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 well studied. The purpose of this study is to quantitatively evaluate the regular and irregular astigmatism of the posterior corneal surface. In 50 normal human eyes, topography of the anterior and posterior corneal surface was measured with the scanning-slit videokeratoscope. Using Fourier series harmonic analysis, dioptric data on a mire ring were decomposed into spherical component, regular astigmatism, asymmetry (tilt or decentration), and higher order irregularity. The obtained values 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 irregularity). 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%, respectively. The ratio of the spherical component was significantly lower than the other three parameters (P<0.001, Wilcoxon signed-rank test), indicating that non-spherical components (regular and irregular astigmatism) of the posterior corneal surface are not negligible in the optical quality of the cornea. The current results can serve as the control data and reference for the future clinical studies of optical characteristics of the cornea as a whole.