Intervertebral disc viscoelastic parameters and residual mechanics spatially quantified using a hybrid confined/in situ indentation method

With advancing age, injury, musculoskeletal pathology or a combination of these, a degenerative cascade of biomechanical, biochemical, and nutritional alterations diminish the intervertebral discs' ability to maintain its structure and function. While the biomechanics of isolated disc tissues has been investigated across this degenerative spectrum, none have attempted to retain the in situ disc-endplate morphology during compressive tissue characterization. The objective of this study was to spatially quantify the viscoelastic parameters of the intervertebral disc throughout degeneration, including the as yet unreported residual stress/strain. This required the development of a hybrid confined/in situ indentation methodology, which preserves the disc structural morphology. At four locations of the disc (anterior-AF, right and left lateral AF, and NP) stress-relaxation tests were performed using the hybrid confined/in situ indentation method, which utilizes the vertebral endplate as the porous indenter tip. This method allows the endplate to remain interwoven with the disc tissue, retaining its native orientation. Healthy disc tissue exhibited significantly higher residual stress values compared to both moderate and severe degeneration in all locations (p<0.0156). Furthermore, the equilibrium stress at 15% strain (stress relaxation) was significantly diminished with advancing disc degeneration (p<0.0241). The equilibrium viscoelastic parameters show healthy discs encounter higher forces at the same strain level, and are able to maintain this force, where degenerated discs are unable to maintain this force throughout time. This morphology-conserved method provides insight into the spatial compressive mechanical properties of the intervertebral disc across the degeneration spectrum and will aid in modeling these tissue changes.