Lecture: IOL Calculation in the 21st Century: New Formulas

Intraocular lens calculation formulas have steadily evolved over the past half-century. The recent popularity of premium IOLs has pushed the envelope of IOL technology and predictive calculations even further, and today we have not only 4th generation formulas but also other technological advancements, such as interferometry and swept source OCT, to assist us with accurately correcting even the most difficult refractive situations encountered during cataract surgery.

Lecturer: Dr. Jose Mendoza, Ophthalmologist, Lima, Peru


DR MENDOZA: Hello. Good morning. Thank you for attending this webinar today. Today I’m gonna talk about IOL calculations. And I put the title: IOL Calculations in the 21st Century, because we’ve come a long time since the first IOL implantations that we made. So let’s talk a little bit about history. The first intraocular lenses have been used since 1949. Ridley actually placed the first intraocular lens in that year. This is actually an electron microphotograph of the IOL that Ridley used at that time. So to calculate the IOL power, measurements of the axial length — axial length — keratometry, the curvature of the cornea, anterior chamber, the lens thickness, here, and white to white are necessary for the correct biometry on calculation of the IOL. Achievement of a target refractive outcome has become an integral part of the cataract surgery. So with the advent of optical biometry and introduction of new IOL calculation formulas, it has improved a lot our ability to accurately predict cataract surgery refractive outcomes. We’ve been using ultrasound for a long time, to do our biometries. Now we’re using other sorts of technologies, such as low coherence base interferometry, that is much more accurate than ultrasound. And less dependent on the operator. Also in the last years, SS-OCT is used to obtain even more precise measurements. We need accurate measurements, because we need to minimize IOL calculation errors. In a review made by Dr. Ronald Melles, of more than 260,000 eyes, it was found that less than 1% of cataract surgeons attained +/- 0.50 diopter accuracy of 92% or better. But the great majority of the surgeons were clustered around the 78% level. If we’re talking about cataract surgery, it’s not a recovery surgery, no more, but it is a refractive surgery, obtaining less than a diopter in the calculation and the refraction after the surgery is considered a goal. And it’s kind of mandatory. If we review some of the formulas, we can see that less than 50% obtain a refraction error of 0 or 0.25. The vast majority obtained between 0.25 and 0.50. And then a lot of people also almost had a diopter error. This is not good right now. So let’s talk a little bit about optical biometries. Does the biometer really make a difference? So I started reviewing some studies that they were making about this. Here’s one that is a comparison of a new swept source optical coherence tomography biometer, plus some other base optical biometers. And the conclusions were that the repeatability and reproducibility of a swept source optical biometer was excellent and agreed with a standard biometer, and the agreement was very high. So swept source and standard biometers perform similarly in this study. In a study from 2015, I and a group of colleagues presented a paper, doing the same comparison. But it doesn’t have swept source OCT biometers in 2015. So we compared three optical biometers. IOL Master, IOL Master 500, and the Aladdin, and the conclusion was that these three biometers perform as well as each other. Except the IOL Master previous version. And this is because the previous IOL Master doesn’t have the composite signal that IOL Master 500 has. And that comes to an interesting point. If the signals — I mean, the peaks that are gotten by the biometer are not quite good, then the biometry won’t be quite good. And that’s an issue when we come to hard cataracts. Here’s another paper, more an actual one — comparison of three optical biometers. IOLMaster 500, Lenstar, and the Aladdin again. There were no differences between these three when measuring axial length, mean keratometry, and ACD. So we could imply that the biometer doesn’t make a difference. But does the biometer really make a difference? When we come to hard cataracts, like this one, and we start analyzing the curves that the biometer makes, if we couldn’t get a good readiness signal, because the cataract is so hard that the signal won’t come through, then we won’t get any good signals of the retina. And the measurements the biometer makes are all between — all these signals. The first one, cornea signal, Descemet’s, and the last one, retina, will take in account the axial length of the eye. So if you got a mild or moderate cataract, any biometer should do. When it comes to hard cataracts, you need some sort of OCT like swept source that has greater wavelength, to get through the opacities and get a good signal. So this comes to our first question. Does the biometer really make a difference? We have five options. And you can vote right now. Yes. No. Optical biometers are superior than ultrasound biometers. A and B. Or none of the above. I leave you a couple of seconds so you can answer this. And let’s see what you think. Okay. We got the answers. And the answer was yes. Yes, it does make a difference. And as we were talking about, it does make a difference only when you get hard cataracts. Now, let’s be sincere. I live in Peru. I’m an anterior surgeon, based in Peru. And we here have hard cataracts. So when we have hard cataracts, we do need a biometer that could take in reliable measurements of these hard cataracts. As I told you before, and all the papers show, when you get moderate or mild cataracts, it doesn’t make a difference. So let’s talk about another important point. Lens constants. The constants must be optimized if you use optical biometers. A constant that the manufacturer gives you on the lens or on the box that comes with the lens are the optical constants — are the constants for ultrasound biometry. If you use that constant without optimizing, in an optical biometer, you get a source of errors. Now, where and how do we optimize these constants? So there is a web page that’s called ULIB. ULIB stands for: The User Group for Laser Interference Biometry. This page was managed by Dr. Walter Haggis until he passed away in 2019. This web page has actually been actualized with new constants, provided by several surgeons, around all the world, and you get the vast majority of IOL constants optimized in this web page. So this comes to our second question. How important are IOL constants optimization? They are very important, they are somehow important, they are not important at all, they are mandatory if you are using optical biometers, or none of the above. I’ll give you a couple of seconds for the audience to answer the questions. And I’ll talk about the importance already about optimizing IOL constants. I’ll also take a minute to talk about Dr. Wolfram Haggis. He was a professor and a close friend, and he was a great guy, and we depend a lot on his work about the optimization of the constants. So all of the work that he did on the ULIB page is still good for us. So how important are the IOL constant optimizations? They are very important? Yes. They are very important. 