contact lens

A pair of contact lenses, positioned with the concave side facing upward.

One-day disposable contact lenses with blue handling tint in blister-pack packaging

This article is written like a personal reflection or essay rather than an encyclopedic description of the subject. Please help improve it by rewriting it in an encyclopedic style. (July 2011)

A contact lens, or simply contact, is a lens placed on the eye. Contact lenses 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.^[1] 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.^[2] Some have estimated that the global market will reach $11.7 billion by 2015. ^[2] As of 2010, the average age of contact lens wearers globally was 31 years old and two thirds of wearers were female.^[3]

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.^[4] 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 typically corrected better by contacts than by glasses.

History

In 1888, Adolf Fick was the first to successfully fit contact lenses, which were made from blown glass

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.^[5]

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.^[6]

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".^[7] 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.^[8] 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.^[9]

Also in 1887, Louis J. Girard invented a similar scleral form of contact lens.^[10] 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.^[11] These lenses were a combination of glass and plastic.

In 1949, the first "corneal" lenses were developed.^{[12][13][14][15]} 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.^[16] 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 that can be used for all these lens types: 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.^[17] 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.^[18][19]

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.^[20]

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 is naturally wettable. The enfilcon A material is 46% water.^[20]

Types of contact lenses

Contact lenses can be classified in many different manners. Contact lenses can be separated by their primary function, material, wear schedule (how long a lens can be worn before removing it), and replacement schedule (how long before a lens needs to be discarded).

Functions

Corrective contact lenses

Corrective contact lenses are designed to improve vision, most commonly by correcting refractive error. This is done by directly focusing the light so that it enters the eye with the proper power for clear vision. Recently, there has been renewed interest in orthokeratology, the correction of myopia by deliberate overnight flattening of the corneal epithelium, leaving the eye without a refractive error during the day.

A spherical contact lens bends light evenly in every direction (horizontally, vertically, etc.). They are typically used to correct myopia and hyperopia. A toric contact lens has a different focusing power horizontally than it does vertically, and as a result can correct for astigmatism. Some spherical rigid lenses can also correct for astigmatism. (See below.) Because a toric lens must have the proper orientation to correct for a person's astigmatism, a toric contact lens must have additional design characteristics to prevent the lens from rotating out of the ideal alignment. This can be done by weighting the bottom of the lens or by using other physical characteristics to rotate the lens back into position. Some toric contact lenses have marks or etchings that can assist the eye doctor in fitting the lens. The first disposable toric lenses were introduced in 2000 by Vistakon.

The correction of presbyopia (a need for a reading prescription that is different from the prescription needed for distance) presents an additional challenge in the fitting of contact lenses. Two main strategies exist: multifocal contact lenses and monovision. Multifocal contact lenses are comparable to bifocals or progressive lenses because they have multiple focal points. Multifocal contact lenses are typically designed for constant viewing through the center of the lens, but some designs do incorporate a shift in lens position to view through the reading power (similar to bifocal glasses). Monovision^[21] is the use single vision lenses (one focal point per lens) to focus one eye for distance vision (typically the person's dominant eye) and the other eye for near work. The brain then learns to use this setup to see clearly at all distances. A technique called modified monovision uses multifocal lenses and also specializes one eye for distance and one eye for near. Alternatively, a person may simply wear reading glasses over their distance contact lenses.

Other types of vision correction

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.^[22][23]

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.^[24] An earlier study showed very significant improvements in color vision and patient satisfaction.^[25]

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^[26] This system has been granted FDA approval in the USA.

Cosmetic contact lenses

A woman wearing a cosmetic type of contact lenses; the enlarged section of the image shows the grain produced during the manufacturing process. Curving of the lines of printed dots suggests these lenses were manufactured by printing onto a flat sheet then shaping it.

