Purpose
Characterize the depth-dependent compressive modulus of central and peripheral human corneas.
Methods
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.
Results
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).
Conclusions
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.