I'm gonna give you something a little bit different. Part of this comes from from my position on the board where I can tell you that there's a number of questions coming up along these topics, but part of it is because I had done an orthopedics residency as well as a neurosurgery residency. The one thing that I see is people do a lot of instrumentation and have absolutely no idea whatsoever why certain implants are selected. You know, for example, like Winston, he goes, why can't we use that pretty blue rod instead of using that ugly gray rod as a major selection for of why we're making an implant selection. So I'm going to talk a little bit about is when we're doing different things there's different reasons for why we may want to do it. There's biomechanical reasons, all these little nice screws and different things you see, there's a reason why they're they're manufactured in a way and for a particular patient how you select what the right implant is really critically important. And what happens is most people in orthopedic training, because so much involves doing things like plating, you know, plating broken fibula or doing things like that, that that's a big focus of what they do. For neurosurgery residents, including almost everything you get in education, it's never touched upon, but it does have a real big impact. So I had my posterior talk, which seemed very similar to Joe's posterior talk and seems very similar to my talk tomorrow, so I figured we'd do something just a little bit different. And these are my disclosures. So what's happening? What are the goals of spinal instrumentation? Why do you actually put this in place? And particularly for a deformity clause, we want to be able to go and we want to correct deformity, do something active, to be able to go and to manipulate the bone and the soft tissues to allow our correction to be achieved. But we also want to be able to go and stabilize that segment, to hold it in place so a fusion occurs. And it's beyond the limits of this talk, but a lot of different things have been shown as far as bone healing, that the better you stabilize it, the more rigid it is, the better chance that a fusion will eventually occur. Particularly in the early stages, a lot of motion goes and works against the eventual fusion from occurring. Then some other things that we're going to talk about when we're looking at our instrumentation is factors such as post-operative imaging and resistance to infection. Now what's happening is, is that when Bob, Harry, and I first started out, there was a lot of discussion, you know, pedicle screws were a relatively new thing. And there were a lot of people that, you know, debated the risks and values of putting wires versus screws. But it's been shown again and again that screw fixation, particularly pedicle screws fixation, has some real advantages. And that's why, I mean, who here has ever put a hook in a patient? Probably, you know, one or two people. And that was something that, you know, that even ten years ago was something that was very, very common, at least particularly in the thoracic spot. And what's shown is that, you know, just for these two people having almost the exact same curve, you can see that the corrections are better using pedicle screws and using those other techniques. Now a screw isn't all a screw. And so the first thing we're going to do is talk about some screw characteristics. And what happens is there's some definitions that, again, most of you probably have never heard, or if you've heard of, really don't know what the implications of them are. So there's something called the thread pitch, then there's the inner diameter of the screw, then there's the outer diameter of the screw. And these things have some real effect as far as what goes on with screws. And again, you know, all the manufacturers do things much better than they used to do in the old days. But when, you know, Bob and I in particular were starting out, there are a lot of people that come in with broken screws. And that was because the way that they were manufactured and some of the concepts that we've come to understand really weren't being implemented. So what's happening is that you notice that there's certain screws that have different thread patterns. And what happens is that this thing right here, the thread pitch, and the pitch, you know, this is the thread pitch. This is the distance between the, the functionally the distance between the threads of the screws. And what's been shown is that as you decrease the pitch, for example, this screw here, in general, this kind of a pitch is used when you're wanting to essentially have more bone in between each of the different threads. And that's generally what's considered a cancellous pitch because it grabs more of the, grabs more of the bone. This tighter pitch is something which they generally say is a cortical pitch. And you can see, for example, for occipital screws, for those people who put occipital plates, they have this kind of a pitch pattern. It doesn't grab as much bone, but it's less likely to go and cause the bone to fracture. And because of that, that's considered a cortical