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Having my topography values, is it possible to double check the calculation leading to the selected IOL strength and what my target refraction will be?
Lens is a Clareon monofocal.
0 likes, 18 replies
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ad12345 BrianCyberEyes
Posted
Are you a doctor? No. So don't ask such questions...
RonAKA BrianCyberEyes
Edited
I would say the short answer is "no". You would need full access to all the measurements that the surgeon has taken, with instruments such as the IOLMaster 700. There are number of different formulas used depending on the specifics of your eye. The IOLMaster 700 has many of these formulas built into the software. The surgeon can select and compare the results of all the formulas and decide which one to use. You depend on the surgeon's experience in selecting the best formula along with other factors such as the position the lens will occupy when the surgery is completed. They fine tune this A factor adjustment based on their own personal experience with each type of lens. Here is an article/site to search for. It is from a Dr. Hill that has developed one of the formulas used for IOL calculation.
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Achieving Accurate IOL Power Calculations Doctor-Hill
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If you go back to the 1970's a very crude formula was used to calculate IOL power. The state-of-the-art for estimating IOL power for emmetropia was to simply add +18.0 D to 1.25 times the pre-cataractous refraction." For example if your prescription was -4.0 D. Then your required IOL power would be +13.0 D. But, back then it was accepted that people would need glasses and precision was not necessary.
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I have done a little research on this and for my eyes, I think the Barrett Universal II formula to be one of the best, along with the Hill-RBF 2.0. My surgeon indicated he would use the Hill RBF. I hope he is right! My eyes are modestly long with no other issues such as prior Lasik etc. Each person depending on their particular conditions are going to be different.
RonAKA BrianCyberEyes
Posted
And as a real example my stable refraction (prior to having cataract issues) in my first eye was -2.25 D. The 1970's rule of thumb formula would indicate that I needed a 15.2 D lens for emmetropia. (-2.25x1.25+18). The surgeon used a 15.5 D lens and my outcome was 0.00 D spherical. So perhaps they were not all that wrong back in the 1970's! Fortunately he did not use a 15.0 D lens as that would have left me somewhat far sighted. I recall having that discussion with him and he said the +15 power would be closer to emmetropia but he was afraid to use it for fear of going over in the + region. He said nobody ever thanks him for leaving them far sighted. It reduces your reading vision.
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That is one thing I would recommend doing. Assuming you want emmetropia have a discussion with the surgeon on what the lens choices are. They only come in 0.5 D steps and the theoretical ideal power will almost always fall between two powers. The normal target to avoid being left far sighted is to target -0.25 D. So, just ask him/her what the two options will leave you at for refraction. Do not go over or too close to 0.00.
BrianCyberEyes RonAKA
Posted
I went with recommendations for RP patients to leave slight myopia of around -0.5D - not really sure how useful that is, but I guestimated that at full distance my retina wouldn't keep up anyway (even with glasses I've never been anywhere close to 20/20), and I'd probably benefit more from slightly more near vision.
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The target refraction was calculated for -.38D in one eye and -0.49D in the other (the next step in the first IOL would have brought it almost to -0.7). So my lens powers now are to be 22D (LE) and 22.5D (RE).
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What bummed me out and made me wonder if I can reverse engineer the calculations, is that the doctor first started to put in the numbers for emmetropia before we changed it to a -.5 target, and I'm certain the doctor called the lens powers for emmetropia as "22D with a target of -0.14D" (LE) and "21.5D with a target of -0.03D" (RE). Now, while the latter is possibe even though it went two steps, the former can't be "22D" for both targets (emmetropia and then slight myopia). Now I'm kinda paranoid that there's an error somewhere and I could get mixed up lens values.
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Now I have to get in touch and basically ask like a chump, 'hey doc, did you misspeak when talking about emmetropia or give me the wrong power for the myopia setup?'
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Please don't judge, I'm a bit autistic and it was a stressful day...
RonAKA BrianCyberEyes
Posted
The discussion with my surgeon went a bit differently. My target for my upcoming surgery in a couple of days is for -1.50 D for monovision purposes. What he said was that he could use a +19.0 D lens (Clareon) and his prediction of outcome was -1.60 D. He also said he could use a +18.5 lens and the predicted outcome would be -1.30 D. Since I am also predicted to have some residual astigmatism, we agreed on using the +18.5 lens.
