The NP:AF ratio was doubled (increasing the NP radius compared to the AF width but keeping the overall IVD width constant) in each of the models to investigate the effect of NP:AF ratio on the mechanics of the disc. Doubling the ratio ensured that there was a clear divergence from the baseline geometry while keeping within the physiological bounds of reported NP:AF ratios (O’Connell et al., 2007; Adam et al., 2015; Newell et al., 2017). In all baseline models the number of rebars/mm was defined based on measurements from Marchand and Ahmed (1990) (4.35 bundles/mm = 0.22 mm interbundle spacing) (Figure 3A). When increasing the NP:AF ratio care was taken to ensure that the total number of fibre bundles, was the same between the baseline, and adjusted NP:AF ratio models (Figure 3). This was achieved by calculating the difference between the circumference of each lamella of the baseline and the altered NP:AF ratio models using the horizontal distance from each lamella to the axisymmetric axis. The fibre bundle spacing was then adjusted to ensure each new model had the same fibre volume as the baseline models (Figure 3).

Schematic of axial cross-section of the IVD demonstrating the change in fibre bundle spacing required to allow a fair comparison between models when adjusting the NP:AF ratio. (A) Shows a baseline NP:AF ratio with a cut out showing the fibre bundle thickness (Tb), and interbundle spacing (Sb), (B) shows an increased NP:AF ratio with larger interbundle spacing. Bundles are only shown in the inner most lamella in both schematics, with a total of 12 bundles shown in both (A,B), although larger interbundle spacing in (B) resulting in the same fibre bundle volume as that shown in (A). Note these schematics are not to scale.

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