Eyeworld

EW CATARACT 30 imaging in ray tracing IOL power calculation was least associated with posterior corneal curvature, thus demonstrating that the introduction of additional measurements to ray tracing calculation would provide accurate IOL power calculation for eyes after corneal refractive surgery. Similarly, Dutch researchers demonstrated that the accuracy of custom ray tracing that includes a modified equivalent refractive index (ERI) for IOL power determination in post-LASIK patients exceeded the current standards for normal eyes. "The ray-tracing procedure that includes an average ERI gave a greater percentage of eyes with an IOL power prediction error within ±0.5 diopter than the Haigis-L (84% versus 52%)," they reported. Their findings were published in the May 2015 issue of the Journal of Cataract & Refractive Surgery. Evaluating other applications in a multicenter retrospective study, Italian investigators in the July 2014 issue of the Journal of Cataract & Refractive Surgery confirmed that corneal ray tracing (performed by rotating Scheimpflug camera-Plac- ido disk corneal topographer) can accurately measure excimer laser surgery-induced corneal refractive changes. Managing patient expectations With the availability of modern tools in ophthalmic surgery, patient expectations have shifted as well, and clinicians today have high expectations to manage. "We cannot promise patients that they are going to have 20/20 vision after the proce- dure," Dr. Sheppard said. Clinicians need to confirm first what their bottom-line impression of the keratometry is with sever- al methods, before proceeding to phacoemulsification in cataract patients. "These patients have a 20% enhancement rate, and they are not candidates for a multifocal IOL; they might certainly be candidates for a toric lens—but options are limited," he added. In current clinical practice, Dr. Sheppard noted the wide range of methods that can be used to procure data and analyze refractive abnor- malities in eyes prior to cataract surgery, from various topographers producing outstanding images and ocular surface imaging, to Scheimp- flug imaging. Indeed, compared to the past, surgeons today have a huge armamentarium even before any RK patient undergoes surgery. "These days, we refrain from using analog tests unless we want to confirm something with manual keratometry," Dr. Sheppard said. "It's the switch from analog to digital. We now have the ability to look at aberrometry, aberration indices, and irregularity indices," he said. Using topographic maps, ac- cording to Dr. Sheppard, clinicians can determine irregularities in the eye. "Rings, reproducibility, stability, environmental and diurnal vari- ability, dry eye and a whole lot of other issues—these are all potential RK-related problems for patients operated on years ago," he said. "It is also key to corroborate several topographic methods in order to ascertain keratometric accuracy for IOL calculations, with or without a toric lens implant." However, in addition to having "regular and irregular astigmatic er- rors," these patients have a different axial length in the morning when pressure is higher compared to in the afternoon, he said. "Thus, we have to be doubly vigilant of topog- raphy and axial length measurments in these patients," he concluded. EW References 1. Canovas C, et al. Effect of the equivalent refractive index on intraocular lens power prediction with ray tracing after myopic laser in situ keratomileusis. J Cataract Refract Surg. 2015 May;41(5):1030–1037. 2. Miyata K, et al. Use of Scheimpflug corneal anterior-posterior imaging in ray-tracing intraocular lens power calculation. Acta Ophthalmol. 2013 Nov;91(7):e546–549. 3. Park HJ, et al. Comparison of the astig- matic power of toric intraocular lenses using three toric calculators. Yonsei Med J. 2015 Jul;56(4):1097–1105. 4. Savini G, et al. Corneal ray tracing versus simulated keratometry for estimating corneal power changes after excimer laser surgery. J Cataract Refract Surg. 2014 July; 40(7): 1109–1115. Editors' note: The sources have no financial interests related to this article. Contact information Preussner: pr.preussner@uni-mainz.de Sheppard: jsheppard@vec2020.com Ray tracing continued from page 29

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