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K.L. Fry, P.S. Hersh, W. Blaker; Spherical Aberration After LASIK and PRK: A Clinical and Theoretical Analysis . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2618.
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Purpose: To analyze cornea asphericity after laser refractive surgery and to test the predictive efficacy of a newly developed model of corneal asphericity changes following excimer laser surgery for myopia. Methods: The cornea topography of 11 patients (20 eyes) was measured before and after LASIK, LASEK or PRK. Preoperative spherical equivalent refractions ranged from –3.75 up to -10.75 diopters. All maps were analyzed using the CTView 4.0, (Sarver & Associates Inc) to determine cornea asphericity. Measured cornea postoperative asphericity was compared to predicted asphericity values calculated using mathematical models that simulated myopic treatments one of which accounted for possible changes in beam fluence across the ablation region. Results: Following excimer laser procedures, all corneas exhibited a positive central asphericity, changing from a more prolate (negative asphericity) to an oblate optical contour. Preoperative mean asphericity was –0.17 (SD 0.14). Following surgery, corneal asphericity averaged +0.92 (SD 0.70). The mean change in asphericity was +1.09 (SD 0.67) of positive asphericity. The range in asphericity change was +0.40 to +2.73 in the direction of a more oblate corneal profile. A trend toward greater asphericity change and more oblateness was observed among eyes requiring higher correction. Taking into account beam fluence changes across the ablation zone, theoretical calculations of cornea asphericity were highly predictive of actual postoperative asphericity measurements. The percent error of theoretical compared to clinical measurement averaged 22.8% (SD 23.4) in the direction of underestimation of the predicted corneal asphericity. Conclusion: Corneal optics within the ablation zone became more oblate after laser refractive surgery. A model of beam fluence taking angle of incidence across the ablation area into account predicts this asphericity change. Possible changes in laser algorithms, delivering more ablation to the peripheral optic zone, could prevent such change in asphericity. Moreover, wavefront guided ablations may need to account for effects of fluence variability across the optical zone in order to correct spherical as well as other aberrations.
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