When choosing an aspheric IOL, aiming for an overall postoperative spherical aberration of +0.10 ?m provides better patient outcomes based on the latest research.
Link to contrast sensitivity
Studies have shown that an increase in spherical aberration is associated with a decrease in visual performance as measured by contrast sensitivity.1-3 For the average population, 83% of aberrations are due to sphere and cylinder, and approximately 17% are due to higher-order aberrations, such as coma, spherical aberration, and trefoil.
In a clinical study of 696 eyes, my research found that the spherical aberration of the cornea is 0.27 ± 0.01 µm. Previous research indicates this changes little with age.
In a paper presented at the American Society of Cataract and Refractive Surgery Annual Meeting in 2004, David Zadok, MD, found that young patients with "super vision," or natural uncorrected visual acuity (UCVA) of 20/12.5, have a mean positive spherical aberration of approximately 0.10 µm,4 so I decided to aim for that value in my study. We wanted to determine whether targeted patients would have better outcomes than patients who were not targeted.
The group targeted for a postoperative spherical aberration of 0.10 µm outperformed the non-selected patients at almost every spatial frequency tested under photopic and mesopic conditions.
Performance of aspheric lenses
Our research then turned to other currently available aspheric lenses and their performance. Patients presenting for cataract surgery were randomly assigned to implantation of either the AcrySof IQ (SN60WF) IOL (Alcon Laboratories) or the SofPort LI61AO IOL (Bausch & Lomb).
These patients were matched for age, spherical equivalent, and postoperative visual acuity to the group of patients who received the Tecnis lenses in the previous study. When comparing AcrySof IQ non-selected group with the Tecnis non-selected group, the AcrySof IQ group outperformed the Tecnis group in contrast testing under both photopic and mesopic conditions. This occurred because the AcrySof IQ lens is designed to bring the average population to a postoperative spherical aberration of 0.10 µm, unlike the Tecnis and SofPort lenses.
When comparing the non-selected AcrySof IQ group with the selected Tecnis group, there was less difference between groups, in terms of contrast testing.