Water properties of hydrogel contact lens materials: a possible predictive model for corneal desiccation staining

A set of properties of the water contained within hydrogel contact lens materials was determined with the aim of developing a model which would predict the propensity of a hydrogel contact lens material to induce corneal desiccation staining. We postulated that materials containing a larger proportion of water with the properties of bulk water would tend to induce corneal desiccation more readily than materials with the same overall water content but containing a larger proportion of water that interacts strongly with the polymer. The water structure [as measured by differential scanning calorimetry (DSC)] and the permeabilities of water and glucose were determined for a series of commercial hydrogel lenses. Both glucose permeability and DSC measurements are sensitive indicators of water structure and able to distinguish between various materials. To illustrate the potential of our model, the results of a short-term clinical study are presented. Lower levels of staining were noted for a material with a lower glucose permeability and a larger amount of water melting below 0 degrees C than for a control lens, even though both materials were similar in water content and water permeability. Further clinical studies are needed to validate this model.