Contact lens
Overview
 

A contact lens, or simply contact, is a lens
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

  placed on the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

. They are considered medical devices and can be worn to correct vision
Corrective lens
A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye...

, for cosmetic
Cosmetics
Cosmetics are substances used to enhance the appearance or odor of the human body. Cosmetics include skin-care creams, lotions, powders, perfumes, lipsticks, fingernail and toe nail polish, eye and facial makeup, towelettes, permanent waves, colored contact lenses, hair colors, hair sprays and...

 or therapeutic reasons. In 2004, it was estimated that 125 million people (2%) use contact lenses worldwide, including 28 to 38 million in the United States. In 2010, worldwide contact lens market was estimated at $6.1 billion, while the U.S. soft lens market is estimated at $2.1 billion.
Discussions
Encyclopedia

A contact lens, or simply contact, is a lens
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...

  placed on the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

. They are considered medical devices and can be worn to correct vision
Corrective lens
A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye...

, for cosmetic
Cosmetics
Cosmetics are substances used to enhance the appearance or odor of the human body. Cosmetics include skin-care creams, lotions, powders, perfumes, lipsticks, fingernail and toe nail polish, eye and facial makeup, towelettes, permanent waves, colored contact lenses, hair colors, hair sprays and...

 or therapeutic reasons. In 2004, it was estimated that 125 million people (2%) use contact lenses worldwide, including 28 to 38 million in the United States. In 2010, worldwide contact lens market was estimated at $6.1 billion, while the U.S. soft lens market is estimated at $2.1 billion. Some have estimated that the global market will reach $11.7 billion by 2015. As of 2010, the average age of contact lens wearers globally was 31 years old and two thirds of wearers were female.

People choose to wear contact lenses for many reasons. Aesthetics and cosmetics are often motivating factors for people who would like to avoid wearing glasses or would like to change the appearance of their eyes. Other people wear contacts for more visual reasons. When compared with spectacles, contact lenses typically provide better peripheral vision, and do not collect moisture such as rain, snow, condensation, or sweat. This makes them ideal for sports and other outdoor activities. Additionally, there are conditions such as keratoconus
Keratoconus
Keratoconus , is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve....

 and aniseikonia
Aniseikonia
Aniseikonia is an ocular condition where there is a significant difference in the perceived size of images. It can occur as an overall difference between the two eyes, or as a difference in a particular meridian.-Causes:Retinal image size is determined by many factors...

 that are corrected better by contacts than by glasses.

History

Leonardo Da Vinci
Leonardo da Vinci
Leonardo di ser Piero da Vinci was an Italian Renaissance polymath: painter, sculptor, architect, musician, scientist, mathematician, engineer, inventor, anatomist, geologist, cartographer, botanist and writer whose genius, perhaps more than that of any other figure, epitomized the Renaissance...

 is frequently credited with introducing the idea of contact lenses in his 1508 Codex of the eye, Manual D, where he described a method of directly altering cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

l power by submerging the eye in a bowl of water. Leonardo, however, did not suggest his idea be used for correcting vision—he was more interested in learning about the mechanisms of accommodation
Accommodation (eye)
Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image on an object as its distance changes....

 of the eye.

René Descartes
René Descartes
René Descartes ; was a French philosopher and writer who spent most of his adult life in the Dutch Republic. He has been dubbed the 'Father of Modern Philosophy', and much subsequent Western philosophy is a response to his writings, which are studied closely to this day...

 proposed another idea in 1636, in which a glass tube filled with liquid is placed in direct contact with the cornea. The protruding end was to be composed of clear glass, shaped to correct vision; however, the idea was impracticable, since it would make blinking impossible.

In 1801, Thomas Young
Thomas Young
Thomas Young may refer to:*Thomas Young , Scottish Presbyterian and author*Thomas Young , member of the Sons of Liberty*Thomas Young , British polymath, scientist and Egyptologist...

, made a basic pair of contact lenses on the model of Descartes. He used wax to affix water-filled lenses to his eyes. This neutralized his own refractive power. He then corrected for it with another pair of lenses.

However, like Leonardo's, Young's device was not intended to correct refraction errors.
Sir John Herschel
John Herschel
Sir John Frederick William Herschel, 1st Baronet KH, FRS ,was an English mathematician, astronomer, chemist, and experimental photographer/inventor, who in some years also did valuable botanical work...

, in a footnote of the 1845 edition of the Encyclopedia Metropolitana, posed two ideas for the visual correction: the first "a spherical capsule of glass filled with animal jelly
Gelatin
Gelatin is a translucent, colorless, brittle , flavorless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning in a similar...

", and "a mould of the cornea" which could be impressed on "some sort of transparent medium". Though Herschel reportedly never tested these ideas, they were both later advanced by several independent inventors such as Hungarian Dr. Dallos with Istvan Komàromy (1929), perfected a method of making molds from living eyes. This enabled the manufacture of lenses that, for the first time, conformed to the actual shape of the eye.

It was not until 1887 that a German glassblower, F.E. Muller, produced the first eye covering to be seen through and tolerated. In 1887, the German ophthalmologist Adolf Gaston Eugen Fick
Adolf Gaston Eugen Fick
Adolf Gaston Eugen Fick was a German ophthalmologist who invented the contact lens. He was the nephew of the German physiologist Adolf Eugen Fick, and the son of the German anatomy professor Franz Ludwig Fick....

 constructed and fitted the first successful contact lens. While working in Zürich
Zürich
Zurich is the largest city in Switzerland and the capital of the canton of Zurich. It is located in central Switzerland at the northwestern tip of Lake Zurich...

, he described fabricating afocal
Afocal system
In optics an afocal system is an optical system that produces no net convergence or divergence of the beam, i.e. has an infinite effective focal length. This type of system can be created with a pair of optical elements where the distance between the elements is equal to the sum of each element's...

 scleral
Scleral lens
A scleral lens is a large lens that rests on the Sclera and creates a Tear-filled vault over the Cornea. Scleral lenses are designed to treat a variety of eye conditions, many of which do not respond to other forms of treatment.- Design :...

 contact shells, which rested on the less sensitive rim of tissue around the cornea, and experimentally fitting them: initially on rabbits, then on himself, and lastly on a small group of volunteers. These lenses were made from heavy blown glass and were 18–21mm in diameter. Fick filled the empty space between cornea/callosity
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 and glass with a dextrose solution. He published his work, "Contactbrille", in the journal
Scientific journal
In academic publishing, a scientific journal is a periodical publication intended to further the progress of science, usually by reporting new research. There are thousands of scientific journals in publication, and many more have been published at various points in the past...

 Archiv für Augenheilkunde in March 1888.

Fick's lens was large, unwieldy, and could only be worn for a couple of hours at a time. August Müller
August Müller
August Müller , born in Mönchengladbach, was a medical student at the University of Kiel, Germany, and a pioneer in the manufacture of contact lenses. In 1889, he presented at the university his doctoral thesis titled Eyeglasses and corneal lenses in which he described his efforts to grind scleral...

 in Kiel
Kiel
Kiel is the capital and most populous city in the northern German state of Schleswig-Holstein, with a population of 238,049 .Kiel is approximately north of Hamburg. Due to its geographic location in the north of Germany, the southeast of the Jutland peninsula, and the southwestern shore of the...

, Germany, corrected his own severe myopia with a more convenient glass-blown scleral contact lens of his own manufacture in 1888.

