop and say "Okay, now I'm there." So I was doing it a little bit because I was kind of doing a teaching point of this and this. I wasn't kind of doing it the standard way I do, usually what will happen is you'll have that sort of bony ledge that extends back like this. And I'll kind of just, you'll kind of forget the space. And I would, again, I would normally have a 4 cutter, and so I would gently kind of touch and come back and look, gently touch, come back and look, gently touch, come back and look. And a lot of times some water will fill up in that area, and you'll actually see a bit of refraction, and you'll see the incus through the water before you actually see it with your eyes. The first reason - I was just showing that I was changing the angle of my view. And the reason for that was for two reasons. One is so that we can actually look into the epitympanum. And the other reason is so that we can - Oh, that is terribly out of focus. Compared to what I'm seeing. That's better. Okay. So one is that we can actually look here into the epitympanum now. And so again, I'm just going to kind of gently remove this ledge. A little bit off the ear canal. A little bit off the tegmen side. I mean, I can see this air cell here, so, I know that I'm safe to come through that. You can see the ear canal there - and kind of thin that down. So here what you would do then is you would kind of - you would touch a little bit of that, and then look, and then touch a little bit, and then look again. We - exposed it far more than we normally would need to, unless we were going to expose our epitympanum for a chronic ear surgery.
So the other reason other than just looking forward into this is - why this view is important because you want to get en face view of the posterior ear canal. Because getting an en face view is really the best way to thin it and get this razor sharp knife edge to the posterior bony ear canal. And it also allows you to be able to see a couple of other things. It allows you to see the disappearance of the air cells in the posterior bony ear canal wall. But I can also see a color change, I can see that that's now pink through that, so if I'm doing any kind of facial recess work… You're going to want that really soon. So that's one of the things I think that residents, you should really practice and work on in the temporal bone lab is thinning the ear canal because this is the safest place for you to get into the ear canal and it'll allow you to kind of get a better feel for that, demonstrating that, I may do it myself, we'll see. Open up these air cells in the mastoid tip a little bit more. Now we're ready to find our facial nerve.
So, I was mentioning before sort of that, you know, our facial nerve, the second genu is going to be just anterior and inferior to the lateral semicircular canal. It should not come out lateral to the lateral aspect of the lateral canal. It might as we go more inferiorly. But it should not at this level. And so, this is why you can kind of safely come with a cutting burr, kind of gently come through these air cells. And so what you're already seeing if I fill up this here - is this is a sentinel air cell of the facial recess. So I'm going to fill the middle ear up with water here, see that? And then I'm going to suction right here. And watch up here what happens. See that? So that - now I know that that's leading into the middle ear space. I'm going to come back and get some more. Thanks Nick. I'm just kind of opening up this area a little better. I was previously trained to find that at some point here, when you get a little bit closer to the lateral canal and close to finding the nerve, to transition from a cutting burr to a diamond burr, we're going to see what happens here in a cadaver because I have not yet done it in a cadaver, but in a live case, I think there's a good argument for continuing to use a cutting burr. And the reason for that is that when you're using a cutting burr, you know, you're really removing the bone. Versus a diamond burr, sometimes it's just pushing the bone away and can result in these sort of not plaques, but very, you know, these white smooshed-in areas of bone. I'm starting to see through here a little bit of a color thing. I'm going to drill behind it a little bit here. That's going to be our fallopian canal here. Clean up some of these other areas briefly. And again, this color change, I may not - I've been relying a lot by using the cutter recently in the operating room, but the color change is a little bit less dramatic here in the cadaver, so we'll see how that pans out. I'll go in front of it a little bit here. We'll go behind it here. Just to give us the space that we need. This area a little bit more as well, the ear canal in that area. I think from here what I'll do is switch over to a 4 diamond. Now we're going to just, again, we're drilling along the path, the expected path of the facial nerve here. We're starting to see that color change right through here. It's more of a white-pink change in the live patient. Here, you can see it's almost more of a gray, dark pink color change that's occurring through the bone. And again, I've got a well-aerated mastoid, so so I expect that the facial recess will be well-aerated as well. So this is actually a good place, I want to pause for a moment. I'm going to point out a couple of things here that we've uncovered as we've drilled. The first of which is a second nerve, that's, or not second nerve, but a offshoot of our facial nerve that we're starting to see and that I'm going to dissect in greater detail here shortly. So as Dr. Chiossone was alluding to, this is going to be our second genu of the facial nerve here. You know, for the purposes of the dissection, ultimately, I am going to take the incus out, but I just wanted to show that this is that incus buttress, right? So you can see that this is very thin. But even when I still push on this, it's still supported well by that posterior ligament, okay? So the ligament is still attached. When I push this - it's not free anymore, it's not free, but it's still supported well. And now that we've drilled out the epitympanum a little bit better, you can see an even greater sort of exposure here of our tympanic segment of the facial nerve. And that's right here. So the other thing I wanted to point out was that now, as we come more distally, this is going to be our chorda tympani nerve that's off-shooting here. Again, even though it's coming off - the facial nerve is running in this direction from - coming out medial to lateral, superior to inferior, the chorda tympani nerve comes off but then runs superiorly in this direction before turning up into the middle ear, okay? So that's going to be our boundaries again of our facial recess: our incus buttress, our facial nerve, and our chorda tympani nerve. And so there's something else that I wanted to point out here because it's nice that we're seeing it. This could be blood vessel, but I think it's worth pointing out that you do have sensory branches from the facial nerve to the posterior bony ear canal. You know, lovingly named after three surgeons from the University of Miami: Dr. Craig Buchman, Dr. Fred Telischi, and Dr. Adrien Eshraghi - the BTE nerve. And so that might be a branch of that coming here. They described in some cadaveric studies how that can come off, above where the chorda tympani never is, but ultimately course over the chorda tympani before going more inferiorly. So that may be a branch of that there. A little bit harder to distinguish whether it's that or maybe a small blood vessel.
