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CONTINUING EDUCATION
In the first of a three-part series concerning lens materials, Dr Karen French looks at a property with a significant influence over lens performance and which should be considered when selecting and assessing contact lenses. CET Module C1807
THERE ARE MANY properties that need to be considered when developing a contact lens material. Optical clarity, dimensional stability, oxygen permeability, wettability and physiological compatibility are all important parameters in determining the success of any new material. The importance of the surface characteristics of materials used in the manufacture of contact lenses should not be underestimated. They govern the interaction of a material with the tear film and any tendency the material may have to become deposited during wear. More importantly, the nature of the material surface can have a vital effect on comfort, wearing time and clarity of vision.
In order to achieve good vision, a stable uniform tear film must be supported over the front surface of a contact lens. A lens that does not have good wetting characteristics will result in a rapid break up of the pre-lens tear film and a consequent reduction in vision quality. A stable pre-lens tear film also provides a lubricating effect, allowing comfortable lid movement over the front surface of the lens. Contact lens material surface characteristics also govern biocompatibility. A wettable contact lens material is more likely to allow a continuous tear film between the back surface of the lens and the corneal epithelium, without which no material can be considered as biocompatible.1 A contact lens surface with poor wettability has a greater tendency to attract tear-film deposits. As the tear film dries out due to evaporation between blinking, the dry spots form areas prone to deposit formation, especially protein, and this in turn further reduces surface wettability. The clinical performance of any contact lens material is governed by its ability to form a stable pre and post-lens tear film, which in turn is governed by its wettability.
Wettability can be thought of as the formation of a continuous fluid film over a solid surface. More specifically it is defined as the adhesion of a liquid to a solid.
Cohesion and adhesion2,3
Cohesion refers to the force of attraction between molecules of...