Gas-permeable lens

Silicone Acrylates. The development of rigid gas-permeable lens materials advanced significantly after the development of polysiloxanylaLkyl acrylates and methacrylates (1), as a component in hard lens materials (56,57), as claimed in a series of patents (58—62). 

Table 1. Partial List of Rigid Gas-Permeable Lens Materials...

Uses Preservative for cosmetics, in hard/rigid gas-permeable lens prods. 

Uses Antimycotic mold inhibitor, yeast inhibitor, bactericide, antimicrobial, preservative in cosmetics, foods, animal feeds, wines, pharmaceuticals OTC dmg active in hard/rigid gas-permeable lens prods. in food pkg. migrating to foods from paper/paperboard... 

The number of contact lens wearers has grown to an estimated 24 million in the United States and 50 million worldwide. Concurrendy, there has been a proliferation of contact lens manufacturers and products. The 1980s saw the widespread introduction of lens products made of more oxygen-permeable materials, ie, rigid gas-permeable (RGP) materials that made PMMA lenses virtually obsolete and high water content hydrogels that competed with HEMA-based lenses. 

Cast molding is an increasingly used manufacturing process for both rigid gas-permeable and hydrogel contact lenses. In this process, two molds, made from a variety of plastics, are used. A female mold forms the lens front surface (convex) and a male mold forms the lens back surface (concave). The plastic molds are made from metal tools or dies that are usually stainless steel, precision lathed, and polished to the specified lens design. A variety of mold materials are used. The polymerized, hardened lens is released from the mated molds and is processed in much the same way as the spin-cast lenses described above. 

Grouping for rigid gas-permeable lenses was published by FDA in 1989. The generic names and oxygen permeabilities of rigid gas-permeable (RGP) lens materials are provided in Table 7 [310]. 

Hard contact lenses are composed of a polymer that repels water because the constituent repeating units (the monomers that link together to form the polymer) are nonpolar, hydrophobic segments. The first hard contact lens was constructed in 1948 from the monomer known as methyl methacrylate (MMA), yielding the polymer poly(methyl methacrylate) or PMMA. This material offers durability, optical transparency, and acceptable wettability for optimal comfort. Today the rigid lens material of hard contact lenses is often constructed by combining MMA with one or more additional hydrophobic monomers to provide better gas permeability. 

Contact Lens Fitting and Management Fluorescein staining of the tear film is a major aid in the fitting of rigid gas-permeable contact lenses.After topical application of fluorescein to the eye the tear layer becomes visible, with a characteristic pattern of green fluorescence. Observation of the fluorescein-stained tear film with an ultraviolet light or the cobalt blue filter of the slit lamp allows determination of the fit of the lens. 

Figure 25-36 Pseudopterygium associated with long-term use of a rigid gas-permeable contact lens.

Figure 26-55 Giant papillary conjunctivitis secondary to rigid gas-permeable contact lens wear. (Courtesy of Pat Caroline.)...

AM = morning CL = contact lens Conj = conjunctival D/C = discontinue RGP = rigid gas permeable SPK = superficial punctate keratitis. FromAUansmith MR, Korb DR, Greiner jy et al. Giant papillary conjunctivitis in contact lens wearers.Am J Ophthalmol 1977 83 700. 

Solvents such as isopropyl alcohol have been incorporated in daily cleaners to aid in removing lipid type of deposits. Such solvents have been reported to affect certain lens materials, especially silicone acrylate rigid gas-permeable lenses. 

LENS CARE PRODUCTS FOR RIGID GAS-PERMEABLE LENSES... 

Solutions formulated to clean hard gas-permeable contact lenses are sterile and pH buffered. They are soaking solutions and include lens-hydrating (wetting) agents (e.g., cationic cellulose derivatives) and preservatives (e.g., edetate disodium, chlorhexidine gluconate). 

Lenses created by leave-behind are generally oriented parallel to the local direction of flow (i.e., the pore-level flow that created them), and do not make the gas phase discontinuous. If leave-behind is the only form of lens or lamella generation, a continuous-gas foam results. Ransohoff and Radke (60) found that foam generated solely by leave-behind gave approximately a five-fold reduction in steady-state gas permeability, whereas discontinuous-gas foams created by snap-off resulted in a several hundred-fold reduction in gas mobility (20, 61). 

Fig. 7. Spectrum of Orthokeratology Using Gas Permeable Rigid Contact Lens...

Piggyback lens systems first emerged in the late 1960s. Westerhout, a pioneer of the system, first published a paper on their use in 1973. The system consists of a rigid gas permeable (RGP) CL worn on top of an SCL liying directly on the cornea (Figure 51.8).i2 ... 

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