Elastic anisotropy of the cornea: comparison of non-contact AμT-OCE with mechanical tests

Abstract

Purpose : Corneal stiffness, along with intraocular pressure (IOP), determines corneal shape, providing a majority of the refractive power to the human visual system. We demonstrate non-contact acoustic micro-tapping (AμT) optical coherence elastography (OCE) of porcine cornea to quantify the shear moduli which contribute to corneal refraction using a nearly-incompressible transversely isotropic (NITI) model based on corneal microstructure.

Methods : The cornea contains layered collagen sheets (lamellae) arranged mostly parallel to the surface, which suggests a transversal isotropy of its mechanical properties. Consequently, corneal shear and tensile behavior (Fig.1) are described by different moduli. We introduce a shear modulus, G, to quantify out-of-plane shear modulus and tensile elastic modulus, E, responsible for corneal deformation.

We perform non-contact AμT-OCE measurements in ex vivo porcine cornea to reconstruct both shear moduli using a nearly incompressible transversally isotropic (NITI) model and compare OCE results with parallel-plate rheometry and strip extensiometry.

Results : Figure 2a shows modulus estimates from AμT-OCE under varying IOP. The OCE test in an exemplary sample yielded a mean Young’s modulus of 16.8 – 33 MPa and shear storage modulus of 50.4 - 120.5 kPa (IOP 5mmHg- 20mmHg). Tensile tests yielded a Young’s modulus (E) of ~2 -25 MPa (1-9% strain) (Fig. 2c). Shear rheometry yielded a storage modulus (G’) of 100- 148 kPa and loss modulus (G”) of 15.7- 31.5 kPa over the low-frequency (1-10Hz) regime (Fig. 2b).

Conclusions : Both AμT- OCE and destructive mechanical tests performed on ex vivo cornea reveal anisotropic elastic behavior with the tensile modulus E differing by orders of magnitude from the shear modulus G. However, mechanical tests are destructive, and each test type measures either G or E, but not both moduli together. On the other hand, AμT-driven OCE is non-contact and non-destructive and is capable of in vivo measurement of both moduli simultaneously.

This is a 2021 ARVO Annual Meeting abstract.

 

Figure 1. Schematic demonstrating corneal response to different loading mechanisms.

 

Figure 2. Moduli estimates from a) the mean and standard deviation of 5 repeat AμT-OCE scans at different IOP, b) complex out-of-plane shear (G) modulus from rheometry and AμT-OCE, and c) Young’s Modulus, E, from tensile testing on the same cornea sample.