 We'll go ahead and get started. This is a really boring lecture if we just spend all of our time on this. So at some point, we're going to switch to sort of a case-based approach. We're going to go through a few of my cases, just look at the thought process that I go through as we're considering that. Health protected information, so we may have to shut this off for that particular aspect. We'll try and go through the concepts first and then look at that from a case-based approach. So the basic components I think we're all familiar with at axial length, corneal curvature, and then specific constants that are used to account for the lens design and the effective lens position, which is probably the biggest variable that we just don't have a great way of predicting or accounting for that effective lens position. That'll be a recurring theme as we're going through things. We're familiar with the different ways to measure things, ultrasound and laser interferometry, either the LENSTAR eye will master using that technology. Ultrasound, Aplenation technology was originally applied, but it was very difficult to control the amount of compression of the cornea. And so it's not as predictable, so immersion was designed down to be more accurate by using a scleral shell, so a fixed structure with saline or a balanced saline solution to allow that to be measured. With ultrasound, it's important to get good spikes with sharp peak that are perpendicular. And sound velocity, it's important to understand with ultrasound measurements that sound passes through different structures at different speeds. And so the measurement is going to be based on that. Most of the time, most of the older machines are just sort of taking an average of that. The newer machines will actually account for the thickness of each of the different structures and the specific speed of sound through each of them to give you the most accurate axial length measurement. We won't worry too much about that. So partial coherence into ferrometry, essentially they're using a beam of light and moving mirrors to measure the axial length. That's both in the lens star and the IOL master, sort of different concepts with that. I won't bore you too much with the details, but that's the basic concept behind it is that that movement or that motion measuring that, knowing how much that arc is subtended in that period of time. We know that laser interferometry is very accurate. It has been essentially calibrated to ultrasound, high precision immersion ultrasound, because most of the IOL calculations were originally designed based on ultrasound measurements. So to keep the formulas the same, all of the work that they had done, they essentially did that. Let's see. So the lens star, some basic difference between the lens star and the IOL master. For the most part, it's the corneal data that's obtained. So the lens star has 32 reference points at two concentric rings, whereas the older IOL master, the 500, had just four points that it was measuring. The newer IOL masters improved upon that ability to have 16 points of corneal data. So it tends to correlate a little bit better with the lens star, but this is one of the reasons that many people like the lens star because of the corneal data that it gains. The interferometry with the lens star are probably not quite as robust. It's a slightly different principle that's used to measure the axle length, not quite as robust as the IOL master in measuring that. The other benefit of the lens star is the robust IOL formula options that it presents, including the Holliday to the Hagus, the Olsen, the Barrett formulas, it's a little more challenging to use with some of the other machines because you have to take the data and enter it in manually into other systems, whereas these are sort of built in to be the lens star software. Let's talk a little bit about corneal power. So cornea is a meniscus lens. Essentially our final measurement is going to be based on anterior radius of curvature and assumptions that are made about the relationship between the anterior and posterior cornea, and we calculate a predicted power that the formula will use based on these assumptions. We know there are a number of different circumstances where those assumptions are violated and we're gonna go over the details of that post-lasic, post-archae, et cetera. So again, the measurement is a measure of the anterior curvature. It's the first Purkinje reflection that it's measuring. Sometimes we can use other types of devices to measure the corneal power or estimate that, whether that's elevation maps, topography, tomography, et cetera. We won't go over too many of these details. This is something that the corneal department should go over in terms of topography interpretation. But most manual or automatic automated charitometers are looking at about this three millimeter zone. If you look at SIMK values from a topographer, it's giving you that same sort of estimated information. So that's where the original sort of K values were developed for the IOL power calculations back in the original forms. Index of refraction is important to keep in mind this sort of the standard index that's typically used 1.3375. Let's keep moving here. It's a long lecture, so I'm just trying to hit the highlights. Let's see. So automated charitometry, a few things why we use it over manual charitometers. Anybody used a manual charitometer in here? You've used one with Dr. Harry? When I was the intern, we measured for a VA patients, we did all of our K values of the manual charitometer and we used the ultrasound measurement. It was not an immersion ultrasound measurement. It was a direct contact ultrasound measurement. That tells you how old I am, I guess, more than anything. So the obviously advantages over manual automated charitometers is just easier to get, less cooperation is required. Any accuracy is pretty comparable to manual with less very inter-observer variability. Again, three millimeter zone. A couple of concepts that are important to understand when you have a flatter cornea, that measurement zone is actually gonna be a little bit further out from the intended measurement. And this is just based