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M. A. Fazio, M. J. A. Girard, J. C. Downs; Anisotropic, Nonlinear Biomechanical Behavior of Posterior Human Sclera. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5558.
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© ARVO (1962-2015); The Authors (2016-present)
To establish the range of nonlinear, anisotropic mechanical properties of posterior sclera from normal eyes of old human donors.
One eye was randomly selected from each of five normal human donors (78.8 ± 2.8 years) and tested within 48 hours post mortem as follows. The intact posterior scleral shell of each eye was pressurized from 5 to 45 mmHg while the 3D displacements of the scleral surface were measured using speckle interferometry. A finite element model of each eye’s scleral shell was constructed using data from a 3D digitizer (topography) and 20 MHz ultrasound (thickness). A fiber-reinforced constitutive model that includes both collagen fiber multi-directionality and stretch-induced stiffening of the fibers was applied to each eye model. A unique set of scleral biomechanical properties were then derived by fitting the simulated model displacements to those obtained experimentally (IOVS 50(11):5226-5237, 2009).
The experimental and model displacements were in good agreement for all eyes. Human posterior sclera exhibited a nonlinear response to elevated IOP and was approximately twice as stiff at 45 mmHg than at 10 mmHg (Figure 1A). Anisotropy was clearly present in both regions as well, but was much higher in the peripheral posterior sclera, wherein the median tangent modulus along the predominant collagen fibril direction was more than double the modulus perpendicular to the predominant fibril direction (Figure 1B).
Human posterior sclera exhibits nonlinear biomechanical behavior and is approximately twice as stiff at an IOP of 45 mmHg compared to 10 mmHg. The observed anisotropic behavior indicates 1) that the sclera is much stiffer in the direction of the predominant collagen fibril direction, and 2) collagen fibrils tend to be more aligned in the peripheral posterior sclera compared to the peripapillary sclera. We are currently using this methodology to elucidate the contribution of age-related and racial variations in scleral biomechanics to glaucomatous susceptibility.
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