76%. And are mandatory if you’re using optical biometers. Both of them are correct. They are mandatory if you’re using optical biometers. Please don’t use a constant of the lens off the box. If you are using an IOLMaster, Aladdin, Lenstar, or any of these optical biometers. Now, let’s talk about mathematics and measurements. This is the formula that we mostly use for this. I’m just kidding. I’m not talking about mathematics right now. It’s kind of hard. We’re surgeons. We actually don’t know that much about mathematics. If someone really knows this mathematics equations, and really likes this, we could have another lecture about this on some other time. But this is not the purpose of this lecture right now. According to Dr. Warren Hill, for axial lengths from 22.5 to 24 millimeters and central corneal powers ranging from 42 to 45 diopters, and a normal anterior chamber depth, most important IOL power calculation formulas will give good outcomes. Dr. Hill, if you don’t know Warren, is the author of several formulas, including one of the most precise ones, the Hill-RBF formula. In this slide, you can see that if you got a normal eye with a normal anterior segment anatomy, all of these formulas perform quite well. The issue with classic formulas are: When you’ve got short eyes, where the vergence formulas doesn’t perform that well, or when you get long eyes. If you also have in the anterior segment some alterations, it will give you some sort of errors in the calculation of IOLs. Now we have these new formulas that we will talk about right now, and see how they perform. Something to consider. As surgeons, we’re being judged by our patients and our peers, by our refractive outcomes. That’s certainly true. It seems a little odd to spend thousands of dollars on the most accurate measurement technology, but continue to rely on calculation methods that are not from this century. So optimal outcomes require the best possible measurement technology, and the best possible calculation methods. So both things are important. One of the new… Actually, it’s not the new methods… But one of the… How should I say this? One of the most important parameters to take in account is effective lens position. The effective lens position has been described with the first generation formulas before 1980s, where the effective lens position was only a constant of 4 millimeters on every patient. And that’s not true. Because the effective lens position has many, many variables. How big is the eye? Is it a myope eye? Is it a hyperope eye? Does the anterior chamber depth is affected? Does the cornea depth is affected? So in the ’80s, they used a single variable predictor as a scaling factor for effective lens position. But they don’t measure it actually. Third generation formulas used two variable predictors. Keratometry, axial length, and improved scaling accuracy of the effective lens position. But it’s only with fourth generation formulas in the ’90s that Olsen and coworkers improved the effective lens position accuracy by adding two more variables. ACD, that’s the anterior chamber depth, and lens thickness. So now if we want to work and we want to use fourth generation formulas, we need to know the ACD value and the lens thickness value. They are important and mandatory for these fourth generation formulas. It’s time for another question. So effective lens position is taken in account with which generation formulas? First generation, second generation, third generation, fourth generation, or all of the above. I’ll give you again a couple seconds for you to answer these questions. But we should notice that since biometers are not that much important, when it doesn’t have this hard cataract, new biometers are important if you want to use fourth generation formulas. And fourth generation formulas are more precise than the other generation formulas. And they work on all sorts of eyes. So the answer was: All of the above. That’s correct. And also fourth generation, because fourth generation formulas are the only formulas that actually need a measurement of the ACD and the lens thickness, so you can get the correct values to get these formulas done. Let’s talk a little bit now about new formulas. I want to talk this morning about five new formulas. The Kane formula, the EVO formula, the Hill-RBF formula, the Barrett Suite, and the Olsen Suite. These last two I have the most experience, so I’ll talk about these two the most. But let’s review some of these interesting formulas also. Here’s another interesting graph that will show you that between the normal eyes all the formulas perform as good. The issue again is when you’ve got these extreme eyes. Big eyes or really small eyes. The Kane formula — here is the link if you want to go and see the IOL formula and how it performs. It’s free and it’s online. So you could get this formula for your calculations. This is a new formula, and it’s based on theoretical optics. It incorporates regression and artificial intelligence to further refine its predictions. A focus of the formula, when it was first made, was to reduce the errors seen at extremes of the various ocular dimensions. We were talking a lot about this. All the formulas perform quite well in normal eyes, with normal anterior chamber, with normal corneas. The issue is the extreme measurements. So this formula accounts for this type of errors to get a best performance on this. It uses axial length, keratometry, anterior chamber depth, and also optional variables of lens thickness and central corneal thickness, and that predicts the refractive outcome a little bit more precise than other formulas. This formula maintains its accuracy at the extremes of axial length, resulting in 25% reduction in absolute error in long eyes. Long eyes stands for 26 millimeters or more of axial length. Compared with SRK/T formula. That’s one of the most used formulas for big eyes. And also got a 25.5% reduction in absolute error in short eyes. That’s less than 22 millimeters. Compared with Hoffer Q. So if we’re talking about these two formulas, SRK/T and Hoffer Q, we do know that SRK/T performs pretty well for big eyes. That’s why they compared it with this formula. And Hoffer Q is a formula that performs the best with smaller eyes. So compared with these two formulas that performs quite well on the extremes, the Kane formula performs even better. There are multiple clinical studies that have been demonstrating that the Kane formula is more accurate than all currently available IOL formulas, including Hill, Barrett, Olsen, Haigis, Hoffer Q, Holladay, SRK/T, EVO, and Holladay 2. So the use of artificial intelligence to refine this formula gives this formula great results. If you go online, this is what you will see. Where you could put the surgeon data, their refractive index, the patient data, ID, and then all the parameters that you need to get here. Axial length, keratometry, anterior chamber depth, lens thickness, and central corneal thickness. This is one of the latest formulas, and this is the one that according to whitepapers performs most — of the best. We talk another formula right now. The EVO 2.0 formula. Here is again the link. It’s another free formula that you can go online and look for it. EVO stands for emmetropia verifying optical. It’s a new thick lens formula that is based on the theory of emmetropization. It generates an emmetropia factor for each eye. So it’s a custom made formula for each eye. It’s based on the factor that corneal growth