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Grand Rounds-Image-Guidance in Spinal Surgery: Indications and Technical Pearls

Eric Nottmeier and Jean-Pierre Mobasser

October 26, 2011


- Good afternoon. This is Jean-Pierre Mobasser with Goodman Campbell Brain and Spine coming from Indianapolis. And today we're discussing another session that AANS operative grand rounds on spinal surgery. With us, we have Eric Nottmeier from Mayo Clinic Florida. Good afternoon, Eric.

- How are you doing today?

- I'm good. Thanks. We're gonna be discussing image-guided spinal surgery set up and techniques today. First, we need to go through disclosures, and you can see Eric's disclosures listed here. Mine are not listed, but include a Medtronic consulting role, some consulting with Cynthia's as well and Emulex and then a royalty with Innomed. So there's a little history behind image-guided navigation. It starts in the '90s, and obviously cranial surgery leading the way for spinal surgery. And let's talk about early uses of navigation. And I think there's still maybe some misconceptions of how much spinal navigation has advanced from these early years.

- Jean-Pierre, point matching was the initial registration modality in spinal image guidance, and this involved picking out points on a preoperative 3D CT scan, and then dissecting those points out on a patient and matching those points to the computer. As you can imagine, this could be quite cumbersome, could be time consuming. Typically, you can only register one segment at a time. It was quite difficult in the revision surgery, especially with patients who've had previous laminectomies, as it was difficult to find points and match points. And so it's no surprise that many surgeons who tried spinal image guidance back in this era quickly abandoned it, never to come back to it again.

- Yeah, in my training, we were doing FluoroNav at that time and it was adding at least two to three hours to a case. And I think a lot of people's early experiences have made them believe that navigation in spine surgery will actually increase the time of surgery, but our experience nowadays is otherwise. Would you agree with that?

- I would agree. I think the image guidance now, especially with the cbCT technology registration takes significant time off of our surgery when compared to doing the same procedure without image guidance.

- And obviously, there are multiple different systems on the market. We're not here to really promote any one over the other, but I think the key is that this cone beam computed tomography gives us an axial cut. So we can be looking at an axial view in addition to whatever trajectory views we wanna see in placement of these instrumentation. Disadvantages with this is obviously, that in people with wide shoulders, heavier patients, there can be some limitations in the cervicothoracic junction. It's not quite the resolution of a true CT scan. However, it's still quite good. Why don't you go over the basics of this real quickly so somebody can understand how this works?

- Talking about how image guidance works is that you're gonna have infrared light emitted from a camera, and that's gonna go ahead and reflect off of spheres on both reference arc in the instruments that the surgeons use. And this light's reflected back to the camera, and this allows a computer to track instruments in 3D space. And so, one of the initial steps with the image guidance in the operating room is placement of the reference arc, and this is a key step to taking out some of the cluster factor of image guidance. Number one, you should always have this arc between the camera as well as the instrument sets you're using. And so if you keep this arc between the camera and the surgeon's instruments, it takes out a lot of line of sight issues that you may experience. Additionally, if you put this reference arc on a level above where you're instrumenting, for example, in this case, I'm doing an L3-L5 fusion, and I got the reference arc on L2, and I try to place this as low as possible to the patient's skin without touching the skin, then that can take out a lot of the line of sight issues also.

- Yeah, and I think you're bringing up some really important issues, and I would add to them the areas where there are pitfalls with navigation. And I think one of these can be, if you don't have anything screwed on properly, if the balls aren't snapped all the way down, if the tap or the screw drivers aren't seated all the way in in the reference handle, you're gonna get some false readings or inaccuracies that could affect what you're doing. So I think having a checklist of all these things is absolutely critical, not to mention the fact that you can have a new scrub tech in your room who accidentally bumps the reference arc without knowing the importance of that. So, here we can see an example of a percutaneously placed reference arc into the posterior superior iliac crest, with the camera at the foot of the bed, again, to keep that in a proper position for line of sight issues. Eric, why don't you talk about what you're showing at this point?

