Wavefront guided ablation

Abstract

A DAPTIVE OPTICS WAS FIRST SUGGESTED IN 1953 BY astronomer Horace Babcock to remove the blurring effects of turbulence in the atmosphere on telescopic images of stars.1 The U.S. Defense Department later invested heavily in the development of adaptive optics technology to improve the effectiveness of laser weapons as part of its Star Wars Program. This information would eventually allow vision scientists to apply this technology to better understand the eye’s optic and retinal image quality. In 1994, Liang and associates used a Shack–Hartmann wavefront sensor to describe higher order aberrations in the human eye.2 In 1997, Liang, Williams, and Miller used the Shack–Hartmann wavefront sensor to detect the eye’s aberrations and then applied an adaptive optics deformable mirror to correct the eye’s lower and higher order aberrations.3 With this system, they noted that adaptive optics provided a sixfold increase in contrast sensitivity to high spatial frequencies when the pupil was large. This study was the first to demonstrate that the correction of higher-order aberrations can lead to supernormal visual performance in normal eyes. The Liang, Williams, and Miller study used monochromatic light.4 Normal viewing conditions usually involve broadband light, and retinal images formed in broadband (white) light are blurred by chromatic aberration, as well as the monochromatic aberrations that adaptive optics can correct. Yoon and Williams showed that, in broadband light which characterizes normal viewing conditions, adaptive optics still provides a twofold increase in contrast sensitivity at high spatial frequencies in typical eyes, even when chromatic aberration is present.5 These findings spurred a ground swell of interest in wavefront sensing and the possibility of coupling it with wavefront correction in the form of customized corneal ablation. In this editorial, we will look at the visual benefit of correcting higher-order aberrations, the limits of the human visual system, and some of the future challenges of the ambitious and sometimes misunderstood world of customized corneal ablation. The wavefront sensor allows the clinician not only to measure the defocus and astigmatism that are the most important determinants of refractive error, but also “higher-order aberrations” as well. Defocus and astigmatism are referred to as second-order aberrations. Higher-order aberrations, such as coma and spherical aberration, refer to aberrations other than defocus and astigmatism. The wavefront sensor, such as that constructed by Liang and Williams,3 can reliably detect as many as 64 higher-order aberrations. Some of these higher-order aberrations had not been previously measured in human eyes and all were usually lumped by clinicians into a single category misleadingly called “irregular astigmatism.” They are better referred to as higher-order aberrations since most have nothing to do with astigmatism. The spectacle correction that provides the best subjective refraction depends not only on defocus and astigmatism but also, to a lesser extent, on higher-order aberrations.6 For this reason, the description of the eye’s wave aberration provided by a wave-front sensor, when properly processed, can provide an especially accurate objective estimate of subjective refraction.