Measuring refractive- surgery-induced change of corneal aberrations

surgery-induced change of corneal aberrations Computerized videokeratography — which measures the shape of the cornea, typically by analysis of the distortion of concentric rings projected on it—is routinely used in clinics to evaluate surgical results (i.e., centration and location of the treatment zone and characteristic patterns of what is known in clinical terms as irregular corneal astigmatism). Applegate et al.2 and Oliver et al.3 used corneal elevation maps from commercial corneal topographers to measure the changes induced in the cornea by different types of refractive surgery; their findings show that refractive surgery is generally followed by an increase in the amount of and an alteration in the distribution of aberrations. The corneal surface and the pattern of aberrations associated with any optical system are typically described as a polynomial expansion. The first-order terms correspond to a prismatic error; the secondorder terms to defocus and astigmatic errors; third-order aberrations include coma, in which a point source appears as a comatic shape; fourth-order aberrations include spherical aberration, or increased focusing error as the pupil dilates. Studies based on corneal topography show that, for large pupils, corneal aberrations increase by approximately a factor of 3. In addition, the distribution of aberrations shifts from third-order dominance (comalike) to fourth-order dominance (spherical). The dramatic increase in spherical aberration is caused by a surgery-induced change in corneal asphericity. Figure 1 shows the distribution of aberrations (in terms of variance) in normal eyes, and one-year after radial keratotomy (RK), photorefractive keratectomy (PRK) , and laser in situ keratomileusis (LASIK). In general, there is an increase in both thirdand fourth-order aberrations, and the increase is correlated to a decrease in contrast sensitivity. optical quality of the human eye.1 Only recently, however, as these procedures have begun reaching a clinical population, has the instrumentation involved captured the attention of refractive surgeons. For the compensation of refractive errors (myopia, hyperopia, and astigmatism), corneal refractive surgery has become a popular alternative to wearing glasses or contact lenses: the operation and recovery are rapid and the procedures are constantly being perfected. The surgery alters the curvature of the central area of the cornea, reducing it in myopes and increasing it in hyperopes. From the lens designer’s point of view, it is clear that a change in only part of the front surface of the cornea (the most refractive element of the ocular optical system) must modify the aberration pattern of the entire optical system. Particularly in the case of large pupils, an increase in optical aberrations—despite the correction of defocus and astigmatism—can frequently be associated with complaints of halos and visual loss at night. Understanding the change in aberrations induced by standard refractive surgical procedures is important for assessment of how vision can be compromised after surgery. It is essential to develop new ablation algorithms to overcome such problems, and perhaps even improve vision. F or more than a century there has been awareness of the fact that the eye is not a perfect optical system and that it suffers from defects or optical aberrations other than defocus and astigmatism. Attempts to measure the wave aberration of the human eye, i.e., phase distortions at the plane of the pupil, date back to 1894. In recent years, new terminology and new technology have invaded the field of refractive surgery and changed the way ophthalmologists and optometrists look at the image-forming capabilities of ocular optics. Wave-frontsensing, aberrometers, high-order aberrations, and Zernike polynomials are only a few of the novelties that have recently entered the field. Over the past two decades, several new methods have been developed to assess the