Basic aspects of contact lens biocompatibility

Abstract

Practically all tissue interfaces that have to be kept moist in the body, such as the mucous membranes, are internal, with the exception of the part of the ocular globe delineated by the palpebral fissure, which is intermittently exposed to the atmosphere between consecutive blinks. This exposed surface, the conjunctival and corneal epithelia have to be coated with a thin fluid layer, the so-called tear film, in order to provide a high optical quality refractive surface and to preserve the well-being of the underlying tissues. An elaborate system exists in the body which ensures the formation and the maintenance of a continuous tear film over the exposed ocular surface. When a contact lens is placed on the eye, two conditions have to be fulfilled to ensure the biocompatibility of the lens. First, the lens should be coated at all times by a continuous tear film complete with the superficial lipid layer. Second, there should be a continuous aqueous tear layer underneath the lens. The oxygen permeability of the lens, a much-stressed requirement, is actually of secondary importance. The thickness of the tear film and that of the pre-lens and the post-lens tear layers fall between 1–10 μm. Thus, the thickness is large as compared to molecular forces, so that the direct interaction between the boundary interfaces can probably be neglected. On the other hand, the film is not thick enough to maintain continuity when the tear—solid interfacial tension is appreciable, so that the tear-film-free energy becomes larger than the surface-free energy of the solid. Under lipid-laden ocular conditions, it is difficult to maintain a low interfacial tension even in the absence of a contact lens. It is thought that the low interfacial tension is maintained in the eye by the presence of a loosely-bound hydrophilic macromolecular layer that can mask lipid contaminants and can be readily removed once it becomes non-functional. The understanding of natural processes which maintain tear film stability over the ocular surface between blinks may enable us to develop better contact-lens materials and contact-lens-wetting solutions that will be more compatible with the ocular environment and will assist rather than hinder the natural processes in maintaining the stability of the pre-lens and post-lens aqueous tear layers and thus improve contact lens biocompatibility.