Silicone Hydrogel Contact Lenses

A new generation of "super-permeable" contact lenses can transmit unprecedented amounts of oxygen to your cornea and, in some cases, enable 30 consecutive days of wear without removal.

Silicone hydrogel contact lenses represent a breakthrough over traditional hydrogel soft contact lenses, because silicone lets so much oxygen (essential for a healthy cornea) pass through the lens. "We're talking about lenses that breathe six to seven times more oxygen than previous lenses," says P. Douglas Becherer, OD, former chairman of the American Optometric Association's Contact Lens and Cornea Section.

Silicone hydrogel contact lenses have caught on with both wearers and eye care practitioners. By 2009, silicone hydrogel lenses are expected to account for more than two-thirds of U.S. soft contact lens sales (in dollars), according to equity research firm Robert W. Baird & Co.

Traditional soft contact lenses are made from hydrogel polymers (soft, water-containing plastics). The plastic itself is not oxygen permeable, so the water performs the job of carrying oxygen through the lens to the eye. But water can carry only so much oxygen — and the more water a lens contains, the greater its tendency to dehydrate after long periods of wear.

Silicone is oxygen permeable. So silicone hydrogel lenses use both their water and polymer content to transmit oxygen to the eye.

The benefits to wearers include comfort and convenience:

• Silicone hydrogel contact lenses contain less water than traditional hydrogel lenses. As a result, they aren't as prone to dehydration while you're wearing them. For some people who wear their lenses for long days, this can mean better end-of-day comfort.
   
• Silicone hydrogels also have made 30-day contact lens wear — sometimes called "continuous wear" — available once again.

The initial heyday of 30-day wear was in the 1980s, but that ended due to health and safety concerns. Today's new silicone hydrogel contact lenses provide much more oxygen to the eye than most conventional soft contact lenses, making 30-day extended wear a safer option than before. As well, the lenses are discarded and replaced monthly, preventing long-term buildup of deposits on the lens surface.

The safety of 30-day silicone hydrogel lens wear is illustrated by a recently completed clinical study. As part of its approval of Night & Day contact lenses for 30-day wear, the U.S. Food and Drug Administration required lens manufacturer CIBA Vision to conduct a post-approval study. For one year, the study followed the experiences of 6,000 people who were prescribed Night & Day for up to 30 nights of consecutive wear. In the study, the incidence of microbial keratitis (an infection that can result in vision loss) was less than 0.18 percent, and the rate of microbial keratitis resulting in reduction of visual acuity was less than 0.04 percent. These rates are higher than for daily wear, but are still considered reasonable by many as a tradeout for the benefits of continuous wear.

In addition, a British study conducted at the University of Manchester and published in 2005 found that people who slept in traditional hydrogel lenses were five times more likely to develop keratitis than those sleeping in silicone hydrogel lenses.
 

Wearing 30-day contact lenses offers benefits similar to those of laser vision correction. Continuous wear contacts may be the right option for people who are considering LASIK but can't afford it, are unwilling to risk a surgical procedure, or aren't very good LASIK candidates because of their vision needs. (Read our comparison of 30-day contact lenses vs. LASIK.)

Because silicone hydrogel lenses provide increased oxygen to your eyes, 30-day wear isn't the only reason to consider using them. And in fact, not all silicone hydrogel brands are approved for 30 days of wear. Currently two brands — Night & Day by CIBA Vision, and PureVision from Bausch & Lomb — can be worn for 30 days.

Two other silicone hydrogels — Acuvue Oasys from Johnson and Johnson, and CIBA Vision's O2Optix — provide higher oxygen levels than traditional soft lenses and are approved for overnight wear of six nights consecutively. Two more, Acuvue Advance and Biofinity, are approved for daily wear only.

If you regularly sleep in your lenses, the case for silicone hydrogels is compelling. For daily wear, it's less so. From a health perspective, the British study mentioned above found no difference in risk of infection for hydrogel and silicone hydrogel lenses when used for normal daily wear. In terms of comfort, some people will find that silicone hydrogel lenses address their dryness and discomfort issues, and others won't see a difference.

Also, silicone hydrogel lenses aren't available "off the shelf" in all lens designs, diameters and curvatures. Toric silicone hydrogels are readily available, for example, but only PureVision comes in a bifocal version. One manufacturer makes custom silicone hydrogel lenses, but they require a two-week delivery time and aren't approved for overnight wear.

Do you like the idea of dealing with your contact lenses only once a month? Before you become too attached to that thought, it's good to be evaluated by an eye care practitioner. Today just about everyone is a candidate for daytime contact lens wear, but not everyone's eyes can adapt to sleeping in contacts. Further, your eyes may be suited for some overnight wear, but perhaps not for 30 consecutive nights. For example, people who have had previous problems with contacts may not be good candidates for 30-day wear.

Your eye care practitioner can help you evaluate the opportunities and risks of 30-day wear, and extended wear in general. If it's not for you, silicone hydrogel lenses for daily wear may be a good option, or you might consider other ultra-convenient options such as daily disposables.