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Reply: Intraocular lens power calculation after photorefractive surgery: Modified double-K method

Saiki, Megumi PhD; Negishi, Kazuno MD; Kato, Naoko MD; Ogino, Rika CO; Arai, Hiroyuki MD; Toda, Ikuko MD; Dogru, Murat MD; Tsubota, Kazuo MD

Journal of Cataract & Refractive Surgery: September 2013 - Volume 39 - Issue 9 - p 1451
doi: 10.1016/j.jcrs.2013.07.017
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Dr. Galvis et al. wrote that the coefficient of determination (R2) of 0.73 for the regression equation between the preoperative mean anterior K values in the 3.0 mm zone (Km) and the postoperative mean posterior corneal power in the 6.0 mm zone (K6mm) in 72 post-LASIK eyes does not guarantee that the model fits the data sufficiently well, because a high R2 can occur in the presence of a misspecified functional form of a relationship or in the presence of outliers that distort the true relationship. However, as shown in Figure 1 in our article, the relationship between the 2 variables in our study was linear and included no outliers, meeting the major assumptions for simple linear regression analysis. Therefore, we believe the simple linear regression model is an appropriate functional form for the data and the parameter estimates are unbiased.

Second, we want to confirm that the constants a0, a1, and a2 for the Haigis-L method and the constant SF constant for the Holladay formula recommended by the ULIB web page were entered into the American Society of Cataract and Refractive Surgery IOL power calculator.

Regarding the postoperative Km, I agree with the comment that the Km is no longer valid theoretically once corneal refractive surgery disrupts the physiologic ratio between the anterior and posterior corneal curvatures, and the total corneal refractive power and true net power of the Scheimpflug system should be more valid to use as the postoperative Km in the double-K method.

Actually, we initially tried to establish our method using the total corneal refractive power or the true net power of the Scheimpflug system to avoid erroneous corneal powers, and we explored appropriate diameters and patterns (zone or ring) of the analysis area. However, the results with the total corneal refractive power or the true net power did not exceed the results with the Km. We ultimately adopted the Km as the postoperative K in our method. Why the Km achieved the best results needs to be clarified in future studies. Considering the corneal topography, we assumed that our study patients who had LASIK included cases with various depths of ablations ranging from shallow to deep. Therefore, we believe that the best results achieved using the Km did not result from bias in the ablation depths among the patients. Further investigation of a larger sample size is necessary to confirm which parameter is best, and we continue to explore an appropriate keratometric value using our method.

© 2013 by Lippincott Williams & Wilkins, Inc.