Biomechanical Analysis of Intervertebral Disc Degeneration

Axial biomechanics are reflective of physiological loading and were used to evaluate NP pressurization, AF tension, and IVD laxity. Creep is known to be sensitive to alterations in short and long-time viscoelastic behaviors. Axial and creep testing used an ElectroForce 3200 instrument (TA Instruments, New Castle, DE). Samples were hydrated in PBS with protease inhibitor tablets (ThermoScientific, Rockford, IL). Motion segments underwent 20 cycles ±8N of tension-compression at 1Hz followed by 60 minutes of compressive creep at −8N and 30 minutes of unloaded rehydration. Compressive stiffness, tensile stiffness, axial range of motion, and axial hysteresis were calculated from the 20th cycle in MATLAB (Figure 3B). Three independent reviewers manually calculated neutral zone (NZ) length and stiffness. Samples with undetectable NZ were excluded from NZ stiffness analysis. Creep was analyzed with MATLAB code measuring total displacement and applying a 5-parameter viscoelastic solid model to calculate elastic response (S_e), fast response (τ₁ and S₁), and slow response (τ₂ and S₂) time constant and stiffness parameters, respectively (39 (link)) (Figure 3C).