Calculating the age of an IOL

Modern intraocular lenses (IOLs) transmit only a part of the spectrum, just like natural lenses, but the transmission of the natural lens changes with age. With increasing age, the lens becomes far less transparent in the violet/blue part of the visible spectrum. Can the transmission of an IOL, therefore, be compared to that of the natural lens at a certain age? In other words, can we assign a virtual age to an IOL? The answer is yes and such an age is far easier to interpret than the complicated transmission curves provided by industry.

Early IOLs were made of completely transparent polymethylmethacrylate (PMMA) material and they not only transmitted all visible light, but also a significant amount of ultraviolet radiation. In the 1980s, however, it was realized that UV radiation might damage the retina and, as a result, all IOLs were provided with a filter that blocked the UV light.

A further modification was made to lens material when scientists hypothesized that, in addition to UV light, short wavelength violet and blue light was also damaging to the retina. Consequently and more recently, certain manufacturers released so-called 'blue-blocker' IOLs onto the market.

To block or not to block?

There is no doubt that short wavelength light can damage the retina. In primates this was first shown in the famous experiments by Ham in the 1970s; the shorter the wavelength, the more dangerous optical radiation proved to be. Such was the importance of Ham's experiments that they resulted in the definition of an action spectrum that is now incorporated into all international standards on the safety of light sources.

There is one important caveat, however - Ham's experiments related to acute damage, caused by very high light levels that are not easily encountered in daily life. In fact, the only natural source that can cause acute light damage to the human eye is the sun. The operation microscope is an important artificial light source that has caused retinal damage.

When discussing spectral filters in IOLs, acute retinal damage is not the concern, the real fear relates to chronic damage that may lead to age related macular degeneration (AMD). Whether this fear is justified, remains to be seen. Epidemiological studies are not equivocal in showing that lifelong exposure to high environmental light levels increases a person's risk of AMD.

And, if the relation between light and AMD exists, what is the action spectrum? This question is nearly impossible to answer. With good reason, however, we might suppose that, as with chronic damage, short wavelength radiation is potentially more dangerous than long wavelength light. Thus, the action spectrum for acute damage is also assumed to apply for chronic conditions.

Industry nowadays advocates IOLs that block at least part of the short visible wavelengths. Understandably, however, the cataract surgeon is thrown into confusion and faces a bewildering variety of transmission curves. How to choose the right one? The answer is simple: compare the filtering properties of an IOL in terms of protection against light damage with that of the natural lens at a certain age.