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So my thoughts are to simply ask the surgeon what outcome you will get with the two closest lenses for each eye, and then agree with him/her on the one to use. I believe the 0.5 step in power at the IOL plane is about 0.35 D at the eyeglass plane. So your predicted outcome should go up/down in about 0.35 D steps.
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Having a discussion with a surgeon about this stuff is hard to do without them getting insulted. When I asked my surgeon about what my lens power choices would be, he essentially questioned why I wanted to know, and then admitted that he had not done the calculation. He only wanted to agree on an outcome of -1.5 D. I was not pleased with his attitude, but finally agreed to it. Then he phoned me back about a hour later and said that he had done the calculation and I now had a decision to make between the two options. I think it kind of sunk in what I was asking about, and that it was not a cut and dried decision. If he had made it without consulting me, and just picked the closed match to the target of -1.5 D I would have gotten the +19 D lens.
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In short I think your concerns are valid. But, I would just keep it simple and ask what each lens power is predicted to give you. The steps in outcome have to go up and down in approximate 0.35 D steps.
BrianCyberEyes RonAKA
Posted
In my case it seems to be closer to -.0.25 per step for the Clareon (.23 and .24), which seems small, but no idea if that's unusual.
I've had an IOL calculated in a different clinic recently, there I have the IOL power table available to me, steps here are closer to .35, but that was for a silicone spherical lens of unknown make and model (that's all my healthcare system would pay for in my country, thus I decided against it and will pay for the whole procedure privately).
One suspicious thing though, for the same eye, here they determined a lens power of 21D (target -.54D) whereas the Clareon in the private practice is calculated to be 22D for a similar refractive target (-.38D). Unsure if the measurements were different or if different lenses and procedures just require different powers.
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Looking at your explanations of different calculation methods, the difference of 1D could be just a result of different formulas. It's a lot though.
RonAKA BrianCyberEyes
Edited
It is hard to say what is going on unless you know how they are doing the measurements, and what formulas are being used. Each lens has an A constant which is used to refine where the lens sits in the eye. Each surgeon's technique is a little different and they refine these A constants based on personal experience. A silicone lens is going to be a lot thicker than a Clareon lens, and will sit in a different position in the eye. But yes, 1 D seems like a lot. It is two power steps different.
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Ideally you want them to be using an Zeiss IOLMaster 700 to take the measurements. I think the Barrett Universal II and Hill-RBF 2 are a couple of the better formulas.
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If you want to look into this in much more detail than I understand google this article.
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Alcon White Paper IOL Lens Constant Optimization
BrianCyberEyes RonAKA
Edited
I'm guessing Zeiss is not the only manufacturer and some surgeons might prefer other devices?
Unfortunately my printouts don't have any info what they were done with. Only distinguishable symbol is a logo with "CS" followed by an eye symbol.
RonAKA BrianCyberEyes
Edited
What I know is that there are two basic ways of measuring the axial length (AL) of the eye, which is the most critical factor in determining the required power. Small differences make significant differences to the outcome. The basic common method is ultrasonic based and is referred to as an A-scan. In Canada this is the method used for healthcare fully covered services. The newer and better method is laser based, and in these jurisdictions you have to pay extra for it. In Alberta the laser method seems to be covered at no extra cost.
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I am not that familiar with the A-Scan instruments but you would probably know if they have been used to measure your eye. They have to touch the surface of your eye with the probe to get an accurate AL. It is difficult to touch the eye without changing the shape of it. Some use an immersion technique where a coupling fluid is used between the ultrasonic probe and the surface of the eye to improve accuracy. You may have to be laying down prone to use this type of measurement.
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In the laser methods the Zeiss IOLMaster seems to be the most common. The older version is the 500, and the newer one the 700. I believe the 500 measures the length of the eye, while the 700 has other measurements including the shape of the cornea. The other instrument that seems to be more common is the Lenstar 900 (LS 900). The IOLMaster 700 seems to be better in measuring the AL if the cataract is advanced and dense. It is also much faster than the LS 900. The patient has to keep their eye open and still for about double the time to get an accurate scan with the LS 900 compared to the IOLMaster.