Also in 1887, Louis J. Girard
Louis J. Girard
Louis J. Girard helped to popularize contact lens fitting as a medical procedure. He was co-author with Whitney J. Sampson, MD and Joseph W. Soper of a text, Contact lenses, published in 1968....

 invented a similar scleral form of contact lens.
Glass-blown scleral lenses remained the only form of contact lens until the 1930s when polymethyl methacrylate (PMMA or Perspex/Plexiglas) was developed, allowing plastic scleral lenses to be manufactured for the first time. In 1936, optometrist William Feinbloom
William Feinbloom
William Feinbloom was an American optometrist considered to be a pioneer in the field of low vision, visual rehabilitation, and the development of low vision devices....

 introduced plastic lenses, making them lighter and more convenient. These lenses were a combination of glass and plastic.

In 1949, the first "corneal" lenses were developed. These were much smaller than the original scleral lenses, as they sat only on the cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 rather than across all of the visible ocular surface, and could be worn up to sixteen hours per day. PMMA corneal lenses became the first contact lenses to have mass appeal through the 1960s, as lens designs became more sophisticated with improving manufacturing (lathe) technology.

Early corneal lenses in the 1950s and 1960s were relatively expensive and fragile, resulting in the development of a market for contact lens insurance
Insurance
In law and economics, insurance is a form of risk management primarily used to hedge against the risk of a contingent, uncertain loss. Insurance is defined as the equitable transfer of the risk of a loss, from one entity to another, in exchange for payment. An insurer is a company selling the...

. Replacement Lens Insurance, Inc. (now known as RLI Corp.
RLI Corp.
RLI Corp. is an American property and casualty insurance company headquartered in Peoria, Illinois, with eighteen branch offices conducting business in all fifty U.S. states. Founded in 1965, RLI was one of the first insurers of contact lenses; the letters RLI originally stood for Replacement...

) phased out its original flagship product in 1994 after contacts became more affordable and easier to replace.

One important disadvantage of PMMA lenses is that no oxygen is transmitted through the lens to the conjunctiva and cornea, which can cause a number of adverse clinical effects. By the end of the 1970s, and through the 1980s and 1990s, a range of oxygen-permeable
Oxygen permeability
Oxygen permeability is a parameter of a contact lens. OP expresses the ability of the lens to let oxygen reach the eye by diffusion. Another parameter is the transmissibility level , the Dk per thickness of the lens, and is generally more used. Typical values are from 25 to 50...

 but rigid materials were developed to overcome this problem. Chemist Norman Gaylord
Norman Gaylord
Norman Gaylord was an American industrial chemist and research scientist credited with playing a key role in the development of permeable contact lens which allows oxygen to reach the wearer's eye....

 played a prominent role in the development of these newer, permeable contact lenses. Collectively, these polymers are referred to as "rigid gas permeable" or "RGP" materials or lenses. Although all the above lens types—sclerals, PMMA lenses and RGPs—could be correctly referred to as being "hard" or "rigid", the term hard is now used to refer to the original PMMA lenses which are still occasionally fitted and worn, whereas rigid is a generic term which can be used for all these lens types. That is, hard lenses (PMMA lenses) are a sub-set of rigid lenses. Occasionally, the term "gas permeable" is used to describe RGP lenses, but this is potentially misleading, as soft lenses are also gas permeable in that they allow oxygen to move through the lens to the ocular surface.

The principal breakthrough in soft lenses was made by the Czech
Czechoslovakia
Czechoslovakia or Czecho-Slovakia was a sovereign state in Central Europe which existed from October 1918, when it declared its independence from the Austro-Hungarian Empire, until 1992...

 chemists Otto Wichterle
Otto Wichterle
Otto Wichterle was a Czech chemist, best known for his invention of modern soft contact lenses.-Biography:His father Karel was co-owner of a successful farm-machine factory and small car plant but Otto chose science for his career...

 and Drahoslav Lim
Drahoslav Lím
Drahoslav Lím was a Czech chemist. He invented polyhydroxyethylmethacrylate, the synthetic material used for contact lenses ....

 who published their work "Hydrophilic gels for biological use" in the journal Nature in 1959. This led to the launch of the first soft (hydrogel) lenses in some countries in the 1960s and the first approval of the "Soflens" material by the United States Food and Drug Administration
Food and Drug Administration
The Food and Drug Administration is an agency of the United States Department of Health and Human Services, one of the United States federal executive departments...

 (FDA) in 1971. These lenses were soon prescribed more often than rigid lenses, mainly due to the immediate comfort of soft lenses; by comparison, rigid lenses require a period of adaptation before full comfort is achieved. The polymers from which soft lenses are manufactured improved over the next 25 years, primarily in terms of increasing the oxygen permeability
Oxygen permeability
Oxygen permeability is a parameter of a contact lens. OP expresses the ability of the lens to let oxygen reach the eye by diffusion. Another parameter is the transmissibility level , the Dk per thickness of the lens, and is generally more used. Typical values are from 25 to 50...

 by varying the ingredients. In 1972, British optometrist Rishi Agarwal was the first to suggest disposable soft contact lenses.

In 1998, an important development was the launch of the first silicone hydrogels onto the market by CIBA VISION in Mexico. These new materials encapsulated the benefits of silicone—which has extremely high oxygen permeability
Oxygen permeability
Oxygen permeability is a parameter of a contact lens. OP expresses the ability of the lens to let oxygen reach the eye by diffusion. Another parameter is the transmissibility level , the Dk per thickness of the lens, and is generally more used. Typical values are from 25 to 50...

—with the comfort and clinical performance of the conventional hydrogels which had been used for the previous 30 years. These lenses were initially advocated primarily for extended (overnight) wear although more recently, daily (no overnight) wear silicone hydrogels have been launched.

In a slightly modified molecule, a polar group is added without changing the structure of the silicone hydrogel. This is referred to as the Tanaka monomer because it was invented and patented by Kyoichi Tanaka of Menicon Co. of Japan in 1979. Second-generation silicone hydrogels, such as galyfilcon A (Acuvue Advance, Vistakon) and senofilcon A (Acuvue Oasys, Vistakon), use the Tanaka monomer. Vistakon improved the Tanaka monomer even further and added other molecules, which serve as an internal wetting agent.

Comfilcon A (Biofinity, CooperVision) was the first third-generation polymer. The patent claims that the material uses two siloxy macromers of different sizes that, when used in combination, produce very high oxygen permeability (for a given water content). Enfilcon A (Avaira, CooperVision) is another third-generation material that's naturally wettable. The enfilcon A material is 46% water.

Corrective contact lenses

Corrective contact lenses are designed to improve vision. For many people, there is a mismatch between the refractive power of the eye and the length of the eye, leading to a refraction error. A contact lens neutralizes this mismatch and allows for correct focusing of light onto the retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

. Conditions correctable with contact lenses include myopia
Myopia
Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

 (near or short sightedness), hypermetropia (far or long sightedness), astigmatism
Astigmatism (eye)
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...

 and presbyopia
Presbyopia
Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...

. Contact wearers must usually take their contact lenses out every night or every few days, depending on the brand and style of the contact. Recently, there has been renewed interest in orthokeratology
Orthokeratology
Orthokeratology , marketed under brand names like "", "MiracLens", "DreamLens", "i-GO OVC", "GOV", "Wake and See", "CRT" and "Emerald", is the use of rigid gas-permeable contact lenses, normally worn only at night, to improve vision through the reshaping of the cornea...

