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By Keith Mulvihill

NEW YORK (Reuters Health) – Hard and soft contact lenses made out of silicone hydrogel may have a lower risk of infections associated with their use than traditional extended-wear lenses, a study suggests.

Recently, the US Food and Drug Administration gave permission to two contact lens manufacturers to market silicone hydrogel lenses for extended 30-day-wear soft contact lenses. However, the material is not yet approved for use in extended-wear hard contact lenses.

"The new material makes for a safer contact lens compared to anything else out there," said lead investigator Dr. H. Dwight Cavanagh, of the University of Texas Southwestern Medical Center at Dallas, in an interview with Reuters Health.

Dr. Cavanagh noted that the new contact lenses should reduce risk of infection by about 10-fold. The risk should drop from 1 infection per 2500 users of daily-wear lenses and 1 infection per 500 users of extended-wear lenses, to 1 in 25,000 and 1 in 5000, respectively.

The success of the new material lies in its ability to let in about 6 to 7 times more oxygen than traditional contact lenses, Dr. Cavanagh explained.

"The lenses also do not interfere with the eyes' own ability to minimize bacteria's ability to bind to the eye," he said. "If the bacteria can't bind to the eye then it can't infect the eye."

In the study, which is published in the January issue of the journal Ophthalmology, the investigators followed 178 patients for 1 year. Some of the patients wore hard or soft contact lenses made of the new material continuously for 6 or 30 days. These subjects were compared with other patients who wore traditional extended-wear soft contact lenses continuously for 6 days.

At 1, 3, 6, 9, and 12 months of contact lens use, all patients visited the researchers and had their eyes irrigated to collect eye-surface cells. Dr. Cavanagh's team then measured the ability of a common eye-infecting bacterium to bind to the cells. This method predicts possible eye infection rates.

The study, which was funded in part by contact lens manufacturers, found that both hard and soft lenses made of silicone hydrogel and worn continuously for either 6 or 30 days produce "significantly less" binding of the keratitis-causing bacteria to cells in the eye.

While the findings suggest that the new material may reduce infection rates for extended-wear contacts, the authors note that future studies should examine specific risks for ulcerative keratitis, including type of lens and the time spent wearing such lenses. The results indicate that over time (about 9 months) the eye adapts to the extended-wear contact lens, and eye cells are less likely to bind to keratitis-causing bacteria.

The current study provides "sound scientific evidence that the high-oxygen transmissible silicone hydrogels are safer and have less of an adverse physiologic effect on the cornea when worn on an extended-wear basis," writes Dr. Peter C. Donshik, a Bloomfield, Connecticut-based physician, in an accompanying editorial.

Not only does the new study indicate that the risk of ulcerative keratitis should be less likely, but "there is no significant difference in the corneal response when these contact lenses are worn for 30 days compared with 7 days," he adds.

These and other findings "are very important in reintroducing the concept of wearing contact lenses on an extended-wear basis," the editorialist concludes.