# Aligning Preoperative and Postoperative Scans in Dental Surgery
## Introduction
[0] Sometimes when we do a surgery, we need to align meshes from a preoperative wax up to a postoperative tissue scan. It can be very difficult depending on the level of bone reduction that was done and the amount of surgical changes. [10] That is why there's the intermediary presurgical marker scan. The mod tissue tracker is a very simple way to accelerate the automatic alignment found in various different softwares such as Exocad between the presurgical scan and the post surgical scan.
## Aligning the Models
[24] Let me show you how we kinda take these models and align them. On the top here, we have a dentate case where we did some pretty aggressive bone reduction, after surgery, and there's a profound difference between the surface of the post surgical scan, tissue scan, to the preoperative scan. The reason why we like to merge these files to the preoperative scan is because oftentimes that's where the wax up is merged to as well and so we wanna be able to use the wax up information.
[64] On the lower we have an edentulous case that has very minor changes between the presurgical and post surgical scan, but the tissue tracker will also help with that. So let me show you how we align these things. We go to tools and expert mode, align meshes. And in align meshes, if you click manually, you could move individual meshes however you want.
## Automatic Alignment
[86] Now typically, there's going to be, in this case, the preoperative model that you don't wanna move because it's locked into the wax up and into the occlusion. Then we go to automatically, and the way that we do this the way that we align meshes is, for example, up here, we're gonna go ahead and align the presurgical marker scan to the preoperative wax up scan in this case.
[113] And we typically use common points for that spread throughout the arch. It really is helpful if they've had a few teeth that you could use hard tissue, spots and also soft tissue spots like that, including even, various different rugae structures that you could come in and mark like this. Now, we have a lot of common data here, especially before anesthesia is placed all in these areas, which could cause the tissue to swell.
## Performing the Alignment
[144] Ideally, the presurgical marker scan, you've only anesthetized the palatal area where you're gonna be putting your tracker before you scan so that you have no deformation of the tissues up here from the anesthesia, local anesthetic. Now what we'd like to do here in this particular instance is go to perform alignment, and then we go to best fit match. And the software is gonna look at all of the data combined and find a best fit match between the various different scans.
[165] And you can see here, it's mostly using the hard tissue data, and we got a pretty accurate merge. Now one thing that you can do let me go to, undo really quick. You could further fine tune that match by telling the software only to focus on certain areas. And the way that you do that is go to matching exclude selected parts, and you go ahead and paint the areas that you want the software to focus on.
## Fine-Tuning the Match
[193] So here, we'll paint maybe the tissues around the teeth. We could paint the similar areas on this model just like this. Maybe we avoid the palate here because that's where anesthesia anesthesia was given and it's gonna be, swollen. We could actually also go up onto the facial in this particular instance.
[222] We got a lot of nice common data like that. And then what we need to do because the blue areas represent areas that Exocad will not consider. And so what we need to do is invert that. So we hit invert markings, and now we can hit perform alignment, best fit match, and it's now only going to look at those painted areas when it does the iterative closest point alignment algorithm.
## Checking the Accuracy
[242] And if you want to know how well, how the accuracy was here, you could unselect this and hit show distances. Now I could already tell you that I know in this particular case that patient was pumped full of anesthesia all around the tissue here, which is why we have no colors around our tissue. So now we have aligned, our I'm gonna click okay. We have aligned our presurgical tissue tracker scan to our preoperative scan that was made probably many weeks or months ago from the day of surgery.
## Aligning Post Surgical Scans
[272] So now we need to align our post surgical tissue scan here. We have sutures, extensive bone reduction to this preoperative scan. And the way that we do that is through the through the intermediary scan, and it's mostly focused exclusively on the tissue tracker. And so, again, what I'm gonna go to do is tools align meshes, and I'm gonna go to, automatically.
[295] And you're gonna select a point on the tissue tracker here. Look for common, areas such as the tip of the triangle and things like that. You wanna do about four or five points that are common between the two the tracker scans, and then you're gonna hit matching exclude selected parts. And now I'm going to have the software only focus on the tracker and maybe some tissue up here that might not have been too badly deformed.
## Finalizing the Alignment
[332] That is it. You could, if you felt confident, you could go ahead and do something like this where you said, okay. I also want you to focus on all of this area like that. Whatever you feel is similar and not distorted between the bone reduction scan and the preoperative or pre surgical scan.
[347] So then again, we have the invert markings, perform alignment, best fit match. And now it's only gonna use the triangles and data points where you have the, scan data visible and not painted blue. And then we could hit here and go to show distances, and we could see that really what you wanna focus on here is the tracker, being all within the acceptable range which is pretty much here we have blue as zero, difference.
## Conclusion
[381] And if the tracker is blue and green, we know that we have a phenomenal merge and that we could rely on the alignment of the model. Which then is aligned to our pre operative wax up. And so that's the beauty of this. Now some of you might be saying well I do stackable guides. I don't need to align anything.
[408] But really what you're doing is if you do stackable guides you have tubes that you're squirting resin around in the mouth and locking on temp cylinders and then polishing and finishing. Really that's the way I did it, for probably about ten years when I first started doing all neck surgeries. But now I find it's more freeing just to to be able to do this and then go direct to MUA with the screw and not pick up anything in the mouth.
[430] Okay. So now if we look I'm gonna click okay. If we look at the the edentulous case here, it's much more simple because we have no bone reduction. We have guided surgery here with just pinpoint openings in the tissue. And really, we could real we could use most all of this minus the anesthesia deformation.
[449] So So all I'm gonna do is go to, again, tools, align meshes, and I'm gonna go ahead and click common points between the presurgical marker scan here, which is done on the day of surgery, like I said, before any implants have been placed. And I'm just gonna look at common data. If I could find any little spots back here in the hamular notch area, that's great.
[471] I'm just gonna put one right here. We're gonna go to perform alignment, best fit match, and it's gonna consider the entirety of this scan. So that's really good. Here, it's it's not a perfect alignment, again, because of anesthesia. So we're gonna click okay. We got a good alignment between these two meshes.
[487] Now we need to align our post surgical scan to our presurgical marker scan. And, again, we're gonna go to tools, align meshes. I'm just gonna go to manually here and just slide this a little bit closer, and then I'm gonna go to automatically. And what I'm gonna do here is focus on my tracker.
[510] So a little star, four five spots, hit matching exclude selected parts, and let's go ahead and just paint the tracker. Perform alignment, best fit match, and now it's only looking at that tracker. If I wanna see the colors, I unclick matching exclusive left to parts and hit show distances, and we could see, we have a really good alignment of our tracker with even, the rest of the model now is is brought in very close alignment, which remember, the reason why we do this is because the original everything's aligned now, to the original scan, which carries all the information of our wax up, our diagnostic wax up, our occlusion, all pre done and ready to go.
[564] So now it's just a matter of once we have this alignment, going ahead and telling Exocad to cut the implant locations in our diagnostic wax up so we could do a direct to MUA or even a tie based type prosthetic. So that's how easy it is. It should take no time at all on the day of surgery. On the mandible, we usually do two or three trackers, retromolar pad area and midline.
[584] Those trackers are a little bit smaller and it's nice to have the triangulation effect on the mandible. So, stay tuned for another video where we do a mandible alignment.
