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Mark Johnson, Joel S Schuman, Larry Kagemann; Trabecular Meshwork Stiffness in the Living Human Eye. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3541. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Trabecular meshwork (TM) stiffness is altered in cadaver glaucomatous eyes as compared with normal eyes (Last, IOVS, 2011; Camras, IOVS, 2012). However, TM stiffness in-vivo is unknown. We here use optical coherence tomography (OCT) to visualize change in dimensions of the TM and Schlemm canal (SC) as IOP is increased, and then use these values to estimate the in-vivo elastic modulus of the TM.
The temporal limbus of 38 eyes of 21 healthy subjects (23-68 years) was imaged by spectral-domain OCT (Cirrus HD-OCT) at baseline and during IOP elevation (ophthalmodynamometer). After scanning, IOP was measured at baseline and during IOP elevation by Goldmann applanation tonometry. TM and SC cross-sectional area and thickness were measured at 5 locations within a 1 mm length of SC using ImageJ. An analytical model of beam-bending under uniform load was applied to estimate the modulus of the TM based on changes in TM and SC thickness.
Baseline IOP (12±2.3 mmHg; mean±SD) was increased to 18±3.2, 21±3.5, and 38±5.7 mmHg with application of 5, 10, and 30g of force, respectively. Cross-sectional area, length and thickness of SC decreased with increasing IOP (p<0.0001), occurring primarily at IOPs higher than 20 mm Hg. Thickness of SC decreased from a baseline of 20.7±0.9 µm to 16.4±0.7 µm after application of 30 grams force to the eye. In contrast, no apparent effect of increasing IOP was seen in TM thickness for any of the forces applied. These data were inconsistent with a model of IOP-generated collapse of SC caused by 1-dimensional expansion of the TM (Fig 1B). Instead, the data were consistent with TM deformation described by 2-dimensional beam-bending where the anterior and posterior aspects of the TM are held in place by the edges of SC and the central aspect collapses into the canal as IOP is raised (Fig 1C). Use of a beam-bending model to describe this deformation allowed us to calculate an average elastic modulus of the human TM in-vivo of 128 kPa.
Collapse of SC with increasing IOP while TM thickness changed little suggests that the TM acts as a membrane under tension. Ciliary muscle likely controls this tension and thereby affects the effective modulus of this tissue. The value we measured for modulus is between that measured in cadaveric eye by Last et al and that by Camras et al. Future studies should examine this parameter in living healthy and glaucomatous eyes while modulating ciliary muscle tension.
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