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Contact lens

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A contact lens (also known as a "contact") is a corrective, cosmetic, or therapeutic lens placed on the cornea of the eye.

Contact lenses usually serve the same corrective purpose of conventional glasses, but are lightweight and virtually invisible—many commercial lenses are tinted a faint blue for visibility. Cosmetic lenses are deliberately and clearly visible and are meant to alter the appearance of the eye.

A pair of contact lenses

History

Adolf Fick was apparently the first to successfully fit contact lenses; first on rabbits, then on himself, and then on a group of volunteers.

Leonardo da Vinci is frequently credited with introducing the general principle 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. Da Vinci, 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.[1]

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 as this would render blinking impossible, the idea was unworkable.

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 da Vinci, Young's device was not intended to correct refraction errors.

Sir John Herschel, in a footnote of the 1845 edition of the Encyclopedia Metropolitana, posed two ideas for the visual correction: the first "a spherical capsule of glass filled with animal jelly", and "a mould of the cornea" which could be impressed on "some sort of transparent medium." Though Herschel reportedly never tested these ideas, they were both later advanced by several independent inventors, seemingly unaware of Herschel's suggestion.

It was not until 1887 that the German ophthalmologist Adolf Eugen Fick constructed and fitted the first successful contact lens. While working in Zürich, he described fabricating afocal scleral contact shells and experimentally fitting them: initially on rabbits, then on himself, and lastly on a small group of volunteers. He published his work in the journal Archiv für Augenheilkunde in March 1888.

Fick's lens was large, unwieldy, and could only be worn for a few hours at a time. The first true contact lens was manufactured by August Muller in Kiel, Germany who corrected his own severe myopia with a glass-blown scleral contact lens in 1888. In 1936 an optometrist, Dr. William Feinbloom introduced plastic, making a lighter, more convenient lens. The development of polymethyl methacrylate (PMMA or Perspex/Plexiglas) allowed plastic scleral lenses to be manufactured for the first time.

In the 1950s, the first 'corneal' lenses were developed - these were much smaller than the original scleral lenses as they sat only on the cornea rather than across all of the visible ocular surface. 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.

One important disadvantage of PMMA lenses is that no oxygen is transmitted through the lens to the cornea with the potential for a range of adverse clinical events during lens wear. 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. Collectively, these polymers are referred to as 'rigid gas permeable' or 'RGP' materials or lenses.

The principal breakthrough in soft lenses was made by the Czech chemist Otto Wichterle who published his work "Hydrophilic gels for biological use" in the journal Nature in 1959[2]. 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 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 making up the polymers.

Usage

People choose to wear contact lenses for various reasons[3]. Many choose to wear them instead of eyeglasses due to cosmesis or because they are more functional for certain sporting activities. [4]

It has been estimated that about 125 million people use contact lenses worldwide[5], including 28 to 38 million in the United States[6][5] and 13 million in Japan [7]. The types of lenses used and prescribed vary markedly between countries, with rigid lenses accounting for over 20% of currently-prescribed lenses in Japan, Netherlands and Germany but less than 5% in Scandinavia[5].

Types of contact lenses

Contact lenses are classified in many different manners[8] [9].

Function

Corrective contact lenses - A corrective contact lens is a lens designed to improve vision. In 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 near (or short) sightedness (myopia), far (or long) sightedness (hypermetropia), astigmatism and presbyopia. 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[10][11]. Other tinted lenses have been used with limited success[12].

A person seen wearing two different styles of cosmetic contact lenses

Cosmetic contact lenses - A cosmetic contact lens is designed to change the appearance of the eye. These lenses may also correct the vision, but some blurring or obstruction of vision may occur as a result of the colour or design. In the United States, the FDA frequently calls non-corrective cosmetic contact lenses decorative contact lenses[13][14][15][16].

Theatrical contact lenses are a type of cosmetic contact lens that are used primarily in the entertainment industry to make the eye appear unusual or unnatural in appearence[17]. These lenses have been used by Wes Borland, Marilyn Manson, Twiztid, and Ray Park as Darth Maul in Star Wars Episode I: The Phantom Menace. Scleral lenses cover the white part of the eye (i.e. sclera) and are used in many theatrical lenses.

Similar lenses have more direct medical applications. For example, some lenses can give the iris an enlarged appearance, or mask defects such as absence (aniridia) or damage (dyscoria) to the iris.

