Eyeworld

75 EW INTERNATIONAL by Claudio Orlich, MD January 2018 Understanding spherical aberration ALACCSA shares a recent article on spherical aberrations from one of its contributors T oday the goal of cataract surgery is to provide pa- tients with the best possible quality of vision. While it is common to see patients post-cataract surgery with a visual acuity (VA) of 20/20, many patients remain dissatisfied with their quality of vision. This is due to several factors, including problems with the ocular surface, pseudophakic dys- photopsia, and optical aberrations in general. Although low order aberrations (myopia, hyperopia, and regular astigmatism) have a greater impact on vision, high order aberrations also play an important role, espe- cially in patients who are candidates for multifocal lenses. Among the high order aberrations that can be corrected with cataract surgery is spherical aberration. There are multiple intraocular lens options that can correct this aberration, but when and how we should use them is a topic of debate and research in recent years. The word "aspheric" is used to describe a surface or in this case a lens that does not have a spherical shape. In an aspherical lens, the rays of light passing through the center do not focus at the same point as the rays passing through the periph- ery of the lens. Naturally, the human cornea is aspheric, usually a bit more curved in the center and flattened toward the periphery; we call this form "prolate." In a prolated cornea, the rays of light that cross the center of the cornea tend to converge or focus on a point anterior to the peripheral rays, and this aberration is called "negative spherical aberration." In an oblate cornea, the central light rays are focused behind the pe- ripheral rays, and this aberration is called "positive spherical aberration" (Figure 1). Spherical aberration is included within the high order aberrations, specifically in the group of fourth order aberrations, along with qua- trefoil and secondary astigmatism. Spherical aberration generally reduc- es retinal image contrast and affects visual quality, especially under mesopic conditions. 1 The average spherical aberra- tion of the anterior cornea surface is slightly positive (between +0.27 and +0.30 μm), remaining stable throughout life. 2 The natural crys- talline compensates for this positive spherical aberration, inducing a neg- ative spherical aberration of –0.20 μm, leaving a slightly positive total aberration of +0.10 μm. Spherical aberration of the lens changes over time unlike spherical aberration of the cornea, going from negative to positive as cataracts develop. This positive spherical aberration of the cataractous lens adds to the spherical positive aberration of the cornea, impairing the visual quality of patients. Based on this concept, intraocular lenses were developed with negative spherical aberration, which simulate a young lens that compensates for the average pos- itive spherical aberration of the cornea. Some studies suggest that it is not necessary to correct spherical aberration completely, and in fact it is recommended to leave a slightly positive residual (+0.10 μm). A study performed on pilots of the American Air Force by Grimson et al. suggests that a quantity of positive spherical aberration can be correlated with visual acuities of 20/15 or better. 3 It is impossible to completely correct the spherical aberration in all our patients as there is an interaction between much more complex aber- rations than a sum of the existing spherical aberration and the in- traocular lens induced aberration. Nevertheless, the objective must be directed to a final low spherical aberration, which allows the patient good contrast sensitivity. In the consultation, we will find that the corneal spherical aberration varies greatly among individuals, especially in the presence of patho- logical corneas or modified ones by post-refractive surgery. In a myo- pic treatment with excimer laser, a central flattening of the cornea is induced, generating an oblate cornea, more flat in the center than in the periphery, inducing a positive spherical aberration. In contrast, a hypermetropic correction increases the central curvature of the anterior surface of the cornea, generating a hyperprolate cornea, inducing greater negative spherical aberration that may contraindicate the use of an aspheric intraocular lens, which instead of correcting would wors- en the existing spherical negative aberration, deteriorating the quality of vision. It is important to differentiate spherical aberration from corneal asphericity or commonly named with the Q coefficient. Both are related but different. Asphericity (Q) is a factor that tells us how much and how the cornea is peripherally flattened from the corneal apex, that is, if the cornea is prolate or oblate, while informing us of the degree of asphericity. A cornea with a Q of –0.20 is not the same as a cornea continued on page 76 Figure 1. A: lens without spherical aberration (neutral); B: lens with negative spherical aberration; C: lens with positive spherical aberration

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