REGULAR AND IRREGULAR REFRACTIVE POWERS OF THE FRONT AND BACK SURFACES OF THE CORNEA

The refractive status of the posterior corneal surface has not been we ll studied. The purpose of this study is to quantitatively evaluate th e regular and irregular astigmatism of the posterior corneal surface. In 50 normal human eyes, topography of the anterior and posterior corn eal surface was measured with the scanning-slit videokeratoscope. Usin g Fourier series harmonic analysis, dioptric data on a mire ring were decomposed into spherical component, regular astigmatism, asymmetry (t ilt or decentration), and higher order irregularity, The obtained valu es for the central 3.0 mm of the posterior corneal surface were -6.55/-0.32 D (spherical component), 0.18+/-0.16 D (regular astigmatism), 0 .40+/-0.22 D (asymmetry), and 0.02 +/- 0.02 D (higher order irregulari ty). The posterior to anterior ratios of these parameters were 13.6 +/ - 0.6%, 35.0 +/- 41.3%, 45.8 +/- 56.9%, and 39.9 +/- 39.8%, respective ly. The ratio of the spherical component was significantly lower than the other three parameters (P < 0.001, Wilcoxon signed-rank test), ind icating that non-spherical components (regular and irregular astigmati sm) of the posterior corneal surface are not negligible in the optical quality of the cornea. The current results can serve as the control d ata and reference for the future clinical studies of optical character istics of the cornea as a whole. (C) 1998 Academic Press.