Although many brands of contact lenses are lightly tinted to make them easier to handle, cosmetic lenses worn to change the colour of the eye are far less common, accounting for only 3% of contact lens fits in 2004[18].

Therapeutic contact lenses - Soft lenses are often used in the treatment and management of non-refractive disorders of the eye. A bandage contact lenses protects an injured or diseased cornea to protect it from the constant rubbing of blinking eyelids thereby allowing it to heal[19]. 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[20]. Contact lenses used for drug-delivery have also been developed[21].

Constructional material

File:Contact lens 246462 2036.jpg
Contact lens inserted in eye.

Arguably, the first reference to the neutralization of defects of the cornea by way of a contact device were made by Leonardo Da Vinci in 1508. However, the first true contact lens was manufactured by August Muller in Kiel, Germany who corrected his own severe myopia with a glass-blown scleral contact lens in 1888. 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 the 1950s, the first 'corneal' lenses were developed - these were much smaller than the original scleral lenses as they sat only on the cornea rather than across all of the visible ocular surface. 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.

One important disadvantage of PMMA is that no oxygen is transmitted through the lens to the cornea with the potential for a range of adverse clinical events during lens wear. 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. 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.

Rigid lenses offer a number of unique properties. In effect, the 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 have astigmatism or distorted corneal shapes as with keratoconus.

Whilst rigid lenses have been around for about 120 years, soft lenses are a much more recent development. The principal breakthrough in this field was made by the Czech chemist Otto Wichterle who published his work "Hydrophilic gels for biological use" in the journal Nature in 1959[2]. 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 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 making up the polymers.

In 1999, an important development was the launch of the first 'silicone hydrogels' onto the market. These new materials encapsulated the benefits of silicone - with its 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.

A small number of hybrid rigid/soft lenses exist. A variant of this is piggybacking contact lenses which refer to the wearing of a smaller, rigid lens atop a larger, soft lens. This is done for a variety of clinical reasons where a single lens will not provide the optical power, fitting characteristics, or comfort required.

Wear time

A daily wear 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. These are increasing in popularity, due to their obvious convenience. Extended- and continuous-wear contact lenses can be worn for such long periods of time because of their high oxygen permeability (typically 5-6 times greater than conventional soft lenses), which allows the eye to remain remarkably healthy.

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 is also a common complication of extended lens wear, however the most common complication of extended lens use is conjunctivitis usually allergic or Giant Papillary Conjunctivitis (GPC) associated with a poorly fitting contact lens.

Disposability

The various soft contact lenses available are often categorized by their replacement schedule. The shortest replacement sechedule is single use lenses, which are disposed of each night. These may be best for patients with ocular allergies or other conditions because it limits deposits of antigens and protein. Single use lenses are also useful for people who use contacts infrequently. More commonly, contact lenses are prescribed to be disposed of on a two-week or monthly basis. Quartley or annual lenses, which used to be very common, have lost favor because more frequent disposable schedule allows for thinner lenses and limits deposits. Rigid gas permeable lenses are very durable and may last for several years without the need for replacement.

Shape

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 comprise the effects of a cylindrical lens, usually in combination with the effects of a spherical lens. People with astigmatism, both myopic (nearsighted) and hypermetropic (farsighted), who have been told they are not suitable for regular contact lenses may be able to use toric lenses. Toric lenses are made from the same materials as regular contact lenses but have a couple of extra characteristics:

  • They have two powers in them, one for spherical correction and the other for the 'cylinder'.
  • They are designed to keep the lens in a stable position regardless of eye movement. Often the lens is weighted more at the bottom and is marked by tiny striations so the wearer can insert them in the correct position, or they are designed in such a way that blinking will reset the lens to the correct orientation.

Number of focal points

Similar to eyeglasses, contact lenses may also be characterized by the number of focal points within the lens. Single vision contact lenses contain only one focal point, and multifocal contact lenses contain more than one focal point.

Multifocal contact lenses are used almost exclusively to correct those with presbyopia or accommodative insufficiency; however, single vision lenses may also be used in a process known as monovision[22]. In monovision, single vision lenses are used to correct one eye's far vision and the other eye's near vision. Alternatively, a person may wear single vision contact lenses to improve distance vision and reading glasses to improve near vision.

Multifocal contact lenses are more complicated and require more skill to fit. All soft bifocal contact lenses are considered "simulatenous vision" because both the distance and near correctios are present simultaneously, regardless of the position of the eye. Commonly these are designed either with the distance correction in the center of the lens surrounded by the near correction, or with the near correction in the center surrounded by the distance correction. Rigid gas permeable contact lenses most commonly have a small lens on the bottom for the near correction. When the patient lowers their eyes to read, the patient views through this seperate lens.