A cosmetic contact lens is designed to change the appearance of the eye. These lenses may also correct refractive error. 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.^[27]

In the USA, the Food and Drug Administration frequently calls non-corrective cosmetic contact lenses decorative contact lenses. As with any contact lens, cosmetic lenses carry risks of mild and serious complications, including ocular redness, irritation, and infection.^[28] For this reason all contact lenses, even purely cosmetic ones, are classified as medical devices in many countries (USA, UK). All individuals who would like to wear cosmetic lenses should have a contact lens examination with an eye doctor prior to first use, and if used long-term, regular aftercare examinations, in order to avoid potentially blinding complications.

Cosmetic lenses can be used to drastically alter the appearance of the eye, as seen in the entertainment industry. Scleral lenses that cover the white part of the eye (i.e., sclera) are used in many theatrical applications. These lenses are typically custom made for a specific production and as a result have very limited availability to the general public. As with any cosmetic lens, if the design changes the clarity of the center of the lens, the lens may interfere with vision.

A new trend in Japan, South Korea and China is the circle contact lens. Circle lenses extend the appearance of the iris onto the sclera. The result is the appearance of a bigger, wider iris.

Cosmetic lenses can have more direct medical applications. For example, some lenses can restore the appearance and, to some extent the function, of a damaged or missing iris.

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.^[29] They are used in the treatment of conditions including bullous keratopathy, dry eyes, corneal abrasions and erosion, keratitis, corneal edema, descemetocele, corneal ectasis, Mooren's ulcer, anterior corneal dystrophy, and neurotrophic keratoconjunctivitis.^[30] Contact lenses that deliver drugs to the eye have also been developed.^[31]

Materials

Contact lenses, other than the cosmetic variety, become almost invisible once inserted in the eye. Most corrective contact lenses come with a light "handling tint" that may render the lens slightly more visible on the eye. Soft contact lenses extend beyond the cornea, and the border is sometimes visible against the sclera.

Rigid lenses

Glass lenses were never comfortable enough to gain widespread popularity. The first lenses to do so were lenses made from polymethyl methacrylate (PMMA or Perspex/Plexiglas). PMMA lenses are commonly referred to as "hard" lenses. A disadvantage of these lenses is that they do not allow oxygen to pass through to the cornea, which can cause a number of adverse clinical events.

Starting in the late 1970s, improved rigid materials which were oxygen-permeable were developed. Lenses made from these materials are called rigid gas permeable or 'RGP' lenses.

A rigid lens is able to replace the natural shape of the cornea with a new refracting surface. This means that a spherical rigid contact lens can correct for astigmatism. Rigid lenses can also be made as a front-toric, back-toric, or bitoric. This is different from a spherical lens in that one or both surfaces of the lens deliver a toric correction. Rigid lenses can also correct for corneal irregularities, such as keratoconus. In most cases, patients with keratoconus see better through rigid contact lenses than through glasses. Rigid lenses are more chemically inert, allowing them to be worn in more challenging environments than soft lenses.

Soft lenses

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" daily 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 biggest improvements to soft lens polymers have been increasing oxygen permeability, lens wetability, and overall comfort.

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. Because silicone allows more oxygen permeability than water, the oxygen permeability of silicone hydrogels is not tied to the water content of the lens. Lenses have now been developed with so much oxygen permeability that they are approved for overnight wear (extended wear). Lenses approved for daily wear are also available in silicone hydrogel materials.^[32] and launched.

Disadvantages of silicone hydrogels are that they are slightly stiffer and the lens surface can be hydrophobic and less "wettable." These factors can influence the comfort of the lens. New manufacturing techniques and changes to multipurpose solutions have minimized these effects. A surface modification processes called plasma coating alters the hydrophobic nature of the lens surface. Another technique incorporates 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.

Hybrid

A small number of hybrid lenses exist. Typically these lenses consist of a rigid center and a soft "skirt". A similar technique is "piggybacking" of a smaller, rigid lens on the surface of a larger, soft lens. These techniques give the vision corrections benefits of a rigid lens and the comfort benefits of a soft lens.