on, if you think about it conceptually, the geometry of the cornea, whether it's steeper or flatter. So with a steeper cornea, it'll be a little bit nearer to the center relative to a flatter cornea. And that plays a little bit of a role in some of the issues with post-lasic calculations. In addition, of course, some of the violations of the anterior-postural relationship. Don't worry about disadvantages. That's the corneal topography. We all know what a topographer is. Essentially, we've got these rings that we're using to measure essentially the first Purkinje image of those rings using software algorithms to sort of create a topographic map with that information. And probably the most important thing with the topography is making sure the corneal surface is pristine to get the most accurate data. So a very dry cornea will give you a lot of problems in terms of getting a good topography, a lot of artifacts due to that issue. Has anybody seen a Norb scan? No, I don't think we have one here anymore. When I was a resident, we had a Norb scan. So Pentacam, I think we're all familiar with that. If you wanna know the details of the history, I'll send out the copy of these slides and you can read through it in a little more detail. But essentially, using slit scanning, rotating slit scans are gonna do with the HR, I think it's 100 scans. Over a very short period of time, they're gonna use those scans to create maps, comparing it to either a best fit sphere, a best fit ellipsoid depends on, a sphere ellipsoid depends on what specific data set you're looking at. But they're taking their data and comparing that to that and then using a software algorithm to give you the actual data that you're seeing. All right, let's see. So the effective lens positioner, enter your chamber depth, with the effective lens positioning, and this is a really important principle in terms of IOL calculations and is probably the variable that's most difficult for us to account for. This accounts for a fair amount of the inaccuracy or the variability that we see with IOL calculations in terms of predicted and actual results. It's represented essentially as a surgeon factor or constant in the different formulas. We're all familiar with those as we look at them with IOL calculations, but essentially those are just trying to account for that position, in the predicted final position of the lens. And they'll use different things to try and predict that. So look at the corneal curvature, a steeper cornea versus a flatter cornea and the intended assumptions, the anatomy of the eye based on that. The axial length, in some cases, enter chamber depth is used to try and help predict that effective lens position. A quick note about sulcus IOL placement. If the lens is placed in the sulcus, what happens knowing optics principles, what happens, the effect of power it increases. And so we have to decrease the power if we want to get the same intended refractive result. A lot of it depends on a few factors with regard to the anatomy of the eye and you can kind of use the lens power that that eye is calling for as a surrogate for that particular aspect of the optics. So a very low power IOL, you may not necessarily need to make any changes. For more standard powers, it's about a half to a full unit. I think it's probably the principle you've probably heard when somebody's talked about in the operating room with high powers, you may actually want to reduce it a little bit further. So that's something to keep in mind. If you opt to capture a lens, you don't have to change the power. It should be very similar to what you would expect if you were placing a lens in the capsular bag. So that's just a quick review of that application there. Let's see. So double methods, the double K, the arm berry method. This is important to understand the concept for post refractive patients. So we're using a post op K. So a post character refractive surgery K value for the IOL power calculation. We're trying to use a pre op K if we can. If it's unknown, then there's an assumption made to calculate the effective lens position. So there's a reason for this. We want to use a post op K because that's the actual K value that's telling us what the current corneal power is to some degree. We want to use a pre op K to help us with the effective lens position calculation because we know a flatter K or a steeper K relative to what is truly there in terms of the anatomy is gonna affect that effective lens position calculation. And so that's the reason that we use those two different K values if we have them available. With a post myopic eczema laser treatments, do you guys know what the typical error is if we did not make any adjustments? So with a post myopic, it is a typically a hyperopic error with a post-hyperopic LASIK patient. It's going to be a myopic error if we don't make the adjustments. So just kind of that's just a rule that you can use for OCAPs if you have a question about that that's what to expect in that scenario. It's the Iowa power formulas. I won't bore you too much with the history. Kind of an understanding of this regression formula P equals A constant plus two and a half times axial length plus 0.9 times K. You can kind of use that to give you an idea of how important the different measurements are in terms of their impact. There's an error in those measurements in terms of their impact on Iowa power calculations. Obviously the axial length and air in the axial length can have a greater effect than the corneal errors although both can play a role in that. So you can see there's multiple different generations of formulas adding additional measurements all of them essentially designed to try and improve the accuracy of the formula and try and improve the ability to predict the effective lens position ultimately there. Yeah. Uh-huh. I do not. And we'll go over that more in detail. The holiday too, I think that we're getting up to the point where we're going to talk about formula accuracy. So let's do that because I think this answers the question that you have, Eileen. The classic teaching for, and this is actually based on studies by Hoffer, less than 22 millimeters. The Hoffer Q or the Holiday 2 are about equivalent in terms of accuracy. 