Global and local contributions to surface curva- ture of healthy corneas

Alan Rubin,Solani David Mathebula

doi:10.4102/aveh.v71i4.82

Abstract

This paper demonstrates for several healthy eyes the application of a simple model to understanding local and global contributions to short-term variation in anterior and posterior corneal curvature. Multiple axial anterior and posterior corneal radii and central corneal thicknesses for the right eyes of 10 young subjects were determined over time using a rotating Scheimpflug camera (Oculus Pentacam). The axial radii were transformed to corneal powers, and also to curvatures that were referred to a mid-corneal surface such that local and global contributions to short-term variation could be analyzed quantitatively.When variation of the anterior and posterior corneal surfaces of several healthy eyes are studied in terms of curvatures (rather than powers) it is the posterior surfaces that are more variable withthe global or macroscopic rather than local effects dominating. (Harris and Gillan found the same for an eye with mild keratoconus.) This finding is opposite to that when variation is considered in terms of dioptric power where the anterior corneal surface usually appears more variable. Possible reasons for this finding includes firstly that the posterior corneal surface has to be measured through the air-tear interface and anterior corneal surface,and thus some uncertainty in measurements of the posterior surface may relate to this limitation. Secondly, no attempt was made here to mathematically align the multiple surfaces as determined per eye and thus we cannot be certain that precisely the same central corneal region was measured each time.Investigators need to carefully consider whether they are more interested in the optical or physical nature of variation in surfaces such as the cornea since studies of the optical effects require theanalysis to be performed in terms of dioptric powers and symmetric dioptric power space whereas studies of physical variation in the topography of the cornea and the possible reasons for such variability require the application of surface curvaturesin surface curvature space. This paper describes the application and significance of both methods to facilitate understanding of short-term variation of the human cornea. It does not, however, attempt to make any definite claims as to what factors (seeabove) may be major contributors to such variability, and this complicated but interesting research issue requires further clarification. (S Afr Optom 2012 71(4) 146-158)

Highlights

The cornea is an extremely complicated and dynamic optical structure of the human eye
One of the ten subjects was randomly chosen to illustrate typical results as obtained and Figure 1a indicates variation of corneal powers for the anterior and posterior corneal surfaces of the right eye of Subject 8
Keratometric measurements appear in this subject to be more variable for the anterior rather than posterior corneal surface and this was similar to what Gillan found for the variation in corneal powers of his keratoconic subject[6]

Summary

Introduction

The cornea is an extremely complicated and dynamic optical structure of the human eye. Perhaps, the simplest level we can model the optical system of the cornea as a single thin lens in air, and of uniform power and no thickness. A slightly more sophisticated approach would be to consider the cornea as a thick lens in air, having both anterior and posterior surfaces of constant power separated by uniform thickness. In reality the cornea varies both in terms of the separation between its surfaces and in the surface powers and shape or topographies. Such variation is in relation to temporal and to spatial factors or geometric location. As Twa et al[9] emphasize, the interpretation of the cornea via methods such as videokeratography remains a major challenge and they suggest the use of automated models to classify corneal shape

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