Contact lens

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A pair of contact lenses, positioned with the concave side facing upward.
Putting contacts in and taking them out.
One-day disposable contact lenses with blue handling tint in blister-pack packaging

A contact lens, or simply contact, is a thin lens placed directly on the surface of the eye. Contact lenses are considered medical devices and can be worn to correct vision, or for cosmetic or therapeutic reasons.[1] In 2004, it was estimated that 125 million people (2%) use contact lenses worldwide, including 28 to 38 million in the United States.[2] 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.[3] Some have estimated that the global market will reach $11.7 billion by 2015.[3] As of 2010, the average age of contact lens wearers globally was 31 years old and two thirds of wearers were female.[4]

People choose to wear contact lenses for many reasons.[5] 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.[6] Other people wear contacts for functional or optical 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. Contact lens wearers can also wear sunglasses, goggles, or other eyewear of their choice without having to fit them with prescription lenses or worry about compatibility with glasses. Additionally, there are conditions such as keratoconus and aniseikonia that are typically corrected better by contacts than by glasses.

History[edit]

Artist's impression of Da Vinci's method for neutralizing the refractive power of the cornea.

Leonardo da Vinci is frequently credited with introducing the idea of contact lenses in his 1508 Codex of the eye, Manual D,[7] where he described a method of directly altering corneal power by either submerging the head in a bowl of water, or wearing a water-filled glass hemisphere over the eye. Neither idea was practically implementable in Da Vinci's time.[8]:9 He did not suggest his idea be used for correcting vision, being more interested in learning about the mechanisms of accommodation of the eye.[7]

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.[8]

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" that 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 Dallos with Istvan Komàromy (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.

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

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 1888, the German ophthalmologist Adolf Gaston Eugen Fick constructed and fitted the first successful contact lens.[11] 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.[12]

Also in 1887, Louis J. Girard invented a similar scleral form of contact lens.[13] 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.[14] These lenses were a combination of glass and plastic. in 1940, German optomestrist Heinrich Wöhlk produced plastic lenses, based on experiments performed during the 1930s.

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

Otto Wichterle (pictured) and Drahoslav Lím introduced modern soft hydrogel lenses in 1959.

The principal breakthrough in soft lenses was made by the Czech chemists Otto Wichterle and Drahoslav Lím who published their work "Hydrophilic gels for biological use" in the journal Nature in 1959.[20]

In 1965 National Patent Development Corporation (NPDC) bought the American rights to produce the lenses and then sublicenced the rights to Bausch & Lomb which started to manufacture them in the USA.[21] The work of the Czech scientists 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.[22][23]

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 that 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.[24]

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.[24]

Types of contact lenses[edit]

Contact lenses can be classified in many different ways. 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[edit]

Corrective contact lenses[edit]

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 hypermetropia. 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 away from 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 (for example bifocal or progressive contact lenses) are comparable to spectacles with 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[25] is the use of 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, thus gaining the benefits of both systems. Alternatively, a person may simply wear reading glasses over their distance contact lenses. Care is advised for persons with a previous history of strabismus and those with significant phorias, who are at risk of eye misalignment under monovision.[26]

Other types of vision correction[edit]

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.[27][28] Red-filtering contact lenses can also be an option for the extreme light sensitivity in some visual deficiences such as achromatopsia.[29]

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.[30] An earlier study showed very significant improvements in color vision and patient satisfaction.[31]

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[32] This system has been granted FDA approval for use in the U.S.

Magnification is another area being researched for future contact lens applications.[33] The embedding of telescopic lenses and electronic components suggests that future uses of contact lenses may become extremely diverse. However, there are still barriers that prevent the transition of these technologies from research and development to practical application and commercial availability.

Cosmetic contact lenses[edit]

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.[34]

In the United States, 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.[35] For this reason all contact lenses, even purely cosmetic ones, are classified as medical devices in many countries (U.S., Canada, UK, Australia). 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 by having a dark tinted area surrounding the iris. The result is the appearance of a bigger, wider iris, a look reminiscent of dolls' eyes.[36]

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 scleral lenses[edit]

Scleral lens, with visible outer edge resting on the sclera of a patient with severe dry eye syndrome.

A scleral lens is a large, firm, oxygen permeable lens that rests on the sclera and creates a tear-filled vault over the cornea. The cause of this unique positioning is usually relevant to a specific patient, whose cornea may be too sensitive to support the lens directly. Scleral lenses may be used to improve vision and reduce pain and light sensitivity for people suffering from growing number of disorders or injuries to the eye, such as severe dry eye syndrome (Kerotanconjuctivis sicca), microphthalmia, keratoconus, corneal ectasia, Stevens–Johnson syndrome, Sjögren's syndrome, aniridia, neurotrophic keratitis (aneasthetic corneas), complications post-LASIK, high order Aberrations of the eye, complications post-corneal transplant and pellucid degeneration. Injuries to the eye such as surgical complications, distorted corneal implants, as well as chemical and burn injuries also may be treated by the use of scleral lenses. [37]

