All right, one of the reasons I was futzing with my slides is that I wanted to update a little bit based on some of the things that I've seen. How many people here actually have taken a course or have had formal lectures in your residency program on biomechanics? Can you raise your hands? All four of you? Everybody else? Have you had any experience in biomechanics? Well, and that's why I kind of wanted to throw this in there into my talk, kind of like what Chris Schaffrey talked about yesterday. It is so important to understand some of the fundamentals. And I'll tell you, at an academic institution, most of our referrals are from other surgeons. And the two most common errors that I see are errors in diagnosis and errors in biomechanics. That is doing all these huge constructs or planning these surgeries, but not really quite understanding what you're putting in, and then leading to the failures of things to the point that you're just like, I don't know what to do, send them to the tertiary center. You know, that's just, and you've seen those come into your centers before, right? So that's why I want to kind of add in a biomechanics primer when I talk about spinal anesthesis. You'll kind of understand why. Nothing to disclose financially, however, I will use the lateral mass screws. Cervical lateral mass screws are only approved, actually, not for the cervical spine. They're only approved in the thoracic spine, typically T1 to T3, which is why they're considered off-label and difficult to add into the courses. And thanks to David Arconco for some of his slides on anesthesis. So the types of spinal anesthesis, I know Praveen had already asked you some questions yesterday, and I'm not going to belabor the point. You know, you guys can read about the various types of spinal anesthesis, whether degenerative or those with PARs defects, ischemic. The only thing I want to highlight is there are certain ones, whether ischemic, traumatic, congenital, or dysplastic, that actually can present with high-grade spinal anesthesis. And the biomechanics of patients with these types of problems are going to be significantly different than those, say, with degenerative spinal anesthesis. So that's why people with a D-gen spinal anesthesis, with the elongated PARs, rarely progress past a grade 2. You know, it's one of these things where you really don't need to intervene as quickly as you would for some of these others. We also talked about the grading of spinal anesthesis. You guys are all familiar with the Myerding classification, you know, the percentage of displacement, right, 0 to 25 percent, up to 50 percent, to grade 2, 75 percent, and then also to grade 5, which is a complete spinal optosis. We typically define a high-grade anesthesis as greater than 50 percent displacement. The only other issue with the high grade is that frequently you'll hear people call things a higher grade than it really is, so that if it's, like, 40 percent, most people will start calling it, like, eh, maybe it's grade 3. And you'll kind of see that. But in general, the Myerding classification is very specific. However, this is not the only way you want to look at spinal anesthesis. All this talks about is a translational component. There's also a very significant angular component, which we call the slip angle, and that's something you shouldn't forget, right? So you want to measure the lumbosacral kyphosis caused by this because a slip angle will actually indicate whether or not your hardware or your construct is going to be more prone to failure or not. And this, again, the reason I'm putting this in there is that this is one of the common parameters that is overlooked in people taking care of patient spinal anesthesis, where it doesn't look like a high-grade anesthesis, maybe a grade 2, but they didn't realize it was a huge slip angle. They just do a simple 5-1 fixation, and only to realize that the S1 screws keep loosening up or failing, and not quite understanding why, and we kind of go over some of that. And this is just an example of a patient with a slip angle to give you an idea that, yeah, you can reduce them back, but if you don't change the slip angle, imagine the forces that's going to be loaded onto the hardware, especially if you're trying to do a short segment construct. And remember, some of your fixation points, like the sacrum, really are not as strong as, say, the L5 pedicle, because you really only have two pedicle walls, right, because the sacrum blends in together into a large cancellous bone mass. And even if you go tricoracal fixation, that's great, it's a second coracal surface, but there's other issues related to that, which we can always talk about. Now the sagittal plane deformity of spinal anesthesis is also huge. So slip angle, again, the high slip angle are those of the patients that you see with almost loss of kyphosis in the thoracic spine. They're trying to compensate so much that they'd kind of bend their back this way, their butt sticking out, their backs arched back to try to