Purpose: To investigate the optimal amount of spherical aberration (SA) in an intraocular lens (IOL) to maximize optical quality.
Setting: Cullen Eye Institute, Baylor College of Medicine, Houston, Texas, USA.
Methods: In 154 eyes of 94 patients aged 40 to 80 years, implantation of aspherical IOLs with different amounts of SA to produce residual ocular SA from -0.50 to +0.50 microm was simulated. Using the VOL-CT program (Sarver and Associates), corneal wavefront aberrations up to 6th order were computed from corneal topographic elevation data (Humphrey Atlas, Carl Zeiss, Inc.). Using the Zernike Tool program (Advanced Medical Optics, Inc.), the polychromatic point-spread function with Stiles-Crawford effect was calculated for the residual ocular higher-order aberrations (HOAs) (3rd to 6th order) for 6.0 mm and 4.0 mm pupils with defocus of 0.00 diopter (D), -0.50 D, and +0.50 D. Five parameters were used to quantify optical image quality, and the optimal ocular SA and IOL SA at which the maximal image quality was achieved in each eye were determined. Stepwise multiple regression analysis was performed to assess the predictors for optimal IOL SA for each eye.
Results: The optimal ocular SA and IOL SA for a 6.0 mm pupil varied widely between eyes. With defocus of 0.00 D, -0.50 D, and +0.50 D, most eyes achieved the best image quality at an ocular SA of -0.10 to 0.00 microm, +0.15 to +0.30 microm, and -0.40 to -0.20 microm, respectively. The IOL SA values that provided optimal visual quality in at least 10% of eyes were -0.45 to -0.20 microm with zero defocus, -0.15 to +0.10 microm with myopia of -0.50 D, and -0.75 microm to -0.45 microm with hyperopia of +0.50 D. The amount of optimal IOL SA could be predicted based on other HOAs of the cornea with multiple correlation coefficients up to 0.952. Among the Zernike terms, 4th-order SA, Z(4,0), made the greatest contribution to the optimal IOL SA, followed by the 6th-order SA, Z(6,0).
Conclusions: The amount of IOL SA producing the best image quality varied widely between patients and could be predicted based on corneal HOAs. Selection of an aspherical IOL should be customized based on the full spectrum of corneal HOAs, not on 4th-order SA alone.