, the correction of myopia
Myopia
Myopia , "shortsightedness" ) is a refractive defect of the eye in which collimated light produces image focus in front of the retina under conditions of accommodation. In simpler terms, myopia is a condition of the eye where the light that comes in does not directly focus on the retina but in...

 by deliberate overnight flattening of the cornea, leaving the eye without contact lens or eyeglasses correction during the day.

For those with certain color deficiencies
Color blindness
Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under lighting conditions when color vision is not normally impaired...

, a red-tinted "X-Chrom" contact lens may be used. Although the lens does not restore normal color vision
Color vision
Color vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect, emit, or transmit...

, it allows some colorblind individuals to distinguish colors better.

ChromaGen lenses have been used and these have been shown to have some limitations with vision at night although otherwise producing significant improvements in color vision. An earlier study showed very significant improvements in color vision and patient satisfaction.

Later work that used these ChromaGen lenses with dyslexics
Dyslexia
Dyslexia is a very broad term defining a learning disability that impairs a person's fluency or comprehension accuracy in being able to read, and which can manifest itself as a difficulty with phonological awareness, phonological decoding, orthographic coding, auditory short-term memory, or rapid...

 in a randomised, double-blind, placebo controlled trial showed highly significant improvements in reading ability over reading without the lenses This system has been granted FDA approval in the USA.

Cosmetic contact lenses

A cosmetic contact lens is designed to change the appearance of the eye. These lenses may also correct the vision, but some blurring or obstruction of vision may occur as a result of the color or design. In the USA, the Food and Drug Administration frequently calls non-corrective cosmetic contact lenses decorative contact lenses. These types of lenses tend to cause mild irritation on insertion, but after becoming accustomed to the lenses, the eyes typically tolerate them well. As with any contact lens, cosmetic lenses carry risks of mild and serious complications, including ocular redness, irritation, and infection. All individuals who decide to wear cosmetic lenses should check with an eye care provider prior to first use, and periodically over long term use in order to avoid potentially blinding complications.

Theatrical contact lenses are a type of cosmetic contact lens that are used primarily in the entertainment industry to make the eye appear altered, most often in horror film and zombie
Zombie
Zombie is a term used to denote an animated corpse brought back to life by mystical means such as witchcraft. The term is often figuratively applied to describe a hypnotized person bereft of consciousness and self-awareness, yet ambulant and able to respond to surrounding stimuli...

 movies, where lenses can make one's eyes appear demonic, cloudy and lifeless, or even to make the pupils of the wearer appear dilated to simulate the natural appearance of the pupils under the influence of various illicit drugs.

Scleral lens
Scleral lens
A scleral lens is a large lens that rests on the Sclera and creates a Tear-filled vault over the Cornea. Scleral lenses are designed to treat a variety of eye conditions, many of which do not respond to other forms of treatment.- Design :...

es cover the white part of the eye (i.e., sclera
Sclera
The sclera , also known as the white or white of the eye, is the opaque , fibrous, protective, outer layer of the eye containing collagen and elastic fiber. In the development of the embryo, the sclera is derived from the neural crest...

) and are used in many theatrical lenses. Due to their size, these lenses are difficult to insert and do not move very well within the eye. They may also hamper the vision as the lens has a small area for the user to see through. As a result they generally cannot be worn for more than 3 hours as they can cause temporary vision disturbances.

Similar lenses have more direct medical applications. For example, some lenses can give the iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

 an enlarged appearance, or mask defects such as absence of (aniridia
Aniridia
Aniridia is the absence of the iris. Aniridia usually involves both eyes. It can be congenital or caused by a penetrant injury. Isolated aniridia is a congenital disorder which is not limited to a defect in iris development, but is a panocular condition with macular and optic nerve hypoplasia,...

) or damage to (dyscoria) the iris.

A new trend in Japan, South Korea and China is the circle contact lens
Circle Contact Lens
A circle contact lens, also known as a big eye contact lens and circle lens, is a cosmetic contact lens that makes the eye's iris appear larger; this product originated from East Asia.- Design :...

. Circle lenses appear to be bigger because they are not only tinted in areas that cover the iris of the eye, but tinted prominently in the extra-wide outer ring of the lens. The result is the appearance of a bigger, wider iris.

Although many brands of contact lenses are lightly tinted to make them easier to handle, cosmetic lenses worn to change the color of the eye are far less common, accounting for only 3% of contact lens fits in 2004.

As a specialist's tool, in the hands of the untrained general public, non-prescription cosmetic contact lenses may represent a health risk.

Therapeutic contact lenses

Soft lenses are often used in the treatment and management of non-refractive disorders of the eye. A bandage contact lens protects an injured or diseased cornea from the constant rubbing of blinking eyelids thereby allowing it to heal. They are used in the treatment of conditions including bullous keratopathy
Bullous keratopathy
Bullous keratopathy is a pathological condition in which small vesicles, or bullae, are formed in the cornea due to endothelial dysfunction....

, dry eyes
Keratoconjunctivitis sicca
Keratoconjunctivitis sicca , also called keratitis sicca, xerophthalmia or dry eye syndrome is an eye disease caused by eye dryness, which, in turn, is caused by either decreased tear production or increased tear film evaporation. It is found in humans and some animals...

, corneal ulcers
Corneal abrasion
Corneal abrasion is a medical condition involving the loss of the surface epithelial layer of the eye's cornea.-Symptoms and signs:Symptoms of corneal abrasion include pain, photophobia, a foreign-body sensation, excessive squinting, and a reflex production of tears...

 and erosion
Recurrent corneal erosion
Recurrent corneal erosion is a disorder of the eyes characterized by the failure of the cornea's outermost layer of epithelial cells to attach to the underlying basement membrane...

, keratitis
Keratitis
Keratitis is a condition in which the eye's cornea, the front part of the eye, becomes inflamed. The condition is often marked by moderate to intense pain and usually involves impaired eyesight.-Types:...

, corneal edema
Edema
Edema or oedema ; both words from the Greek , oídēma "swelling"), formerly known as dropsy or hydropsy, is an abnormal accumulation of fluid beneath the skin or in one or more cavities of the body that produces swelling...

, descemetocele, corneal ectasis, Mooren's ulcer, anterior corneal dystrophy, and neurotrophic keratoconjunctivitis. Contact lenses that deliver drugs to the eye have also been developed.

Materials

The first contact lenses were made of glass
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...

, which caused eye irritation, and were not wearable for extended periods of time. But when William Feinbloom
William Feinbloom
William Feinbloom was an American optometrist considered to be a pioneer in the field of low vision, visual rehabilitation, and the development of low vision devices....

 introduced lenses made from polymethyl methacrylate (PMMA or Perspex/Plexiglas), contact lenses became much more convenient. These PMMA lenses are commonly referred to as "hard" lenses (this term is not used for other types of contact lens).

PMMA lenses have certain disadvantages: no oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 is transmitted through the lens to the cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

, which can cause a number of adverse clinical events. In the late 1970s, and through the 1980s and 1990s, improved rigid materials — which were also oxygen-permeable
Oxygen permeability
Oxygen permeability is a parameter of a contact lens. OP expresses the ability of the lens to let oxygen reach the eye by diffusion. Another parameter is the transmissibility level , the Dk per thickness of the lens, and is generally more used. Typical values are from 25 to 50...

 — were developed. Lenses made from these materials are called rigid gas permeable
Rigid gas permeable
Rigid gas permeable lenses are rigid contact lenses made of oxygen-permeable polymers. Initially developed in the late 1970s, and through the 1980s and 1990s, they were an improvement over prior 'hard' lenses that restricted oxygen transmission to the eye....

 or 'RGP' lenses. RGP lenses are not hydrophilic and do not absorb vapours or liquids, making them suitable for use in some industrial environments.

