A soft contact lens

A contact lens (also known as "contact", for short) is a corrective or cosmetic lens placed on the cornea of the eye atop the iris.

Contacts can come in a number of varieties, including hard and soft (extended-wear and disposable, respectively). The most commonly used contact lenses today, however, are of the soft variety, invented in 1961 by the Czech chemist Otto Wichterle (1913–1998).

Contact lenses (both soft and hard) are made of various types of polymers, usually containing some variant of silicone hydrogel. Previously, hard contact lenses were made of a polymer known as PMMA. They have since been replaced by rigid gas-permeable (RGP) contact lenses. Many contact lenses are made of hydrophilic (water-absorbing) materials, thereby allowing oxygen to reach the cornea, and make the lens more comfortable to wear.

Heavily tinted contacts are tinted to change the color of the iris, and are used for cosmetic reasons. Some standard contact lenses are slightly tinted in order to make them more visible for handling purposes.

Corrective contact lenses

The specific dioptre that is required to treat the patient's condition can be found with the help of an optometrist and provided by an oculist. The thickness and shape of the contact lens will also vary with the increase in dioptres, and according to the condition that is being treated: Nearsightedness (myopia), farsightedness (hypermetropia), or astigmatism.

The idea of applying a corrective lens directly to the surface of the eye was first proposed and sketched by René Descartes in 1636, but in was not until 1887 that the German physiologist Adolf Eugen Fick constructed the first successful contact lens.

Toric lenses

People with astigmatism, both myopic (nearsighted) and hypermetropic (farsighted), who have been told they are not suitable for regular contact lenses may be able to use Toric lenses. Toric lenses are made from the same materials as regular contact lenses but have a couple of extra characteristics:

• They have two powers in them, one for spherical correction and the other for astigmatism.
• They are designed to keep the lens in a stable position regardless of eye movement. Typically, the lens is weighted more at the bottom and is marked by tiny striations so the wearer can insert them in the correct position, or they are designed in such a way that blinking will reset the lens to the correct orientation.

Cleaning and disinfection products

While daily disposable lenses require no cleaning, other types require regular cleaning and disinfecting in order to retain clear vision and prevent infections. There are a number of products that can be used to perform these important tasks:

• Saline solution - used for rinsing the lens after cleaning and preparing it for insertion.
• Daily cleaner - used to clean lenses on a daily basis. Usually one puts a few drops of cleaner on the lens and rubs for about 20 seconds (check directions) on each side. One must be extra careful in this step if one has long fingernails.
• Multipurpose solution - used for rinsing, disinfecting, cleaning and storing the lenses. Many people typically only use it for disinfection and storage, relying on other products for rinsing (e.g. saline) and cleaning (e.g. daily cleaner).
• Hydrogen peroxide solution - used for disinfecting the lenses. One must ensure they rinse any lens taken out of hydrogen peroxide with another solution such as saline.
• Enzymatic cleaner - used for cleaning the protein off of lenses, usually on a weekly basis. Typically, this cleaner is in tablet form. Using only the daily cleaner is not sufficient to prevent protein deposits, making the lens very uncomfortable and possibly leading to eye damage.

Some products may contain preservatives such as thimerosal. However, about 10% of contact lens wearers have problems with these products, a reason why several brands no longer use it. Such thimerosal-free products are sometimes labelled "for sensitive eyes". Products that do not contain any preservatives usually have shorter expiration dates. For example, non-aerosol preservative-free saline solution typically only last two weeks once opened.

See also

• Intraocular lens