title: Centration axis in refractive surgery.
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.
authors.affiliation: SCHWIND eye-tech-solutions, Kleinostheim, Germany ; Recognized Research Group in Optical Diagnostic Techniques, University of Valladolid, Valladolid, Spain ; Department of Ophthalmology and Sciences of Vision, University of Oviedo, Oviedo, Spain.
authors.forename: Samuel
authors.initials: S
authors.lastname: Arba Mosquera
authors.affiliation: SCHWIND eye-tech-solutions, Kleinostheim, Germany.
authors.forename: Shwetabh
authors.initials: S
authors.lastname: Verma
authors.affiliation: Flinders University, Adelaide, South Australia Australia ; Wenzhou Medical University, Wenzhou, Zhejiang China.
authors.forename: Colm
authors.initials: C
authors.lastname: McAlinden
keywords: Alpha; Angle Kappa; Asymmetric offset; Centration; Corneal light reflex; Corneal vertex; Lambda; Line of sight; Neural axis; Optical axis; Pupil centration; Refractive surgery