Soft lenses

No particular contact lens type or product is considered universally superior. In some regions of the world hard lenses dominate the market, eg, some European countries and Japan in other regions, eg. North America and Scandinavia, soft lenses dominate. Contact lens practitioners select their preferred type of lens using criteria other than just lens material properties. However, among soft lenses, HEMA-based lenses are prescribed most often, and among hard lenses, siUcone—acrylate RGP lenses are most common. 

Most contact lenses are worn for optical reasons as an alternative to spectacles. Contact lenses are of two types, namely hard lenses, which are hydrophobic, and soft lenses, which may be either hydrophilic or hydrophobic. The surfaces of lenses mnst be wetted before nse, and wetting solntions (section 4.5.1) are nsed for this pnrpose. Hard and, more especially, soft lenses become heavily contantinated with protein material dnring nse and must therefore be cleaned (section 4.5.2) before disinfection (section 4.5.3). Contact lenses are potential sonrees of eye infection and conseqnentiy nticroorganisms should be removed before the lens is again inserted into the eye. Lenses mnst also be clean and easily wettable by the lacrimal secretions. Contact-lens solntions are thns sterile solutions of the varions types described below. Apart fiom... 

The first soft contact lenses were also constructed with a polymeric material containing a single monomeric unit. The added pliability of the soft lens was derived from the more hydrophilic nature of the monomer, enhancing the ability of the polymer to absorb water and provide greater comfort to the lens wearer. This monomer is a derivative of MMA known as hydroxyethyl methacrylate (HEMA). A number of hydrophilic monomers are used in soft lenses today these materials are referred to as hydrogels because of their ability to absorb significant amounts of water yet remain insoluble. 

Contact lenses come in a wide variety there are soft lenses and hard lenses there are some you can wear night and day for a month or more, and disposable ones that you need wear only for a day there are lenses that let your eyes breathe there are bifocal lenses and there are even lenses to correct astigmatism, a condition of the lens of the eye which makes everything seem blurred. All are products of years of research by chemical companies like Novartis, Ciba Vision, and Wesley Jessen. There are even coloured lenses that can make blue eyes bluer, and sport lenses that enable tennis players to keep their eye on the ball by enhancing the colour of a yellow tennis ball relative to its surroundings. 

Fluorexon is not recommended for use with highly hydrated soft lenses having a water content of 60% or higher. In such cases the lens can absorb significant amounts of dye, resulting in unwanted lens discoloration. 

Acanthamoeba ocular infection was first described in 1915. Acanthamoeba keratitis can occur in both healthy and immunocompromised individuals and is initiated by contact with contaminated water. Most Acanthamoeba keratitis cases described in the mid-1980s involved daily-wear soft contact lens wearers who were using saline made from distilled water and salt tablets. Cases have also been described in extended-wear soft contact lens wearers and rigid contact lens wearers. In a survey of corneal specialists, it was found that 85% of the reported cases were in contact lens patients using primarily daily-wear or extended-wear soft lenses. 

Prior to the commercial availability of pharmaceutically prepared saline solutions, they were prepared by the lenswearer using salt tablets and distilled water. This method was undesirable and created many problems as the pH and osmolarity of such solutions were not controlled, which often resulted in parameter changes in some soft lenses. The major problem... 

Because RGP lenses are not as porous or water absorbing as soft lenses, wearers do not experience the problems that are specific to soft lenses, resulting from preservatives penetrating and concentrating within the lens polymer matrix, which often cause toxic and hypersensitivity-type reactions. However, certain preservatives bind with RGP surfaces and can create clinical problems. Binding may involve ionic and/or hydrophobic interactions. 

