Abstract

Contact Lenses Contact lenses are small, hemispherical-shaped optical devices placed in contact with the transparent tissue at the front of the eye called the cornea. Contact lenses are used to correct vision deficiencies such as myopia (nearsightedness), hyperopia (farsightedness), presbyopia (loss of near focusing power with age), and astigmatism. An increasingly common use of contact lenses is to change the color of a normal eye or to improve the appearance of an eye disfigured by injury or disease. Contact lenses can also be used as therapeutic devices in the medical treatment of certain eye diseases or injuries. The 1980s saw the widespread introduction of lens products made of more oxygen-permeable materials, ie, rigid gaspermeable (RGP) materials that made PMMA lenses virtually obsolete and high water content hydrogels that competed with HEMA-based lenses. Research continues for materials having improved oxygen permeability, deposit resistance, and comfort. In addition, there is the search for breakthrough lens designs, such as bifocal contact lens products. Competition includes developing new surgical techniques, eg, excimer laser refractive surgery that modifies the shape of the cornea to correct vision, potentially without the use of eyeglasses or contact lenses. No particular contact lens type or product is considered universally superior. To remain safe and efficacious on the eye, contact lenses must maintain clear and wetted surfaces, provide an adequate supply of atmospheric oxygen to and adequate expulsion of carbon dioxide from the cornea, allow adequate flow of the eye's tear fluid, and avoid excessive abrasion of the ocular surface or eyelids, all under a variety of environmental conditions. Clinical experience has shown that certain types of lens materials are more prone to deposit problems. All contact lenses can be divided according to the wearing modality. It is convenient to classify lenses according to whether they are rigid (hard) or flexible (soft). Some newer materials give rise to lenses that can be termed semirigid, semisoft, and the combination soft–rigid. The flexibility of a contact lens material dictates, to a large extent, the design of the lens. Not every polymer can be manufactured successfully into a contact lens. Several important properties for both ocular physiology and patient handling are required of a material for a contact lens application. Various lenses are discussed: rigid (hard) lenses; hydrogel contact lenses (includes silicone and fluorohydrogels); flexible nonhydrogel lenses; Tinted lenses; and surface-modified lenses. Today, most contact lens materials, including xerogels, ie hydrogels in their dry state, can by produced in a disklike form (button) that allows lathe cutting for accurate manufacture of contact lenses. Cast molding is an increasingly used manufacturing process for both rigid gas-permeable and hydrogel contact lenses.