As Dr. Chiossone was mentioning, the thing that we're going to start seeing through here now as I'm softening this, this is our digastric ridge. And the muscle through that. Now for your head and neck surgeons or for your neurosurgeons, when they're, you know, exposing things externally on the mastoid cortex or behind that, they're going to see the digastric groove, which is where the muscle is attaching into the bone. But, of course, we're seeing it from the inside aspect of it. So that groove to them is going to be a ridge to us. Yeah, and I'm not going to remove the tip. I just wanted to kind of give us - so, if we were doing a, you know, a temporal bone resection, or if we were going to be finding the facial nerve for our head neck colleagues, if they had a bad cancer out in the parotid gland and needed to find the nerve more proximally, or if we were doing a canal wall down surgery, we would take the tip off. And the way that we would do that would be to identify the digastric muscle, and then come through the posterior aspect of the mastoid cortex and the anterior aspect of the mastoid cortex where the digastric muscle is - that frees up the mastoid tip, and then you can cut off all the soft tissue and ultimately remove that, and that's what will allow you to soften up at the mastoid foramen, and trace the facial nerve out distally from there. What I'm going to do is I'm going to thin all the bone on the tegmen, on the sigmoid sinus, on the posterior fossa dura we're going to find the endolymphatic sac. And then we'll do our facial recess dissection when we start using some smaller burrs. So I love the diamond and normally if I was this close in an actual specimen, or excuse me, in an actual patient, I would switch to a diamond here, but the bone's still a little bit hard, so I'm going to for the sake of time and effort, I am going to continue to use the cutting burr here. You'll hear me telling you that, Zack, when we… I'm just going to come through these veins here. Interestingly, those veins are still within the bone. They're not actually on the tegmen itself, because that still has bone on it. That chipped off. So that's soft.
So I'm going to show you one of the tech - you know, I've - I've been lucky enough to train under you know, a multitude of phenomenal surgeon who have been trained by, you know, their mentors in very different ways. And so, you know, you can thin the bone off the tegmen, egg shell it, and then take it all off. Alternatively, you can get an area that's soft like this, and sort of get the whole superior ridge of it nice and soft, is what we're going to do here. Yep, yeah, this is brain right here. Covered with dura? Yeah, still covered with dura. And this is what we do in a skull base or a translab case. So this isn't just for the sake of the dissection. So sometimes this will deepen here. And you can imagine if I was doing this in an actual specimen, just how bloody this would have been because of all these veins here. Ultimately, if we were doing it, and we know that we have to decompress, we would be coming through them, and they would just - we would just have to control them with bone wax, or Surgiflo, or whatever else we had handy. So, we've got this nice area kind of thinned up here. I'm going to come to that a little bit more. And again normally, this will be done with a diamond, but… I'm going to thin up this bone here. But instead, what I'm going to do is I'm going to sort of wedge out a nice piece of bone. So I've got this off here, I'm going to find an anterior - kind of going this way - medial, and then posterior. And we'll wedge out a larger piece that way. In a little while still. Getting this down. Into our sinodural angle here. You can see our dura is all still intact, even though we have been drilling with a cutting burr. I'm only going to do this for this part just to kind of show you an example of how this bone can be removed. Because, you know, we're not dissecting a large tumor or anything like that. So this sort of part of the dissection isn't really critical, in that sense. And you can take a freer. We'll take a little penfield here. And find the edge of that. All right. So, the reason why we do this, and this is why I wanted to do it in this case is that if you're - as you work more medially, ultimately you're confined by your dissection laterally. And that's why we talk about saucerization for mastoidectomy surgery. Because if you're working in a space here, but you had only opened up an area like this, then you would not be able to work your instruments around in there. So, when you saucerize, and you open things up widely like this, even if you're working in a small area here, you're now able to have a greater degree of freedom in terms of how you're moving your instruments. So you can imagine if we were working more medially, or we had a large tumor that we were working on, you would want to be able to push this up out of the way, right? And this is actually great. You can see some, I mean, I'm going to assume it's still some sort of fluid in the process of fixing. But you can see that there. Do you see the little air bubble? So we know there's no leak. But the dura is all still intact. What that allows you to do, which we'll do it when we open up our sinodural angle here in a little bit, I'll switch over and do some of the endolymphatic sac and facial recess stuff and save the skull base for later. Because you can dissect and do that, and