Differential effects of trabecular meshwork stiffness on outflow facility in normal human and porcine eyes

Purpose. The study was designed to determine trabecular meshwork (TM) stiffness and its relationship to outflow facility (C) in perfused normal human and porcine eyes. Methods. Human and porcine eyes were perfused at pressures of 10, 20, 30, and 40 mm Hg to determine C and how outflow resistance (R = 1/C) varied with the pressure. Following perfusions, TM tissue segments were dissected and stretched uniaxially to determine the circumferential bulk Young's modulus (E). The statistical significance of difference between different groups was evaluated using a two-tailed Student's t-test or Mann-Whitney U test. Results. A larger E correlated with a higher C measured at 10 and 20 mm Hg (P < 0.05), and a similar trend was observed at 30 and 40 mm Hg in human eyes (n = 7). Additionally, a higher C correlated to a lower variance of R, and a stiffer TM correlated to a lower variance of R in human eyes (P < 0.05). For porcine TM, E was inversely correlated to a cross-sectional area (P < 0.003, n = 11), and its value (24.9 and 1.5 kPa; geometric mean and geometric SE) was lower than E of human TM (515 ± 136 kPa; mean ± SE) (P < 0.01). C and variance of R were not significantly different between the species. Conclusions. A higher circumferential stiffness of the TM correlated with a higher outflow facility and less IOP elevation-induced variation in outflow resistance in normal human eyes, but not in porcine eyes. For future studies, these correlations need to be evaluated in glaucomatous eyes to better understand normal and abnormal TM functions.