Intervertebral disc degeneration increases surface strain on metastatic vertebrae in compression and flexion but not in torsion

Despite the fundamental role of intervertebral discs (IVDs) in the biomechanics of the spine, their condition has never been considered among the possible factors affecting the mechanical behaviour of metastatic vertebrae. The structure of the IVDs changes over the course of life, leading to alterations of their biomechanical behaviour. This study aimed to assess if IVD degeneration affects the strains experienced by the adjacent healthy or metastatic vertebrae. Eight human spine segments consisting of four vertebrae, with a healthy and a metastatic vertebra in the middle and mildly degenerated IVD, were prepared. The segments were biomechanically tested under different loading configurations: axial compression, flexion, and torsion. An enzymatic IVD degeneration was induced by injecting a collagenase solution. The degenerated specimens were tested again, following the same loading protocol. Surface vertebral strains were measured with a 3D-Digital Image Correlation (DIC). IVD degeneration was found to influence the strain distributions in the adjacent vertebrae. In particular, IVD degeneration resulted in a significant increase of the median compressive strains experienced by the cortical shell of the metastatic vertebrae, in both axial compression (+25.6 %) and flexion (+43.7 %), with larger strains close to the degenerated IVD. Conversely, control vertebral showed less relevant variations between the two conditions. Negligible strain differences were, instead, observed in torsion, for both metastatic and control vertebrae. This study showed the ability of the healthy vertebrae to withstand loads transmitted in different directions and highlighted the susceptibility of metastatic vertebrae to even minor alterations in boundary conditions.