Implantation

Much different than contact lenses that are placed on the surface of the eye, an intraocular contact lens, also known as an implantable contact lens, is a small corrective lens that is surgically placed in the eye's posterior chamber behind the iris and in front of the lens to correct higher amounts of myopia and hyperopia.

Manufacturing of contact lenses

Most contact lenses are mass produced.

  • Spin-cast lenses - A spin cast lens is a soft contact lens manufactured by whirling liquid plastic in a revolving mold at high speed[23].
  • Lathe cut - A lathe cut lens is a contact lens produced by the lathe method of cutting and grinding[23].
  • Molded
  • Hybrids

Although many companies make contact lenses, there are four major manufacturers: Vistakon, Ciba, Bausch & Lomb, and CooperVision[24].

Prescribing contact lenses

The prescribing of contact lenses is usually limited to appropriately qualified eye care practitioners. (In the United States, all contact lenses are deemed to be medical devices by the Food and Drug Administration.) In countries such as the United States, 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 interchangable.

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 successfully wear contact lenses.

The parameters specified in a contact lenses prescription may include:

  • Brand
  • Material (eg. Oxygen Permeability/Transmissibility (Dk/L, Dk/t), water content, modulus)
  • Base curve radius (BC, BCR)
  • Diameter (D, OAD)
  • Power in dioptres - Spherical, Cylindrical and/or reading addition)
  • Cylinder axis
  • Centre Thickness (CT)

Complications due to contact lens wear

Complications due to contact lens wear are relatively common, affecting roughly 5% of contact lens wearers each year [25]. Overwear of contact lenses, particularly overnight wear, are associated with the most serious safety concerns[6]. Problems associated with contact lens wear may affect the eyelid, the conjunctiva, the various layers of the cornea, and even the tear film that covers the outer surface of the eye[25].

Eyelid

Conjunctiva

Cornea

  • Bacteria
  • Acanthamoeba

Tear film

Contact lens care: cleaning and disinfection products

While daily disposable lenses require no cleaning, other types require regular cleaning and disinfecting in order to retain clear vision and prevent infections by various microorganisms including bacteria, fungi, and Acanthamoeba. There are a number of products that can be used to perform these tasks:

  • Multipurpose solution - The most popular cleaning solution for contact lenses. Used for rinsing, disinfecting, cleaning and storing the lenses. This is arguably the easiest way to clean contacts, as it eliminates the need for multiple solutions.
  • Saline solution - Used for rinsing the lens after cleaning and preparing it for insertion. Saline solutions do not disinfect the lenses.
  • Daily cleaner - Used to clean lenses on a daily basis. Usually one puts a few drops of cleaner on the lens and rubs for about 20 seconds (check the cleaner's directions) on each side. One must be extra careful in this step if one has long fingernails.
  • Hydrogen peroxide solution - Used for disinfecting the lenses, and available as 'two-step' or 'one-step' systems. If using a 'two-step' product, one must ensure that the lens taken out of the hydrogen peroxide is neutralized before it is worn, or else wear will be extremely painful. Saline must not be used to rinse away the peroxide. Some peroxide solutions, such as CIBA Vision's Clear Care, come with a special storage case that contains a catalyzing disk. If soaked in the solution with the disk for at least six hours, the hydrogen peroxide decomposes and the remaining solution is a saline that will not harm the eye. People with extremely sensitive, irritable eyes often use these types of cleaning solutions.
  • Enzymatic cleaner - Used for cleaning the protein off of lenses, usually on a weekly basis, if the daily cleaner is not sufficient. Typically, this cleaner is in tablet form. Protein deposits make the contact lenses uncomfortable, and may lead to various eye problems.

Some products may only be used with certain types of contact lenses; thus, it is important to check the product label to make sure that it can be used for a given type of lens. Additionally, it is important to follow the product's directions carefully to reduce risk of eye infection or eye irritation.

