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Silicone Hydrogel Contact Lenses

By Liz Segre; reviewed by Joseph T. Barr, OD

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 allows much more oxygen (essential for a healthy cornea) to pass through the lens. In fact, one recent study shows silicone hydrogel lenses deliver three times the oxygen to the cornea, compared with the leading hydrogel soft contact lens brand.

Other data show silicone hydrogel lenses allow up to 5 times more oxygen through the lens than traditional soft lenses (source: CIBA Vision).

Silicone hydrogel contact lenses have caught on with both wearers and eye care practitioners. In 2009, silicone hydrogel contacts accounted for approximately 60 percent of new contact lens fittings and lens refittings, according to equity research firm Robert W. Baird & Co.

How Silicone Hydrogel Lenses Work

Traditional soft contact lenses are made from hydrogel polymers (soft, water-containing plastics). The plastic itself is not oxygen permeable, so the water content 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.

30-Day Contact Lens Wear

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 clinical study conducted by lens manufacturer CIBA Vision. As part of its approval of Night & Day contact lenses for 30-day wear, the FDA required CIBA Vision to follow the experiences of 6,000 people who were prescribed Night & Day for up to 30 nights of consecutive wear.

After one year, 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.

Silicone Hydrogel Lenses: A LASIK Alternative

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.)

Different Wearing Options for Silicone Hydrogel Lenses

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 continuous wear:

• Two brands — Air Optix Night & Day by CIBA Vision, and PureVision from Bausch + Lomb — can be worn for up to 30 days.
• Four other silicone hydrogels — Acuvue Oasys from Johnson & Johnson Vision Care, Air Optix from CIBA Vision, and CooperVision's Biofinity — are approved for overnight wear of six nights consecutively.
• Acuvue Advance, PureVision2 and CooperVision's Avaira are approved for daily wear only; in addition, J&J's TruEye is a daily disposable lens.

Most silicone hydrogel lenses have a replacement schedule of two to four weeks. Your optometrist or ophthalmologist will advise you about how frequently you should replace your lenses, based on how your eyes respond to wearing the lenses.

Should Everyone Be Wearing Silicone Hydrogels?

If you regularly sleep in your lenses, the case for silicone hydrogels is compelling. For daily wear, it's less so.

Want to wear your contacts for up to 30 days without removing them? Silicone hydrogel contact lenses could make that possible.

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.

Silicone hydrogel lenses are available "off the shelf" in many lens designs, diameters and curvatures, including toric silicone hydrogels for correction of astigmatism. Several companies make custom silicone hydrogel lenses that must be ordered, and aren't approved for overnight wear, but can be manufactured in just about any prescription.

In addition, Bausch + Lomb's PureVision and CIBA Vision's Air Optix silicone hydrogel lenses come in bifocal versions.

30-Day Wear: Not for Everyone

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.

If your eyes are not suited for 30-day wear or even shorter periods of consecutive overnight wear, daily disposable contact lenses often are a good alternative. These lenses offer nearly the same convenience of 30-day extended wear contacts, including little or no need for contact lens solutions.

In June 2010, Johnson & Johnson Vision Care introduced 1-Day Acuvue TruEye, the first FDA-approved silicone hydrogel daily disposable contact lens, in the United States market. Acuvue TruEye lenses transmit significantly more oxygen to the eye than other daily disposable lenses, which may mean fewer red and dry eyes during contact lens wear, according to the company.

Resources:

Finding the comfort zone with the newest silicone hydrogel technology. Contact Lens Spectrum. March 2004.

Contact Lenses 2009. Contact Lens Spectrum. January 2010.

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[Page updated June 2011]