Eyeworld Daily News

13 EW SHOW DAILY 2018 ASCRS•ASOA Annual Meeting, Washington, D.C. ensure patient satisfaction. Doing topography is not easy— there is no consensus protocol, and each practice has to make their own, Dr. Cohen said—and despite every- one's best efforts, the results may not be as intended. Moreover, every component of the multipart process contributes such that while a per- fected component might not lead to significantly different outcomes, an incorrect component will guarantee a "refractive miss." Thus, Dr. Cohen said, staff and surgeon must not accept preop mea- surements at face value and must all be experts in preop measurements. Technicians and nurses need to un- derstand and question preoperative test data as well as establish proto- cols with the surgeon. EW Editors' note: Dr. Cohen has no finan- cial interests related to his comments. between eyes in terms of these pa- rameters warrants repeat testing. In terms of operative issues, Dr. Cohen discussed astigmatism. Astigmatism, he said, is mea- sured by keratometry using auto- keratometry or an IOLMaster or by topography, used to take simulated keratometry (SimK, measuring the anterior corneal surface), total corne- al power (accounting for both ante- rior and posterior corneal surfaces), and posterior corneal astigmatism. He then described the effect of the posterior corneal curvature on astigmatism, saying that an eye with greater curvature along one meridian than the other, for exam- ple, more curved in the 90-degree vertical meridian than the 180-de- gree horizontal meridian, means more should be subtracted from the 90-degree vertical meridian than the 180-degree horizontal meridian. This is the result of the differ- ences in the indices of refraction as light rays pass through air, bending as they cross the anterior surface into the cornea, then bend again as they cross the posterior surface into the aqueous fluid medium inside the eye. Ray tracing through the cornea takes this effect into account to pro- vide a measurement of total corneal power. It is also important to consider whether the anterior astigmatism is with- or against-the-rule; with-the- rule astigmatism will result in lesser total corneal power while against- the-rule astigmatism will result in greater total corneal power. Dr. Cohen summed up the mea- surements to calculate IOL power as anterior corneal topography and ax- ial length; real ray tracing of 30 rays based on Snell's law; IOL geometry and refractive index; and an itera- tive multidimension optimization process to minimize spot size and wavefront error. Technicians and nurses also need to know that keratometry devices take measurements differ- ently, with devices taking measure- ments from different numbers of points around variable diameters of cornea. He qualified that taking measurements from more points doesn't necessarily mean more accurate measurements, though the advantage of the topographer over other devices is the fact that, again, it accounts for the posterior corneal surface. Preoperative imaging is also used to evaluate the retina, with as- sessment of retinal cell layers, foveal architecture, and retinal distortions all potentially affecting postopera- tive vision and other postoperative outcome parameters. All of these help cataract sur- geons choose the appropriate IOL for each patient and provide them with realistic prognostication to

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