Wear schedule

A "daily wear" (DW) contact lens is designed to be worn for one day and removed prior to sleeping. An "extended wear" (EW) contact lens is designed for continuous overnight wear, typically for up to 6 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). Extended and continuous wear contact lenses can be worn overnight because of their high oxygen permeability. While awake, the eyes are typically open, allowing oxygen to from the air to dissolve into the tears and pass through the lens to the cornea. While asleep, oxygen is supplied from the blood vessels in the back of the eyelid. A lens that interferes with the passage of oxygen to the cornea can cause corneal hypoxia which can result in many complications, including a corneal ulcer, which has the potential to permanently decrease vision. Extended and continuous wear contact lenses typically 5–6 times more oxygen than conventional soft lenses, allowing the eye to remain healthy, even when the eyelid is closed.

Wearing lenses designed for daily wear overnight has an increased risk for corneal infections,corneal ulcers, and corneal neovascularization. The most common complication of extended wear lenses is giant papillary conjunctivitis (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, 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.

Implantation

Some intraocular lenses are known as implantable contact lenses. While these implants are used to correct refractive error, because of their surgical implantation in the eye, they are not true contact lenses.

Manufacturing of contact lenses

Typically, soft contact lenses are mass produced while rigid lenses are made-to-order. This is because the size and shape of a rigid lens is made to exact specifications for each and every patient.

• Spin-cast lenses – A spin-cast lens is a soft contact lens manufactured by whirling liquid silicone in a revolving mold at high speed.^[33]
• Diamond turning – A diamond-turned contact lens is cut and polished on a CNC lathe.^[33] The lens starts out as a cylindrical disk held in the jaws of the lathe. The lathe is equipped with an industrial-grade diamond 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. This process can be used to shape rigid lenses, but can also be used to make soft lenses. In the case of soft lenses, the lens is cut from a dehydrated polymer that is rigid until water is reintroduced.
• 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.^[34] The contact lens is kept moist throughout the entire molding process and is never dried then rehydrated.

Although many companies make contact lenses, in the US there are four major manufacturers:^[35]

• Bausch & Lomb: Makers of Boston rigid lens materials and care system as well as various soft lens brands and the ReNu line of multipurpose solutions.
• Ciba Vision: Owned by Novartis; when Novartis purchased Alcon in August 2010, Ciba Vision was absorbed by Alcon, though products continue to carry the Ciba name.
• CooperVision: Makers of Biofinity and Proclear brand lenses.
• Vistakon: Makers of Acuvue brand lenses, and a subsidiary of Johnson & Johnson

Contact lens prescriptions

The parameters specified in a contact lenses prescription may include:

• Material / Brand name
• Base curve radius (BC, BCR)
• Diameter (D, OAD)
• Power in diopters
• Center thickness (CT)

Prescriptions for contact lenses and glasses may be similar, but are not interchangeable. While every country is different, the prescribing of contact lenses is usually restricted to various combinations of ophthalmologists optometrists and opticians. An eye examination is needed to determine an individual's suitability for contact lenses. This typically includes a refraction to determine the proper power of the lens and an assessment of the health of the anterior segment of the eye. Many eye diseases prohibit contact lens wear, such as active infections, allergies, and dry eye. Keratometry is especially important in the fitting of rigid lenses.

United States

Contact lenses are prescribed by ophthalmologists, optometrists, or specially licensed opticians under the supervision of an eye doctor. The higher profitability of surgery compared to contact lenses is one reason why ophthalmologists prescribe fewer contact lenses than optometrists.

Contact lenses can typically be ordered at the office that conducts the eye exam and contact lens fitting. The Fairness to Contact Lens Consumers Act^[36] give consumers the right to obtain a copy of their contact lens prescription, allowing them to fill that prescription at the business of their choice, including online discount sites.

Contact lens prescriptions expire yearly. This is to ensure that the patient's eyes are still healthy enough to support contact lens wear and that the current lenses are not causing any adverse effects. However, the policies of online vendors make it possible for expired and fraudulent prescriptions to be filled without verification by the prescribing doctor.^[36] This can be very unsafe and potentially cause permanent damage to the eye.

