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F. Manns, Y. Lee, J.–M. Parel; Comparison Of Gaussian, Keratometric And Clinical History Formulas For The Calculation Of Corneal Power Before And After Refractive Surgery In An Optical Model Of The Cornea . Invest. Ophthalmol. Vis. Sci. 2006;47(13):571.
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To evaluate the theoretical accuracy of keratometry and the clinical history method for the calculation of corneal power before and after laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK).
A paraxial optical model of the cornea with two spherical surfaces was used for the analysis. The model uses the same refractive indices and central thickness as the Gullstrand model (nstroma=1.376, naqueous=1.336). The value of the anterior radius was varied from 7.7 to 10.21 mm to simulate a cornea before and after LASIK or PRK for myopia ranging from 0 to 12D. The radius of the posterior surface was calculated by multiplying the preoperative anterior radius by a factor, k, equal to 0.81 (Dubbelman et al, 2001) or 0.889 (Gullstrand model). Four formulas were used to calculate the pre– and postoperative corneal power: the Gaussian optics formula, keratometric formulas with indices of 1.3375 (Javal) and 1.332 (Hartinger), and the clinical history formula. The keratometric and clinical history results were compared to the values obtained with the Gaussian optics formula, which represents the true corneal power of our model.
Keratometric formulas overestimate the corneal power and underestimate the change in corneal power after refractive surgery. To obtain the true corneal power, keratometric powers must be multiplied by 1.114–0.118/k (Javal) and 1.132–0.12/k (Hartinger), where k is the ratio of the posterior to anterior corneal radius of curvature. The clinical history method also overestimates the postoperative power of the cornea, by an amount corresponding to 2% of the preoperative keratometry (0.78D) when Ra=7.7mm and k=0.883 (Gullstrand model), independent on the amount of correction.
There are inherent errors in the formulas corresponding to the clinical methods used for the calculation of postoperative corneal power. To obtain the true postoperative corneal power, the keratometry values used in the clinical history formula must be multiplied by a correction factor that depends on the ratio of the posterior to anterior corneal radius of curvature.
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