May 8, 2001 Freedom to move around is a great thing. Just ask a group of researchers who have created a model they hope will lead to better contact lenses. In their eyes, greater lens mobility is needed to improve the safety of extended wear soft lenses.
Their findings are presented in the July issue of Industrial & Engineering Chemistry Research, a peer-reviewed journal of the American Chemical Society, the world's largest scientific society. The study was led by Clayton Radke, Ph.D., at the University of California, Berkeley.
Researchers and lens manufacturers have known that soft lenses need to be able to move when the eye blinks in order to extend the length of time a lens may be worn safely. Up-and-down lens movement is bothersome, Radke says. Lenses that allow a greater degree of in-and-out, or squeezing, motion work better. The in-and-out motion is less noticeable and permits higher levels of lubricity, helping to flush out impurities that can cause eye problems, according to Radke.
Contact lenses for use up to 30 days without removal were approved by the U.S. Food and Drug Administration in 1981. But increased eye infections and other problems led the FDA to revise its recommendation downward to seven days. Design and material improvements could lengthen that period, Radke said.
Researchers focused on the thin layer of moisture between the lens and the eye known as the "posterior lens tear film." Less than a tenth as thick as a human hair, it can trap bacteria, debris and other irritants, causing problems ranging from bloodshot eyes to conjunctivitis, according to Radke.
He said a lens with greater mobility could result in fewer problems. Soft contact lenses currently in use move very little -- typically less than a tenth of a millimeter per blink, he added.
Radke compared a previously published 23-person clinical experiment involving soft lens wearers with his computer models to gauge the performance of the posterior lens tear film. He found that the tear film liquid under a soft contact lens is typically flushed in about 30 minutes, while bacteria and inflammatory debris take a bit longer. Unfortunately, however, even half an hour may be too long for such material to remain in the eye, he said.
Accordingly, new lens designs are needed, he said. "This study is important to understanding the origin of adverse responses some people have with wearing soft contact lenses and to seeking better lens designs to alleviate them. However, there is always a compromise between increased lens motion and comfort."
For this reason, more study is needed to find the range of motion people can tolerate and to design new lenses that can improve the flushing of eye contaminants, he added.
Of the nearly 25 million Americans who wear contact lenses, approximately 80 percent wear soft lenses, according to the American Optometric Association. That percentage includes daily wear, extended wear and disposable contacts.
Most disposable contacts use the same materials as daily wear soft contact lenses. As a result, they also are vulnerable to irritant problems. Newly introduced silicone hydrogel soft contact lenses provide improved oxygen transfer through the lens to the eye but do not eliminate all adverse responses in extended wear, Radke said. An exception is hard (rigid, gas permeable) lenses, which can move up and down by several millimeters per blink and clear the eye of irritants quicker because of their smaller diameter. Their greater movement and smaller size, however, mean hard contact lenses tend to be more uncomfortable and less popular.
The research cited above was partially funded by Bausch and Lomb and Ciba-Vision Corporation.
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