The dynamics of tear flow within a phenol red impregnated thread.

A method for the laboratory production of a phenol red impregnated thread is described. Using the thread, tear flow was measured in the right eye of 2 groups of normal subjects: group I, n = 20, average age 26.2 years (+/- 5.6); and group II, n = 20, average age 56.4 years (+/- 13.8). Data were collected by placing a thread over the lower lid margin for a time period of 15 s. A further seven threads were inserted at 5 min intervals for periods increasing by 15 s up to 120 s. A significant difference in mean thread wetting between the two age groups was found for thread insertion times of 30, 60 and 90s. For each eye, the rate of thread wetting was exponentially related to the period of thread insertion. Thread wetting and the constant of the exponential were not found to be gender dependent. Simulating the tears in the lower fornix, using a bath of saline, it was concluded that the exponential constant was a property of thread material and not tear fluid. For practical purposes, the rate of thread wetting reached equilibrium by 120 s where mean total thread wetting was 19.5 mm (+/- 6.5). The typical flow of tears y in mm/s, along the phenol red thread can be described by: y = 0.16 + 0.47e-0.0283t where t is the period of thread insertion.