DEC 2012

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

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Page 28 of 72

26 EW REFRACTIVE SURGERY December 2012 Refractive editor���s corner of the world Posterior corneal astigmatism vital to calculating correct total astigmatism by Erin L. Boyle EyeWorld Senior Staff Writer E very now and then, we see information that makes perfect sense, yet has often been overlooked. For years, many cataract surgeons have noticed a tendency to overcorrect with-the-rule astigmatism and to undercorrect againstthe-rule astigmatism. But there has not been a clear understanding as to why this may occur, until now. The posterior corneal surface���for years it remained the great unknown. Surgeons didn���t understand it; diagnostics couldn���t image it. Refractive surgeons have learned that the posterior cornea is not to be ignored. In fact, it is often considered essential in screening for candidacy for corneal refractive surgery to assist in identifying patients at higher risk for ectasia or the development of irregular astigmatism. Cataract surgeons are just beginning to understand the posterior corneal surface. They are beginning to screen for appropriate candidates for corneal refractive surgery at the time of cataract surgery. Now, with new information, we may be beginning to understand the refractive effect of the posterior corneal surface, which may play a role with refractive outcomes following corneal relaxing incisions (LRIs, arcuate incisions) and toric IOLs. Congratulations to Drs. Koch and Wang for their pioneering work and helping to shed some light as we look to better understand our refractive outcomes. Kerry Solomon, M.D., refractive editor This journal article also is highlighted in the EyeWorld Journal Club column on pages 45 and 46. N ot measuring the posterior corneal astigmatism could result in incorrect estimation of total corneal astigmatism, hindering toric IOL selection through overcorrection in with-therule astigmatism and undercorrection in against-the-rule astigmatism, researchers found. Douglas D. Koch, M.D., professor and the Allen, Mosbacher, and Law Chair in ophthalmology, Cullen Eye Institute, Baylor College of Medicine, Houston, and Li Wang, M.D., associate professor, Cullen Eye Institute, Baylor College of Medicine, Houston, are researching the effect of posterior corneal astigmatism and toric IOL selection in cataract surgery cases. Dr. Wang said both posterior and anterior corneal astigmatism measurements are important to all cases undergoing cataract surgery. ���It would be best to measure posterior corneal astigmatism,��� she said. ���The magnitude of posterior corneal astigmatism cannot be predicted based on the amount of anterior corneal astigmatism. If there is no access to a device that measures the posterior corneal astigmatism, the average value of the posterior corneal astigmatism may be used.��� Drs. Koch and Wang and colleagues published study results on the topic in the Journal of Cataract & Refractive Surgery. They evaluated 715 corneas of 435 consecutive patients, calculating total corneal astigmatism using ray tracing, corneal astigmatism from simulated keratometry, anterior corneal astigmatism, and posterior corneal astigmatism. They found that toric IOL selection based on anterior corneal measurements only could lead to problems. ���Patients who have anterior with-the-rule astigmatism���in other words, the cornea is steep at 90 degrees anteriorly���tend to have, on average, 0.5 diopter (D) of steepness vertically along the posterior cornea, and because the posterior cornea is a minus lens, steepness vertically translates into power horizontally or against-the-rule effect refractive power at 180,��� Dr. Koch said. ���So you might measure a patient who has 2 D on the anterior cornea. And when all is said and done, that patient may only have 1.3 or 1.4 D Posterior corneal astigmatism Baylor toric IOL nomogram Source (all): Douglas D. Koch, M.D., and Li Wang, M.D. on the total corneal power because the posterior cornea throws in about 0.5 or 0.6 D in the other direction.��� Measuring devices Measuring posterior corneal astigmatism is a challenge, Dr. Koch said. Two devices on the market, the Galilei Dual Scheimpflug Analyzer (Ziemer, Port, Switzerland) and the Pentacam (Oculus, Lynnwood, Wash.), measure it ���moderately accurately,��� he said. ���I think that our measurements could improve,��� Dr. Koch said. ���We do find that even the Galilei, which has a wonderful dual Scheimpflug mechanism for measuring the back, does not always seem to capture all of the posterior corneal astigmatism, and especially in patients [who have] with-the-rule astigmatism, it still seems to underestimate the amount of posterior corneal astigmatism based on our actual refractive outcomes.��� Dr. Koch has created a nomogram that incorporates: 1) the mean posterior corneal astigmatism in eyes having either with-the-rule or against-the-rule astigmatism and 2) the effect of against-the-rule drift that occurs with age. He said that their data indicate that the new nomogram greatly improves accuracy with toric IOLs. In addition, manufacturers are interested in providing clinicians with this information because they are finding similar results retrospectively in their data, he said. However, to disseminate a new nomogram themselves, they would have to validate it in a clinical trial with the U.S. FDA, which could slow the approval process.

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