- One of the issues we've had with traditional retractor systems like a Gelpi retractor or a Wiltse retractor is that once you have your span and your registration, trying to get these retractors in, in the superior part of the wound above your reference arc has been difficult. And I just find that these crank type of retractors, you can go ahead and place the arm without the blades, and thereby not touching or disturbing the reference arc. And you can put the blades on while the arms are in the wound and then do your distraction. And so I found this type of retractor, this cranked type of retractor very valuable and retracting on the superior aspect of the wound around the reference arc.

- And I think this is another critical point you bring up. Getting the retractors in underneath the reference arc can be very tricky to do without touching it. And I've certainly seen some of these retractors bump up against the frame and throw off the accuracy. So I think having all of these issues addressed and continuing to remind yourself and everybody involved the importance of them is critical to the success of the navigation. So let's talk about posterior cervical cases, occipital cervical, and placement of reference arcs. Why don't you talk about your preferences and how you arrived at those?

- You know, Jean-Pierre, placement of the reference arc in these cases is challenging, especially at C1-2 fusion cases. So posterior arch of C1 is not a good place to anchor the arc. There is not a lot of bone there to catch. If you place the arc on the C2 spinous process, and this can overlie the entry points so that your C2 laminar screws, and also get in the way of the trajectory of your C1 lateral mass screws. And so, and especially in revision cases, when you have wiring and bone graft that's already in place, you're not gonna have good options to place your reference arc in those cases.

- Yeah, and I think this is an important point. I tend to place my arc on C2. It can be difficult if I'm doing an occipital cervical fusion in accessing the keel, because the stem of my reference arc will block it. I tend to place C2 pars screws, so I don't have as much of an issue as you would if you're doing these translaminar screws. So I think the point of this is there's several different ways to do it, but you have to find a way that the arc is not gonna get in your way, or get in your way as little as possible, and certainly in a place where it's safe. We also have to remember the further away from the spine it is, the potential inaccuracy exists.

- That's correct.

- So let's continue on the posterior cervical discussions, and we can see several different posterior cervical constructs here, Some of which obviously wouldn't require any sort of navigation to do.

- That's correct. These are two cases that I did in which, these are pseudarthrosis that were sent in. Patients with previous wiring in place, one with stainless steel wiring. So that's definitely something you're not wanting to remove. And so another example of, there's some cases where it's gonna be very difficult to place a reference arc on C1 or C2, and therefore, looking at other alternatives, which I'll talk about putting on the Mayfield head holder, are good alternatives in these particular types of cases.

- So let's look at this case and the patient set up that you have, and it really shows, for neurosurgeons, this should be nothing unusual. This is not an unusual setup for any cranial case that we all trained to do during our residencies. Would you agree?

- I agree. I mean, this is the basic posterior cervical, even posterior occipital set-up of a patient.

- The only thing I would tell you that we've modified in doing these is nowadays on a Jackson table, we'll bring the head of the bed to the top notch and the feet, leave it on the bottom. So then when we raise the head, we can almost get the neck to a flat parallel plane to the floor, which has been something that's helped us a little bit.

- Correct, and you know, Jackson table's very tricky in some aspects, especially as we'll talk about here. You really wanna try to minimize your intersegmental emotion in the cervical spine. It's a very dynamic part of the spine, and there's a lot of things that can move the dorsal elements of the cervical spine relative to the reference arc. And so one of those is actually Trendelenburg positions of the OR bed. There's a significant movement in the spine that will throw off navigation if you go ahead and register the patient at one angle and then go ahead and change the patient to another angle. And that's with, you have your reference arc in the Mayfield head holder. And so we try to get that bed in the position that we're gonna be operating in and registering in, and then try not to move at least the angle of the head of the bed. You can move the bed up and down parallel to the floor, or even with an angle, as long as you don't adjust the angle. But we've noticed that once you start adjusting head to bed angle, then you're gonna start getting some navigation inaccuracy if you have your reference arc on the Mayfield head holder.

- Absolutely, and so we do all our adjustments before spinning an image, and I've always said that the first thing I do is I try and do my navigated screw, drill, whatever it is, as soon as I've spun the image. So if my most difficult screw is gonna be C2, that's the screw that I'm gonna at least drill and tap first, where my accuracy is the best. And as time goes on and you're working on different levels and there are some adjustments, I think there's always gonna be a slight change in your accuracy, maybe not of significance, but I wanna do my most difficult, challenging trajectory first when I've done a spin.

