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Yoshiaki Yasuno, Satoko Nagase, Masahiro Yamanari, Kotaro Ishii, Shinichi Fukuda, Tetsuro Oshika, Masahiro Miura; Impact Of Dynamically Applied Mechanical Force On Scleral Birefringence. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2798.
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It has been reported that birefringence of the sclera is sensitive to ultrastructure of the sclera and correlated with the elasticity. We had investigated a relationship between the scleral birefringence and its mechanical properties by using polarization-sensitive optical coherence tomography (PS-OCT) in order to make PS-OCT an early diagnostic tool for glaucoma. This previous study showed significant correlation between scleral elasticity and its birefringence at a relaxed state. However, the lack of knowledge about the scleral birefringence under mechanical force/tension limits the understanding of the mechanism behind this correlation. In this paper, we investigate how acute mechanical stress influences the scleral birefringence.
Porcine sclera was dissected at temporal region 5-mm apart from the optic nerve head (ONH) to be a rectangular shape with a width of 4 mm. To apply one of two orthogonal extensions to each scleral sample, the long sides of the rectangular regions were oriented to meridional or its perpendicular directions. The sample was mounted on a uniaxial translation stage using two clamps that initially had 5-mm distance. Preconditioning was applied from 0 to 6.5% strain for 10 cycles of loading and unloading. Subsequently, the birefringence of the sample was measured by PS-OCT at the central 3-mm region of the sample between the clamps before and after applying stretching of 6.5% strain. Twenty and 19 porcine eyes were stretched to meridional or its perpendicular directions from the ONH, respectively.
When the sclera was stretched to the meridional direction, statistically significant increase of birefringence was found (p = 0.001, paired t-test). Averaged difference of the birefringence was +0.06 deg/um. Also with the stretch perpendicular to the meridional direction, the increase of the birefringence of +0.03 deg/um was found with statistical significance (p = 0.047).
The results showed that uniaxial transversal tension does affect to the birefringence of the sclera, and this effect has bi-directionality. This bi-directionality would be associated with the orientation of collagen fibers in the sclera. This finding provides a further insight to understand the relationship between the scleral birefringence and its mechanical properties, and enhances the utility of PS-OCT as an investigation tool for glaucoma.
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