Abstract
Two centuries ago, Pellier de Quengsy suggested the implantation of a glass plate in the cornea, and the Chevalier de Tadini the replacement of the crystalline with a glass lens. This was the beginning of ophthalmic prostheses. Since, material sciences have made tremendous progress with respect to ocular implantation. As postoperative implant rejection and complications have to be minimized, several requirements on the materials to be selected can be given: In order to minimize stress locations and local tissue deformation, the physical properties of the implants should be similar to those of the replaced tissue. The material should remain stable in time and not degrade and, in some instance, they should be colonizable. The implants should not induce reactions to surrounding tissues, and therefore should be non-carcinogenic, nonallergenic and non-immunogenic. When the material is implanted within the optical path, its optical properties should be similar to those of the replaced tissue. Especially, a high transparency is required in the visible part of the spectrum (400-700 nm) with a low scattering. As the retina and the lens need to be protected from UV radiations and, to a lesser extend, from radiations in the deep-blue, the materials should also present a high degree of absorption at those wavelengths. To avoid potential complications at long term, an absorption as large as that of the eye tissues should also exist in the infrared part of the spectrum. Because light has to be focused sharply on the retina, the implant geometry and index of refraction have to accurately selected. In addition, the surface quality and the homogeneity of the implants have to be as high as possible. Ideally, to respect the natural physiology of the eye, the materials' index of refraction should match that of the tissue it is design to replace. Consequently, whenever possible the implant material should mimic the properties of ocular tissues. Certain glasses and crystals (sapphire) mounted on metal alloys or ceramics fixation devices (haptic), have been used for the fabrication of intraocular lenses and keratoprostheses. However, the most well-known material for ophthalmic implantation is the thermoplastic polymer PMMA (Polymethylmethacrylate) obtained by polymerization of its monomer, methylmethacrylate. Its ocular stability was accidentally demonstrated after the second world war [1]. Ridley took first advantages of this material for intraocular lenses [2] and, a few years later, MacPherson and Anderson for intracomeal implants [3]. Synthetic polymers are made of very large molecules consisting of either three-dimension (bulk) or two-dimension (linear chains) crosslinked repeating monomers. Thermoplastics are organic polymers having in common a long repetitive carbon chain [-C-]. The mechanical properties of these polymers depend principally on the van der Waals intermolecular forces. In some other polymers the various chains are chemically linked to each other by covalent bonds. Carbon, however, is not the only atom possessing the ability to form very large molecule with other atoms. Silicon is another element that exhibits similar properties. The chain is made in this case of repetitive siloxane units [-Si-O-], whereby the molecular weight of the polymer is determined by the chain's length and type of cross-linking. At low molecular weight, a few thousands centistokes (ctsk), and end-crosslinking, polymers are fluid. Because of their viscosity and their ability to repel water, they are often referred to as silicone oils [4]. Gels are another form of matter, an intermediate between solid and liquid, that can also be used for the fabrication of implants. Gels consist of long chain molecules (generally polymers) that are bulk cross-linked to create a topological complex network immersed in a liquid medium. The latter prevents the polymer network for collapsing while the former traps the liquid. Such gels have been found to be very sensitive to external parameters such as temperature, pH, pressure and salt concentration. The rigidity of a gel is function of the number of crosslinks per linear chain composing the bulk. Most ocular tissues, such as the vitreous, a loose low rigidity network made of proteins (<99 % H20), and the corneal stroma (or the sciera), a dense network composed of collagen (<75%H20),could be considered as gels.
