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G.-M. G. Dai; Effects on Refractive Surgical Treatment Targets by the Variable Refractive Index of the Human Cornea. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4198.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the potential effect on the variability of the refractive index of the human cornea as a function of ablation depth during a laser refractive surgery
It is well known that the refractive index of the cornea is higher on the anterior surface than on the posterior surface, ranging from 1.380 to about 1.373, with a rate of change of about 0.000015 per µm tissue depth. A model has been built to consider ablation targets as an ensemble of many differential layers where the refractive index within a layer is considered as constant. The ablation targets are originally given in optical path difference (OPD) and the depth of each differential layer is converted to tissue depth. The differential layers in tissue depth are recombined to obtain an accurate representation of the treatment target. This target is then compared to the treatment target using a commonly-used fixed refractive index of 1.377. The error is measured as the difference between the two treatment targets.
Three groups of input data were used in the simulation: (1) each of the Zernike coefficients excluding piston and tilt with one micron wavefront root mean square (RMS) in OPD; (2) eight low-order-only wavefronts with refractions ranging from -15 D to +6 D that covers spherical myopia, hyperopia, myopic and hyperopic astigmatism and mixed astigmatism; (3) five wavefronts with normal high order aberrations (HOAs) and little low order aberrations. For all three groups, very little residual refractions are induced. However, high order aberrations, measured as RMS and peak-to-valley (PV), are not negligible. The RMS values for the three groups are 0.042±0.004, 0.090±0.047, and 0.044±0.006 µm, respectively. For PV, they are 0.138±0.006, 0.268±0.052, and 0.134±0.003 µm, respectively. The RMS errors as a percentage of the input wavefront RMS are approximately 5%, 1%, and 10%, respectively. Therefore, it is expected to contribute to the induction of HOAs after refractive surgery, whether it is conventional or wavefront-driven
Neglecting the variability of refractive index of the cornea in designing treatment targets in refractive surgery appears to be a contributing factor for the induction of HOAs in vision correction, especially in wavefront-guided surgeries
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