Silicone Hydrogel Soft Contact Lenses - Up to 30 Day Wear
The development of silicone hydrogel contact lenses has revolutionized the contact lens industry. This new generation of ‘super-permeable’ contact lenses allows much more oxygen to reach the cornea than its predecessors – which is a key to maintaining healthy eyes. This breakthrough has allowed the creation of ‘extended wear’ contact lenses which can be worn 24 hours a day for up to a month at a time. Silicon hydrogel is so impressive that more than half the contact lenses on the market are now made of this material.
What's so special about silicone hydrogel?
Traditional soft contact lenses are made from hydrogel polymers which are soft, water-containing plastics. The plastic itself is not oxygen-permeable; it's the water content that allows oxygen to pass through it and into the eye. Trouble is, the water content can only carry so much oxygen, and the more water content a contact lens has, the more likely it is to dehydrate after long periods of wear.
Silicone is an oxygen permeable substance, so oxygen can be transmitted to the eye through both the water content and the silicone, allowing for much more oxygen transmission. Because of this, silicone hydrogel lenses can contain less water than traditional hydrogel lenses, and are less likely to dehydrate, improving end-of-day comfort. Silicone hydrogel has also allowed continuous wear contact lenses to be safe and practical for up to 30 days. (Note: While silicone hydrogel has allowed for the development of 30 day lenses, not all silicone hydrogel lenses are approved for 30 day continuous wear.)
Safety concerns about extended wear lenses
Extended wear contact lenses were introduced in the 1980s but safety concerns soon saw them withdrawn from the market. But thanks to the development of silicone hydrogel contact lenses, extended wear for up to 30 days is once again approved by the FDA. After 30 days these lenses are discarded and replaced so long-term build-up of deposits is avoided.
As part of the FDA approval process, CIBA Vision (the maker of Night and Day silicone hydrogel lenses) was required to track 6000 patients who were prescribed their contact lenses to be worn continually for 30 days. After one year, the incidence of microbial keratitis, an eye infection known to cause vision loss, was less than 0.18% and the rate of decreased visual acuity from this infection was 0.04%. While higher than rates reported for daily wear lenses, these rates were considered reasonable. In addition, a 2005 study published by the University of Manchester in Britain showed that those who slept in traditional hydrogel lenses were five times more likely to develop keratitis than those who slept in silicone hydrogel lenses.
Extended wear lenses are so effective that many patients choose than rather than having LASIK surgery. Some people may choose silicone hydrogel lenses to address their dryness and discomfort issues. If worn only during the day there is no difference in the risk of infection between silicone hydrogel lenses and traditional hydrogel lenses. However, like any treatment, they're not right for everyone, so be sure to discuss all your options with your eye doctor before making a final decision. Some eyes just can't tolerate continuous wear, or are better suited for shorter periods of extended wear.
Types of silicone hydrogel lenses
Not all types of silicone hydrogel contact lenses are recommended for 30 days of continuous wear. Night and Day and PureVision are two brands of lenses which have been approved by the FDA for such long term use. Other brands are approved for wearing up to six nights consecutively, and some for daily wear only. Silicone hydrogel lenses typically have a 2-4 week replacement schedule.
Silicone hydrogel lenses are not yet available to treat all vision problems, but there are toric lenses (for astigmatism) on the market, as well as bifocal lenses. The silicone hydrogel technology is still fairly new, so other versions of these lenses will likely be available in the future.
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