is mainly complete in infancy, and the majority of eye growth occurs in the posterior segment instead of the anterior segment. For a specific corneal power, there’s a specific axial length and effective lens position to achieve emmetropia. So here’s another formula that takes into account effective lens position. So you will see that this is one of the most important factors that we are using in all modern formulas. The EVO 2.0 formula uses axial length keratometry, anterior chamber depth, and also optional variables of lens thickness and white to white distance. In a study published in Ophthalmology last year, Dr. Mellers and collaborators used the EVO formula, and they said that it was more accurate than the Hill-RBF 2.0. Right now we are on the Hill-RBF 3.0. But it was less accurate than the Kane, the Olsen, and the Barrett formulas. So right now, we see two formulas and several papers that say that the EVO 2.0 formula — the Kane formula, the Olsen, and the Barrett are formulas that perform quite well. The performance of the EVO suffers in the short and long axial length eyes, indicating that this emmetropization concept may break down at extremes of the axial lengths. Again, we got the same issues. We got big eyes. We got small eyes. And the formula does not perform quite well in these types of eyes. So the EVO 2.0 formula, even though it’s one of the newest formulas, it performs quite good, but not as good as other fourth generation formulas. If you go online, the IOL calculator for the EVO 2.0 formula will ask you for the same parameters. Axial length. K1, K2, ACD, lens thickness, central corneal thickness, and the target refraction. If you’ve got patients with postmyopic LASIK or PRK, this formula will measure this, if you got the data, and the refractive history. So it will improve the way it performs in postsurgical patients. Because — let’s be sincere again. We’ve been performing laser surgical or refractive corrections on patients for a long time. And the patients that we first start operating, where they were made with a laser, for example, radial keratotomy, now has a lot of irregularities in the cornea. So for these patients, the axial length will be the same. But the radial keratotomy or the LASIK surgery that we perform will induce changes in the anterior surface of the cornea. And that will induce errors on the calculation of the formula if we do not take into account these types of measurements. So we’ll talk about that in a minute. It’s another important factor. I know there are some Q and A questions coming in on the chat. We’ll have time certainly at the end of this to answer all your questions. So let’s talk about Dr. Hill and the Hill-RBF 3.0 formula. This is where you could get this formula online. And also it comes with all these new biometers. The Hill-RBF calculator is an advanced self-validating method for IOL power selection. It employs pattern recognition and a sophisticated form of data interpolation. This formula has been optimized for use with the Lenstar LS 900 optical biometer in combination with the Alcon biconvex intraocular lens, the MA60MA, but it also works with other biometers, with other biconvex models. Dr. Hill has a contract with the people from Lenstar, Haag-Streit. That’s why it includes that formula and works with that biometer. As it says, it will work with other biometers as well, and you have the RBF calculator online. I want to take one minute to talk about errors that we make when we move data from a biometer or keratometer to these online formulas. Sometimes when you get all the formulas on the biometer, for example, the Aladdin, that’s got the Olsen formula, got the Barrett formula, and all the other third generation formulas, you will minimize the error of making the data or changing the data from one machine to another. I mean, if you’re performing ultrasound, and then you have to write it down, and you’ve got a busy clinic, then you could have some errors extrapolating the data from one machine to another. If you’ve got all the data measuring on one device and all the formulas on that device, this will minimize this error. So the Hill-RBF 3.0 is more accurate than the third generation formulas. All the formulas we’ve been talking about are more accurate than that. It indicates that it has improved, compared with the original version. The original version is the 1.0. But nevertheless, it is still less accurate than the Kane, Olsen, Barrett, and the EVO formulas, which are all based on optic principles. Suggesting that the artificial intelligence regression model that Dr. Hill is using on this formula is not yet as accurate as the optical models. If you go online, Dr. Hill’s RBF formula will look like this. You have all the same parameters to introduce, and then you get the results. Let’s talk about the Barrett Suite. Here is the link for the Barrett Suite. As I told you. Some of the biometers incorporate the Barrett Suite. I work in a busy clinic here in Peru. Mainly the clinic that operates most cataract patients here in Lima. We perform around… 60 to 80 cataract surgeries per week. So we’ve got a really busy clinic. And since we have this Barrett Suite formula, to be honest with you, this is the only formula that we are using, because it performs quite well in every situation. We’re talking about the suite, because this is a combination of five different formulas. For any type of eye or any type of situation that you could get in, there is a solution on the Barrett Suite. The first one is called Barrett Universal II. This is for non-toric IOL calculation with keratometry values. The second one is the Barrett Toric. This is for toric IOL calculations. Then we’ve got the Barrett True K. This is for non-toric IOL calculations in patients post-laser vision correction. LASIK, LASEK, PRK, RK. If you’ve got the keratometry values, all of them could get into this formula. The fourth one is the Barrett True K Toric. This is the same. Post-op people of laser vision correction, but got a toric refraction reminder. Calculation with total keratometry values for this toric calculation on post-op. This is not the same here. This is virgin eyes. This is post-op. And we also have the Barrett Rx. Rx is for surprises that you got on the OR. If you want to exchange a lens, because it’s an older version of the lens, or the patient got a refractive error. If you want to add a lens, doing a piggyback, all these selections will be taken into account with the Barrett Rx formula. According to Dr. Barrett, when using the True K formula after refractive surgery, he recommends entering the optional values pre-LASIK refraction and post-LASIK refraction, if known, because then the formula will perform even better. This is the Barrett Universal II. You see here are all the five formulas. Remember, Universal II non-toric IOL calculation with K values. This is the Barrett Toric calculation. Barrett Toric is for toric IOL with keratometry values. You will see here that you even get a simulation of where to place the incisions. This is almost the same calculator that you got from the manufacturer of the toric IOLs. So you will get the same information. Then we’ve got the Barrett True K formula. Barrett True K is for post-op of laser vision correction. Remember, here you’ve got the correction type. You should select which type of correction has your patient been performed. And you’ve got the pre-LASIK refraction and post-LASIK