- Yeah, that's correct. And what I noticed when I started first using image guidance for C1 and C2 fusions is that there's a lot of dynamism at C1 and C2. And so I would drill my lateral mass hole, tap that hole and place that screw. And sometimes, by the time I got to the other side or to C2, I was off with my accuracy because of the dynamism of the cervical spine. So what I found very helpful now, which significantly decreases the need for registration is going ahead and drilling all your holes first. Both your C1 lateral mass and your C2 holes, checking those holes with an image-guided probe, verify that they're good holes. And then go ahead and tap those holes and place your screws. You're gonna be moving that C1-2 junction with tapping the bone and putting the screws in. But as long as you have your holes drilled with good navigation, and then you can be off after that, it doesn't really matter.

- Yeah, and I think that's a great point because we have found the exact same thing. And I think it's great for people to be able to learn these pearls from somebody else who's gone through that learning curve rather than have to figure all these things out themselves. The issues you list here, I think are all very important. We scan with all our cervical cases within retractors in place as well, because when you crank those retractors open, you can actually see the cervical spine move. We always hold respirations for cervical and thoracic cases. We drill all our holes first, and then we don't change the bed after our scan. So I like the list that you have here, and I think this is really important for doing these cases. Let's move on to anterior cervical and reference arc. Again, we can see a similar placement here. Why don't you go over some different possible anterior cervical uses for this?

- Yeah, I don't use image guidance for the routine anterior cervical discectomies. I've found guidance useful in these anterior cervical procedures, especially if there's a corpectomy involved or if it's a revision case. And so, what I typically do is use the short board of the Jackson table. And I lay the head right off the short board. We lock in an extended position, put that cranial attachment for the Mayfield on, and then I'll go ahead and register that patient before I even prep. And it's very unusual for you to get down to the anterior cervical spine in those cases and be off with your registration and navigation. And so I'll go ahead and register before we even prep and make the incision. I'll use a guidance to mark the incision over the levels I wanna come down on. And then typically when I get down to that anterior border of the spine, you wanna always check accuracy, pointing on the anterior border of the spine at landmarks and verifying your accuracy is very important to do that. And this image on the right here, you can see, you can get this reference arc kind of up and out of your way, and in a position where it doesn't impede the surgeon or assistant's working room, and there's typically no line of sight issues there.

- And tell us quickly what you're utilizing navigation for in any anterior cervical case. What is the purpose that you see the value-

- The only reason I really use it is to mark the midline in the anterior cervical spine. In older people, revision cases, with a lot of osteophytosis over the anterior border, you can get off on your midline with your corpectomy and have dire vascular consequences from that. And even in patients with virgin necks and young patients, people can still get off the midline. And so when I get down to the anterior border of the spine and mark my midline on the corpectomy levels and also on the levels above and below that. And then as I drill down that corpectomy level, I'll mark my trough borders. And as I'm drilling down the trough, I'll check occasionally, then see how close I am to the vertebral artery. And so these are the kinds of cases, at least in my hands, where it's really sped up the corpectomy portion of the cases. You know exactly where you are, how much leeway you have, and it allows you to go forward more forcefully and quickly and efficiently in doing that corpectomy.

- Yeah, and I think as surgeons, there's always been cervical cases where you'd like to reverify where midline is, but you don't wanna take the time to bring the CR man, do an anterior review. You've gotta wear lead step back. So I think that if you need this, if there's like you said, a difficult case, a revision case, I think having the ability to find the vascular structures, find the foramen transversarium, know where midline is. It's nice to be able to do all this without wearing lead, without having to wait for a CR image.

- And the slide just previous to that just shows that it's very important to verify accuracy, whether in your cervical or the lumbar spine. And you can do that by dragging over the anterior border of the cervical spine and the lumbar spine, just dragging that pointer off the transverse process laterally as well as superiorly and medially, and letting that pointer fall off that transverse process. Takes two or three seconds to do, and it's very important. I think when you go across levels and go to a separate level for instrumentation to verify accuracy very quickly and make sure you're still accurate at that level.

