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Lucinda Camras, W Daniel Stamer, David Epstein, Pedro Gonzalez, Fan Yuan; Circumferential Trabecular Meshwork Stiffness in Glaucomatous Eyes. Invest. Ophthalmol. Vis. Sci. 2013;54(15):50. doi: https://doi.org/.
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Our previous work indicated that a larger bulk Young’s modulus (E) of trabecular meshwork (TM) correlated with a higher outflow facility (C) in normal donor eyes (Camras, et al, IOVS. 2012). This study investigated the influence of TM stiffness on C in glaucomatous donor eyes.
The left eyes of eight glaucomatous donors were perfused within 48 hrs postmortem time (PMT). With the exception of cataract surgery, there were no previous ocular surgeries. Outflow rate was measured at pressures of 10, 20, 30, and 40 mmHg to determine C and how outflow resistance (R=1/C) varied with pressure. The TM was then isolated and cut into 5-10 mm segments. Optical coherence tomography was used to determine cross-sectional area of the segments. Uniaxial tensile stress was applied longitudinally to TM segments at a rate of 0.1 % stretch per second to generate stress-strain curves. E was calculated at 0.0% strain to represent the circumferential stiffness of the TM at a relaxed state. Regression analysis was used to determine statistical significance of correlations.
Glaucomatous eyes had lower outflow facilities than normal eyes, which reduced (0.17 ± 0.02 to 0.11 ± 0.02 uL/min/mmHg; arithmetic mean ± SE, n=8) with increasing pressure from 10 to 40 mmHg. The distribution of E was right-hand skewed and logarithmically converted for linear regression analysis. The geometric mean and standard error of E was 121 & 1.4 kPa. The glaucomatous TM was about 5 times softer than normal TM (n=7) observed in our previous study and showed a similar correlation between stiffness and C. A larger E correlated to smaller cross-sectional area (p<0.03). There were no significant correlations between E and PMT, age, C, or variance of R.
Glaucomatous TM showed a greater variability in stiffness and a lower average stiffness compared with normal TM. Additionally, E was dependent upon its cross-sectional area, indicating the stiffness heterogeneity within layers of glaucomatous TM. Due to its variability, there were no correlations between stiffness and outflow facility in glaucoma. However in combination with our previous data, it appears that softer TM led to impaired outflow, potentially due to an increased risk of Schlemm’s canal collapse. Prospective studies are needed to confirm the influence of TM stiffness on outflow regulation.
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