Editing Contact lens (section)

===Functions===

====Correction of refractive error====
[[Corrective lens|Corrective]] contact lenses are designed to improve vision, most commonly by correcting [[refractive error]]. This is done by directly focusing light so it enters the eye with the proper [[Dioptre|power]] for clear vision.

A spherical contact lens bends light evenly in every direction (horizontally, vertically, etc.). They are typically used to correct [[myopia]] and [[hypermetropia]].

There are two ways that contact lenses can correct astigmatism. One way is with [[toric lens|toric]] soft lenses that work essentially the same way as eyeglasses with cylindrical correction; a toric lens has a different focusing power horizontally than vertically, and as a result can correct for [[astigmatism]]. Another way is by using a rigid gas permeable lens; since most astigmatism is caused by the shape of the cornea, rigid lenses can improve vision because the front surface of the optical system is the perfectly spherical lens.<ref>{{Cite web |title=Astigmatism |url=http://www.aoa.org/Astigmatism.xml,%20http://www.aoa.org/Astigmatism.xml |access-date=2020-08-06 |website=www.aoa.org |language=en}}{{Dead link|date=December 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Both approaches have advantages and drawbacks. Toric lenses must have the proper orientation to correct for astigmatism, so such lenses must have additional design characteristics to prevent them from rotating out of 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, but these mechanisms rarely work perfectly, so some misalignment is common and results in somewhat imperfect correction, and blurring of sight after blinking rotates the lens. Toric soft lenses have all the advantages of soft lenses in general, which are low initial cost, ease of fitting, and minimal adjustment period. Rigid gas permeable lenses usually provide superior optical correction but have become less popular relative to soft lenses due to higher initial costs, longer initial adjustment period, and more involved fitting.<ref>{{Cite web |title=Advantages and Disadvantages of Various Types of Contact Lenses |url=https://www.aoa.org/patients-and-public/caring-for-your-vision/contact-lenses/advantages-and-disadvantages-of-various-types-of-contact-lenses |access-date=2020-08-06 |website=www.aoa.org |language=en}}</ref><ref>{{Cite web |date=2013-01-26 |title=A Complete Guide to Gas Permeable Contact Lenses |url=https://www.eyehealthweb.com/rigid-gas-permeable-lenses/ |access-date=2020-08-06 |website=EyeHealthWeb.com |language=en}}</ref>

====Correction of presbyopia====
Correction of [[presbyopia]] (a need for a reading prescription different from the prescription needed for distance) presents an additional challenge in the fitting of contact lenses. Two main strategies exist: multifocal lenses and monovision.

Multifocal contact lenses (e.g. bifocals or progressives) are comparable to spectacles with [[bifocals]] or [[progressive lens]]es because they have multiple [[focus (optics)|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<ref>{{Cite journal |last=Lebow |first=KA |last2=Goldberg |first2=JB |year=1975 |title=Characteristic of binocular vision found for presbyopic patients wearing single vision contact lenses |journal=Journal of the American Optometric Association |volume=46 |issue=11 |pages=1116–23 |pmid=802938}}</ref> is the use of single-vision lenses (one focal point per lens) to focus an eye (typically the dominant one) for distance vision and the other 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 the other for near, thus gaining the benefits of both systems. 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.<ref>{{Cite journal |last=Pollard |first=Zane F. |last2=Greenberg |first2=Marc F. |last3=Bordenca |first3=Mark |last4=Elliott |first4=Joshua |last5=Hsu |first5=Victoria |date=September 2011 |title=Strabismus Precipitated by Monovision |journal=American Journal of Ophthalmology |volume=152 |issue=3 |pages=479–482.e1 |doi=10.1016/j.ajo.2011.02.008 |pmid=21669405}}</ref>   Studies have shown no adverse effect to driving performance in adapted monovision contact lens wearers.<ref>{{Cite journal |last=Wood |first=Joanne M. |last2=Wick |first2=Kristan |last3=Shuley |first3=Vicki |last4=Pearce |first4=Brendon |last5=Evans |first5=Dean |date=6 May 1998 |title=The effect of monovision contact lens wear on driving performance |journal=Clinical and Experimental Optometry |volume=81 |issue=3 |pages=100–103 |doi=10.1111/j.1444-0938.1998.tb06727.x |pmid=12482258 |s2cid=38392636}}</ref>

Alternatively, a person may simply wear reading glasses over their distance contact lenses.