refraction. That’s important. They are not mandatory. Here’s a tick that you could mark with no history if you don’t have the history of the patient. But if you do have it, the outcomes of the formula would be even better. Now we have the Barrett True K Toric. Barrett True K Toric is almost the same, but now we are looking for toric lenses. So these are for patients… I’m sorry… These are for patients that have had LASIK correction surgery. But also have a toric component. Some astigmatism. After this correction. And at last, we’ve got the Barrett Rx. Now, the Barrett Rx will ask you… I mean, the biometer will measure everything. Axial length, ACD, lens thickness, the refraction error, but you should need to know which model of IOL is implanted. Which model, which power, if it’s a toric, the refraction, the pre-op keratometry, and then you could pick from a piggyback selection IOL, or to exchange the IOL. So as you will see, the Barrett Suite will walk you through all these different situations that you could get from a post-op patient, for a virgin eye. That it’s performing the cataract surgery. The Barrett Suite formula performs quite well over all types of eyes. Short eyes. Normal eyes. And long eyes. It performs as well as the Olsen formula that we will be talking about in a minute. So these new generation formulas, Olsen and Barrett, perform better than old generation formulas, and even perform better than fourth generation formulas such as EVO and Hill-RBF. That’s why we’re using these formulas the most. Now it’s time for another question. Which of these are part of the Barrett Suite? Universal II, Universal II Toric, True K and True K Toric, Rx, and all of the above? I’ll give you a couple of minutes — couple of seconds, actually — for you to answer this question. This formula, in my experience, is the formula that performs the best of all the formulas that we’ve been talking about. In every situation. I want to assess that when we’re talking about postoperative corneas, all of the above — that’s the correct answer. Remember that the Barrett Suite includes all these five formulas. And as we were talking about, effective lens position is one of the things that we needed the most to review. On post-op patients, the changes on the anterior surface of the eye will take into account for errors if we do not measure them correctly. Some of the biometers that we have right now were measuring only 5 or 7 points on the central part of the cornea. And that works okay for most of the eyes. That hasn’t got refractive surgery. Because as we’ve been talking, refractive surgery will change the anterior part of the eye. So if you take only 6 or 7 points to measure the anterior surface, then you won’t be measuring the whole cornea, and you could get some errors or surprises after the surgery. So it’s a good point to have included in your biometer. Also some sort of topographer. If it doesn’t come in with the biometer, you have to do the measurements on a separate machine. A topographer, Pentacam, some sort of anterior chamber analyzer. But when the biometer comes with that topographer incorporated, that topographer, not a keratometer, it will get you more information when you get these corneas that have already been having another surgery. Let’s talk about the Olsen Suite. It’s available on the ASCRS web page. Also some of the other formulas are on the ASCRS web page. You go to tools, and then you can get all these formulas. The Olsen formula uses ray tracing and thick lens considerations to account for the true physical dimensions of the eye’s optical system. It uses the same technology employed by physicists to design telescopes and camera lenses. One key feature of the Olsen formula: It’s the accurate estimation of the IOL’s physical position using a newly developed concept that’s called the C-constant. So right now, we’re talking of another constant. We have the A constant. That’s the manufacturer constant. But the Olsen formula also takes into account this concept of the C constant. The C constant can be thought of as a ratio by which the empty capsular bag will encapsulate and fixate an IOL following in-the-bag implantation. So we all know that the IOL, after the surgery, will contract. It will start to encapsulate and fixate the IOL. And doing this, if you’ve got, for example, a big eye, it could move forward or backwards, because you’ve got a lot of room, a lot of space there, to move. And that will affect the effective lens position. On an optical system, if you take a lens and you move it closer or far away from your target, you will change the refraction. And you will change the visual acuity of the patient. So this new formula, taking into account this new concept of the C-constant, will provide one of the most exact positions and one of the best results after surgery. Here’s a graph that compares the Olsen formula that you’ll see here on axial length between 22 and 32. That’s a really long eye. And you see that it’s less than half a diopter, compared with Holladay, for example, that again we know on extreme eyes will start having some troubles. So as input parameters, apart from the axial length and the K measurements, Olsen used ACD and length measurements. Now we talk about five formulas. And most of them use these two parameters. ACD and lens thickness measurements. *lens thickness measurements. A couple of years ago, before we were using these new formulas, lens thickness wasn’t one of the measurements that we considered. Because we were thinking… We’re gonna get rid of the lens. Why do we have to measure it? And the reason is that you’re not getting rid of all the lens. You’re just getting rid of the central part of the lens. The cortex. The nucleus. You’re getting the capsular bag intact, or at least, that’s what you want to do on a phaco surgery. So if you don’t measure the lens thickness, you won’t calculate correctly or estimate correctly the effective lens position, and we already know that it will give a lot of errors afterwards, if we do not take into account this. So Dr. Hill, Dr. Meier are currently assessing the performance of the Olsen formula in comparison with the Holladay 2 formula. The Holladay 2 is this one. Okay? Fourth generation multivariable IOL calculation method using lens thickness measurements as a parameter for improved IOL prediction accuracy. In a clinical series of more than 1700 eyes, Dr. Olsen assessed the performance of the Olsen formula as compared to standard formulas like Holladay and SRK/T. It’s time for the last question. Which formula uses ray tracing technology? Actually, this is an easy one. We just talked about it. So Hill-RBF, Barrett, Olsen, Haigis, or none of the above. So a part of the formula used in ray tracing technology — remember, this formula has a new concept, that’s a C-constant, that takes into account the effective lens position after the surgery, measuring two important parameters such as ACD and lens thickness. So the Olsen formula. Perfect. Now… Which formula should I use? According to Dr. Holladay’s corneal power decision tree, that’s one of the best decision trees I’ve found about this, it takes into account a lot of measurements that usually biometers don’t take into account. Keratometry. Has the patient got dry eye? Does the treatment improve the refraction, the dry eye? Does the patient got a huge amount of astigmatism? Does it need a toric implantation? Does the topography on the 4.5 