Contact lenses are a thin, plastic device placed directly on the surface of the eye to correct visual defects and sometimes reshape the cornea of the eye. All contact lenses, even purely cosmetic ones, are considered medical devices and require a prescription only obtained through a proper contact lens fitting.
Types of contact lenses
Contact lenses are not all the same. They are available in different forms and designed to meet different purposes. These forms can range from daily, every two weeks, monthly, quarterly or annually. They also fit different needs, such as contacts for astigmatism, contacts for nearsightedness, and more.
Contact lenses come in different strengths. If you wear contacts, take a look at the box or your prescription. You’ll see either plus or minus signs, followed by numbers. Without getting too in-depth, these convey the visual shape of the lenses. Different powers correct different vision problems.
Soft Toric lenses, which are more weighted at the bottom, help correct astigmatism. Spherical lenses, which are the same all around, help correct nearsightedness and farsightedness. There are also lenses. That are bifocal and progressive which help with presbyopia, or changes in near vision associated with age.
Contact Lens Materials
- Soft lenses are made of specifically designed plastic.
- Silicone hydrogel lenses are an advanced type of soft contact lenses that are more porous than regular lenses and allow even more oxygen to reach the cornea. Introduced in 2002, silicone hydrogel contact lenses are now the most popular lenses prescribed in the United States.
- Gas permeable lenses — also called GP or RGP lenses — are rigid contact lenses that look and feel like PMMA lenses (see below) but are porous and allow oxygen to pass through them. Because they are permeable to oxygen, GP lenses can be fit closer to the eye than PMMA lenses, which were the original contact lens options, making them more comfortable. Since their introduction in 1978, gas permeable contact lenses have essentially replaced nonporous PMMA contact lenses. GP contacts often provide sharper vision than soft contacts — especially if you have astigmatism.
- Hybrid contact lenses are designed to provide wearing comfort that rivals soft or lenses, combining the crystal-clear optics of gas permeable lenses with a soft lens comfort. Hybrid lenses have a rigid gas permeable central zone, surrounded by a “skirt” of soft or silicone hydrogel material. Despite these features, only a small percentage of people in the U.S. wear hybrid contact lenses, perhaps because these lenses are more difficult to fit and are more expensive to replace than soft and silicone hydrogel lenses.
Daily vs. Extended Wear
Until 1979, everyone who wore contact lenses removed and cleaned them nightly. The introduction of “extended wear” enabled wearers to sleep in their contacts. Now, two types of lenses are classified by wearing time:
- Daily wear — must be removed nightly
- Extended wear — can be worn overnight, anywhere from seven to thirty days consecutively without removal
“Continuous wear” is a term that’s sometimes used to describe 30 consecutive nights of lens wear — the maximum wearing time approved by the FDA.
Schedule a consultation with one of our optometrists
Union Square Eye Care has a team of leading ophthalmologists, optometrists, and opticians. If you are interested in getting contacts or renewing your contacts, contact us today.