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Effect of astigmatism on multifocal intraocular lenses
G Ravalico1, F Parentin, F Baccara
1Istituto di Clinica Oculistica, Universitá di Trieste, Ospedale Maggiore, Italy.
Purpose:
To study the effect of astigmatism on multifocal intraocular lens (MIOL) function.
Setting:
Laser Laboratory, Department of Electronics, Electrotechnics and Informatics, University of Trieste, Italy.
Methods:
Using an experimental optical system, this study compared the division of a laser beam in the focal spots of 1 monofocal IOL (Pharmacia 722A), 1 bifocal IOL (Pharmacia 811E), and 2 MIOLs (AMO Array MPC25NB and Domilens Progress 1). The model consists of a helium-neon laser and an optical system: a triangular optical bench with a precision collimator, a micropositionable immersion stage to support the IOL, and a digital image-processing system. Astigmatism was induced by interposing a +1.0 diopter (D) cylinder lens between the IOL and the television camera. Astigmatism was corrected by adding a -1.0 D cylinder lens in front of the IOL on the same axis.
Results:
Astigmatism creates pairs of focal lines, 1 for each focal spot in the IOL. In the multifocal IOL, the posterior focal line of the nearest focus interfered with the anterior line of the next focus. Correcting the astigmatism led to a significant reduction (mean 20%) in light intensity.
Conclusions:
Astigmatically neutral surgery or surgical correction of pre-existing astigmatism is essential in MIOL implantation to minimize the decrease in contrast sensitivity.