pitch. So basically what happens is you, as you go and you have the space bigger, it grabs more bone. And as you go and you have the minor diameter, the inner diameter of the screw gets smaller, it allows more bone to be, to be, to be gripped. But it makes the screw substantially weaker. And these are some of the trade-offs that's done. So as in general, for people, if you're using an osteoporotic, if people are osteoporotic, what you'd like for people to have is in the cancellous bone to have a distance, a wider pitch. Now you can see a number of the different manufacturers are now making screws that have two different thread pitches. They have a narrow pitch for it to grab the cortical bone in the pedicle, and they have a bigger pitch to go that's in the cancellous bone. That's an attempt to optimize all these different, different factors. Now several of the manufacturers make screws so that they're conical in shape, that they're bigger, that they're bigger as you get more distally in screws. You get near the tip, they're narrower. And this used to be a very common thing. And what happens is, again, as you advance that screw, it gives a feeling of increasing insertional torque. And one of the, and what happens is they behave about equivalently in the bone. The difficulty is if you ever have to back out one of these conical screws, you lose the vast majority of your purchase. And that's why many of the manufacturers have gone away from it. Basically, if you look at most models, the size of the screw, the outer diameter of the screw, is the single most important factor for pull-out of a screw. So basically, as you go from 4.5 to 5.5 to 6.5 to 7.5, each of those things markedly increases the screw pull-out. As mentioned to you, the pitch has some relationship of displacement before failure. If you grab more bone, and as you can see, a lot of the different manufacturers, as you can see now, that when I was first starting out, for example, the Synthes USS screw, which was very, very common, had a very narrow pitch. And it was good because it was an exceptionally strong screw, but also it was more apt to pull out. All the screws that you see now, the pitch and the thread design are meant to optimize the bone purchase. And so there are some of the things to consider. Basically, the pull-out of screws occurs by shearing the material between the different threads. So what happens, you basically break that little ridge of bone that sits between each of them. So basically, again, the most important factors are the outer diameter screw, the length of the screw, and how strong your bone is. Now the screw strength itself is dependent upon the inner diameter. That's the area right here along this sort of shaft, which the threads are attached to. And as you go and you thicken that inner diameter of the screw, it makes it stronger. And you can see that some of the screw patterns, you pick up a screw and you look at them now, many of them have that the outer diameter stays constant, but the inner diameter gets wider as you move towards the shaft of the screw. And that's a change that has been made to go and optimize the strength because the majority of times when screws used to break was the junction of the screw head to the shaft of the screw. And by thickening this inner diameter in this location, it resists that fracture. So basically, this just shows how much if you get a screw that's bigger, how much dramatically stronger that screw is. Now basically, the next thing that we're going to talk about is insertion of the screw. And at least to my table, this is probably a little bit of an important point. And so what happens is some rationale as far as how far you want to insert the screw, what's your angle of your screw, and how you want to actually place it. So basically, some of the things that we're going to talk about is the depth of penetration, the insertion angle, the hole preparation, whether it should be tapped or not, and how much it should be tapped. And again, this is dependent upon the bone quality. So this study went and compared inserting a screw to 50% of the depth of the vertebral body compared to doing it 80% of the depth of the vertebral body. By increasing that down to 80%, it went and increased almost by a third the pullout strength of the screw. And I think ideally for everybody, you really should shoot to optimize your screw depth to about 80% of the depth of the vertebral body. And so many of the cases that get sent to me with failures, and I see a lot of failures, people are putting these really very, very short screws. And so what I typically do is I go and I use my little feeler probe and I put it against the anterior cortex, take five millimeters less than that, and that just makes it a very consistent way to get you right about at that 80% screw depth as far as things are concerned. The second thing is it's important to triangulate your screws. And particularly a lot of times when you're doing open surgery, you're kind of working against your muscles. And a lot of people, there's a tendency to put your screws too vertical. And again, by going in, putting your screws in in a triangulated