Eyeworld

EW NEWS & OPINION 26 January 2017 A-constant causes your favorite for- mula to produce that −2.00 in this case and run the same formula for the second eye using that A-con- stant. Similarly, you could run every formula you have access to and choose the one that comes closest to calling for −2.00 for the first eye and embrace that formula for the second eye. This approach has the theoretical advantage of using a formula that may be best suited for that particular eye's combination of measurements, but practically speaking for "normal" eyes modern theoretical formulas seldom differ by this degree. If one such formula did in fact project that –2.00 result, perhaps an earlier generation formu- la, this is in practice very similar to developing an individualized A-con- stant for that patient. If you were to do this full com- pensation, however, what if the er- ror has more to do with chance than a factor that will predictably occur with the second eye? You could im- plant a lower power to compensate for that unexpected −2.00 result and end up with the second eye behav- ing more in line with expectations with a +2.00 result instead. For years many ophthalmolo- gists have split the difference with a "fudge factor." This approach has served many well, and now we have, thanks to a paper by Renu Jivrajka, MD, Maya Shammas, MD, and H. John Shammas, MD, 1 evidence that an approximately 50% adjustment factor can improve the results of the second eye more than other methods. Two respondents use a 75% adjustment factor for the second eye. Several declined to answer this question because an error of these magnitudes with modern devices and formulas should be due to error. In the next installment on this topic, we will further explore how we approach additional subtleties in IOL selection and how to do it. EW Reference 1. Jivrajka RV, et al. Improving the second-eye refractive error in patients undergoing bilateral sequential cataract surgery. Ophthalmology. 2012;119:1097–1101. Editors' note: Dr. Gossman is in private clinical practice at Eye Surgeons & Physicians, St. Cloud, Minnesota. He has no financial interests related to this article. Contact information Gossman: n1149x@gmail.com rently difficult to measure, error in axial length, and, most importantly, the uncertainty in the final effective lens position in the eye. Theoretical formulas have come a long way in helping determine the projected effective lens position (ELP) based on white-to-white, anterior chamber depth, lens thickness, and other pa- rameters, but no formula is perfect in all cases. As a result, the challenge is determining which of these many factors was responsible. If we knew for certain it was due to calculated ELP differing from reality, one could argue that forging ahead and operat- ing on the second eye without mak- ing any modifications would likely result in history repeating itself. You could theoretically determine that patient's individual A-constant by determining what arbitrary lens the pale, imagine a surprise where you normally make an adjustment for the second eye, be it 0.50, 1.00, or 1.50, etc." See Table 2. When a refractive surprise occurs and human error has been excluded, we are left blaming cir- cumstances out of our control such as uncertainty in measurement of anterior corneal power, unusual posterior corneal power that is cur- Art continued from page 25 ©2016 Clarity Medical Systems Inc. PN 99-100396 Rev A 12/16 Only HOLOS IntraOp ™ Wavefront Aberrometer delivers Continuous Real-time ™ actionable data to help you confirm target refractions, deliver precise astigmatic correction, and predict IOL power using the exclusive HOLOS Barrett IOL Formula ™ . HOLOS is available now. HOLOS.com holos @ claritymsi.com The New Standard in Wavefront Aberrometry • Accurate intraoperative IOL prediction with the exclusive HOLOS Barrett IOL Formula ™ • Continuous Real-time ™ feedback at 90 refractions per second • Precise guidance during toric IOL rotation for enhanced astigmatic correction Accurate IOL prediction, coming at you in real time.

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