EyeWorld Asia-Pacific March 2015 Issue

March 2015 IOL Calculations 25 EWAP SECONDARY FEATURE is the modified double-K SRK/T formula based solely on the current measurement of the corneal peripheral power. It uses the estimated pre-LASIK K value, which is calculated based on the post- LASIK posterior corneal curvature of the central zone, instead of the actual pre-LASIK K value in the double-K method. In the modified A-P method, to prevent a hyperopic shift, it simply shifts the results using the A-P method to 0.98 D of myopia based on the third quartile of the hyperopic error of the A-P method (0.98 D). Corneal peripheral power approach A newer approach that Dr. Negishi has helped to develop is the central-peripheral corneal curvature method (C-P method), which is a modified double-K SRK/T formula based solely on the current measurement of the corneal peripheral power. This approach uses the estimated pre- LASIK K value, which is calculated based on the post-LASIK peripheral corneal curvature. “The concept of this method is very similar to the A-P method,” Dr. Negishi said. “It allows predictable outcomes of IOL power calculations in eyes that underwent corneal refractive laser surgery for myopia.” Accurate measurement of the peripheral corneal data is important in the C-P method, and it should be strictly checked before calculating the IOL power. However, it is sometimes hard to maintain the width of the palpebral fissure during measurement, especially in older patients with a narrow palpebral fissure. In such cases, Dr. Negishi shifts to the modified A-P method because it only needs the data of the central zone, which is easier to obtain. The modified A-P method provided greater accuracy than the original A-P method in published results.² The approach has provided Dr. Negishi even better results, with 63% of eyes within 0.5 D of target results and 91.3% within 1 D. In comparison, Wolfgang Haigis, PhD , University Eye Hospital, Wurzburg, Germany, said his use of the Haigis-L formula for the IOLMaster (Carl Zeiss Meditec, Jena, Germany) provides refractive outcomes within 0.5 D of target about 55% of the time. Addressing surprise outcomes Dr. Haigis said that in cases of a surprise error after implantation, he also uses the Haigis-L formula to determine the correct power for an IOL exchange after having rechecked all measurement data of the patient. Dr. Negishi does not use any specific formula to determine the correct power for IOL exchange and instead usually calculates the correct power based on the power of the implanted IOL, its target refraction, and the postoperative refractive error. “I use the original calculation sheet and estimate the correct power using the difference between postop refractive error and the target refraction,” Dr. Negishi said. For example, if the power of the implanted IOL was +15.0 D, the preop estimated target refraction was –1.11 D, and the postop subjective refraction was +0.02 D, the IOL power calculation error was +1.13 D (hyperopic shift) in subjective refraction. The IOL power for IOL exchange is calculated to be +16.5 D on the basis of the corrected estimated refraction in the primary calculation sheet (Figure 1). Although Dr. Geggel has not had to replace any IOLs because of power miscalculations, he suggested surgeons of patients who had postoperative hyperopia multiply the amount of hyperopia by 1.5 to determine the number of diopters to add to the replacement IOL. “That’s how we handle overcorrections or undercorrections on anybody, whether they’ve had previous refractive surgery or not,” Dr. Geggel said. EWAP References 1. Geggel, HS. Intraocular lens power adjustment after myopic excimer laser surgery: validation studies for Geggel ratio and consensus group. Cornea 2013;32:1009–1014. 2. Saiki M, Negishi K, Kato N, Ogino R, Arai H, Toda I, Dogru M, Tsubota K. Modified double-K method for intraocular lens power calculation after excimer laser corneal refractive surgery. J Cataract Refract Surg. 2013;39(4):556–62. Editors’ note: Drs. Geggel and Negishi have no financial interests related to their comments. Dr. Haigis has financial interests with Carl Zeiss Meditec. Contact information Geggel: ophhsg@vmmc.org Negishi: fwic7788@mb.infoweb.ne.jp Haigis: wh@ocucalc.de IOL - from page 22 their analysis, using the C-constant for “unbiased prediction of the true position of the IOL,” the Olsen formula also modifies exact ray tracing to correct for spherical aberrations of the cornea and IOL. This approach corresponded to a 15% reduction in mean absolute errors, and the number of large errors (1.0 D) dropped 39% in public hospital series and 85% in private clinic series when compared to the SRK/T formula. “A great deal of the improved accuracy was the result of improved accuracy in predicting the IOL position,” in normal eyes over a large axial length range, Dr. Olsen said. Both physicians said the future looks promising for even more accurate IOL power calculations. EWAP Editors’ note: Dr. Olsen is the founder of IOL Innovations and has financial interests with Haag-Streit (Koniz, Switzerland). Dr. Savini has no financial interests related to his comments. References 1. Savini G, Hoffer KJ, Casamenti V. Clinical Comparison of Methods to Calculate IOL Power After Myopic LASIK and PRK. Paper presented at: American Society of Cataract & Refractive Surgery. April 2014, Boston. 2. Olsen T, Hoffmann P. C constant: New concept for ray-tracing assisted intraocular lens power calculation. J Cataract Refract Surg. 2014 2014;40:764–773. 3. Olsen T. Use of fellow eye data in the calculation of intraocular lens power for the second eye. Ophthalmology. 2011;118(9):1710–1715. 4. Olsen T, Funding M. Ray-tracing analysis of intraocular lens power in situ. J Cataract Refract Surg. Apr 2012;38(4):641–647. Contact information Olsen: tkolsen@dadlnet.dk Savini: giacomo.savini@alice.it