ABSTRACT
ABSTRACT: Cushioning is the essential function of the spine to protect itself, and the major form for the spine to resist the external load. Viscoelasticity is one of the most important indicator of the cushioning capacity of the spine. By comparing the creep characteristics before and after the fusion, this paper examines the influence of fusion on the cushioning capacity of the spine. Ten ovine lumbar two-level segments were continuously compressed under an axial pressure of 0.4 Mpa for 30 minutes. Each sample was used twice in intact and fused groups respectively. Fused groups were internally fixed rigidly without curvature changes. The deformation of the adjacent segment were recorded using an optics automatic extensometer built in the Instron mechanical testing system. The creep curves of the whole samples were also recorded and a 5-parameter rheological model was used for curve fitting. The equilibrium displacement and time were calculated using the fitting functions. There were no significant differences between axial and lateral creep deformations, while the total creep displacement decreased significantly. L/SE, L/S2 and τ2 were significantly smaller, but there was no significant difference on L/S1 and τ1. S1 and S2 increased significantly, and the equilibrium displacement and time reduced significantly. Based on these results, we can draw the following conclusions. The fusion with only stiffness increased had no effect on creep displacement of the adjacent segment. The viscosity of the whole spine decreased but the stiffness increased after fusion. The shortened equilibrium time illustrates that the whole cushioning capacity decreased after fusion. Fast and slow responses correspond bone tissue and Intervertebral Disc (IVD) tissue respectively, and fusion mainly changed the viscoelasticity properties of IVD tissue, while no significant change was found on viscoelasticity properties of bone tissue.
