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Yusuke Inoue, Yuta Ueno, Takahiro Hiraoka, Fumiki Okamoto, Tetsuro Oshika; Quantitative evaluation of refractive components of posterior corneal surface using Fourier analysis: an attempt to determine a normal range of values for each Fourier index. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1623.
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© ARVO (1962-2015); The Authors (2016-present)
Fourier analysis has been used to quantify irregular astigmatism. Although a normal range of values has already been determined for the anterior surface of the cornea, it has not been established for the posterior surface yet. This study aims to determine the normal distribution of each refractive component of the posterior corneal surface by Fourier analysis using anterior segment OCT.
Three hundred normal eyes were enrolled in the study. The anterior/posterior corneal height data were obtained with an anterior segment Fourier-domain OCT (CASIA, TOMEY, Nagoya, Japan). Four components of the posterior corneal surface, such as spherical power, regular astigmatism, asymmetry and higher order irregularity, were calculated from the obtained data using Fourier analysis. A normal range of values was defined as the mean ± 2SD for each component. Moreover, 40 eyes with astigmatism higher than -2.00D, 50 eyes with keratoconus, 25 eyes submitted to penetrating keratoplasty, and 12 eyes submitted to DSAEK were also evaluated, and the ratio of those included within the normal range for each studied component was calculated.
In normal eyes, the normal range of posterior corneal surface values was -6.60 ~-5.70D for spherical power, 0 ~ 0.27D for regular astigmatism, 0 ~ 0.10D for asymmetry, and 0 ~ 0.03D for higher order irregularity. In pathologic corneas, the percentage of eyes included within the normal range for spherical power, regular astigmatism, asymmetry and higher order irregularity was respectively 83, 55, 90, 93 % in the high astigmatism group, 4, 8, 0, 2 %, in the keratoconus group, 25, 12, 4, 0 %, in the penetrating keratoplasty group, and 17, 42, 0, 8%, in the DSAEK group.
We determined, for the first time, a normal range of values for the refractive components of the posterior corneal surface using Fourier analysis. These numbers could be used for comparison purposes with pathologic corneas. Also, the ratio of keratoconic eyes showing values within the normal range was extremely low for all components in the posterior corneal surface, suggesting these measurements could be useful in the diagnosis of this pathology.
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