Abstract
The human eye is an asymmetric optical system and the real cornea is not a rotationally symmetrical volume. Each optical element in the eye has its own optical and neural axes. Defining the optimum center for laser ablation is difficult with many available approaches. We explain the various centration approaches (based on these reference axes) in refractive surgery and review their clinical outcomes. The line-of-sight (LOS) (the line joining the entrance pupil center with the fixation point) is often the recommended reference axis for representing wavefront aberrations of the whole eye (derived from the definition of chief ray in geometrical optics); however pupil centration can be unstable and change with the pupil size. The corneal vertex (CV) represents a stable preferable morphologic reference which is the best approximate for alignment to the visual axis. However, the corneal light reflex can be considered as non-constant, but dependent on the direction of gaze of the eye with respect to the light source. A compromise between the pupil and CV centered ablations is seen in the form of an asymmetric offset where the manifest refraction is referenced to the CV while the higher order aberrations are referenced to the pupil center. There is a need for a flexible choice of centration in excimer laser systems to design customized and non-customized treatments optimally.
Highlights
The human eye is an optical system comprising four main non coaxial optical elements, an aperture stop and an imaging film in the form of a light sensitive tissue layer called the retina, but conforming a robust aplanatic design compensating the spherical aberrations and coma through non-planar geometry
Defining the optimum center for laser ablation is difficult with many available approaches, each of them claiming to provide good results
The problem comes from the fact that the real cornea is not a rotationally symmetrical volume, and the human eye is an asymmetrical optical system [85]
Summary
The human eye is an optical system comprising four main non coaxial optical elements (anterior and posterior corneal and lens surfaces), an aperture stop (pupil) and an imaging film in the form of a light sensitive tissue layer called the retina, but conforming a robust aplanatic design compensating the spherical aberrations and coma through non-planar geometry. They found an error of 0.50.8 mm when referencing the visual axis, which probably arose from the use of corneal light reflex as a sighting point or from inadvertent monocular sighting in techniques requiring binocular sighting They explained that for an ideal centration, the patient should fixate at a point that is coaxial with the surgeon’s sighting eye and the cornea is marked with the center of the patient’s entrance pupil ignoring the corneal light reflex. They concluded that for the best optical results, the procedure must be centered on the LOS and the entrance pupil of the eye.
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