Refractive Surgery Feature Story

SEPTEMBER 2010 CATARACT & REFRACTIVE SURGERY TODAY EUROPE 35 T he introduction of wavefront-sensing techniques to ophthalmology has redefined the clinical meaning of refractive error. Now, all imperfections in the components and materials within the eye that cause light rays to deviate from their desired path are referred to as optical aberrations. Lower-order aberrations (defocus and astigmatism) account for 90% of overall wave aberrations in the eye. Currently, lower-order aberrations can be well corrected with spectacles, contact lenses, or excimer laser surgery. Although higher-order aberrations make a smaller contribution to the eye’s total aberrations, their correction can improve visual performance substantially. The purpose of this article is to review methods of measuring wave aberrations in the eye and to illustrate the objective metrics that describe both optical and image quality. Aberrometers allow detailed measurements of the eye’s overall wave aberrations, as well as accurate measurements of the distribution and contribution of each higher-order aberration. However, the former metrics used to describe these aberrations, chiefly the Zernike polynomials, do not suitably depict visual quality because they do not directly relate to retinal image quality. New objective optical quality metrics could improve our ability to interpret the eye’s aberrations. There are a variety of approaches for the description of complex optical performance, including objective metrics that quantify the quality of the optical wavefront in the plane of the pupil (ie, pupil-plane metrics) and others that quantify the quality of the retinal image (ie, image-plane metrics). These metrics are derived from the wave aberration of the individual eye, as measured by corneal or ocular wavefront sensors.