   #copyright

Contact lens

2007 Schools Wikipedia Selection. Related subjects: Engineering

   A pair of contact lenses when not inserted in the eye. They are
   positioned with the concave side facing upward.
   Enlarge
   A pair of contact lenses when not inserted in the eye. They are
   positioned with the concave side facing upward.

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

   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 to make them more
   visible when immersed in cleaning and storage solutions. Cosmetic
   lenses are deliberately colored for altering the appearance of the eye.

   It has been estimated that about 125 million people use contact lenses
   worldwide (2%), including 28 to 38 million in the United Statesand 13
   million in Japan . 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 reasons. 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. Additionally, ophthalmological
   conditions such as keratoconus and aniseikonia may not be accurately
   corrected with glasses.

History

   In 1887, Adolf Fick was apparently the first to successfully fit
   contact lenses, which were made from brown glass
   Enlarge
   In 1887, Adolf Fick was apparently the first to successfully fit
   contact lenses, which were made from brown glass

   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. Leonardo, however, did not
   suggest his idea be used for correcting vision — he was more interested
   in learning about the mechanisms of accommodation of the eye.

   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 1887 that the German physiologist Adolf Eugen Fick
   constructed and fitted the first successful contact lens. While working
   in Zürich, he described fabricating afocal scleral contact shells,
   which rested on the less sensitive rim of tissue around the cornea, and
   experimentally fitting them: initially on rabbits, then on himself, and
   lastly on a small group of volunteers. These lenses were made from
   heavy brown glass and were 18-21mm in diameter. Fick filled the empty
   space between cornea/callosity and glass with a grape sugar 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 few 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.

   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, 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 Czech chemist
   Otto Wichterle who published his work "Hydrophilic gels for biological
   use" in the journal Nature in 1959. This led to the launch of the first
   soft (hydrogel) lenses in some countries in the 1960s and the first
   approval of the 'Soflens' material by the United States Food and Drug
   Administration (FDA) in 1971. These lenses were soon prescribed more
   often than rigid lenses, mainly due to the immediate comfort of soft
   lenses; by comparison, rigid lenses require a period of adaptation
   before full comfort is achieved. The polymers from which soft lenses
   are manufactured improved over the next 25 years, primarily in terms of
   increasing the oxygen permeability by varying the ingredients 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.

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 colour deficiencies, a red-tinted "X-Chrom"
   contact lens may be used. Although the lens does not restore normal
   colour vision, it allows some colorblind individuals to distinguish
   colors better. Other tinted lenses have been used with limited success.
   A man seen wearing two different styles of cosmetic contact lenses
   Enlarge
   A man 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.

   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 appearance. These lenses have been used by Bill
   Bixby and Lou Ferrigno durng the TV show The Incredible Hulk Wes
   Borland, Marilyn Manson, Twiztid, World Wrestling Entertainment (WWE)
   wrestlers Kane, Rey Mysterio and Viscera, and Ray Park as Darth Maul in
   Star Wars Episode I: The Phantom Menace, among others. Scleral lenses
   cover the white part of the eye (i.e. sclera) and are used in many
   theatrical lenses. Recently, contact lens using patterns of Sharingan
   become pupular with fans of the anime Naruto.

   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.

   Therapeutic contact lenses - Soft lenses are often used in the
   treatment and management of non-refractive disorders of the eye. A
   bandage contact lens protects an injured or diseased cornea from the
   constant rubbing of blinking eyelids thereby allowing it to heal. 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. Contact lenses to
   deliver drugs to the eye have also been developed.

By constructional material

   Contact lenses, once inserted in the eye, become almost invisible
   (except cosmetic contact lenses).
   Enlarge
   Contact lenses, once inserted in the eye, become almost invisible
   (except cosmetic contact lenses).

