Contact lenses

Very thin, shell-like, transparent discs fitted on the cornea of the eye to correct defective vision. Generally, contact lenses are used to correct myopia (shortsightedness) and hypermetropia (longsightedness). There are several types. Hard plastic lenses give good vision, are long-lasting and durable, inexpensive, and easy to maintain. However, they are sometimes difficult 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 lenses include rigid, scleral lenses that cover the whole of the front of the eye and are used to disguise disfigurement due to injury or disease; bifocal contact lenses; and toric contact lenses with an uneven surface curvature to correct astigmatism. Hard plastic contact lenses may cause abrasion of the cornea if they are worn for too long. Soft lens wearers sometimes develop sensitivity of the eyes and lids. Other problems that may occur with any type of contact lens include infections and redness of the eye.