Optimal refraction with monofocal intraocular lenses: no beneficial effect of astigmatism

PURPOSE: This study aimed to determine the optimal spherocylindrical refraction in the monofocal, pseudophakic eye using power vectors in dioptric space. METHODS: For parallel incident light the defocus of a spherocylinder may be described in dioptric space as: Defocus equivalent for distance fixation = D((SEP,M,x=∞)) = √SEP(2)+(1/2M)(2), where SEP = spherical equivalent power in dioptres (D) and M = astigmatic magnitude in D. In the pseudophakic eye the defocus for any fixation distance x is: Defocus equivalent for the fixation distance x = D((SEP,M,x)) = √(SEP-1/x)(2)+ (1/2M)(2). The cumulative defocus over a fixation interval is the integral of D((SEP,M,x)) . A minimal value for cumulative defocus will indicate a maximal unaided visual acuity (VA) over the chosen fixation interval. We calculated the summated defocus for various spherocylinders for fixation distances ranging from 0.5 m to 6.0 m. RESULTS: Minimal cumulative defocus was present for pure spheres of -0.25 D to -0.5 D. No beneficial effect of the presence of astigmatism was detected. CONCLUSIONS: In monofocal pseudophakia the highest possible VAs over the most extended fixation ranges may be achieved with slight myopic refractions without astigmatic components.