It is important to ensure that the product does not become contaminated with microorganisms. Hence, the tips of the containers for these solutions should never touch any surface, and the container should be closed when not in use. To counteract minor contamination of the product and kill microorganisms on the contact lens, some products may contain preservatives such as thimerosal, benzalkonium chloride, benzyl alcohol, and other compounds. In 1989, thimerosal was responsible for about 10% of problems related to contact lenses[26]. For this reason, many products no longer contain thimerosal. If a person is allergic to one solution and suspect the preservative for causing the reaction, they may look for a solution that contains a different preservative. If they suspect they are allergic to most preservatives, there are some products that are completely free of preservatives to reduce the risk of these problems-- such products are sometimes labeled as being "for sensitive eyes" or "preservative-free". However, products that do not contain any preservatives usually have shorter expiration dates. For example, non-aerosol preservative-free saline solutions typically last only two weeks once opened.

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

References

  1. ^ 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.
  2. ^ a b Wichterle O, Lim, D. "Hydrophilic gels for biological use". Nature. 1960;185:117-118.
  3. ^ Sokol JL, Mier MG, Bloom S, Asbell PA. "A study of patient compliance in a contact lens-wearing population." CLAO J. 1990 Jul-Sep;16(3):209-13. PMID 2379308
  4. ^ Athletes and Contact Lenses
  5. ^ a b c Barr, J. "2004 Annual Report". Contact Lens Spectrum. January, 2005.
  6. ^ a b Dixie Farley. "Keeping an Eye on Contact Lenses: Safety, Options Shape Contact Lens Decisions." U.S. Food and Drug Administration: FDA Consumer. March-April 1998; revised August 1998.
  7. ^ National Consumer Affairs Center of Japan. NCAC News Vol. 12, No. 4. NCAC News. March, 2001.
  8. ^ "What are the types of contact lenses?"
  9. ^ "Different types of contact lenses."
  10. ^ Hartenbaum NP, Stack CM. "Color vision deficiency and the X-Chrom lens." Occup Health Saf. 1997 Sep;66(9):36-40, 42. PMID 9314196.
  11. ^ Swarbrick HA, Nguyen P, Nguyen T, Pham P. The X-Chrom lens. On seeing red. Surv Ophthalmol. 1981 Mar-Apr;25(5):312-24. PMID 6971497.
  12. ^ Swarbrick HA, Nguyen P, Nguyen T, Pham P. "The ChromaGen contact lens system: colour vision test results and subjective responses." Ophthalmic Physiol Opt. 2001 May;21(3):182-96. PMID 11396392.
  13. ^ U.S. Food and Drug Administration. "FDA Warns Consumers Against Using Decorative Contact Lenses Obtained Without a Prescription or Professional Fitting." FDA News. October 21, 2002.
  14. ^ U.S. Food and Drug Administration. "FDA Warns Consumers Not to Use Decorative Contact Lenses Without Proper Professional Involvement." FDA News. October 10, 2003.
  15. ^ Woo E. "Flying blind with decorative contact lenses." Nursing. 2003 Nov;33(11):70. PMID 14650382.
  16. ^ U.S. Food and Drug Administration. "FDA Public Health Web Notification: Non-Corrective Decorative Contact Lenses Dispensed Without a Prescription." (Public health notification.) October 23, 2002.
  17. ^ 99mm Special Effects website
  18. ^ Morgan PB et al."International Contact Lens Prescribing in 2004: An analysis of more than 17,000 contact lens fits from 14 countries in 2004 reveals the diversity of contact lens practice worldwide." Contact Lens Spectrum. January 2005.
  19. ^ EyeMDLink.com
  20. ^ "45 COVERAGE ISSUES - SUPPLIES - DRUGS 11-91 45" (PDF). Centers for Medicare & Medicaid Services. Retrieved 2006-03-01.
  21. ^ "Contact Lenses Employed for Drug Delivery."
  22. ^ Lebow KA, Goldberg JB. "Characteristic of binocular vision found for presbyopic patients wearing single vision contact lenses." J Am Optom Assoc. 1975 Nov;46(11):1116-23. PMID 802938
  23. ^ a b Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990.
  24. ^ Federal Trade Commission. "The Strength of Competition in the Sale of Rx Contact Lenses: An FTC Study". February, 2005.
  25. ^ a b John Stamler. "Contact Lens Complications." eMedicine.com. September 1, 2004.
  26. ^ Wilson-Holt N, Dart JK. "Thiomersal keratoconjunctivitis, frequency, clinical spectrum and diagnosis." Eye. 1989;3 ( Pt 5):581-7. PMID 2630335

Sources

  • Efron, Nathan (2002). Contact Lens Practice, Elsevier Health Sciences. ISBN 075064690X.

See also

Products

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