Contact Lens Materials
Ultraviolet-Absorbing Contact Lenses
Ambient ultraviolet (UV) radiation is ubiquitous. We're all exposed to this form of radiant energy in one form or another. UV exposures for the most part occur via solar radiation. While we all enjoy the cosmetic and psychological benefits of a suntan, we as eyecare practitioners need to always be mindful of the harmful effects of UV radiation.
Potential for Damage
UV radiation is broken down by wavelength. UVC is classified as encompassing 100nm to 280nm, UVB encompasses 280nm to 315nm and UVA encompasses 315nm to 380nm (although some consider the upper extent to be 400nm).
Electromagnetic radiation is generally considered damaging if it contains enough energy to ionize molecules. Shorter wavelength radiation has more energy than longer wavelength radiation does. Thus, UV radiation has more energy than visible light and infrared radiation do. Molecular changes can occur with as little as 3.1eV, which is associated with 400nm radiation (although the retina is sensitive to 380nm).
We don't typically focus too much on UVC because the atmosphere filters it. Most of our focus should remain on UVA and UVB, which have the most potential for tissue-related damage. Tissue damage from UV radiation is generally associated with photochemical processes leading to damage that the system may repair or to damage that causes a permanent defect or mutation.
We should have significant concerns for our patients regarding ocular tissue damage caused by UV radiation. Generally, of all UV radiation to which the eye is exposed, the cornea absorbs approximately half, while the crystalline lens absorbs the other half.
It's important to note that UV radiation really doesn't reach the retina (in phakic individuals), so our concern focuses on the anterior segment and crystalline lens. Anterior segment pathologies associated with UV radiation include photokeratitis, pterygium and conjunctival squamous cell carcinoma. Cataracts are a major concern; in particular, direct epidemiological and basic links exist showing UVB's etiological role in cataractogenesis.
We must clinically assess our patients' exposure levels, including both recreational and occupational exposures. The biggest preventive strategies include patient education about risks and damage. Other general recommendations include the use of appropriate clothing and brimmed caps, proper spectacle protection (both in clear and tinted materials) and even contact lenses.
Contact Lenses and UV Rays
Unlike spectacle lenses, there are no ANSI guidelines associated with UV-blocking contact lenses, although the FDA does allow for UV blocking labeling. In this regard, a Class 2 contact lens absorbs 70 percent of UVA and 95 percent of UVB (the same as the ANSI standard for a sunglass lens). A Class 1 contact lens absorbs 90 percent of UVA and 99 percent of UVB.
Several manufacturers incorporate UV-absorbing characteristics into their polymers, and to my knowledge the first hydrogel associated with this novel technology was the Precision UV lens (now CIBA Vision). Today, Class 2 materials are manufactured by Vistakon, CooperVision, CIBA Vision, Dosco and Sauflon. Vistakon manufactures Class 1 materials.
Contact lenses are an attractive means of complete internal ocular protection given the coverage by the lens of the entire cornea and entrance pupil of the eye. This once again highlights the tremendous benefits contact lenses afford our patients.
Dr. Nichols is assistant professor of optometry and vision science at The Ohio State University College of Optometry.