Two-Year Clinical Experience With a Photochromic IOL
It is common for the human crystalline lens to become yellow with age.1-3 There is a hypothesis that the gradual darkening of the human crystalline lens with age may provide a mechanism for blocking harmful violet and blue light from reaching the retina, thus protecting eyes from age-related diseases, such as macular degeneration. Based on this hypothesis, yellow IOLs that block both violet and blue light have been developed for cataract surgery. These include the AcrySof IQ Natural (model SN60WF; Alcon Laboratories, Fort Worth, Texas) and AF-1 UY lens (model YA-60BB; Hoya Healthcare Corp., Tokyo, Japan).
Yellow IOLs are believed to provide additional protection for the retina compared with clear UV-blocking IOLs. Although no human clinical study has provided direct evidence for this additional benefit of yellow IOLs, some studies indirectly suggest that blocking both violet and blue light may reduce the risk for macular degeneration or its progression.4-10 Yellow IOLs have been demonstrated to protect lipofucsin-containing retinal pigment epithelial cells from being damaged by blue light.7-9
At present, however, ophthalmologists are divided on the optical performance of yellow IOLs.11-14 Although they are touted as providing additional benefits to cataract patients for retinal protection, yellow IOLs have been shown to decrease color perception in at least one study,15 and they are believed to compromise scotopic vision, which is important for common evening activities, such as driving at night.11,12
The concern that a yellow IOL compromises night vision is based on the fact that rod-mediated scotopic sensitivity increases from the violet- to the blue-light range and peaks at 507 nm in the blue-light range. Therefore, a yellow IOL that blocks blue light inevitably decreases the scotopic sensitivity of cataract patients. Consequently, their night vision may be compromised.
In a recent report by Mester et al,15 in an intraindividual comparison of a blue-light–filtering IOL with a nonblue-light–filtering control IOL, color perception with the blue-light-filtering IOL was significantly reduced under mesopic conditions over 12-month follow-up and under photopic conditions over 6 months.
To overcome the potential disadvantages of poor color perception and reduction of scotopic and mesopic vision with a blue-light–filtering IOL, researchers at Medennium (Irvine, California) have developed an IOL with photochromic properties. This photochromic IOL, the Matrix Aurium, is based on Medennium's Matrix Acrylic, with a patented hydrophobic acrylic material in a three-piece, sharp-edged design. The hydrophobic acrylic material has a refractive index of 1.56.
The Aurium is colorless and behaves like a normal UV-blocking IOL in the absence of UV light. When exposed to UV light, (eg, sunlight), the IOL turns yellow in a few seconds. When the UV stimulus is removed, the IOL switches back to colorless, again in a few seconds.14 The yellow color of the Aurium IOL can be repeatedly turned on by the presence of UV light and turned off by the absence of UV light (Figure 1).
The design principle for the Aurium is based on that fact that in sunlit conditions, blue-light intensity is more than 10 times stronger than in indoor lighting conditions (Figure 2).16 Accordingly, in strong blue-light conditions, an intelligent IOL should become yellow to filter the blue light, while in weak blue-light conditions, such as scotopic or mesopic conditions, it should become colorless to allow all blue light to pass through the lens.
Researchers at the University of Utah14 observed repeated switching-on and switching-off of this photochromic property in rabbit eyes. At the end of the study, explanted photochromic IOLs showed no detectable deterioration in their photochromic property.
According to Medennium, the photochromic property of the Aurium lens has been demonstrated to be stable for up to 23 years in a solar exposure simulation experiment, exceeding the International Organization for Standardization (ISO) requirement for photostability of 20 years for IOL materials. The UV-visible–light curve of the Aurium with and without the presence of UV light is shown in Figure 3.
CLINICAL EXPERIENCE
We performed a prospective intraindividual comparison of BCVA with a photochromic IOL (Matrix Acrylic Aurium) versus a nonphotochromic counterpart (Matrix Acrylic) in human eyes. In this prospective comparative eye study, 10 patients underwent bilateral IOL implantation, with the Aurium randomly selected for one eye and the Matrix Acrylic in the contralateral eye. BCVA for both eyes was measured using Snellen charts under various lighting conditions. The lighting conditions started at a low level, such as 11 lux, and gradually increased to photopic conditions (1,000 lux). The mean of the 10 patients' BCVA at specific lighting conditions is plotted against the lighting intensity in Figure 4.
Ten patients completed 24 months of follow-up. The mean postoperative BCVA (in logMAR) in eyes with both the study IOL and the control IOL increased as the illuminance levels increased, especially significantly in low illuminance conditions. The mean postoperative BCVA with the Aurium IOL was found to be similar to that for the control IOL (Figure 4).
Follow-up at 2 years indicated that the Aurium appears to be as safe and efficacious as the control IOL.
Guillermo Avalos, MD, is in practice at the clinic Laser Oftalmico in Guadalajara, MÈxico. Dr. Avalos states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +52 33 36410972; fax: +52 33 36410978; e-mail: gavalosu@gmail.com.
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