manner, putting them in from lateral to medial, you again almost increase your strength to pull out by almost a third when doing that. And what's happening is one of the real advantages, I think, of minimally invasive techniques like Praveen and others do, is that those techniques really encourage a lateral to medial placement of the screw. It makes the screws more biomechanical. Now this is a real critical, I think a real critical slide. And what's happening is in the thoracic spine, everybody's so really paranoid, with good reason, about putting a screw into the spinal canal. And a lot of times, people like Winston and others will come and look and say, Dr. Schaffer, I just measured the pedicles in the middle of the thoracic spine and they're only three millimeters. And what happens is if you do a slightly more lateral starting spot, you can go and put your screw in the area between the pedicle and the lateral aspect of the rib. And this is what's called a rib pedicle screw. And a lot of times, even if you have a three or four millimeter pedicle, the distance between the medial cortex and the lateral aspect of the rib is oftentimes a centimeter. You usually have a very, very big space. This study went and compared the screws that were placed and contained completely within the pedicle versus those that were placed in a rib pedicle position. And what's happening is by putting a rib pedicle screw compared to completely containing it in the screw, you lose about a third of your actual grip for whatever size screw you use. The advantage, a lot of times, of using a rib pedicle screw, though, is you can put a bigger screw in. So, for example, in the thoracic spine, I almost never put less than a 5-5 screw. And the reason why is I use this type of a technique. And even though it isn't quite as strong as completely containing within the pedicle, the fact is is if you go and you increase it by a one millimeter size, the pullout's equivalent. So, for example, if I had to, by just looking at the size of the pedicle, I had to put a 3-5 screw in. And I can put a 5-5 screw putting a rib pedicle screw. The fact is I actually have a stronger construct. Because a 3-5 pedicle screw is equivalent to about a 4-5 rib pedicle screw. And this is something when you're going and you're contemplating your deformity correction techniques is something to consider. Now, what's happening is that in a lot, particularly in Europe, a lot of people feel strongly that the initial preparation technique should be with a drill. And some people have said that a drill is more effective than using a probe like a Lenke-type pedicle probe that I think most of you all use to put in your thoracic screws. And what happened, this particular study really didn't show much of a difference between the two different techniques. Basically, this looks like, this is again another study looking at the insertional torque. And basically, this particular study showed that, again, if you contain it completely within the pedicle, whether you drill it or you use a probe, that the insertional torque is higher. One of the things which always comes up, and my own residents will ask, well, why sometimes do you tap the bone and sometimes you don't? And this is a study that went and compared the use of a tap versus not tapping an osteoporotic bone. And it was shown that, again, your insertional torque and your pull-out strength is better if you do not tap it. And this is the results of the study. What happens if you tap it line to line? And that means if you're going to put a 5.5 screw, if you tap to 5.5, if you reduce that, if you under tap by one millimeter, you increase your insertional torque by 93%. So what's happening is, if you've got someone who doesn't have good bone quality, you do not want to tap it to the size of the screw you use. You can either go and under tap by at least one millimeter, or you can just insert the screw directly. Usually, again, if you use a lanky pedicle probe, that's about a four millimeter path that you're making. So if you're going to use a 5.0 or a 5.5 screw, it's the best way to go and maintain the integrity of it. I will, on occasion, if someone has extremely poor bone, I'll use a combination of both screws and hooks. The posterior elements of the spine are usually the last area to be significantly involved with osteoporosis. And this study just shows you the dramatic benefits you have if you add both a hook to a screw using offset hooks to increase the strength in osteoporotic bone. This is really, again, something that makes a lot of biomechanical sense, and I think that most people here have gradually gone away from it. Again, when Bob and I were first kind of starting out, the use of crosslinks was something that people used really a lot. And the fact is that they do increase the biomechanical rigidity of different constructs. And at least as far as torsional stiffness is concerned, torsional stiffness is concerned, crosslinks increase this by about 44%. Now, what's happening is if somebody, again, has very poor osteoporotic bone, adding a crosslink can improve that torsional stiffness. It's the rotation back and forth and over