millimeter central zone is exact toric calculation? Or is the back surface of the cornea, for example, in patients with keratoconus — is it having any trouble in the calculations? After all these decisions, Dr. Holladay will talk about Holladay 2, his formula. So which formula should I use? I already talked about this. In the clinic, we use the Barrett formula for almost 98% of our cases. The other 2%, we use the Olsen formula. And in my opinion and in my experience, these are the two fourth generation formulas that performs the best right now. So in summary: Achieving a target refractive outcome is an essential and complex aspect of cataract surgery. Accurate biometry is one of the most important steps in calculating the IOL power. IOL constants must be optimized. If you’re using an optical biometer. Never use the A constant that comes in the box with the lens. You must go into the ULIB web page and look for the optimized value of that IOL. Sometimes it’s easier to ask the manufacturer which is the optimized constant for that lens. Usually they know, but sometimes if you don’t ask, you won’t get that answer. Now, we know the importance of having a healthy corneal surface. We talk a lot about this. Dry eye, LASIK surgery, anterior segment rings for keratoconus, or even the keratoconus or high astigmatism, irregular astigmatism, will — it is an important measurement that should be taken into account. Fourth generation formulas or prior formulas don’t take it into account that much. Remember, they just need K1 and K2 values. K1 and K2 could be messed up, if you’ve got an… A cornea that’s got a lot of aberration. So it is important to have not only a good biometer, but also some type of topographer. And I will also set… If you’re planning to implant multifocal IOLs or trifocal IOLs that are dependent on the pupil, you do not only need a good biometer and a good topographer, but you should also look for a good pupillometer. Right now, some of the biometers such as the Aladdin came in with the topographer, and the pupillometer all in one machine. So it will minimize extrapolating the data from several machines to get the formula. Is another formula the solution? Well, it depends. We talk a lot about this. But you certainly don’t want to be left behind in the race toward accuracy. We need the best formula, and we need the best measurements for that. Of course, the formula you choose will depend in part on the equipment you are currently using. If you’re using ultrasound biometry, and you don’t have a topographer, but you’ve got a keratometer, you can use one of the third generation formulas. You could measure ACD or you could not measure lens thickness. So you could use some of the latest formulas. But if it’s not a big eye, 23.5, 24 millimeters, it will perform quite well. Remember: If it’s a virgin eye. If that eye… Has been performed LASIK surgery prior to that, you will need fourth generation formulas. Otherwise, you’ll be lost. So you also don’t want to jump off a formula if it’s working well in your practice. So evaluate your own results. We got the surgeon factor. We all know that every hand is different. So if a formula is working for you, for example, if you’re using SRK-T in all your patients and you’re getting good results, you don’t have a reason to change the formula. So evaluate your own results as one of the best advices I could tell. How does a clinician know when a measurement is likely to be correct or incorrect? So Dr. Hill says: A measurement is only as good as your ability to know what it means. So we could measure a lot of things in the eye. But if we don’t know the exact meaning of this measurement, for example, anterior surface of the cornea, that wouldn’t make a difference on the calculation of the IOL. Right now… I’m sorry. Right now the idea of the universal IOL power formula that works in every eye of all shapes and sizes — it’s really an attractive one. But it is unlikely, unrealistic, at least, for now. So that’s what I have to share with you today. Thank you very much for having me, again, here. It is always a pleasure. And let’s see some of your questions. To see if we could answer some of them. Let me jump out of my presentation. There you go. So I have already answered four. What kind of formula best for sort axial length should medium axial length and long axial length — I think we talked about it several times. All the formulas perform well in normal eyes. The issue is big eyes or small eyes. If you’re using third generation formulas, Hoffer Q is the one that works the best for small eyes. And SRK/T is the one that works the best for long eyes. If you’re using one of the fourth generation formulas, Olsen or Barrett should be the best for any type of eyes. At least for right now. Why unable to calculate measurements with IOL Master mature… Oh, okay. Depends on which IOL Master you use. IOL Master 3.4 doesn’t perform that well, because the composite signal that it’s using doesn’t have the power enough to get through opaque media such as hard cataract. IOL Master 500, on the other hand, it’s got the composite signal, and this composite signal will get us a lot more eyes that we could measure, because the signal has more power to get through this hard material. If we’re talking about the IOL Master 700, IOL Master 700 works with swept source OCT. Swept source OCT has a bigger wavelength. It’s around 1050 nanometers. 1050 nanometers could get through opaque media. Such as corneal opacities, such as hard cataracts. So if you’re looking for an IOL Master that will get you through all these cataracts, you will want to look for IOL Master 700. But I want to say IOL Master 700 is a really expensive machine. Because swept source technology is expensive technology right now. If I only have ultrasound available, should I use manufacturer constants, or are there… Yes. If you are using only ultrasound, you could work with the A constant provided by the manufacturer of the lens. But one thing. A constants provided by the manufacturer only works… I mean… Not only works… Let me rephrase this. Are made for ultrasound biometers when you are using the cup. When you’re not using the direct contact. If you’re using ultrasound with direct contact, you could have a lot of errors. If you’re using immersion, then it will work quite well with this type of constant. Ask a mathematician! Yes, we should ask a mathematician. I don’t have a mathematician friend. I do have a couple of physics friends. Maybe I should ask them. Ha-ha. Are these new formulas categorized into the fourth generation itself? Yes, they are. They are categorized into fourth generation themselves. Now, what difference does gender of the patient have for calculation by Kane formula? I exactly don’t know. I should ask Dr. Kane maybe about this. But it doesn’t seem… When I was looking for this, I tried to get… Why gender will make any type of difference. I actually don’t know. I will get an answer for you. Sorry. An anonymous attendee. I’ll try to get an answer for that. Which formula best works with pediatrics? Well, it depends. How old are the pediatric patients? If we’re talking about, for example, a baby, that has congenital cataract, well… It’s a really small eye. You shouldn’t use SRK/T for really small eyes, because you won’t get that good measurements. In pediatric patients, we should consider that when the eye starts developing and starts growing, the lens that we first implant