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Claudio Macaluso, Alberto Neri, Patrizia Scaroni, Rosachiara Leaci, Marianna Malori, Elisabetta Delfini; Real Corneal Power Measurements with 3D Anterior-Segment Swept-Source Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2012;53(14):3611.
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To evaluate the accuracy and repeatability of real corneal power (Pr) measurements obtained by direct evaluation of anterior and posterior corneal curvature with swept-source anterior-segment OCT (AS-OCT) in normal and keratoconic patients, and to compare it with keratometric power (Pk), measured with the same instrument.
25 normal (N) subjects and 25 keratoconus (KC) subjects were examined in their right eye with a swept-source OCT dedicated to anterior segment examination (Casia SS-1000, Tomey Co., Japan), with a fully automated topography routine that derives curvature data by fitting OCT data scans along 16 meridians. Examinations were carried out in the same session twice, in order to assess repeatability. Pk was obtained using keratometric index=1.375; Pr was obtained using refractive indices of Gullstrand eye model and the thick lens formula of Gaussian optics. Statistical analysis was performed with SPSS 18.0.
Pr measurements (mean±SD) were 42.83±1.30D (diopters) and 47.28±3.06D, and Pk measurements were 43.86±1.32D and 48.63±3.58D, respectively in N and KC subjects. Repeatability (ICC) of Pr measurements was 0.995 in N and 0.989 in KC; Coefficient of Repeatability (CR=1.96*SD of test-retest differences) was 0.26D in N and 0.89D in KC. Correlation (ICC) between Pr and Pk measurements was 0.757 in N and 0.877 in KC (see figure). Pr measurements resulted significantly lower than corresponding Pk measurements both in N and in KC, with larger difference in KC (p<0.001). In accordance with these results, the measured ratio between anterior and posterior corneal curvature radius was 0.844 in N and 0.789 in KC eyes. Both results are significantly inferior to the 0.883 ratio of the Gullstrand model eye (p<0.001). Moreover, Bland-Altman plots showed that in KC eyes the difference between Pr and Pk measurements was proportional to the degree of corneal ectasia (r=0.587, p<0.01) (see figure).
Real corneal power measurements entirely derived from AS-OCT evaluation of both anterior and posterior corneal curvature showed excellent repeatabilty. They deviated slightly but significantly from keratometric power, and in KC the deviation was proportional to the degree of corneal ectasia.
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