University of California, San Diego, Department of Cellular and Molecular Medicine, La Jolla, CA 92093-0686, USA.
J Exp Biol. 2010 Nov 15;213(Pt 22):3934-40. doi: 10.1242/jeb.042960.
Articular cartilage is the soft tissue that covers contacting surfaces of bones in synovial joints. Cartilage is composed of chondrocytes and an extracellular matrix containing numerous biopolymers, cations and water. Healthy cartilage functions biomechanically to provide smooth and stable joint movement. Degenerative joint diseases such as osteoarthritis involve cartilage deterioration, resulting in painful and cumbersome joint motion. Temperature is a fundamental quantity in mechanics, yet the effects of temperature on cartilage mechanical behavior are unknown. This study addressed the questions of whether cartilage stiffness and stress relaxation change with temperature. Samples of middle-zone bovine calf patellofemoral cartilage were tested in unconfined compression first at 24°C and then again after heating to 60°C. The data reveal that when temperature increases: (1) both peak and equilibrium stiffness increase by 150 and 8%, respectively, and (2) stress relaxation is faster at higher temperature, as shown by a 60% decrease in the time constant. The increases in temperature-dependent stiffness are consistent with polymeric mechanisms of matrix viscoelasticity but not with interstitial fluid flow. The changes in the time constant are consistent with a combination of both fluid flow and matrix viscoelasticity. Furthermore, we discovered a novel phenomenon: at stress-relaxation equilibrium, compressive stress increased with temperature. These data demonstrate a rich area of cartilage mechanics that has previously been unexplored and emphasize the role of polymer dynamics in cartilage viscoelasticity. Further studies of cartilage polymer dynamics may yield additional insight into mechanisms of cartilage material behavior that could improve treatments for cartilage degeneration.
关节软骨是覆盖滑膜关节骨接触表面的软组织。软骨由软骨细胞和含有许多生物聚合物、阳离子和水的细胞外基质组成。健康的软骨在生物力学上具有功能，可提供平滑稳定的关节运动。退行性关节疾病，如骨关节炎，涉及软骨恶化，导致关节运动疼痛和不便。温度是力学中的基本量，但温度对软骨力学行为的影响尚不清楚。本研究探讨了软骨刚度和应力松弛是否随温度变化的问题。首先在 24°C 下，然后在加热至 60°C 后，对中区域牛犊髌股关节软骨样本进行无约束压缩测试。数据显示，当温度升高时：（1）峰值和平衡刚度分别增加 150%和 8%，（2）较高温度下的应力松弛更快，时间常数减少 60%。温度依赖性刚度的增加与基质粘弹性的聚合机制一致，但与间隙流体流动不一致。时间常数的变化与流体流动和基质粘弹性的结合一致。此外，我们发现了一种新现象：在应力松弛平衡时，压缩应力随温度升高而增加。这些数据展示了一个以前未被探索的丰富的软骨力学领域，并强调了聚合物动力学在软骨粘弹性中的作用。进一步研究软骨聚合物动力学可能会深入了解软骨材料行为的机制，从而改善软骨退化的治疗方法。
