Modeling the vertical motion of a soft contact lens

Purpose. The motion of a soft contact lens in the up-down or vertical (inferior-superior) and in-out (anterior-posterior) directions drives mixing in the post lens tear film.(1) Thus, it is important to obtain an accurate assessment of lens motion. The commonly used experimental technique to measure the vertical motion, video microscopy, only gauges motion during the interblink period. Since most of the eyelid force, which drives the motion, is exerted during the blink, it is reasonable to assume that the majority of the lens motion in the up-down direction takes place during the blink and is, therefore, hidden from view. Thus, experimentally measured values of vertical lens travel are currently underpredicted. Methods. In this paper, we use a simple mechanical force balance on the lens to predict its motion in the vertical direction. The forces included in our model are due to the upper and the lower eyelids, gravity, elasticity, and viscous stresses. We input the lens physical parameters, the various tear film thicknesses, and the upper eyelid velocity. Then we integrate a macroscopic force balance to obtain the lens vertical position as a function of time. Results. The proposed model predicts that the downward lens motion during a blink is about 2-3 times the downward motion during the interblink (centration). Conclusions. The up-down motion observable during the interblink period is only a small fraction of the total lens vertical travel.