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S. Norrby, H.-B. Fam, P. Piers, K.-L. Lim; Corneal Asphericity and Wavefront: Comparison Between Chinese and Caucasians. Invest. Ophthalmol. Vis. Sci. 2007;48(13):3522.
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To investigate if there are any differences between Chinese and Caucasians with regard to corneal topography.
Two data sets were compared. One with both eyes of predomintly Chinese (Singapore) patients (n=304), measured with Orbscan II prior to refractive surgery. Another with one eye of predominatly Caucasian (Sweden) patients (n=71), measured with Orbscan I prior to cataract surgery. The Chinese Orbscan files were analyzed by CTView for Zernike fitting of anterior corneal shape and the resulting wavefront. This data was already available for the Caucasian eyes. Corneal central curvature and asphericity (Q-value) were calculated from the surface Zernike coefficients. Fitting was done for various pupil sizes, but the comparisons in this abstract are limited to 6 mm pupils.
The Chinese data showed no tendency in Q-value with regard to preop refraction (spherical equivalent -5.40 ±2.92 D; mean ±standard deviation) or age (34 ± 8 years). Nevertheless right eyes of patients older than 40 years (mean 45, range 40 to 59) were selected (n=65) for comparison with the older Caucasian patients (mean 72, range 35 to 94). Q for the Chinese was +0.03 ±0.09 and for the Caucasians -0.06 ±0.21. The difference was statistically significant (P<0.002). A normalized Zernike spherical aberration coefficient (C[4,0]) of 0.31 ±0.23 µm was found for the Chinese, compared with 0.27 ±0.20 µm for the Caucasians. This difference was not statistically significant (P=0.31). The only statistically significant difference was in vertical coma (C[3,-1]), which in the Chinese was -0.27 ±0.45 µm, while in the Caucasians it was +0.00 ±0.31 µm. Analyzing the full Chinese data set an increase in C[4,0] with age was found, however not statistically significant. A quite normal central corneal radius of 7.70 ±0.26 mm was found for the Chinese, suggesting their myopia was due to axial length.
The groups compared were quite different in age distribution. The Chinese sought surgery to cure severe myopia, while the Caucasians being normal cataract patients can be assumed close to emmetropia. However, given the absence of trend with age and refraction in the Chinese group it can be reasonably inferred that Chinese in general have slightly oblate corneas, while those of Caucasians are slightly prolate, and that Chinese have considerably more vertical coma than Caucasians.
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