Geometrical correspondence between first and second corneal surface

Abstract

The cornea is the principal refracting element and contributor to the aberrations in the human, so an accurate description of the anterior and the posterior corneal surfaces is needed in order to model the eye as an optical system. Some eye models treat the cornea as one single refracting surface, i.e. a diopter which separates air (n = 1) from the aqueous humour with an equivalent keratometric index typically established in 1.3375. Some studies have stated that this index is too high and results in inaccurate measures of corneal optical power. Moreover, many ophthalmologic interventions on the cornea like ortho-keratology or corneal ablation, needs precise information about the second surface and the one surface description results inaccurate. Most work has been done in analyzing the second corneal surface geometry and central and peripheral corneal thickness. In some studies high correlation between central curvature radii and asphericities between the two corneal surfaces has been stated. In this work we extend this study to the whole corneal surface. Availability of commercial rotating Scheimplug cameras allow obtaining point to point pachymetry of the cornea and thus analyzing the second corneal surface and its correlation to the first surface. We have analyzed and compared the geometry of both corneal surfaces. We have used 42 eyes from 42 subjects measured with Oculus Pentacam. Corneal heights of both surfaces have been decomposed in low order Zernike polynomials and we have studied correlation between the different components. Our study shows that there is not only a strong correlation between curvature radii, but also between astigmatic components. Strong correlation between both corneal sides in healthy eyes can be explained by biomechanical stability of the cornea and can be of the utmost importance in planning optical surgery. If this correlation is not taken into account for astigmatic corrections, ablating the anterior cornea can break the equilibrium in the cornea structure.