- And Eric, I agree with you. I think what you're bringing up is that this is just a tool. It does not replace your thought process, and you need to confirm that things are accurate and that they're making sense. So this does not replace the judgment that we use and the experience we've developed as surgeons, but this is more of just an adjunct or a tool that helps us take care of patients. So obviously, there are lots of different ways to drape. I don't think we'll spend too much time on this. Some people drape the image, the machine itself. Typically that's done if you're gonna leave the machine in place, but if you're just gonna do a spin and take the image out, then you can drape the patients with sheets, clamps, towels, and other things just to keep it sterile. The only case that I've tend to keep the arm in for nowadays is a kyphoplasty. So that's the only case where I use this drape. What cases have you used the drape for to keep in the field during the surgery.

- We've pulled this in for some direct lateral approaches of the spine. I have pulled it in for some kyphoplasties where you need image guidance and additional live fluoroscopy. I really try to avoid this at all possible. It's a very painful experience trying to drape this thing and deal with a sagging drape. And so, I really, there are cases where if I need live fluoro, I may just bring it in separately. I know some people who are doing this kind of a draping on all of their routine image guidance cases, and very quickly, they're turned off on image-guided spinal surgery. And so I would suggest you drape the patient, you get to a cbCT device and get your spin and get it out of your field and then start navigating and driving screws, because this can be a time consuming and frustrating process. But I agree with you, Jean-Pierre, is that you need to do it sometimes when you need a live fluoroscopy in addition to the guidance.

- And at our facility, we have probably four or five spine rooms going at a time. So our x-ray tech will come in, spin an image, take the O-arm or the navigation machine out of the room and take it to the next person or room while we're using it. They'll do this four or five times, and then come back around to check post placement accuracy of all our instrumentation. So, this device does not have to stay in your room for any reason while you're doing a lot of these procedures and can actually be used in multiple rooms at the same time. So why don't you give us the basics of the room set up for a basic spin?

- So when we bring the cbCT device in after draping a patient, the radiology tech will position that. And what you want is the levels at your instrument and to be centered in the APM lateral fluoroscopic plane of the cbCT device. Our OR staff will typically go outside into a separate sterile room to protect from any radiation exposure. Obviously, anesthesiologists and the surgeon stay in the room as well as their radiology tech, and will stand behind this lead shield about 10 feet away from the cbCT device. We've measured our radiation exposure at this position, and there's been no radiation exposure to anyone after multiple spins of the cbCT device in this position here. So you have to worry about radiation scatter coming back and affecting the surgeon. And so once that spins completely, go ahead and remove the cbCT device. And as Jean-Pierre had stated, other rooms are using that and they'll take that to another room.

- And sometimes you need to remind some anesthesiologists who are used to sitting at the head of the bed behind the lead wall that they really should move, as that's where the highest amount of radiation occurs.

- Yeah. Correct.

- So we make sure they come with us, unless we really don't like them, then we leave 'em there. All right. So let's go on to a multi-spin registration in a procedure like this. Obviously, the O-arm has a pretty big field of view or any of the cbCT devices do, but sometimes you're gonna have to do more than one spin to get in all the anatomy.

- That's correct. I think this case is a T10 at O-arm fusion. And so what we'll do is I'll put that reference arc on the T9 spinous process. Again, the more superior level, getting it up out of your way of your field. And then we'll start to spin by registering T10 to L2 or L3. And then, we do not undrape the patient and do those levels, and then bring the cbCT device back in. We go ahead and just move that device down right after our first spin, and then accomplish the spin on the lower lumbar sacral and ilial segments. That's typically done in one spin also. And so you're obtaining two data sets at the same draping of the patient. This saves time, gets your registration done. You bring the cbCT device out, and you always wanna start the most distal to the arc. So you'll start with your iliac screws or S2 iliac screws or whatever you place at that level, and then work your way up, that's one L5, towards the reference arc itself. And that's the best way you can maintain accuracy at the more distal levels as you start with the distal levels first and work your way towards the reference arc.