won’t work after some years. So maybe the Barrett formula will give you all the information you need. Because you need to exchange that lens afterwards. And the Barrett formula is the only one that’s got the Rx formula where you could add a piggyback or add another or exchange the lens. So maybe that will work best for you. If you’re talking about a pediatric patient that is around 13, 14 years old, the growth of the eye is almost the same as an adult. So you could use… Depending on the axial length… And it’s sort of other formulas. But pediatric patients are really, really hard. Which single formula do you prefer in cases of keratoconus? Rx. Well, for this patient, okay. So… Let me try to answer this in two parts. For postrefractive, post-keratorefractive surgery, long myopic eyes, short eyes, I use the Barrett formula. It works perfect. If we see keratoconus, it depends on how big is the conus. If it’s a grade one or grade two, we could work with Olsen or Barrett formula. If it’s three or four, what we do actually is… Perform post-penetrating keratometry on these patients and exchange the lens. But if we want to take keratoconus into account, I would use another machine. Not only a topographer, but tomographer, such as the Pentacam, that will also measure the posterior surface of the cornea. So IOL calculation post-penetrating keratoplasty and in stable keratoconus… We just talked about this, just on the previous one. What is the meaning of True K? True K means the true corneal power. It takes into account not only the anterior surface of the eye but the posterior surface of the eye. You will ask me… Well, then, does this biometer measure the posterior surface of the eye? And the answer is no. They do not measure the posterior surface of the eye except the ones that are working with Scheimpflug technology. That could measure the posterior surface of the eye. Such as the Pentacam AXL and the Galilei… It’s the G6 or G8, actually, I think. Okay? So True K stands for that. How important is the posterior corneal surface measurement? I think I answered these questions already. So it is really important. If we are talking about a patient that has had performed refractive surgery or has keratoconus or any of these. If you’re using fourth generation formulas such as Barrett or Olsen — Olsen, by mathematic preparations, algorithms, the posterior surface of the cornea. I personally think that we should measure it directly. I’ve got a meeting with Dr. Olsen to ask about it, and he says… His formula performs pretty well without the need to measure indirectly the posterior surface of the eye. Of the cornea, I mean. I don’t want to say that I disagree with him. But I do think that it’s better to measure mathematically. That’s my opinion. What is the difference between keratometry, total keratometry, and total corneal power? Which is the best parameter for IOL calculation? It depends. If you’ve got a normal cornea, just K1 and K2 would be fine. If you’ve got a high aberration cornea, I would suggest to use a topographer or tomographer to get a whole cornea instead of just the central part of the cornea. Keratometry does anterior surface. It only measures K1 and K2. That’s for the post curvature part of the eye. Total keratometry is the whole corneal power of the whole anterior surface of the eye and total corneal power will be the anterior surface of the cornea minus the posterior surface of the cornea. That’s the total corneal power. Melissa. What then would be your recommended IOL formula for children? I think we answered that already. AL refers to central anterior corneal surface or central posterior surface? AL is axial length. Axial length is the whole measure of the eye. Axial length… Depending on which machine you’re using… Could be measured from the anterior surface of the cornea to the anterior surface of the retina. Other machines measure from the posterior surface of the cornea to the anterior surface of the retina. So most of the machines, if they change the way they measure the cornea, if they include the cornea or not, will have like… 500 microns difference between each product. Most of them measure from the anterior part of the cornea to the anterior part of the retina. What about Kane formula? What about Kane formula? We talked about Kane formula. It’s a great formula. It uses the artificial intelligence. It performs quite well. I don’t really have that much experience with Kane formula. It seems that it performs outstanding. But as we talked about, in my practice, we’re using mostly Barrett and the Olsen formula. And as I told you, on the last slide of the presentation, if that’s working for you, there is no need to be changing. I would like to see how it performs, the Kane formula. So we can give it a try. Thank you, Marco. Melissa. How do you decide when you need to use Olsen rather than the Barrett calculations? I only use Olsen instead of Barrett when I’ve got corneas that have a high aberration. Most of the time, I told you, 98% of our patients I measure with the Barrett calculations instead of Olsen. In my opinion, in my experience, both of them will work quite well. So if you need to change a lens, you should go for Barrett, because you’ve got all that information. If you don’t need to exchange a lens, or it’s a virgin eye, you could go with Olson, without any hesitation. Both perform outstanding. That I can tell you. What was the indication for using Olsen over the 2% on Barrett? Well, I told you, it depends on the cornea and if this patient got any other type of surgeries. I think we’ve answered that too. Should I personalize my constant, or is it okay to use a constant from websites? You could personalize your constant. That will be… That will make a more custom made surgery for you. If you’ve got the time and the interest, there is a sheet on the ULIB website that you could fill out with your patient, with your results, and they could provide you personalized constants, with your own surgeon factor, if that works really good for you. You need… I don’t want to say a lot… But you need some amount of post-op data from patients, if you want to personalize your constant for yourself. What is the definition of target refractive? For what reasons? Why don’t we choose zero? That’s a really interesting question. I personally set the target refraction for 0.5 diopters. That is because I do prefer, if something goes wrong, that my patient be a little myope instead of a little hyperope. Because when you, after the surgery, became hyperope, and you’ve never been a hyperope before, then you’ve got a really unhappy patient. Besides that, when I target my refraction to 0.5, and the patient… And I’m placing a spheric or monofocal IOL, the patient could use that small myopic shift that I gave them for reading a little bit. At least the watch or the cell phone. So that’s why I target for 0.5. Some of the doctors target for 0. That’s okay if your surgeon factor — it’s okay. I mean, do what is working for you. Don’t try to change it. I mean, if it’s not working for you, change it. But if it’s working for you, go ahead. In my hands, 0.5 is the target I always choose. For Barrett Toric from which investigation should we use? I don’t understand the question. From which investigation? For Barrett Toric, you need the parameters provided by your topographer. If it comes with — the biometer comes with the topographer itself, you should do that. I suggest for any toric