do that, and that's a different patient than someone who is compensated overall. Now this is all nice and good, but how many people really understood what I'm talking about, about why slip angle gets into the, you know, come into play? See, no one's raising their hands, they're kind of giving me a bunch of answers, which is why we're jumping into a primer in biomechanics, okay? This is to complement what Chris had talked about yesterday, which is more of a higher level biomechanics component in biomaterials, and I'm gonna give you a primer so you can understand really what we're talking about when we use terms like Young's modulus or stiffness, et cetera. So you really understand that biomechanics actually have a significant amount of comorbidities. Unlike craniotomies, which we do as neurosurgeons, spine's different now, because it's a load-bearing structure. So if you wanna do a craniotomy and you leave the bone flap off, eh, that's nice. Maybe you'll put them in a helmet, maybe you won't, but it's not a huge deal, right? All you gotta worry about is the anatomy and the physiology. Now in biomechanics, if you wanna do a corpectomy, take off a piece of the bone in the spine, that's fantastic. You wanna leave it off, well, the patient can't sit up anymore, they can't weight-bear across their spine, and so this becomes a huge issue. In general, what we're dealing with is forces, and these are all linear components, or forces across a certain distance, which causes rotation, moment in torques, displacement, how much does it move when you apply a force, based on the material properties, and at the end of the day, it's all about energy, it's all about transfer of energy, and I'm gonna explain why that is such a key concept, because that's really what we're talking about. We're not talking about load preservation or motion preservation and all that other stuff, which again, is kind of a pet peeve, because we focus on motion, but implants don't fail because of motion, implants fail because of excessive loading across it, right? Your chair can swivel, that's not gonna cause it to break. You put too much weight on your chair and it breaks, you're like, yeah, what'd you expect? And that's the same issue that you deal with when you're talking about your spinal constructs. Some examples, like this is a simple ACDF, just to throw you a curveball, where you see an x-ray like this, you're like, wow, that looks pretty good, right? Nice cages, nice, tall inner spaces, plates put on perfectly, but then it progresses to failure, and you're like, eh, what can you do? These things happen, right? But does it really? Because this is really a complication of not understanding the compressive forces across that area, leading to subsidence and plate issues related to that. So to start off with, what we're talking about forces is linear forces as a through variable, right? So for every force, there's an equal and opposite reaction. So for example, me standing on this floor right now, my body weight has an equal and opposite force coming up through the floor to mitigate that body weight. Otherwise, I'd be falling, free-falling into the center of the earth. And so that's pretty simple. Mass times acceleration is really the concept of force. Now the reason why I kind of bring this up as far as mass is that when we talk about weight, we're really confusing the picture in medicine, because weight, when you talk to engineers or you're talking about spinal biomechanics, when you're talking about newtons or weight or pounds-force, you're really talking about the force. But in medicine, when we say, how much do you weigh, we really are asking, what's your body mass? Which is not weight, right? It's just mass. So that's why we calculate body mass index versus body weight index off of kilograms, because it really is not the weight. We have to still multiply that by gravity, which we assume is pretty constant. However, this becomes a problem, because when we start taking that concept of kilograms and mass, using it as weight, and then we start putting it into scenarios where acceleration is not gravity, someone fell off a ladder, someone got hit by a car, we actually start miscalculating what we really need in that scenario because of this issue. The other thing I want to point out is the difference between quasi-static and dynamic. So quasi-static is, if I put a book on a Styrofoam cup, that's a quasi-static loading. And when you look at a plain x-ray, that's all you're seeing is just a person standing there with an x-ray, what we call standing films or loading films. But if I took that book and dropped it a few centimeters above the Styrofoam cup, would you be surprised if this happened and it broke? Or would you sit there and go, how did that happen? You know, it was fine a minute ago. Or would you say, yeah, what did you expect? If you dropped it for a few centimeters, the force is different, it broke it. So how is that any different than a patient that you put a construct in that walks at 4 to 8 hertz, that