Contact lens

A contact lens, or simply contact, is a lens placed on the eye. They are considered medical devices and can be worn to correct vision, for cosmetic 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 that 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 and aniseikonia that are corrected better by contacts than by glasses.

History

Leonardo Da Vinci 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 corneal 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 of the eye.

René Descartes 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, 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, 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", 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 (1929), who 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 constructed and fitted the first successful contact lens. While working in Zürich, he described fabricating afocal scleral 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 and glass with a dextrose solution. He published his work, "Contactbrille", in the journal 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 in Kiel, 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 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 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 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. Replacement Lens Insurance, Inc. (now known as RLI Corp.) 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 but rigid materials were developed to overcome this problem. Chemist Norman Gaylord 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 chemists Otto Wichterle and Drahoslav Lim 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 (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 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—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.

Types of contact lenses

Contact lenses are classified in many different manners.

Functions

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. Conditions correctable with contact lenses include myopia (near or short sightedness), hyper metropia (far or long sightedness) astigmatism and presbyopia. 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, the correction of myopia 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, a red-tinted "X-Chrom" contact lens may be used. Although the lens does not restore normal color vision, 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 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 accustoming to the lenses, the eyes are typically well tolerated. 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 theentertainment industry to make the eye appear confusing and arousing in appearance, most often in horror film and zombie 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 lenses cover the white part of the eye (i.e. sclera) 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 an enlarged appearance, or mask defects such as absence of (aniridia) or damage to (dyscoria) the iris.

A new trend in Japan, South Korea and China is the circle contact lens. 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, dry eyes, corneal ulcers and erosion, keratitis, corneal edema, 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, which caused eye irritation, and were not wearable for extended periods of time. But when William Feinbloom 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 is transmitted through the lens to the cornea, 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 — were developed. Lenses made from these materials are called rigid gas permeable 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 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.

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 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 FDA in 1971. Soft lenses are immediately comfortable, while 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. Theoxygen permeability 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, fitting characteristics, or comfort required.

In 1998, 'silicone hydrogels' became available. Silicone hydrogels have both the extremely high oxygen permeability of silicone 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, 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 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 ulcers, primarily due to poor care and cleaning of the lenses, tear film instability, and bacterial stagnation. Corneal neovascularization 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 or giant papillary conjunctivitis (GPC), sometimes associated with a poorly fitting contact lens.