  The cornea provides the largest refractive power for the human visual system.
Its stiffness, along with intraocular pressure (IOP), are linked to several
pathologies, including keratoconus and glaucoma. Although mechanical tests can
quantify corneal elasticity ex vivo, they cannot be used clinically. Optical
coherence elastography (OCE), which launches and tracks shear waves to estimate
stiffness, provides an attractive non-contact probe of corneal elasticity. To
date, however, OCE studies report corneal moduli around tens of kPa,
orders-of-magnitude less than those (few MPa) obtained by tensile/inflation
testing. This large discrepancy impedes OCE's clinical adoption. Based on
corneal microstructure, we introduce and fully characterize a
nearly-incompressible transversally isotropic (NITI) model depicting corneal
biomechanics. We show that the cornea must be described by two shear moduli,
contrary to current single-modulus models, decoupling tensile and shear
responses. We measure both as a function of IOP in ex vivo porcine cornea,
obtaining values consistent with both tensile and shear tests. At pressures
above 30 mmHg, the model begins to fail, consistent with non-linear changes in
cornea at high IOP.