An RGP lens is able to replace the natural shape of the cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

 with a new refracting surface. This means that a regular (spherical) rigid contact lens can provide good level of vision in people who require strong correction, have astigmatism or suffer from diseases which distort the cornea, such as keratoconus
Keratoconus
Keratoconus , is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve....

.

While rigid lenses have been around for about 120 years, soft lenses are a much more recent development. The principal breakthrough in soft lenses made by Otto Wichterle
Otto Wichterle
Otto Wichterle was a Czech chemist, best known for his invention of modern soft contact lenses.-Biography:His father Karel was co-owner of a successful farm-machine factory and small car plant but Otto chose science for his career...

 led to the launch of the first soft (hydrogel) lenses in some countries in the 1960s and the approval of the 'Soflens' material (polymacon) by the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

 FDA in 1971. Soft lenses are immediately comfortable, while rigid lenses require a period of adaptation before full comfort is achieved. The polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

s from which soft lenses are manufactured improved over the next 25 years. The oxygen permeability
Oxygen permeability
Oxygen permeability is a parameter of a contact lens. OP expresses the ability of the lens to let oxygen reach the eye by diffusion. Another parameter is the transmissibility level , the Dk per thickness of the lens, and is generally more used. Typical values are from 25 to 50...

 has been increased by varying the polymer composition.

A small number of hybrid rigid/soft lenses exist. An alternative technique is piggybacking of contact lenses, a smaller, rigid lens being mounted atop a larger, soft lens. This is done in cases where a single lens will not provide the optical power
Optical power
Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...

, fitting characteristics, or comfort required.

In 1998, 'silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....

 hydrogels' became available. Silicone hydrogels have both the extremely high oxygen permeability
Oxygen permeability
Oxygen permeability is a parameter of a contact lens. OP expresses the ability of the lens to let oxygen reach the eye by diffusion. Another parameter is the transmissibility level , the Dk per thickness of the lens, and is generally more used. Typical values are from 25 to 50...

 of silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....

 and the comfort and clinical performance of the conventional hydrogels. These lenses were initially advocated primarily for extended (overnight) wear, although more recently daily (no overnight) wear silicone hydrogels have been approved and launched.

While it provides the oxygen permeability
Oxygen permeability
Oxygen permeability is a parameter of a contact lens. OP expresses the ability of the lens to let oxygen reach the eye by diffusion. Another parameter is the transmissibility level , the Dk per thickness of the lens, and is generally more used. Typical values are from 25 to 50...

, the silicone also makes the lens surface highly hydrophobic and less "wettable." This frequently results in discomfort and dryness during lens wear. In order to compensate for the hydrophobicity, hydrogels are added (hence the name "silicone hydrogels") to make the lenses more hydrophilic. However the lens surface may still remain hydrophobic. Hence some of the lenses undergo surface modification processes by plasma treatments which alter the hydrophobic nature of the lens surface. Other lens types incorporate internal rewetting agents to make the lens surface hydrophilic. A third process uses longer backbone polymer chains that results in less cross linking and increased wetting without surface alterations or additive agents.

Wear Schedule/Wear Indicator

A daily wear (DW) contact lens is designed to be removed prior to sleeping. An extended wear (EW) 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, extended wear lenses are discarded after the specified length of time, according to the replacement schedule (see next section). Extended- and continuous-wear contact lenses can be worn for such long periods of time because of their high oxygen permeability
Oxygen permeability
Oxygen permeability is a parameter of a contact lens. OP expresses the ability of the lens to let oxygen reach the eye by diffusion. Another parameter is the transmissibility level , the Dk per thickness of the lens, and is generally more used. Typical values are from 25 to 50...

 to the cornea (typically 5–6 times greater than conventional soft lenses), which allows the eye to remain healthy even when the eyelid is closed.

Extended lens wearers may have an increased risk for corneal infections and corneal ulcer
Corneal ulcer
A corneal ulcer, or ulcerative keratitis, is an inflammatory condition of the cornea involving loss of its outer layer. It is very common in dogs and is sometimes seen in cats...

s, primarily due to poor care and cleaning of the lenses, tear film instability, and bacterial stagnation. Corneal neovascularization
Corneal neovascularization
Corneal neovascularization is the excessive ingrowth of blood vessels from the limbal vascular plexus into the cornea, caused by a low reception of oxygen, which is generally not received from the bloodstream, but through the air. One of the most common causes is contact lens wear, and to a...

 has historically also been a common complication of extended lens wear, though this does not appear to be a problem with silicone hydrogel extended wear. The most common complication of extended lens use is conjunctivitis, usually allergic
Allergy
An Allergy is a hypersensitivity disorder of the immune system. Allergic reactions occur when a person's immune system reacts to normally harmless substances in the environment. A substance that causes a reaction is called an allergen. These reactions are acquired, predictable, and rapid...

 or giant papillary conjunctivitis
Conjunctivitis
Conjunctivitis refers to inflammation of the conjunctiva...

 (GPC), sometimes associated with a poorly fitting contact lens.

Replacement Schedule

The various soft contact lenses available are often categorized by their replacement schedule. The shortest replacement schedule is single use (1-day or daily disposable) lenses which are disposed of each night. Shorter replacement cycle lenses are commonly thinner and lighter, due to lower requirements for durability against wear and tear, and may be the most comfortable in their respective class and generation. These may be best for patients with ocular allergies or other conditions because it limits deposits of antigens and protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

, and is considered the healthiest wear schedule due to the most frequent replacement. Single use lenses are also useful for people who use contacts infrequently, or for purposes (e.g., swimming or other sporting activities) where losing a lens is likely.

More commonly, contact lenses are prescribed to be disposed of on a two-week or 4-week basis. Quarterly or annual lenses, which used to be very common, have lost favor because a more frequent replacement allows for increased comfort and fewer on-lens deposits. Rigid gas permeable lenses are very durable and may last for several years without the need for replacement. PMMA hard lenses were very durable, and were commonly worn for 5 to 10 years. Interestingly, a careful analysis of the materials used to manufacture many 'daily' disposable lenses show that they are often manufactured from the same material as the longer life disposables (4-week replacement for example), from the same company. Although the materials are the same, the manufacturing processes by which the respective contact lenses are made is what differentiates a 'daily disposable' lens from a lens recommended for two-week or 4-week replacement.

Contrary to popular belief, replacement schedule is not determined by the Food & Drug Administration (FDA). Replacement schedule is recommended only by the manufacturer of that contact lens. The only FDA-approved measure of contact lens wear is the 'wear indication' or 'wear schedule' (extended wear or daily wear) as was discussed in the previous section.

