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
Categories of 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
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
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
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.
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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