   The first contact lenses were made of glass, which caused eye
   irritation, and so were not able to be worn for extended periods of
   time. But when 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 FDA in 1971. Soft
   lenses are immediately comfortable, while rigid lenses require a period
   of adaptation before full comfort is achieved. The polymers from which
   soft lenses are manufactured improved over the next 25 years, 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 piggybacking of contact lenses, a smaller, rigid lens
   being mounted atop a larger, soft lens. This is done for a variety of
   clinical situations where a single lens will not provide the optical
   power, fitting characteristics, or comfort required.

   In 1999, ' silicone hydrogels' became available. Silicone hydrogels
   have both the extremely high oxygen permeability of silicone and the
   comfort and clinical performance of the conventional hydrogels. These
   lenses were initially advocated primarily for extended (overnight)
   wear, although more recently daily (no overnight) wear silicone
   hydrogels have been launched.

   While it provides the oxygen permeability, the silicone also makes the
   lens surface highly hydrophobic and less "wettable." This frequently
   results in discomfort and dryness during lens wear. In order to
   compensate for the hydrophobicity, hydrogels are added (hence the name
   "silicone hydrogels") to make the lenses more hydrophilic. However the
   lens surface may still remain hydrophobic. Hence some of the lenses
   undergo surface modification processes which cover the hydrophobic
   sites of silicone. Some other lens types incorporate internal rewetting
   agents to 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 ulcers, primarily due to poor care and cleaning of the
   lenses, tear film instability, and bacterial stagnation. Corneal
   neovascularization has historically also been a common complication of
   extended lens wear, though this does not appear to be a problem with
   silicone hydrogel extended wear. The most common complication of
   extended lens use is conjunctivitis usually allergic or giant papillary
   conjunctivitis (GPC), sometimes associated with a poorly fitting
   contact lens.

By disposability

   The various soft contact lenses available are often categorized by
   their replacement schedule. The shortest replacement schedule is single
   use (daily disposable) 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, or for
   purposes (e.g. swimming or other sporting activities) where losing a
   lens is likely. More commonly, contact lenses are prescribed to be
   disposed of on a two-week or monthly basis. Quarterly or annual lenses,
   which used to be very common, have lost favour 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 does not, the patient may be told to use a
   spherical lens in one eye and a toric lens in the other. Toric lenses
   are made from the same materials as regular contact lenses but have a
   couple of extra characteristics:
     * They correct for both spherical and cylindrical aberration.
     * They have a specific top and bottom, as they are not symmetrical
       around their centre and must not be rotated. Lenses must be
       designed to maintain their orientation regardless of eye movement.
       Often lenses are weighted more at the bottom and are marked by tiny
       striations so the wearer can insert them in the correct
       orientation, or they are designed so that the correct orientation
       is restored when the user blinks. Some do both.

By number of focal points

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

   For correction of presbyopia or accommodative insufficiency multifocal
   contact lenses are almost always used; however, single vision lenses
   may also be used in a process known as monovision: 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 centre 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 lenses, also known as an implantable contact
   lenses, are special small corrective lenses surgically implanted in the
   eye's posterior chamber behind the iris and in front of the lens to
   correct higher 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.
     * Lathe cut - A lathe cut contact lens is cut and ground on a lathe.
     * Molded
     * Hybrids

   Although many companies make contact lenses, there are four major
   manufacturers: Vistakon/ Johnson & Johnson, CIBA Vision, Bausch & Lomb,
   and CooperVision.

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
   devices by the Food and Drug Administration), the United Kingdom and
   Australia, optometrists are usually responsible. In France and eastern
   European countries, ophthalmologists play the major role. In other
   parts of the world, opticians usually prescribe contact lenses.
   Prescriptions for contact lenses and glasses may be similar, but are
   not interchangeable.

   The practitioner or contact lens fitter typically determines an
   individual's suitability for contact lenses during an eye examination.
   Corneal health is verified; ocular allergies or dry eyes may affect a
   person's ability to successfully wear contact lenses. Especially above
   the age of 35 years dry eyes often cause wearing contact lenses too
   risky, especially soft lenses.

