Hard and soft contact lenses made out of a material called silicone hydrogel may have a lower risk of infections associated with their use than traditional extended-wear lenses.

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. Daily-wear contact lenses and particularly extended-wear lenses -- those worn even when sleeping -- can increase a person's risk of getting eye infections, including the very serious condition known as ulcerative keratitis.

The new contact lenses should reduce risk of infection by about 10-fold. The risk should drop from 1 infection per 2,500 people who use daily-wear lenses and 1 in 500 for extended-wear lenses, to 1 in 25,000 and 1 in 5,000, 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.

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

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.

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.

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.

Ophthalmology January 2002;109:27-40