Contact lens

A pair of contact lenses, positioned with the concave side facing upward.
One-day disposable blue color contact lens in packaging

A contact lens (also known simply as contacts for a pair) is a corrective, cosmetic, or therapeutic lens usually placed on the cornea of the eye. Leonardo da Vinci is credited with describing and sketching the first ideas for contact lenses in 1508, but it was more than 300 years later before contact lenses were actually fabricated and worn on the eye. Rigid ones were produced and marketed first.[citation needed] Modern soft contact lenses were invented by the Czech chemist Otto Wichterle and his assistant Drahoslav Lím, who also invented the first gel used for their production.[1][2][3] Some soft contact lenses are tinted a faint blue to make them more visible when immersed in cleaning and storage solutions. Some cosmetic lenses are deliberately colored to alter the appearance of the eye. Some lenses now have a UV protection surface treatment to reduce UV damage to the eye's natural lens.[4]

It has been estimated that 125 million people use contact lenses worldwide (2%),[5] including 28 to 38 million in the United States[5] and 13 million in Japan.[6] The types of lenses used and prescribed vary markedly among countries, with rigid lenses accounting for over 20% of currently-prescribed lenses in Japan, the Netherlands and Germany but less than 5% in Scandinavia.[5]

People choose to wear contact lenses for many reasons, often due to their appearance and practicality.[7] When compared with spectacles, contact lenses are less affected by wet weather, do not steam up, and provide a wider field of vision. They are more suitable for a number of sporting activities. Additionally, ophthalmological conditions such as keratoconus and aniseikonia may not be accurately corrected with glasses.

Contents

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.[8]

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, while conducting experiments concerning the mechanisms of accommodation, scientist Thomas Young constructed a liquid-filled "eyecup" which could be considered a predecessor to the contact lens. On the eyecup's base, Young fitted a microscope eyepiece. 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".[9] 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.[10] 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.[11]

Also in 1887, Louis J. Girard invented a similar scleral form of contact lens.[12] 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.[13] These lenses were a combination of glass and plastic.

In 1949, the first "corneal" lenses were developed.[14][15][16][17] 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.[18] 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.[19] 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.[20][21]

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.[22]

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.[22]

Types of contact lenses

Contact lenses are classified in many different manners.[23][24]

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), hypermetropia (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.[25][26]

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.[27] An earlier study showed very significant improvements in color vision and patient satisfaction.[28]

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[29] 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 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 the entertainment industry to make the eye appear confusing and arousing in appearance,[citation needed] 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.[30]

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.[31]

As a specialist's tool, in the hands of the untrained general public, non-prescription cosmetic contact lenses may represent a health risk.[32]

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.[33] 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.[34] Contact lenses that deliver drugs to the eye have also been developed.[35]

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.

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. The oxygen 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[36] 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.

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.

Design

A spherical contact lens is one in which both the inner and outer optical surfaces are portions of a sphere. A toric lens 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 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:

Like eyeglasses, contact lenses can have one (single vision) or more (multifocal) focal points.

For correction of presbyopia or accommodative insufficiency multifocal contact lenses are almost always used; however, single vision lenses may also be used in a process known as monovision:[37] 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 lenses, also known as an implantable contact lenses, are special small corrective lenses surgically implanted in the eye's posterior chamber behind the iris and in front of the lens to correct higher degrees of myopia and hyperopia.

Manufacturing of contact lenses

Most contact lenses are mass produced.

