Contact lenses are medical devices that, when placed on the eye can correct shortsightedness, farsightedness and astigmatism. They can also be worn for cosmetic or therapeutic reasons.
People choose to wear contact lenses for many reasons. Aesthetics and cosmetics are often motivating factors for people who would like to avoid wearing glasses or would like to change the appearance of their eyes. Other people wear contacts for more visual reasons. When compared with spectacles, contact lenses typically provide better peripheral vision, and do not collect moisture such as rain, snow, condensation, or sweat. This makes them ideal for sports and other outdoor activities.
Additionally, there are conditions such as keratoconus and aniseikonia that are better corrected with contact lenses than glasses.
Although contact lenses have been available for over a century, the development of PMMA (polymethyl methacrylate) or Perspex promoted the development of plastic corneal hard contact lenses. In 1971 the first soft contact lens was approved by the American Food and Drug Administration. Since then the advancements in both material and design have made contact lenses first choice for over 15% of the world's population.
The prescribing of contact lenses is restricted to eye-care practitioners (Optometrists and Ophthalmologists). The best type of lens will be prescribed based on your visual requirements, eye structure and tear quality. Although your contact lens prescription is usually similar to your spectacle prescription the two are not interchangeable.
The many types of contact lenses currently available can be grouped in various ways according to:
Prescription contact lenses
Corrective contact lenses are designed to improve vision. Conditions correctable with contact lenses are myopia (shortsightedness), hypermetropia (farsightedness), astigmatism and presbyopia.
Cosmetic contact lenses
A cosmetic contact lens is designed to change the appearance of the eye. These lenses are available with or without prescription and carry the same risk of complications as any other contact lens. It is for this reason that even zero-powered lenses are classified as medical devices and a contact lens examination is essential before first use.
Therapeutic contact lenses
Contact lenses may be used in the management and treatment of various disorders of the eye. For example, a bandage contact lens is used to prevent the eyelid rubbing against an injured or diseased cornea.
Hard lenses are made from a rigid plastic material called PMMA. Their main disadvantage is that no oxygen is transmitted through the lens to the cornea.
RGP (Rigid Gas Permeable) lenses started development during the 1980s. Their rigid structure and high oxygen transmission make them ideal for higher prescriptions and patients with keratoconus. In addition, RGP lenses are not hydrophilic and do not absorb vapours or liquids, making them suitable for use in many industrial environments.
Soft lenses are made from gel-like, water-containing plastics. Whilst rigid lenses require a period of adaptation, soft lenses are immediately comfortable.
Daily Wear lenses are designed to be removed prior to sleeping.
Extended Wear lenses can be worn overnight, usually for 6 or more nights. These lenses can be worn for such long periods of time because of their high oxygen permeability to the cornea, which allows the eye to remain healthy even when the eyelid is closed. However, 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.
Daily disposable lenses - Discard after a single day of wear.
Shorter replacement cycle lenses are commonly thinner and lighter, due to lower requirements for durability against wear and tear, and may be the most comfortable in their respective class and generation. These may be best for patients with ocular allergies or other conditions because it limits deposits of antigens and protein, and is considered the healthiest wear schedule due to the most frequent replacement. 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.
Disposable lenses - Discard every two to four weeks as prescribed by your practitioner.
Many lens designs are available to correct various types of vision problems:
Spherical contact lenses are prescribed to correct myopia (shortsightedness) or hyperopia (farsightedness).
Toric contact lenses are used to correct astigmatism.
Multifocal contact lenses contain different zones for near and far vision to correct presbyopia.
Lenses are available in a multitude of combinations of diameter, curvature and material. Your Optometrist will judge the best lens for you based on good vision, fit, comfort and wearing schedule. He/she will also ensure that the final lenses prescribed will not interfere with the physiology of the eye.
Cleaning and disinfecting contact lenses is usually carried out by storing in multipurpose or preservative-free hydrogen peroxide solutions.
Daily disposable contact lenses do not need cleaning or disinfecting. They are stored in sterile containers and discarded after use.
People react differently to various lens materials and cleaning solutions. In most cases, if a problem develops, a change in lens type, material or cleaning solution is all that is needed to provide comfort, good vision and healthy eyes.
If you experience any discomfort, remove your lenses and consult your Optometrist.