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Short story is that if you want the most accurate measurement it is better to use one of the laser methods rather than the basic A-Scan ultrasonic method. That may account for some of the difference between the first place you saw, compared to the second one.
BrianCyberEyes RonAKA
Posted
The private clinic did refraction (sitting at an apparatus, looking at an image of a small house), then IOP, later I had to sit down at 5 more similar devices - 3 had just simple red lights to look at, one had a blue cross, and one had a red light like the others, but surrounded by huge red circles (for guidance I think, they were turned off for the test). I don't remember which ones were done before and which after eye drops (I know that a retina scan was after + slit lamp at the end). No lying down, no contact at any point. Whole visit was pretty drawn out, I was there maybe 3 hours, but any of the tests were just seconds, I wasn't even aware what part of it was calibration or the scan itself.
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It was very similar to my visit there two years ago, just the topography printouts look like they had an design update.
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For the healthcare covered visit the other clinic did 3 tests: IOP, refraction, then they had me lying down after eye drops and used a hand held scanner to my eye (no contact).
rwbil BrianCyberEyes
Posted
There are different machines they use. I can tell you when I had mine done the IOLMaster machine had a label stating IOLMaster.
In my case my cataract was so dense they could not get a reading from the IOLMaster Machine, so they went to a SonoMed blue light pen to the eye to get Axial Length. And they did it again and again as it never gave a great reading. IMHO they can get almost any reading off that depending on how hard they push that pen and into your eye. Luckily for me they had measured that eye when I had my other eye done and they were able to dig up the records out of storage (one reason I now tell people to get a copy of their IOL Master results). If we had used the SonoMed reading my refractivity would have been off.
Also I will add my astigmatism readings were not consistent so I had them redone.
In addition to that there is the Corneal topography machine that creates create pictures and there is also a manual one they use, but I forgot what it is called.
And of course there is the eye pressure machine.
BrianCyberEyes RonAKA
Posted
"Spectralis Tracking Laser Tomography (Heidelberg Engineering)" is labeled on some printouts (RNFL Trend Report, Overview Report, Single exam report, Thickness map change report), there's also something from "EM-4000", which apparently is a "Specular Microscope". No idea if any of those are equivalents to an IOLMaster and if they are any good.
RonAKA BrianCyberEyes
Posted
On a quick look they appear to be imaging diagnostic instruments. The former seemingly for the retina, and the later the cornea. I don't think either are biometry instruments to measure the eye for IOL power calculation.
BrianCyberEyes RonAKA
Edited
Can the AS-OCT printout be from a biometer or is that something else entirely?
Is a biometer always used or just supplemental?
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I found two clinics in my country that definitely have the IOL Master 700 (others either don't or do not advertise in a google friendly manner). Not sure though how valuable it would be to travel around the country to have the IOL power calculated somewhere I might not have the surgery at.
BrianCyberEyes RonAKA
Edited
I think this study compares the IOL Master to the manufacturer my clinic seems to be using (Heidelberg Engineering):
https://www.escrs.org/marrakech2020/Programme/free-papers-details.asp?id=35211&day=0
One of the manufacturing companies funded that study, not sure though which.
RonAKA BrianCyberEyes
Edited
If you were to travel to get measurements done with the IOLMaster 700 you would need a good printout of all the data needed for the particular formula to be used for calculating the power. It would be best to ask your surgeon what data they need. Or, you would want those who use the instrument to come up with a power recommendation.
RonAKA BrianCyberEyes
Edited
Yes that Anterion instrument seems to be a similar instrument to the IOLMaster 700. I found this brochure which explains what it can do.
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Multimodal Imaging Platform Optimized for the Anterior Segment Anterion pdf
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You have to look through the google results to find the pdf version. I did not immediately see from the information if it is capable of integrating the various power calculation formulas into the software. It may. The IOLMaster allows various formulas to be integrated and displays the output of each for comparison.