What Is a Rigid Gas Permeable Contact Lens?
There are three types of contact lenses: PMMA (hard) contact lenses, soft contact lenses, and rigid gas permeable lenses (RGP) (today often referred to as GP lenses).
- Hard contact lenses are made of a hard plastic material. Hard lenses (also called PMMA lenses) were the first mass-market contact lenses. A big drawback of these lenses is that they do not allow oxygen to pass through the lens to the eye. Few people still wear these lenses.
- Soft contact lenses are made of a highly flexible gel material that contains water. Soft lenses do permit oxygen to reach the eye.
- Rigid gas permeable contact lenses, also called RGP or GP contact lenses, are made from a firm plastic material and contain no water. Unlike hard lenses, RGP lenses permit oxygen to pass directly through the lens to the eye so that it may "breathe." Because they transmit oxygen these lenses are referred to as gas permeable.
Why Is Gas Permeability Important?
A contact lens rests on the cornea, the clear front "window" of the eye. In this position, the contact lens is a barrier between the cornea and oxygen in the air. When the cornea does not get enough oxygen, it tends to swell and become cloudy. This condition can lead to blurred vision or more serious complications. RGP lenses allow oxygen to pass right through the lens. In addition, an RGP lens moves each time you blink, circulating oxygen-rich tears under the lens. Thanks to these two factors, the gas permeability of the lens and the movement of tears under the lens during linking-the cornea beneath an RGP lens can get all the oxygen it needs.
Are All RGP Lenses the Same?
No. RGP lenses come in different designs and are made from different materials. There are many materials to choose from. In general, materials that allow more oxygen through the lens are more flexible but less durable than materials that allow less oxygen through the lens. Having a broad range of materials and designs available allows your contact lens fitter to devise a lens that meets your precise needs. For example, if you must keep your lenses in for long periods of time, the fitter can pick a material that allows the highest amount of oxygen to pass through the lens. In other cases, a very thin lens could be made from a less oxygen permeable material; this lens would be very comfortable (due to its thinness) and very durable (due to the material). In addition, the lens surface can be treated to increase comfort. The variety of designs, materials, and surface treatments available for today's RGP lenses make successful wear possible for people who previously had difficulties with soft lenses or earlier rigid lenses.
What Are the Advantages of RGP Lenses?
Because RGP lenses are custom made, they can be designed to fit your eye exactly. The fitter can make the lens larger or smaller, so that the lens will work perfectly with your eyelids. Also, the lens can be ground to precisely match the shape of your cornea. RGP lenses are excellent for astigmatism. People with astigmatism have corneas that are not perfectly round. Many people with astigmatism are unsatisfied with the vision they get with soft lenses. Because RGP lenses don't bend, they can give the cornea with astigmatism a more round shape, which provides crisp vision. RGP lenses are more durable and last longer than soft lenses. The hard surface of an RGP lens resists deposits better than a soft lens. Its greater toughness makes RGP lenses better able to withstand the wear and tear of cleaning also. RGP lenses are also easy to handle. Unlike soft lenses, RGP lenses can usually be modified by your fitter. A soft lens that doesn't fit cannot be used. In many cases though, an RGP lens can be reworked to improve its fit. Your fitter can also adjust the RGP lens edge to improve comfort. Plus, RGP lenses can usually be polished (by a professional) to get them super-clean. None of this is possible with a soft lens.
What Are the Disadvantages of RGP Lenses?
RGP lenses take longer to get used to. Many beginning RGP lens wearers can feel the lens edge and find that feeling uncomfortable. However, the overwhelming majority of people who try RGP lenses adapt quite well to them within 2-3 weeks. In many cases, the fitter can adjust the lens edge or design to improve comfort. In some cases, the lens might adhere to the cornea leaving an impression on the cornea when removed. This can usually be taken care of by changing the lens parameters or going to a different lens design. RGP lenses can "pop-out" of the eye more readily than a soft lens during sports activity, but this is rare.
Are there People Who Must Wear RGP Lenses?
Yes. People with scarred, warped, or irregular corneas must wear rigid lenses to get acceptable vision. Eyes with keratoconus (a disease in which the cornea develops a cone-like shape) usually require an RGP lens. People who had to give up soft lenses because they developed an allergic condition called giant papillary conjunctivitis (GPC for short), can often go back to contact lens wear by switching to RGP lenses.
How Often Must I Replace My RGP Lenses?
With careful cleaning and disinfection, many RGP lenses can last two years or more. Because they require less care and last much longer than soft lenses, RGP lenses usually, but not always, cost less over time than soft lenses.
The above information is taken, with some revision, from the CLAO Patient Information Pamphlet titled RIGID GAS PERMEABLE CONTACT LENSES. Pamphlet Advisor was Richard G. Lembach, MD. Copyright 1994-2004, Contact Lens Association of Ophthalmologists, Inc. Thanks to Contact Lens Docs for content used in the creation of this website. All rights reserved. Reproduction other than for one-time personal use is strictly prohibited.
