Tuesday, February 07, 2012
Contact Lenses & Corneal Oxygen
A Unique tissue - The only living tissue in the human body that contains no blood
vessels is the transparent cornea of the eye. The cornea is the firm, smooth outer shell
that arcs across in front of the iris and pupil. The cornea forms the eye's "front window"
and normally contains no blood vessels because it must be perfectly clear.
Without blood to provide oxygen and nutrients, the cornea must get them directly from the
air. The oxygen first dissolves in the tears and then diffuses throughout the cornea to keep
it healthy. Equally important, a waste product of a healthy cornea is carbon dioxide which
must be eliminated. In a reverse process, carbon dioxide must diffuse out of the cornea and
into the atmosphere.
Placing an improperly fitted contact lens onto the cornea can slow or even stop the exchange
of oxygen and carbon dioxide. Without enough oxygen the cornea can warp, become less
transparent, less able to detect pain and may develop scars. Additionally, new blood
vessels from the sclera (the white part of the eye) can grow into the cornea and cause
loss of transparency and corneal scarring.
"Hard" vs. "Soft" lenses
"Hard" lenses were the original contact lenses made several decades ago from a plastic
called PMMA. For a long time they were the only kind of lens available. A big drawback of
PMMA was that it didn't allow oxygen to pass through the lens. PMMA lenses have become
much less common as newer lens materials have become available.
When people say "hard lenses" today, they are most likely referring to the gas permeable
type introduced in 1978. Rigid Gas Permeable (RGP) lenses are similar to hard lenses in design
nd appearance. As the name suggests, RGP materials are permeable to gases (most importantly
oxygen and carbon dioxide) and they contain no water. "Soft" contact lenses are slightly
larger, flexible and made of materials which absorb water. The soft lens plastic itself is
not gas permeable; the uptake of water is what allows these lenses to transmit oxygen to
the cornea. The uptake of water is also what makes them flexible; if you let a soft lens
dry out, it can becomes brittle and easily chipped or broken.
High water content lenses
As mentioned above, soft lenses absorb water, and it this water which allows the eye to
"breathe" through the contact lens. The more water a lens absorbs, the easier it is for
oxygen and carbon dioxide to pass through it.
The original soft contact lenses, introduced in 1971, are now often referred to as
"Low Water Content" lenses and contain roughly 38% water when hydrated. Many lenses now
contain between 50% and 60% water and some contain over 70%. Simple math reveals an
interesting fact; only 30% of a 70% water content lens is actually plastic.
Even though they transmit more oxygen, there are some significant disadvantages to a
higher water content lens. Because higher water content lenses contain less plastic, they
are more fragile. They are usually made thicker to compensate for the increased fragility.
Unfortunately, this reduces the effect of the higher water content - thick lenses transmit
less oxygen than thin ones. Also, higher water content lenses tend to attract deposits
more quickly making them harder to keep clean. Finally, the higher the water content,
the easier it is for that water to be evaporated from the lens causing the lens to become
uncomfortable and less clear.
Silicone hydrogel Lenses
The newest of the soft lens family is the hybrid silicone hydrogel lens. These lenses
feel like a soft lens and behave like a soft lens but allow as much oxygen to get to the eye
as some of the very best gas permeable rigid (RGP) lenses. Many patients find they have
improved tolerance to contact lens wear with this material.
Disposable lenses
"Disposable" lenses are those lenses which are designed to be replaced more often than
regular lenses. Almost all disposable lenses are of the soft variety. The advantage of
disposable lenses is that they are thrown away before getting too dirty and are therefore
more healthy for your eyes. Because they do not have to last as long they can be made
thinner, which improves the comfort and increases the ability of Oxygen to pass through
them.
Contacts and astigmatism
Think you can't wear contact lenses because you have astigmatism? Even if you have
unsuccessfully tried lenses designed to correct astigmatism in the past, be sure to ask
your eye doctor whether you may be a candidate. Today, there are several types of contact
lenses which may provide excellent vision, even though you have a significant degree of
astigmatism.
Contacts and presbyopia
If you're over 40, you may be experiencing difficulty reading or seeing fine print on
medicine bottles, etc. This condition is a normal part of the aging process called
presbyopia. While presbyopia does present an additional challenge, you might still be a
contact lens candidate. In some cases, you might be able to wear conventional lenses with
or without supplemental reading glasses. If you have trouble reading but are highly
motivated to wear contact lenses, be sure to discuss your situation with your eye doctor.
This is just a brief overview of the basic types of lenses available. Whether contacts are
right for you and which type can help you depends on many factors such as your prescription,
the health of your eyes, and lifestyle. A consultation with your eye doctor is always a good
place to start. When you have all the facts, making a good decision is much easier.
A Word About Glasses for Contact Lens Wearers
If your prescription changes, it's generally wise to replace your glasses at the same time
as you update your contact lenses. Some people cannot comfortably wear contact lenses every
day, all their waking hours. Glasses are absolutely preferred when working around
vapor-producing materials like many paints and cleaners... If you can smell it, you should be
wearing glasses!
Finally, If you do have trouble with your lenses (from wearing them too long, eye
infection, damaged lenses, etc.) then you will probably have to stop wearing contacts and
rely on your glasses for a time. If you cannot wear your contacts and your vision is bad,
could you safely drive or work effectively without a pair of glasses?
Sources:
- National Eye Institute
- American Optometric Association
Back to Top
