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L. Wang, D. D. Koch, A. M. Mahmoud, C. J. Roberts; Total Corneal Power Estimation by Ray Tracing vs. Gaussian Optics. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5101.
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© ARVO (1962-2015); The Authors (2016-present)
In normal corneas and corneas with prior LASIK/PRK, to evaluate the differences in total corneal powers calculated using ray tracing method and Gaussian optics formula based on topographic data.
With the ray tracing method, the total corneal power (TCP) is ray-traced through the anterior and posterior surfaces using Snell’s law, while the Gaussian formula calculates Gaussian equivalent power (GEP) of the central cornea by combining two lenses (anterior and posterior corneal surfaces) separated by the corneal thickness and assuming paraxial imaging. With the Galilei Dual Scheimpflug Analyzer (Ziemer), in 94 normal eyes, 61 eyes following wavefront-guided myopic LASIK/PRK, and 9 eyes following hyperopic LASIK/PRK, TCP was calculated over the central 0-4 mm zone. GEP was calculated using the central average of instantaneous curvature (0-4 mm). Differences between TCP and GEP (TCP - GEP) were calculated. Correlation analysis was performed.
The mean differences between TCP and GEP were +0.61 ± 0.32 D (range -0.25 to +1.29 D) in myopic LASIK/PRK eyes, +0.83 ± 0.21 D (range +0.36 to + 2.05 D) in normal eyes, and +1.41 ± 0.50 D (range +0.47 to +2.07 D) in hyperopic LASIK/PRK eyes (all P<0.05). The differences decreased with increasing anterior instantaneous radii of curvature, and increased with increasing ratio of posterior/anterior radius of curvature (all P<0.05).
Differences of more than 0.60 D were found between the total corneal power calculated with the ray tracing method and the Gaussian Equivalent Power, calculated with the thick lens formula. We believe that these findings are due to the use of posterior power from topographic data, which does not take into account that the incoming rays have been refracted by the anterior surface.
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