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Letter

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Roberts, Cynthia J. PhD; Dupps, William J. Jr. MD, PhD

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Journal of Cataract & Refractive Surgery: November 2014 - Volume 40 - Issue 11 - p 1942-1943
doi: 10.1016/j.jcrs.2014.09.024
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We appreciate the interest and response to our editorial. Although we agree that LASIK is safe and postoperative changes to the posterior surface are small, we disagree that the changes that occur are clinically irrelevant. Their relevance lies in the ability to recognize the tomographic signs of ectasia and differentiate them from the normal postoperative appearance. The posterior surface changes demonstrated by the authors’ data with a paired analysis decreased postoperatively from 1 week to 1 month. Conversely, changes characteristic of ectasia would increase over time. Clinically, postoperative tomography is often not done unless the patient has a visual complaint. The data from this study demonstrate that comparing a single postoperative timepoint to preoperative data would not allow a definitive diagnosis of ectasia as the early postoperative posterior surface is distinct from the preoperative surface. An increasing postoperative posterior change would be required to diagnose ectasia. It is critical to recognize normal changes in the posterior surface, which decrease with time, in order to recognize abnormal changes that signify evolving ectasia, which increase with time.

The authors claim that their conclusions from their original statistical analysis did not change. However, the change between a nonsignificant difference with unpaired statistical analysis and a significant difference with a paired analysis will clearly affect the conclusions. The authors originally concluded that no changes to the posterior surface occurred, which was altered with a paired analysis. The authors claim that overlapped points, which would not reproduce with scanning the original figure, might affect the analysis. However, even if points overlapped in the published figures, the point distribution is clearly below the zero difference line. Overlapped points would not alter the final result. In addition, we would be happy to reanalyze the original data, rather than scanning and reproducing the figures, if the authors would agree to share their data.

Additional references are provided by the authors to support their claims. However, the Hashemi and Mehravaran study1 reported results from only 23 subjects, which is likely underpowered to detect small changes in the posterior surface.2 Ciolini and Belin3 investigated a larger dataset but reported a mean difference between preoperative and postoperative changes in the posterior surface without a statistical analysis, so the differences in the posterior surface that were measured are difficult to interpret. Both references reported a statistically significant difference between the 2 devices used to make the measurements. The authors also suggest that the statistically significant posterior surface steepening reported by Smadja et al.,4 which occurred in proportion to the amount of anterior tissue disruption during myopic LASIK and regressed during the 1 to 3 month postoperative interval, is “minute and clinically irrelevant.” We disagree with this characterization of the conclusions since the study demonstrates with yet another imaging technology the tendency of photoablative severing of stromal lamellae to produce peripheral corneal thickening and central posterior surface steepening in the early postoperative period.

In conclusion, consistent changes to the posterior surface after refractive surgery have been reported with multiple tomographic devices, although the magnitude of the changes differs by device. These changes decrease over time and are associated with a normal postoperative outcome of a safe procedure. They are also consistent with our proposed biomechanical response to refractive surgery.5

References

1. Hashemi H, Mehravaran S. Corneal changes after laser refractive surgery for myopia: comparison of Orbscan II and Pentacam findings. J Cataract Refract Surg. 2007;33:841-847.
2. Wilhelmus KR. Beyond the P. III: possible insignificance of the nonsignificant P value. J Cataract Refract Surg. 2004;30:2425-2426.
3. Ciolino JB, Belin MW. Changes in the posterior cornea after laser in situ keratomileusis and photorefractive keratectomy. J Cataract Refract Surg. 2006;32:1426-1431.
4. Smadja D, Santhiago MR, Mello GR, Roberts CJ, Dupps WJ Jr, Krueger RR. Response of the posterior corneal surface to myopic laser in situ keratomileusis with different ablation depths. J Cataract Refract Surg. 2012;38:1222-1231.
5. Dupps WJ Jr, Roberts C. Effect of acute biomechanical changes on corneal curvature after photokeratectomy. J Refract Surg. 2001;17:658-669.
© 2014 by Lippincott Williams & Wilkins, Inc.