that allows you to now identify a free ridge to drill on. There we go. Thank you. I'm not sure if it's - sometimes when it gets foggy for me, I'm not sure if it's foggy because there's drill on it or if it's out of focus, so thank you. So again, when you do this, this allows you to have free edges to then drill on. You don't always have to do that for skull base cases, but it's certainly nice to. So the next thing that we're going to do is find our endolymphatic sac. All right. So we've got our lateral semicircular canal. And what we can do now is trace this posteriorly. I'm going to remove some of these air cells above the lateral semicircular canal here. And I'm already starting to see, because I dissected it out a little bit earlier… Is that still in focus for you guys? The posterior semicircular canal, you see that here? So this is great because again, it's well-aerated, and it's allowing us the opportunity to really remove these air cells around it and show their relationships to one another. And so again, trabeculated bone, hard, smooth bone of the posterior canal.
And so, there's an important landmark when we're looking for the endolymphatic sac. Or not landmark, but I guess a means of helping us localize it, and that's called Donaldson's line. And Donaldson's line is an imaginary line that's drawn along the plane of the lateral semicircular canal, bisecting the posterior semicircular canal and continuing on to the posterior fossa dura. Does that makes sense? A line like this coming posteriorly. And what Donaldson's line does is approximates the location of the superior limit of the endolymphatic sac. And so what I'm going to do to dissect the posterior fossa dura is turn the patient towards me. I know where the facial nerve is. It's important to identify the facial nerve, I think, when you do these endolymphatic sac surgeries. You don't necessarily have to, I suppose, but I think that it's wise to do so.
And so now, we're going to remove the trabeculated bone. I'm going to take some off the sigmoid here real quick just to see that a little bit better. So we're going to remove this bone over the posterior fossa dura. Again, he's very, very well-aerated. You'll probably hear me say that at least 10 more times during this case. As I'm removing this trabeculated bone - again, when it starts to smooth out like you're seeing here, you know that there's something underlying it. And the thing that's going to be underlying it in this case is going to be the posterior fossa dura that's anterior to the sigmoid sinus - anterior and medial to the sigmoid sinus. And so, the facial nerve is up here, so I can drill this. These are the retrofacial air cells. Can trace that up now. I keep my posterior semicircular canal in view. All right, so we're going to thin this bone down. Some of the sigmoid. So Zack, do see how this is starting to look here now too? Smooth and flat. So there's posterior fossa dura under that, okay? And do you know what structure is under all that stuff here? Yeah, but what - what neural structure is right there? So if this is the temporal lobe, what's going to be down here? Occipital is going to be more posterior. Is it your cerebellum. Cerebellum, exactly, there you go. Two points to Angela. All right, so I'm going to thin this off. Which I probably took the bone right off this back here honestly. All right. We're getting all this bone thinned down. We're about to remove it until we can take a look at the structures beneath. So I'm going to thin the bone here first. Okay. So, hopefully we've got this thin enough now. I'm going to take a freer here. It's not a very sharp freer, unfortunately. So here's what we're going to do. So sometimes what you can do is you can thin all the bone, and you can either remove all the bone, or you can leave an island of thin bone overlying the sigmoid that allows you to then compress it - referred to as Bill's island. So I'm going to, probably, we'll see how this goes, I'm debating what I'm going to do in this case, whether we remove all of it or have an island. And all this - getting it nice and thin. We got bone there. You can see when you leave that little rim of bone, and then come all around it, it allows us to compress the sigmoid like so. All right? So now that we're able to do that… And I set that down. Come through that. Through that. And remove this bone. So now we've got our sigmoid sinus compressible. Here is going to be our superior petrosal sinus, which we'll find, you know, much later. I'm not as concerned about doing that right now. Donaldson's line would estimate that our endolymphatic sac should be about right here. And I do believe we're starting to see that, so I'm going to thin a little bit more of this bone. But notice the sort of very kind of purple-pinkish color, thin dura here. And as you get down this way, you're starting to see this double layer of dura, or the thickened dura that's here. And they're all kind of running - all the fibers are all kind of running in the same direction. And so that's usually indicative that you're near the sac. I think the sac is going to be right under my freer, there. So I'm going to remove this bone, and we're going to prove that to ourselves - that that is indeed… Okay, again, facial nerve is here. And our facial nerve is there, and I'm going to drill under - almost under here for a second. And that's going to allow us to remove - the bone here - up to the posterior canal, which is right there.
