38 Aspheric Intraocular Lenses Photographers have known for many years that the key to great images is the optical properties of the lenses used in the cameras, almost all of which are aspheric. We have adopted the same concept in ophthalmology, with the introduction of precisely made intraocular lenses (IOLs) with aspheric optics. Using an aspheric lens, particularly at large pupil sizes (or large camera apertures) ensures that all light rays entering the lens, both in the center and in the periphery, are focused at the same point or plane (Fig. 38.1). This gives a better image quality in terms of both sharpness as well as contrast (Fig. 38.2). The traditional lenses are spherical because they have the same, constant curvature on the optic. In essence, they can be thought of as being carved out of a sphere of glass. This spherical lens has a higher power in the periphery than in the center, thereby inducing positive spherical aberration (SA). An aspheric lens with zero SA has a variable curvature but produces a lens with an even power from center to edge. It can be more technically challenging to make, but it produces better optical quality. An aspheric lens can also be designed to have a negative degree of SA wherein the power of the lens is higher in the center than it is in the periphery (Fig. 38.3). Incorporating aspheric IOLs in cataract surgery can help to restore a better quality of vision to our patients. Although the most important optical function of an IOL is correcting the lower order aberrations, such as sphere and cylinder, the ability to modulate the SA of the eye can lead to further improvements in vision. Lens power calculations depend on accurate biometry of the corneal power and axial length, but also on the ability to predict the effective lens position of the IOL after surgery. This last factor ends up being the source of much of the variance in refractive outcomes in our cataract surgery patients. Ideally, we would like to address the spherical component of the refraction, the astigmatism, as well as the SA with cataract surgery and the IOL implanted. Optical aberrations, as best explained by the Zernike polynomials, can be divided into lower order aberrations and higher order aberrations. Patient satisfaction and visual outcomes are very well associated with achieving precise results in correcting these lower order aberrations, such as myopia, hyperopia, and astigmatism. In most eyes, it is far more important to accurately address the lower order aberrations than to focus on SA, which is just one of the many higher order aberrations. Higher order aberrations are more important in eyes with larger pupils. At a pupil size of 3 mm or below, SA plays only a minor role in image quality.1 Fig. 38.1 In a spherical lens, the rays in the periphery of the lens focus differently than the rays in the center of the lens, thereby producing spherical aberration (SA) and degrading image quality. The aspheric lens enables all rays from the center to the periphery to focus at the same point. Fig. 38.2 Image quality taken through an intraocular lens (IOL) with aspheric optics and traditional optics showing the difference in image quality. Studies done by Li Wang and Douglas Koch’s group2,3 demonstrate that when we look at the average of the various corneal aberrations in a sample of normal eyes, the only one that has a nonzero mean is SA. The average cornea has a small degree of positive SA, which the young crystalline lens typically neutralizes because it has a small degree of negative SA. This produces an eye with a total of zero SA. As we age, the lens changes toward a less negative SA and the eye as a whole develops a small degree of positive SA. This may be helpful because it may increase the depth of field as the eye becomes presbyopic and loses accommodation. In selecting an IOL for the eye, we have a choice of lenses with negative SA, such as the AcrySof line (Alcon, Fort Worth, TX) and the Tecnis line (Abbott Medical Optics [AMO], Abbott Park, IL), or those with zero SA, such as the EnVista, Akreos, Sof-Port, and Crystalens lines (Bausch and Lomb, Rochester, NY) (Fig. 38.4). In eyes in which we want to maximize the depth of field, there may be a benefit to the zero SA IOLs. In eyes in which we want to prioritize image quality at a specific focal point, the negative SA IOLs would be selected. Of particular importance is to remember that in order for a negative SA IOL to address the positive SA of the cornea, the IOL must be very well centered in the visual axis. Decentration of negative SA IOLs can induce other higher order aberrations such as coma, whereas zero SA IOLs are relatively immune to mild decentration and do not induce coma.4 Because the center of the visual axis is slightly nasally displaced when compared with the center of the capsular bag, it is helpful to orient the IOL in the vertical axis, with the haptics at the 12 and 6 o’clock positions, so that the axis can be nudged nasally to achieve optimal centration. We know that lenticular SA changes as the eye ages. By age 60, most eyes have excessive positive SA, which degrades the quality of vision. The beauty of an aspheric IOL is that it can restore the sharper image quality of the young eye. By addressing the SA, in addition to the lower order error, we can offer the patient improved contrast sensitivity and better visual performance in suboptimal lighting (Fig. 38.5).
Lower Order Versus Higher Order Aberrations
Addressing Spherical Aberration
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