Abstract
Purpose: :
Trabecular meshwork (TM) stiffness has been postulated to affect outflow facility (C) and outflow obstruction coefficient (Q), but has yet to be demonstrated experimentally; we tested this hypothesis in the present study using perfused normal human and porcine eyes.
Methods: :
Seven human globes were perfused within 48 hours postmortem (PMT) at pressures of 10, 20, 30, and 40 mmHg to determine C and Q. The TM was then isolated and cut into 1-2 cm segments. Optical coherence tomography was used to determine cross-sectional area of the TM segments. Uniaxial tensile stress was applied longitudinally to TM segments at a rate of 0.1 % stretch per second to generate stress-strain curves. The bulk Young’s modulus (E) was calculated at 0.0% and 0.5% strain to represent the circumferential stiffness of the TM at a relaxed state and when Schlemm’s canal (SC) is collapsed. Results from the human data were compared to eleven porcine eyes (perfused within 6 hours PMT) tested under identical experimental conditions. Regression analysis was used to determine statistical significance of the relationships. Normally distributed data were analyzed with two-tailed Student t-test, while non-normally distributed data were analyzed with Mann-Whitney U tests.
Results: :
In human eyes (n = 7), a larger E correlated with a higher C at 0.0% (p < 0.05) and 0.5% (p < 0.1) strain. Additionally, a larger C correlated to a lower Q (p < 0.05). In human eyes lower Q correlated to a thicker and stiffer TM at 0.0% (p < 0.07) and 0.5% (p < 0.05) strain. There was no significant correlation between PMT and age. Porcine eyes (n = 11) showed a similar trend between Q and E at 0.0% and 0.5% strain (p < 0.06) and a significant correlation between a larger cross-sectional area and a lower E at 0.0% and 0.5% strain (p < 0.005). E at 0.0% (p < 0.01) and 0.5% (p < 0.005) strain were more than 15-fold higher in human (515 ± 136 kPa and 571± 129; mean ± SE) than porcine (24.9 ± 13.1 kPa and 29.5± 15.2) eyes, respectively. C and Q were not significantly different between species.
Conclusions: :
Bulk stiffness of soft-tissue is dependent upon the magnitude, location, direction, and velocity of the applied forces. We hypothesize that a circumferentially stiffer TM prevents outflow obstruction by reducing the collapse of SC and facilitates proper separation in the JCT to aid outflow in normal human eyes. Based on the organization of the aqueous plexus in porcine eyes, stiffer TM may only prevent collapse and obstruction, but not aid JCT separation unlike human eyes. Future studies investigating the TM stiffness under different stress conditions and in glaucoma eyes may provide new insight into TM function and IOP homeostasis.
Keywords: outflow: trabecular meshwork • trabecular meshwork • anterior segment