Gas Permeable (GP)
Contact Lenses

By Nancy Del Pizzo; reviewed by Joseph T. Barr, OD

Soft contact lenses are most common, but there is another lesser-known category of contact lens materials: gas permeable (GP) contact lenses, also known as RGPs, rigid gas permeable, or oxygen permeable lenses.

GP contact lenses are rigid, but they shouldn't be confused with old-style "hard" contact lenses, which are now obsolete. Hard contact lenses were made of a material known as PMMA. Before 1971, when soft contact lenses were introduced, just about all contact lenses were made from PMMA.

The problem with PMMA lenses is that they are difficult to get used to and somewhat uncomfortable to wear. Also, PMMA does not allow oxygen to pass through it, and healthy eyes need plenty of oxygen.

One big difference between soft lenses (top) and GP lenses (bottom) is their size: GP lenses have a smaller diameter. (GP lens photo courtesy of GP Lens Institute.)

What Makes GPs Different?

GP lenses were first introduced in the late 1970s; they are actually a newer technology than soft lenses. Most GPs incorporate silicone, which makes them more flexible than PMMA. And silicone is oxygen permeable, so oxygen can pass through GP lenses, resulting in greater comfort and better eye health. In fact, GPs transmit more oxygen to the eye than do traditional soft contact lenses (although some silicone hydrogel soft lenses are comparable to GPs in oxygen transmission).

GPs can also provide better vision, durability, and deposit resistance than soft contact lenses. They can be easier to clean, and since they're long-lasting, they can be less expensive in the long term than soft lenses.

You'll Need to Adapt

So why doesn't everyone wear GPs? Primarily because soft lenses are instantly comfortable to wear, and GPs require an adaptation period before they can be comfortable. Some other downsides of RGPs are discussed later.

The Benefits of GPs

GPs offer some outstanding benefits over soft lenses. For one, because an RGP is made of a stiff material, it retains its shape well when you blink, which tends to provide crisper vision than would a soft lens.

GPs are extremely durable. Although you can break them (for instance, if you step on them), you can't tear them easily, like soft lenses. And they're made of materials that don't contain water (as soft contact lenses do), so protein and lipids from your tears do not adhere to GPs as readily as they do to soft lenses. With a little care, they may last for years, as long as you don't require a prescription change.

Niches Where GPs Excel

GPs are made from permeable materials that allow oxygen to reach your eye.

GPs are frequently the answer for people who don't obtain acceptable vision with soft lenses. This includes:

• Individuals who are very fussy about the quality of their vision.
   
• Some people with astigmatism for whom soft contacts don't produce the desired visual acuity. (Read about contacts for astigmatism.)
   
• People with presbyopia, because GPs come in numerous bifocal and multifocal designs. Different bifocal designs work well for different people, so having many choices is a real plus. Also, many people find that the best combination of near and distance acuity is obtained with GP bifocals.
   
• People who have a condition called keratoconus, where the cornea is cone-shaped and causes extreme visual distortion.
• People who need contact lenses after refractive surgery.

GPs are also used for ortho-k, where specially designed contacts are worn during sleep to reshape the cornea and improve vision.

The Downside of GP Contact Lenses

Unlike soft lenses, to achieve maximum comfort with GPs, you need to wear them regularly (although not necessarily every day). If you don't wear your soft lenses for a week, they'll still be comfortable when you put them on a week later. But if you don't wear your GPs for a week, you'll probably need some time to get comfortable again.

Also, some people experience "spectacle blur" with GPs: when lenses are removed, vision is blurry, even while wearing glasses. This effect is temporary, but it can necessitate full-time GP wear. This can make GPs an all-or-nothing proposition, which some people will find inconvenient.

Finally, GPs do require care, since they are reused for one year or more.

SynergEyes hybrid contact lenses have an RGP center and a soft outer skirt.

Best of Both Worlds?

Since comfort is the primary barrier to GP use, one interesting innovation is the SynergEyes lens, which was FDA-approved in 2005. This lens combines a GP center with a soft outer ring; the idea is that this configuration can provide the comfort of a soft lens and the visual acuity of a GP. Newer versions of the lens are for presbyopia, keratoconus and post-surgical use.

For more information about GPs, visit the GP contacts educational website provided by the Contact Lens Manufacturers Association. This site includes a video about GP lenses which aired on PBS television stations. You'll also want to consult with your eye care practitioner to determine if GPs make sense for your needs.

[Page updated December 2008]