All right, so - we'll prove it to ourselves here that that is indeed the sac. We got all the bone off. I'm going to take a little curette here. This is our endolymphatic sac here, which I'm going to open up and prove to you that it's a double-layered sac here in just a moment. But look what happens when I push down here, like so this is just, you know, the cerebellum posterior fossa. But look at this right here. Do you see how this is getting tented right here? That's going to be the entrance into the bone of the endolymphatic duct, which has a name called the operculum. And so, that's a really nice example of the endolymphatic sac here, and the endolymphatic duct going into the operculum right here at the posterior semicircular canal. So again, this is for the purposes of the educational value. So I'm going to incise this dura here, okay? See how there's cerebellum coming out now? So, that's a single layer of dura, and then all of a sudden - boom, I was right there on cerebellum, right? Let me let him get this picture. Go for it.
So I cut that to prove - to demonstrate what's going to happen when we open up the actual sac here. So when I cut through this dura… What do we have? A second layer. Look at how beautiful that is. All right. Do you see that Dr. Chiossone? So, my question to you: what do you do then? Nothing. Okay. All right, so this is going to bring up an interesting point here of endolymphatic sac decompression. Okay, that's a decompression. I have seen decompression. I have seen decompression with cauterization - with bovie, or excuse me, with bipolar cauterization of the endolymphatic sac. I've seen decompression with incision. And I've seen decompression with incision with a just sort of triangular silastic shoved in, and I've seen decompression incision opening and a little T-shaped stent that gets rolled over on each other. Cal Cunningham does that technique.
I did not know. So this is a nice example here. So I want to show this. And I'm going to do this before I open the facial recess or much more of the chorda tympani nerve. I do think that we should just take this down. So, again, we had the bony covering on the incus buttress. Sorry, is that in focus for everybody? Okay. I was going to take it out later to show the tympanic segment. So, this is what we were saying before, where this is still a little piece of bone here. And you can see it's become more lax with time because this ligament - this is that posterior incudal ligament right here. I'm taking away from some of its bony support into that fossa incudus, right? So as I've done that, it's had less of a - remember before I - It took me a fair bit of pressure to actually get it to do that. And then again too - look this, this is that other - that's that superior malleolar suspensory ligament here. So I'm going to cut through that. So this is the malleolar - the incudomalleolar joint right here. So, again, we've got that. The one thing I wanted to show you from a technique standpoint here, guys, which is true for the facial nerve as well. It's true for any nerve that you do. But, when you have nerve, right here, then you just want to kind of follow it. And you follow it by drilling the last kind of right on the last place that you can see. And so, we're going to trace this out. So I can see nerve right here, all right? And so then I just kind of drill in the expected path of it. And you see, now I can see it here. So I'm going to drill just past that. Get some of this bone out of the way. Oh, man, much better. I think the ICC just keeps spinning around on me. Is that in focus? Then I drill a little bit more. See, now I see it here. I'm going to take some of this down. Even if we get into canal, again, this is for anatomic dissection here. And drill that down. So, then I drill here. Now I see it there. So, you see that's where it's going into the middle ear. So now what I'm going to do is I'm going to stay posterior to it. I've got my eye on facial nerve. And I'm going to drill between the two, okay? If you're using a drill, like a Stryker drill, that doesn't have a protected shaft, you need to be cognizant of that as you start to drill more medially. This still generates heat. And if you haven't resting on the facial nerve, you can cause thermal damage. Okay? All right. So… We're about ready to identify some structures in the middle ear.
