The device, which is being developed by Jin-Hui Shen, an ophthalmologist at Vanderbilt University's School of Medicine in Nashville, Tennessee, contains six overlapping lenses. As the muscles in the eye relax, the overlap increases, allowing patients to focus on closer objects.
Millions of people are given artificial lenses to replace their own lenses when they become clouded by cataracts (right). But these consist of a single, fixed-focus lens, so patients can only focus at a set distance.
"I find road signs particularly difficult," says British cataract patient Valerie Kingsland, who has an artificial lens. She can only focus on objects about two metres away, and has to wear glasses to read or drive. Being able to focus between these extremes would be "enormously useful", she says. Other lens implants, like some contact lenses, are "multifocal", but they still only focus at fixed distances, says Shen. But he has now patented a device containing six overlapping lenses arranged like the petals of a flower (see below). Like other artificial lenses, the device would be inserted into the lens capsule of the eye after the cloudy lens had been removed. "It would be almost the same procedure," Shen says.
Flexible springs connect each lens to the edge of the lens capsule. When the rim of the device is squeezed by the relaxing eye muscles, the degree of overlap between the lenses increases, allowing the eye to focus on closer objects. When the muscles contract, pulling on the rim of the implant, the lenses spring back out again to allow focusing on more distant objects.
"A limited change [in shape] results in a large amount of focus change," Shen says. He has calculated that these changes in the shape of the lens capsule should produce a focal range of 10 dioptres-as good as a healthy lens in a young person.
"In principle it could work," says George Waring, an ophthalmologist at Emory School of Medicine in Atlanta. But cells left behind after surgery can proliferate and cause the lens capsule to stiffen, he says. For variable focus lenses, this could be a problem.
Shen has yet to build lenses that could be implanted in humans, so it will be at least 18 months before trials are carried out. If such trials are successful, however, some patients might not need to wear glasses at all-unlike most elderly people, who gradually lose the ability to focus at different distances as the natural lens becomes more rigid with age.
Author: Duncan Graham-Rowe
New Scientist issue: 30 June 2001
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