time causes the implants to loosen. And occasionally, again, on very osteoporotic patients, I do use them. The fact is that if you use iliac fixation, that also goes and increases substantially your rotational stiffness of your overall construct. And I'm going to show you at the end of the talk why I don't use crosslinks as much, because crosslinks, even though they increase the rotational stiffness, they're a really big stress riser in your rods. And these very long constructs, many times that's where the rods break is where you've placed a crosslink. So you have to balance the need for it. Generally, shorter constructs with poor bone, I think the cross links are relatively good. Very long constructs, you have to balance the advantage of notching your rod and causing trouble. Again, this just shows the advantages of the torsional stability with the use of that. What happens is this is an additional study that went and looked and it says that as your constructs get longer and longer and longer, you have to add more and more cross links to get the same effect for it. And again, this is a reason why people have drifted away from doing this. Basically, when someone has osteoporotic bone, it's always a challenge and a lot of different things have been tried over time. And these are all considered off-label uses, but these are all different biomechanical studies that have gone and looked at ways to supplement the fixation. And you can see here, this is basically how some of the different testing methods they use to go and to evaluate things. Basically, when you have a failure, failure is different depending upon what your bone quality is. Basically, if you have a screw that fails in healthy bone, it's usually a fracture through the healthy bone. It usually results in screw migration through osteoporotic bone when that occurs. The insertional torque really highly correlates with what the quality of your bone is. And what happens is that that feel that you actually get is really a good estimation of how good the bone actually is. This is just a thing about the pedicle morphometry. We had talked earlier today, at least in my group, about the angles of the screws and the sizes of the screws. You can see in the upper thoracic spine, the pedicles are quite big, usually in the six to eight millimeter range. In the mid thoracic spine, they can be three, four, five millimeters. And again, at T11, T12, and T10, the screw sizes are usually big. At L1 and L2, they shrink back down again. And a lot of people ask me, why don't I routinely stop my constructs at L1? It's usually because it's really not a very good pedicle versus going up two levels to T11, you have a much bigger pedicle size. This is probably the last one. Again, which I was showing at my table, but I think is really important to recognize, is what the transverse, the angle that you need to go from lateral to medial when you're inserting your screws. Again, at T1, T2, and T3, it's a pretty substantial degree of medial angulations, about 30 degrees at T1, about 20 degrees at T2, pardon me, T1, T2, and T3. And then it becomes almost straight up and down at the T12 segment. And it's just real important as you're angling in. Most times when you're having a thoracolumbar construct, at the very top of the spine, you're working against the muscles as the end of your incision, and you really need to be thoughtful about getting your hand out lateral enough so that screw doesn't break out. Because if you don't get good fixation, you break out the lateral wall of the screw, it can be a real problem. Basically, as far as what do you do if you rip a screw out, this is just, again, a model where they went and did a screw failure technique. Basically, the use of methamethacrylate is something that, at least biomechanically, adds substantially to the strength of the screw, but it is considered an off-label usage. And I think that case that Justin showed earlier today shows some of the downsides of getting this into the spinal canal. A lot of people have tried other things, other types of bone cement, and they also increase the strength, but they also have some significant potential side effects. There's been some real strong pulmonary problems by people using the bone cement material that they use for cranioplasties in pedicles, and you really probably shouldn't do that. If you use anything, it should be PMMA. Basically, there's been several studies looking, and it's been shown the best way to salvage a screw is to go and to increase the diameter of the screw. Just putting a screw longer rarely goes and reestablishes the integrity. So basically, if you fail something, put a slightly longer, but definitely wider diameter screw to be able to go do this. This particular study here went and salvaged a 7.0 screw with an 8.0 screw and showed that that even further increased the pull-out compared to the 7.0 screw alone. Another salvage technique is to be able to realign the screws. Most people now use what's called a straight-ahead technique for the placement of a thoracic pedicle screw, which parallels the end plate, but you also can put a screw down the true cord of the pedicle as