Eyeworld

EW CATARACT 32 May 2017 Device focus by Lauren Lipuma EyeWorld Contributing Writer that considers the effective lens posi- tion with the Abulafia-Koch formula adjustments, he said. Dr. Abulafia also uses direct measurements of the posterior cornea by an additional instrument to validate his toric cal- culations. Dr. Greenwood uses topography measurements and the LENSTAR (Haag-Streit, Köniz, Switzerland) to measure the anterior corneal curva- ture. During surgery, he uses intra- operative aberrometry to measure the total corneal power. Dr. Yeu uses the Cassini LED topographer (i-Optics, The Hague, Netherlands) to measure the posteri- or corneal curvature preoperatively. "My work with the Cassini demonstrates that having this ability at your fingertips does improve your ultimate outcomes in terms of what your postoperative residual astig- matism will be because you have a better insight into what is specifical- ly going on in the posterior cornea itself," Dr. Yeu said. Using the Cassini has improved her outcomes considerably in the past year, she said. Ninety-three per- cent of her post-toric IOL outcomes are within a half diopter of the pre- dicted refractive astigmatism. Using the Baylor nomogram alone, only 75% of her patients were within a half diopter. "That's to be expected, because the nomogram can't provide you with any information about your oblique astigmatism patients and upwards of 20% of patients do not follow the convention of vertical- ly-oriented astigmatism posteriorly," she said. "To have that direct insight into what's truly going on with the posterior corneal curvature, that helps you to really help you to pick your toric power accordingly." Surprises after surgery With refractive cataract surgery, it's important to get patients into the end zone, Dr. Greenwood said. "If there's any residual refractive error, we address it." To address any refractive sur- prises, Dr. Greenwood uses astigma- The ideal scenario would be to obtain accurate direct measure- ments of the posterior cornea, said Adi Abulafia, MD, Shaare Zedek Medical Center, Jerusalem, Israel. Technologies such as Scheimpflug, swept-source OCT, and LED reflec- tion technology are being developed to directly measure the posterior corneal curvature. Although these measurements are becoming more predictable, they're not yet precise enough for surgeons to rely on them exclusively, Dr. Abulafia said. "Regression formulas that predict the net corneal astigmatism based on anterior corneal measure- ments serve as place-holders until advances in technology allow us to accurately measure total corneal power," he said. Best devices and methods Dr. Abulafia's preferred method to perform toric IOL calculations is to take high-quality corneal curvature measurements with several devices, using validation data from Warren Hill, MD, Mesa, Arizona, combined with either the Barrett toric calcula- tor or a standard toric IOL calculator "If you're not accounting for that, it can certainly lead to an overcorrection of your WTR patients and an undercorrection of your ATR patients," Dr. Yeu said. "That alone can be that residual amount that can end up leaving a patient with less than satisfied postoperative uncor- rected vision." Is it essential to measure it? "There are certain components of astigmatism management that are really important for us to be aware of because they can lead to refractive surprises," Dr. Yeu said. With posterior corneal astigma- tism, surgeons now have multiple tools at their fingertips to better manage this aspect of vision cor- rection. It's not crucial to buy a device to measure posterior corneal astigmatism, but it is important to understand it and take it into ac- count during surgery, said Michael Greenwood, MD, Vance Thompson Vision, Fargo, North Dakota. Surgeons can measure posterior corneal astigmatism directly or use a nomogram or formula to indirectly incorporate the posterior cornea into toric calculations. Experts discuss their preferred methods for considering posterior corneal astigmatism during refractive cataract surgery C orrecting astigmatism with toric IOLs has become standard practice for many eye surgeons, but visual outcomes are not always predictable. One source of uncertainty in the surgical process is posterior corneal astigmatism. Here, three refractive cataract surgery experts discuss the importance of accounting for and measuring pos- terior corneal astigmatism and the tools they use to get the best visual outcomes after surgery. How important is posterior corneal astigmatism? The ophthalmology community is fortunate that Doug Koch, MD, Houston, "rediscovered" the effects of posterior corneal astigmatism, said Elizabeth Yeu, MD, in private practice, Virginia Eye Consultants, and assistant professor of ophthal- mology, Eastern Virginia Medical School, Norfolk, Virginia. "In the past, when we only had corneal relaxing incisions to use at the time of cataract surgery to help with astigmatism management, the unpredictability of the outcomes were attributed to the fact that [re- laxing incisions are] somewhat of an inexact science and more of an art," Dr. Yeu explained. But once toric IOLs became available, we realized that patients were experiencing residual astigmatism and surprise outcomes with a flipped axis because of the effects of the posterior cornea, she said. 1 Dr. Koch's 2012 study found that between 70 and 87% of eyes have with-the-rule (WTR) posterior astigmatism on the order of 0.3 to 0.5 D. 2 As a result, the posterior cor- nea actually increases the amount of total ocular against-the-rule (ATR) astigmatism. Measuring posterior corneal astigmatism Factoring in posterior corneal astigmatism is crucial to improving outcomes during refractive cataract surgery. Sources: Doug Koch, MD, and Li Wang, MD

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