Design

A spherical contact lens is one in which both the inner and outer optical surfaces are portions of a sphere. A toric lens
Toric lens
A toric lens is a lens with two different powers in two orientations perpendicular to each other. One of the lens surfaces is shaped like a "cap" from a torus , while the other one usually is spherical...

 is one in which either or both of the optical surfaces have the effect of
a cylindrical lens, usually in combination with the effect of a spherical lens. Myopic (nearsighted) and hypermetropic (farsighted) people who also have astigmatism and who have been told they are not suitable for regular contact lenses may be able to use toric lenses. If one eye has astigmatism
Astigmatism (eye)
Astigmatism is an optical defect in which vision is blurred due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. There are two types of astigmatism: regular and...

 and the other does not, the patient may be told to use a spherical lens in one eye and a toric lens in the other. Toric lenses are made from the same materials as regular contact lenses but have a few extra characteristics:
  • They correct for both spherical and cylindrical aberration.
  • They may have a specific 'top' and 'bottom', as they are not symmetrical around their centre and must not be rotated. Lenses must be designed to maintain their orientation regardless of eye movement. Often lenses are thicker at the bottom and this thicker zone is pushed down by the upper eyelid during blinking to allow the lens to rotate into the correct position (with this thicker zone at the 6 o'clock position on the eye). Toric lenses are usually marked with tiny striations to assist their fitting.
  • They are usually more expensive to produce than non-toric lenses; therefore they are usually meant for extended wear. The first disposable toric lenses were introduced in 2000 by Vistakon.


Like eyeglasses, contact lenses can have one (single vision) or more (multifocal) focal points
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...

.

For correction of presbyopia
Presbyopia
Presbyopia is a condition where the eye exhibits a progressively diminished ability to focus on near objects with age. Presbyopia’s exact mechanisms are not known with certainty; the research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the...

 or accommodative insufficiency
Accommodative insufficiency
Accommodative insufficiency involves the inability of the eye to focus properly on an object. Approximately 80 percent of children diagnosed with CI also demonstrate AI....

 multifocal contact lenses are almost always used; however, single vision lenses may also be used in a process known as monovision: single vision lenses are used to correct one eye's far vision and the other eye's near vision. Alternatively, a person may wear single vision contact lenses to improve distance vision and reading glasses to improve near vision.

Some contact lenses have small text written around the edge, such as 123 or AV. These are used to identify if the contact lens is put on correctly or not.

Rigid gas permeable bifocal contact lenses most commonly have a small lens on the bottom for the near correction, when the eyes are lowered to read, this lens comes into the optical path. RGPs must translate (move vertically) to work properly, and thus the gaze of the eye can change from the near to the distant sections, much like bifocal eyeglasses.

Multifocal soft contact lenses are more complex to manufacture and require more skill to fit. All soft bifocal contact lenses are considered "simultaneous vision" because both far and near vision corrections are presented simultaneously to the retina, regardless of the position of the eye. Of course, only one correction is correct, the incorrect correction causes blur. Commonly these are designed with distance correction in the center of the lens and near correction in the periphery, or vice versa.

Implantation

Intraocular lens
Intraocular lens
An intraocular lens is an implanted lens in the eye, usually replacing the existing crystalline lens because it has been clouded over by a cataract, or as a form of refractive surgery to change the eye's optical power. It usually consists of a small plastic lens with plastic side struts, called...

es, also known as an implantable contact lenses, are special small corrective lenses surgically implanted in the eye's posterior chamber
Posterior chamber
The posterior chamber should not be confused with vitreous chamber. The posterior chamber is a narrow chink behind the peripheral part of the iris of the lens, and in front of the suspensory ligament of the lens and the ciliary processes. The Posterior Chamber consists of small space directly...

 behind the iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...

 and in front of the lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

 to correct higher degrees of myopia and hyperopia.

Manufacturing of contact lenses

Most contact lenses are mass produced.
  • Spin-cast lenses – A spin-cast lens is a soft contact lens manufactured by whirling liquid silicone in a revolving mold at high speed.
  • Diamond turning
    Diamond turning
    Diamond turning is a process of mechanical machining of precision elements using lathes or derivative machine tools equipped with natural or synthetic diamond-tipped tool bits...

     – A diamond-turned contact lens is cut and polished on a CNC lathe
    Lathe
    A lathe is a machine tool which rotates the workpiece on its axis to perform various operations such as cutting, sanding, knurling, drilling, or deformation with tools that are applied to the workpiece to create an object which has symmetry about an axis of rotation.Lathes are used in woodturning,...

    . The lens starts out as a cylindrical disk held in the jaws of the lathe. The lathe is equipped with an industrial-grade diamond
    Diamond
    In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...

     as the cutting tool. The CNC lathe may turn at nearly 6000 RPM as the cutter removes the desired amount of material from the inside of the lens. The concave (inner) surface of the lens is then polished with some fine abrasive paste, oil, and a small polyester cotton ball turned at high speeds. In order to hold the delicate lens in reverse manner, wax is used as an adhesive. The convex (outer) surface of the lens is thus cut and polished by the same process.
  • Molded – Molding is used to manufacture some brands of soft contact lenses. Rotating molds are used and the molten material is added and shaped by centrifugal forces. Injection molding and computer control are also used to create nearly perfect lenses.
  • Hybrids


Although many companies make contact lenses, in the US there are four major manufacturers:
  • Acuvue
    Acuvue
    Acuvue is a brand of disposable contact lenses. They are made by Vistakon, a subsidiary of Johnson & Johnson.-Overview:Acuvue lenses got their start at Frontier Contact Lens Company, a small company that started in the 1950s and opened a branch in Jacksonville...

    /Vistakon (Johnson & Johnson
    Johnson & Johnson
    Johnson & Johnson is an American multinational pharmaceutical, medical devices and consumer packaged goods manufacturer founded in 1886. Its common stock is a component of the Dow Jones Industrial Average and the company is listed among the Fortune 500....

    )
  • Ciba Vision
    CIBA Vision
    CIBA VISION is a company that researches, develops, and manufactures contact lenses and lens care products. A Novartis company, it is headquartered in Atlanta, Georgia in the Johns Creek Technology Park. Manufacturing plants are also located in Germany, Singapore, Indonesia, and Malaysia...

     (Novartis
    Novartis
    Novartis International AG is a multinational pharmaceutical company based in Basel, Switzerland, ranking number three in sales among the world-wide industry...

    )
  • Bausch & Lomb
    Bausch & Lomb
    Bausch & Lomb, an American company based in Rochester, New York, is one of the world's leading suppliers of eye health products, such as contact lenses and lens care products today. In addition to this main activity, in recent years the area of medical technology has been developed...

  • CooperVision
    CooperVision
    The Cooper Companies is an American medical company. Subsidiary CooperVision was founded in 1979 and is the 4th largest contact lens maker in the world and the number one manufacturer of toric lenses...


Silicone Hydrogel Lenses

Silicone is oxygen permeable. Silicone hydrogel lenses use both their water and polymer content to transmit oxygen to the eye.

The benefits to wearers include comfort and convenience. Silicone hydrogel contact lenses contain less water and deliver more oxygen to the eye than traditional hydrogel lenses. As a result, they aren't as prone to causing dehydration. For some people who wear their lenses for long days, this can mean better end-of-day comfort and allow for overnight wear. Some brands of silicone hydrogel lenses are approved for 30 days of continuous wear.

Hydrogel materials

  • Alphafilcon A
  • Asmofilcon A
  • Balafilcon A
  • Comfilcon A
  • Enfilcon A
  • Etafilcon A
  • Galyfilcon A
  • Hilafilcon A
  • Hilafilcon B
  • Hioxifilcon A
  • Hioxifilcon D
  • Lotrafilcon B
  • Methafilcon A
  • Omafilcon A
  • Phemfilcon A
  • Polymacon
  • Senofilcon A
  • Tetrafilcon A
  • Vifilcon A
  • POLY HEMA

Contact lens prescriptions

The prescribing of contact lenses is usually restricted to appropriately qualified eye care practitioners
Eye care professional
An eye care professional is an individual who provides a service related to the eyes or vision. It is a general term that can refer to any healthcare worker involved in eye care, from one with a small amount of post-secondary training to practitioners with a doctoral level of education.-Current...