a way to be able to salvage things as well. And if you look here from these signs, it shows that these can both be very good salvage techniques that are present. Basically, sort of the biomechanics, the screw characteristics, the outer diameter is most important for the pull-out. The inner diameter is most important for the strength. The insertional technique is really important to get up near 80%. Triangulation, you want to under-tap. If you crack a cortex, it diminishes the pull-out strength. Bone mineral density is really, really important for the overall construct integrity. And if you're gonna salvage something, use a bigger screw. If you can't, then the use of methamethacrylate is probably the most important things to do. Basically, I'm gonna skip through this. I think this is one thing that's really kind of important and something that I rarely hear discussed. And one of the things that when you select an implant is what's the biomaterial that you're gonna use. And back, again, 15 years ago, 95% of all implants that were used were stainless steel. And what happened, it was routinely known, at least for kids with idiopathic scoliosis, that you would have a two to 3% delayed infection rate. People would look like they healed up in a year or two years or three years down the road. They would puss out their wounds. And so many of the things that you hear about people coming to your clinic saying, my dentist wants to know whether I need to have antibiotics because I had a spine fusion done, relates to this era. And there were some reported cases, particularly with stainless steel implants, of people developing wound infections after teeth cleanings or colonoscopies or other things due to the seating. Now, what's happening is as we've moved away from stainless steel, the incidence of that has decreased. And this was a study looking at, this was a study looking, a very detailed study looking at what's something called a biofilm. And a biofilm is the ability of a material to go with some type of seating to get a coating that bacteria can live in that is very, very difficult to clear. So basically what is happening is they went and compared a lot of different substances. This is from stainless steel, commercially pure titanium, titanium alloy, cobalt chrome, cobalt chrome, and peak in a polycarbonate material. They went and they looked at all of these, all these different things. They compared the surface treatments of both of them. And what they basically, what they basically found with, is they look through these different, they look through these different ones, is that basically that certain materials have a higher ability to maintain biofilms than others. And what it was shown was that titanium has the least biofilm adherence of any material. And that's why dentists can use it in implants on the mouth. The next highest in the biofilm ability to resist it is cobalt chromium. The one after that is stainless steel. And the material that has the least capability, one that's most apt to maintain biofilms is peak. And I know that my partner, Justin, one day said, why don't you always use titanium for your inner body fusions? And I said, well, you know something, if you get one of those peak ones infected, it's gonna be an absolute nightmare for you. And after Justin had a couple peak inner body infections, suddenly Justin all of a sudden, all of a sudden is a big believer in the use of that. And one of the things is really important is a lot of times when you're seeing some of these revision cases, where they've had a non-union, and particularly ones who have had peak implants, if you pull the peak implant out and you go and you send that off for culture, the number of people that have a very low resistant infection that are in those things is higher. And it's something to consider and peak, like anything, nothing's bad. You have to decide the benefits and risks of everything you're doing. But why I personally have a tendency to use titanium, because I'm using really big cases. These are open and exposed for a long period of time. And they're more apt, I think, historically to get an infection. So I wanna use materials that are gonna go and give me the least chance of getting it. The last little thing I'm gonna touch on is the impact of what happens with rods and the different materials on this. And basically, what happens is if those people in particular are doing big operations, the fact is that if you don't get a fusion pretty early, you're putting a tremendous amount of force on these rods. And what was the total fracture rate, Justin, that we did for the ISSG for that? It was, what? For the PSO that were used on different sites, 10% fracture rate of rods and 30% fracture rate overall of seven. Right, so what's happening is, is that these things can fraction. There's certain things that are more associated with fractures. If you look at these pictures, if you look at these pictures here, this is a notch that's made on the rod from a French rod bender. You know, when any of us go and bend the rods, it does a little bit of notching. And can you see where it's fractured? Right through the center of the French