- Yeah, and I would agree completely with that. The one thing we've added to some of these cases, particularly if there's a flexible spine or fracture, we'll typically place two reference arcs along the way and cover one so that we have a proximity to our arc whenever we're placing a screw. There's some debate as to whether or not you truly lose accuracy the further you get away from the arc. But even, there are simple things like the camera seeing the devices when they're much further away and having to move the camera. So a lot of times, we'll use two reference arcs and move from one to the other as we're doing our distal ends. So let's go over, how long does this really take to do?

- Yeah, it's multiple tasks when you're doing a cbCT registration that includes applying the arc and draping, and undraping the patient. There's data transfer issues after you have performed the spin. And so we measured this with the iso C, and average time, about eight and a half minutes for that whole process. And with newer cbCT devices may be a little bit faster in the spin time and the data transfer time, and they knock a minute or two off that. And so there's always been a big complaint with some surgeons about how image guidance adds to the procedure, and as we've talked about with the point matching registration techniques, that may be the case, but in these techniques here, we're always doing a confirmatory spin. And so if you're looking at a two or three level fusion, do one spin to register the spine and another spin at the end of the case to confirm registration. And it's gonna add really only 12 to 15 minutes to our case, total.

- Yeah, and I would echo that. And what I'd also say is, particularly in our longer segment cases, is that we make that time up and save time in having the ability to place our screws in the time it takes to place them with navigation versus doing it with fluoroscopy. So while we may have a little more time in this setup and placement, we tend to get that back in the procedure itself.

- Agree.

- So we'll move on to some posterior cervical uses. And we touched on this a little bit earlier, but I think a nice view here is that in placing occipital keel screws, sometimes the surface of that occipital bone is not as obvious as we'd like it to be, and there's some irregularities to it. And I've certainly found that the navigation can really help me find the midline perfectly for placement of these keel screws.

- Yeah, I agree. Allows you to find the midline and also you can measure the depth of those screws and set your drill guide appropriately, and it gives you less of a chance of getting a CSF leak and you know exactly how deep you're gonna be going at that angle.

- So why don't we briefly go over what we're looking at with these different navigation pictures?

- Moving down from the occiput to the C1-2 junction, after you expose the posterior aspects of the C1 lateral masses, you can just put your probe or your image-guided drill guide on that dorsal aspect and line up your trajectory in a medial lateral in the superior or inferior plane. And so trajectory views or inline views are the views that I think are most useful, and these will give you kind of an axial inline view of your screw trajectory, and a sagittal inline view of your screw trajectory. And then you can tell from this information how long of a screw you're gonna use in this case. It's maybe a little short of a 20 millimeter screw. And so, a lot of information from this just moved here, and size of screw, as well as direction and trajectory. And down below here, we got the C2 level, just putting a C2 laminar screw in. Same technique, finding your entry point, sizing your screw, determining the length of the screw, and then drilling the hole, tapping the hole and place your instrumentation.

- Yeah, I think there are a few cases that I always found a little bit hair-raising and thrilling, and that was placement of C2 screws and T1, T2 screws. And really in my practice, navigation has made those cases very straightforward to the point where I don't really worry about them anymore. Obviously, I'm still paying as much attention as possible to make sure everything's accurate, but the nervousness that I used to have in the drilling process has really gone away from me.

- I agree. It's doing the C2 screws and the T1 and T3 screws now are a cheap shot with this technology. As long as you maintain your accuracy and verify your accuracy, it's a very easy screw to place.

- I think that the issue I have to guard now against is really complacency and having to make sure that my accuracy is correct in verifying and being diligent, because this has almost lulled me into a position where I'm not worried about them any more and lose that heightened sense of diligence.

- I agree with you. And that's why I kind of make a rule on myself, is whenever I go from one segment to another, very quickly just drag that pearl on the TP or other anatomic landmark. Very quickly verify accuracy. And 99% of the time, you maintain accuracy, but every now and then you'll see that you've been off for some reason. It's really important to know that, especially at the C2 and upper thoracic levels.

- Tell us a little bit about your utilization for transarticular screws in the cervical spine.