22 to 24 and a half Holiday 1 or Hoffer Q. SRKT was pretty close as well. So in these more typical rent, that normal range of axial lengths, most of the formulas perform fairly similarly. Although the Holiday 2 tends to be a little bit less accurate and it seems to be more applicable to extremes, if you will, that additional data. A little bit longer ISRKT in Holiday 1 and then the longer ISRKT tends to be the sort of the classically taught one. This was when I say classic is back when I was a resident. So we can look at some of the more recent studies this Hoffer studies where that information is essentially based on this JCRS August 2000 paper. Some more recent papers that have looked at this concept, Navarez, they looked at using immersion ultrasound and manual care tomatry. All of them essentially performed the same, short to long eyes. They really didn't find much of a difference in terms of the accuracy of the formulas. We look at this group in England, Aristomedeu et al in 2011. They used the IOL master. What they found with really short eyes is the Hoffer Q performed slightly better with really long eyes. The SRKT performed slightly better with typical eyes, the Holiday 1 was probably the most accurate formula. And I believe they included the Holiday 2 in their analysis. If we look at very long axial lengths, the Haggis and the SRKT tend to perform better but there was a tendency for a hypo-opic result if you aim for plano and so they would recommend aiming for just a slight myopic result. The greater the axial length, the more myopic result you want to aim for. So you maybe started a quarter and move it up by a quarter unit about per millimeter of additional axial length. When we looked at really short eyes, this Korean group looked at that the Haggis was most accurate in their study. Suggest taking that with a grain of salt. This is the Korean journal of ophthalmology. I don't know how rigorous it is in terms of getting something published. But it looked like it was a good study from what I evaluated on that a few years ago. Aristomidaeo, the same group, they looked at Iowl optimization and they did optimization of their Iowl constants with their entire practice, so a lot of different surgeons. They also personalized their constants for each individual surgeon. They did not show any improvements in the accuracy of their calculations going from just generalized optimization for the group versus personalization for each surgeon. So there may be a benefit when you're in a large group to just optimize with the group and not worry about personalizing for each individual surgeon. Let's see, so a couple of other things I think that are useful. So this covert group and then the Aristomidaeo group looked at using the refractive error from the first eye. So knowing the predicted and actual result from the first eye and then trying to use that information to see if they could improve the outcome in the second eye. And what they found is applying approximately a 50% difference from the predicted to actual results. So let's say it was a diopter off, predicted versus actual in the first eye. You take half of that, so 0.5 diopters and apply that to the calculations for the second eye and make that adjustment and that would give you a little bit better accuracy in terms of the actual outcome for that second eye. So that's something that can be done. That's one of the reasons I advocate for trying to refract patients at the VA after the first eye, before the second eye to kind of be able to apply that information to help with that. It doesn't often apply all that much if we're within a half diopter, 55 to 75% of the time. Imagine a quarter diopter change, in most cases probably isn't gonna push you to change your Iowa power, but can be useful if you get a little bit more of a stray result. Assuming that the eyes are fairly similar in terms of their axial length and K values, this can be applied and used to help you improve the outcome of the second eye. Olson, who is considered an Iowa guru, his study showed that his formula is the most accurate. Surprise. And then for long axial lengths, the Wang-Coke adjustment, this is the, I think this is sort of the standard that most people use now for longer axial lengths. We talked about SRKT being a formula that's most accurate in some of the older studies. Dr. Koke and his group demonstrated that making this adjustment for axial lengths longer than 25 millimeters, and they have the formula as they're published. You can use those and mathematically apply them. The LENSTAR automatically applies them. And they showed an improvement in the accuracy and you did, essentially it accounts for the hyperopic results. You don't have to aim for a myopic result in these cases. They'll make that adjustment for you mathematically and you can just think about the Iowa calculations the same way you would in a typical patient. The formulas that were most accurate with that was the adjusted, I believe, holiday one. I was just keeping in mind when you're looking at it, it's the holiday one adjusted that you're looking at to get that data. All right, let's do a few more minutes. So formula of personalization, we can talk about that some of the time individually. Staffelomas create more issues with ultrasound measurements and with IOL master or interferometry measurements. Silicone oil with ultrasound measurements, something to keep in mind. Silicone oil sound, the speed of sound is significantly different in that scenario and that can impact your calculations quite significantly with laser interferometry, not a big issue. Unilateral magnetropia, I think we all know a little bit about that, PKP, that's another big deal. So let's talk about radial cartotomy and its impact. So one of the big problems with radial cartotomy, if you've, I'm sure all of you have seen this, with these radial cuts in the cornea, oftentimes the optical zone is very small and so many cartometers are measuring right at that knee, that junction point between those incisions and the flattening effect that it has on the center of the cornea. So it creates a lot of variability in terms of those