Figure 1-6 Zernike polynomial representation of trefoil. (Courtesy of Tracey Technologies.)

Fourier analysis is an alternative method of evaluating the output from an aberrometer. Fourier analysis involves a sine wave–derived transformation of a complex shape. Compared with shapes derived from Zernike polynomial analysis, the shapes derived from Fourier analysis are more detailed, theoretically allowing for the measurement and treatment of more highly aberrant corneas.

Lower-Order Aberrations

Myopia, hyperopia, and regular astigmatism are all lower-order (second-order) aberrations that can be expressed as wavefront aberrations. Myopia produces positive defocus (see Fig 1-2), whereas hyperopia produces negative defocus. Regular (cylindrical) astigmatism produces a wavefront aberration that has orthogonal (ie, facing at right angles) and oblique components (see Fig 1-3). Other lower-order aberrations are non–visually significant aberrations known as first-order aberrations, such as vertical and horizontal prisms and zero-order aberrations (piston).

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 or laser in situ keratomileusis (LASIK) 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 aberrations

When peripheral light rays impacting a lens or the cornea focus in front of more central rays, the effect is called spherical aberration (see Fig 1-4A, B). Clinically, this radially symmetric fourth-order aberration is the cause of night myopia and is commonly increased after myopic LASIK and surface ablation. It results in halos around point images. Spherical aberration is the most significant higherorder 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. The effective image resembles a comet, having vertical and horizontal components (see 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 and produces less degradation in image quality than does coma of similar RMS magnitude (see 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, both wavefront-optimized and wavefront-guided 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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