Contact lens
Leonardo da vinci is the first to introduce the general peinciple of contact lenses in 1508, where he described a method of directly altering corneal power by submerging the eye in a bowl of water.
A contact lens 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. It has been estimate that about 125million people use contact lenses worldwide. People choose to wear contact lenses for various reasons.
The functions of contact lenses can be classified as

Corrective contact lenses
A corrective contact lens is a lens designed to improve vision. Conditions correctable with the contact lenses include short sightedness(Myopia), far sightedness(Hypermetropia), astigmatism and presbyopia.
Cosmetic contact lenses
These lenses are designed to change the appearance of the eye.

Therapeutic contact lenses
Soft lenses are often used in the treatment and management of no n refractive disorders of the eye. A bandage contact lens protects an injured or diseased cornea from constant rubbing of blinking eye lids thereby allowing it to heal.
Types of contact lenses by material
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. The introduction of lenses made from polymethyl methacrylate(PMMA) becomes much more convenient and these lenses are commonly referred as "Hard" lenses. However PMMA lenses doesn't transmit oxygen to the cornea which can cause a number of adverse clinicle events.

In the late 1970 improved rigid materials which were also oxygen-permeable were developed. These lenses were referred as RGP lenses. In effect, the lens is able to replace the natural shape of the cornea with a new refracting surface and provides good level of vision in people wgo have astigmatism or distorted corneal shapes as with keratoconus.
The principle breakthrough in soft lenses made by Otto Wichterle led to the launch of the first soft lenses in the 1980's. Soft lenses are immediately comfortable, while RGP lenses require a period of adaptation. The polymers from which soft lenses are manufactured improved over the next 25 years, primarily in terms of increasing the oxygen permeability. In 199, silicone hydrogels became availabkle. Silicone lenses have extremely high oxygen permeability and were advocate primarily for extended(overnight) wear. 

A daily wear lenses is designed to be removed prior to sleeping. An extended wear(EW) lenses is designed for continuous overnight wear, typically for 6 or more consecutive nights. Newer materials such as silicone hydrogels allow for even longer periods of up to 30 consecutive nights, these lenses were referred as continuous wear(CW).
The various soft contact lenses available are often categorized by their replacement schedule.More commonly contact lenses are prescribed to be disposed of on a two week or monthly basis. Quarterly or annual lenses, which are used to be very common, have lost favor because a more frequent disposal schedule allows for thinner and limits deposits. 
Complecations due to contact lenses wear affect roughly 5% of contact lens wearers each year. Excessive wear of contact lens, particularly overnight wear. Problems associated with contact lens may affect the eyelid, conjunctiva and the various layers of the cornea.
Tips To Use Contact Lenses
All lenses require regular cleaning and disinfecting in order to retain clear vision and prevent discomfort and infections. There are number of products that can be used to perform these tasks

Tips for trouble free lens wear
When you get you contact lenses, your eye care professional will teach you all about cleaning, disinfecting, storing and replacing your lenses according to the prescribed schedule.
Always wash and rinse your hands thoroughly before you handle your lenses.
To avoid getting your mixed up, always apply or remove your lenses, one4 at a time in the same order.
Never use eye drops or solutions that arn't specifically for contact lenses.
Don't use saliva to try and wet or clean your lenses.
 Don't sleep with your lenses unless if it is recommended.
Clean,rinse and disinfect your lenses every time you remove them.
Keep all your solution bottles tightly capped when you're not using them.
Never re-use lens solutions.
Replace your lens case once in three months.


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