Rigid Contact Lenses

The rigid contact lenses, as well as the FFP lenses, fit loosely on the cornea and move with the blink more or less freely over the tear film that separates the lens from the corneal surface. The mechanical properties of rigid and FFP contact lenses must be such that any flex on the lens provoked by the blink must recover instantaneously at the end of the blink.

The first widely available contact lenses were made of poly-(methyl methacrylate), which is an excellent optical biomaterial in almost all respects except for its virtual impermeability to oxygen. Several materials that were specially developed for the manufacture of rigid gas-permeable (RGP) contact lenses are copolymers of methyl methacrylate with siloxanylalkyl meth-acrylates (Refojo and Dabezies, 1984). To compensate for the hydrophobic character imparted to the polymer by the high siloxane content of these copolymers (required for oxygen permeability), the copolymer also contains some hydrophilic com-onomers. The most commonly used hydrophilic comonomer in rigid lenses is methacrylic acid. There are also minor ingredients and cross-linking agents. A diversity of RGP contact lenses, consisting of different but closely related comonomers used in a variety of proportions to obtain the most desirable properties, are commercially available (Table 2). However, any subde change in the chemistry of a contact lens material might strongly affect its clinical performance. As a general rule, the oxygen permeability coefficient of the siloxanylalkyl methacrylate contact lens materials is inversely proportional to the density.

The development of the fluorine-containing contact lenses and the realization that the fluoroderivatives may improve oxygen permeability and resistance to deposit formation caused contact lens chemists to include a fluoroalkyl methacrylate or a similar fluorine-content monomer as an additional ingredient in the siloxanylalkyl methacrylate-comethyl methacrylate RGP contact lens materials. These perfluoroalkyl-siloxanylalkyl-methyl methacrylate contact lenses have high oxygen permeability and, supposedly, better surface properties than the non-fluorine-containing rigid contact lenses.

Cellulose acetate butyrate (CAB) is also used as a rigid oxygen-permeable contact lens material. However, CAB not only has relatively low oxygen permeability compared with the siloxanylalkyl methacrylate copolymers but also has low scratch resistance and tends to warp with humidity changes.

Other copolymers useful as contact lens materials are iso-butyl and isopropyl styrene, with hydrophilic comonomers of the HEMA or vinyl pyrrolidone type.

Polymacon 38

Ocufilcon A 44

Ocufilcon C 55

Etafilcon A 58

Tetrafilcon A 43

Perfilcon A 71

Bufilcon A 45

Bufilcon B 55

Crofilcon A 39

Lidofilcon A 70

Lidofilcon B 79

Vifilcon A 55

TABLE 2 Composition of Some Rigid Gas-Permeable Contact Lenses

Polymer

USAN11

Cellulose acetate dibutvrate

S-[3,3,5,5,5-pentamethyl-Ll-bisfpentamethyldisiloxanyboxyl t ri si lox a n y i| p ropy I methacrylate, with methyl methacrylate (MMA'i, methacrylic acid (MAA) and tetraethvleneglycol dimethacrylate (TECDMA)

MM A with MAA, EGDMA, 3(3,3,3,-trimethyl- f, I -bis(trimethylsiloxy)disiloxanyIj propyl methacrylate (TRIS) and N-(l,l-dimeth y 1-3 oxybutyl )acrylam ide.

VP with HEMA, TRIS, allyl methacrylare and u-methacryloyl-oi-imethacryioxy) polyioxyettiyleue-co-oxy(dimethylsilylenc)-co-oxyethyienc.

TRIS with MM A, dimethyl itaconate, MAA and TEGDMA.

TRIS with 2,2,2,-rrifliioro-l-(triihioromethyl) ethyl methacrylate, 1 vinyl-2-pyrrohdone (VP), MAA and cthyletieglycol dimethacrylate (EGDMA),

I RIS with 2,2,2-trifluoroethy! methacrylate, MAA, MM A, VP with EGDMA.

Poro foco n C a bu foco n

Silafocon

Ne foco n

Mesifilcon Itafocon

Mela foc on Pa flu foco n adopted name corneal surface and is reepitheiialized with the recipient corneal epithelium (Werblin et a!., 1987). A modification of this technique attempts to obtain similar results with an artificiaf material that would heal into the donor cornea and be able to grow the epithelium of the donor cornea on its surface (Fig, 2).

An epithelium that has become irregular through swelling and proliferation has been replaced by an artificial epithelium made of a hard plastic contact lens glued with a cyanoacrylate adhesive to the corneal stroma (Fig. 2). This procedure has not been successful mainly because of failure of the glue to maintain a tight attachment of the prosthesis to the corneal stroma and also because of epithelial penetration between the prosthesis and the cornea.

0 0

Post a comment