Complications

Complications due to contact lens wear affect roughly 5% of contact lens wearers each year.^[37] Most complications arise when lenses are worn differently than prescribed (improper wear schedule or lens replacement) Sleeping in lenses not designed or approved for extended wear is a common cause of complications. Many people go too long before replacing their lenses, wearing lenses designed for 1, 14, or 30 days of wear for multiple months or years. While this does save on the cost of lenses, it risks permanent damage to the eye and loss of sight.

Improper use of contact lenses may affect the eyelid, the conjunctiva, and the various layers of the cornea.^[37] Poor lens care can lead to infections by various microorganisms including bacteria, fungi, and Acanthamoeba. Long-term (over 5 years) use of contact lenses may "decrease the entire corneal thickness and increase the corneal curvature and surface irregularity."^[38] Long-term wear of rigid contact lens is associated with decreased corneal keratocyte density^[39] and increased number of epithelial Langerhans cells.^[40]

All contact lenses sold in the United States are studied and approved as safe by the FDA when specific wear schedules and replacement schedules are followed.

Usage

Before touching the contact lens or the eye, it is important to thoroughly wash & rinse hands with soap. Soaps that contain moisturizers or potential allergens should avoided as these can cause irritation of the eye if the hands thoroughly rinsed before handling the lenses. Next the lens should be cleaned, rinsed, and inspected for defects.

Care should be taken to ensure that soft lenses are not inside-out. The edge of a lens that is inside out will have a different appearance, especially when the lens is slightly folded. Insertion of an inside-out lens for a brief time (less than one minute) should not cause any damage to the eye, but the discomfort will help identify that the lens is not in the proper orientation. Some brands of lenses have markings that make it easier to tell the front of the lens from the back.

The technique for removing or inserting a contact lens varies depending upon whether the lens is soft or rigid. There are many subtle variations to insertion and removal techniques. Because of differences in anatomy, manual dexterity, and visual limitations, every person must find the technique that works best for them. In all cases, the insertion and removal of lenses requires some training and practice on the part of the user.

Insertion

Inserting a contact lens

Contact lenses are typically inserted into the eye by placing them on the index finger with the concave side upward and then using the index finger to place the lens on the eye. Rigid lenses should be placed directly on the cornea. Soft lenses may be placed on the conjunctiva (the white part of the eye) and slid into place. The other hand may be used to keep the eye open. Problems may arise if the lens folds, turns inside-out, slides off the finger prematurely, or adheres more tightly to the finger than the surface of the eye. A drop of solution may help the lens adhere to the eye.

When the lens first contacts the eye, it should be comfortable. A brief period of irritation may be caused by a difference in pH and/or salinity between the lens solution and the tears. This discomfort fades quickly as the solution drains away and is replaced by the natural tears. If the irritation persists, the cause could be a dirty, damaged, or inside-out lens. Removing the lens, cleaning it, and inspecting it again for damage and proper orientation should correct the problem. If discomfort continues, the contact lens should not be worn. In some cases, taking a break from lens wear for a day may correct the problem. If the discomfort is severe, or does not resolve the next day, the person should be seen by an eye doctor to rule out potentially serious complications.

Removal

Rigid contact lenses may be removed by pulling the eyelid tight and then blinking. With one finger on the outer corner of the eyelids, or lateral canthus, the person stretches the eyelids towards the ear. The increased tension of the eyelid margins against the edge of the lens allows the blink to break the capillary action that adheres the lens to the eye. The other hand is typically cupped underneath the eye to catch the lens. This technique can also be used for soft lenses.

A soft lens may be removed by pinching the edge between the thumb and index finger. Moving the lens off the cornea first can improve comfort during removal and reduce the risk of scratching the cornea with a fingernail. It is also possible to push a soft lens far enough to the side or bottom of the eye to get it to fold out of the eye without pinching it. Using these techniques on a rigid lens will likely scratch the cornea.

