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The role of spherical aberration or those with prior hyperopic LASIK who likely have a cornea with negative SA. Correcting SA is even more import- ant for the optical performance of multi- focal IOLs. Because diffractive multifocal lenses reduce contrast sensitivity, we don't want to compound the effects on image quality by also leaving the patient with significant positive SA. There is some debate about whether to err on the negative or positive side when we can't get to precisely zero SA. A small amount of negative SA will improve near focus, particularly when the pupil constricts with near effort and forces more light rays through the parax- ial cornea. However, the near vision that is gained will still be aberrated. Some surgeons adjust their refractive target depending on the predicted residual SA, opting for a slightly myopic final refrac- tion if they expect to leave the eye with some positive SA, or slightly hyperopic if they plan to leave it with negative SA. It is important to remember that SA always affects the quality of the retinal image. In an ideal world, we would choose IOLs that fully correct each patient's actual SA while also achieving a plano refraction to maximize both acuity and retinal image quality. –0.27 µm of SA (Figure 2). 6 It was later confirmed that this amount of SA correction did indeed result in better contrast sensitivity. 7,8 Over the past decade, asphericity has become a standard IOL feature, with contemporary IOLs featuring –0.27 D µm of SA (Tecnis platform, Abbott Medical Optics, Abbott Park, Ill.), –0.1 to –0.2 µm of SA (AcrySof platform, Alcon, Fort Worth, Texas, and Hoya Surgical Optics, Chino Hills, Calif.), or 0 SA (Bausch + Lomb IOLs, Bridgewater, N.J., and Lenstec, St. Petersburg, Fla.). All of these options lessen SA compared to positive SA IOLs that add to the cornea's positive SA. In my practice, I try to target zero total SA to achieve that "perfect" balance of the 20-year-old eye. The average cor- neal SA of +0.27 µm is derived from pop- ulation data, so of course individual eyes can vary around that average. I measure each patient's actual SA using Pentacam tomography (Oculus, Arlington, Wash.), which I find to be a better measure of corneal SA than topography-based devices. Most patients have +0.25 µm or more, so I usually choose lenses that of- fer the highest amount of SA correction (–0.27 um). However, I alter that plan in patients who already have closer to 0 SA Aspheric lenses have become widely accepted for their contribution to improved pseudophakic image quality L ight rays passing through a perfect lens will all focus at the same point. The human eye, however, does not represent a perfect lens. There are multiple refracting surfaces that all contribute to either positive or negative spherical aberration (SA). Positive SA occurs when light rays passing through the steeper periphery of a lens focus in front of the flatter central rays. Negative SA occurs when the periphery of the lens is flatter than the central portion, focusing light rays farther. In the young eye, the positive SA of the cornea is countered by negative SA in the lens. At around age 20, everything is in perfect balance, providing the eye with zero SA and optimal visual quality. But over time, the lens develops more positive SA, resulting in a gradual loss of contrast sensitivity and visual quality. More than a decade ago, Pablo Artal and colleagues compared the vision of pseudophakes with that of age-matched and younger phakic adults. 4 They realized that while visual acuity in the pseudophakes was similar to that of the younger adults, retinal image quality was considerably worse in the pseudophakic patients, about the same as that of the older adults who had not yet undergone cataract surgery. Given the objectively high optical quality of the IOLs implant- ed, this was rather surprising. But it turns out that the reason for the poorer optical performance in the pseudophakes was largely due to the fact that IOLs at the time replicated the positive spherical ab- erration of the aged eye rather than the balanced SA of the young eye. 5 Aspheric IOLs Further studies demonstrated that the average SA of the human cornea at a 6.0- mm optical zone is +0.27 µm, meaning that most patients would benefit from an IOL that eliminated total SA by inducing Jeremy Kieval, MD Figure 2: Approximately 90% of patients (those to the right of the red bar) benefit from an IOL with –0.27 microns of SA. 3

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