Gas Permeable (GP) Contact Lenses

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.

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 newer 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.

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.

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.

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

Some eyecare practitioners use RGPs to perform ortho-k, a nonsurgical procedure intended to produce good vision without glasses or contacts. Learn more about ortho-K.

• 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 more 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. (Read more about bifocal contacts.)
   
• People who have a condition called keratoconus, where the cornea is cone-shaped and causes extreme visual distortion. (Read more about keratoconus.)

Recent studies also have shown that fitting nearsighted children with GPs may slow the progression of their nearsightedness somewhat.

GPs are also used for orthokeratology, where specially designed contacts are worn during sleep to reshape the cornea and improve vision. (Read more about orthokeratology.)
 

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, and they require rubbing during the cleaning process. (Read more about caring for GP contact lenses.)

Since comfort is the primary barrier to GP use, one interesting innovation is the SynergEyes lens, which was FDA-approved in 2005 (a multifocal version was released in 2007). This lens combines a GP center with a soft outer ring; the idea is that this can provide the comfort of a soft lens and the visual acuity of a GP.

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