Jascha Wendelstein, MD, of the Institute for Refractive and Ophthalmic Surgery, spoke with Ophthalmology 360 at the American Society of Cataract and Refractive Surgery Annual Meeting about the future of corneal power calculations.
Jascha Wendelstein, MD:
Hi, my name is Jascha Wendelstein. I’m currently working at 2 places. I’m working in Munich in Germany at the university clinic and I am working at the IROC Zurich in Switzerland, an institute founded by Professor Seiler who invented PRK and cross-linking. I am presenting today a study we did on the power of the epithelium and its role for either laser or IOL calculations. We based the study upon measurements by the MS-39, which is a hybrid placebo and OCT tomography device. We use maps to get 3 different corneal models to calculate corneal power, a model based on one refractive surface, only the anterior surface, a model based on 2 refractive surfaces, the anterior and the posterior surface. Then we had a third model which calculated the corneal power based on 3 refractive surfaces, which is the anterior power, the anterior stromal power, and the posterior power of the cornea.
We used the height map data from the tomography data we got and used floating best fit spheres to get the corneal radii to calculate it. We had 4 different cases. We had an exemplary normal eye. We had an exemplary eye after myopic laser vision correction and an exemplary eye after hyperopic laser vision correction and we had an eye with keratoconus. We then compared corneal power measured with 2 refractive surfaces, three refractive surfaces and compared that to 1 refractive surface. In the normal eye, the differences were not spectacular. It was a difference of about 0.05 diopters between the 2 refractive surfaces and the 3 refractive surfaces, so there’s not much to gain there. More interesting with the other eyes, the eye after myopic laser vision correction. We had a difference of around 0.4 diopters, so a 10-fold difference as compared to the normal eye and compared to the one refractive surface corneal power calculation, there was a bit more than a diopter of difference.
For the hyperopic eye, it was not as much. It was around 0.8 diopters difference between 1 surface and 3 surfaces. If we compared the 2 surface corneal, so only anterior and back to the three surfaces we were around 0.37, 0.38 again, that benchmark of around 0.4 diopters difference for the hyperopic laser vision correction. Now the most drastic differences we saw on the keratoconus eye where epithelium seems to play the highest role or the highest mistakes, if you want to put it like that.
We had differences of somewhere around 0.4 diopters for the more central regions and we had even higher differences of up to 0.9 diopters going outwards into the periphery, showing how much of a deviation there is between the 2 and the 3 refractive surface model. Very interestingly, comparing the 3 refractive surfaces to the one refractive surface, the difference was much, much more drastic. We had a difference of around 3.6 to 4 diopters between 3 surfaces and 1 surface. I think that goes to show that the future of corneal power calculations that can be used either for IOLs or for laser profiles is in the 3 refractive surface models, which can much more accurately display corneal power and can maybe try to correct some of those small mistakes that we still have in keratoconus eyes and eyes after laser vision correction. Thank you so much.
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