Dr. David Huang: This particular talk it’s not very new. I think I’ve been talking on this topic for the past three years, but what’s really new this year is that it’s now clinically available. You can actually get this module and use it for intraocular lens power calculation, and it works well, and I want to talk about our latest results

I do have financial interest in Optovue in terms of equity and patents, and I also have financial interest in Zeiss

So, the problem with keratometry after laser vision correction is that conventional keratometry no longer works. So, the way conventional keratometry works is that you project a circle or other patterns on the interior corneal surface. You get a reflection that allow you to measure the slope of the cornea at two points. And there are formulas that converse this to corneal power that’s basically equivalent to extrapolation on the interior and posterior corneal curvature. For a virgin eye that never had any surgery that works fine because these structures are very irregular. Post-Lasik you modify the interior curvature without modifying the posterior curvature. So, when you extrapolate from the interior measurements it no longer works. And that causes a problem with post-Lasik intraocular power calculation

With OTC we can measure both the interior and posterior corneal boundaries and derive curvature from that and compute the total corneal power. This was FDA approved towards the end of 2011, and it’s available now on the RTVue machine

When you compare the RTVue OCT measure net corneal power with the automated keratometer, such as one you get from IOL-Master, what you see is that they are not the same even for a normal cornea. The OCT power is lower by about 0.66 diopters. And why is that? One is that we use the first principle plane as a reference plane in OCT and conventional keratometry is defined on the back vertex, but another difference is that conventional keratometry based on the keratometric index at 1.3375 actually underestimate the posterior corneal curvature. That’s an index that’s been assumed a long time ago. So, this--it’s an historical error. Unfortunately, most of our intraocular lens power calculation formulas are calibrated to this keratometric index. So, in order to properly use OCT data for IOL power calculation we need a special IOL formula that’s calibrated for OCT

So, that’s what we have done. The formula that we have developed in my research group is very similar to all the other theoretic formulas, and it’s based on virgin’s calculations and an effective lens position calculation based on basic biometry [sp] information and this has been previously published

So, in this formula the [unintelligible] variables include interior chamber depth and axial length measurement from another device such as ultrasound, A-scan or partial coherence barometer [sp]. And then, from the Fourier-domain OTC, the RTVue system, you input net corneal power, interior corneal power, posterior corneal power and the central corneal thickness. So, you notice here that some of these parameters are new that that’s not conventional formulas such as the posterior corneal power or corneal thickness

So, this is new information that’s valuable for calculations in Post-Lasik and post-PRK patients. We’ve done prospective studies on the performance of this formula. We have data from both Cullens Eye Institute in Huston and Doheny Eye Institute in Los Angeles. And the primary outcome measure is a post-operative manifest refraction evaluated at one month. And this has been recently published. In our study we used two OCT measurements and averaged the values and this increases the precision of the measurement

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The accuracy of OCT base intraocular lens power calculation compares well to the best other formula in this study, which was the Haigis-L, with 82 percent of the OCT formula within 82 percent with a mean absolute error of 0.57 diopters and 82 percent within one diopter. And you can see on the plot that the spread is slightly better than Haigis-L. The difference was not statistically significant. We need more eyes to detect a small difference

And the accuracy of OCT based calculation was also compared to historical--clinical history methods and contact lens over refraction methods and our Doheny subgroup and these were significantly--these older methods are significantly worse than the OCT based calculations

How about hyperopic eyes? Well, in these eyes OCT did also better with a mean absolute error of only 0.26 diopters and 100 percent were within one diopter; however this is a relatively small study. So, again, we need more eyes for clinical--for statistical significance

So, this is a case example, a 53-year-old female with Pre-Lasik myopia of more than eight diopters, and you can see that because she had high myopic correction. The corneal power estimate from the various methods varied widely with an OCT measurement at 35.7 diopters and the others ranging from a low of 34 diopters with clinical history and a high of 39 diopters in contact lens

So, this kind of case is of course it’s very vexing for the cataract surgery who wants to aim for ametropia post-op. So, this surgeon basically hedged and took the average measurements and average IOL power recommendation from all these measurements and put in the lens. And the outcome was hyperopic by 1.25 diopters. And you can see that from the predication error that if this was pretty much what OCT based calculations predicted. And if the surgeon had used the OCT formula he would have been right on

So, this case is fairly typical. I think OCT based IOL calculation had its highest value in these eyes that had very high myopic correction where if you try to extrapolate using other formulas, rather than directly measuring posterior corneal curvature, you could be relatively far off

So, the conclusion is that predictive accuracy of OCT based intraocular lens power calculation is equal to, or better than, current standards for post laser vision correction eyes

And I want to acknowledge the data from Dr. Koch and Dr. Li Wang at Cullens Eye Institute who we work with on this grant project sponsored by NIH and RPB and also material support from Optovue

We have posted our intraocular lens power calculation formula on our website at www.COOLLab.net. And unlike some other formula this one is actually free. You can just go on to our website and download the spreadsheet and there’s no royalty for using it, and in fact a lot of people are downloading it. I haven’t tallied our recent result yet, but in February we had 800 visits and more than 100 downloads. So, a lot of people are using this formula

I actually met somebody in Taiwan--some people in Taiwan that I recently visit and they already are having a pretty good experience with using the formula. And I want to acknowledge Dr. Maolong Tang in my research group who had developed this formula

And this talk will be posted on COOLLab.net