Contact lenses
Contact lenses are now widely used for the correction of many different types of vision deficiencies. A contact lens is generally classified into a hard contact lens and a soft contact lens. Further, the hard contact lens is classified into an oxygen-non-permeable hard contact lens formed of a homopolymer or copolymer of methyl methacrylate (MMA) and a rigid gas permeable (RGP) hard contact lens formed of a copolymer formed of siloxanyl methacrylate (SiMA), MMA and fluoroalkylmethacrylate (FMA) as main components. Rigid, gas permeable contact lenses are made from materials including polymerized comonomers such as cellulose acetate butyrate or acrylate-silicone esters. These rigid, gas permeable lenses, as the term implies, are permeable to oxygen in varying degrees, thus reducing or eliminating corneal edema. Contact lenses having a toric optical zone (toric contact lenses) are used to correct refractive abnormalities of the eye associated with astigmatism. The toric optical zone provides cylindrical correction to compensate for the astigmatism. The toric surface may be formed in either the posterior lens surface (back surface toric lens) or in the anterior lens surface (front surface toric lens). Bifocal contact lenses are lenses with two zones having different optical powers, or more than two zones in which case they are also called multifocal lenses. A far power zone provides the optical power that is needed for the wearer's distance vision and a near power zone, sometimes called a segment, provides the optical power that is needed for the wearer's near vision. The power zones together with an optional transition zone form the bifocal portion of the lens. Bifocal contact lenses generally are classified into two types, segmented and concentric. Modern contact lenses are formed of a plastic composition that, while providing much comfort for the wearer, also requires some degree of care. The lenses may be required to be removed for cleaning. Some contact lenses require that they be removed while the wearer sleeps. Contact lenses have been available in many colors, for many years, in both hard, including rigid gas permeable (RGP), and soft contact lenses. Colored contact lenses are often used for cosmetic purposes to change the apparent color of a wearer's irises. A leading cause of contact lens discomfort and shortened wearing time is the failure of the wearer to keep the contact lenses free of surface deposits and coatings. During handling and wear, contact lenses are susceptible to the accumulation of a variety of materials which may adhere to the surface of the lens and/or lodge within and adhere chemically and/or spatially to the inner bulk matrix of the lens. Contact lenses often require periodic treatment, for example, disinfecting, cleaning, soaking and the like, to provide substantial performance advantages. Such treatments often occur by immersing the lenses in a liquid contact lens care composition, for example, solution, while the lenses are held in a contact lens case. Such cases can be used to store contact lenses between use periods. Contact lenses are fabricated from a suitable plastic material which is extremely pliable and flexible. Based on the use of such lenses, the user is required to clean the lenses at frequent intervals and in doing so, the lenses are placed in a case in which they may be stored or otherwise treated during the cleaning procedure. Both types must be kept in their respective preserving solutions while they are not being worn, in order to preserve their delicate shape and performance. In particular, water-containing soft contact lenses should be stored in physiological saline to keep a constant water content thereby maintaining the characteristic softness that assures comfort in use.
Contact lenses categories
Bifocal contact lens
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