Types of Contact Lenses and Their Characteristics1
3 ■ Types of Contact Lenses and Their Characteristics1 The basic categories of contact lenses currently acetate butyrate [CAB], PMMA-silicone, and in use are the original hard (polymethylmethacry- silicone). There are variations within each group, late [PMMA]) lenses, soft hydrogels (hydro- but their respective properties are similar enough xymethylmethacrylate [HEMA] and other mate- to consider them as essentially the same type of rials), and gas-permeable hard lenses (cellulose lenses. HARD PMMA LENSES As discussed in the previous chapter, hard rial and safe for use in the eye. It can be molded PMMA lenses were the first lenses introduced or lathed into lenses with a high degree of preci- onto the market, and corneal PMMA lenses simi- sion. Once made, PMMA lenses can be reworked lar to those currently in use have been available and modified to customize them to an individual’s since the early 1950s. Compared to later types of requirements. The result is a safe lens of excellent lenses, PMMA lenses can be difficult to adapt to, visual properties which very closely conforms to and perhaps as many as half of the people fitted patient requirements. PMMA lenses require min- do not become long-term wearers. Since PMMA imal use of cleaning, soaking, and wetting solu- is not water- or gas-permeable, wearers must rely tions. They may be tinted to reduce excessive light on the “tear pump” action of the eye to provide sensitivity, to make them easier to find when oxygen to the covered portion of the cornea. As dropped, or for appearance reasons. They are the wearer blinks, tear interchange occurs from durable, can be renewed by polishing away mi- outside the lens to beneath it, providing the nec- nor surface scratches, and often last 5 to 7 years essary oxygen.
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