The effect of static in vivo bending on the murine intervertebral disc.
BACKGROUND CONTEXT: Intervertebral disc cell function in vitro has been linked to features of the local environment that can be related to deformation of the extracellular matrix. Epidemiologic data suggest that certain regimens of spinal loading accelerate disc degeneration in vivo. Yet, the direct association between disc cell function, spinal loading and ultimately tissue degeneration is poorly characterized. PURPOSE: To examine the relationships between tensile and compressive matrix strains, cell activity and annular degradation. STUDY DESIGN/SETTING: An in vivo study of the biologic, morphologic and biomechanical consequences of static bending applied to the murine intervertebral disc. SUBJECT SAMPLE: Twenty-five skeletally mature Swiss Webster mice (12-week-old males) were used in this study. OUTCOME MEASURES: Bending neutral zone, bending stiffness, yield point in bending, number of apoptotic cells, annular matrix organization, cell shape, aggrecan gene expression, and collagen II gene expression. METHODS: Mouse tail discs were loaded for 1 week in vivo with an external device that applied bending stresses. Mid-sagittal sections of the discs were analyzed for cell death, collagen II and aggrecan gene expression, and tissue organization. Biomechanical testing was also performed to measure the bending stiffness and strength. RESULTS: Forceful disc bending induced increased cell death, decreased aggrecan gene expression and decreased tissue organization preferentially on the concave side. By contrast, collagen II gene expression was symmetrically reduced. Asymmetric loading did not alter bending mechanical behavior of the discs. CONCLUSIONS: In this model, annular cell death was related to excessive matrix compression (as opposed to tension). Collagen II gene expression was most negatively influenced by the static nature of the loading (immobilization), rather than the specific state of stress (tension or compression).