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Contact lenses usually serve the same corrective purpose as 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. It has been estimated that about 125 million people use contact lenses worldwideBarr, J. "2004 Annual Report" . ''Contact Lens Spectrum''. January, 2005., including 28 to 38 million in the United StatesDixie 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. and 13 million in Japan National Consumer Affairs Center of Japan. NCAC News Vol. 12, No. 4 . ''NCAC News''. March, 2001.. 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 . People choose to wear contact lenses for various reasonsSokol 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 . Many consider their appearance to be more attractive with contact lenses than with glasses. 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 Athletes and Contact Lenses . Additionally, Ophthalmological conditions such as Keratoconus and Aniseikonia may not be accurately correctable with glasses. HISTORY 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 Cornea l 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.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. 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 unworkable, 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 da Vinci'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". 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 ''Archiv für Augenheilkunde'' in March 1888. Fick's lens was large, unwieldy, and could only be worn for a few hours at a time. August Muller in Kiel, Germany, corrected his own severe myopia with a more convenient glass-blown scleral contact lens of his own manufacture 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 1936 an optometrist, Dr. William Feinbloom introduced plastic lenses, making them lighter and more convenient. These lenses were a combination, however, of both plastic and glass. 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, which can cause a number of adverse clinical events. 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. The principal breakthrough in soft lenses was made by the 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—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. TYPES OF CONTACT LENSES Contact lenses are classified in many different manners "What are the types of contact lenses?" "Different types of contact lenses." . By 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 betterHartenbaum NP, Stack CM. "Color vision deficiency and the X-Chrom lens." ''Occup Health Saf.'' 1997 Sep;66(9):36-40, 42. PMID 9314196.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.. Other tinted lenses have been used with limited successSwarbrick 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.. 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 color or design. In the United States, the FDA frequently calls non-corrective cosmetic contact lenses ''decorative contact lenses''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.U.S. Food and Drug Administration. "FDA Warns Consumers Not to Use Decorative Contact Lenses Without Proper Professional Involvement." ''FDA News.'' October 10, 2003.Woo E. "Flying blind with decorative contact lenses." ''Nursing.'' 2003 Nov;33(11):70. PMID 14650382.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.. Theatrical contact lenses are a type of cosmetic contact lens that are used primarily in the ''. Scleral Lens es 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 color of the eye are far less common, accounting for only 3% of contact lens fits in 2004Morgan 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.. 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 to protect it from the constant rubbing of blinking eyelids thereby allowing it to heal EyeMDLink.com . They are used in the treatment of conditions including Bullous Keratopathy , Dry Eye s, Corneal Ulcer s and Erosion , Keratitis , corneal Edema , Descemetocele , corneal ectasis, Mooren's Ulcer , anterior corneal dystrophy, and neurotrophic keratoconjunctivitis1. Contact lenses to deliver drugs to the eye have also been developed "Contact Lenses Employed for Drug Delivery." . By constructional material The first contact lenses were made of Glass , which caused Eye Irritation and it was not possible to wear these lenses very long. But when Dr. William Feinbloom introduced lenses made from Polymethyl Methacrylate (PMMA or Perspex/Plexiglas ), contacts become much more convenient. These PMMA lenses are commonly referred to as "hard" lenses (this term is not used for other types of contacts). However, PMMA lenses have their own side effects: 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. Collectively, these Polymers are referred to as 'rigid gas permeable' or 'RGP' materials or lenses. 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 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 Food And Drug Administration in 1971 . Soft lenses are immediately comfortable while, rigid lenses require a period of adaptation before full comfort is achieved. The Polymer s 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. A small number of hybrid rigid/soft lenses exist. An alternative technique is Piggyback ing of contact lenses, a smaller, rigid lens being mounted 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. In 1999 , ' Silicone Hydrogel s' became available. Silicone hydrogels have both extremely high Oxygen Permeability of Silicone and the comfort and clinical performance of the conventional Hydrogel s. These lenses were initially advocated primarily for extended (overnight) wear although more recently, daily (no overnight) wear silicone hydrogels have been launched. The property of high oxygen permeability of silicone hydrogel lens materials is due to the presence of silicone. However, the disadvantage of having silicone is that it makes the lens surface highly hydrophobic making the lenses less wettable. This frequently results in patients complaining of discomfort and dryness during lens wear. In order to compensate for the hydrophobicity caused due to silicone, hydrogels are added (hence the name "silicone hydrogels") in order to make the lenses more hydrophilic. However, even after the addition of these hydrogels the lens surface still remains hydrophobic. Hence some of the lenses undergo surface modification processes which cover the hydrophobic sites of silicone. While some other lens types incorporate internal rewetting agents which make the lens surface hydrophilic. By 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 Ulcer s, 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. By 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. Quarterly or annual lenses, which used to be very common, have lost favor because a more frequent disposal 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. By 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 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 doesn't, the patient may be told to use a spherical lense in one eye and a toric lense in the other. Toric lenses are made from the same materials as regular contact lenses but have a couple of extra characteristics:
By number of focal points Similarly to eyeglasses, contact lenses can have one (single vision) or more (multifocal) Focal Point s. 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''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: 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 complex to manufacture and require more skill to fit. All soft bifocal contact lenses are considered "simultaneous vision" because both far and near vision are corrected simultaneously, regardless of the position of the eye. Commonly these are designed with distance correction in the center of the lens and near correction in the periphery, or viceversa. Rigid gas permeable 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. Implantation Intraocular Contact Lens es, 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 amounts of myopia and hyperopia. Orthokeratology (ie Corneal Refractive Therapy) See main page: Orthokeratology MANUFACTURING OF CONTACT LENSES Most contact lenses are mass produced.
Although many companies make contact lenses, there are four major manufacturers: Vistakon , Ciba , Bausch & Lomb , and CooperVision Federal Trade Commission. "The Strength of Competition in the Sale of Rx Contact Lenses: An FTC Study". February, 2005.. PRESCRIBING CONTACT LENSES 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 Device s 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 Eye s may affect a person's ability to successfully wear contact lenses. The parameters specified in a contact lenses prescription may include:
COMPLICATIONS , the Conjunctiva , the various layers of the Cornea , and even the Tear Film that covers the outer surface of the eye. Eyelid: Conjunctiva: Cornea:
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 discomfort and infections by various Microorganism s including Bacteria , Fungi , and '' Acanthamoeba '', that form a Biofilm on the lens surface. There are a number of products that can be used to perform these tasks:
Some products must only be used with certain types of contact lenses: it is important to check the product label to make sure that it can be used for a given type of lens. It is also 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 s such as Thimerosal , Benzalkonium Chloride , Benzyl Alcohol , and other compounds. In 1989, thimerosal was responsible for about 10% of problems related to contact lensesWilson-Holt N, Dart JK. "Thiomersal keratoconjunctivitis, frequency, clinical spectrum and diagnosis." ''Eye.'' 1989;3 ( Pt 5):581-7. PMID 2630335: because of this, many products no longer contain thimerosal. If a person has problems with a solution and suspects the preservative, it is worth trying a solution with a different preservative. If they suspect they are sensitive to most preservatives, try completely preservative-free products, described as "for sensitive eyes" or "preservative-free". However, preservative-free products usually have shorter shelf life. For example, non-aerosol preservative-free saline solutions can typically be used for only two weeks once opened. Before touching the contact lens or one's eyes, it is important to thoroughly Wash & Rinse Hands with a Soap that does not contain Moisturizer s or Allergen s such as Fragrance s. REFERENCES AND SOURCES Sources:
References: SEE ALSO EXTERNAL LINKS
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