====Other types of vision correction====
For those with certain [[color blindness|color deficiencies]], a red-tinted "X-Chrom" contact lens may be used. Although such a lens does not restore normal [[color vision]], it allows some color-blind people to distinguish colors better.<ref>{{Cite journal |last=Hartenbaum |first=NP |last2=Stack |first2=CM |year=1997 |title=Color vision deficiency and the X-Chrom lens |journal=Occupational Health & Safety |volume=66 |issue=9 |pages=36–40, 42 |pmid=9314196}}</ref><ref>{{Cite journal |last=Siegel |first=IM |year=1981 |title=The X-Chrom lens. On seeing red |journal=Survey of Ophthalmology |volume=25 |issue=5 |pages=312–24 |doi=10.1016/S0039-6257(81)80001-X |pmid=6971497}}</ref> Red-filtering contact lenses can also be an option for extreme light sensitivity in some visual deficiencies such as [[achromatopsia]].<ref>http://www.achromatopsia.info/red-contact-lenses/ Red contact lenses for achromats</ref>

ChromaGen contact lenses have been used and shown to have some limitations with vision at night although otherwise producing significant improvements in color vision.<ref>{{Cite journal |last=Swarbrick |first=HA |last2=Nguyen |first2=P |last3=Nguyen |first3=T |last4=Pham |first4=P |year=2001 |title=The ChromaGen contact lens system: Colour vision test results and subjective responses |journal=Ophthalmic & Physiological Optics |volume=21 |issue=3 |pages=182–96 |doi=10.1046/j.1475-1313.2001.00583.x |pmid=11396392 |s2cid=40409461}}</ref> An earlier study showed very significant improvements in color vision and patient satisfaction.<ref>Harris D "Colouring Sight: A study of CL fittings with colour enhancing lenses" 'Optician' 8 June 1997</ref>

Later work that used these ChromaGen lenses with [[Dyslexia|people with dyslexia]] in a randomised, double-blind, placebo-controlled trial showed highly significant{{Clarify|reason=vague|date=June 2017}} improvements in reading ability over reading without the lenses.<ref>Harris DA, MacRow-Hill SJ "Application of ChromaGen haploscopic lenses to patients with dyslexia: a double masked placebo controlled trial" Journal of the American Optometric Association 25/10/99.</ref> This system has been granted FDA approval for use in the United States.<ref>{{Cite press release |title=Cision - FDA Cleared ChromaGen Lenses |url=https://www.prnewswire.com/news-releases/fda-cleared-chromagen-lenses-are-a-life-changing-aid-for-people-struggling-with-dyslexia-and-other-reading-problems-200289391.html |url-status=live |archive-url=https://web.archive.org/web/20130703043949/http://www.prnewswire.com/news-releases/fda-cleared-chromagen-lenses-are-a-life-changing-aid-for-people-struggling-with-dyslexia-and-other-reading-problems-200289391.html |archive-date=3 July 2013}}</ref>

[[Magnification]] is another area being researched for future contact lens applications.<ref>{{Cite magazine |title=Telescopic contact lens magnifies vision by 2.8 times on demand |url=https://www.wired.co.uk/news/archive/2013-07/24/magnifying-contact-lenses |url-status=dead |archive-url=https://web.archive.org/web/20150402151345/http://www.wired.co.uk/news/archive/2013-07/24/magnifying-contact-lenses |archive-date=2 April 2015 |access-date=26 March 2015 |magazine=Wired UK}}</ref> Embedding of telescopic lenses and electronic components suggests that future uses of contact lenses may become extremely diverse.

====Cosmetic contact lenses====
[[File:Contact Grain.jpg|thumb|Woman wearing a cosmetic type of contact lens; enlarged detail 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 eye color are far less common, accounting for only 3% of contact lens fits in 2004.<ref>Morgan PB et al.[http://www.clspectrum.com/article.aspx?article=12741 "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.</ref>

In the United States, the FDA labels non-corrective cosmetic contact lenses as ''decorative contact lenses''. Like any contact lens, cosmetic lenses carry risks of mild to serious complications, including ocular redness, irritation and infection.<ref>[http://www.mc.vanderbilt.edu/news/releases.php?release=1670 Vanderbilt University Medical Center – Vanderbilt Eye Doctors Warn of the Dangers of Cosmetic Contact Lenses]. Mc.vanderbilt.edu (19 April 2010). Retrieved on 2013-07-21.</ref>

Due to their medical nature, colored contact lenses, similar to regular ones, are illegal to purchase in the United States without a valid prescription. Those with perfect vision can buy color contacts for cosmetic reasons, but they still need their eyes to be measured for a "plano" prescription, meaning one with zero vision correction. This is for safety reasons so the lenses will fit the eye without causing irritation or redness.

Some colored contact lenses completely cover the iris, thus dramatically changing eye color. Other colored contact lenses merely tint the iris, highlighting its natural color. A new trend in Japan, South Korea and China is the [[circle contact lens]], which extend the appearance of the iris onto the sclera by having a dark tinted area all around. The result is an appearance of a bigger, wider iris, a look reminiscent of dolls' eyes.<ref>{{Cite web |title=How Do You Find the Right Circle Lens? |url=http://www.eyecandys.com/news/finding-the-right-circle-lenses |url-status=dead |archive-url=https://web.archive.org/web/20131213060734/http://www.eyecandys.com/news/finding-the-right-circle-lenses |archive-date=13 December 2013 |access-date=26 March 2015 |website=EyeCandy's}}</ref>

Cosmetic lenses can have more direct medical applications. For example, some contact lenses can restore the appearance and, to some extent the function, of a damaged or [[aniridia|missing]] [[Iris (anatomy)|iris]].

