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Rafael Grytz, John T Siegwart, Thomas T Norton; Changing Material Properties of the Tree Shrew Sclera during Minus Lens Compensation and Recovery. Invest. Ophthalmol. Vis. Sci. 2014;55(13):3041.
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To estimate two material properties (collagen fibril crimp angle and elastic modulus) of the remodeling tree shrew sclera during monocular -5 D lens wear and recovery.
Tensile tests were performed on 3-mm wide scleral strips (0-50 g, 30 sec) obtained from juvenile tree shrews exposed to three different visual conditions: (i) normal development (24, 28, and 35 days of visual experience; n = 2, 3, and 3, respectively); (ii) monocular -5 D lens wear to induce axial elongation and myopia (1, 2, 4, and 11 days of lens wear; n = 5 per group); and (iii) recovery from the myopia (1, 2, 4, and 10 days of recovery with no lens after 11 days of lens wear; n = 3 per group). Collagen fibrils are crimped in the unloaded sclera and uncrimp as the tissue stiffens under load. Inverse numerical analyses were performed to estimate the collagen fibril crimp angle (unloaded) and elastic modulus using a microstructure-based constitutive model.
The fitted crimp angle was significantly higher in the treated eye vs. control eye after 2 days and peaked after 4 days of -5 D lens wear (p < 0.05). This difference was reduced but remained significantly higher after 11 days. In contrast, the difference in crimp angle rapidly decreased after the lens was removed and was not significant after 1 day of recovery. A rapid increase in the elastic modulus (up to 2-3 fold) was seen in both eyes (control and treated) after starting or stopping the -5 D lens wear. The increase was highly transient during lens wear, but more sustained during recovery. Compared to normally developing eyes, this stiffening effect was significant during the first 2 days of monocular lens wear in both eyes, while it remained significant up to day 4 and 10 of recovery in the treated and control eye, respectively.
The estimated change in the crimp angle of scleral collagen fibrils is temporally associated with the change in axial elongation rate during myopia development and recovery. This finding suggests that axial elongation may be controlled by a remodeling mechanism that modulates the collagen fibril crimp as well as creep rate (Siegwart and Norton, Vision Res. 1999;39:387-407). The binocular changes in scleral stiffness during monocular lens treatment and recovery are not temporally associated with the change in axial elongation, indicating that scleral stiffening may not be causally related to axial elongation in myopia.
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