A contact lens (also known as a "contact") is a corrective, cosmetic, or sometimes protective lens placed on the cornea of the eye.

History

Leonardo da Vinci had proposed the idea of applying a corrective lens directly to the surface of the eye as early as 1508, and similar concepts surfaced from René Descartes in 1636, but it was not until 1887 that the German physiologist Adolf Eugen Fick constructed the first successful contact lens.

Usage

It has been estimated that between 32 and 35 million Americans wear contact lenses[1]. In 1971, Bausch & Lomb introduced the first soft contact lenses.

Types of contact lenses

Contact lenses are available in a number of varieties.

Corrective vs. cosmetic contact lenses

A corrective contact lens is a contact lens designed to improve vision, whereas a cosmetic contact lens is a contact lens designed to change the appearance of the eye. The first cosmetic contact was designed by optometrist named frankie for the price of about $5000 to a playboy model for a halloween photo shoot. (please fill in the name of this woman and date, its the photo of her wrapped with a snake)

Contact lenses typically correct vision by refracting or bending light to focus on the eye's retina. An optometrist can find the specific dioptre required to treat the patient's condition, and an oculist will provide it. The thickness and shape of the contact lens will vary with the increase in dioptres and the condition that is being treated. Conditions treatable with contact lenses include near (or short) sightedness (myopia), far (or long) sightedness (hypermetropia), and astigmatism. Some contact lenses correct nearsightedness by flattening the cornea rather than refracting light. (See main article: Orthokeratology.)

Heavily tinted contacts are 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 and, therefore, easier to handle.

Some companies produce contacts that can change the tint or color appearance of the eye, or can place various designs on it (opaque lenses). Some cosmetic contacts can give the iris an enlarged appearance, or can mask various defects, such as absence (aniridia) or damage (dyscoria) to the iris. Cosmetic contacts are rarely used day-to-day, although many performers, such as Wes Borland, Marilyn Manson, Twiztid, and Ray Park as Darth Maul from Star Wars Episode I: The Phantom Menace, use them for artistic purposes. These types of contacts can even correct vision like other contacts, although some blurring or obstruction of vision may occur as a result of the specific contact lens design being used. Some contacts cover the whites (or sclera) of the eye; these are referred to as scleral lenses.

Soft vs. hard contact lenses

Contact lenses may also be classified as either soft or hard. Hard contacts are typically not disposable, while soft contacts often are. Some soft contacts are also known as extended wear lenses. The most commonly used contact lenses today 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, the latest containing some variant of silicone hydrogel. Previously, hard contact lenses were made of a polymer known as PMMA. Rigid gas-permeable (RGP) lenses have replaced them. Many contact lenses are made of hydrophilic (water-absorbing) materials, thereby allowing oxygen to reach the cornea, and making the lens more comfortable to wear.

Piggybacking contact lenses refers to the wearing of a smaller, rigid lens atop a larger, soft lens. This is done for a variety of clinical reasons where a single lens will not provide the optical power, fitting characteristics, or comfort required.

Rigid contact lenses are also used to help correct vision in patients with corneal conditions, such as keratoconus, where soft contact lenses or glasses prove ineffective. Rigid contact lenses trap tears beneath the lens. Since the refractive index of tears is very close to that of the cornea, the tears pass light through the affected cornea with minimal distortion. The result is a lens that acts as a regularly shaped "artificial cornea," and improves vision.

Daily vs. extended wear contact lenses

A daily wear contact lens is designed to be removed prior to sleeping. An extended wear contact lens is designed for continuous overnight wear, typically for 6 or more consecutive nights. Newer materials, such as silicone hydrogels, allow for even longer wear periods of up to 30 consecutive nights; these longer-wear lenses are often referred to as continuous wear (CW). Generally, EW lenses are discarded after the specified length of time. These are increasing in popularity, due to their obvious convenience. Extended- and continuous-wear contact lenses can be worn for such long periods of time because of their high oxygen permeability (typically 5-6 times greater than conventional soft lenses), which allows the eye to remain remarkably healthy.

An example of a soft contact lens made from silicone hydrogel that's currently available is Bausch & Lomb PureVision™, which has innovative aspheric optics to deliver highly refined detail due to its enhanced optical design to correct spherical aberration, a natural occurrence in which light rays are refracted at different angles as they pass through the eye. Left uncorrected, spherical aberration can result in less than optimal visual quality.

Extended lens wearers may have an increased risk for corneal infections and corneal ulcers, primarily due to poor care and cleaning of the lenses, tear film instability and bacterial stagnation.

Spherical vs. toric contact lenses

A spherical contact lens is one in which all meridians of the lens have the same power correction. A toric contact lens is one in which the power correction differs from meridian to meridian. 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 the 'cylinder'.
• They are designed to keep the lens in a stable position regardless of eye movement. Often 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 tasks:

• Multipurpose solution - The most popular cleaning solution for contact lenses. Used for rinsing, disinfecting, cleaning and storing the lenses. This is arguably the easiest way to clean contacts, as it eliminates the need for multiple solutions.
• 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 the cleaner's directions) on each side. One must be extra careful in this step if one has long fingernails.
• Hydrogen peroxide solution - Used for disinfecting the lenses, and available as 'two-step' or 'one-step' systems. If using a 'two-step' product, one must ensure that the lens taken out of the hydrogen peroxide is neutralised before it is worn, or else wear will be extremely painful. Do NOT use saline to rinse away the peroxide. Some peroxide solutions, such as CIBA Vision's Clear Care, come with a special storage case that contains a catalyzing disk. If soaked in the solution with the disk for at least six hours, the hydrogen peroxide decomposes and the remaining solution is a saline that will not harm the eye. People with extremely sensitive, irritable eyes often use these types of cleaning solutions.
• Enzymatic cleaner - Used for cleaning the protein off of lenses, usually on a weekly basis, if the daily cleaner is not sufficient. Typically, this cleaner is in tablet form. Protein deposits make the contact lenses uncomfortable, and may lead to various eye problems.

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

Parameters

A contact lens can be characterised by

• Base curve radius (BC, BCR)
• Diameter (D, OAD)
• Power (Dioptres - Spherical, Cylindrical and/or reading addition)
• Cylinder axis
• Centre Thickness (CT)
• Material (eg. Oxygen Permeability/Transmissibility (Dk/L, Dk/t), water content, modulus)
• Replacement schedule

Products

• Acuvue

See also

• Base curve radius
• Intraocular lens
• Refraction error
• A simple eye test

External links

• Contact Lenses: A Timeline
• The History of Contact Lenses (Article)
• Decorative contact lenses (WildEyes brand by CIBA Vision)
• Contact Lenses Guidance
• Contact Lens catalog via Lenscraftersde:Kontaktlinse