Rigid gas-permeable lenses

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]. 

Table 7 FDA Grouping of Hydrophobic Hard and Rigid Gas-Permeable Lenses... 

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... 

Alexandra J.Taylore, Simon D.R. Wilson, 1994- Post-Lens Tear Film Thinning in Rigid Gas Permeable Lenses , Optometry and Vision Science,Vol.7 2, No. 12, p849-856... 

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. 

Rigid gas-permeable polymers containing perfluorocyclobutane substituents, (IV), were prepared by Schorzman et al. (4) and used in contact or intraocular lenses. 

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. 

The first methacrylate-silicone hybrid to achieve widespread commercialization was introduced by Syntex (now Wesley-Jessen) for rigid gas permeable (RGP) lenses based on the technology disclosed by Gaylord [4]. In the simplest example, methacryloxypropyltris(trimethylsiloxy)silane (1) or similar highly siloxy substituted methacrylates are copolymerized with other methacrylate monomers, by which means the oxygen permeability of siloxanes is combined with the mechanical and optical properties of the methacrylates [5]. 

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. 

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