Gas Permeable (GP)
Gas permeable contact lenses are rigid lenses made of durable plastic that transmits oxygen. These lenses also are called GP lenses, rigid gas permeable lenses, RGP lenses and oxygen permeable lenses.
GP contact lenses are rigid, but they shouldn't be confused with old-fashioned 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.
Though PMMA hard contacts had excellent optical qualities, they were not oxygen-permeable, and the front surface of the eye needs plenty of oxygen to stay healthy. Also, old-fashioned hard lenses were not as comfortable as modern GP lenses.
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One big difference between soft lenses (top) and GP lenses (bottom) is their size: GP lenses generally have a smaller diameter. (GP lens photo courtesy of GP Lens Institute.)
What Makes Gas Permeable Lenses Different?
Gas permeable contacts were first introduced in the late 1970s; they are actually a newer technology than soft lenses. Most GP lenses 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, gas permeable contacts transmit more oxygen to the eye than do traditional soft contact lenses (although some silicone hydrogel soft lenses are comparable to GP lenses in oxygen transmission).
RGP lenses also provide better vision, durability, and deposit resistance than soft contact lenses. And because they last longer than soft lenses, they can be less expensive in the long term.
Adapting to RGP Lenses
So why doesn't everyone wear gas permeable lenses? Primarily because soft lenses are instantly comfortable, and GP lenses require an adaptation period before they are as comfortable as soft contacts.
Some other downsides of RGP lenses are discussed below.
The Benefits of RGP Lenses
Gas permeable contact lenses offer some outstanding benefits over soft lenses. For one, because GP lenses are made from a firm plastic material, they retain their shape when you blink, which tends to provide sharper vision than pliable soft lenses.
GP lenses also are extremely durable. Although you can break them (for instance, if you step on them), you can't tear them easily, like soft lenses.
Niches Where GP Contact Lenses Excel
Oxygen permeable contacts frequently are 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 GP lenses 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.
Gas permeable contacts also are used for ortho-k, where specially designed GP lenses 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 gas permeable contacts, you need to wear them regularly (though 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 GP lenses for a week, you'll probably need some time to get comfortable again.
Also, GP lenses are smaller in size than soft lenses, which means there is a greater risk of gas permeable lenses dislodging from the eye during sports or other activities.
And because gas permeable lenses are designed to move on the eye when the wearer blinks, there is a higher risk of dust and debris getting under the lenses, causing discomfort or a possible abrasion to the cornea.
Finally, GP lenses do require care, since they are reused for one year or more.
Hybrid Contacts: The Best of Both Worlds?
Since comfort is the primary barrier to greater use of gas permeable lenses, one interesting innovation is the SynergEyes hybrid contact lens, which was FDA-approved in 2005.
SynergEyes hybrid contact lenses have an RGP center and a soft outer skirt.
The central optical zone of this lens is made of a GP material and is surrounded by a "skirt" of soft lens material. The goal of hybrid lenses is to provide the crisp optics of a GP lens, combined with wearing comfort comparable to soft lenses.
SynergEyes (Carlsbad, Calif.) also manufactures hybrid contact lenses for the correction of presbyopia and keratoconus, and offers a hybrid lens specially designed for post-surgical eyes. In 2011, the company introduced a second-generation hybrid lens called Duette, which is designed to correct astigmatism and features a GP center surrounded by a highly oxygen-permeable silicone hydrogel material.
For more information about gas permeable lenses, visit the GP contacts educational website provided by the Contact Lens Manufacturers Association.
You'll also want to consult with your eye care practitioner to determine if RGP contact lenses make sense for your needs.
[Page updated March 2011]
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