Therapeutic soft lenses[edit]

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.[38] 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.[39] Contact lenses that deliver drugs to the eye have also been developed.[40]

Materials[edit]

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[edit]

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[edit]

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 usually comfortable shortly after insertion, 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.[41]

Disadvantages of silicone hydrogels are that they are slightly stiffer and the lens surface can be hydrophobic, and thus, 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[edit]

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 are often chosen to give the vision correction benefits of a rigid lens and the comfort benefits of a soft lens. As of 2014, this is a new technology that only a few companies provide.[42]

Wear schedule[edit]

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 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 allow for the transfer of 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[edit]

The contact lenses are often categorized by their replacement schedule. Single use lenses (typically called 1-day or daily disposable) are discarded after one use. Because they do not have to stand up to the wear and tear of repeated uses, single use lenses can be made thinner and lighter. This can improve the comfort of the lens. Lenses replaced frequently gather fewer deposits of allergens and germs, making these lenses preferable for patients that have ocular allergies or are prone to infection. Single-use lenses are also useful for people who wear contacts infrequently, or when losing a lens is likely or not easily replaced (such as when on vacation). They are also considered useful for children, because no cleaning or disinfecting are needed, leading to improved compliance.

Other disposable contact lenses are designed for two-week or 4-week replacement. Quarterly or annual lenses, which used to be very common, have been discontinued by manufacturers. 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, but had other drawbacks.

Lenses with different replacement schedules can be made of the same material. Although the materials are the same, differences in the manufacturing processes determines if the resulting lens will be a "daily disposable" lens or a lens recommended for two-week or 4-week replacement.

In the United States, contact lens manufacturers must prove to the Food & Drug Administration (FDA) that a lens is safe before it can be sold to the public. Typically, the prescribing doctor instructs patients to follow the same limitations used during those studies. This included wear time and replacement schedule.

Manufacturing of contact lenses[edit]

Typically, soft contact lenses are mass-produced, while rigid lenses are custom-made to exact specifications for specific individual patients.

  • Spin-cast lenses – A spin-cast lens is a soft contact lens manufactured by whirling liquid silicone in a revolving mold at high speed.[43]
  • Diamond turning – A diamond-turned contact lens is cut and polished on a CNC lathe.[43] 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.[44] The contact lens is kept moist throughout the entire molding process and is never dried and rehydrated.

Although many companies make contact lenses, in the US there are four major manufacturers:[45]

Contact lens prescriptions[edit]

The parameters specified in a contact lenses prescription may include:

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 can prohibit contact lens wear, such as active infections, allergies, and dry eye.[46] Keratometry is especially important in the fitting of rigid lenses.

United States[edit]

Contact lenses are prescribed by ophthalmologists, optometrists, or specially licensed opticians under the supervision of an eye doctor. 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[47] gives 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 some vendors make it possible for expired and fraudulent prescriptions to be filled without verification by the prescribing doctor.[47] This can be very unsafe and potentially cause permanent damage to the eye.

Complications[edit]

CLARE, or contact lens associated red eye, is a group of inflammatory complications from contact lens wear.

Contact lenses are generally safe as long as they are used correctly. Complications due to contact lens wear affect roughly 5% of contact lens wearers each year.[48] Improper use of contact lenses may affect the eyelid, the conjunctiva, and the various layers of the cornea.[48] Poor lens care can lead to infections by various microorganisms including bacteria, fungi, and Acanthamoeba.

Many complications arise when lenses are worn not as 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.

One of the major factors that causes contact lens complications is that the lens is a barrier to oxygen. The cornea needs a supply of oxygen to function and it normally gets that oxygen from the surrounding air while awake and from the blood vessels in the back of the eyelid while asleep. The most prominent risks associated with long-term, chronic low oxygen to the cornea include corneal neovascularization, increased epithelial permeability, bacterial adherence, microcysts, corneal edema, endothelial polymegethism and potential increase in myopia.[49] That is why much of the research into the latest soft and rigid contact lens materials has centered around improving oxygen transmission through the lens.

Mishandling of contact lenses can also cause problems. Corneal abrasions can increase the chances of infection.[50] When combined with improper cleaning and disinfection of the lens, the risk of infection further increases. Decreased corneal sensitivity following extended contact lens wear may cause a patient to miss some of the earliest symptoms of such complications.[51]

The way contact lenses interact with the natural tear layer is a major factor in determining lens comfort and visual clarity. People that suffer from dry eyes are particularly vulnerable to discomfort and episodes of brief blurry vision. Proper lens selection can minimize these effects for some patients.

