Eyeworld

JUNE/JULY 2020 | EYEWORLD | 31 R Contact Dupps: bjdupps@outlook.com Huang: huangd@ohsu.edu "There is something about this tapered pro- file that's done at the surface of the cornea that I think is friendlier from a structural standpoint vs. a flap that is being cut straight down through a lot of fibers and then you have the ablation," Dr. Dupps said. "I have a higher percentage of PRK in my practice compared to some people. It's about 35%." Dr. Dupps described a software program in testing called SpecifEye (not commercially avail- able) that runs patient-specific simulations of the various refractive surgery procedures, giving a report on the stresses and strains, structural risk metric, and a refractive outcome. Dr. Dupps said there has been an evolution in corneal imaging technologies. Fifteen years ago, he said, tomography was not the standard of care; most refractive surgery screening at that time included topography and ultrasound pachymetry at the center of the cornea. A decade ago, tomography took its place as the standard of care and anterior OCT has more recently become a thing, Dr. Dupps said. "I think most young doctors won't have been exposed to the epithelial mapping feature and how helpful that can be," Dr. Dupps said. While Dr. Dupps acknowledged that he has access to a range of available—and some not yet available—technologies to screen for refrac- tive surgery, he stressed the importance of solid tomography, if little else. It's important to learn how to use it, mostly by learning what normal, slightly abnormal, and frankly abnormal maps look like. "The more cases you look at and see how people have interpreted those, the more comfortable you are going to be assessing new patients," he said. "The other thing I would say is … when someone sets up the device, make sure you are there. … Some of these features you have to understand yourself to teach techs how to use them." "We can start to sift these corneas into structurally robust corneas vs. those that might be at higher risk for corneal ectasia," Dr. Dupps said. The Ocular Response Analyzer (Reichert) also provides information on corneal biome- chanics. "There is significant interest in char- acterizing corneal biomechanical properties in the hopes of advancing screening methods for refractive surgery candidates," Moshifar et al. wrote in a 2019 literature review. 2 Based on this review, the authors concluded that "it is war- ranted to consider in vivo mechanical assess- ment as an appropriate approach for screening of corneal ectasia" because "changes in biome- chanical properties may occur before disease becomes apparent via tomography or topog- raphy." The study authors also wrote that this review showed several clinical applications for biomechanical parameters, including assisting in screening of surgical candidates and tracking postop changes. Dr. Huang cautioned that biomechanical measurements can overlap between normal and keratoconic eyes and thus may not be the most reliable in detecting keratoconus. As such, he said he doesn't use technologies that measure corneal biomechanics. When asked about the different refractive procedures and their impact on corneal bio- mechanics, both Drs. Huang and Dupps said PRK and SMILE have a lesser effect on corneal biomechanical stability. "SMILE I think carries some advantage over LASIK in terms of the structural pres- ervation, and we know from the way it works it should be an advantage for most patients to not cut through those anterior structures," Dr. Dupps said, adding that this is true in simula- tions as well. He went on to say that ectasia risk is also lower with PRK. References 1. Schallhorn JM, et al. Dis- tinguishing between contact lens warpage and ectasia: usefulness of optical coherence tomography epithelial thickness mapping. J Cataract Refract Surg. 2017;43:60–66. 2. Moshirfar M, et al. Advances in biomechanical parameters for screening of refractive surgery candidates: A review of the literature, Part III. Med Hypoth- esis Discov Innov Ophthalmol. 2019;8:219–240. Relevant disclosures Dupps: Alcon, Carl Zeiss Meditec, CorneaGen, Glaukos Huang: Optovue

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