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" 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 and 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.

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?