In clinical practice, myopia, hyperopia, and astigmatism have long been the familiar refractive errors (i.e., lower-order optical aberrations) for which patients have received corrective glasses and contact lenses. However, the optical system of the eye displays many other imperfections, referred to as higher-order aberrations. They include such errors as spherical aberration and coma. These have not been addressed in routine office work because they could not be measured in clinical practice and there were no means to correct them optically. Also in the normal eye, these aberrations do not degrade the image quality below 20/20 vision.
With the advent of laser refractive surgery, higher-order aberrations have come into focus both because the laser procedures induced unwanted aberrations into the eye and because improvements in the lasers and the software that runs them now make it possible to correct some of the eye’s higher-order aberrations.
Laser refractive surgery induces various amounts of spherical aberration and coma in treated eyes. For example, the procedure may correct myopia perfectly but at the same time introduce an increase in spherical aberration. These aberrations are generally not significant when the pupil is small, but they become important at low light levels when the pupil dilates. As a result, driving at night may become a problem because induced spherical aberration can cause glare, halos, and ghosting. The optical problems caused by spherical aberration in dim illumination cannot be corrected with glasses and, although a laser retreatment may be attempted, there is no guarantee that a second operation will eliminate the patient’s complaints.
With the growing realization of the importance of higher-order aberrations in laser refractive patients, new measuring devices have been introduced to help the surgeon plan the procedure more accurately. These devices are based on well-known systems used for many years in optical engineering and physics. Referred to as wavefront sensors or aberrometers, they can measure the aberrations of the entire system of the eye. By documenting the aberrations preoperatively, the surgeon can theoretically aim at correcting not only the spherocylindric errors of the eye but also some of the higher-order aberrations. Also by obtaining a more accurate picture of the preoperative optical system of a given eye, the laser ablation can be tailored to that eye (“custom ablation”), reducing the amount of aberrations induced, possibly avoiding the induction of such aberrations altogether, and even eliminating some of the preoperative higher-order aberrations.
Such a scenario sounds enticing, but is difficult to accomplish because it calls for micrometer precision and a fairly perfect conversion of the aberrometer data into an ideal ablation pattern. Imperfections in the laser beam and unpredictable individual healing responses add to the problems of designing a custom ablation.
The current intense research and development activity in the field of custom ablation has produced some systems (laser + aberrometer) that are precise enough to correct an impressive part of the eye’s higher-order aberrations. However, this precision does not always translate into highly accurate outcomes because the response of the treated eyes is variable. For example, during the laser procedure, the hydration of the cornea may vary, thus limiting the accuracy of tissue removal even when an extremely precise system is used. The cornea’s healing response after laser surgery is a further confounding factor; postlaser epithelial thickness may vary from patient to patient (epithelial thickness changes occur not only after surface ablation but also, surprisingly, after laser-assisted in situ keratomileusis [LASIK]) and can thereby influence the refractive outcome of a procedure. Stromal remodeling after laser ablation and postoperative biomechanical changes of the cornea do not follow standard patterns and may derail the most meticulously planned custom ablation. Also aside from the difficulties in predicting the individual eye’s healing response, aging changes—especially those of the lens—may degrade the effect of the aberration correction over time.
There are further stumbling blocks when considering the discrepancy between an ideal custom ablation scenario and realistic expectations. Studies of the aberrations of normal human eyes (eyes without disease and with minor or no refractive errors) show that there appears to be a certain ratio between the magnitudes of different higher-order aberrations in normal eyes and that this ratio seems to shift for different pupil diameters. This implies that changing such a ratio (e.g., by reducing spherical aberration without a proportional reduction of other higher-order aberrations) could in fact increase an eye’s total aberrations.
A further observation made was that even a normal eye displays a substantial increase in spherical aberration as the pupil dilates from 5 to 7 mm (as it does in low luminance). In view of that, it may be unrealistic to expect that custom ablation can improve on nature by substantially decreasing spherical aberration for 7-mm pupils in excimer-treated eyes.
Aberrometers work on the principle that light can be defined as an electromagnetic wave and that the propagation of such a wave can be described as a wavefront ( Fig. 38.1 ). If the eye were optically perfect (i.e., aberration-free) and focused at infinity, a wavefront entering the eye would exit unchanged as a flat plane. In the presence of aberrations (as is the case in the normal eye), a deviation from the ideal wavefront is registered. In other words, the exit wavefront deviates from that of the plane wave. A wavefront aberration of the eye is thus defined as the deviation of the actual wavefront from an ideal reference wavefront.