EyeWorld India March 2014 Issue

32 EWAP CAtArACt/IOL March 2014 Posterior corneal astigmatism’s role increasingly recognized by Erin L. Boyle EyeWorld Senior Staff Writer Measurement of posterior corneal astigmatism is important in achieving the best toric IOL results, experts say W hen a toric lens has been implanted but results are not visually optimal— or expected—posterior corneal astigmatism might play a role in that discrepancy, experts say. “I think many cataract surgeons who have used toric lenses can remember cases where they were scratching their heads postoperatively,” said D. Rex Hamilton, MD , director, UCLA Laser Refractive Center, and associate clinical professor of ophthalmology, Jules Stein Eye Institute, Geffen School of Medicine at UCLA, Los Angeles, Calif., U.S. “You look in the eye with dilation, you can see the orientation of the lens, it’s exactly where you wanted to put it, and yet the patient ends up with residual astigmatism.” Posterior corneal astigmatism can explain that issue, he said. New findings and ways of measuring the back of the eye have discovered that the posterior corneal surface contributes to the total corneal astigmatism. “The effect of the posterior corneal astigmatism, whose meridian may be different from the axis of the anterior corneal surface meridian, explains why in some eyes the results with toric IOLs are less than ideal despite seemingly appropriate IOL selection and 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 With-the-rule posterior astigmatism shape subtracts from with-the-rule anterior astigmatic shape, reducing overall with-the-rule astigmatism power. placement based on good anterior corneal measurements,” said Dilraj S. Grewal, MD , and Surendra Basti, MD , Department of Ophthalmology, Northwestern University Feinberg School of Medicine, Chicago. They conducted a study examining the relationship between the anterior and posterior corneal astigmatism in a cohort of normal eyes with >3 diopters of astigmatism (28 patients) and ectatic eyes (22 patients). In the study, they found a greater contribution of the posterior surface to the total corneal astigmatism in normal corneas (31±8.5%) when compared to keratoconic eyes (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, NY, U.S.) 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 continued on page 34