is cyclic, that goes and plays sports, that does activities? Are you going to be surprised when bones don't hold up the way you want them to, when constructs loosen up a little bit? Or are you going to sit there and go, no, that makes sense, you know, because dynamic loading and quasi-static loading are different. And so this is just an example. So if you think about this case, the simple ACDF case, you start looking at this and go, well, I get it now. I over-distracted the disc spaces. Look how tall the disc spaces are relative to that. So anything greater than 10% really creates a lot of elastic recoil from the remaining ligaments that you stretch out overall. We shaved off the end plates so that we took off the cortical end plates, leaving a lot of cancellous bone exposed, which is a weaker component. And that's what Chris had talked about yesterday about the modulus of elasticity and why some people try to choose peak implants to mimic that. And I can talk about that later. What else did we do in here? Well, we used a dynamic, we used a variable angle screw system, right? So by using a variable angle screw system at all the levels, we ended up overloading the bottom because nothing else is mitigating or stress shielding or absorbing any of the load. So how can we avoid this in the future? Well, we can not over-distract the inner bodies. We can use a hybrid construct with fixed angle screws to offload some of the forces across the multiple levels. And so those are some of the things that you can think about as an example of when you look at your constructs and saying, you know, why did it fail like that? Now distraction forces are important. So things can either compress or they can distract, otherwise known as tensile. Now when we distract forces, you're like, yeah, I get it. I pull on it, it moves apart. And that's why it's important to understand that when you're looking at, for example, in trauma, for example, if you're looking at a patient with a cervical spine injury, you're not really looking for bony injuries for distraction forces. This is where ligaments come in, right? So I know you want to focus on things like the transverse ligament to look for C1, C2 injuries. But what's the second strongest ligament in that area? Any guesses? Alar ligament, right? Right. So, well, basically it's the extension of the, I'm sorry, it's the extension of the posterior longitudinal ligament coming up into the tectorial membrane. And so basically that area becomes the second strongest region. And so when that's disrupted on your MRI scan, you can pretty much guess that by putting someone in traction, it's going to cause problems. And so this is something to also kind of keep in mind of what you're looking for for pathomechanics. So if you see an injury, you know what's going to happen if you apply a load. Guess what? That's exactly what I was saying about the spectrum of understanding osteotomies, where now you know what do you need to violate, i.e. bone, joints, ligaments, in order to get the spine to move the way you want it to move. And really you're creating an unstable situation, right, to manipulate the spine. So shear, or translational, is a horizontal force. And this is something, just a horizontal force pushing across like that cup. And that's important to know because, for example, that's why if you look at endontoid screws, that's why you need the fractures to be perpendicular to the long axis of the screw that you're putting the lag across. Any other angle will start creating a shear. So for example, this person that put in this screw, in this type of angled situation, you could already look at that fracture ahead of time and know that surgery was destined to failure. Right? There's actually not even a debate about that, because from the biomechanics standpoint, that surgery would never have worked. So why do we even try that surgery in a situation like this, right? So that's something kind of... Now there is no rotational forces or curved force vectors. And so when we talk about things like moment or torque or things like that, what we're really talking about is a load across a distance. And so this is just an example of a post-laminectomy kyphosis in a cervical spine, where the weight of the skull's coming down. The center of gravity of the gravity line is separated from where it's the center of rotation and at angles. Now that's a good segue into kind of explaining the difference between center of rotation and Tyler alluded to the instantaneous axis of rotation. Center of rotation is a two-dimensional model. So that if you were a two-dimensional concept, which is if you're looking at a plane x-ray on a lateral spine and you have someone flex and extend, you can calculate where the spine is moving or what parts of the spine is moving relative to a center or point where it's rotating around. Right? That's different than an instantaneous axis of rotation, which is a three-dimensional concept of a single vertebral body or rigid body moving in space. Okay? Now you may