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Abstract

We thank Dr Lanza for his comments on our article in which we described the use of optical coherence tomography (OCT) to accurately measure corneal refractive power change after laser refractive surgery. As Dr Lanza notes, it has previously been shown that using automated or simulated keratometry, there is an underestimation of corneal power change using these keratometric techniques after myopic laser refractive surgery with the error increasing as the myopic correction increased.1Hugger P. Kohnen T. La Rosa F.A. et al.Comparison of changes in manifest refraction and corneal power after photorefractive keratectomy.Am J Ophthalmol. 2000; 129: 68-75Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar, 2Rosa N. Capasso L. Lanza M. et al.Reliability of the IOLMaster in measuring corneal power changes after photorefractive keratectomy.J Cataract Refract Surg. 2004; 30: 409-413Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar, 3Rosa N. Furgiuele D. Lanza M. et al.Correlation of changes in refraction and corneal topography after photorefractive keratectomy.J Refract Surg. 2004; 20: 478-483PubMed Google Scholar In the hyperopic population, the error was more variable depending on the amount of correction and duration of follow-up.4Rosa N. De Bernardo D. Borrelli M. et al.Reliability of the IOLMaster in measuring corneal power changes after hyperopic photorefractive keratectomy.J Refract Surg. 2011; 27: 293-298Crossref PubMed Scopus (17) Google Scholar In our study, we did not segregate these different refraction populations. However, for the purpose of this discussion, we have reformatted our myopic data to reflect the linear regression analysis used in the previously described keratometric studies. In our myopic population, the simulated keratometry data (Fig 1, top) is similar to the previously reported regression of y = 0.7495x − 0.6127 with underestimation of the corneal power change (slope < 1).2Rosa N. Capasso L. Lanza M. et al.Reliability of the IOLMaster in measuring corneal power changes after photorefractive keratectomy.J Cataract Refract Surg. 2004; 30: 409-413Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar With OCT (Fig 1, bottom), the linear regression slope is nearly 1, which is much closer to a 1:1 relationship between the values measured by OCT and the change in refraction. This correlation could potentially be further improved with continued development of the technology. Because there were only 2 hyperopic subjects in our study, we did not reformat those data. More subjects undergoing hyperopic correction would help verify the performance of OCT for those corrections. Although the preoperative refractive status could be inferred from the reported refractive changes, our total study population had a preoperative mean spherical equivalent manifest refraction, vertexed to the corneal plane, of −3.63±2.22 diopters with a range of 2.84 to −7.09 diopters. Overall, we think that Figure 1 further illustrates the benefit of using OCT over keratometric techniques that rely solely on measurements of only the anterior cornea. Optical coherence tomography can properly measure and integrate the refractive contributions of the posterior corneal surface and corneal thickness with the resultant improvement in measuring corneal power change from laser refractive surgery. Thus, in contrast to Dr Lanza's statement, we believe that the conclusions of the article are clear, are expected to be of great practical use, and can be the guiding method for future studies. Re: McNabb et al.: Optical coherence tomography accurately measures corneal power change from laser refractive surgery (Ophthalmology 2015;122:677-86)OphthalmologyVol. 123Issue 1PreviewI read with interest the article by McNabb et al1 regarding the measurement of corneal power in eyes that have undergone refractive surgery. One of the study's limitations was that myopic and hyperopic eyes were analyzed in a single group. It is known that after refractive surgery there is an overestimation of the corneal power in the eyes that had surgery for myopia,2,3 and there is an underestimation of the power of the cornea in eyes that had surgery for hyperopia.4 Evaluating the overall differences in corneal power for both refractive defects may introduce bias in assessing the accuracy of the variations before and after surgery. Full-Text PDF