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Rigid lens materials have played an important role in the development of rigid contact lenses generally and occupy a small but significant place in the range of currently available products. Much of the relevant information is contained in the patent literature, which is analysed in some detail elsewhere ( ).
To appreciate the way in which rigid, as distinct from soft, materials have developed, it is necessary to go back to the period following World War II. The new availability of plastics materials, specifically polymethyl methacrylate (commonly known as PMMA), led to the design and development of the first corneal lens as a replacement for glass scleral lenses. The PMMA lenses were prepared by polymerization of methyl methacrylate with a free-radical initiation system ( Fig. 11.1 ) to form rods or buttons from which a lens was obtained by lathing and polishing. Fig. 11.1 represents the assembly of n methyl methacrylate units to form a PMMA chain n units long. PMMA was an ideal candidate for use as a hard contact lens material because it had a similar appearance and ease of fabrication to glass, acceptable surface wettability and excellent durability. The lenses were compared favourably with scleral lenses in that they were thin and lightweight, could be worn more comfortably and gave excellent visual correction.
By the 1960s a greater appreciation of the effects of contact lens wear on the anterior eye was developing and the fact that PMMA is essentially a barrier to oxygen transport became widely recognized. Corneal physiologists at that time were not only able to carry out theoretical calculations on the effect of contact lenses on corneal respiration but were also able to carry out well-differentiated experiments using PMMA and a very different material, silicone rubber ( Fig. 11.2 ).
Some aspects of the structure and behaviour of polymers, including these two materials, have been outlined in Chapter 4 . The most important property to consider in the context of the present discussion is the difference in their oxygen permeabilities. PMMA is a glassy thermoplastic material, which has advantageous optical clarity, processability and ease of sterilization, but the disadvantage, as a contact lens material, of virtual impermeability to oxygen. Silicone rubber, on the other hand, belongs to a group of materials known as synthetic elastomers, which are not only flexible but also show rubber-like behaviour (i.e. they are capable of being compressed or stretched and when the deforming force is removed they instantaneously return to their original shape). They consist of polymer chains that possess high mobility and are cross-linked at intervals along the polymer backbones. Because of this chain mobility, oxygen is able to diffuse rapidly through the structure. These polymers have oxygen permeabilities more than 100 times greater than that of PMMA. Silicone rubber is the most significant member of the group, with an oxygen permeability around 1000 times greater than that of PMMA. This extremely high oxygen permeability arises from the backbone of alternate silicone and oxygen atoms, which confers not only great freedom of rotation but also a much higher solubility for oxygen than is possessed by rubbery polymers with simple carbon backbones.
Silicone rubber lenses, surface treated to give acceptable wettability, were developed in the mid-1960s ( ) and found clinically to have a little deleterious effect on corneal respiration ( ). The problems of maintaining adequate surface properties, which were initially encountered in its routine clinical use, have never been fully overcome, however, and silicone rubber lenses are used only rarely. The uniquely high oxygen permeability of the silicon–oxygen backbone has, however, been harnessed into two distinct types of contact lens material: silicone hydrogels ( Chapter 4 ) and the so-called rigid gas-permeable materials described here. Because PMMA is now essentially redundant as a contact lens material, the term ‘rigid gas-permeable’ is equally redundant. All rigid lenses manufactured today (aside from PMMA) are gas permeable. For this reason, the term ‘rigid lens’ is used throughout this book and is intended to refer to all rigid lenses apart from PMMA, unless the latter is specifically designated.
Elastomers, such as silicone rubber, are in many ways intermediate between thermoplastics such as PMMA and hydrogels such as poly(hydroxyethyl methacrylate). Thus they possess to a degree the toughness associated with the former group of materials and the softness of the latter, and in this sense, they are ideal candidates for contact lens usage. Unfortunately, however, they all possess the same inherent disadvantage: the molecular features required for true elastic behaviour invariably produce polymers with hydrophobic surfaces. All polymers in this group – not only the silicone-based materials – require some form of surface treatment to render them sufficiently hydrophilic for use as contact lenses, but because of the ease of chain rotation, the surfaces slowly revert to their untreated state. This problem is made worse by the virtually instantaneous elastic recovery of the materials, which causes them to ‘grab’ the cornea after being deformed by the blink. This in turn displaces the posterior tear film and leads to lens binding. Despite the attempts to harness almost every available elastomeric material, as witnessed by the patent literature, no true elastomer has been successfully used as a commercial contact lens material.
Once the need for a contact lens material with higher oxygen permeability than PMMA was established, a wide-ranging search began. There was, additionally, some belief that a more flexible material than PMMA would confer enhanced comfort. At first sight, the task of finding an improved material would not seem too difficult as almost all thermoplastics are less rigid and more oxygen permeable than PMMA. Several flexible thermoplastic materials have been suggested as being suitable for contact lens manufacture in the patent literature, but none of these has achieved clinical significance. The most promising results were obtained with poly (4-methylpent-l-ene), a form of which is known commercially as TPX, and with cellulose esters such as cellulose acetate butyrate (CAB). TPX and CAB resemble each other in many ways. Both polymers are less rigid and less brittle than PMMA – they can conveniently be described as ‘tougher’. The oxygen permeability of both materials is appreciably (of the order of 20 times) greater than that of PMMA ( ) and both were capable of being fabricated by moulding techniques, which are inherently cheaper than lathe cutting, this still being the most widely used method of contact lens fabrication in the 1970s. Even the fact that TPX required a surface treatment step and that both materials (especially CAB) lacked the dimensional stability of PMMA did not seem to be inhibitors to their commercial future. The fact that they became, fairly quickly, curiosity materials was largely due to the appearance, and almost instant success, of the silicone acrylates.
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