corneal values. It doesn't change the anterior to posterior relationship. Both the anterior and posterior cornea are flattened with this technique, and so when we look at that versus laser gets a little bit different concept in the way that flattens the cornea. And of course the central part of the cornea is greatest in terms of the flattening effect and obviously more incisions equals greater flattening with RK surgery. We all know that there's some potential regression or further effect over time. One of the studies that looked at RK, I think it was over 50% of patients would have continued hypo-opic drift after the procedure and so many surgeons will advocate aiming for some mild myopia, understanding that there's a reasonable chance that patient will see some drift through the rest of their lifetime and they'd like to try and avoid a hypo-opic refractive error for a significant portion of that. That's the concept behind that. I think we're all familiar with the SCRS calculations, looking at post-RK as a good formula, looking at that. The aura is another technology that allows us to potentially improve our accuracy of those measurements using that data intraoperatively. Let's see. So post-lasic or PRKI, so there are three causes of the errors that we see with these eyes. The instrument error we're looking, as I mentioned, with post-myopic-lasic or with a flatter cornea, you're measuring a more peripheral corneal value, so we're starting to get a change in that. We're not getting the same ring that we were specifically looking at because of that excessive flattening or extra flattening, if you will. An index of refraction error, the index of refraction is based on assumptions made between the anterior and posterior curvature of the cornea, so those are violated because with the X-ray laser, we are changing the anterior curvature, but essentially not making any significant change to the posterior cornea directly. And then, of course, formula errors, they're using K-values, the curvature of the power, to predict the effective lens position, so we've essentially flattened or steepened, depending upon the type of lasic you've done, the cornea relative to the anatomy of the eye, and that's violated some of the assumptions regression formulas that have been designed for that. So there are a number of different ways to calculate things. Some of the original options was a contact lens or clinical history method. Studies that have looked at the ESCRS online calculations essentially has found that these pre-historical formulas are not as accurate as some of the other formulas that have been since designed, so when you're looking at those, I think they've essentially, I believe they recently removed some of the, they had three columns originally, they basically removed that first column because it was shown to be significantly less accurate than the other two columns that they had, but originally they had three columns with this clinical history method, contact lens over fraction method. Let's see about that. Talked about the double K. That's kind of a G-Wiz question. Toric eye wells. So I think it's an important concept with Toric intraocular lenses, the amount of terricity required for a lens is somewhat dependent on the anatomy of the eye, so when you think about that, there's a fixed ratio that's used in some of the calculation, online calculators. Other calculators take into account the interior chamber depth, axolength of the eye, so with decreasing eye well power, so in other words, longer eye and increasing effective lens position, in other words, deeper chamber, the ratio increases, so you'll see a change where you have to have an increase in the Toric power for a given eye relative to a standard eye. That usually isn't a big issue for typical eyes, it's when you're looking at a very long eye or a very short eye that you have to think about that formula adjustment. Some of the calculators, I think the Barrett calculator already builds this in, I believe some of the Lens Star calculators built this in. The Alcon form calculator online doesn't, I believe the AMO does account for this adjustment. Again, in most cases, it's not a huge adjustment, you don't have to worry about it, but if you've got sort of the extremes, a very long eye or a very short eye, you should be looking at that and thinking about that and how that affects the Toric power requirements. Let's see, so we've talked about the Aura, so you could use that to measure both the eye well power required and Toric correction that might be required that could potentially enhance the accuracy of your Toric calculations. And if you happen to have sort of a Toric surprise, so to speak, post-operatively, there is an online calculator using vector analysis, you can calculate the effect of rotating that lens to determine if just rotating the lens would help to reduce the amount of residual astigmatism to a point that you'd be satisfied with that. So if a patient's unhappy and you're unhappy with that result, too much astigmatism, residually, you can use that to kind of determine would just rotating the lens make enough of a difference to improve the outcome versus do we have to do something else to try and manage the residual astigmatism. Let's see, pediatric eyes just know that the formulas are very challenging in terms of applying them to these eyes. You know that the rate of growth is logarithmic, but it's quite variable from individual to individual and so the end result is somewhat like a dartboard game, nothing better than that, unfortunately, at this point. So we try to aim for some hyperopia and assume that as they grow, their eye enlarges that we're gonna lose that, but oftentimes we end up with significant myopic errors when they're fully grown. Let's see, we won't worry about piggyback IOLs. So that's the lecture portion. Again, I'll send this, we'll pick a resident to be in charge of that, I'll send it to them and they can distribute it to those who are interested in reviewing this, having this available to you to kind of think about or review and refresh your memory on IOL calculations and concepts. All right, does anybody have any questions they wanted to make sure?