Determination of Optic Axes by Corneal Topography among Italian, Brazilian, and Chinese Populations

Bernardo T Lopes,Riccardo Vinciguerra,Renato Ambrósio,Ahmed Elsheikh,Ashkan Eliasy,Fangjun Bao,Ahmed Abass,Paolo Vinciguerra,Mohamed Elhalwagy

doi:10.3390/photonics8020061

Abstract

This study aims to describe a new universal method to identify the relative three-dimensional directions of visual, pupillary, and optical axes of the eye and the angles between them using topography elevation data. The method was validated in a large clinical cohort, and ethnical differences were recorded. Topography elevation data were collected from 1992 normal eyes of 966 healthy participants in Italy, Brazil, and China. The three main axes were defined as follows: optical axis (OA) was defined as the optimal path of light that passes through the ocular system without refraction. The pupillary axis (PA) line was defined using X and Y coordinates of the pupil centre with the chamber depth, in addition to the centre of a sphere fitted to the central 3 mm diameter of the cornea. The visual axis (VA) was taken by its best approximation, the coaxially sighted corneal light reflex. The alpha angle was measured between the VA and OA, and the kappa angle between the VA and PA. The average values of kappa and alpha angles were 3.41 ± 2.84 and 6.04 ± 2.43 in the Italian population, 2.6 ± 1.53 and 5.87 ± 2.3 in the Brazilian population, and 2.09 ± 1.22 and 3.85 ± 1.48 in the Chinese population.

Highlights

The visual axis is the line that passes through the fixation point, front nodal point, rear nodal point, and fovea of the eye [5], whereas the line that passes through the centre of the entrance pupil while normal to the corneal surface is defined as the pupillary axis
We evaluated the distribution of the alpha and kappa angles in three different ethnic groups, which may play an important role in certain refractive procedures [10]
We proposed and validated the consistency in a big cohort of clinical cases of a new universal method to evaluate the different positions of the ocular axes

Summary

Introduction

Among many other reasons for this non-alignment, the temporal location of the fovea and the relative nasal pupil deviation are of great importance. This leads to the presence of three main distinct axes: the visual, the optical, and the pupillary axes [4]. The visual axis is the line that passes through the fixation point, front nodal point, rear nodal point, and fovea of the eye [5], whereas the line that passes through the centre of the entrance pupil while normal to the corneal surface is defined as the pupillary axis. The optical axis is the line that passes through the central corneal and stays normal to its surfaces [6]. The difference between the visual and the pupillary axes forms the kappa angle (κ), and between the visual and optical axes forms the alpha angle (α) [8]

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Discussion

Conclusion