Stone, R. Contact Lens Spectrum 1988; 3(12): 38-41.
The paper entitled Why Contact Lens Groups? Written by Ralph Stone is an attempt to explain the evidence that led to the separation of contact lens materials into four separate groups and to offer additional comments on the earliest work with this grouping system. In his paper, he states that the concept of lens grouping was first presented as a part of the July, 1985, FDA Draft Guidelines for Testing Contact Lens Care Products. These guidelines resulted from collaboration between the FDA and contact lens industry, which was facilitated by the Contact Lens Institute and the Contact Lens Manufacturers Association.
It is outlined in the paper that the monomers used in conventional contact lens polymers can be categorized into three classes: hydrophilic monomers to interact with water to form the basic hydrogel component; hydrophobic monomers to add mechanical strength; and crosslinking agents to increase mechanical strength and thermal stability. The author states that it is the hydrophilic monomers which are largely responsible for the varied performance found between materials and individuals. For hydrogel lenses, the main hydrophilic monomers which are used alone or in combination are hydroxylethyl methacrylate (HEMA), glyceryl methacrylate (GMA), vinyl pyrrolidone (VP), and methacrylic acid (MA).
With respect to the separation of lenses into groups, the primary mechanism was based on lens hydration, which is primarily related to the ionic nature of the material. As described in the paper, a non-ionic polymer is a molecule which can interact with other polar molecules, such as water, without resulting in a formal charge. The well-known example of a non-ionic polymer is HEMA. For an ionic material, a monomeric acid, usually MA, is added to result in a charged molecule, which increases lens hydration. Ionic materials are sensitive to changes in pH and osmolality, which subsequently makes them more sensitive to the components in a lens care system compared to non-ionic materials. Ionic materials have also been shown to demonstrate a greater interaction with the ocular environment, such as increased uptake of lysozyme and other positively charged ocular proteins.
The second mechanism described for the separation of lenses into groups was related to water content. Low water non-ionic content lenses, between 38% to 45%, typically contain HEMA, HEMA-VP mixtures or GMA. High water non-ionic content lenses, between 70% and 79%, generally contain VP-based polymers. The addition of hydrophobic monomers and crosslinking agents tends to result in a decrease in water content, despite the benefit of increased polymer strength. Therefore, different concentrations of monomeric acid are added to increase the water content, resulting in both low and high water ionic materials. The addition of greater amounts of monomeric acid increases water content and subsequently leads to a heightened interaction with tear proteins as well. The end result is a four group system, with lenses separated into ionic and non-ionic groups and further subdivided according to their water content. As reported in the paper, research using these lens groupings proved to substantiate this system.
What is apparent from this paper is that the development and use of this grouping system was not an easy task and required a great deal of collaboration between the contact lens industry and contact lens community. Silicone hydrogel lenses represent a new family of biomaterials with very unique material properties, differing greatly from conventional hydrogels with respect to their polymer bulk chemistry and surface characteristics. It has become all too apparent that they do not interact in the same way as conventional hydrogels with current lens care products. In light of this, with a growing number of silicone hydrogel lens materials and lens care products emerging, the creation of a separate grouping system for silicone hydrogel lenses could prove to be a very useful and worthwhile endeavour.