say, well, what the heck does that mean? Think of it this way. If I had a football and I threw a spiral and the football was arcing across the sky, you know, in a spiral, the center of rotation is how the football's rotating. Right? That's the instantaneous axis of rotation. The reason it's instantaneous is that at every point of that arc as it flies across the air, it's tangent. The axis rotates and changes in an instantaneous fashion. So that's what we're talking about. We're just talking about how does the piece of bone move relative to another bone, but it also requires you to have a fiducial or another point of reference to compare that to. So when you look at aspects of moments, for example, every force has an equal and opposite reaction. A guy dives and hits his head onto the ground. His body weight is driving it through the spine, the equal and opposite reaction of the ground into a skull. The distance between the two becomes a center of rotation, right? And the distance between where the force is and where that center is becomes a moment arm, force times distance. Now don't confuse moment arms with work. So work is force times distance. And that's how you calculate Newton meters. That's the energy of how do you take care of something. Force times distance in a moment arm is different because in order to get work, you have to multiply this by radians, which is unitless, which is the angle of that. I don't want to get too much into the details, but much is just to remember that moment is not the same thing as work, even though it seems like the same calculation that you do. But once you understand this, are you surprised that when a person lands on their head, that the C5 region seems to be where the center of rotation is and why typically teardrop fractures occur at the C5 body? And so when you think about the mechanics of this, you're like, yeah, that makes sense. I get it now. I see why when someone lands on their head, it's pretty common to get a teardrop fracture because of the biomechanics that is being imparted into the cervical spine. And that's all we're doing in deformity. Spinal deformity, in my mind, is nothing more than lining up force vectors. If someone has a positive sagittal balance because your gravity line, your center of gravity is pulling down, the equal and opposite reaction of the reaction force is coming up, but it's separated, all we're trying to do is get this line over this line so it's zero. When someone has a significant truncal shift, they have imbalanced coronally, all we're doing is taking this force vector, their body weight, and lining it up over the equal and opposite reaction so that they can then mitigate the forces. And that's all we're really doing is mitigating moments, if you think about it, in spinal deformity surgery. And that's what Duboiset had talked about. The whole idea, the cone of economical function, is all about minimizing loads, minimizing moment arms. So when you're standing straight up and down, the distance is zero. The more you tilt, your center of gravity starts moving off from where your feet is, or feet are, and that becomes an issue. And so you can apply that a number of ways. So sometimes when you have two separate situations, this is one patient who had a corpectomy in a plate that did fine, another patient had a corpectomy in a plate and it started failing for the same type of vertebral body fracture. And you start thinking, why were they different? Was it the construct? Was it this? And you start realizing that different path mechanics, different tissue injury. Not every deformity is the same. You have to individualize it to the patient that's in front of you. So for example, this patient here has a compression fracture, right, of the vertebral body. The posterior elements, the ligaments around it, were not disrupted. So therefore, by putting a corpectomy graft in here in a cage, or in a plate rather, all you had to do was create another area of axial loading to support it. That's different than a teardrop fracture, where now the ligaments are disrupted. Facet joints are ripped apart. All the surrounding support structures are off. And so now it's not going to hold and shear. You put that cage in there, it's going to start slipping loose over time. Now, Chris had alluded to material properties, stress, and strain. And just some basic definitions. Stress is just the force over the area. Strain is the change of length over the original length. And so when you look at that component, that's called a material property because it assumes a homogeneous thing. The problem is that anatomy is not homogeneous, OK? It's heterogeneous. So unlike a piece of metal, you can't really take it down to the true material properties in the spine. However, stress-strain curve, when you plot it out, the most linear area of the stress-strain curve is called Y or Young's modulus. So when we talk about the modulus of elasticity, all we're talking about is the slope of that curve. If you don't have the area, and you don't have the