. In countries such as the United States (where all contact lenses are deemed to be medical device
Medical device
A medical device is a product which is used for medical purposes in patients, in diagnosis, therapy or surgery . Whereas medicinal products achieve their principal action by pharmacological, metabolic or immunological means. Medical devices act by other means like physical, mechanical, thermal,...

s by the Food and Drug Administration), the United Kingdom and Australia, optometrists
Optometry
Optometry is a health care profession concerned with eyes and related structures, as well as vision, visual systems, and vision information processing in humans. Optometrists, or Doctors of Optometry, are state licensed medical professionals trained to prescribe and fit lenses to improve vision,...

 are usually responsible. In France and Eastern European countries, ophthalmologists
Ophthalmology
Ophthalmology is the branch of medicine that deals with the anatomy, physiology and diseases of the eye. An ophthalmologist is a specialist in medical and surgical eye problems...

 play the major role. In other parts of the world, opticians usually prescribe contact lenses. Prescriptions for contact lenses and glasses
Glasses
Glasses, also known as eyeglasses , spectacles or simply specs , are frames bearing lenses worn in front of the eyes. They are normally used for vision correction or eye protection. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or...

 may be similar, but are not interchangeable.

The practitioner or contact lens fitter typically determines an individual's suitability for contact lenses during an eye examination
Eye examination
An eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes....

. Corneal health is verified; ocular allergies or dry eyes may affect a person's ability to wear contact lenses successfully.

The parameters specified in a contact lenses prescription may include:
  • Material (e.g., Oxygen Permeability/Transmissibility (Dk/L, Dk/t)
    Oxygen permeability
    Oxygen permeability is a parameter of a contact lens. OP expresses the ability of the lens to let oxygen reach the eye by diffusion. Another parameter is the transmissibility level , the Dk per thickness of the lens, and is generally more used. Typical values are from 25 to 50...

    , water content, modulus; optional)
  • Base curve radius
    Base curve radius
    Base curve radius or simply base curve, abbreviated BCR or BC, is the measure of one important parameter of a lens in optometry. On a spectacle lens, it is the flatter curvature of the front surface and on a contact lens it is the curvature of the back surface. Typical values for a contact lens are...

     (BC, BCR; required)
  • Diameter
    Diameter
    In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

     (D, OAD; required)
  • Power
    Optical power
    Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. It is equal to the reciprocal of the focal length of the device. The dioptre is the most common unit of measurement of optical power...

     in dioptre
    Dioptre
    A dioptre, or diopter, is a unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres . It is thus a unit of reciprocal length. For example, a 3-dioptre lens brings parallel rays of light to focus at metre...

    s (Spherical required, cylindrical only for correction of astigmatism, reading addition required only for bifocal prescription)
  • Cylinder axis (Often just "Axis"; required only if a cylindrical correction is present)
  • Center thickness (CT; optional and seldom provided)
  • Brand (Optional)


Note that while a contact lens prescription may specify diameter and base curve of the ideal lens for a particular individual, many manufacturers only produce lenses at very limited intervals. Johnson & Johnson
Johnson & Johnson
Johnson & Johnson is an American multinational pharmaceutical, medical devices and consumer packaged goods manufacturer founded in 1886. Its common stock is a component of the Dow Jones Industrial Average and the company is listed among the Fortune 500....

, which manufactures the "Acuvue 2" soft lens, only makes such lenses with base curves of either 8.3mm or 8.7mm, and only at a diameter of 14.0mm—such lenses are designed to fit a "target audience" that includes the vast majority of individuals who might fit such lenses or might at least find them sufficiently comfortable to wear.

Many people already wear contact lenses ordered over the Internet. In the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, the Fairness to Contact Lens Consumers Act
Fairness to Contact Lens Consumers Act
The Fairness to Contact Lens Consumers Act |codified]] at et seq.), also known as FCLCA, is a United States federal law that aims improving consumer protection and ocular health for contact lens users.-Provisions:...

, which went into effect
Effective date
An effective date or as of date is the date upon which something is considered to take effect. This may be different from the date upon which the event actually occurs or is recorded....

 in 2004, was intended to ensure the availability of contact lens prescriptions to patients. Under the law consumers have a right to obtain a copy of their contact lens prescription, allowing them to fill that prescription at the business of their choice. Some controversy has arisen over the fact that online vendors are allowed to fill a prescription if the originating prescriber doesn't respond within eight business hours, as verifications are often sent during the prescriber's closed hours, allowing even an expired prescription to be filled.

Complications

Complications
Complication (medicine)
Complication, in medicine, is an unfavorable evolution of a disease, a health condition or a medical treatment. The disease can become worse in its severity or show a higher number of signs, symptoms or new pathological changes, become widespread throughout the body or affect other organ systems. A...

 due to contact lens wear affect roughly 5% of contact lens wearers each year. Excessive wear of contact lenses, particularly overnight wear, is associated with most of the safety concerns. Problems associated with contact lens wear may affect the eyelid
Eyelid
An eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...

, the conjunctiva
Conjunctiva
The conjunctiva covers the sclera and lines the inside of the eyelids. It is composed of rare stratified columnar epithelium.-Function:...

, the various layers of the cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...

, and even the tear film
Tears
Tears are secretions that clean and lubricate the eyes. Lacrimation or lachrymation is the production or shedding of tears....

 that covers the outer surface of the eye.

Studies conducted on side effects from long-term wearing of contact lenses, i.e. over 5 years, such as by Zuguo Liu et al., 2000, concludes that "Long-term contact lens wear appears to decrease the entire corneal thickness and increase the corneal curvature and surface irregularity."

Long-term wear of rigid contact lens is associated with decreased corneal keratocyte
Corneal keratocyte
Corneal keratocytes are specialized fibroblasts residing in the stroma. This corneal layer, representing about 85-90% of corneal thickness, is built up from highly regular collagenous lamellae and extracellular matrix components. Keratocytes play the major role in keeping it transparent, healing...

 density and increased number of epithelial
Corneal epithelium
The corneal epithelium is made up of epithelial tissue and covers the front of the cornea. It acts as a barrier to protect the cornea, resisting the free flow of fluids from the tears, and prevents bacteria from entering the epithelium and corneal stroma.The corneal epithelium consists of several...

 Langerhans cells.

Cornea

  • Epithelium
    Epithelium
    Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

    • Corneal abrasion
      Corneal abrasion
      Corneal abrasion is a medical condition involving the loss of the surface epithelial layer of the eye's cornea.-Symptoms and signs:Symptoms of corneal abrasion include pain, photophobia, a foreign-body sensation, excessive squinting, and a reflex production of tears...

    • Corneal erosion
    • Contact lens acute red eye (CLARE)
    • Keratoconus
      Keratoconus
      Keratoconus , is a degenerative disorder of the eye in which structural changes within the cornea cause it to thin and change to a more conical shape than its normal gradual curve....

  • Corneal endothelium
    Corneal endothelium
    The corneal endothelium is a single layer of cells on the inner surface of the cornea. It faces the chamber formed between the cornea and the iris....


