Shop Now Find Eye Doctor Search HomeContact LensesOther Contacts | En Español Gas permeable contact lenses (RGP or GP contacts) By Gary Heiting, OD

What makes gas permeable lenses different? Adapting to RGP lenses The benefits of RGP lenses Where GP contact lenses excel Limitations of gas permeable lenses

Hybrid contacts: The best of both worlds?

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Oxygen passing through a gas permeable contact lens Gas permeable contact lenses are rigid lenses made of durable plastics that allow oxygen to pass through the lens. 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 essentially obsolete. Hard contact lenses were made of a type of plastic called polymethyl methacrylate (PMMA). Before 1971, when soft contact lenses were introduced, just about all contact lenses were made from PMMA, which is also called acrylic or acrylic glass, as well being referred to by the trade names Plexiglas, Lucite, Perspex and others.

PMMA has excellent optical properties and was developed as a lightweight and shatter-resistant alternative to glass for many applications. But it is impermeable to oxygen and other gases, and the clear front surface of the eye (cornea) needs a significant supply of oxygen to stay healthy.

Soft lenses: Soft and silicone hydrogel lenses are thin, pliable lenses that cover the entire cornea and a small portion of the sclera.

GP lenses: Gas permeable contact lenses are rigid lenses that float on a layer of tears and typically cover about 75 percent of the cornea.

Since oxygen cannot pass through a PMMA contact lens, the only way for this vital element to reach the cornea when wearing them was for tears to wash underneath the lens with each blink. In order for this blink-induced, tear-pumping action to occur, PMMA lenses had to be made relatively small in size. Also, there had to be a significant gap between the edge of the lens and the surface of the cornea.

These design characteristics made many people very aware of PMMA lenses on their eyes or caused discomfort that made wearing the lenses impossible. In some cases, these features also caused problems with PMMA lenses popping off the eye, especially during sports.

What makes gas permeable lenses different? Most gas permeable lens materials include silicone, which makes GP lenses more flexible than PMMA hard contacts. And silicone allows oxygen to pass directly through GP lenses to keep the cornea healthy without having to rely solely on oxygen-containing tears to be pumped under the lens with each blink.

In fact, modern rigid gas permeable contacts allow more oxygen to reach the cornea than most soft contact lenses (although some silicone hydrogel soft lenses are comparable to GP lenses in oxygen transmission).

Because gas permeable contact lenses allow oxygen to pass through them, GP lenses can be made larger than

PMMA hard contact lenses, and the edges of GP lenses can be fitted closer to the surface of the eye. These design changes make modern rigid GP lenses more comfortable and easier to get used to than old-fashioned hard contacts. They also help GP contacts stay more securely on the eye when worn during sports and other activities.

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.