Material Properties

The properties of each material used in the models are shown in Table 1. The AF matrix and the NP were assigned non-linear hyperelastic material properties (Mooney-Rivlin). The strain energy function for this material model is shown in Eq. 1, where W is the strain-energy density function, I₁ and I₂ are strain invariants, and C₁₀ and C₀₁ are material constants (Mooney, 1940):

Baseline material properties for components of each of the FE models.

The AF fibre bundles were modelled using tension only rebar elements with a Young’s modulus (YM) of 415 MPa which was obtained from the results of an optimisation study (Newell et al., 2017). The fibre bundles were aligned at ±30.45° to the transverse plane which was an average of six measurements taken from different coronal MRI slices at regular intervals from the inner to outer AF (Figure 1B). The cross-sectional area of each fibre bundle was set to be 3.212 × 10^–2 mm², and spacing of the bundles was set to 4.35 bundles/mm (Marchand and Ahmed, 1990; Adam et al., 2015). The bulk modulus of the NP and AF ground matrix was set at 2,000 MPa to ensure near incompressibility.