|
Hard Contact Lenses
These were the first type of contact lenses available and they
were composed of plastics called polymethylmethacrylte, or PMMA,
also known as Plexiglass. They are not much in use today though
they have several advantages. They can be inserted in the eye
with ease and taken out without much problem. Also, since they
are made up of hard plastics, they do not get contaminated easily
and do not cause eye infections. They are easier to maintain and
are less expensive than the more popular soft contact lenses,
which are prescribed more often nowadays. Also for some people,
eyesight improves with their use.
However, these hard lenses covered the entire cornea and were
made of PMMA, they did not permit much oxygen to reach the cornea,
which is a clear dome in front of the eyeball and posed risks
of a damaged cornea. They were invariably uncomfortable to wear
especially during the initial periods of use.
Gas Permeable Contact Lenses
By the end of the 1970’s and throughout the 80’s
and 90’s a range of oxygen permeable, but rigid, materials
were created to counter the problems faced due to hard lens usage.
These gas permeable lenses were known together as RGP lenses.
These lenses are known as rigid lenses and are easier to maintain
and to care for than soft lenses. Also gas permeable lenses can
correct many vision problems.
However, the period for adjustment for these types of lenses
is much longer than with soft contacts. And though some newer
gas permeable lenses are approved for extended wear, doctors usually
advise that it is better to remove contacts during sleep. It is
always advisable to consult the eye specialist before using gas
permeable lenses for extended wear.
Soft Contact Lenses
Despite continual improvements in hard lenses or RPG’s,
these remained uncomfortable for most people to wear. The Czech
chemist Otto Wichterle was the inventor of hydrogel lenses, which
were made with soft water absorbing plastic known as hydroxethyl
methacrylate, first produced in 1951. Contact lenses using these
materials were approved by the U.S. FDA by 1971 and marketed by
Bausch & Lomb. Unlike RGP’s and hard lenses, soft contacts
do not cause any initial discomfort and the adjustment time period
is much less. Soft lenses can also correct various eyesight problems.
They are an ideal type of lens for sportsmen or for people whose
activities would be hampered if hard lenses or gas permeable lenses
were used. Moreover, the soft lenses have much less chance of
slipping out of the eye accidentally.
However, inserting soft lenses in the eye require a bit of practice
initially and they are more difficult to maintain than hard lenses.
They have to be very carefully handled, cleaned and sterilized
with extreme care. Because they have several advantages over hard
or RPG lenses, soft lenses are invariably more expensive though
less durable. Also, they need to be replaced more frequently.
People using soft contact lenses have to invest in cleaning and
storing solutions for soft lenses.
People using soft contacts are prone to more allergies and infections
especially if they use soft lenses for an extended period of time.
Infections leading to corneal ulcers are considered extremely
harmful to the eyes. People having dry eye disease are unable
to wear soft contacts.
Disposable Contact Lenses
These are soft contact lenses, which are used for a specific
period of time for e.g. a week, a day or a month and then discarded.
Never should these disposable contacts be used again after discarding.
They have less chance of getting contaminated and infected as
they are replaced at short intervals. However, if worn beyond
the specified time period they will adversely affect the health
of the eyes.
These lenses are extremely convenient as they provide lucidity
and comfort that can be experienced with new lenses. They will
help people who are allergic to contact lens solution or protein
deposits as these are discarded before deposits are allowed to
infect the eye. Due to all these advantages and comfort, they
cost much more than regular soft lenses.
Bifocal Contact Lenses
These contact lenses are used to correct presbyopia of the eyes.
That means they are meant for people who have a distance, as well
as a reading, prescription in each eye. These come in two types
- soft lenses or gas permeable lenses and they can be either daily
wear, nightly removal or extended wear types. However, a period
for trial and adjustment is required.
Other Types
Other types of lenses are soft lenses to treat astigmatism, a
condition present in about 25% of the population, known as toric
lenses. Extended wear lenses, which are used overnight are also
used by some people. A number of colored and tinted contact lenses
for cosmetic use have also become popular in recent years.
|
