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F Manns, A Ho, W Culbertson, J-M Parel; Calculation Of Tolerances For Wavefront-guided Corneal Ablations . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2051.
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Purpose:To calculate the ablation precision required to produce a controlled postoperative ocular aberration state after wavefront-guided corneal reshaping. Methods:The Navarro eye model with aspheric surfaces (Navarro et al, 1985) was used for the calculations. The anterior corneal surface of the Navarro eye model has a radius of curvature of Ri=7.72 mm and an asphericity factor of Qi=-0.26. The eye model was rendered myopic, with 0 to 10D of myopia, by shifting the position of the retinal plane. The corneal radius of curvature (Rf) required to correct defocus and the corneal asphericity (Qf) required to produce a postoperative eye with zero spherical aberration were calculated. Assuming Ri=7.72 mm and Qi=-0.26, the ablation profile required to produce a cornea with radius of curvature Rf and asphericity Qf was calculated for a 6 mm diameter ablation zone. The variation of the postoperative ocular spherical aberration coefficient as a function of ablation depth was then calculated. Results:The postoperative corneal asphericity required to correct spherical aberration was found to vary from -0.45 to -0.47 for corrections of myopia of up to 10 D. For a 6 mm diameter ablation zone, a precision of 0.3 µm per diopter of correction is required at the center of the ablation zone to produce a tolerance of one-quarter wavelength on the spherical aberration coefficient. Conclusion:Precise control of ocular aberrations after wavefront-guided corneal reshaping requires submicron precision in the postoperative corneal shape, which is produced by the ablation itself, and by factors such as stromal remodeling, hydration changes during ablation and biomechanical effects. Supported in part by Quantel Medical, SA, France; Australian Cooperative Research Centre (CRC) Scheme for Eye Research and Technology, Sydney ; Henri and Flore Lesieur Foundation; Florida Lions Eye Bank; and Research to Prevent Blindness, New York, NY.
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