Hi, my name is David Cooke. Today, we're going to discuss how to choose an intraocular lens. And mainly I'm going to center on lenses chosen with formulas that are available for free. Mainly in those that are in United States machines that come for free. I'll mainly discuss spheres and these are the questions that I want to answer. What do you do if you have several formulas? How do you choose which formula to use? What if you have no formulas at all on your device? I was in Honduras and they handed me a sheet that said, here's the axial length on the patient, here's the k reading, here's the a constant and here's your calculator. And figure out which lens we should use for the patient using the SRK 2 formula. Is that the best thing to do? What if you have partial measurements? I was in Zimbabwe and I was entering the country, I have my A-scan and my suitcase. And I had a separate autorefractor, they saw that unit, they confiscated it at the border. And I was only there about a week, they kept it most the whole time. So all I had was the A-scan. What would you do if that's all you have? What do you do if you don't have measurements? What if their eyes are very short or very long. And what if you don't have the proper lens constant for the formula that you are using. I'll try to answer all those questions today. And so if you're only interested in these, you could run to the end of it. At the very end, I'll discuss real briefly what do you if you have eyes that higher astigmatism or eyes that are post-LASIK. Here we go. If you have partial biometry measurements so you're entering a country and they take away one of your units, what do you do? There was a nice paper that came out in 2012 by a group from the United States Navy, they do humanitarian missions. And apparently they run into the same problem, because they published a paper that looked at this particular question. And if you only have axial lengths, then the best thing to do. You don't have a keratometer, you take this formula. And you take 1.3 times the axial length, and you add 75 to it. And you use that number instead of your Ks. That works better than just guessing. What if you have only Ks or you don't have any measurements? They found that there was no formula that will work for this, the best thing to use was just the mean Intraocular lens power for that particular population. Well, how do you know that power? They published another formula and this was also for humanitarian missions. And this was also out of the United States Navy. This particular author right here was on both papers. And this one came out a few years before, this was in 2009. They showed the average mean IOL power for several different populations. And if you look at it, it's about a 21.5 diopter lens. Now, for the Taiwan population, it was 20.0 because that population is a more myopic population. So a more myopic population is going to require a lower lens power. These A-Constants were about a 118.5. So if you have a lens that has a 119, you'll want to go up about a half diopter. So you would use about a 22.0, mean IOL power for your population. And then my friend in Zimbabwe when he operates on men, he uses a 21.0 because he didn't have any equipment to measure. And when he operated on women, he used one diopter higher 22.0. And the reason was he knew that the women did more near tasks. So he gave them a little bit higher willpower. What do you do if you have several formulas? How do you know which one to use? There are several formulas available. And the four that are on most machines are listed here. There are two other formulas that are available for free. They're on the web, we'll talk about those in a minute. And then there are these two formulas that are available for purchase. And then there's the last one that I list to the SRK-II. I have one comment on the SRK-II and that's don't use it, it's not as good as the other formulas. So these four formulas are the main formulas that we'll talk about. The Haigis requires an anterior chamber depth, so it requires an extra reading. You may not have that, so you may be limited to the other three. The SRK/T does not do well with steep corneas, the T2 might be a better option. Dr. Haigis in 1993, published this paper that showed there was a problem with the SRK/T in steep corneas. This was finally resolved in 2010 by a group led by Richard Sheard. They developed the T2 formula and that addresses the steep cornea problem. So back to the question, which formula is best? Dr. Aristodemou Petros, he and others put out a paper that discussed this particular question. They used national health service data from England. So they had a lot of eyes, they had 8,000 eyes. They did not have anterior chamber depth in most of these eyes. So they only looked at three formulas. I have four formulas on here, because they also went back later and added the T2 formula. This is an unpublished slide using the T2 formula. They gave it to me and I'm using it with their permission. There's a lot on these slides, so I'll go through it slowly. On the y axis, is a mean absolute error. And you'll see from this that, the lower numbers are at the bottom, and that's where you want. If your formula is really accurate, the value would be zero. So you want the values low. The lower on this list that the line goes, the better that formula is. On