Effect of tear additives on the shear stress and normal stress acting on the ocular surface

Based on lubrication theory, an elastohydrodynamic model of the human tear film is presented. Using this model we inves-tigate the effect altering the viscosity of the tears has on the stresses acting on the cornea during a blink. In order to model the compliant cornea and eyelid surface, a mattress model is employed. This model is then coupled with the lubrication model of the tear film leading to a one-dimensional nonlinear partial differential equation governing the fluid pressure in the lubrication film. The differential equation is solved numerically using a finite-difference scheme. The results indicate that typ-ical tear additives will lead to an increase in the shear stress acting on the cornea. This in turn may effect the shedding rates of corneal epithelial cells. The model is also of use in predicting the drag force the eyelid exerts on a contact lens and in assess-ing how tear additives may effect the movement and rotation of the contact lens