Biological and mechanical consequences of transient intervertebral disc bending

Degenerative mechanisms for the intervertebral disc are unclear, particularly those associated with cumulative trauma. This research focuses on how mechanical loading at levels below those known to cause acute trauma can lead to cellular injury. Mouse-tail discs were subjected to static bending for 1 week, then allowed to recover unloaded for 3 weeks and 3 months. Discs were analyzed using histology, in situ hybridization (collagen and aggrecan gene expression), TUNEL assay for apoptotic cell death, and biomechanics. The bent discs demonstrated loss of annular cellularity on the concave (compressed) side, while the nucleus and convex annulus appeared normal. Chondrocyte-like cells were apparent within the inner, concave annulus on the recovered discs, with evidence of proliferation at the annulus/endplate interface. However, annular architecture and biomechanical properties for the recovered discs were not different from controls, suggesting that restoration of physiologic tissue stress prevents the inner annular degradation noted in previous compression-induced degeneration models. These data demonstrate that cellular injury can be induced by transient compressive stress, and that recellularization is slow in this avascular tissue. Taken together, this suggests that cellular damage accumulation may be an important injury mechanism that is distinct from acute mechanical failure.