We've got our facial nerve. We've got our chorda tympani nerve. We've got our short process of the incus, posterior incudal ligament, and the incus buttress. So our facial recess delineated by incus buttress, facial nerve, chorda tympani. Notice that we always think of these things as a triangle, right? And it is, but not all legs of the triangle are in the same plane. Does that make sense? So this is in kind of a - these two are in a relatively similar plane. But look at the chorda, you know, the chorda comes off in the lateral aspect of the facial nerve, and so it's coming off, and then it's traveling, and then it's diving medially, so they're not always in that same plane. So we've got our incudomalleolar joint. And then here, we've got this bony protuberance called the pyramidal process, okay? Within the pyramidal process is the stapedial muscle, which is giving rise here to the stapedius tendon, okay? You see the stapedius tendon here? And the stapedius tendon… Is that in focus? The stapedius tendon is attaching to the capitulum of the stapes. This is a lovely view here. Just a few little middle ear adhesions. So, stapedial tendon, capitulum. This is the posterior crus of the stapes, okay? Anterior crus you can kind of see right here. And then underneath it is the footplate? That would be the footplate, correct. Sitting on the? Oval window. So this is a great relationship I want you to burn into your memory Zack. And I think you'll get asked about this on your rotation. And that is - there's a relatively constant distance between the oval and round windows. Okay? So if you're doing chronic ear surgery, you're having a tough time finding where, you know, the stapes is eroded, you don't know where the footplate is, but you know where the round window is, you can use this relationship, okay? And so, let me point it out, I guess, first. So there's the round window niche. So this is the round window niche here. There's a little bit of a false - probably a false membrane here over the round window membrane. But this relationship is important. So I have a 2-mm burr. Okay? So the distance from the oval window to the round window is approximately 2 mm. Do you see that? Oval window, round window. So that's just a good thing to keep in mind, okay? Now I think for the purposes now of our dissection, I'm going to remove the incus.
So what I'm going to kind of do a bit blindly here… But because I want to leave, for now I want to leave the stapes on the oval window. Because I'm going to separate the IS joint. I'm trying to kind of slide into it. Again, you remember earlier, Zack, when we were talking about the steps of the stapes surgery and dividing the joint? You want to try to divide the joint in a posterior to anterior direction. And the reason for that is because you're using the tendon as your - yeah, as your counterweight, your counter-tension rather. So, hopefully I didn't disarticulate the staples with that. But again, not why we're here. So that joint is separated. And oftentimes this is done, this part here is done blindly. Because you don't have the epitympanum open as much yet when you're doing a chronic ear surgery. You can imagine, if the incus - so if we weren't able to separate the joint, and even when we had, if I came to separate the IM joint, and I came in from the back like this. It's fine to do that. But you can imagine, if I push - if I were to push up on this, look what happens to the stapes. See how it gets pushed in? So ideally, when you do this separation, you actually want to separate from above because then, as you put pressure down, see how the stapes is not moving when I'm doing this now? And so I'm trying to kind of just work it into that joint. And then pull it out. Okay? So that's the incus. The incus, also lovingly known as our anvil. It looks like an anvil, also, I think looks like a predator head. It doesn't go with a hammer? Yeah, exactly. So, great, another great relationship to see here is, again, do you know what this is here now? Sorry, check out - look at the TV screen there. Do you know what this is here? So this is going to be the tympanic segment of our facial nerve, right here. So the facial nerve is coming out, and then turning. So I'm going to remove the incus buttress. We're going to do some good middle ear exposure because I think seeing these relationships is what really helps solidify that. I know it's not necessarily by the book in terms of the manual, but I want this relationship to be emblazoned because it's going to help you with understanding middle ear surgery as well, okay? We're going to take down the incus buttress now. And I'm thinning this, again, much more than I normally would. But I want you to see the course of the facial nerve, okay? Once you see it in it's full length and sort of progression, I think that's when you start to understand exactly what it's doing. It's really hard to conceptualize and visualize when you have everything else in the way still, don't you think? So, if I went in - if I went much more here, I would go into the annulus and the tympanic membrane, which I'm probably close to doing right here. Anyways, okay. Water off. 5 Suction. So, now… So now we have the entire, not entire, but now we have the tympanic segment of our facial nerve starting here, coming out, it's going inferior and lateral. It's making this turn at the second genu, and then it's going distally towards the stylomastoid foramen. So this is another important thing. So tomorrow, when we do our stapes surgery look at how close the facial nerve is to the oval window and the stapes. You see that? And I'm going to leave this for now. I don't - I think it's still attached. Maybe partially disarticulated, that anterior crus is very thin. So do either of you know what this structure is below the facial nerve here, anteriorly? This is one of Dr. Telischi's favorite questions. And I want to make sure I tell the residents that so that they can know it now. It's a very, very important relationship. So this structure, right here - is the cochleariform process. Within the cochleariform process and coming out of it, is the tensor tympani tendon attaching to the neck of the malleus. So, this is our tensor tympani tendon. Do you see that that's soft that I'm pushing here. And below it is hard? Exactly, so this… Yep, that's hard. That soft. So what I'm going to do is I'm going to take a pair of Bellucci scissors. I'm going to cut that tendon. Is there ever, like, a surgery where you would just get rid of the incus buttress like that? Yes, and if that's - so, removing the incus buttress and opening this up is called an extended facial recess dissection. And you do that in chronic ear surgery. It promotes you greater access to kind of seeing - so the malleus is still going to be attached to the under-surface of the tympanic membrane, but I kind of want to try to get it out here. One, so that you can see it. Two, so that we can, again, have better access to the anatomy of the middle ear here. So that's the tympanic membrane that I just took with it. Okay? Is that the TM right there? This is a little part of the TM. So do you know what this attachment would be to this part of the malleus here of the tympanic membrane - what would we call that? It's called the umbo. The umbo, yeah, exactly. So we have our manubrium. Right? So this is our manubrium, or our long process. This is our lateral process, this is the neck, and this is the head of the malleus. So in this ear, it normally would have been sort of sitting here like - one like we just saw, right? The angle - the, you know, the - excuse me, the incus was attached right here. So you have the lateral process, and that's where that pars flaccida is of the tympanic membrane, right? We see pars flaccida retraction pockets and cholesteatomas. Okay. So we got incus and the malleus out. I'm going to leave the stapes in now and I'll show you why here in just a second. I don't think there's anything else I want to do in the middle ear right now or show you in the middle ear right now. And that's the cut end of our tensor tympani muscle. So I think we're going to move on now to the labyrinthectomy.