parameters to measure the original length or dimension changes, you get a force-disflection curve. And a slope of that line, which is similar, is now called K or stiffness. And so something that's stiffer has a higher force for less deformation. Something that's ductile or elastic can have a high deformation with lower forces. None of them are talking about the area I need to curve. All we're talking about is the slope of the line, right? Make sense? Pretty straightforward on that. And the reason why this is important is that this is just another example of a patient who had a fracture, an ACDF was performed, and then it failed. And the question is, why did it fail? What biomechanics was overlooked? And you start realizing that because of the way the fracture pattern is, and this whole construct, now they use the fixed-angle plate system with the plate with the screws kind of short within the body. There's no way this thing would have held up well in the first place, right? So think about it, if you think about this whole construct now, it's like a Christmas tree. You have a tall Christmas tree and a very small base. Are you surprised that that Christmas tree tips over? So what are your two options if you had to fix this or readdress this again in the same situation? Well, you can either use longer screws or more screws at the bottom, that is create a larger base for your Christmas tree, or you can go posteriorly and tie it up. That is, take your Christmas tree, tie a rope around the top of it, and tie it to some anchor that's higher up, what we call a tension band and spine. And so those are the concepts that you want to see as you're planning these constructs. Putting in the hardware alone or putting the bone graft alone isn't quite sufficient without understanding what your goal is. So biomechanics is a little bit more than just quasi-static force vectors, and that's why I'm saying it's all about energy. If you take a force deformation curve and you do an area underneath the curve, and I know this is like college calculus again, right, that actually is energy. Energy transfer is what leads to things failing, okay, because you can't absorb it. So when you do biomechanics, for example, in the cervical discs of artificial discs, you're realizing that when you're talking about motion alone, you're not really talking about the force, and therefore when things fail, things fail not because of motion but because of excess loading across that area. And if things do fail, you have to kind of take a step back and think about it. And this is pretty common, right? So if I ask you, what is trauma? Trauma is the disease of excess mechanical energy. If someone came up and just gave me a small little punch in the arm, not a big deal, not much energy transfer, but if someone hit me with a baseball bat and broke my arm, well, that's trauma, right? And you wouldn't be surprised by that. Guess what? Trauma is either potential energy, right? Someone falls, mass times gravity times height, or someone gets hit by a car, kinetic energy transfer, or by a bullet, one-half mv squared. And if you think about it, this whole idea of energy transfer is not unusual. Because what is this? This is gray. Gray is radiation. When someone gets radiation damage, radiation myelitis, it's because they have excess radiation energy, which is joules per kilogram, a gray is the amount of radiation energy in joules per kilogram, or newton meters, over one kilogram of tissue. Too much radiation per kilogram of tissue, you get tissue damage. Thermal energy, calories, guess what it does again? It's all based on how much energy in newton meters to raise water by one degree Celsius, and the same thing, too much energy, thermal, you get a burn. So you start kind of getting this concept that's all about excess energy transfer that leads to it. Yeah? Is it also over time? Because you can imagine that it implies a certain amount of force, or a long amount of time, or a certain amount of heat, or a certain amount of radiation, and it actually is all over time as well, so how quickly is that longer? It can be. And so, what Dan's asking is, energy transfer also over time? The answer is yes. You know, that's why mechanical energy can either be really fast, or it could be a slow crushing injury, or just a weight on top of you, over time. And that we call subsidence. Subsidence is when something deforms, when an object does not deform, but the environment does. So a metal pole in a field of mud that sinks into the mud ground is called subsidence. Creep is a deformation of the material versus the environment, so that if you had a cardboard box, and I put a stack of books on it, and a cardboard box starts deforming, that's called creep. And so, those are all time constraints of that. So, the biomechanical pain hypothesis that I try to espouse is the whole idea that surgery should be load sparing, not motion sparing. And all we're trying to do in the spinal deformity surgery is minimize the amount of energy that's put into the system. That's