Biaxial Mechanical Evaluation of Human Sclera

Abstract

Purpose: : Biomechanics of the optic nerve head (ONH) is strongly affected by the mechanical properties of sclera. However, there is a paucity of information about the variation of scleral mechanical properties within eyes and between individuals. We thus used biaxial testing (pulling tissue samples in two orthogonal directions) to measure scleral stiffness and better understand the biomechanical properties of the human scleral.

Methods: : Ten eyes from 5 human donors (age 55.4 ± 3.5 years; mean ± SD) were tested. Square scleral samples (6 mm on a side) were cut from each ocular quadrant 3-9 mm from the ONH centre and were mechanically tested equi-biaxially using an extensional tissue tester (BioTester 5000, CellScale Biomaterials Testing, Waterloo). Stress-strain data in the latitudinal (toward the poles) and longitudinal (circumferential) directions, here referred to as directions 1 and 2, were fit to the four-parameter Fung constitutive equation: W = c(e^Q-1), where Q = c₁E₁₁²+c₂E₂₂²+2c₃E₁₁E₂₂ and W, c’s and E_ij are the strain energy function, material parameters and Green strains, respectively. Fitted material parameters were compared between samples.

Results: : The parameter c₃ ranged from 10^-7-10^-8, but did not contribute significantly to the accuracy of the fitting and was thus fixed at 10^-7. The products c*c₁ and c*c₂, measures of stiffness in orthogonal directions, were 3.7 ± 2.5 MPa and 3.6 ± 2.4 MPa respectively, and were not significantly different (2-sided t-test; P =0.877). The level of anisotropy (ratio of stiffness in orthogonal directions) was 1.065±0.33. No statistical differences were found between the stiffness of the right and left eyes (P=0.874), between different quadrants of an eye (P=0.913), or between eyes from different individuals (P=0.34). Weak correlations between sample thickness and stiffness were found (correlation coefficients = 0.115 and 0.123 in directions 1 and 2, respectively).

Conclusions: : Human sclera showed near-isotropic and nonlinear stiffness properties over the scale of our samples. Variability in stiffness between individuals was more pronounced than that between right and left eyes or between different locations of an individual eye.

Keywords: sclera • stress response • computational modeling