All right, so when we're doing a labyrinthectomy, the first thing that we're going to do is we're going to open the lumen of the lateral semicircular canal. You know, usually, we wouldn't have all this exposed. It's great to be able to do and great to be able to see, but one of the lessons and pivotal things to keep in mind when you're going to do a labyrinthectomy, is to leave the inferior shell of bone. That's what's going to protect you from the facial nerve, okay? So, you see the lumen of the - excuse me, the lateral canal here, right? So we're going to open that. We're going to open it up to the ampullated end, which is anterior. All right, so that's now open. We're going to trace it posteriorly. We're going to thin this little rim of bone on the facial nerve. You see how close that is, right? So again, because I'm doing an anatomic dissection, I am going to actually kind of show you probably more than I normally would take. So when you open the canal, it's typically lined by, like, a membrane, right? No, you - honestly, this is not that different from what it would look like in the live - you know, it's not - and so it's not lined by a membrane, it's actually contains the membranous labyrinth, which is filled with fluid. But the diameter of the membranous labyrinth is only about 25% of the total diameter of the bony labyrinth, okay? So, what I've done is I've gone through the lateral semicircular canal, and I've gone posteriorly, and that's now identified the lumen of the posterior semicircular canal. I'm going to trace the posterior semicircular canal inferiorly, I'm going to - real, real close to the facial nerve here. Very, very thin on the second genu here, you got to be careful about that, okay? So I'm going to trace this down to its ampullated end here, okay? Sometimes you can have a very high jugular bulb, which will come up down in this area here. Thankfully, in this patient, that is not something we have to worry about. I'm just opening this up for the sake of it here. So we were talking about the pyramidal process earlier, right? And the stapedius muscle that was in that. Yeah, the pyramidal… And we were talking about that housing the stapedius muscle, right? So the stapedius muscle is running medial to the facial nerve, and so that's actually the stapedius muscle right there. Okay? And then in the distance here, what you're starting to see is the jugular bulb. That's the jugular bulb down there, okay? So if we were doing a glomus case, like a glomus jugulare, we would be tracing the sigmoid all the way down and out to the jugular bulb. You can see down here, this is some marrow-filled space, not uncommon down in this sort of retrofacial and mastoid tip area. You know, it's got that kind of brownish color to it, that's just marrow, okay? So again, that's not important in this case, I just wanted to kind of show - to give you a sense of that here. So that's the jugular bulb in there, okay? So we're thinning - we've got our posterior canal that's in here. We go down to the ampullated end, which is right there. Okay. Then we follow the posterior canal superiorly. So what we're going to do here - I'm sorry, that's - excuse me, that's still the lateral semicircular canal. I apologize. Is that focused? So, lateral semicircular canal, posterior semicircular canal was right here. So now what I'm going to do is I'm going to follow the lumen of the posterior semicircular canal to the common crus. Or crus commune, depending on how you like to say it. Going to follow that. And we're able to do that. So this is the common crus here. It was giving rise to the posterior semicircular canal. But it's not called the common crus for nothing because it also gives rise to the superior semicircular canal, right? So I'm going to thin this bone here. Again, that's what's nice sometimes, when you - see how when you decompress the tegmen, and then you can kind of just gently push it? Allows you to thin that down, so I'm going to just kind of keep thinning this bone. We're getting close on the water there. And this is just so I can get the exposure that I need. Okay. So now, I'm going to follow the limit of the superior semicircular canal, which is there, right? I still see it there. I'll try to leave as much of it in view as I can. Yeah, I'm kind of fighting against a fixed brain here. You see this is still the lumen of the superior canal. So I'm trying to kind of leave that medial wall. So, this I - can we change this? Is that okay? Thank you. So this structure - so we've got the superior semicircular canal here, right? That lumen. And then here, we have the subarcuate artery. That's very, very nice here in this case. So that's the subarcuate artery. Oftentimes, when you're drilling this in vivo, this will give you some bleeding, and honestly you kind of just have to keep drilling through it until it stops. Now we're going to create what are known as the snake eyes. The snake eyes are the ampullated ends of the lateral and superior semicircular canals. Right? Because we know that the superior vestibular nerve is what provides innervation to the utricle, the lateral semicircular canal, and the superior semicircular canal. So it would make sense that the ampullated ends of the superior and - excuse me, of the superior and lateral canals would be adjacent