DEC 2013

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

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Page 42 of 74

40 EW FEATURE February 2011 2014 December 2013 What's ahead in Posterior continued from page 39 (22.6±9.2%, p=0.02), highlighting the importance of measuring both corneal surfaces to determine total corneal astigmatism. Drs. Grewal and Basti said their findings show more evidence that the posterior corneal astigmatism measurement should be considered during toric IOL calculations. Their study is currently under review for publication. Dr. Hamilton said the ability to measure posterior corneal astigmatism accurately is one of the newest developments in the field. The Galilei Dual Scheimpflug Analyzer (Ziemer Ophthalmic Systems, Port, Switzerland) has made it easier to locate data and has enhanced accuracy, he said. He and colleagues published a study this year comparing the Orbscan (Bausch + Lomb, Rochester, N.Y.) to the Galilei in 78 patients undergoing LASIK. They found that the Galilei was more accurate and reproducible in measuring the back surface of the cornea. They also conducted a study examining standard cataract patients who had undergone monofocal toric IOL implantations and the residual astigmatism following those implantations. The study, currently under review, looked at three ways of measuring astigmatism preop, comparing those to postop results, with the IOLMaster (Carl Zeiss Meditec, Jena, Germany) and two methods of the Galilei. Dr. Hamilton and colleagues found that the IOLMaster, on average, did slightly better in minimizing residual astigmatism. "However, there was a significant bias toward overcorrection of with-the-rule astigmatism and undercorrection of against-the-rule astigmatism when using the standard IOLMaster, which is simply the keratometry readings. That bias goes away when you use the total corneal power and the total astigmatism," he said. Importance The traditional way of measuring corneal astigmatism was based on the anterior surface, examining the fixed anterior-posterior curvature ratio and ignoring the posterior surface's contribution, Drs. Grewal and Basti said. But ignoring the posterior corneal astigmatism could be a crucial mistake, a key researcher and colleagues in the field cautioned. In his 2012 ASCRS Innovator's Lecture and the 2012 study "Contribution of posterior corneal astigmatism to total corneal astigmatism," Douglas D. Koch, MD, and colleagues concluded, "Ignoring posterior corneal astigmatism may yield incorrect estimation of total corneal astigmatism." In a recent interview, Dr. Koch elaborated. "We now know that posterior corneal astigmatism contributes in a much more significant way to ocular astigmatism than we had ever imagined, and it has important implications for how we correct patients' astigmatism with either relaxing incisions or, more importantly, with toric IOLs," said Dr. Koch, professor and the Allen, Mosbacher, and Law Chair in Ophthalmology, Cullen Eye Institute, Baylor College of Medicine, Houston. He said the posterior cornea is vertically steep in most eyes, causing a net against-the-rule astigmatism. This is because the cornea is a minus lens—if it is steep vertically then it creates power horizontally, he said. With-the-rule posterior astigmatic shape subtracts from with-the-rule anterior astigmatic shape, reducing overall with-the-rule astigmatism power. "The consequences of that are if we ignore posterior corneal astigmatism, patients who have with-therule astigmatism initially will tend to get overcorrected," he said. "For example, if a patient has 2 D of with-the-rule astigmatism on the front of the cornea, and you implant a toric lens that corrects 2 D, you may flip the axis and leave that patient with a half diopter or more of against-the-rule astigmatism. Our clinical study3 confirms the Baylor nomogram that we have developed." That difference matters, Dr. Hamilton said, because posterior corneal astigmatism's impact can be a quality of vision issue. Residual astigmatism does not always result in a Snellen line change, but the quality of vision is affected, he said. "We're talking about relatively small numbers here," Dr. Hamilton said. "The accuracy is important. It's not like we're talking about 5 D of difference, we're talking about things on the order of half a diopter. So you've got to be really precise with those measurements." Discovery One reason that posterior corneal astigmatism's importance was discovered, Dr. Koch said, was the creation of toric lenses—they assisted in establishing an idea that had long been suspected by researchers. "What's striking is that this was anticipated 143 years ago by Javal in a paper in which he showed that refractive astigmatism is not the same as corneal astigmatism, and he assumed that this was either from the posterior cornea or the crystalline lens. Now we know that much of this is certainly from the posterior cornea," Dr. Koch said. He became interested in the topic because he was finding puzzling results with some toric IOL implantations. "Patients who had with-the-rule astigmatism, I was overcorrecting, and patients who had against-therule astigmatism, I was undercorrecting. I was stunned by these results, having gone back and remeasured the anterior cornea and confirmed the refractions, and from that concluded the only possibility was it must be emanating from the posterior cornea," he said. He emphasized that other authors had shown the importance of posterior corneal astigmatism in excellent studies before he and his colleagues investigated the topic. Their work differed in that they examined varying age groups and the amount of with-the-rule and against-the-rule astigmatism, accounting for the contributions of factors such as differing patients and age. Dr. Koch developed the Baylor nomogram, a regression nomogram, to assist surgeons in compensating for posterior corneal astigmatism based on population averages. "Based on Dr. Koch's study, we now know that the steep meridian of posterior corneal astigmatism is oriented vertically in about 87% of eyes, a measurement not factored into what is typically measured as corneal astigmatism," Drs. Grewal and Basti said. "This omission results in an overestimation of astigmatism (as measured by devices that calculate corneal astigmatism by measuring only the anterior surface) by about 0.50 D for with-the-rule astigmatism and underestimation by about 0.30 D for against-the-rule astigmatism." continued on page 43 With-the-rule posterior astigmatic shape adds to against-the-rule anterior astigmatic shape, increasing overall against-the-rule astigmatic power. Source (all): D. Rex Hamilton, MD

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