- So usually, in the subaxial spine, if I'm placing instrumentation there, I'm typically going to C2, to the upper thoracic spine or both. It's just a fraction of my practice. And so I've noticed that if you're ending a construct at the C7 level, or if you're getting some short C7 lateral mass screws secondary into anatomy, C6-7 transarticular screws are very beneficial. It gives you some added purchase of C7 across the C6 lateral mass with one screw. And then it is very easily connected up to the construct. It's a very different angle than the lateral mass screw. It's more of a medial to lateral and inferior angle, but very easy to do with guidance as long as you verify your accuracy. You just drill your hole, you can tap across that hole. It's very important to make sure that you actually drill sharply across this cortical surface, because if you don't do that, you can actually push this joint apart, affect your accuracy, and end up with a screw that has just pushed this other lateral mass instead of going across the joints. So once you verify you're across the joint, go ahead and tap and put your screw. It's very easy to do.

- And then the other thing that's nice is being able to see with your probe or with your tap that you've crossed that, and whether you wanna get bicortical purchase or not. So we've mentioned briefly T1, T2, T3 pedicle screws, which certainly doable with or without navigation. But I think this picture that you have here shows what we see in the operating room, which is a really accurate and pretty view of the trajectory through the T1, T2, T3, upper thoracic pedicle screws, and how much easier it is to do this portion of the case nowadays.

- That's correct, and this is a kind of case, the C2 to T3 that not uncommon in our practice. And you can register all these levels just with one cbCT spin, get your registration. And then again, always starting from T3, distal, moving the proximal. And we'll do these type of cases typically with the Mayfield reference arc attachment. And sometimes we'll put it on C2 spinous process if the anatomy allows a simple application of it there, but we'll usually put the reference arc on the Mayfield and still find that we maintain very good accuracy, even starting in the mid thoracic spine.

- We discussed sort of anterior applications previously. Do you wanna show a revision type anterior case where this has helped you a lot?

- Yeah, this is a morbidly obese woman who was sent in from another institution and had a anterior or posterior cervical procedure, which she initially did well from then it started developing myelopathy and she's kyphosing above this with a bone spur pressing against the spinal cord, collapsing, subsiding her graft, tilting her graft. And so here's a woman who's morbidly obese. You're gonna do a redo approach to the cervical thoracic junction. You have a lot of osteophytosis over the anterior border of the spine. And also you have this slanted graft that you can see how you could be off on your... You can be midline down here and superiorly be completely off on it. So this is a, I think, a perfect case for guidance. And so you just do your spin, come down. And what you do as I pointed out before is we're just marking the midline. We're just gonna mark our corpectomy borders. And we also wanna make sure that we don't do this big procedure without taking out this three or four millimeter bone spur, which is really the source of all of her problem.

- Yeah, and there've been certain anterior cases where people have significant degeneration, large anterior osteophytes, or previous surgery, where if you were to come down and see a large osteophyte and lose sight of where midline is, it's sort of nice to have this ability. We sometimes may not plan to utilize it in a case, but if we run into some difficult anatomy that doesn't make sense, it's nice to have the ability to pull it into the room and really not add too much time in the case to get that additional comfort, knowing where we are. So obviously, a very nice postoperative result considering what you started with. Another example of a good degenerative use. Why don't you go over this case?

- A woman with progressive myelopathy who had very advanced degenerative changes in her mid cervical spine. And so yet another case where here, your anatomy is gonna be off a little bit, I wanted to try to keep this as a two-level corpectomy in this lady and plant on this superior vertebral body surface. And so, interoperatively you get in there and this little disc space is not that easy to see. And sometimes you end up drilling down into the cancellous bone. And so guidance helpful here for many reasons. Of course, maintaining your midline, making sure you're not leaving any pieces of bone back in the canal, and also drilling parallel to this end plate, so you can keep that corpectomy, a two level corpectomy and not leave any of these superior vertebral body in the canal. And so we turned it over and did some C2 to T2 or T3 instrumentation. You can verify your cage placement and plate placement and posterior surgical placement before you leave the operating room. And here's another example where you see, as you're doing your corpectomy, you can get your pointer out towards the vertebral artery. Make sure you're not getting too close to the vert as you're drilling down on your bone there.