to one another, right? Because they're going to be sharing that. One of the things to be cognizant of is you don't want to - when you're working here, and you don't have all this stuff open here, is you want to leave a thin rim of bone along the anterior aspect of these, and that's what's going to protect you from your labyrinthine segment of the facial nerve because that's very, very close there, okay? So we've got now that open. So now I'm going to open up the vestibule. Let me see here. Just let me get this TM out of the way. It's going to be - this is going to be a really - a really cool point here. And that's why I left the stapes where I did. All right, is that in focus? Okay. So now, I'm going to follow this sort of common crus and non-ampullated end of the lateral semicircular canal into the vestibule. Let's take out that intervening bone there. I'm going to gently… Thin this bone. All right, so this now - now we've opened the vestibule. I'm going to finish opening that here, hold on. Where you can see the spherical and elliptical recesses, I'm not sure the plural of that. So again, the lateral and superior semicircular canal ampullated ends are going to share the superior vestibular nerve with the utricle, right? And this might still be some of that neuroepithelium of the utricle here that we're seeing. The inferior, or posterior, semicircular canal is going to have its innervation from the inferior vestibular nerve and is also going to be innervated by, or is also going to innervate the saccule, right? That's going to be right here. So the reason why I wanted to keep that in place, or excuse me, the reason why I wanted to keep the stapes in place was to show you this. So we have the vestibule opened, we know that the stapes sits on the oval window, and that the oval window is overlying the vestibule, correct? So then what will happen if we push the stapes through the oval window? Woah. Inception. Right? And I'm sorry that it broke. It would have been a heck of a lot cooler if it hadn't. But, you know, that's just a great relationship, right?
So, the reason why you want to keep in mind the - this you'll see a lot in the House dissection manual when they talk about the IAC is… Right? We talked about what innervates the lateral and superior semicircular canal, correct? The superior vestibular nerve. So this is going to demarcate the superior extent of our internal auditory canal, right? Nothing should be above the superior vestibular nerve. Much in the same way that down here, the ampullated of our posterior semicircular canal, innervated by the inferior vestibular nerve is going to demarcate the inferior extent. Oh, and this is beautiful here. I, this is, watch this. Remember how we were talking about the operculum? And the bone of the endolymphatic duct earlier? So that… Is this the endolymphatic duct? Correct, yeah. So I'm actually skeletonizing here. See that? So this, when we opened it up earlier, endolymphatic sac, endolymphatic duct, in the operculum. And look at where the fluid - can you all see that fluid bubbling right there? So the bone that I'm removing right now is bone within that sinodural angle, right? There's a big blood vessel in the sinodural angle. Do either of you know what that's called? What was that, Zack? No, transverse is going to be proximal. Superior petrosal. Superior petrosal drains the cavernous sinus to this TS junction, okay? So transverse sinus would be back here. Superior petrosal sinus, on the other hand, will be in here. Again… That's something I would use a cutting burr for typically. So now I'm going to remove all this bone out of the way here. I'm going to leave some of it actually because it's helping me keep the brain out of the way. I'm going to come through the duct now. So, again, that's where that is. So I'm going to create kind of a little trough. Again, this patient we don't think has a tumor, right? I mean, they maybe had a - I don't know, it was never diagnosed, but we aren't expecting a tumor. So the other thing to keep in mind about the internal auditory canal is that as it leaves the, as it enters the porus and travels towards the fundus - so as it's going from medial to lateral, it's going in a posterior to anterior direction. So it's not oriented like this, actually, the way to think about is it's in a similar angle as the external auditory canal. So it should be angled like this. All right? So I'm going to find it, hopefully, here at this portion of it. As we come through the media wall, of the vestibule here. And again, pardon - please don't hate me my mentor, for me doing this with a cutting here, but for the purposes of moving this along. You see how this is starting? It may not show up as well for you guys. There's starting to be a subtle color change right there. No, what that's actually going to be is our internal auditory canal. And that's one of our nerves right there. Do you see that? What I'm going to do is now trace it medially. All this bone is going to have to go. A little bit more of this bone away. To be able to see that a little bit better. You'll see why that's important here in just a few minutes. So this - has got to go. Okay. Now we're going to keep tracing this medially. This is always very, very thick bone here. So once we get the internal auditory canal