A Novel Virtual Fields Method to Measure the Biomechanical Properties of Human Optic Nerve Head Tissues

Abstract

Purpose : To develop a novel and fast virtual fields method (VFM) to measure the patient-specific biomechanical properties of optic nerve head (ONH) tissues.

Methods : Our method was designed to extract the biomechanical properties of human ONH tissues (choroid, peripapillary sclera, prelamina, and lamina cribrosa), given their full-field IOP-induced deformations. The latter can be derived through 3D tracking of in vivo optical coherence tomography (OCT) images (Girard et al., J R Soc Interface. 2013; 10:20130459). To verify our technique, we first generated ‘artificial’ ONH deformation data from predetermined (known) ONH tissue biomechanical properties using finite element analysis (Figure 1a-b). Using such deformations, if we are able to match back the known biomechanical properties, it would indicate that our VFM (Pierron, F., & Grédiac, M. 2012. Springer Science & Business Media) technique is accurate. We assumed that the prelamina, the choroid and the lamina cribrosa can be described with a single stiffness parameter (elastic moduli: E_p, E_c and E_l); the peripapillary sclera can be described with 2 stiffness parameters representing the stretch-induced stiffening of the collagen fibers (c₃ and c₄), and 2 microstructural parameters representing the main orientation (θ_p) and the degree of alignment (k) of the collagen fibers. The ‘artificial’ IOP-induced ONH deformations were fed into the VFM algorithm (custom written in C++) to extract back the biomechanical properties. The computational speed of VFM was then compared to that of a gold standard stiffness measurement method (inverse finite element).

Results : From the given ONH deformations, VFM successfully matched back the biomechanical properties of ONH tissues with high accuracy and efficiency. For all parameters, the percent errors were less than 1% (Figure 1c). Our method was also able to recover the highly-aligned circumferential organization of the collagen fibers in the peripapillary sclera. Finally, VFM was found 60 times faster than gold-standard stiffness measurement methods.

Conclusions : VFM may be capable of measuring the biomechanical properties of human ONH tissues with high speed and accuracy. It has potential in identifying patient-specific ONH biomechanical properties in the clinic if combined with OCT.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.