Comparing a refractive multifocal IOL with a diffractive multifocal IOL showed that the diffractive multifocal lens provides excellent intermediate visual acuity despite theoretical predictions to the contrary.
"Multifocal IOLs can have either a refractive or diffractive design," said Dr. Beiko, assistant professor of ophthalmology, McMaster University, and lecturer in ophthalmology, University of Toronto. "Comparisons of the two designs conducted previously have found that refractive multifocal IOLs provide good distance and intermediate vision, while diffractive multifocal IOLs provide excellent distance and near vision.
"However, those comparisons were confounded because the comparisons invariably involved multifocal lenses not only of different optical designs (refractive versus diffractive) but also of different IOL materials, optic edge designs, and/or IOL designs," he added.
However, there are differences between the two IOLs, Dr. Beiko said.
"The near vision mechanism depends on either refraction, by which 100% of light is used, or diffraction, by which 82% of light is used and 18% is lost," he said. "The refractive multifocal IOLs are pupil dependent while diffractive multifocal IOLs are pupil independent."
In addition, the diffractive multifocal IOL has a full anterior aspheric design that corrects for 0.27 μm while the refractive multifocal IOL has asphericity of –0.37 μm under photopic conditions. Both of the IOLs are made of the same low-chromatic-aberration acrylic material. However, the diffractive design of the diffractive multifocal IOL reduces the chromatic aberrations at near even further.
Refractive multifocal IOLs use the bending of light as it passes through materials of differing refractive indices; these IOLs often use alternating zones that work independently-each zone creates one focal point. Refractive multifocal IOLs of the progressive-addition type incorporate a range of foci from near to distance. In contrast, diffractive multifocal IOLs use the spreading of light as it encounters an edge or a step. These IOLs use a series of steps (gratings) and each step creates multiple focal points, according to Dr. Beiko.