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M. Shirayama, L. Wang, D. D. Koch; Ratio of Posterior and Anterior Corneal Radii of Curvature in Eyes With Radial Keratotomy. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4221.
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© ARVO (1962-2015); The Authors (2016-present)
To assess the ratios of posterior and anterior corneal radii of curvatures measured with the Galilei Dual-Scheimpflug corneal analyzer in patients after radial keratotomy (RK) and in normal subjects.
Retrospectively, twenty-six eyes of 16 patients with previous RK, and 26 eyes of 24 normal subjects were reviewed in this study. The average anterior and posterior corneal radii of curvatures of 0-, 1-, 2-, 3-, 4-, 5-, and 6-mm annular zones from the Galilei instantaneous maps were obtained in each subject. The ratios between posterior and anterior corneal radii of curvatures in both groups were calculated and were compared between groups.
In eyes with previous RK, the average radii of curvature on the anterior corneal surface were 9.17, 9.10, 8.84, 8.46, 8.01, 7.53, and 7.09 mm for 0-, 1-, 2-, 3-, 4-, 5-, and 6-mm annular zones respectively, while the average radii of curvature on the posterior corneal surface were 8.22, 8.04, 7.50, 6.90, 6.36, 5.96, and 5.75 mm for respective annular zones. The ratio values of posterior and anterior radii of curvature were 0.889, 0.879, 0.848, 0.816, 0.794, and 0.790, and 0.808 for 0-, 1-, 2-, 3-, 4-, 5-, and 6-mm annular zones respectively. In normal population, the posterior / anterior ratio was 0.818, 0.820, 0.831, 0.849, 0.874, 0.906, and 0.943 for respective annular zones. Compared to normal virgin corneas, the posterior / anterior ratio was significantly greater in post-RK corneas for the 0- and 1-mm annular zones and significantly smaller for the 4- to 6-mm annular zones (all P<0.05 with Bonferroni correction). Compared to normal subjects, the anterior surface of post-RK corneas flattened by 17%, 16%, 13%, 8%, 2%, -6%, and -14% for the respective annular zones; in contrast, the posterior surface flattened by 28%, 25%, 16%, 4%, -7%, 17%, and 25%.
Although the corneas with previous RK were flattened in the central zones of both the anterior and posterior surfaces, the ratio of posterior to anterior radii of curvature was found to be greater in the central zone and smaller in the paracentral zone, compared to normal subjects. This indicates that, in the RK cornea, the anterior and posterior surfaces flatten by different amounts in different zones. This could have important implications for calculating corneal refractive power in post-RK eyes.
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