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T. Nakagawa, N. Maeda, Y. Hori, T. Inoue, M. Saika, Y. Hirohara, T. Mihashi, Y. Tano; Higher-Order Aberrations From Posterior Corneal Surface in Patients With Keratoconus. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1032.
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To measure the higher-order aberrations (HOA) caused by the posterior corneal surface in patients with keratoconus and to compare it to that in normal eyes.
Twenty-eight keratoconic eyes and 24 normal control eyes were studied. The anterior and posterior corneal elevation and pachymetric data were obtained with a rotating Scheimpflug camera. (Pentacam Oculus, Wetzlar.) Higher-order aberrations for 6-mm pupils were calculated separately for the anterior and posterior corneal surfaces from the differences between the elevation data and the best-fit sphere. The refractive index of the cornea and aqueous humor were 1.376 and 1.336, respectively. The reference axes of the measurements were aligned with the primary line of sight. The magnitudes, axes of trefoil, and coma were calculated by vector analysis.
The mean corneal total HOAs, trefoil, coma, and spherical aberration due to anterior/posterior surface (root mean square, µm) were significantly higher in the keratoconic eyes (4.38/1.10, 0.775/0.194, 3.57/0.857, and 1.16/0.344) than in normal control eyes (0.461/0.146, 0.090/0.048, 0.337/0.076, and 0.090/0.037; Mann-Whitney Rank Sum test, P ≤0.001), respectively. The mean axes of the coma caused by the anterior surface (81.2 degrees) and the posterior surface (253.2 degrees) were in opposite directions.
The corneal higher-order aberrations in keratoconic eyes were higher than in control eyes not only on the anterior surface but also on the posterior surface. The coma caused by the anterior corneal surface is compensated for, in part, by the coma due to the posterior corneal surface. The residual irregular astigmatism in keratoconic eyes wearing rigid gas-permeable contact lens can be estimated with the direct measurement of the HOA due to posterior corneal surface.
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