implantation, not only for Barrett, but any toric implantation, that you don’t use a keratometer. You don’t just measure six points in the center of the cornea. Try to measure a bit more. Try to use a topographer. Try to give more information of the cornea. For toric, remember that toric IOLs only tolerate a decentration of 5 degrees. If that much. So the best information about astigmatism you could get from that patient would be the best. How do you advise a cataract surgeon who operates in a resource deficient environment and aims at achieving patients’ desired refractive outcomes, and has no tool for this measurement but only doing refraction. Is there hope for this surgeon? I want to say yes, Chika. I want to say yes, there is a hope for those surgeons. But if you’re operating in such conditions, most of the time patients come with no vision or with really, really diminished vision. I would say… 20/400 or worse. So on those patients, if you give them the ability to at least have some sort of vision or to improve their vision, they will be so, so happy. We’re having issues with patients in big cities — I mean, if you want to operate on a CEO of a big company, that man wants to be zero. He wants to be emmetropic. He wants to have multifocal IOL. He wants to perform as he was as a 20-year-old. When you’re doing surgeries on the other sort of patients… If you do a great surgery, and they got some vision, they will be happy. Some advice: To never leave a patient aphakic. Try to place an IOL in every circumstance, unless it couldn’t be done. Thank you for the information. You’re welcome. Do you have any concept about Panacea IOL calculator? I don’t have any concept about that. Sorry. I’ve heard about it. But I haven’t used it. Thank you for your lecture. You’re welcome. You say you use a Barrett 98 and 2% will be more dependent on Olsen? We’ve already talked about it. Thank you. Thank you. What is the lowest SNR value accepted for a mature cataract? I don’t have the right answer right now, Diana. SNR value is signal to noise ratio. So it depends on how much noise you’ve got in the background and how hard or how deep is your signal. So if you get a good signal, and you don’t have that amount of noise on the backend, you could get a good calculation. But SNR value… I couldn’t answer that right now. I think it depends actually which biometer you’re using. Could we still use Barrett’s formula even without optical biometer? And use other biometric — yes, you can. Yes, you can. You can use Barrett’s formula if you’re doing ultrasound biometry. But remember which Barrett formula you are aiming for. If you are talking about universal 2 and it’s a virgin eye, there won’t be an issue. If we’re talking about the other type of eye, long eyes, post-op, surgeries, or anything… I think that you should consider using a topographer, a biometer, or a machine like the Aladdin, for example, that’s got topographer, pupillometer, and also the biometer all in one. I enjoyed synopsis. I will go for Barrett’s as well. But because of resource limitations, I’m stuck with SRK-T for now. Okay, Boniface. If that’s working for you, I think that you should stick to it. But remember, SRK-Ts do not perform well on small eyes. Please don’t use that formula for eyes less than 23 millimeters. Use Hoffer Q instead. You will get better results with that. But for medium eyes and long eyes, SRK-T is the third generation formula that performs the best. What difference does the gender of the patient — I don’t know. We’ll talk about that. I’m sorry. I will ask Dr. Keane next time I’ll be able to. How about the Haigis L formula for post-LASIK? That’s a great question. Haigis-L is one of the formulas that works the best for post-LASIK. It’s accurate. It works with IOLMaster 500. It’s a really, really accurate formula. It doesn’t take into account some factors. But remember, all of Dr. Haigis’s formulas, instead of just one A constant, they work with three A constants: A0, A1, and A2. So that will take into account a little bit of the effective lens position, not as well as the other formulas, because you’ve got more data. But Haigis L was the formula that we were using. Prior to Olsen or Barrett being used. So it’s a really great formula for myopic eyes. Which formula do you use in keratoconus? We answered that. It depends on the degree of the keratoconus. It depends on if it’s stable or not. Depends on several factors. But again… You should measure if you’ve got keratoconus anterior surface of the cornea and posterior surface of the cornea as well. I will recommend the Holladay report from the Pentacam for this type of patient. If it’s available in your region, Alexander. What about Panacea? I haven’t got any experience with Panacea. Sorry about that. Why are some formulas called third and other fourth generations? Because third generation formulas were the prior formula that doesn’t take into account effective lens position. Remember, the fourth generation formulas do take into account effective lens position, and they measure it. You measure the central corneal power — I mean the central corneal thickness. You measure the ACD. And you measure lens thickness. If you don’t measure this, you won’t be using a fourth generation formula. You will be using third generation formulas. Thank you for the lecture. Thank you for attending. Great presentation. Gladys Rx. Does ACD refer to anterior cornea? ACD refers to anterior chamber depth. It’s the amplitude of the anterior chamber. It will be measured from the posterior part of the cornea to the anterior surface of the lens. What is Rx? Rx stands for refractive surprises. So that’s the part of the Barrett Suite that you will use when you need to exchange a lens or to put a piggyback lens. Do the two IOLs… Do two IOLs with nearly same lens factor… Will always have same IOL power for an eye? So it’s advisable to put IOL same power to nearly the same lens factor without taking in from the biometer? I do not recommend that. Every eye, even if it’s on the same patient, performs differently. Again, effective lens position is one of the measurements that you should take into account for this. So I do recommend always measure. Can you talk about Panacea? I’m sorry. There’s a lot of questions about Panacea. I’ve never worked with it. I’m so sorry. How about calculating keratoconus using total corneal power? Yes. That’s the way. You should use total corneal power. If you’ve got a machine that will measure the anterior surface of the cornea and the posterior surface of the cornea, that will be the correct approach. Remember, for example, the Pentacam has this Holladay report that will allow you to select which type of IOL is better for these sorts of patients. Marco. How about calculating keratoconus using total corneal power? I think that’s the one we’ve just answered. Thank you. Thank you. What about operating pterygium and IOL… Oh, pterygium. Okay. So you referred to after post-op pterygium. It will change the anterior surface of the cornea. As well as refractive surgeries. So again, you need to look at the topography. You need to look at the whole cornea topography. Not just the central part of it. It depends how big was the pterygium prior to surgery. If it’s only grade one, small one, maybe it doesn’t affect that much. But remember, after you operate on a pterygium from a patient, you do not give them prescriptions after the surgery. You wait for time for it to