EyeWorld Asia-Pacific March 2014 Issue

34 EWAP CATARACT/IOL March 2014 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, Texas, U.S. 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. “The consequences of that are if we ignore posterior corneal astigmatism, patients who have with-the-rule 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 study 3 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-the-rule 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.” Their study has added to this finding by “demonstrating that this relationship is dynamic and also varies with the magnitude of astigmatism and is different among regular and ectatic corneas.” Future possibilities Improvements in toric calculators could be an important future innovation in the topic. Dr. Hamilton said IOL power calculators would most likely measure total corneal power, taking the front and back surfaces into account. In the future, techniques for measuring corneal astigmatism will also likely improve, Dr. Koch said, with more accurate and reproducible devices that will measure cases on an individual basis. “I am confident that we will have the ability, on a patient-by- patient basis, to precisely measure corneal refracting power and its astigmatism, front and back, and that this will be a major step forward for us in all of our IOL calculations, not just our selection of astigmatic correction techniques,” Dr. Koch said. EWAP References 1. Koch DD, Ali SF, Weikert MP, Shirayama M, Jenkins R, Wang L. Contribution of posterior corneal astigmatism to total corneal astigmatism. J Cataract Refract Surg. 2012;38(12):2080-2087. 2. Sy ME, Ramirez-Miranda A, Zarei- Ghanavati S, Engle J, Danesh J, Hamilton DR. Comparison of posterior corneal imaging before and after LASIK using dual rotating Scheimpflug and scanning slit-beam corneal tomography systems. J Refract Surg. 2013;29(2):96-101. 3. Koch DD, Jenkins R, Weikert MP, Yeu E, Wang L. Correcting astigmatism with toric intraocular lenses: the effect of posterior corneal astigmatism. J Refract Surg. 2013. Editors’ note: Dr. Koch has financial interests with Alcon (Fort Worth,Texas, U.S.), AMO (Santa Ana, Calif., U.S.), i-Optics (the Netherlands), and Ziemer. Drs. Basti, Grewal, and Hamilton have no financial interests related to this article. Contact information Grewal : Dilraj@gmail.com Basti : sbasti@northwestern.edu Hamilton : Hamilton@jsei.ucla.edu Koch : dkoch@bcm.edu Posterior - from page 32