all we're doing. The less energy we have to put in, the better. So, remember, pain is a physiologic response to tissue damage. Tissue damage is from the inability to absorb the excess energy, and guess what? We can try to minimize mechanical pain by doing that. And that's how we get our outcomes in patients, by minimizing the amount of energy they have to expend in order to be upright. And so, when we're using all these parameters, whether it's Cobb angle, all we're doing is figuring out the greater the angle, the higher the moment arm, right? Same thing with pelvic, when we look at these pelvic parameters. I kind of said yesterday that if you look at the pelvic incidence, all we're looking at the pelvic incidence is the angle of the sacrum, the sacroiliac joint angle is really what we're talking about. That's a fixed angle. So that doesn't change because it's just the angle of the joint you're born with, right? But if you start looking at these other parameters, you start realizing that they all have a role. For example, pelvic tilt. The larger your pelvic tilt, the larger your moment arm across your femoral heads, which is where the energy has to go to get to your feet. Sacral slope, it's the same thing. Sacral slope is an indication of your shear energy, which is why I'm talking about this in spondylolisthesis, right? I.e. your slip angle. A sacral slope of 0 degrees is pretty much like a flat ground. You can put something on it, it stays, L5-S1 fusion, nothing moves, right? But how about if I had a sacral slope of 90 degrees? It's like trying to stick a Christmas tree to the side of a cliff. It just slips off, right? Would you be surprised if the loads were different as far as the difference in sacral slope? And so now you're starting to understand, yeah, I get it now. So that if I had a patient with an L5-S1 translation and I'm trying to pull them back at a sacral slope of 0, the forces across that L5-S1 joint is going to be a lot different than if I had a sacral slope of 90 degrees at the other extreme. And that how I plan my hardware and my construct is going to be a little bit different. Now, there are other things besides biomechanics, and that's the neurostructures. And so, for example, things like high-grade listhesis, there is an incidence of neurologic issues, especially when you try to stretch the nerves overall. And I don't want to make you think that it's all purely on biomechanics. So the surgical options you have for listhesis are anything from in situ fusion to instrumented posterior fusions, circumferential fusions, all the way down to doing dome osteotomies, etc. Just to kind of go over some of this. So that if you have a patient with an isolated spondylolisis, PARS defect, maybe a grade 1 slip, not much, very small slip angle, your sacral slope's within normal parameters, boy, that's the easiest case to do. You can do this however you want. There's almost no wrong way to do it. In situ fusion, A-lift, posterior, MIS. And so this is a case that we did MIS because there's not a lot of decompression that needed to be done. And you can get by with using shorter screws, maybe bicortical, you know, but you don't have to be perfect anymore in how you anchor the hardware. And you can even use smaller diameter rods. You can get by with a lot of stuff because it's not going to be a lot of force. The issue is that what happens if you start having a listhesis like this, that's a little bit more aggressive, that now is going to be loaded a little bit differently, and with an aggressive decompression, you're going to destabilize that a little bit more. And so in a situation like this, you're going to have to start thinking about your anchor points, including putting your screws across this space to kind of give that a little bit of additional support to anchor that. And that's similar to a Bowman technique that I'll talk about here in a second. And now you can do that from in situ without reduction. But what if you want to reduce those types of slips? Knowing what we know about the biomechanics this last two days, you start realizing that you can reduce this from a number of ways. One of the older ways that was commonly done, but isn't as common these days, is using another set of screws. For example, putting the screws at L4, creating this bar where you can then reduce or pull this up in. Now, the only thing I have to say about that, and it's okay for a L5-S1 area or a mono-segmental fusion, is that remember, the more you reduce in this fashion by using a persuader to pull things up, the more you lose your lordosis. Does that make sense? So that if you had a patient with lordosis and you kept pulling the middle screws up, you'll actually put them in flat back or kyphosis. If you want to introduce a lordosis, you would actually be pushing the middle screw heads forward. And so you would then be reducing on the two end screws overall. So this is why it's important to understand these concepts as you're applying them to your patient, especially if you're trying to