Nearly-incompressible transverse isotropy (NITI) of cornea elasticity: model and experiments with acoustic micro-tapping OCE

TL;DR

This paper introduces a NITI biomechanical model for the cornea that aligns OCE measurements with traditional tests, improving clinical assessment of corneal elasticity by accounting for its nearly-incompressible, transversally isotropic nature.

Contribution

The study develops and validates a NITI model for corneal biomechanics, revealing the need for two shear moduli and reconciling OCE with tensile testing results.

Findings

Corneal elasticity varies with intraocular pressure.

The NITI model accurately predicts shear and tensile responses.

Model fails at high IOP above 30 mmHg.

Abstract

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.