There are also small tools specifically for removing lenses. Usually made of flexible plastic, these tools can resemble small tweezers, or plungers that suction onto the front of the lens. Typically these tools are only used with rigid lenses.

Care

Lens case to store contact lens

Lens care varies depending on material and wear schedule. Daily disposable lenses are discarded after a single use and thus require no cleaning. Other lenses require regular cleaning and disinfecting to prevent surface coating and infections.

There many ways to clean and care for contact lenses, typically called care systems or lens solutions:

• Multipurpose solutions

Multipurpose solutions are the most common method for rinsing, disinfecting, cleaning and storing soft lenses. In 2002, concerns were raised that some multipurpose solutions are not effective at disinfecting Acanthamoeba from the lens.^[41] In May 2007, one brand of multipurpose solution was recalled due to a cluster of Acanthamoeba infections.^[42][43] Newer generations of multipurpose solutions are effective against bacteria, fungi, and acanthamoeba. The latest multipurpose solutions also contain ingredients that improve the surface wetability and comfort of silicone hydrogel lenses.

• Saline solution

Sterile saline is used for rinsing the lens after cleaning and preparing it for insertion. Saline solutions do not disinfect, so it must be used in conjunction with some type of disinfection system. One advantage to saline is that it can not cause an allergic response, so it is well suited for individuals with sensitive eyes and/or strong allergies.

• 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. The lens must then be rinsed. This system is commonly used to care for rigid lenses.

• Hydrogen peroxide systems

Hydrogen peroxide can be used to disinfect contact lenses.^[44] Care should be taken not to get hydrogen peroxide in the eye because it is very painful and irritating. With "two-step" products, the hydrogen peroxide must be rinsed away with saline before the lenses may be worn. "One-step" systems allow the hydrogen peroxide to react completely, becoming pure water. Thus "one-step" hydogen peroxide systems do not require the lenses to be rinsed prior to insertion, provided the solution has been given enough time to react.

• 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.
• 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. These devices are not usually available in optic retailers but are in some electro-domestic stores.^[45][46][47]

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.^[48]

Contact lens solutions often contain preservatives such as thiomersal, benzalkonium chloride, and benzyl alcohol. In 1989, thiomersal was responsible for about 10% of problems related to contact lenses.^[49] As a result, many products no longer contain thiomersal. Preservative-free products usually have shorter shelf lives, but are better suited for individuals with an allergy or sensitivity to one or more preservatives.

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 and that the most predominant microbial pathogen is Pseudomonas aeruginosa.^[50] 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 species (aureus and epidermidis) and the Streptococcus species, among others.^[51][52] Microbial keratitis is a serious focal point of current research due to its potentially devastating effect on the eye, including severe vision loss.^[53]

One specific research topic of interest is how microbes such as Pseudomonas aeruginosa 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.^[54] One way to alleviate hypoxia 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,^[55] 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 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.^[56] 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.^[57][58][59] 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-impregnated lens case has been developed which helps to eradicate any potentially contaminating microbes that come in contact with the lens case.^[60] Additionally, a number of antimicrobial agents are being developed that have been embedded into contact lenses themselves. Contact lenses with covalently attached Selenium molecules have been shown to reduce bacterial colonization without adversely affecting the cornea of a rabbit eye^[61] and octylglucoside used as a contact lens surfactant significantly decreases bacterial adhesion.^[62] 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

• List of soft contact lens materials
• Bionic contact lens
• Corrective lens
• Eyeglass prescription
• Visual acuity
• Effects of Long-Term Contact Lens Wear on the Cornea

References

Further reading

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

External links

Wikimedia Commons has media related to: Contact lens

• British Contact Lens Association
• Contact Lens & Anterior Eye Journal
• Video How Contact Lenses are Manufactured
• Contact Lens History
• [1] Free Contact Lenses
• Contact lenses treatment overview
• "Glass Disks Under Eyelids Replace Spectacles" Popular Mechanics Monthly, July 1930, left-bottom pg 31