Formation and rupture of the tear film

The formation of the preocular tear film successive to each blink is discussed in terms of surface chemical characteristics of the lid-cornea system. The interaction of conjunctival mucin with water and lipids is found to be the most important factor responsible for the spontaneous, rapid spreading of the fluid phases and for the stability of the resulting tear film. The mechanism of the tear film formation can be speculated from the dynamic aspects of mucin-lipid interaction. The transport of mucin to the superficial lipid layer— aqueous tear interface most likely occurs through spreading at the water-lipid interface. Dynamic measurements of the spreading of mucin-lipid mixtures indicate that the spreading of the lipid is the primary event which is then immediately followed by the spreading of the mucin. This mechanism establishes a stable tear film within a second upon the opening of the eye lids and effectively restricts the deleterious protein-lipid interaction. Evaporation—even though retarded by the superficial lipid layer—results in a slow net decrease in the thickness of the tear film. Approximately 5–10 min of continuous evaporation would be needed to eliminate the tear film completely under normal conditions. The thinning tear film would be stable at least down to the thickness of a few hundred Angstroms provided that the corneal surface would remain hydrophilic. Clinical observations show, however, that the tear film breaks up over the cornea within less than a minute and dry spots of increasing area form spontaneously. In vitro experiments suggest that thin, aqueous films over hydrophobic solids, which are confined by a suitable frame, rupture suddenly when the liquid film thins beyond a certain critical thickness a few hundred microns in magnitude. Thus even a thicker-than normal tear film would break up as soon as the corneal epithelium became hydrophobic. Such a decrease in wettability could result from contamination of the mucin adsorbed on to the corneal epithelium by the lipids of the superficial lipid layer. The wettability of adsorbed meibomian secretion and the contact angle of the tear film at the dry spot-tear film boundary support such a hypothesis of dry spot formation.