contact lenses

The contact lenses - Thin, shell-like, transparent discs fitted over the cornea of the eye that are used to correct defective vision. Contact lenses are most commonly used for the correction of myopia (shortsightedness) and hypermetropia (longsightedness). In addition, noncorrective lenses are available for cosmetic use, for example to change eye colour.

TYPES:
There are several types. Hard plastic lenses give good vision; they are also long-lasting and durable, inexpensive, and easy to maintain. Sometimes, however, these lenses are difficult for the wearers to tolerate and may fall out. Hard gas-permeable lenses are more comfortable because they allow oxygen to pass through to the eye, but are less durable. Soft lenses are the most comfortable because of their high water content. Disposable soft lenses are for single use only; extended-wear lenses are worn for up to a month. Other types of contact lenses include rigid scleral lenses, which cover the whole of the front of the eye and are used to disguise disfigurement; bifocal contact lenses; and toric contact lenses, which have an uneven surface curvature and can correct astigmatism.

PROBLEMS:
Hard plastic lenses may cause abrasion of the cornea if worn for too long. Soft lens wearers sometimes develop sensitivity of the eyes and lids. Any type of contact lens may cause redness of the eye. The most serious complication of using lenses is infection, which can occasionally cause permanent damage to the cornea and affect vision; meticulous hygiene lowers the risk.