Hard lenses

Hardgrove index Hard lenses Hard metals Hardness... 

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. 

Hard lenses can be defined as plastic lenses that contain no water, have moduli in excess of 5 MPa (500 g/mm ), and have T well above the temperature of the ocular environment. Poly(methyl methacrylate) (PMMA) has excellent optical and mechanical properties and scratch resistance and was the first and only plastic used as a hard lens material before higher oxygen-permeable materials were developed. PMMA lenses also show excellent wetting in the ocular environment even though they are hydrophobic, eg, the contact angle is 66°. 

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... 

These are used to hydrate the surfaces of hard lenses after disinfection. Since they must also cope with chance contamination, they mnst contain a preservative as well as a wetting agent. They may be isotonic with lacrimal secretions and be formulated to a pH of about 7.2 for compatibility with normal tears. 

Most everyone is familiar with the contrasting resilient, pliable nature of soft contact lenses and the brittle character of hard lenses. Both lenses are constructed from polymers, but the differing chemical composition of each polymer leads to considerably different physical properties. 

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. 

Hard lenses are made from gas-permeable materials that contain silicon and fluorine. They can correct most optical defects and have diameters of 8-10 mm. 

Some lenses combine soft and hard materials, including hard lenses with soft surfaces and lenses with a hard center and a soft periphery. 

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. 

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,... 

Soft lenses differ from PMMA hard lenses in several aspects. Their main feature is the capacity to absorb water, which allows softness and permeability, making them considerably more comfortable than hard lenses. Wichterle and Lim [22] obtained the first soft contact lenses in the late 1960s. They produced the hydrophilic monomer hydroxyethyl... 

So-called soft contact lenses are actually made from a polymer hydrogel with a composition of up to about 50% water (the older hard lenses were a solid piece of plastic). If you are a contact lens wearer, you will know what happens when you leave a soft lens to dry out it shrinks and becomes hard like plastic. Dried up lenses will regain their former shape if soaked in water as the polymer network swells and refills with water. Some of the most current contact lens technologies, such as overnight or continuous wear lenses, are based on silicone hydrogels (polymers with a silicon backbone). 

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