EyeWorld India December 2020 Issue

FEATURE 10 EWAP DECEMBER 2020 devices vary, not unusual for post-refractive surgery eyes,” Dr. Barrett said. The default mode of the True K Toric uses the theoretical PCA, but utilizing measured PCA provides better accuracy for both spherical and toric prediction, particularly when no refractive history is available. In Dr. Barrett’s own small series of 23 eyes, the True K Toric calculator using measured PCA achieved a greater percentage of outcomes within 0.5 D of target (73.9%) than using predicted PCA (69.6%) (Figure 2). “The ability of the True K formula to utilize the measured posterior cornea has improved the outcomes in my patients that have undergone previous refractive surgery and I hope you also find these features helpful,” Dr. Barrett said. Another gamble in prediction The next new feature of the Suite is the ability to predict IOL power in what Dr. Barrett called ”another gamble in prediction”: cases with keratoconus. In keratoconus, he said, the Ks are steep and irregular, the cornea thin and ectatic, and again the normal relationship between anterior and posterior cornea is disrupted, similar to post- refractive eyes but, Dr. Barrett said, “in its own unique fashion.” A recent review in the Journal of Cataract and Refractive Surgery 5 indicated that outcomes tend to be hyperopic. The suggested solution is to compensate by aiming for more myopic targets and using the SRK/T formula, which tends to have more myopic outcomes with steep Ks. The True K TK formula now includes an algorithm for keratoconus, again best used with the measured posterior cornea. Instead of myopic or hyperopic RK, simply select keratoconus, now included in the drop-down menu. There’s no need to enter refractive change; if you do, the data will be ignored. Select the PCA option and device used to measure the keratoconus in brackets. The formula will calculate the posterior cornea and use this in the formula. Yokrat Ton and Ehud Assia shared with Dr. Barrett a data set of 32 eyes from Israel, comparing the new formula with the standard formula as well as Figure 2. Retrospective analysis of 23 eyes comparing the use of predicted vs. measured PCA. Source: Graham Barrett, MD the Kane formula, which also has an option for keratoconus (Figure 3). Normal formulae all have a hyperopic prediction error which is less with SRK/T (mean error 0.31, SD 0.87), Kane (0.57, SD 0.95), and the Kane KC (0.12, SD 0.58). The True K TK has a mean error close to 0 (–0.05, SD 0.48), 87.5% within 0.5 D vs. BUII 59.4%, SRK/T 59.3%, Kane 56.3%, and Kane KC 53.1%. “Improvement in prediction of the True K with the KC option was statistically significant, depends on utilization of measured posterior cornea, and is not derived from dataset,” Dr. Barrett said. “I hope this new feature will help reduce uncertainty in predicting outcomes in patients with keratoconus who are undergoing cataract surgery.” EWAP Editors’ note: Dr. Barrett is the author of various IOL formulas that have been licensed to several companies and has been a consultant to both Alcon and Zeiss. References 1. Abulafia A, et al. Accuracy of the Barrett True-K formula for intraocular lens power prediction after laser in situ keratomileusis or photorefractive keratectomy for myopia. J Cataract Refract Surg. 2016;42:363–369. 2. Vrijman V, et al. ASCRS calculator formula accuracy in multifocal intraocular lens implantation in hyperopic corneal refractive laser surgery eyes. J Cataract Refract Surg. 2018;45(5):582–586. 3. Turnbull AMJ, et al. Methods for Intraocular Lens Power Calculation in Cataract Surgery after Radial Keratotomy. Ophthalmology. 2020;127(1):45–51. 4. Lawless M, et al. Total keratometry in intraocular lens power calculations in eyes with previous laser refractive surgery. Clin Exp Ophthalmol. 2020;48(6)749–756. 5. Garzón N, et al. Intraocular lens power calculation in eyes with keratoconus. J Cataract Refract Surg. 2020;46(5):778–783. Figure 3. Comparing different IOL power calculation formulas for keratoconus in 32 eyes. Source: Yokrat Ton, MD, and Ehud Assia, MD, as presented by Graham Barrett, MD