Original Research

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

Alan Rubin, Solani David Mathebula
African Vision and Eye Health | South African Optometrist: Vol 71, No 4 | a82 | DOI: https://doi.org/10.4102/aveh.v71i4.82 | © 2012 Alan Rubin, Solani David Mathebula | This work is licensed under CC Attribution 4.0
Submitted: 09 December 2012 | Published: 09 December 2012

About the author(s)

Alan Rubin, Department of Optometry, University of Johannesburg, South Africa
Solani David Mathebula, Department of Optometry, University of Limpopo, South Africa

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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 the
analysis 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 curvatures
in  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 (see
above) 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)


Keywords

Dioptric power; curvature; cornea; variance; vector spaces; multivariate statistics

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