Contact Lenses [2]
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Wikipedia-Article "Contact Lenses [2]"
A contact lens (also known as a "contact") is a corrective, cosmetic, or sometimes protective lens placed on the cornea of the eye.
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History
Leonardo da Vinci first proposed the idea of applying a corrective lens directly to the surface of the eye as early as 1508. Similar concepts surfaced from René Descartes in 1636, but it was not until 1887 that the German physiologist Adolf Eugen Fick constructed the first successful contact lens. In 1971, Bausch & Lomb introduced the first soft contact lenses.
Usage
It has been estimated that about 125 million people use contact lenses worldwide including 38 million in the United States[1] and 13 million in Japan[2]. 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[3].
Types of contact lenses
Contact lenses are available in a number of varieties.
Corrective vs. cosmetic contact lenses
A corrective contact lens is a contact lens designed to improve vision, whereas a cosmetic contact lens is a contact lens designed to change the appearance of the eye. Sometimes the two overlap. Although many people wear contact lenses for functional reasons, they are frequently referred to as a cosmetic alternative to eyeglasses. Conversely, some people wear cosmetic contact lenses for functional benefits.
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. The power of a contact lens neutralises this mismatch and allows for correct focussing of light onto the retina. Conditions correctable with contact lenses include near (or short) sightedness (myopia), far (or long) sightedness (hypermetropia), astigmatism and presbyopia. There has been recent interest in the corretion of myopia by deliberate overnight flattening the cornea, leaving the eye corrected without contact lenses during the daytime. (See main article: Orthokeratology.)
Much attention is often paid to contact lenses which seek to change the colour of the eyes; however, these lenses accounted for only 3% of contact lens fits in 2004[4]. Some standard contact lenses are slightly tinted in order to make them more visible and, therefore, easier to handle.
Cosmetic contact lenses can have more direct medical applications. For example, some lenses can give the iris an enlarged appearance, or can mask various defects, such as absence (aniridia) or damage (dyscoria) to the iris. Coloured lenses are also used artisically; examples here include Wes Borland, Marilyn Manson, Twiztid, and Ray Park as Darth Maul from Star Wars Episode I: The Phantom Menace. These types of contact lenses can also offer vision correction, although some blurring or obstruction of vision may occur as a result of the specific contact lens design being used. Some contact lenses cover the white (or sclera) of the eye; these are referred to as scleral lenses.
The prescribing of contact lenses is usually limited to appropriately qualified eye care practitioners. In some 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.
Soft vs. hard contact lenses
Contact lenses may also be classified as either soft or hard. Hard contacts are typically not disposable, while soft contacts often are. Some soft contacts are also known as extended wear lenses. The most commonly used contact lenses today are of the soft variety, invented in 1961 by the Czech chemist Otto Wichterle (1913–1998).
Contact lenses (both soft and hard) are made of various types of polymers, the latest containing some variant of silicone hydrogel. Previously, hard contact lenses were made of a polymer known as PMMA. Rigid gas-permeable (RGP) lenses have replaced them. Many contact lenses are made of hydrophilic (water-absorbing) materials, thereby allowing oxygen to reach the cornea, and making the lens more comfortable to wear.
Piggybacking contact lenses refers 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.
Rigid contact lenses are also used to help correct vision in patients with corneal conditions, such as keratoconus, where soft contact lenses or glasses prove ineffective. Rigid contact lenses trap tears beneath the lens. Since the refractive index of tears is very close to that of the cornea, the tears pass light through the affected cornea with minimal distortion. The result is a lens that acts as a regularly shaped "artificial cornea," and improves vision.
Daily vs. extended wear contact lenses
A daily wear contact lens is designed to be removed prior to sleeping. An extended wear 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, EW 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.
Spherical vs. toric contact lenses
A spherical contact lens is one in which all meridians of the lens have the same power correction. A toric contact lens is one in which the power correction differs from meridian to meridian. 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.
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. 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.
- 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 neutralised before it is worn, or else wear will be extremely painful. Do NOT use saline 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 contain preservatives such as thimerosal. However, about 10% of contact lens wearers have problems with these sorts of preservatives, a reason why several brands no longer use them. Such thimerosal-free products are sometimes labelled as being "for sensitive eyes". 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.
Parameters
A contact lens can be characterised by
- Base curve radius (BC, BCR)
- Diameter (D, OAD)
- Power (Dioptres - Spherical, Cylindrical and/or reading addition)
- Cylinder axis
- Centre Thickness (CT)
- Material (eg. Oxygen Permeability/Transmissibility (Dk/L, Dk/t), water content, modulus)
- Replacement schedule
Products
See also
External links
- Contact Lenses: A Timeline
- The History of Contact Lenses (Article)
- Decorative contact lenses (WildEyes brand by CIBA Vision)
- Contact Lenses Guidance
- Contact Lens catalog via Lenscrafters