====Therapeutic scleral lenses====
[[File:Scleral lens worn on an eye.jpg|thumb|upright|alt=See caption|[[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, transparent, oxygen-permeable contact 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 is too sensitive to support the lens directly. Scleral lenses may be used to improve vision and reduce pain and light sensitivity for people with disorders or injuries to the eye, such as severe [[dry eye syndrome]] (keratoconjunctivitis sicca), [[microphthalmia]], [[keratoconus]], [[corneal ectasia]], [[Stevens–Johnson syndrome]], [[Sjögren's syndrome]], [[aniridia]], neurotrophic keratitis (anesthetic corneas), complications post-LASIK, high order [[aberrations of the eye]], complications post-corneal transplant and [[Pellucid marginal degeneration|pellucid degeneration]]. Injuries to the eye such as surgical complications, distorted corneal implants, as well as chemical and burn injuries also may be treated with scleral lenses.<ref>{{Cite journal |last=Caceres |first=Vanessa |date=June 2009 |title=Taking a second look at scleral lenses |url=http://www.eyeworld.org/article-taking-a-second-look-at-scleral-lenses |url-status=dead |journal=ASCRS EyeWorld |archive-url=https://web.archive.org/web/20190228004334/https://www.eyeworld.org/article-taking-a-second-look-at-scleral-lenses |archive-date=28 February 2019 |access-date=18 May 2014}}</ref>

====Therapeutic soft lenses====
Soft lenses are often used in the treatment and management of non-refractive disorders of the eye. A bandage contact lens allows the patient to see<ref>{{Cite journal |last=Alvarez-Lorenzo |first=Carmen |last2=Anguiano-Igea |first2=Soledad |last3=Varela-García |first3=Angela |last4=Vivero-Lopez |first4=María |last5=Concheiro |first5=Angel |date=January 2019 |title=Bioinspired hydrogels for drug-eluting contact lenses |journal=Acta Biomaterialia |volume=84 |pages=49–62 |doi=10.1016/j.actbio.2018.11.020 |pmid=30448434 |s2cid=53780024}}</ref> while protecting an injured or diseased cornea<ref>{{Cite journal |last=Zidan |first=Ghada |last2=Rupenthal |first2=Ilva D. |last3=Greene |first3=Carol |last4=Seyfoddin |first4=Ali |date=2018 |title=Medicated ocular bandages and corneal health: potential excipients and active pharmaceutical ingredients |journal=Pharmaceutical Development and Technology |volume=23 |issue=3 |pages=255–260 |doi=10.1080/10837450.2017.1377232 |pmid=28875742 |s2cid=32765975}}</ref> from the constant rubbing of blinking eyelids, thereby allowing it to heal.<ref>{{Cite web |title=Eye Health Guide – Eye Diseases, Eye Problems and Eye Conditions |url=http://eyemdlink.com/terminology/bandage_contact_lens.htm |url-status=dead |archive-url=https://web.archive.org/web/20080409214021/http://eyemdlink.com/terminology/bandage_contact_lens.htm |archive-date=9 April 2008 |access-date=26 March 2015 |website=All About Vision}}</ref> They are used in the treatment of conditions including [[bullous keratopathy]], [[Keratoconjunctivitis sicca|dry eyes]], [[corneal abrasion]]s and [[recurrent corneal erosion|erosion]], [[keratitis]], corneal [[edema]], [[descemetocele]], [[corneal ectasia]], [[Mooren's ulcer]], anterior corneal dystrophy, and neurotrophic keratoconjunctivitis.<ref>{{Cite web |title=45 COVERAGE ISSUES – SUPPLIES – DRUGS 11–91 45 |url=http://new.cms.hhs.gov/manuals/downloads/Pub06_PART_45.pdf |url-status=dead |archive-url=https://web.archive.org/web/20060304101146/http://new.cms.hhs.gov/manuals/downloads/Pub06_PART_45.pdf |archive-date=4 March 2006 |access-date=1 March 2006 |publisher=[[Centers for Medicare and Medicaid Services]] |df=dmy-all}}</ref> Contact lenses that deliver drugs to the eye have also been developed.<ref>{{Cite web |title=Qmed is the world's only directory of pre-qualified suppliers to the medical device and in vitro diagnostics industry. - Qmed |url=http://www.devicelink.com/mddi/archive/05/01/011.html |url-status=dead |archive-url=https://web.archive.org/web/20081205140248/http://www.devicelink.com/mddi/archive/05/01/011.html |archive-date=5 December 2008 |access-date=26 March 2015}}</ref>