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What are hard contact lenses?
Hard contact lenses are made from rigid materials like glass
and plastics. The earlier types are rarely prescribed now, though
they had several advantages. They were easy to wear and remove
and easy to maintain. They were durable and cheap. Today the lenses
made from rigid gas permeable materials are practically the only
hard lenses available.
When are hard contact lenses used?
Like all contact lenses they are used for correcting vision in
near sightedness, far sightedness and in some astigmatism cases.
Hard contact lenses, by and large, provide good vision correction.
What are the types of hard contact lenses?
Glass contacts, PMMA contacts and RGP contacts are the three
main types. The first two are almost obsolete. Presently RGP contacts
are practically the only hard contacts still used.
Glass contact lenses: The first contact lenses, made from glass,
caused a lot of irritation and could be worn for only limited
periods. These were soon discarded.
Polymethyl methacrylate or PMMA lenses: Glass lenses were replaced
by lenses made from a mixture of plastic and glass called polymethyl
methacrylate or PMMA and commonly known as plexiglass. These were
the first contact lenses with mass appeal. They were lighter and
more convenient. But they came with their own side effects.
PMMA lenses did not allow oxygen to enter the cornea, thus increasing
the risk of corneal and other infections. This lack of oxygen
permeability in PMMA lenses was a serious disadvantage. Also PMMA
lenses were quite uncomfortable for the first few weeks of wear.
Despite these disadvantages, PMMA lenses are still occasionally
worn but have become largely obsolete. They are cheaper than the
latest contact lenses.
Rigid gas permeable or RGP lenses: These are the modern version
of the hard lenses. Today when people talk about hard contact
lenses they mean RGP lenses. After the 70s, superior plastic materials,
which had higher oxygen permeability and were also rigid, were
developed. Lenses made from such materials are called Rigid Gas
Permeable or RGP lenses to distinguish them from soft contact
lenses that are also oxygen permeable.
What are the benefits of RGP lenses
- Rigid gas permeable lenses are less flexible than the
soft lenses, since they are made from a mixture of semi-rigid
materials. This means they retain their shape even when you
blink. They provide better vision than what you would get with
soft contacts.
- They are very durable and provide excellent vision
clarity and
can serve you for up to a year.
- Since they are longer lasting,
RGP lenses can be cheaper than soft lenses in the long term.
- They
can be worn occasionally or regularly but need to be cleaned,
disinfected and stored properly.
What are the unique benefits of RGP lenses?
RGP lenses offer a number of unique benefits, they are:
- These lenses can replace the natural shape of the cornea
with a new refracting surface. This allows RGP lenses to provide
good level of vision in people who have astigmatism or distorted
corneal shapes as in keratoconus. Conventional glass spectacles
and soft contacts cannot correct such visual impairments.
- People
who are very fussy about their clarity of vision prefer RGP
lenses.
- According to many people, the best bifocals are made
from RGP material. It is of special benefit to people with presbyopia
since RGP lenses come in several bifocal and multifocal designs.
- For children,
RGP lenses can slow down the progression of near sightedness.
- RGPs
are used for orthokeratology where specially designed lenses
are worn during sleep to correct the corneal shape and improve
the vision.
What are the disadvantages of RGP lenses?
- These lenses can take longer than soft lenses to get
used to.
- Unlike soft contacts, you will have to wear RGP, if
not daily, at least very regularly to get maximum comfort. Soft
contacts
worn after a week don’t need time to adjust. But RGP lenses
worn after a similar period will certainly need some time to
feel comfortable.
- Some people experience what is called ‘spectacle
blur’,
a blurring of vision when the lenses are removed that persists
even after wearing glasses. Though a temporary phenomena, it
makes RGP a full time wear causing inconvenience for some people.
- RGP
lenses need greater care and more rubbing during cleaning.
Conclusion
RGP lenses, although needing more time to adjust, are as good
as soft contacts. Some vision problems can be corrected by only
RGP lenses. But they require greater care than soft contacts.
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