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Contact lenses

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

Contact lenses are available in a number of varieties, including hard and soft. 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. They have since been replaced by rigid gas-permeable (RGP) contact lenses. Many contact lenses are made of hydrophilic (water-absorbing) materials, thereby allowing oxygen to reach the cornea, and make the lens more comfortable to wear.

Heavily tinted contacts are tinted to change the color of the iris, and are used for cosmetic reasons. Some standard contact lenses are slightly tinted in order to make them more visible for handling purposes.

History

The idea of applying a corrective lens directly to the surface of the eye was first proposed as early as 1508, by Leonardo da Vinci and 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.

Corrective contact lenses

The specific dioptre that is required to treat the patient's condition can be found with the help of an optometrist and provided by an oculist. The thickness and shape of the contact lens will also vary with the increase in dioptres, and according to the condition that is being treated: Near (or short) sightedness (myopia), far (or long) sightedness (hypermetropia), or astigmatism.

Extended wear lenses

Extended wear lenses are designed to be worn continuously for 6 or more consecutive nights. Newer materials, such as silicone hydrogels, allow for even longer wear periods 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 owing to their obvious convenience. Such contact lenses are able to be worn for this extended period 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 generally have an increased risk for corneal infections and corneal ulcers primarily due to tear film instability and bacterial stagnation.

Toric lenses

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. Typically, 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.

Other types of contact lenses

A person seen wearing two different styles of cosmetic contact lenses
A person seen wearing two different styles of cosmetic contact lenses

Some companies produce contacts that can change the tint or color appearance of the eye, or can place various designs on it (opaque lenses). There also exist contacts that can give the iris an enlarged appearance, or can be used to mask congenital defects (iris coloboma), absence (aniridia) or damage (dyscoria) to the iris. These contacts are rarely seen on people, although many performers, both in music and movies, commonly use them for artistic purposes. This type of contacts can also have all the features common of corrective contacts, although some blurring or obstruction of vision may occur based on the specific contact lens design being used.

Some notable musicians that have been known to use such contacts are Wes Borland, Marilyn Manson, and Twiztid.

These lenses were also featured in Star Wars Episode I: The Phantom Menace worn by Ray Park, who played Darth Maul.

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.

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 important tasks:

  • 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 directions) on each side. One must be extra careful in this step if one has long fingernails.
  • Multipurpose solution - used for rinsing, disinfecting, cleaning and storing the lenses. Many people typically only use it for disinfection and storage, relying on other products for rinsing (e.g. saline) and cleaning (e.g. daily cleaner).
  • Hydrogen peroxide solution - used for disinfecting the lenses. Available as 'two-step' or 'one-step' systems. If using a 'two-step' product, one must ensure that they neutralise any lens taken out of hydrogen peroxide with 'Step 2' before wearing the lens otherwise it can be an extremely painful experience! (do NOT use saline to rinse away the peroxide). Some such 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 simple saline solution that will not harm the eye. This type of cleaning solution is used commonly by people with extremely sensitive eyes that are irritated easily by standard solutions.
  • Enzymatic cleaner - used for cleaning the protein off of lenses, usually on a weekly basis. Typically, this cleaner is in tablet form. Using only the daily cleaner is not sufficient to prevent protein deposits, making the lens very uncomfortable and possibly leading to eye damage.

Some products may contain preservatives such as thimerosal. However, about 10% of contact lens wearers have problems with these products, a reason why several brands no longer use it. Such thimerosal-free products are sometimes labelled "for sensitive eyes". Products that do not contain any preservatives usually have shorter expiration dates. For example, non-aerosol preservative-free saline solution typically only last 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

 


 

 

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