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Categories of Contact Lenses There are three main categories of contact lenses: Soft Disposable lenses, Rigid Gas Permeable lenses (RGPs), and Hard Plastic (PMMA) Lenses. Figure 1 illustrates the relationship between the categories; a brief description of each will follow. Figure 1: Categories of Contact Lenses
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Soft Contact Lenses |
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The majority of contact
lens wearers chose soft lenses. The soft lenses of
choice are disposables, which are available in daily,
weekly or monthly packages. Soft lenses are made from
a gel-like plastic, and have a high water content.
When they absorb liquid, they become softer and can
mold to the shape of the eye. In addition, they allow
a great deal of oxygen to pass through to the eye.
They are known to be very comfortable to wear. Most
soft contact lenses can correct Myopia
(nearsightedness), Hyperopia (farsightedness), and
certain types of Astigmatism (irregular shaping of the
cornea causing skewed vision). Soft contact lenses
differ from RGP's (Rigid Gas Permeable lenses). Soft
contact lenses differ from RGP's because they are more
comfortable, fall out less, are easier to insert, and
require less break-in time. Break in time is the time
required for a patient to get used to wearing contact
lenses.
Soft contact lenses are not for everyone. Although there are benefits of wearing a soft contact lens over wearing a RGP lens, there are also some disadvantages. Soft contact lenses are more expensive than RGP lenses, and they require more replacements as they are less durable. Also, wearers of soft lenses are more likely to get eye infections than wearers of rigid lenses. Maintenance of soft contact lenses may also require several cleaning and storing sterilization chemicals. Only an eye care professional, like your local optometrist can help you decide if soft contact lenses are the right fit for you. Your eye care professional must also fit your contact lenses as the eye curvature in each patient differs. |
Rigid Gas Permeable Lenses (RGPs) |
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Rigid Gas Permeable
contact lenses (RGPs) are worn by about 15 percent of
contact lens wearers. Unlike hard plastic lenses, RGPs
are gas permeable. That is, they allow oxygen to pass
through to the cornea. They are less expensive and
more durable than soft contact lenses, and provide
wearers with clearer, sharper vision.
Unlike soft contact lenses, RGPs have a low water content and therefore, resist protein deposits and bacteria. Because they are rigid, they hold their shape when the wearer blinks, allowing for crisper vision. Other benefits include ease of handling and ease of care. Because they do not contain water, proteins from the eye do not build up on the lens, so the lens stays cleaner, and requires less maintenance. RGP's are not for every patient. Because of their rigid form, they require more break in time for the patient to become comfortable with wearing the lenses. Also, if you discontinue wearing your RGP's for a few days, it may be difficult to become used to wearing them again. RGP's differ from soft contacts in that becoming comfortable with wearing a rigid lens requires the patient to wear their lenses all the time. Soft contact lens users may discontinue wearing their lenses for a few days, and immediately be comfortable with having them inserted again. Rigid Gas permeable are also known as Oxygen Permeable contact lenses. The plastic of these lenses is breathable, and they must be custom fit to the shape of the cornea. Only your eye care professional can help you decide which contact lenses are right for you. |
Hard Plastic Lenses (PMMA) |
| Hard Plastic (PMMA) lenses do not allow oxygen to pass through to the cornea. They require a long adjustment period, and cannot be worn for more than 12 hours. These high-maintenance lenses were the first contact lenses on the market, but are now considered obsolete and are rarely used. |
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