Theoretical analysis examines depth of focus with aspheric IOLs

April 30, 2007

"Everybody's interested in aspheric IOLs, and one of the things you hear commonly said is, 'There's a big trade-off if you put in an aspheric IOL, because you're going to lose depth of focus (DOF).' We wanted to look at whether that's really true," said Douglas D. Koch, MD, professor of ophthalmology and holder of the Allen, Mosbacher, and Law Chair in Ophthalmology, Cullen Eye Institute, Baylor College of Medicine, Houston.

"Everybody's interested in aspheric IOLs, and one of the things you hear commonly said is, 'There's a big trade-off if you put in an aspheric IOL, because you're going to lose depth of focus (DOF).' We wanted to look at whether that's really true," said Douglas D. Koch, MD, professor of ophthalmology and holder of the Allen, Mosbacher, and Law Chair in Ophthalmology, Cullen Eye Institute, Baylor College of Medicine, Houston.

To examine the issue, he and colleagues performed theoretical analysis using Zernike tool software (Advanced Medical Optics) that enabled them to model different amounts of spherical aberration (SA) and defocus. Their work incorporated the Stiles-Crawford effect.

Two IOLs (AcrySof and AcrySof IQ, Alcon Laboratories) were included.

"We used two different kinds of cornea in this model," Dr. Koch said. "We used a so-called perfect cornea that had no higher-order aberrations (HOAs), [and] . . . we also looked at average corneas" of real-world patients aged 40 to 80 years.

In the latter group, the researchers found no apparent significant loss of DOF with a monochromatic light source. With a polychromatic light source, "the best result in these eyes was a little bit of negative SA," Dr. Koch said.

Regarding a reduction in SA by the aspheric lens, he said, "There's a modest increase in modulation transfer function (MTF), the largest benefit is monochromatic, there is improvement in quality of vision, there's minimal if any decrease in DOH, [and] it's highly dependent on the other HOAs of the eye. For best quality of vision, on average, we found that net SA for the eye as a whole may provide the best-quality vision. If you reduce all HOAs, eliminate them, you get the largest increase in MTF, but you do get some loss of DOF."