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M. J. Girard, J.-K. F. Suh, C. F. Burgoyne, J. C. Downs; Biomechanical Alterations of Monkey Posterior and Peripapillary Sclera Following Chronic Experimental IOP Elevation. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5217.
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© ARVO (1962-2015); The Authors (2016-present)
To characterize posterior and peripapillary scleral biomechanics in both eyes of 8 adult monkeys in which one eye had been given chronic, laser-induced intraocular pressure (IOP) elevations of modest to substantial magnitude and duration.
Following enucleation, the intact posterior scleral shell of each eye was mounted on a custom-built pressurization apparatus, then IOP was elevated from 5 to 45 mmHg while the 3D displacements of the scleral surface were measured using speckle interferometry. Each scleral shell geometry was digitally reconstructed from data generated by a 3D digitizer (topography) and 20 MHz ultrasound (thickness). An inverse finite element method incorporating a collagen fiber-reinforced constitutive model was used to extract a unique set of biomechanical properties for each eye. Biomechanical properties for the 8 adult (17.3 ± 5.1 yo) normal eyes were compared to previously reported values in 4 young (1.5 ± 0.7 yo) and 4 old (22.9 ± 5.3 yo) bilaterally normal monkeys. Data for each glaucomatous eye was compared to its contralateral normal and only differences exceeding inter-eye differences in the bilaterally normal animals were considered significant.
For both the normal and glaucomatous eyes, the sclera exhibited inhomogeneous, anisotropic, nonlinear biomechanical behavior. Scleral biomechanical properties in the eight adult normal eyes fell between those from young and old monkeys (ARVO 2008, 49:4058). Biomechanical changes caused by chronic IOP elevation were complex and individual-specific. Specifically, we observed that 1) sclera with initially large tangent modulus or thickness were less prone to biomechanical changes; 2) the tangent modulus associated with an IOP of 10 mmHg decreased following minimal IOP elevation; 3) the tangent modulus associated with IOPs of 30 and 45 mmHg increased following moderate IOP elevation; 4) collagen fiber reorientation did not occur in glaucomatous eyes.
Sclera from adult monkeys is stiffer than that from young monkeys, but less stiff than that from old monkeys. Significant stiffening of the posterior and peripapillary sclera follows exposure to moderate to substantial IOP elevations in the majority of adult monkey eyes. Scleral hypercompliance may precede stiffening or be a unique response to minimal IOP elevation in some eyes. The biomechanical changes seen in the sclera from glaucomatous eyes are likely the result of extracellular matrix remodeling. The effect of these alterations on the susceptibility of the axons within the lamina cribrosa remains to be determined.
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