Eyeworld Supplements

Clinical relevance So why should we pay attention to CA in IOL material and design? Does it really matter to our patients? I would argue that it matters a great deal, for three reasons. The first is simply the pursuit of "super" vision, or the highest possible quality of vision. Since the images we process consist of all the colors of the spectrum, reducing CA increases con- trast, allowing images to be as sharp and as crisp as they can be. To improve CA, we want to use IOL materials with low dispersion and an Abbe number that, if possible, exceeds that of the human lens (47). Increasing the Abbe number of optic materials has been shown to improve overall pseudophakic optical performance. 9 If we can theoretically optimize vision in the daytime, bringing all wavelengths in focus and minimizing CA, then images will also be in focus at night when blue light is more dominant. Less CA reduces myopic shift and could potentially reduce difficulty with night driving. Finally, a starting point of higher visual quality and sharper contrast pro- vides patients with the ability to better withstand other challenges to their vision, whether intentional (multifocal IOLs) or unintentional (dry eye, age- related macular changes). By choosing IOLs with low disper- sion or design features that deliberately diffract light in such a way as to reduce CA, we can actually improve quality of vision after cataract surgery. 5 Figure 4: Chromatic aberration and IOLs Blocking portions of the spectrum Sumit "Sam" Garg, MD I ntraocular lenses have been designed with the intention of blocking specific wavelengths of light that have been considered harmful or potentially harmful, including ultraviolet, violet, and blue light. Ultraviolet The natural crystalline lens does a very good job of blocking ultraviolet (UV, <400 nm) light, and since at least the mid-1980s, IOLs have tried to replicate that ability. We know that UV radiation is harmful to the skin and can cause damage to the external surfaces of the eye, such as the eyelids, cornea, and conjunctiva. UV exposure is associated with cataract development and likely plays a role in retinal conditions like macular degeneration, although this is less well documented. Most IOLs effectively block UV-B (280–315 nm) radiation, which is believed to be responsible for UV-related ocular pathology and UV-C (200–280 nm), most of which is absorbed by the atmosphere. Over the years, there has been some variation in how well different IOLs block UV-A (315–400 nm) light, 10 and an early version of the Crystalens (Bausch + Lomb, B+L, Bridgewater, N.J.) provided very little UV protection. Blocking UV is a reasonable strategy and a desirable lens characteristic given the potential risks and the fact there is no detriment to blocking non-visible light. Violet At one time, B+L offered IOLs that blocked violet (400–440 nm) light, in addition to UV light. UV phototoxicity is highest in the ultraviolet range, still relatively high in the violet range, and drops off through the blue portion of the spectrum. The purported value of the violet-shield lenses was reduction of oxidative stress on the retina that causes cellular damage. 11,12 However, this idea never really caught on with clinicians or IOL manufacturers. Blue There are currently two types of IOLs that block portions of the blue (440–500 nm) spectrum: the Alcon (Fort Worth, Texas) AcrySof platform and the Hoya (Chino Hills, Calif.) AF-1. Since blue-blocking lenses were first introduced in the 1990s, proponents have argued that blocking blue light could protect the retina from oxidative stress and prevent age-related macular degeneration, while critics have argued that they nega- tively affect circadian rhythms and color vision and reduce scotopic sensitivity. Studies have failed to conclusively prove that blue-blocking lenses provide any significant benefit or harm. 13,14 Given that there are no quality of vision or clear health advantages to blocking blue light transmission, I prefer to use clear lenses that more closely mimic the young human lens and transmit blue light fully. In my experience, there can be a subtle impact of blue blocking on color perception, particularly in highly discerning patients, so I also avoid mixing blue-blocking and clear lenses in the same patient. In evaluating how IOLs transmit light, we should consider the potential positive and negative effects. A good principle is to mimic the properties of the crystalline lens but avoid unnecessarily blocking any portion of the visible spectrum.

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