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When the performance of four different aspheric IOLs in scotopic light conditions was evaluated using a computer model, the results favored one three-piece lens (Tecnis, Advanced Medical Optics) for providing the best compromise between spherical and chromatic aberrations and depth of focus.
"Our results demonstrate that it is insufficient to design an aspheric lens that maintains a certain degree of spherical aberration in order to obtain a certain degree of pseudoaccommodation. In fact, if the quantity of light focused along the optical axis in front of and behind the retina does not guarantee sufficient illumination, this light is useful only to decrease the quality of the main image," said Dr. Franchini, professor of ophthalmology , University of Florence, Italy.
"As our study shows, some residual sphero-chromatic aberrations in aspheric lenses may enhance the depth of focus by decreasing image sharpness or the amount of light on the fovea."
Dr. Franchini told attendees that the introduction of the three-piece IOL as the first aspheric pseudophakic implant represents one of the most important innovations in cataract surgery considering its benefit for increasing quality of vision secondary to reduction of spherical aberration. Subsequently, other leading IOL manufacturers introduced their own versions of an aspheric IOL, but the various available lenses differ with regard to their design and the amount of spherical aberration they correct.
Recognizing that spherical aberration affects depth of focus and pseudoaccommodation, Dr. Franchini undertook a study using computer-assisted analysis to compare the aforementioned three-piece lens with three other aspheric IOLs: a one-piece lens (AcrySof IQ, Alcon Laboratories) and two other three-piece lenses (KS-3Ai, Canon-Staar; SofPort, Bausch & Lomb). The study was recently published [J Cataract Refract Surg. 2007;33:497-509]. Testing was performed using a pseudophakic eye model and sophisticated ray-tracing software with a 5.0-mm pupil diameter to simulate scotopic vision.
"In fact, all lenses guarantee good depth of focus through a small pupil," noted Dr. Franchini.
Each of the four lenses was subjected to a defocus of ±0.8 mm. The behavior of geometrical aberrations was evaluated through spot diagrams for three colors; monochromatic point spread function for yellow, red, and blue; and monochromatic modulation transfer function.
Depth of focus behavior of the various IOLs was analyzed by determining the variation of the Strehl ratio for the three colors in each IOL. The results showed the Tecnis lens had a good depth of focus and Strehl ratios for all colors with good overlapping of the yellow and blue curves, which are most relevant to mesopic vision. In contrast, the AcrySof IQ IOL had a very good Strehl ratio but poor color overlap, whereas color overlap was good for the SofPort lens, but it had poor Strehl ratios. The KS-3Ai IOL had intermediate Strehl ratios and poor color overlap.
In further analyses conducted to gain a better understanding of the results, a threshold of Strehl ratio was established at 10%, and the corresponding depth of focus for the three colors or yellow and blue was computed. Then, the Gauss formula was used to define the range over which each lens provided clear distance vision.
Considering the blue and yellow wavelengths, the Tecnis IOL offered depth of focus at 0.16 mm, which corresponds with clear vision from 1.829 m to infinity. The range of distinct vision for the AcrySof IQ lens was from 1.7 m to infinity but began at 9.65 m for the SofPort IOL and at 4.45 m for the KS-3Ai lens.
For the analyses based on the three colors, the findings indicated that the Tecnis IOL would provide clear vision from 3.3 m to infinity. The results could not be evaluated for the AcrySof IQ or SofPort lenses. The depth of focus determined for the KS-3Ai IOL corresponded with distinct vision from 4.45 m to infinity.