Although many companies make contact lenses, in the US there are four major manufacturers:[40]

Extended Wear Contacts 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

Contact lens prescriptions

The prescribing of contact lenses is usually restricted to appropriately qualified eye care practitioners. In countries such as the United States (where all contact lenses are deemed to be medical devices by the Food and Drug Administration), the United Kingdom and Australia, optometrists are usually responsible. In France and Eastern European countries, ophthalmologists play the major role. In other parts of the world, opticians usually prescribe contact lenses. Prescriptions for contact lenses and glasses 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. 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:

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, 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, the Fairness to Contact Lens Consumers Act, which went into effect in 2004, was intended to ensure the availability of contact lens prescriptions to patients.[41] 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.[41]

Complications

Complications due to contact lens wear affect roughly 5% of contact lens wearers each year.[42] 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, the conjunctiva, the various layers of the cornea,[43] and even the tear film that covers the outer surface of the eye.[42]

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,[44] 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 density[45] and increased number of epithelial Langerhans cells.[46]

Eyelid

Conjunctiva

Cornea

Usage

Before touching the contact lens or one's eyes, it is important to thoroughly wash & rinse hands with a soap that does not contain moisturizers or allergens such as fragrances[citation needed]. 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[citation needed]. 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

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 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 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. Irrigation may help during this period, 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. 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, then blinking to cause the lens to lose adhesion. 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 plungers 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

Lens cover to store contact lens

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 microorganisms including bacteria, fungi, and Acanthamoeba, that form a biofilm on the lens surface. There are a lot of products that are used to cleans contact lenses:

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.[54] 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 preservatives such as thiomersal, benzalkonium chloride, and benzyl alcohol. In 1989, thiomersal was responsible for about 10% of problems related to contact lenses:[55] because of this, many products no longer contain thiomersal. Preservative-free products usually have shorter shelf lives. 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 and that the most predominant microbial pathogen is Pseudomonas aeruginosa.[56] 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.[57][58] Microbial keratitis is a serious focal point of current research due to its potentially devastating effect on the eye, including severe vision loss.[59]

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.[60] 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,[61] 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.[62] 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.[63][64][65] 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.[66] 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[67] and octylglucoside used as a contact lens surfactant significantly decreases bacterial adhesion.[68] 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.

See also

References

  1. ^ Wichterle O, Lim, D. "Hydrophilic gels for biological use". Nature. 1960; 185:117–118.
  2. ^ Otto Wichterle and Drahoslav Lim, "Process for producing shaped articles from three-dimensional hydrophilic high polymers", US Patent 2,976,576, March 28, 1961
  3. ^ Otto Wichterle and Drahoslav Lim, "Cross-linked hydrophilic polymers and articles made therefrom", US Patent 3,220,960, November 30, 1965
  4. ^ http://www.eurograduate.com/arch_article.asp?id=3054
  5. ^ a b c Barr, J. "2004 Annual Report". Contact Lens Spectrum. January, 2005.
  6. ^ National Consumer Affairs Center of Japan. NCAC News Vol. 12, No. 4. NCAC News. March, 2001.
  7. ^ Sokol, JL; Mier, MG; Bloom, S; Asbell, PA (1990). "A study of patient compliance in a contact lens-wearing population". The CLAO journal : official publication of the Contact Lens Association of Ophthalmologists, Inc 16 (3): 209–13. PMID 2379308. 
  8. ^ Heitz, RF and Enoch, J. M. (1987) "Leonardo da Vinci: An assessment on his discourses on image formation in the eye." Advances in Diagnostic Visual Optics 19—26, Springer-Verlag.
  9. ^ "The History of Contact Lenses." eyeTopics.com. Accessed October 18, 2006.
  10. ^ Contact Lens Council
  11. ^ Pearson, RM; Efron, N (1989). "Hundredth anniversary of August Müller's inaugural dissertation on contact lenses". Survey of ophthalmology 34 (2): 133–41. PMID 2686057. 
  12. ^ Hellemans, Alexander; Bunch, Bryan (1988). The Timetables of Science. Simon & Schuster. pp. 367. ISBN 0671621300. 
  13. ^ Robert B. Mandell. Contact Lens Practice, 4th Edition. Charles C. Thomas, Springfield, IL, 1988.
  14. ^ U.S. Patent No. 2,510,438, filed February 28, 1948.
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