Types of Contact Lenses
Contact lenses are tiny, curved plastic discs which are microthin, very light and barely visible. There are two major types: hard and soft.
Hard Lenses
The hard lens covers about two-thirds of the cornea and may be clear or tinted. This lens has been improved so that it can be made into a bifocal, toric or bitoric lens.
The hard, gas-permeable lens resembles the hard contact lens but is made of a material that allows oxygen to cross through the lens to the cornea, which is healthier for the cornea. Today, virtually all hard lenses are made in gas permeable plastics.
The scleral contact lens covers the whole front of the eye and is still used for certain medical conditions. The latest version of this lens design is used in the management of advanced Keratoconus.
The X-chrome lens is a hard, red contact lens which, when fitted on the non-dominant eye, allows people who have a particular kind of "color blindness" (those who can't discriminate between red and green) to partially distinguish colors. It is worn in only one eye (the weaker, "non-dominant" eye) and may be made with a prescription for patients who would also need eyeglasses to correct for vision.
Soft Lenses
The soft, or hydrophilic, lens is made of a special thin and very flexible plastic which absorbs water. It is larger than the hard lens and covers all of the cornea and extends to the sclera. Soft astigmatic bifocal and colored lenses are available.
Daily-wear lenses were the first type of soft lenses available. These lenses are approved to be worn and removed on a daily basis. Patients may not sleep or nap with these lenses and are expected to clean and disinfect them whenever handled. A wide variety of soft daily-wear lenses are currently available on the market, including colored, bifocal and toric.
Extended wear lenses have been approved for patients after cataract surgery and for patients with myopia, hyperopia and astigmatism. These are soft lenses that allow better oxygen transmission to the cornea and need not be removed on a daily basis. They provide the visual benefits of contact lens wearing for many patients who cannot or do not wish to handle daily wear lenses. Because of the nature of these lenses, more frequent return visits are necessary. Extended-wear, hard, gas-permeable lenses are also available.
The disposable contact lens is one of the latest soft lenses available and may be worn as extended wear, daily wear or single-use lenses.
As extended wear lenses, they are designed to be worn for one week and then thrown away. They are replaced with a new pair. No maintenance is required in caring for the lenses, but because they are dispensed in three-month or six-month supplies, wearers must follow a strict schedule of checkups to be re-supplied with lenses. Extended-wear lenses are available for myopic (nearsighted) and hyperopic (farsighted) patients.
As daily-wear lenses, these are designed to be worn for one to two weeks. A specific schedule will be determined by the doctor. When removed daily, they must be soaked in disinfecting solution overnight. They are available for myopic and hyperopic patients.
Single-use lenses are designed to be worn for one day and then thrown away. These lenses are currently available only in limited prescriptions.
The latest innovation in soft lenses is the Silicone gel lens. These lens have a silicone backbone and are much more biocompatible than traditional soft lens. These lenses are used for extended wear, dry eyes and eyes that need more oxygen.
