Two-part analysis reveals differences among multifocal IOLs

July 15, 2004

San Diego-Experiments in model eyes that examined the patterns of light transmittance and image quality point out the differences in how various multifocal IOLs work and how some designs may have advantages over others.

San Diego-Experiments in model eyes that examined the patterns of light transmittance and image quality point out the differences in how various multifocal IOLs work and how some designs may have advantages over others.

The results of these studies were presented here at the annual meeting of the American Society of Cataract and Refractive Surgery by Thom Terwee, who is manager, biophysical research, AMO-Groningen BV, Groningen, The Netherlands.

In the experiments, various IOL designs were placed in a model eye that was filled with liquid to simulate the refractive properties of aqueous fluid and to allow the pattern of transmitted light to be seen. The model incorporated an artificial "cornea" designed to impart the same degree of spherical aberration as an average human cornea. An artificial iris was set at a default diameter of 5 mm to approximate mesopic viewing conditions, and in some experiments was varied to simulate the function of IOLs at other pupil sizes.

"The Tecnis ZM001 is a combination of the spherical-aberration-correcting design of the monofocal Tecnis lens on the anterior optic surface with a multifocal diffractive pattern on the posterior side," Terwee said. "The diffraction pattern creates two major focal points that are 4 D apart. This difference is equivalent to almost 3 D in the spectacle plane, which would create a near focus at the reading distance of about 35 cm."

The light coming into the eye was evenly divided between the two focal points after passing through the Tecnis multifocal lens, Terwee said.

Light transmitted by the Array refractive multifocal IOL, on the other hand, consisted of several bundles refracted by the various zones of the lens optic that converged at near and far focal points.

"The most widely used multifocal IOL today is the AMO Array multizone refractive IOL," Terwee noted. "This lens has five zones that refract light into two main focal points 3.5 D apart, corresponding to a near reading distance of about 40 cm. The effect of the various refracting zones on light distribution is to focus 60% of the effective light at far and 40% at near."

This difference was evident when examining the patterns of green light transmittance in the model eye.

"When we compare the light paths of the Tecnis and Array multifocal lenses at high magnification, the effect of the conceptual differences between the lenses could be seen," Terwee said. "The entire optic of the Tecnis contributed evenly to both far and near focal points, while the multizone Array created more discrete light distribution."

In another test, the transmitted image of a U.S. Air Force target was captured using a CCD camera. Subjective assessment of image quality was then made.

According to Terwee, the Tecnis ZM001 IOL gave the best image quality at a pupil size of 5 mm. At both near and far distance, the image quality was superior to the images transmitted by monofocal IOLs.

"The CeeOn 811E diffractive IOL performed very well as a multifocal from an optical point of view," Terwee said. "However, the superiority of the image with the spherical aberration correction of the Tecnis was seen clearly."

The quality of the image was also relatively constant for all pupil sizes, he added.

"The Array provides no near focus in pupils smaller than about 2.5 mm because of the distance-dominant center design, and modulation transfer function values are lower for the Array than the Tecnis at all pupil sizes," he said.

The comparison of image quality with the Tecnis multifocal versus the spherical monofocal CeeOn Edge 911 led to an unexpected result.