Contact lenses: uses, types and peculiarities
Contact lenses are often worn by people with poor eyesight who prefer lenses placed on their eyes to glasses for some reason. Since contacts are intended for vision correction and for therapeutic or cosmetic reasons, they are looked upon as medical devices.
There are different reasons for which people wear contact lenses. Some of them guided by cosmetic principles would like to change the appearance of their eyes. Others are willing to forget about glasses and place their contacts on their eyes every morning for aesthetic reasons. Other people use contacts according to their intended purpose of vision correction. Advantages of contacts over glasses are as follows:
- better peripheral vision;
- moisture resistance (protection from snow, rain, condensation or sweat);
- better treatment of certain eye disorders (aniseikonia and keratoconus).
Contact lenses are classified into two big groups by their material: rigid lenses and soft lenses.
The first contact lenses which gained widespread popularity were made of polymethyl methacrylate (PMMA or Perspex/Plexiglas). They were invented in 1930s. PMMA lenses are usually called “hard” lenses. They have a significant drawback: they don’t let oxygen through, so that it doesn’t reach the cornea, which can be fraught with clinic consequences.
In 1970s oxygen-permeable lenses, an improved version of PMMA lenses called rigid gas permeable or “RGP” lenses, were invented.
Compared to a rigid lens, a soft one is a relatively new development. Invented by Otto Wichterle in 1960s, the first soft hydrogel lenses became widespread in some countries. Soon scientists got daily material polymacon which was approved by the United States FDA in 1971. The year of 1998 brought around silicone hydrogels which were notable for their high oxygen permeability and comfort.
The major advantage of a soft lens over a rigid one is that the former doesn’t require a period of adaptation, which accounts for its immediate comfort. The downside is soft lens probable resistance to water causing discomfort for an eye. However, the latest technologies have minimized this effect.
There are also hybrid lenses which combine benefits of both rigid and soft lenses.
Contacts can be classified by their primary function – vision correction. Such corrective lenses are divided into two types: spheric and toric, by their focusing power.
Spheric contact lenses refract light in every possible direction, vertically, horizontally and so on. Such eye illnesses as myopia and hyperopia are well treated with these lenses.
Toric lenses are used to treat astigmatism as their focusing power in a horizontal direction differs from that in a vertical one. Toric lenses must be ideally aligned to correct a patient’s astigmatism. To exclude any rotating out of the ideal alignment, such lenses have special physical characteristics which help to rotate a toric lens back into position.
There exist two main strategies: multifocal contact lenses and monovision. As stems from their names, the former has multiple focus points whereas the latter uses only one. As a rule, in case of multifocal lenses a human’s eye views through the center of the lens. When a person wears lenses based on the monovision principle, his one eye is focused on distance vision while the other eye is focused on near work.
Those who suffer from color deficiencies are recommended to wear red-tinted “X-Chrom” contact lenses which allow to distinguish colors, but they don’t provide full-fledged color vision.
According to some research in 2004, more than 125 mln. people wore contact lenses for different reasons. Now this number is obviously much larger as lenses are getting more and more popular with people of different ages.