The influence of protein deposition on contact lens tear film stability

When contact lenses (CLs) are worn, they are subject to deposition of the surrounding biomolecules found in the tear film (TF) of the eye. There is a correlation between protein deposition on CLs and feelings of discomfort in patients, but it has not been well understood if these feelings of discomfort arise solely from immunogenic reactions to the protein deposits or a physical instability of the tear film on protein-fouled CLs. This study compared two hydrogel CLs: etafilcon A (polyhydroxyethylmethacrylate-based hydrogel) and senofilcon A (silicone hydrogel with internal wetting agent) to elucidate how lysozyme and mucin sorption affect the wettability of CLs and understand the potential impact on TF stability in vivo. Here, we use "wettability" to refer to the stability of a film of phosphate buffered saline on the CL surface. A custom-built platform was used to conduct experiments that monitored the stability of phosphate-buffered saline (PBS) and artificial tear solution (ATS) on clean and fouled CLs. PBS was more stable (wettable) on etafilcon A than senofilcon A, and both CLs showed increased wettability after protein-fouling. However, surface wettability in PBS did not correlate with the stability of ATS on the CLs. The viscoelastic interface of ATS slowed drainage, making evaporation the primary thinning factor, in addition to the presence of a disjoining pressure that stabilized the thin film. From this, we conclude that protein deposition increases CL wettability, but does not alter tear film stability and we predict that CL susceptibility to evaporation is a better predictor of TF stability than wettability.