x axis, we have short eyes on the left, 20 diopters, and long eyes on the right. Most formulas if you look at eyes from about 21.5 millimeters to eyes that are 26 millimeters in length. So that's most eyes, most formulas are about the same for most eyes. Short eyes you see a difference here. One formula, the Hoffer Q, has a lower mean absolute error so that's a better formula, and several papers have shown this as well. Long eyes are quite a spread. When you see that the blue and green lines, the Hoffer Q and the Holladay 1 do not do well in long eyes. The best one was the SRK/T. It was statistically significant better for long eyes. Now the really long eye like they were only about 20 in this group so it can't say much about these really long eyes. There was another group mainly out of bailer. That look at the question about really long eyes and said for eyes that are longer than 25, can we correct those by just adjusting the Axial Length? And they came up with different formulas to use for each of those lens formulas. I've looked at about 500 of these eyes, and in my data the best one was the Holladay 1 adjuster and it worked really quite well. So that's the only one I'm going to list in this paper, but you could look up any of the others if you're interested the reference is at the bottom of the screen. Basically, the intraocular lens is determined by adjusting the AxialLlength. You take the IOLMaster axial length. Well you take 88% of the IOLMaster length. You add to it 2.9, and you use that as your new axial length and put it in the Holladay 1 formula. So now there's another question, what do you do if you have no access to any formula like in Honduras where they handed me a calculator? You're way better off to look on the web, there are couple formulas available for free on the web. This one is the T2 formula calculator, is available at richardsheard.net for free. Another one and that was out of England, this one is out of Australia. This is by Dr. Barrett, Grant Barrett, this is the universal2 formula. It's about the same as the T2, if all you have is axial length in case. But it's better if you have length thickness and white to white We'll review what we've done. If you have several formulas, we discussed what formulas should you use. If you have no formula for your device, what do you do? You can look up some formulas. If you have partial measurements, if there's some adjustments. If you have short eyes or long eyes, use different formulas. What do you do if you don't have the proper lens constant? So let's say, you have measurements, you have formulas, but you don't know what the lens constant is. You could start with the a constant on the box. You have to be a little careful a lot of the out con lenses were designed with the SRK2 formula. And so those lens constants aren't quite the same as for an SRK/T. So you could go on the web and you could look at the ULIB website. This is run by Dr. Haigis. And on it there are lens constants for all these formulas. They're designed for use with the IOL-Master, and they really are pretty good. What if you have let say, you have measurements and you have Holiday 1 formula, you'd like to use that. But you only have the eight constant, you don't have the surgeon factor that you need. That's a different concept. You need the surgeon factor if you going to use the Holiday 1, where do you find that? How do you get that? Well if you have an A-constant, you can use this lens constant calculator also in the ULIB website. It's a different webpage. So I have the address here for you. You can put in the SRK A-constant, SRK/T A-constant, and it will give you the surgeon factor, you can then use that in your Holladay 1 formula. So the summary, if your eyes are shorter than 21.5 mm, use the Hoffer Q formula. If they are greater than 26 millimeters use the SRK/T or the Haigis formula. If they are greater than 27 millimeters adjust the axial length and use the Holiday 1 formula. Don't use the SRK II, don't use the SRK/T for steep corneas. If you have no measurements at all, use a mean IOL. Often that's at 21.5 diopters, using an A-constant of 118.5. If you have no Ks, you could substitute a value using your axial length. If you have no formula and you want to look online you could find two formulas they're both good. And lastly, this is not a summary, but if you've had an eye that's had prior LASIK or PRK or RK, you can go online to IOLcalc.org. That's the website that's run by ASCRS. And on that website it gives good information as to how to adjust things, to get a better result than you would have with a normal formula. If you have an eye with a lot of astigmatism and you'd like to include posterior corneal astigmatism. This is something we've not talked about today. But there is a website at pcacalculator.org that walked you through how to do this. More help is available on a wonderful website by Dr. Hill and are listed on here for you. And most of there interested in intra occulence calculations. And sometimes I can be helpful, sometimes not. If you'd like to email me with a specific question about choosing an intraocular lens, I've listed my email address here for you as well. Thank you very much for watching this course, I hope it's been helpful. Good luck in choosing intraocular lenses, and I hope you'll find the rest of the course helpful as well.