Ocular Aberrations Before and After Myopic Corneal Refractive Surgery

Abstract

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. Although, the optical surfaces are aligned almost co-axially, the deviations from a perfect optical alignment results in a range of optical and neural axes and their inter relationships. The sharpest vision of a target is realized when it is in line with the fixation target and the fovea of the retina (visual axis). Displacing the pupil or the target object from this axis results in reducing the optical and visual properties of the system. Refractive surgery has become a popular procedure to treat refractive errors, alternative to conventional spectacle or contact lens wear. The development of new surgical techniques and the improvements in the laser systems have run parallel to a tremendous increase of the number of patients undergoing refractive surgery. Narrow-beam (flying-spot) laser systems and the incorporation of eye trackers for compensation of eye movements during surgery have lead to better results and to a higher degree of satisfaction by the patients (although complaints of seeing glare and halos at night are not uncommon). Routine postoperative evaluation consists mainly of measuring, under photopic conditions (small pupil sizes), subjective refraction and visual acuity. Some visual acuity loss, particularly for