- Yeah. So let's move on to thoracic and lumbar open pedicle screw placement. And I think this is a good picture, which really says what we see in the operating room. And you can get an idea of what width pedicle, what diameter pedicle screw you wanna utilize, the length of the pedicle screw, all by just placing your navigated probe on the entry point for the pedicle. This shows then tapping down that trajectory through the pedicle, and then with navigation, taking a virtual screw and placing it down into that trajectory and having a very good sense of where things are. One of the limiting factors of this is making sure that your screwdriver is very tight, because if there's any laxity in it, you could bend your screwdriver one way or the other and see a totally different trajectory for where that screw could be. So just one of the pitfalls with navigation that you have pay attention to.

- I just wanted to point out this diagram too, is that I've always erred, try to err a little more lateral on that screw and get more of a toenail type of medial angulation on that. And so even one or two millimeters from the medial border, accepting if you have a small lateral breach or close to the lateral reaches and of concern to me, I think if you err to laterally and superiorly in these screws, then that decreases your chances of a nerve root injury if the system's off.

- So let's discuss some of your other utilizations here in the lumbar spine.

- We see a fair amount of patients in who, revision surgery, they come in and they've had the instrumentation done and pseudarthrosis, and you find that a couple of their screws are medial. Sometimes they're symptomatic, sometimes they're not, but sometimes it's hard completely redirect that screw without using the same hole or crossing that screw path. And so I think in these cases, image guidance is very helpful. And in verifying that you're putting a new screw down a separate trajectory, which is a safe trajectory for the patient. Some of these people who are symptomatic from these misplaced screws are very angry about it. And the last thing you wanna do is slip a screw right down that same hole. We do see people who've had a pedicle, these chronic pedicle fractures. These would be very difficult pedicles to just probe with a gear shift and try to feel that. And so, in a lot of these cases, we'll bring the drill guide out and go ahead and drill across this fracture, this chronic fracture line, and then sequentially tap across it and perpendicular to the line. You don't wanna come across diagonal and try and shear it or anything like that. And so these are just the type of cases where we found the guidance particularly useful. And when you get have to do some more creative screws in pedicles such as this.

- We're gonna move on to percutaneous pedicle screws. We've got another presentation on the AANS site on percutaneous screws. So we won't spend too much time on it, but I think we can go over the basics with this. And I think one of the nice things you see here is really, it tracing out a trajectory or an angle down your pedicle all the way up to the skin's surface. So you can get that skin opened up and stay in a parallel axis down that pedicle so that you're not having it tug on the skin or the fascia. Have you found that to be beneficial?

- Yeah, especially in these minimal invasive TLF cases, I'll do my spin and I'll just drag that probe along the skin surface. And I wanna find one point that's gonna give me a good shot to this space, as well as the pedicle superior and inferior to that, where I can center my stab incision. It's very easy to do with 3D guidance, dragging your probe across the spin and finding that one site that's gonna give you good access to the pedicles in the disc space.

- And I think in discussing minimally invasive surgery, we know that it is really critical to have that trajectory accurate. Otherwise, if you're fighting soft tissues or tough fascia, it can make the case extremely difficult.

- Here's a diagram that's showing that you're at one point, good shot to the disc space, and then good shot to the L5 pedicle from the same point. You're just pivoting that one, and then good shot to the S1 pedicle. And so that's where I'm gonna make my stab incision and go forward with placing the screw. And we place our screws solely with guidance. We don't use live fluoroscopy, we don't use K-wires. You don't need to use K-wires if you're versed in image guidance. You can just find that pedicle hole with subsequent instruments by going to the base of your plan.

- And this is an example of a percutaneous procedure, and you can see the percutaneous reference arc. We didn't really spend much time on this, but obviously in an open case, you don't need to place these reference arcs into the PSIS, or you can actually just use the spinous process clamp. But in these cases, when you don't have that access, they have this nice alternative, which is a little bit less invasive. And again, just showing the placement of the screws down through the sacrum, utilizing navigation. So this is obviously a minimally invasive case with the passing of the rods through that screw extenders and the screw heads. Why don't you talk about this a little bit? We don't spend a lot of time on sacral alar screws.

