Different types of contact lenses

Originally, all contact lenses were made of a hard plastic called polymethyl methacrylate (PMMA). This is the same plastic used to make Plexiglas. But hard lenses don't absorb water, which is needed to help oxygen pass through the lens and into the cornea. Because the eye needs oxygen to stay healthy, hard lenses can cause the wearer a lot of irritation and discomfort. The upside to these lenses is that they are relatively easy to clean because bacteria don't stick very well to them. Soft contact lenses are more pliable and easier to wear than hard lenses because they're made of a soft, gel-like plastic. Soft lenses are hydrophilic, or "water loving," and absorb water. This allows oxygen to flow to the eye and makes the lens flexible and more comfortable to wear. Many companies bill their soft contact lenses as "breathable" because they transmit a high amount of oxygen to the eye. Letting more oxygen reach the eye means that you can wear soft contact lenses for longer periods with less chance of irritation.

Several different types of soft contact lenses are available:

• Daily-wear lenses - You remove these each night before going to bed.
• Extended-wear lenses - You can wear these for several days or weeks without removing them.
• Disposable lenses - You can wear these lenses for a day, a week, or even several weeks before throwing them away.
• Color-tinted or cosmetic lenses - These lenses change the color of your eyes or tint them for clearer vision outdoors. Other cosmetic lenses drastically change the eye's appearance. Many people use them for the theater or for Halloween.
• Ultraviolet (UV) protection lenses - These lenses protect against harmful ultraviolet rays from the sun.
• Corneal reshaping lenses - You wear these lenses overnight and they gradually reshape the cornea to correct your vision. When you take them out, you can temporarily see clearly without contact lenses.
• Rigid, gas-permeable lenses, which were developed in the late 1970s, are made from a combination of PMMA, silicone, and fluoropolymers (a type of porous plastic). These lenses combine some of the features of hard and soft contact lenses. They're more durable than soft lenses, but they still allow oxygen to pass into the eye, making them more comfortable to wear than hard lenses. Because they don't contain water, they are less likely to develop bacteria and cause infection than soft lenses. The rigidity of the lens provides clear, crisp vision.

Non-standard contacts
Contact lenses aren't for everyone. Some people have conditions that make them more difficult to fit, and others can't wear them at all. Contacts aren't recommended for people who:

• Have a history of corneal infections
• Work in an industry where they are exposed to chemical fumes, dust, or dirt
• Have allergies to lens-care products
• Are diabetic
• Are under the age of nine

Many people with dry eye syndrome can't wear regular contact lenses, because the lens dries up on their eye too quickly. Soft lenses that contain very little water work better for people with this condition, because they don't dry up as fast as normal lenses. There are also certain brands, such as Proclear Compatibles, that are designed specifically for people with dry eyes.

People with keratoconus have a thinning of the cornea that causes a cone-shaped bulge. A hard, gas-permeable contact lens is most effective for people with this condition, because it stays fixed on the cornea. People with extremely distorted corneas may need a piggybacking technique, in which they wear two lenses on each eye. A soft lens sits right on top of the cornea, and a gas-permeable lens is placed on top of it.

Giant papillary conjunctivitis is an inflammatory condition in which the eyes secrete proteins. Soft contact lenses don't tend to work well for people with GPC, because proteins deposit on the lens of their eyes and cloud them over. Daily disposable daily contacts may work better, because the proteins don't have enough time to accumulate. Hard, gas-permeable lenses also can work because the proteins don't tend to stick to them.