title: Biaxial mechanical testing of human sclera.
abstract: The biomechanical environment of the optic nerve head (ONH), of interest in glaucoma, is strongly affected by the biomechanical 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 to measure scleral stiffness in human eyes. Ten eyes from 5 human donors (age 55.4+/-3.5 years; mean+/-SD) were obtained within 24h of death. Square scleral samples (6mm on a side) were cut from each ocular quadrant 3-9 mm from the ONH centre and were mechanically tested using a biaxial 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=c(1)E(11)(2)+c(2)E(22)(2)+2c(3)E(11)E(22) 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. The parameter c(3) ranged from 10(-7) to 10(-8), but did not contribute significantly to the accuracy of the fitting and was thus fixed at 10(-7). The products cc(1) and cc(2), measures of stiffness in the 1 and 2 directions, were 2.9+/-2.0 and 2.8+/-1.9 MPa, respectively, and were not significantly different (two-sided t-test; p=0.795). The level of anisotropy (ratio of stiffness in orthogonal directions) was 1.065+/-0.33. No statistically significant correlations between sample thickness and stiffness were found (correlation coefficients=-0.026 and -0.058 in directions 1 and 2, respectively). Human sclera showed heterogeneous, near-isotropic, nonlinear mechanical properties over the scale of our samples.
authors.affiliation: Department of Mechanical and Industrial Engineering, University of Toronto, Canada.
authors.forename: Armin
authors.initials: A
authors.lastname: Eilaghi
authors.forename: John G
authors.initials: JG
authors.lastname: Flanagan
authors.forename: Inka
authors.initials: I
authors.lastname: Tertinegg
authors.forename: Craig A
authors.initials: CA
authors.lastname: Simmons
authors.forename: G
authors.initials: G
authors.lastname: Wayne Brodland
authors.forename: C
authors.initials: C
authors.lastname: Ross Ethier
mesh_terms: D016880:Anisotropy; D003198:Computer Simulation; D055119:Elastic Modulus / Q000502:physiology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D012590:Sclera / Q000502:physiology*; D013718:Tensile Strength / Q000502:physiology