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Joseph Park, Andrew Shin, Joseph L Demer; Optical Birefringence Correlates with Tensile Properties of Human Sclera. Invest. Ophthalmol. Vis. Sci. 2017;58(8):3165. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Past research has shown glaucomatous eyes have abnormal mechanical properties, but those demonstrations generally entail destructive ex vivo testing. Hence, we propose that birefringence, an optical property of anisotropic material, may provide a non-invasive in vivo indicator of stress in collagen that is found in many biological tissues. Polarization sensitive optical coherence tomography (PSOCT) is an interferometric imaging technique that sensitive to birefringence. Since mechanical loading alters the orientation and geometry of collagen fibrils, we sought to determine if birefringence changes can be used as a noninvasive optical method to infer local mechanical properties of ocular connective tissues.
A linear motorized uniaxial tensile loading cell was employed to measure the change of the mechanical stress and strain of the specimens during simultaneous infrared (1300 nm) PSOCT imaging (Thorlabs 1300). Fresh scleral specimens were prepared from rabbits, cows, and 8 pairs of human eyebank eyes. Specimens were obtained from 3 sites: Region 1 near the limbus, Region 2 at the equator, region 3 near the optic nerve head. Specimens were stretched to the failure at constant rate of 0.01mm/s while cross-sectional PSOCT images were concurrently taken every 117ms. Birefringence was calculated for each frame using MATLAB (Mathworks, Natick, MA). Birefringence modulus (BM) was defined as the slope of birefringence change with strain, and tangent modulus (TM) as the slope of stress change between 0 to 9% strain.
: BM and TM in all 3 human scleral regions were positively correlated with slopes of 0.47±0.12, 0.52±0.11 and 0.61±0.11, respectively. Pearson r values (p value) were 0.512 (<0.001), 0.576 (<0.001), and 0.694 (<0.001) for each region. Additionally, mean BM was proportional to TM as both decreased progressively from region 1 to 3. In bovine and rabbit sclera, correlation slopes were 0.34 and 0.19, with Pearson r values (p value) 0.630 (0.038) and 0.635 (0.027), respectively.
Birefringence and tensile elastic moduli of sclera in humans and lower mammals are positively correlated, but the correlations differ for various scleral regions. These results suggest that it may be possible to mechanically characterize the sclera in vivo using PSOCT.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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