Eyeworld

EW FEATURE 58 All about IOL calculations • January 2017 Monthly Pulse IOL calculations T he topic of this Monthly Pulse survey was "IOL calculations." We asked, "Which formula do you use to determine IOL power in a 'normal eye'?" The majority of respondents to this survey said SRK/T; Holladay 2 was the second most popular answer. When asked how they determine axial length and IOL power, a large majority of respondents said they use optical biometry. The third question in the survey was, "When a patient has undergone previous myopic LASIK, how do you calculate the IOL power?" The most popular answer was the ASCRS Post-Refractive IOL Calculator. Finally, we asked, "For a patient undergoing cataract surgery with 1 D of corneal cylinder, what technique would you use to reduce postoperative astigmatism?" The majority of respondents said a toric IOL. An on-axis incision was the second most popular answer. what has been termed 'the credit card test.' In this test, the surgeon places the long axis of a credit card on what is visually determined as the steepest meridian on topography and can read the axis of astigmatism directly." Dr. Tipperman noted that the K values will always differ since dispa- rate devices measure in varying ways and at different corneal locations. "The surgeon should analyze the data, and they should 'make sense' so that in most cases—with the exception of "lenticular astig- matism"—a patient with 2 D of astigmatism at axis 90 in his or her spectacles would be expected to have this approximate magnitude and axis of astigmatism when mea- sured with a diagnostic device," Dr. Tipperman said. "In cases of marked differences, the surgeon should care- fully analyze the Placido ring images and look for distortion of the rings, which could indicate ABM dystro- phy or keratitis sicca." When biometry measurements seem unreliable, Dr. Tipperman has found it is often helpful to treat the patient aggressively with artificial tears for a week or two and then repeat all the measurements. Calculators' role Stephen Klyce, PhD, adjunct pro- fessor, Department Ophthalmology, Icahn School of Medicine of Mount Sinai, New York, said that although not all devices yield identical keratometry values, most should provide accurate measurements for normal healthy corneas. However, for eyes that have un- dergone surgery—keratorefractive or intraocular—standard keratometry values can provide measurements that don't reflect the true central corneal average curvature, Dr. Klyce said. To address this challenge, sever- al corneal topographers have devel- oped indices that form an average curvature of the central cornea over the pupil, and these can improve the prediction accuracy of IOL calcula- tions. Dr. Klyce co-authored one of the first papers to demonstrate such use of indices, and various investigators have contributed their methods to the ASCRS IOL calculator (iolcalc.ascrs.org). 2 Dr. Klyce noted that many of the IOL equations have been refined with data from specific devices, so it is important to follow the recommen- dations provided for the different approaches. Dr. Rubenstein agreed that it is important to use methods such as the ASCRS IOL calculator to deter- mine a possible IOL power preop. However, among patients who have had previous refractive corneal surgery, he always employs intraop aberrometry. "I still obtain preop corneal measurements in the usual fashion, however, I realize that most of the corneal testing will overestimate cor- neal power," Dr. Rubenstein said. Tips continued from page 54 Surgically induced astigmatism The measurement of surgically induced astigmatism (SIA) is essen- tial in predicting postop astigmatic results. Jack Holladay, MD, clinical professor of ophthalmology, Baylor College of Medicine, Houston, has found the best way to measure SIA is the vector difference between the preop Ks and the postop refraction. For example, if the Ks are spher- ical and the postop refraction is 0.50 D against-the-rule (ATR), then the SIA is 0.50 at 180, not zero. "Using the preop Ks and postop refraction includes everything: an- terior corneal astigmatism, posterior Location of standard deviation of corneal radii on IOLMaster printout

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