Long-term wear (over 5 years) of contact lenses may "decrease the entire corneal thickness and increase the corneal curvature and surface irregularity."[52] Long-term wear of rigid contact lens is associated with decreased corneal keratocyte density[53] and increased number of epithelial Langerhans cells.[54]

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

Usage[edit]

Before touching the contact lens or the eye, it is important to wash hands thoroughly with soap and rinse well with water. Soaps that contain moisturizers or potential allergens should be avoided as these can cause irritation of the eye.[55] Drying of hands using hand towels or tissues, prior to handling the lenses, can transfer lint ('fluff') to the users hands and, subsequently, to the lenses, causing irritation upon insertion. Hand towels, unless freshly laundered on a high temperature wash, are frequently contaminated with large quantities of bacteria and, as such, should be avoided when handling lenses. Dust, lint and other debris may collect on the outside of lens cases. Again, hand contact with this material, prior to handling the lenses, may transfer it to the lenses themselves. Rinsing the case under a source of clean running water, prior to opening it, can help alleviate this problem. Next the lenses should be removed from their case and inspected for defects (e.g. splits, folds, lint). A 'gritty' or rough appearance to the lens surface may indicate that a considerable quantity of proteins, lipids and debris has built up on the lens and that additional cleaning is required; this is often accompanied by unusually high irritation upon insertion.

Care should be taken to ensure that soft lenses are not inside-out prior to insertion. 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[edit]

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 sclera (the white part of the eye) and slid into place. The other hand may be used to keep the eye open. Alternatively, the user may close their eye and then look towards their nose, sliding the lens into place over the cornea.

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.[56][57] 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 as soon as possible by an eye doctor to rule out potentially serious complications.

Removal[edit]

Young woman removing contact lenses from her eyes in front of a mirror

Removing contact lens incorrectly could result in damage to the lens and injury to the eye, so care must be taken during 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. Extreme care must be exercised when using mechanical tools or finger nails to insert or remove contact lenses.

Care[edit]

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 are 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 multipurpose solutions are not effective at disinfecting Acanthamoeba from the lens.[58] In May 2007, one brand of multipurpose solution was recalled due to a cluster of Acanthamoeba infections.[59][60] Since then, studies showed that multipurpose solutions are ineffective against Acanthamoebae.[61][62][63] The latest multipurpose solutions also contain ingredients that improve the surface wetability and comfort of silicone hydrogel lenses.[citation needed]
Hydrogen peroxide can be used to disinfect contact lenses.[64] 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" hydrogen peroxide systems do not require the lenses to be rinsed prior to insertion, provided the solution has been given enough time to react. An exposure time of 2-3 hours to 3% H
2
O
2
(non neutralized solution) is sufficient to kill bacteria, HIV, fungi, and Acanthamoeba.[65][66] This can be achieved by using a "two-step" product or a "one-step" tablet system if the catalytic tablet isn't added before 2-3 hours.[66] However, the "one-step" catalytic disk systems are not effective against Acanthamoeba.[66]
  • 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.[67][68][69]
  • 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.

Contact lenses can be mechanically cleaned of more substantial protein, lipid and debris build up by rubbing them between the clean pad of a finger and the palm of a hand, using a small amount of cleaning fluid as a lubricant. However, this method does not sterilize the lenses and so should only be performed prior to a full sterilization cycle (e.g. when putting the lenses away the night before).

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.

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.[70] 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[edit]

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.[71] 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.[72][73] Microbial keratitis is a serious focal point of current research due to its potentially devastating effect on the eye, including severe vision loss.[74]

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.[75] 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,[76] 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 unworn silicone hydrogel contact lenses than conventional hydrogel contact lenses and that adhesion of Pseudomonas aeruginosa was 20 times stronger than adhesion of Staphylococcus epidermidis.[77] This might help to explain one reason why Pseudomonas infections are the most predominant. However, another study conducted with worn and unworn silicone hydrogel and conventional hydrogel contact lenses showed that worn silicone hydrogel contact lenses were less prone to Staphylococcus epidermidis colonization than conventional hydrogel contact lenses.[78]

Another important area of contact lens research deals with patient compliance. Compliance is a major issue[79] pertaining to the use of contact lenses because patient noncompliance often leads to contamination of the lens, storage case, or both.[80][81][82] However, careful users can extend the wear of lenses through proper handling: there is, unfortunately, no disinterested research on the issue of "compliance" or the length of time a user can safely wear a lens beyond its stated use. 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.[83] 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[84] and octylglucoside used as a contact lens surfactant significantly decreases bacterial adhesion.[85] 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. Bionic lenses are visual displays that include built-in electric circuits and light-emitting diodes and can harvest radio waves for their electric power. Bionic lenses can display information beamed from a mobile device overcoming the small display size problem. The technology involves embedding nano and microscale electronic devices in lenses. These lenses will also need to have an array of microlenses to focus the image so that it appears suspended in front of the wearer’s eyes. The lens could also serve as a head-up display for pilots or gamers.[86]

Drug administration through contact lenses is also becoming an area of research. One application is a contact lens that releases anesthesia to the eye for post-surgery pain relief, especially after PRK (photorefractive keratectomy) in which the healing process takes several days. One experiment shows that silicone contact lenses that contain vitamin E deliver pain medication for up to 7 days compared with less than 2 hours in usual lenses.[86]

See also[edit]

References[edit]

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Further reading[edit]

External links[edit]