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DZ Reinstein, S Srivannaboon, RH Silverman, HF S Sutton, DJ Coleman; Limits of Wavefront Guided Customized Ablation: Biomechanical and Epithelial Factors . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3942.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: To determine to what extent epithelial and biomechanical changes in LASIK may affect the accuracy of LASIK and hence wavefront-based customized corneal ablation. Methods: 52 routine LASIK cases (-1.00 to-10.25) were analyzed by 3D very high-frequency digital ultrasound (VHFU) scanning (1-micron precision) and Orbscan pre- and at least 3-month post-op. Epithelial thickness (ET) and residual stromal bed thickness (RST) 3D maps were produced from the VHFU data. Anterior and posterior corneal best-fit sphere to the central 3-mm zone (BFS) were obtained by Orbscan. The curvature of Bowman's surface was calculated from the anterior BFS and the epithelial thickness profile. Gradient optics and lens formulae were used to calculate total corneal power from anterior, Bowman's and posterior corneal interfaces. Back surface curvature change was treated as the bioelastic or "bowing" factor. Curvature change of the anterior corneal surface was dissociated into epithelial and "bowing" components. Corneal power change (CPC) from pre- to post-op was calculated with permutations removing "bowing", epithelial changes or both. Linear regression and paired t-tests were used to determine significant epithelial and/or "bowing" contributions to the final refraction. Results: The post-operative refractive error was correlated to the amount of measured epithelial power shift (p<0.05, R=0.29) with average 0.25D of epithelial power shift per diopter of refractive error post-op. Post-operative refractive error was highly correlated with borderline significance to residual stromal thickness (RST) (p=0.056, R=0.7) for cases with RST below 290-microns. Fifteen percent of the flattening produced by ablation was lost to "bowing" of the cornea. Five percent of flattening produced by ablation was lost to epithelial profile changes. Conclusion: Notable biomechanical and epithelial effects were demonstrated. Biomechanical and epithelial effects may need to be accounted for to accurately achieve desired curvature changes in the cornea with LASIK.
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