Mechanism of tear film rupture and formation of dry spots on cornea

The rupture of the tear film covering the cornea and the formation of dry spots is of importance in various pathological states associated with a dry eye. The purpose of this paper is to identify its mechanism and to compute the time required for such a rupture. A two-step, double-film model is proposed which accounts for the real structure of the tear film as composed of a mucous layer and an aqueous layer. The mechanism identifies the key step in the process of rupture to be the instability and eventual rupture of the mucous layer of about 200 to 500 Å thickness, which covers the epithelium, due to the dispersion interactions. This in turn exposes the aqueous layer of the tear film to the hydrophobic cornea, resulting in the observed end result of a spontaneous tear-film rupture or "beading-up." The proposed mechanism is shown to be consistent with the range of observed breakup times and other clinically observed characteristics of the tear-film rupture. The implications of this mechanism in the normal wearing and tolerance of contact lenses are also indicated