Higher-Order Aberrations
Wavefront aberration is highly dependent on pupil size, with increased higher-order aberrations apparent as the pupil dilates. Higher-order aberrations also increase with age, although the clinical effect is thought to be balanced by the increasing miosis of the pupil with age. Although lower-order aberrations decrease after laser vision correction, higher-order aberrations, particularly spherical aberration and coma, may increase after conventional surface ablation, laser in situ keratomileusis (LASIK), or radial keratotomy (RK) for myopia. This increase is correlated with the degree of preoperative myopia. After standard hyperopic laser vision correction, higher-order aberrations increase even more than they do in myopic eyes but in the opposite (toward negative values) direction. Compared with conventional treatments, customized excimer laser treatments may decrease the number of induced higher-order aberrations and provide a higher quality of vision, particularly in mesopic conditions.
Spherical aberration
When peripheral light rays impacting a lens or the cornea focus in front of more central rays, the effect is called spherical aberration (Fig 1-4). Clinically, this radially symmetric fourth-order aberration is the cause of night myopia and is commonly increased after RK and myopic ablation. It results in halos around point images. Spherical aberration is the most significant higher-order aberration. It may increase depth of field but decreases contrast sensitivity.
Coma and trefoil
With coma, a third-order aberration, rays at one edge of the pupil come into focus before rays at the opposite edge do (Fig 1-5). As can be seen by examining the illustrations, light rays entering the system do not focus on a plane; rather, one edge of the incoming beam focuses either in front of or behind the opposite edge of the beam. If one were to examine the image generated by an incoming light beam passing through an optical system with a coma aberration, the image would appear “smeared,” looking somewhat like a comet with a zone of sharp focus at one edge of the image tailing off to a fuzzy focus at the opposite edge of the beam. Coma is common in patients with decentered corneal grafts, keratoconus, and decentered laser ablations.
Trefoil, also a third-order aberration, can occur after refractive surgery. Trefoil produces less degradation in image quality than does coma of similar RMS magnitude (Fig 1-6).
Other higher-order aberrations
There are numerous other higher-order aberrations, of which only a small number are of clinical interest. As knowledge of surgically induced aberration increases, more of the basic types of aberrations may become clinically relevant.
Effect of excimer laser ablation on higher-order aberrations
Whereas use of conventional (non–wavefront-guided) excimer laser ablations typically increases higher-order aberrations, wavefront-optimized, wavefront-guided, and topographyguided ablations tend to induce fewer higher-order aberrations and may, in principle, be able to reduce preexisting higher-order optical aberrations.
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Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.