I implant the AcrySof Natural IOL (Alcon) in all of my patients, and have been doing so ever since this IOL first became available, when I switched immediately from using clear lenses. I have never used any other coloured IOL because I have never seen any advantages to other lenses or had a reason to change.

I last implanted a clear lens about six months ago, when I was working on a patient who already had a clear IOL in the eye I wasn't operating on; the patient was a very compulsive, detail-oriented engineer. I knew he would notice a difference and I thought it would be very difficult for me to explain to him why a blue-light filtering lens is better. Other than isolated incidents like this, I implant coloured IOLs in all my patients. Moreover, I believe that in future, all IOLs will be designed to filter blue light.

Filtering blue light is natural

For me, there is no question that a blue-light filtering IOL is much more natural for the human eye than a clear IOL. Only an aphakic eye is exposed to the same light spectrum as an eye with a clear IOL; mediated by a crystalline lens, blue light is filtered. Not having a colour filter is a very unnatural state for the human eye.

In a crystalline lens of any age, even that of a four-year-old, there is filtration of blue light beyond what a non-filtering clear IOL provides. Additionally, as we age and develop nuclear sclerosis, the crystalline lens yellows even further: this is a protective rather than a detrimental mechanism, because as we age we build up lipofuscin in the retina. Blue light activates the lipofuscin, releasing free radicals and damaging the retina, so the yellowing is the lens's attempt to minimize this.

Because of the inherent yellow tint of the crystalline lens, filtering blue light artificially allows us to create a light-stimulation of melanopsin that is similar to what we've evolved to experience as human beings. If one looks at the melanopsin absorption curve — assuming that melanopsin is indeed a trigger for melatonin and therefore a significant influence on circadian rhythms — filtering blue light will, contrary to some suggestions, lead to maintenance of healthy circadian rhythms. It is true that excessive yellowing of the lens could theoretically decrease the activation of melanopsin and therefore interfere with circadian rhythms, but implanting a blue-filtering lens and erring on the side of caution nevertheless allows a more natural light exposure, which makes more sense to me than exposing the patient to wavelengths of light that the retina has never seen.

Implanting a coloured IOL will therefore not only maintain the protection against lipofuscin activation offered by a crystalline lens in its natural state, but will also preserve the body's natural rhythms.

AMD protection: small but important

Blue-light filtering lenses also have a potential — and, I think, probably real — benefit in protecting against age-related macular degeneration (AMD). It is not the most efficient way of protecting against AMD, as any protection that the lens provides is small, but it is one of the very few factors that the surgeon can control. If I were forced to pick between having a patient stop smoking or implanting a filtering lens, I'd say stop smoking. If it's a matter of them changing their genetic make-up or me putting in a lens, I'd say "change your genetic make-up". I don't have control over those things, but I can control which model of lens I choose to implant. Even though the protective effect may only be small, I do not believe that there is any demonstrated downside to implanting coloured IOLs, and so I do not know why anyone would opt not to implant a blue-light filtering lens.