Next aberration you need to know

Key Points

Ten years ago when the concept of an aspheric foldable IOL was first introduced, many surgeons and other members of the ophthalmic community doubted that an aspheric design would have much success. Nevertheless, by 2009, 81% of cataract surgeons in the United States listed an aspheric lens as their preferred hydrophobic acrylic IOL, according to a survey of members of the American Society of Cataract and Refractive Surgery.¹

Given the unprecedented growth of interest in correction of monochromatic optical aberrations to improve the quality of pseudophakic vision, it is natural that the next step in our evolution will be to investigate polychromatic light and its inherent aberrations.

Chromatic aberration develops out of the variation of an optical material's refractive index with the wavelength of light. The common clinical red-green duochrome test relies on chromatic aberration. Chromatic aberration occurs because different wavelengths of light are bent to a different extent. The longer wavelength (red) is refracted less than the shorter (green), and therefore the red light is focused further posterior (hyperopia) while the green light is focused more anterior (myopia). If the letters on the red side stand out more, the subject needs more minus power; if the letters on the green side stand out more, the subject needs more plus power. Neutrality is reached when the letters on both backgrounds appear equally distinct.²

In terms of optical materials, the Abbe (pronounced "abby") number is used to quantify chromatic aberration, the value (often symbolized by the Greek letter n or the letter V) expressing the dispersion of an optical medium, which is directly proportional to the chromatic quality of a lens. Its value is given by the expression:

V = n = (n_d – 1)/(n_F – n_C)
where n_d, n_F, and n_C are the refractive indices pertaining to the wavelengths of the Fraunhofer lines 587.6, 486.1, and 656.3 nm, respectively.

The Abbe number of an optical glass mixture is carefully monitored and controlled by manufacturers to achieve target dispersion values, which usually range between 20 and 60. A higher number means less dispersion and therefore reduced chromatic aberration.

Chromatic aberration came to the attention of IOL manufacturers because the acrylic and silicone polymers used in the production of IOLs vary in regard to their Abbe number. For example, Negishi et al.³ studied 51 eyes of 33 patients who underwent cataract surgery. The eyes were divided into three groups according to the material from which their IOL was made: group 1, polymethylmethacrylate (PMMA); group 2, silicone; and group 3, an acrylate/methacrylate copolymer.

Ten normal phakic control eyes (group 4) underwent the same examination. Best-corrected distance visual acuity and contrast sensitivity were measured under white light and monochromatic light with wavelengths of 470 nm, 549 nm, and 630 nm, with the best correction under white light.

The authors found that there were no significant differences in best-corrected visual acuity and contrast sensitivity under the 549-nm monochromatic light in any group. However, under both white polychromatic light and 470- and 630-nm monochromatic light, the mean contrast sensitivity in group 3 tended to be lower, sometimes significantly, than in the other IOL groups. These results suggested that longitudinal chromatic aberrations of some IOLs may degrade the quality of the retinal image.³

Franchini⁴ analyzed spot diagrams to capture light dispersion, monochromatic point-spread function (PSF), and monochromatic modulation transfer function (MTF) to evaluate image contrast. He also evaluated depth of focus at best focus and –0.08 mm and +0.08 mm from best focus for four aspheric monofocal IOLs (Tecnis, Abbott Medical Optics; AcrySof IQ, Alcon Laboratories; SofPort AO, Bausch + Lomb; and KS-3Ai, STAAR Surgical).⁴

The spot diagrams for the Tecnis IOL were well focused within a diameter of 10 µm.