No Free Lunch in Optics

Binkhorst Lecture focuses on compromises and challenges in optics. Dermot McGrath reports from the 39th Congress of the ESCRS in Amsterdam.

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Binkhorst Lecture focuses on compromises and challenges in optics. Dermot McGrath reports from the 39th Congress of the ESCRS in Amsterdam. Every intraocular lens designed to correct presbyopia, irrespective of its design properties and material, involves a trade-off in visual performance once implanted in the eye, said Gerd Auffarth MD, FEBO, during the annual Binkhorst Medal Lecture. In a wide-ranging lecture focused on the inherent compromises in multifocal and presbyopic-correcting IOLs, Professor Auffarth stressed the importance of neuroadaptation and brain function in the performance of any implanted lens, adding it was important to appreciate how intraocular lenses perform under real-life conditions once inside the eye. “If an intraocular lens company tells you that its new lens is perfect and that everything is great, we should be immediately on our guard as the reality is a lot more complex than that—there really is no such thing as a free lunch in IOL optics,” he said. Turning to the question of optical performance, Prof Auffarth noted presbyopia correction remains one of the greatest challenges in ophthalmology. He said any attempt to correct presbyopia, a natural but complex process of physiological insufficiency of accommodation associated with eye ageing, needs to take due account of three interrelated concepts: visual quality, depth of field, and dysphotopsia. “If we want to increase the depth of field, we will reduce visual quality and increase dysphotopsia. We have to play with the aberrations of the eye and other factors to maximise visual quality and balance out a certain amount of depth of focus in order to achieve our goal,” he said. Dysphotopsias include a wide range of visual symptoms such as halos, glare, and starbursts, but not all of them are necessarily caused by the implanted IOL, he noted. “The optics of an intraocular lens are really the reason for the halos. However, glare and starbursts can also be introduced by refractive error, defocus issues, ocular surface diseases, or other opacification in the optical pathway,” he said. Chromatic aberration is another important influencing factor in IOL performance. It occurs when light rays pass through an optical media at different points according to their wavelength. Longitudinal chromatic aberration (LCA) causes shorter wavelengths to focus in front of longer wavelengths resulting in a difference of focus. The cornea, the crystalline lens, and the implanted IOL can all contribute to chromatic aberration. The type of IOL material used also plays a role in the extent of the chromatic aberration, Prof Auffarth explained. “IOL material affects chromatic aberration and has a direct impact on it. For instance, hydrophilic IOLs have a lower LCA than hydrophobic lenses. While a refractive optic produces a given chromatic aberration, with a diffractive optic, the chromatic aberration can be manipulated and reversed to reduce the dispersion.” While this sounds beneficial in principle, the reality is LCA correction actually has a negative impact on the depth of focus in pseudophakic eyes, Prof Auffarth concluded. “Once again we are back to the reality of there being no free lunch in optics,” he said. Prof Auffarth said the variety of lenses on the market, using different optical principles, and the terminology used to describe them—enhanced monofocal, extended depth of focus (EDOF), extended range of vision (ERV), and trifocal lenses—was understandably confusing for many ophthalmologists. “These categories are overlapping and interacting, and it is really difficult sometimes to match the patient profile with the lens category being proposed,” he said. He noted that as a general rule, the further one moves away from monofocal designs towards EDOF-ERV and then trifocal lenses, the greater the propensity for dysphotopsias to occur. One compelling solution to try to maximise the specific advantages of different lenses is by adopting a mix-and-match approach Prof Auffarth calls “binocular trifocal”. “We can for instance put a bifocal lens in one eye for distance and near vision, and an EDOF lens in the other eye for distance and intermediate vision. Or we can try blended vision with an EDOF lens in both eyes but use different power additions to target different distance ranges,” Prof Auffarth said. Combining different lens types in an individualised manner can achieve better binocular visual outcomes with enhanced depth of focus and reduced incidence of dysphotopsias, he concluded.