Tear-film considerations with and without contact-lens wear

The pre-ocular tear film coats the surface of the eye or contact lens, providing lubrication, a smooth refractive surface for vision, and protection against disease. When the eye is open during the interblink period, the tear film remains intact in normal healthy eyes. Through numerical modeling, we demonstrate how the regions of localized thinning at the lid margins contribute to the stability of the tear film. These regions, called “black lines,” keep the tear film trapped on the cornea for extended periods, and lid action is required to disturb the resulting “perched” tear film.; Thus, it is the action of the lid sweeping across the cornea that initiates the tear-refreshment mechanism. Researchers have studied tear flow and drainage using sodium fluorescein. We repeat the physiologic tear-turnover measurements using higher molecular weight FITC-dextran. The results give a much lower tear exchange than reported previously, suggesting that the use of sodium fluorescein affects the mixing measurement.; The presence of a contact lens on the eye formally splits the aqueous tear film into a pre-lens tear film and a post-lens tear film (PoLTF). The blink still plays a role in tear removal from behind the lens. Lid-induced, in-out, and up-down lens motions causes tear mixing in the PoLTF. We give clinical evidence supporting the motion-assisted diffusion model for fluid exchange in the PoLTF. When blinking stops, e.g. during sleep, there is a build-up of metabolic debris under the lens. If the debris, sandwiched between the contact lens and the cornea, is not quickly removed upon waking, each movement of the lens mechanically agitates the epithelium. For some contact-lens wearers, this process may induce adverse responses.; We hypothesize that timely removal of debris following overnight contact lens wear is a requirement for safe extended-lens wear. In this report, we use currently available information to construct a model that describes the basic physics of tear mixing, we examine lens parameters that affect tear exchange, and we use theoretical and empirical data to suggest new directions for optimizing tear flow under a soft contact lens. Specifically, we test the hypothesis that fenestrations and channels in soft lenses increases tear mixing in the PoLTF.