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.
Soft Contact Lenses
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)
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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Click to enlarge
Figure 1
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