Topographic Distribution Of Blink Induced Shear Stress (BLISS) On The Corneal Surface

Abstract

Abstract: : Purpose: Inner surface of the upper eyelid wipes over the corneal surface during blink action. The force facilitating the eyelid motion causes laminar shearing of the underlying tear film resulting in BLISS on the corneal surface. Previously, the magnitude of BLISS was shown to be 14 dynes/cm2 when averaged over an entire blink interval based on solutions to unsteady state Navier-Stokes equation (Mutharasan et al., ARVO; 2001) for both Newtonian and Non-newtonian flow. Since the eyelid motion entails acceleration and deceleration, the magnitude of BLISS is variable along the locus of the eyelid motion on the corneal surface. In this study, we have determined the temporal averages of BLISS on the corneal surface to predict locations of maximum shear stress and hence sites of potential ocular surface damage. Methods: Velocity profiles for the upper eyelid motion were assumed to follow models for the eyelid kinetics by Berke and Mueller (Adv Exp Med Biol; 438:417-24, 1998). Tear film was assumed to consist of two layers: (1) Watery layer of 6 mm thickness and a Newtonian viscosity of 1 cP, and (2) Mucous layer of 1 mm thickness and a Newtonian viscosity of 1.7 cP. Results: (1) Instantaneous BLISS was found to be maximum (242 and 121 dynes/cm2 for downward and upward motions, respectively) at 0.5 cm from the start of the eyelid motion (called start position). This position corresponds to the location of the maximum blink velocity for both downward and upward motions. (2) The time integral of local BLISS over a full blink cycle (TI-BLISS) was found to be maximum at x = 0 (15 dynes.sec/cm2) at the start position and decreased linearly to 0 at x = 1 cm. (3) On the other hand, time-averaged local BLISS during downward motion over the time of shear exposure (TA-BLISS) varied from 160 dynes/cm2 at x = 0 to 0 dynes/cm2 at x = 1 with a maximum of 175 dynes/cm2 at x = 0.2 cm. Similarly, TA-BLISS for upward motion varied from 80 dynes/cm2 at x = 0 to 0 dynes/cm2 at x = 1 with a maximum of 89 dynes/cm2 at x = 0.2. Conclusion: The maximum TA-BLISS occurs NOT at the point of maximum velocity (i.e., at x = 0.5 cm) but at 0.2 cm from start of the eyelid motion. Excessive shear stress, therefore, is likely to manifest above the equator and 0.2 cm from the start position of the blink. If the blinks are NOT full, the damage may manifest at x < 0.2 cm. Furthermore, x=0.2 is also vicinity for maximum static friction.

Keywords: 370 cornea: basic science • 410 eyelid • 437 inflammation