heal out. And then you do that. So it’s kind of the same for IOL calculations. Ultrasonic best machine is (inaudible)? Oh, okay. I’m not gonna agree or disagree with that. Have you used EDOF lenses? Do you have an opinion of them? Yes, I use EDOF lenses. EDOF stands for enhanced depth of focus. They do work well, but you should select your patients quite well, if you want to use a premium IOL. Premium IOL should be EDOF, trifocals, multifocals. Any sort of IOL. I won’t place that lens without doing the best measurements of the eye possible. And I will talk with the patient, because our eyes are not used for that. Our eyes do accommodation. And these are not accommodative IOLs. So it doesn’t work the way our eyes are used to. So not all patients benefit from multifocal IOLs. Not all patients benefit from EDOF IOLs. You should talk with your patient. You should ask your patient… What labor does he make or does she make. If it’s airplane pilot, I don’t place an EDOF IOL in those guys. If it’s a CEO of a company, maybe yes. Because he wants to look at the computer and look at his cell phone. Everything all together. So… My suggestion… Select pretty well your patients. Some biometers take into consideration posterior corneal power? Yes. The Pentacam AXL and the Galilei G8, they take into account the posterior corneal power, because they do measure it. Most of the topographers, using Placido discs, just measure the anterior surface of the cornea. Can we consider some K values with posterior corneal power? It will integrate… Yes. That’s true. True net power or total corneal power. K1, K2 referred to steep and flat cornea? Yes. K1 and K2 is the steepest meridian and the flattest meridian of the cornea. Is Summation A-Scan still the gold standard? Oh. It is the standard used for IOL manufacturers to provide the A constant. But I do think that right now most of the clinics and the clinicians are made by optical biometers. So I want to say no. But I think it’s yes. Can we use sim-K values for frontal power… To take… Power pulse refractive surgery in calculation? Yeah. Sim-K? No. I won’t use Sim-K. I would prefer you use in meridians. In these patients, I will also use a pupillometer to look how big the pupil contracts or moves. Because that will change the effective power of the IOL. So instead of Sim-K values, I prefer meridians, or you’ve got the opportunity to measure anterior and posterior, you should do that. Does any formula use vitreous length? Vitreous length, does it need to be accounted? I don’t know. Vitreous length? No. I don’t think so. I don’t think the vitreous will be a factor to consider. We do change on biometry when we have vitreous or silicone oil in the eye. And that’s because of the refraction index. That should be changed. But about the vitreous itself? I don’t think so. What about SRK-T? Where is it? Oh, SRK-T. José Antonio Fuentes. Oh, I’ll say this in Spanish. (speaking Spanish) What about SRK-T formulas for the best IOL calculations? SRK-T, as we talk a lot, they work pretty well on big eyes. The third generation formulas, that works the best on big eyes. It doesn’t perform good on small eyes. So I will use SRK-T formula just for normal or big eyes. Never for small eyes. Thank you very much. Thank you. Do you have any experience with IOL Station from NIDEK? What do you think? No. I haven’t got any experience with IOL Station from NIDEK, but I heard that it performs pretty well. That’s the only thing I could say for that. I’ve worked with NIDEK machines. They do perform well. I love the OPD-Scan. It is one of the best machines that NIDEK got. But I haven’t been working with this IOL Station. I hear good things about it, though. Thank you. Thank you, Danny. I missed your answer about ACD due to network. ACD stands for anterior chamber depth. Anterior chamber depth is the amplitude of the anterior chamber, and it’s measured from the posterior part of the cornea to the anterior surface of the lens. Great lecture. Thank you. Thank you. Thank you. Does your center do training of cataract surgeons? Yes. I would like to see your name. But it says anonymous. We do have a residence program in our clinic. My clinic mainly is OftalmoSalud. It’s the biggest eye clinic in (audio drop). We do have a training program, and we also got a fellow program for anterior segment surgeons, for cornea, refractive surgery, for retina, and also for image and diagnostic. That’s the thing that I do the most. Which formulas will be your first choice to calculate IOL power in an eye with silicone? Uh-huh. Any formula will work in an eye with silicone. But you do need to change the refractive index on your biometer. Or in your ultrasound machine. Because the refractive index of silicone oil is quite different than the one on vitreous. Is there any means by which IOL power can be elicited in the operated eye? I don’t quite understand. IOL power can be… Ah. After you implant the IOL. Well, there is a mathematician way to get the power. But the best thing is to look at the chart and look which lens has been implanted. (speaking Spanish) I’m sorry to answer this in Spanish. (speaking Spanish) Can I apply newer generation formulas programmed on all biometric instruments or not? Well, I don’t think so. It depends on which biometer you’re working with. But I don’t think you could add a new formula to an old instrument. Maybe you should ask the vendor of your biometer. I do know that if you’re using some ultrasound machines, you could do that. But optical biometers… I really don’t think so. Wonderful lecture. Oh, thank you very much. (speaking Spanish) Can you share your email, sir? Sure. I will share my email. Why some IOLMasters can’t IOL calculate mature cataracts? We already answered that. It’s due to signal. IOLMaster 3.4 doesn’t perform well on hard cataracts. IOLMaster 500 performs better, because of the composite signal. Actually, hard cataracts… The nucleus is not the factor that will affect it the most. When you have these cataracts that have a lot of cortex tissue, the cortex will start doing diffraction of the signal. And that’s why you won’t get a good signal. That’s why it doesn’t measure quite well. Informative. Thank you. Muchas gracias. Thank you. Thank you too. (speaking Spanish) I missed your answer about IOL calculation on operated eye due to network. What was the question? I’m sorry. IOL calculation in operated eyes? I have to go back. SNR varies in different optical biometers? Yes, it varies in different optical biometers, and it varies also in the same biometer, because it’s a mathematical function between signal and noise ratio. So if you got a good signal and a low noise ratio, you will have a big number. If you’ve got a lot of noise and less strong signal, you won’t have that variable. So that’s why it changes. For aphakic eyes, will you prefer anterior ICL or iris claw fixated lens? We use Artisan for aphakia. So we do prefer iris claw. Instead of anterior chamber lenses. They’ve got their haptics on the angle, actually. And when you’ve got the haptic on the angle, and it moves, it starts sending some pigment into the trabecular meshwork. And you could get some issues after that. So that’s why we prefer iris claw IOLs. Thank you very much for your assistance here. Thank you very much for Cybersight, for having me again. And I hope to all of you have a wonderful day.

Download Slides


September 9, 2021

Last Updated: October 31, 2022

Leave a Comment