exposed, we'll skeletonize it a little bit better, we'll open it up. I'll show you guys the nerves, and then we'll call it there, okay? Always have to be cautious here, especially as you get out towards the fundus. Because you're going to have the takeoff of the labyrinthine segment of the facial nerve, okay? Hopefully I'll be able to safely find here. Yep, absolutely, thank you. Better? Yes. Great. This is going to be the takeoff here at the porus, we're getting a little bit closer there, see how it's kind of getting into this little wedge of bone? I'm kind of trying to push that away. This burr probably needs a longer shaft around it. It doesn't chatter like that, but… I don't want to get impatient here at the end. I got to say, I kind of am. Just because this is kind of the coup de grace of opening up the IAC and, you know, looking at all the nerves and everything. So you can see why it's important to protect the facial nerve here. But how even a little millimeter or two bone on the second genu can really impede your visualization of that inferior trough. So I'm almost kind of coming under - you have to be careful to drill under the facial nerve because then again, it's going medially here, right? So, just something to be cognizant of. So I feel good about that trough. I'm going to finish our superior trough here. You can tell, we're kind of coming around it. Just doing a little bit of a… You probably pointed this out before, but this is the...? Oh, sorry. That's the internal auditory canal. Yeah, so I'm coming around it now. Because remember, it's completely encased within the bone of the petrous portion of the temporal bone, right? And so - that's why, once you identify it, you trace it, and then you have to get around it because when we dissect the tumor out, you want to be able to come around to the tumor - oh shit, I got into it there, okay, so - what we're going to do now - I'm going to get this a little bit thinner, and then we're going to transition away from - we're going to open it up. So, I'm going to point out a couple things here. It's not perfect, but you know, I want to be expeditious about this in some ways. So, it's hard to appreciate, and I'm sure it's going to be hard to appreciate on your screens, but this is lateral, this is medial, this is anterior, this is posterior, right? So as it goes from - it goes from a medial, posterior position to a lateral, anterior position, okay? So I'm going to finish getting some of this bone off. So, again, if we want to talk about identifying structures… (indistinct). I don't disagree with that. A little more bone here. So this is the superior vestibular nerve, okay? This structure right here. Looks like - I don't know if I cut that out or not, I may have, again, I may have accidentally already pulled out some of the nerve as we were opening this. Or I think this is probably superior vestibular nerve. Out here towards the fundus, you have a crest called the transverse crest, which separates the superior and inferior vestibular nerves. See that right there? And Bill's bar, the vertical crest is what's going to divide… I need to get more of that bone out. Is going to divide the superior vestibular nerve from the facial nerve. So this - I want you guys to look here. So, this was the inferior vestibular nerve that I had pulled down. So, inferior vestibular nerve. I'm pulling that back. This is the transverse crest here. See that? Is that in focus, I hope? Yeah, good. So transverse crest, so - see how I'm separating this now? This nerve here. I'm separating that from the - likely the cochlear nerve there. All kind of mushed right now. Which is fine. So that's the inferior vestibular nerve that we just pulled down. For vestibular schwannomas, is it usually inferior vestibular? No, I do not know of any studies that show predilection for one or the other. So just so we can see the bony anatomy this, I'm saying this on purpose. I'm going to pull the cochlear nerve out now too, okay? There goes the cochlear nerve. Focused? Better? Okay. That's facial nerve right here. See this? See - because - it's going - this is that labyrinthine segment here. So it just didn't separate well. Okay, so that's that labyrinthine facial nerve that you're pulling? Yeah, right here is labyrinthine facial nerve. Is what you're pulling off of it the… The superior vestibular nerve. Well, hold on, let me just make sure here. Yeah, because this is going to be that vertical crest right here that was separating the two before as that kind of pulled back. And it's a little bit challenging this sort of delineate that in this cadaver. I thought this was going to more of the ampullated ends of the canals, but I guess I was wrong. So what we'll do, and I'm going to just avulse that for now. All right, so that's all squished down. So we can appreciate all of these sections now of the facial nerve, okay? So, what we did is we had the intrameatal, or intracanalicular, segment of the facial nerve. It exited through the fundus to become the labyrinthine segment of the facial nerve. It makes this first genu, or turn, at the geniculate ganglion becoming the tympanic segment here. It makes the second genu into the descending, or mastoid, segment all the way out into the stylomastoid foramen, all right? And that is your facial nerve.