Ta electroforce 3200

Biomechanical testing was performed on the L5/6 motion segments (vertebra-IVD-vertebra) from each animal (Fig. 1b,c). Testing consisted of 20 cycles of axial tension–compression sinusoidal loading at ± 8 N and 1 Hz, followed immediately by 1 h of compressive creep at − 8 N. After 30 min of unconstrained rehydration in saline, 20 cycles of torsional rotation was applied at ± 10° and 1 Hz, with 8 N axial static compression^{32 (link)}. Axial and creep testing were done using a TA ElectroForce 3200 instrument (TA Instruments, New Castle, DE) and torsional testing with an AR2000ex rheometer (TA Instruments, New Castle, DE). Axial and torsional properties were calculated from the 20th loading cycle using MATLAB^{32 (link)} and creep parameters were determined by applying a 5-parameter viscoelastic solid model^{32 (link),78 (link)}. The viscoelastic model is the sum of a rapid and slow exponential decay and an elastic component.

Mechanical properties were determined as described previously (Thomas et al., 2020 (link)). Briefly, three-point bending was performed on a mechanical testing machine while the bones were fully hydrated with PBS (TA ElectroForce 3200, TA Instruments, Eden Prairie, MN, USA). Bones were tested with a 7 mm support span in anterior–posterior direction with the posterior surface in compression. The loading point was placed directly at the midshaft location, the bone was preloaded to establish contact, and then testing occurred at 0.025 mm/s to failure. The yield point was determined using the slope of the stress-strain curve, then implementing the 0.2% offset method. The ultimate point was determined as the maximum force recorded. The failure point was determined as when the bone broke. Whole-bone (extrinsic) properties included yield and ultimate force and displacement, yield and total work, and stiffness, based on the force-displacement curve. Material (intrinsic) properties included yield and ultimate stress and strain, modulus, resilience and toughness based on the stress-strain curve. Cortical geometry was used to normalize the stress-strain curve from the force-displacement curve.