Relevance of Wavefront Aberrations of the Human Eye in Corneal Laser Surgery

Abstract

Summary The classical optical errors of the human eye – myopia, hyperopia, and astigmatism – are known to reduce the visual performance of the patient eye and can be corrected by means of glasses, contact lenses, or corneal laser surgery. However, the optic of each individual eye includes also imaging errors of higher order (wavefront aberrations or optical aberrations) such as spherical aberration or coma like aberrations that can decrease the visual performance. In contrast to the classical optical errors, the wavefront aberrations of higher order can not be corrected by means of glasses or contact lenses. However, modern laser systems used in corneal laser surgery possess the potential to correct the optics of an individual eye for such higher order imaging errors. The correction of higher order aberrations improving imaging quality requires a precise knowledge of the relevance of such aberrations. Therefore, we performed a clinical trail to measure the higher order wavefront aberrations in a large population of 130 eyes that were scheduled for corneal laser surgery. The averaged human eye was found to be of good optical quality. On average all Zernike coefficient are statistically not significant different from zero, except the vertical coma and the spherical aberration for 5 and 7 mm pupil size. None of the individual eyes had ideal optical performance with an rms- wavefront error smaller than the Marechal criterion (diffraction limit). Fifty percent of the investigated eyes had significant optical aberrations at pupil sizes >3.2 mm. This critical pupil size represents the smallest pupil diameter at which diffraction limit is achieved. A statistical cluster analysis demonstrates a significant increase of the rms-wavefront errors in the eyes of patients over 40 years old. In conclusion, the averaged human eye has a rather good optical quality exceeding the Marechal criterion (diffraction limit) only by a factor of 2. However, none of the individual eyes achieved the optical quality of “averaged eye”. Thus, the human eye is in general designed to have an almost perfect optical performance, whereas the individual eye deviates significantly from this ideal imaging quality. Optical aberration become significant at a pupil individual size above >3.2 mm and as a consequence optical quality decreases drastically at larger pupil diameters as they occur under dimmed light conditions (mesopic vision). As the pupil diameter can be as large as 6 mm in diameter under such low light conditions, modern corneal laser surgery uses optical relevant treatment zones of 6.0mm or larger and, consequently, the corrections should include the higher order wavefront aberrations. Surgeons and patients should be aware that the corrections of the higher order aberrations are not stable over time because optical performance decreases also with age.