Effect of pupil size and astigmatism on contrast acuity with monofocal and bifocal intraocular lenses

We conducted a bicenter study to evaluate the effect of pupil size and corneal astigmatism on best corrected contrast acuity (BCCA) in patients with the True Vista™ bifocal intraocular lens (IOL) and compared the results with those in a matched group of monofocal patients. Best corrected contrast acuity was measured using the Regan Charts (96%, 50%, 25%, 11 %) at four pupil sizes (2.0, 2.5, 3.5, 6.0 mm) in three groups (Group 1: corneal astigmatism 0–0.50 D [n = 11]; Group 2: 0.75–1.50 D [n =11 ]; Group 3: 1.75–2.50 D [n = 4]). In monofocal eyes, BCCA between these astigmatic groups did not differ significantly at any contrast step or at any pupil size tested. However, BCCA decreased slightly with increasing pupil size in each group, and differences between 2.0 mm and 6.0 mm pupils were significant at each contrast level. In all bifocal groups, BCCA decreased significantly with increasing pupil size and also decreased with increasing corneal astigmatism; differences were significant at most pupil sizes and contrasts tested. In Group 1, BCCA was lower in eyes with bifocal IOLs than in eyes with monofocal IOLs at 25% and 11 % contrast with a 2.5, 3.5, and 6.0 mm pupil; with a 2.0 mm pupil it did not differ significantly. In Groups 2 and 3, BCCA in eyes with bifocal IOLs was lower at all contrast steps and all pupil sizes tested. Our results indicate that corneal astigmatism in excess of 0.50 D reduces quality of vision in eyes with bifocal IOLs, while eyes with monofocal IOLs are not affected. It may be important to minimize postoperative corneal astigmatism in patients with bifocal IOLs.