   The parameters specified in a contact lenses prescription may include:
     * 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)
     * Brand

   Many people already wearing contact lenses order contact lenses over
   the internet at their own risk. Often they use an old prescription. It
   is also possible to convert the power of a glasses prescription to the
   power of contact lenses with the following formula:

          F_{contact\ lens} = F_{glass}/(1 -d\ F_{glass}) ,

   where
     * d is the distance of the glassses to the eye and
     * F = 1 / D are the focal length of the contact lense and the glass,
     * D is the lens power in diopters.

   The formula above is implemented in this online calculator.

Complications

   Complications due to contact lens wear are relatively common, affecting
   roughly 5% of contact lens wearers each year. Excessive wear of contact
   lenses, particularly overnight wear, is associated with the most
   serious safety concerns. 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.

   Eyelid:
     * Ptosis

   Conjunctiva:
     * Contact dermatitis
     * Giant papillary conjunctivitis
     * Superior limbic keratoconjunctivitis

   Cornea:
     * Epithelium

          + Corneal abrasion
          + Corneal erosion
          + Corneal ulcer
          + Hypoxia

     * Stroma

          + Infection and keratitis

               o Bacteria
               o Protozoa: Acanthamoeba
               o Fungal: Fusarium

          + Contact lens acute red eye (CLARE)
          + Keratoconus

     * Endothelium

Usage

   Before touching the contact lens or one's eyes, it is important to
   thoroughly wash & rinse hands with a soap that does not contain
   moisturizers or allergens such as fragrances. The technique for
   removing or inserting a contact lens varies slightly depending upon
   whether the lens is soft or rigid.

Insertion

   Contact lenses are typically inserted into the eye by placing them on
   the index finger with the convex side upward and raising them to touch
   the cornea. The other hand may be employed to keep the eye open.
   Problems may arise particularly with disposable soft lenses; if the
   surface tension between the lens and the finger is too great the lens
   may turn itself inside out; alternatively it may fold itself in half.
   When the lens first contacts the eye, a brief period of irritation may
   ensue as the eye aclimatises to the lens and also (if a multi-use lens
   is not correctly cleansed) as dirt on the lens irritates the eye.
   Irrigation may help during this period, which generally should not
   exceed one minute.

Removal

   A soft lens may be removed by holding the eyelids open and grasping the
   lens with opposing digits. This method can cause irritation, could risk
   damage to the eye and may in many cases be difficult, in part due to
   the blink reflex. If the lens is pushed off the cornea it will buckle
   up (due to the difference in curvature), making it easier to grasp.

   Rigid contact lenses may be removed by pulling with one finger on the
   outer or lateral canthus, then blinking to cause the lens to lose
   adhesion. The other hand is typically cupped underneath the eye to
   catch the lens. There also exist small tools specifically for removing
   lenses, which resemble small plungers made of flexible plastic; the
   concave end is raised to the eye and touched to the lens, forming a
   seal stronger than that of the lens with the cornea and allowing the
   lens to be removed from the eye.

Care (cleaning and disinfection)

   While daily disposable lenses require no cleaning, other types require
   regular cleaning and disinfecting in order to retain clear vision and
   prevent discomfort and infections by various microorganisms including
   bacteria, fungi, and Acanthamoeba, that form a biofilm on the lens
   surface. There are a 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. Using this product eliminates the need for
       protein removal enzyme tablets in most cases.
     * 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. A few drops
       of cleaner are applied to the lens while it rests in the palm of
       the hand, then the lens is rubbed for about 20 seconds with a
       fingertip (check the cleaner's directions) on each side. Long
       fingernails can damage the lens, so care should be taken.
     * 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 protein deposits off lenses,
       usually weekly, if the daily cleaner is not sufficient. Typically,
       this cleaner is in tablet form. Protein deposits make use of
       contact lenses uncomfortable, and may lead to various eye problems.

   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 microorganisms: the tips of the containers for these solutions
   should never touch any surface, and the container should be kept 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: because of this, many
   products no longer contain thimerosal. 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.
   Retrieved from " http://en.wikipedia.org/wiki/Contact_lens"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
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