- Yeah, we noticed that there's some cases where you can't really salvage a S1 pedicle screw in a pseudarthrosis case, or say, you're going with, we use these far as people we take up to L2 or L3. It just gives us some adjunctive fixation down in the sacrum. These go to the anterior inferior portion of the sacred alar. You're not going by cortical, but you can catch the cortex. You don't wanna go too posterior with this screw to violate the SI joint. Anteriorly, you have the L5 nerve root camping out here. And so what we found is that on longer level fusions, but not above L2, up to L2, good adjunct to S1 screws. We use them in place of S1 screws when the S1 screws can't be salvaged. Additionally, if we have patients with low set lumbosacral junctions, which are very difficult to get a medial trajectory of those S1 screws, and we'll just go ahead and use these S2 alar screws instead.

- And I think what this is showing is a lot of people who are well-versed in navigation are starting to utilize it, to learn and do other procedures that are beyond the scope of what they started with. I've certainly found that there are things that I'm pushing the envelope on, and learning to do differently, because I have the navigation and what it affords me, and the views it affords me to be able to do these types of things.

- There's a case where we just did a S2 alar screw in the place of an S1 screw on that side because it was struggling against the iliac crest and with trajectory. So I just shot an S2 alar screw instead. And a case where we had both sides. Very difficult, fighting the iliac crest. So said, "Forget it, we'll just put S2 alar screws on both sides." And you see this guy, he is very solidly in and did very well.

- So obviously, now we're gonna move over to some iliac screws.

- Yeah, so if we go above L3 or L2 and go up into the thoracic spine, then instead of these alar screws, we'll just angle a little bit more laterally and do these S2 iliac screws. The advantage of these, I think over iliac screws is that you're gonna get four layers of a cortical purchase. You can take this screw all the way to the external cortex of the ilium if you want to. And so I think it's a really solid screw. You do traverse the SI joint, you bridge the SI joint with an iliac screw in which you're... Usually you're only gonna get one layer of cortical purchase at best. And so we found these very valuable and these long level fusions, we do a fair amount of them down here. And also another advantage is that you can run that rod almost straight into the screw in most cases. You may have to incorporate the S1 screws with a cross connector, but I think they're easier to hook up than an iliac screw. And I think if they need to be taken out, we haven't taken any out as of yet, then it'd be much easier, just to do a minimally invasive tube right down the axis of the screw and drill the rod and take it out.

- So let's go over a few other revision applications.

- Yeah, so in addition to the S2 iliac, excuse me, talked about, there are some cases where you're ending a fusion at L2. As we know, L1 and L2 can be fraught with some very narrow pedicles. And if you're ending a fusion at that level, it's very important to really get a screw in on your first shot. Because if you trash that pedicle, you're gonna have to go up to L1 and then you'd have issues with that, then may have to go in the lower thoracic spine. So we found it very helpful when stopping at those levels and putting a screw right down the pipe on these smaller pedicles. Revision cases. When you have bone caked on the dorsal aspect of the spine, very easy to put screws into that bone, into the pedicle. And also for pedicle subtraction, osteotomies, we found is very helpful in guiding not only retractor placement for the osteotomy, but performance of the osteotomy itself in addition to the instrumentation placement. And so a very useful tool in revision and deformity cases. And I think as we talked about with other cases with image guidance, it takes off a lot of time off our surgery when compared to not using image guidance or using fluoroscopy or standard techniques.

- Yeah. And I think this has been an excellent talk discussing navigation and really giving people an idea of the different things that we can do with it. I think the pitfalls that can occur with navigation can all be guarded against if you're diligent, if you have a checklist of things you run through, if the people assisting you are very aware of the arc itself and how to avoid it, because those are where things can run into trouble. All the little things that we talked about, making sure that the reference arrays are snapped onto the instruments tightly, that the als or the screwdrivers are all seated firmly in the reference handles so there's no gap. And each time you're placing this in the wound, checking and verifying that in your mind, what you're seeing matches what the screen is telling you. We went over some line of sight issues and how to avoid these as best as possible, and then how to utilize these machines so that they're not in your way and not creating problems, but so that they're helping you and really speeding up the case itself. Eric, I wanna thank you so much for joining us today. I think this has been a great talk and really appreciate you spending the time sharing this with us.

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