Department of Orthopaedic Surgery, University of Pennsylvania, Philadelphia, PA, United States.
J Mech Behav Biomed Mater. 2011 Oct;4(7):933-42. doi: 10.1016/j.jmbbm.2011.02.002. Epub 2011 Feb 22.
The intervertebral disc maintains a balance between externally applied loads and internal osmotic pressure. Fluid flow plays a key role in this process, causing fluctuations in disc hydration and height. The objectives of this study were to quantify and model the axial creep and recovery responses of nondegenerate and degenerate human lumbar discs. Two experiments were performed. First, a slow compressive ramp was applied to 2000 N, unloaded to allow recovery for up to 24 h, and re-applied. The linear-region stiffness and disc height were within 5% of the initial condition for recovery times greater than 8 h. In the second experiment, a 1000 N creep load was applied for four hours, unloaded recovery monitored for 24 h, and the creep load repeated. A viscoelastic model comprised of a "fast" and "slow" exponential response was used to describe the creep and recovery, where the fast response is associated with flow in the nucleus pulposus (NP) and endplate, while the slow response is associated with the annulus fibrosus (AF). The study demonstrated that recovery is 3-4X slower than loading. The fast response was correlated with degeneration, suggesting larger changes in the NP with degeneration compared to the AF. However, the fast response comprised only 10%-15% of the total equilibrium displacement, with the AF-dominated slow response comprising 40%-70%. Finally, the physiological loads and deformations and their associated long equilibrium times confirm that diurnal loading does not represent "equilibrium" in the disc, but that over time the disc is in steady-state.
椎间盘在外部施加的负载和内部渗透压之间保持平衡。流体流动在此过程中起着关键作用，导致椎间盘水合和高度的波动。本研究的目的是量化和模拟非退变和退变人腰椎间盘的轴向蠕变和恢复响应。进行了两项实验。首先，对 2000N 施加缓慢压缩斜坡，卸载以允许恢复长达 24 小时，然后重新施加。对于恢复时间大于 8 小时的线性区域刚度和椎间盘高度与初始条件的偏差在 5%以内。在第二项实验中，施加 1000N 的蠕变载荷 4 小时，监测 24 小时的卸载恢复，然后重复蠕变载荷。使用由“快”和“慢”指数响应组成的粘弹性模型来描述蠕变和恢复，其中快响应与核髓核（NP）和终板的流动有关，而慢响应与纤维环（AF）有关。研究表明，恢复比加载慢 3-4 倍。快速响应与退变相关，表明与 AF 相比，NP 中的退变引起的变化更大。然而，快速响应仅占总平衡位移的 10%-15%，而以 AF 为主导的缓慢响应占 40%-70%。最后，生理负荷和变形及其相关的长平衡时间证实，昼夜负荷不能代表椎间盘的“平衡”，而是随着时间的推移，椎间盘处于稳态。
