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Stephen Rogers Sloan, Manuel Alejandro Ramirez Garcia, Yousuf Khalifa, Mark Raymond Buckley; Depth-Dependent Mechanical Properties of the Human Cornea under Compression. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1108.
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© ARVO (1962-2015); The Authors (2016-present)
Characterize the depth-dependent compressive modulus of central and peripheral human corneas.
Unpaired central (n=3) and peripheral (n=3) corneal buttons 3 mm in diameter were punched from donor human corneas, then stained in acridine orange (a nuclear dye) to enable strain-tracking. Specimens were subjected to a stress relaxation test in an Optisol-GS bath with a microscope-mounted mechanical testing device (TDIS; Sloan et al., IOVS, 2014) under fluorescence imaging. A loading rate of 1 um/sec was applied until a peak force of 2.9 N was reached, then the specimens were allowed to relax for 30 minutes.<br /> <br /> Two-dimensional digital image correlation software (Jones, Exp. Mech., 2014) was utilized to calculate the location-dependent Lagrangian compressive strain. Force measurements at the equilibrium state (taken to be at the 30 minute mark) were divided by the cross-sectional area to calculate stress. Compressive modulus was calculated by dividing stress over strain.
In general, the compressive modulus varied continuously with depth for both locations and was highest at d/T ~ 0.6, where d is depth from the anterior surface and T is the tissue thickness. In the central specimens, a peak compressive modulus of 66 +/- 11 kPa was found at 60% depth, while the peripheral specimens exhibited a peak compressive modulus of 55 kPa at 40% depth. At the equilibrium state, central specimens measured a relaxation thickness of 472 +/- 14 um, while peripheral specimens were 569 +/- 44 um (mean +/- SEM).
Compared to our previously reported corneal shear modulus profiles that peaked at d/T ~ 0.25, the compressive modulus peaks substantially closer to the central stroma. These differences likely reflect distinct structural components of the cornea that contribute to different modes of mechanical loading.
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