correct these parameters. In this case, this technique, what you do is you can reduce the L5 up, cut the rods, remove the L4 screws as in this example, and this is just the x-ray here. But now that you know this concept, you can also do other things, right, if you want to reduce it up. If you anchor your S1 screws pretty firmly and you use fixed angle or mono-axial heads that don't move, you can then use that as a cantilever beam and reduce off your L5 and pull it up by locking in your rod almost like a springboard effect. And if you didn't have mono-axial heads to do that, you can still lock in your S1 screws, your lower areas, anchor it, leave on your rod holder or your screw holder to give you a little bit of lever arm and then still reduce up on the L5 or whatever level you're trying to reduce. So this is how you can apply these biomechanical concepts in using slip reductions. And so this is just another example, just cantilever beam of anchoring this first and then just pulling up on the area that you want to slip. If you're worried about high-grade listhesis, someone with a high slip angle, this is where you start needing to think, what do I need to put in that L5-S1 disc space because otherwise I can't anchor it in. If I put in just a metal cage, it's still going to slip. It's going to slide over that. So I want something that doesn't slip or slide overall. And typically this is a Bowman technique with a fibular strut graft where you have a patient with a high listhesis and then you put in a fibular graft across it. My technique's changed. Now actually instead of using fibula, I use titanium cages. And so this is an example of how I would use it. This is a patient I took care of actually this past year, a 30-year-old, with a BMI of 57.2. So now you have a guy with spondylotopsis. Here's his films. He's becoming more debilitated. He's tried everything. He's becoming disabled. Now presents with bilateral foot drops with a film like this. But God, it's a BMI of 57 with a high slip angle. What construct are you going to apply? So what we did is this. You can barely see it here, but this is a cage. What I do now is I do a sacral laminectomy. I use one of those acetabular reamers that they have in Ortho with a K-wire to drive it through. It does a nice core across that area from the posterior to anterior across the disc space. And I put in that cage, and this is where the cage sits in the middle. And so what that does is it helps mitigate that slip angle, that shear force. That's really what I'm trying to mitigate across, right? And so this is just an example of how, when you're in these situations, to try to minimize that, you certainly want to use what tools you have available. You can also do VCRs. This is popularized by GAINS. It's called the GAINS technique from Missouri. And so it is technically challenging only because of the amount of dissection you have to do to not inadvertently knock out your L5 nerve roots on your reduction. And so this is just an example of that where you just do a partial resection of L5 vertebral body. You do your sacral dome osteotomy. You reduce back, and you plant that together. It's a very solid technique. And so this is a patient of mine that I took care of in 2004 who had a high-grade listhesis, almost spondyloptosis. Well, it is spondyloptosis, right? So you can see how, if you follow the spine down, you're like, huh, it's not matching up to the sacrum anymore. So that's a pretty high grade, and this is her MRI scan. Now, the reason I'm pointing this out is that if you do your sacral dome osteotomies and you lift your bone up and then you impact them together, there's such a friction fit that you don't need to anchor a ton of other screws. And for some reason on this patient, I can't remember if it was her pelvis, the shape of her pelvis or something, we decided not to go with iliac fixation. But by doing a sacral dome osteotomy, we're able to stick with just S1 fixation. And this is a film that actually we just took this past year, because I still see her every year, just some routine follow-up. So I've been following her up now for 10 years for this. And so this is just an idea that depending on the scenarios that you have, you have to learn these different techniques as tools and understand the biomechanics so that you don't end up over- or under-treating. Iliac fixation is something that we've talked about ad nauseum, so I'm not going to go into that, but understanding the whole concept of the moment arm to need iliac fixation to support that. And in conclusion, just remember, it's all about understanding the biomechanics, the implant materials that you're hearing about over and over again, understanding the goal of your surgery, and making sure that you choose the appropriate construct and plan it appropriately versus just putting in a ton of hardware and hoping that it works. All right, thank you very much. Thank you.

biomechanics

spinal surgery

diagnosis errors

forces

stress and strain

surgical options

individualized approach