Errors in Treatment of Lower-order Aberrations and Induction of Higher-order Aberrations in Laser Refractive Surgery.

Abstract

The purpose of refractive surgery is to reduce dependence on corrective lenses. In laser keratorefractive procedures, this is achieved by altering the shape of the cornea to change the refractive state of the eye. Historically, sphere and cylinder were the only components of a refractive error that could be measured and systematically corrected. Procedures designed to treat these lower order aberrations often achieved emmetropia, but with bothersome visual symptoms such as glare, halos, starbursts and ghost images that could not be corrected with glasses or contact lenses. These symptoms have since been attributed to induction of higher-order aberrations (HOAs).1–4 Wavefront analysis is a method by which the aberrations of an optical system are measured. Several methods exist to assess ocular aberrations including Tscherning aberrometry, Shack-Hartmann wavefront sensing, ray-tracing, optical path difference aberrometry, and spatially resolved refractometry. Currently, it is most commonly performed using devices based on the Hartmann-Shack wavefront sensor. This technology was developed in the 1960’s and 70’s to improve the images of satellites captured by telescopes from Earth and later adapted to measure aberrations of the human eye.5 Such devices use a low-power laser beam focused on the retina and analyze the reflected rays of light. The light passes outward through the optical system of the eye, through an array of lenses and onto a detector. In a perfect optical system, these rays would emerge parallel and focus at a single plane. Given the complex nature of the eye’s optics, this is not the case. The degree that each image deviates from the expected focal point of an individual lens in the system represents the aberration or “wavefront error.” The wavefront error is computed and broken down into components that visually and mathematically describe distinct elements of the overall aberration. These components are most commonly expressed as Zernike polynomials, and encompass both lower- and higher-order aberrations. Lower (second) order aberrations include positive defocus (myopia), negative defocus (hyperopia), and regular astigmatism. Visually significant higher (third and fourth) order aberrations include coma, trefoil, and spherical aberration. These aberrations can have both positive and negative values. Coma, or comatic aberration occurs when light rays from one edge of the pupil comes to focus before those from the opposite edge. This third order aberration has the effect of “smearing” an image or making it appear to have a tail like a comet. Trefoil, another third order aberration, has less effect on image quality than an equal amount of coma. Spherical aberration is a fourth order aberration that occurs when rays from the peripheral pupil focus in front of those from the central cornea. Spherical aberration results in halos around point light sources and decreased contrast sensitivity. Other lower-order aberrations and those above fourth-order are considered to be relatively less visually significant.6 The most visually significant lower- and higher-order aberrations are shown in Figure 1. The Zernike polynomial system allows the deconstruction of any aberration structure, no matter how complex, into predefined, fundamental building blocks. Open in a separate window Figure 1 Visually significant optical aberrations displayed as Zernike polynomials. Each is labeled with their common name and Z(x,y) notation where x is the order of the aberration and y is the angular frequency or number of times the wavefront pattern repeats itself. (Images courtesy of Dr. Ronald Krueger, M.D.)