Usage

Before touching the contact lens or one's eyes, it is important to thoroughly wash & rinse hands with a soap
Soap
In chemistry, soap is a salt of a fatty acid.IUPAC. "" Compendium of Chemical Terminology, 2nd ed. . Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford . XML on-line corrected version: created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN...

 that does not contain moisturizer
Moisturizer
Moisturizers or emollients are complex mixtures of chemical agents specially designed to make the external layers of the skin softer and more pliable, by increasing its hydration by reducing evaporation. Naturally occurring skin lipids and sterols as well as artificial or natural oils,...

s or allergen
Allergen
An allergen is any substance that can cause an allergy. In technical terms, an allergen is a non-parasitic antigen capable of stimulating a type-I hypersensitivity reaction in atopic individuals....

s such as fragrances . The soap should not be antibacterial due to risk of improper hand washing and the possibility of destroying the natural bacteria found on the eye. These bacteria keep pathogenic bacteria from colonizing the cornea . The technique for removing or inserting a contact lens varies slightly depending upon whether the lens is soft or rigid.

In all cases, the insertion and removal of lenses requires some training and practice on the part of the user, in part to overcome the instinctive hesitation against actually touching the eyeball with one's fingertip.

Insertion

Contact lenses are typically inserted into the eye by placing them on the index finger with the concave side upward and raising them to touch the cornea. The other hand may be employed to keep the eye open. Problems may arise particularly with disposable soft lenses; if the surface tension
Surface tension
Surface tension is a property of the surface of a liquid that allows it to resist an external force. It is revealed, for example, in floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects to run on the water surface...

 between the lens and the finger is too great the lens may turn itself inside out; alternatively it may fold itself in half. When the lens first contacts the eye, a brief period of irritation may ensue as the eye acclimatizes to the lens and also (if a multi-use lens is not correctly cleansed) as dirt on the lens irritates the eye. This may be relieved by placing a drop of saline or multipurpose solution into the lens prior to insertion, this also allows the lens to conform to the eye more quickly by lubricating both contact lens and eye surfaces. After insertion, irrigation may help if irritated, which generally should not exceed one minute. It may be noted that although with some types of contact lenses it is easy to tell if you have inserted the lens backwards (as it is usually painful and vision is impaired) you are able to determine the lens's correct position beforehand by holding the lens on the tip of your finger and squeezing the bottom of it with two fingers from your other hand, you will know you have it the correct way if the edges of the lens curve inward like a taco. If they curve out you need to flip the lens. With some types of lenses however, this is difficult as both sides look very much the same. With many lenses it is hard to tell whether they are inside out or not even when they are in the eye itself. This is because the vision and feel of the lens can be very similar for both sides. For these reasons many people try to ensure they keep visual track of the different sides of the contact lenses from the day they are open, if they suspect the lens is inside out they can always change its orientation at a later stage. It is never advisable to wear the lenses inside-out even if they feel comfortable and vision is good when doing so.

Removal

A soft lens may be removed by holding the eyelids open and grasping the lens with opposing digits. This method may cause irritation, could risk damage to the eye and may in many cases be difficult, in part due to the blink reflex
Corneal reflex
The corneal reflex, also known as the blink reflex, is an involuntary blinking of the eyelids elicited by stimulation of the cornea , or bright light, though could result from any peripheral stimulus. Stimulation should elicit both a direct and consensual response...

. If the lens is pushed off the cornea (by touching the lens with your forefinger and looking towards your nose, moving the lens) it will buckle up (due to the difference in curvature), making it easier to grasp.

As an alternative method to grasping, once the lens is moved off the cornea to the inner corner of the eye, it can be pushed out of the eye by pressing downwards on the upper eyelid with a finger. With this method there is less risk of touching the eye with the fingers, and it may be easier for people with long fingernails.

Rigid contact lenses may be removed by pulling with one finger on the outer or lateral canthus
Canthus (anatomy)
Canthus is either corner of the eye where the upper and lower eyelids meet. More specifically, the medial and lateral canthi would be described as the medial and lateral ends/angles of the palpebral fissure....

, then blinking to cause the lens to lose adhesion
Adhesion
Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....

. The other hand is typically cupped underneath the eye to catch the lens. There also exist small tools specifically for removing lenses, which resemble small plunger
Plunger
A plunger is a common device that is used to release stoppages in plumbing. The tool consists of a rubber cup with an attached stick "shaft", usually made of wood or bronze. Before use, any objects such as hair in the plug grate should be removed and, if possible the overflow hole should be...

s made of flexible plastic; the concave end is raised to the eye and touched to the lens, forming a seal stronger than that of the lens with the cornea and allowing the lens to be removed from the eye.

Care

While daily disposable lenses require no cleaning, other types require regular cleaning and disinfecting in order to retain clear vision and prevent discomfort and infections by various microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...

s including bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, fungi, and Acanthamoeba
Acanthamoeba
Acanthamoeba is a genus of amoebae, one of the most common protozoa in soil, and also frequently found in fresh water and other habitats. The cells are small, usually 15 to 35 μm in length and oval to triangular in shape when moving. The pseudopods form a clear hemispherical lobe at the anterior,...

, that form a biofilm
Biofilm
A biofilm is an aggregate of microorganisms in which cells adhere to each other on a surface. These adherent cells are frequently embedded within a self-produced matrix of extracellular polymeric substance...

 on the lens surface. There are a lot of products that are used to cleans contact lenses:
  • Multipurpose solutions – The most popular cleaning solution for contact lenses; these are suitable for rinsing, disinfecting, cleaning and storing lenses, and in most cases eliminate the need for protein removal enzyme tablets. Some multipurpose solutions are not effective at disinfecting Acanthamoeba
    Acanthamoeba
    Acanthamoeba is a genus of amoebae, one of the most common protozoa in soil, and also frequently found in fresh water and other habitats. The cells are small, usually 15 to 35 μm in length and oval to triangular in shape when moving. The pseudopods form a clear hemispherical lobe at the anterior,...

    from the lens. In May 2007, one brand of multipurpose solution was recalled due to a cluster of Acanthamoeba infections. Newer generations of multipurpose solutions are effective against bacteria, fungi, and acanthamoeba and are designed to condition the lenses while soaking.
  • Saline
    Saline (medicine)
    In medicine, saline is a general term referring to a sterile solution of sodium chloride in water but is only sterile when it is to be placed intravenously, otherwise, a saline solution is a salt water solution...

     solution – Used for rinsing the lens after cleaning and preparing it for insertion. Saline solutions do not disinfect.
  • Daily cleaner – Used to clean lenses on a daily basis. A few drops of cleaner are applied to the lens while it rests in the palm of the hand, and the lens is rubbed for about 20 seconds with a fingertip (depending on the product) on each side. Long fingernails can damage lenses.
  • Hydrogen peroxide
    Hydrogen peroxide
    Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...

     solution – Used for disinfecting the lenses, and available as 'two-step' or 'one-step' systems. With 'two-step' products, the peroxide must be rinsed away with saline before the lenses may be worn, because hydrogen peroxide is an irritant and strong oxidizer.
  • Enzymatic cleaner – Used for cleaning protein deposits off lenses, usually weekly, if the daily cleaner is not sufficient. Typically, this cleaner is in tablet form. Protein deposits make use of contact lenses uncomfortable, and may lead to various eye problems.
  • Ultraviolet, vibration or ultrasonic devices – Used to both disinfect and clean contact lenses. The lenses are inserted inside the portable device (running on batteries and/or plug-in) for 2 to 6 minutes during which both the microorganisms and protein build-up are thoroughly cleaned. Saline solution is typically used as multi-purpose solutions are not necessary. These devices are not usually available in optic retailers but are in some electro-domestic stores.


Some products must only be used with certain types of contact lenses. Water alone will not adequately disinfect the lens, and can lead to lens contamination and has been known in some cases to cause irreparable harm to the eye. Proper lens cleaning is important in warding off biofilm formation.

