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T. Kawamorita, H. Uozato, Y. Iida, K. Shimizu; Effect of Aging and Corneal Shape on Individual Effective Refractive Index of Human Cornea. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3525.
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To compare estimated K value regarded as a single refractive surface with the effective refractive index 1.3375 to True Net power (TNP) calculated in consideration of posterior corneal surface and corneal thickness, and whether individual effective refractive index depends on aging and corneal shape or not.
Ninety eyes from 90 subjects (mean age 38.8±21.3) with no known abnormalities were included in the study. Measurements of shape of the cornea were performed with Pentacam device (Oculus, Germany), which is comprised of a rotating Scheimpflug camera and short-wavelength slit light. As the refractive index of cornea, aqueous humor and air, we used 1.376, 1.336, and 1.000, respectively. Also, multiple regression was performed to analyze the impact on the individual effective refractive index. The independent variables investigated were aging, anterior corneal curvature, corneal thickness, and corneal astigmatism. Statistical significance was defined as P<.05.
The average estimated K value for the central cornea 3.0 mm (42.94±1.45D) were significantly larger than the average TNP (41.77±1.44D) (P<.001, Wilcoxon Signed Rank Test).The multiple regression of analysis of variables showed that the individual effective refractive index was related to aging and corneal astimgmatism, and corneal thickness(P=.003, P=.002, P=.012, respectively). However, it was not related to corneal curvature (P=.076).
The corneal refractive power estimated with the conventional effective refractive index 1.3375 was overestimated approximately 1.2 D in the central corneal power. Our results suggest that individual effective refractive index depends on aging, corneal astigmatism, corneal thickness significantly, impacting slightly on the precision of postoperative refractive error in cataract surgery.
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