rod bender area. So basically, this was some different studies looking to go and to assess the different characteristics that went and that made rod fracturing higher. And basically what they did, this is something called a Nisola bender for people who do a lot of deformity, particular pediatric deformity, this is a way to bend the rod. You grip it at both ends and you sort of bend the rod in its entirety. And what this was thought to do was to create less notching. It does put a little mark on the rod at the spot where the bender was, but the middle portion of the rod, it's a smooth bend. This is a French rod bender here. I think everybody's seen one of those things. And what happens is that with the French rod bender, both in this area, this area, and this area, it creates this type of notching. This is an in-situ bender and a lot of people use these, including me, to be able to go and to correct the rod. And again, you can see the notching that's on the rod from in-situ bending. And then different companies make different pre-lordosed rods that are bent by the manufacturer. And then they're annealed in this particular shape that goes and makes them stronger. So here, these group of rods are the ones that tested bent by the pre-lordosed rod. This is one using an Isola bender. This is one that was used a French bender, and this was an in-situ bender. They basically went and compared those and tested them out. And you can see that the pre-lordosed rods are substantially stronger than any of the ones where you go and we bend them as surgeons. And you can see the difference between these is really great. If you look at the millions of cycles and the forces across them, that it's a way stronger way to go and to bend the rod. You can also see that straight rods are maybe a little bit better than the pre-lordosed rods, at least early on with the forces that are present. So you can see that basically if you go and you use any of the different benders, that it does have an effect on the rods. What it was shown is that the commercially pure titanium are the weakest of the rods, but they do tolerate bending a little bit better than titanium alloy. If once you go and use a French rod bender or any other benders, you reduce the strength of a titanium alloy rod by 50%, but a commercially pure titanium rod reduces it by 35%. So basically, if you have a pre-bent rod, the most common place where they fracture is right next to your set screw, okay? That's a stress concentration area. A lot of times if you see somebody who has one broken rod and the patient's having a lot of pain, if you open them up, you'll see right under where that set screw is where another rod is broken that you can't see it as much. So realize this is a real site where rods break. For people that have had, the Isola rod typically just broke in the middle of the rod, okay? Pre-lower dose rods broke at the set screw place. And you can see anyone that had a French rod bender almost always broke at the area where the apex of the French rod bender was present or on the site of the in-site tube bending. So basically, just to realize that when you can use a pre-contoured rod, that's got some advantages. There are times that you do have to bend them. And what a lot of people who are doing these big deformity constructions, you'll see tomorrow, a lot of times at the area where the highest area of stress is, I'll contour the rod, get my correction, then I'll put satellite rods with a pre-contoured rod in that area to go and reduce the risk of fracture in the overall construct strength. So basically, of all the different materials, titanium is the one that is most notch sensitive. That's why a lot of people have moved to cobalt chrome rods. I sort of skipped over it. If your rod gets bigger, the strength of the rod becomes dramatically stronger. A 6.0 rod is dramatically stronger than a 5.5 rod. And you just have to make your decisions. So basically, let me sort of do my conclusions. So basically, taking all these different factors into consideration, I generally use titanium screws for the imaging capability. I use cobalt chrome rods, 5.5 or 6.0, because they are a bit stronger. I have a tendency to use satellite rods in areas that have a very high risk of fracture. I use titanium inner body for deformity, and I use the titanium cages because I think that they're most infection resistant. So that's a little bit, I know this is sort of a little dry and boring, but the fact is you have all these different selections of implants, and at least that way, if you go back and you say you want to use it because it's more infection resistant, or hey, I'm really concerned about the notch sensitivity of this cobalt chrome rod, you'll sound better to the people, at least maybe to your orthopedic colleagues when you're back home. If you say it, because as Winston told me, I like that pretty blue rod better than that ugly gray one, then the, if you have purple, then you have a little less credibility about your decision making. All right, thank you. Thank you.

implants

biomechanical factors

screws

implant selection

fusion outcomes

insertion techniques

spinal fusion