Soft lenses are made of hydrophilic hydrogel polymers that contain 36-74% water. Other plastics and copolymers are added to alter the physical characteristics of the lens. The diameter is 10.5-15.5 mm and the thickness at the center 0.03-20 mm. Soft lenses can correct most optical defects, including myopia, hyperopia, and astigmatism. Bifocal lenses are also available. They can be colored with either transparent hnes or opaque patterns to change apparent eye color or to mask malformations of the cornea or iris. They are available for daily, weekly, and twice-weekly disposable nse, 1-3 months frequent replacement, and annual replacement. 

Soft contact lenses, especially extended-wear lenses, carry a significantly higher risk of keratitis than hard lenses do. In a case-control stndy, the relative risk for overnight wear soft lenses was 21, for daily-wear soft lenses 3.6, and for polymethylmethacrylate hard lenses 1.3, as compared with gas-permeable hard lenses (5). 

Cases of allergic conjnnctivitis and blepharitis have been reported with the preservatives benzalkonium chloride and mercnrial salts, althongh benzalkonium at the commonly nsed concentration of 0.01% produces no evident damage. These adverse reactions can be prevented by rinsing hard lenses in clean water before insertion in the eye and by boihng soft lenses in normal saline after cleaning. 

The nse of chlorobntanol is hmited to pHs below 6. It is snbject to thermal degradation and can be adsorbed on to the walls of containers. However, it has no major adverse effects. Chlorobntanol has no effect on wetting of the cornea or contact lenses, as surface-active agents do. When tested with soft lenses, chlorobntanol, concentrated in the lenses, canses mild conjunctivitis. 

Cetrimide is also used as a cleanser and disinfectant for hard contact lenses, although it should not be used on soft lenses as an ingredient of cetrimide emulsifying wax, and in o/w creams (e.g. cetrimide cream). 

Most contact lenses are worn for optical reasons as an alternative to spectacles. Contact lenses are of two types, namely hard lenses that are hydrophobic, and soft lenses, which may be either hydrophilic or hydrophobic. The surfaces of lenses must be wetted before use and wetting solutions are used for this purpose. Hard, and more especially,... 

All eye drops for allergic conjunctivitis contain the preservative benzalkonium chloride, which is absorbed into soft contact lenses and released onto the cornea during wear, causing inflammation and irritation. Soft lenses should not be worn while using these products gas-permeable lenses may be inserted 30 minutes after using the eye drops. 

The use of contact lenses, initially made from glass, to correct vision has been known since the 19th century. The first polymeric contact lens was a hard contact lens made in 1936 from polymethylmethacrylate (PMMA). " However, these lenses had to be taken out at night to prevent eye irritation and much research has been directed to making those that could be worn for much longer periods of time. This led to the development of the so-called soft lenses. 

In humans, a contact lens lies in the conjunctival sac of the eye. In a closed eye, this sac is a slitlike space between the conjunctiva-covered eyeball and the eyelids. Contact lenses are small polymer bowls that float on tears superficial to the corneal eye layer and correct existing visual deficiencies similarly to glasses. While the idea of the contact lens was formulated as early as 1508, it was not until the 1800s that contact lenses became a reality. Hard plastic contact lenses were invented around 1936, and although soft lenses were invented in 1960, they were not available on the commercial market until 1971. 

Next-generation soft contact lenses, dental polymers, surface coatings, and similar materials are produced from compounds of varying structure and reactive functionality. For example, currently in development are new soft lenses that will be manufactured from monomers synthesized with dimethylsil-oxane backbones. The dimethylsiloxane backbone is terminated with a methacryloxy functionality that supplies the site for polymerization. The siloxane provides lens softness. Occasionally the functionality is formed on both ends of the monomer, resulting in undesired properties. The compound BisGMA is a monomer that is polymerized to form hard dental structures. In the monomer synthesis process impurities are coproduced that interfere with the polymerization. Finally, diacetone acrylamide used in a copolymerization process is another specialty monomer that is occasionally contaminated with difficult-to-remove impurities. These three monomers are quite reactive at modest temperature and cannot be purified by distillation. The three examples that are presented here derive from as yet unpublished research (Krukonis, 1982c). 

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