To keep the cleaning product as clean as possible, and to counteract minor contamination of the product and kill microorganisms on the contact lens, some products contain preservative
Preservative
A preservative is a naturally occurring or synthetically produced substance that is added to products such as foods, pharmaceuticals, paints, biological samples, wood, etc. to prevent decomposition by microbial growth or by undesirable chemical changes....

s such as thiomersal
Thiomersal
Thiomersal , and commonly known in the US as thimerosal, is an organomercury compound. This compound is a well established antiseptic and antifungal agent....

, benzalkonium chloride
Benzalkonium chloride
Benzalkonium chloride, also known as alkyldimethylbenzylammonium chloride and ADBAC, is a mixture of alkylbenzyldimethylammonium chlorides of various even-numbered alkyl chain lengths. This product is a nitrogenous cationic surface-acting agent belonging to the quaternary ammonium group...

, and benzyl alcohol
Benzyl alcohol
Benzyl alcohol is an organic compound with the formula C6H5CH2OH. The benzyl group is often abbreviated "Bn", thus benzyl alcohol is denoted as BnOH. Benzyl alcohol is a colorless liquid with a mild pleasant aromatic odor. It is a useful solvent due to its polarity, low toxicity, and low vapor...

. In 1989, thiomersal was responsible for about 10% of problems related to contact lenses: because of this, many products no longer contain thiomersal. Preservative-free products usually have shorter shelf lives
Shelf life
Shelf life is the length of time that food, drink, medicine, chemicals, and many other perishable items are given before they are considered unsuitable for sale, use, or consumption...

. For example, non-aerosol preservative-free saline solutions can typically be used for only two weeks once opened. The introduction of silicone-hydrogel soft contact lens materials in 1999 made selection of the proper disinfecting solution more important.

Current research

A large segment of current contact lens research is directed towards the treatment and prevention of conditions resulting from contact lens contamination and colonization by foreign organisms. It is generally accepted by clinicians that the most significant complication of contact lens wear is microbial keratitis
Keratitis
Keratitis is a condition in which the eye's cornea, the front part of the eye, becomes inflamed. The condition is often marked by moderate to intense pain and usually involves impaired eyesight.-Types:...

 and that the most predominant microbial pathogen is Pseudomonas aeruginosa
Pseudomonas aeruginosa
Pseudomonas aeruginosa is a common bacterium that can cause disease in animals, including humans. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, and has, thus, colonized many...

. Other organisms are also major causative factors in bacterial keratitis associated with contact lens wear, although their prevalence varies across different locations. These include both the Staphylococcus
Staphylococcus
Staphylococcus is a genus of Gram-positive bacteria. Under the microscope they appear round , and form in grape-like clusters....

species (aureus and epidermidis) and the Streptococcus
Streptococcus
Streptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek στρεπτος streptos, meaning...

species, among others. Microbial keratitis is a serious focal point of current research due to its potentially devastating effect on the eye, including severe vision loss.

One specific research topic of interest is how microbes such as Pseudomonas aeruginosa
Pseudomonas aeruginosa
Pseudomonas aeruginosa is a common bacterium that can cause disease in animals, including humans. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, and has, thus, colonized many...

invade the eye and cause infection. Although the pathogenesis of microbial keratitis is not well understood, many different factors have been investigated. One group of researchers showed that corneal hypoxia exacerbated Pseudomonas binding to the corneal epithelium, internalization of the microbes, and induction of the inflammatory response. One way to alleviate hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...

 is to increase the amount of oxygen transmitted to the cornea. Although silicone-hydrogel lenses almost eliminate hypoxia in patients due to their very high levels of oxygen transmissibility, they also seem to provide a more efficient platform for bacterial contamination and corneal infiltration than other conventional hydrogel soft contact lenses. A recent study showed that Pseudomonas aeruginosa
Pseudomonas aeruginosa
Pseudomonas aeruginosa is a common bacterium that can cause disease in animals, including humans. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, and has, thus, colonized many...

and Staphylococcus epidermis adhere much more strongly to silicone hydrogel contact lenses than conventional hydrogel contact lenses and that adhesion of Pseudomonas aeruginosa was 20 times stronger than adhesion of Staphylococcus epidermidis. This might help to explain one reason why Pseudomonas infections are the most predominant.

Another important area of contact lens research deals with patient compliance. Compliance is a major issue surrounding the use of contact lenses because patient noncompliance often leads to contamination of the lens, storage case, or both. The introduction of multipurpose solutions and daily disposable lenses have helped to alleviate some of the problems observed from inadequate cleaning but new methods of combating microbial contamination are currently being developed. A silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...

-impregnated lens case has been developed which helps to eradicate any potentially contaminating microbes that come in contact with the lens case. Additionally, a number of antimicrobial
Antimicrobial
An anti-microbial is a substance that kills or inhibits the growth of microorganisms such as bacteria, fungi, or protozoans. Antimicrobial drugs either kill microbes or prevent the growth of microbes...

 agents are being developed that have been embedded into contact lenses themselves. Contact lenses with covalently attached Selenium
Selenium
Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...

 molecules have been shown to reduce bacterial colonization without adversely affecting the cornea of a rabbit eye and octylglucoside used as a contact lens surfactant significantly decreases bacterial adhesion. These compounds are of particular interest to contact lens manufacturers and prescribing optometrists because they do not require any patient compliance to effectively attenuate the effects of bacterial colonization.

A recent area of research is in the field of bionic lenses. LED lights and circuitry have been designed into recent contact lenses (http://news.cnet.com/2300-11393_3-6227089.html) based on the early research of Eric Booth in the 70s, who specialized in both train engineering and electrical engineering. He attempted to design transistor circuitry in early rigid contact lenses, but not until 2011 was the research perfected with the use of red LED lighting.

See also

  • Bionic contact lens
    Bionic contact lens
    Bionic contact lenses are being developed to provide a virtual display that could have a variety of uses from assisting the visually impaired to the video game industry. The device will have the form of a conventional contact lens with added bionics technology...

  • Corrective lens
    Corrective lens
    A corrective lens is a lens worn in front of the eye, mainly used to treat myopia, hyperopia, astigmatism, and presbyopia. Glasses or "spectacles" are worn on the face a short distance in front of the eye. Contact lenses are worn directly on the surface of the eye...

  • Eyeglass prescription
    Eyeglass prescription
    An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist or ophthalmologist, that specifies the value of all parameters the prescriber has deemed necessary to construct and/or dispense corrective lenses appropriate for a patient.If an examination indicates that...

  • Visual acuity
    Visual acuity
    Visual acuity is acuteness or clearness of vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....

  • Bates method
    Bates Method
    The Bates method is an alternative therapy aimed at improving eyesight. Eye-care physician William Horatio Bates attributed nearly all sight problems to habitual strain of the eyes, and felt that glasses were harmful and never necessary...

  • Effects of Long-Term Contact Lens Wear on the Cornea
    Effects of Long-Term Contact Lens Wear on the Cornea
    Long-term contact lens use leads to alterations in corneal thickness, stromal thickness, curvature, corneal sensitivity, cell density, and epithelial oxygen uptake, etc. Other changes include the formation of epithelial vacuoles and microcysts as well as the emergence of polymegathism in the...

  • Advantage and Disadvantage of contact lenses

Further reading

  • Efron, Nathan (2002). Contact Lens Practice, Elsevier Health Sciences. 0-7506-4690-X.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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