Nieminen Heikki J, Julkunen Petro, Töyräs Juha, Jurvelin Jukka S
Department of Physics, University of Kuopio, Kuopio, Finland.
Ultrasound Med Biol. 2007 Nov;33(11):1755-66. doi: 10.1016/j.ultrasmedbio.2007.06.001. Epub 2007 Aug 13.
Ultrasound elastography is a method that can be used to determine the elastic properties of soft tissues and it has been recently applied to study of articular cartilage. While ultrasound elastography techniques assume a constant ultrasound speed in tissue under mechanical compression, ultrasound speed in articular cartilage has been found to vary depending on the loading conditions. This may limit the quantitative use of the technique for determination of the elastic properties of articular cartilage along the axis of ultrasound propagation. The aim of the present study was to investigate the origin of the load-related variation in ultrasound speed. Samples of human and bovine articular cartilage (n = 82) were mechanically and acoustically tested during unconfined compression. A statistically significant (p < 0.05) variation of ultrasound speed was found in cartilage during a stress-relaxation test. A finite element model was constructed by exploiting microscopically determined collagen and proteoglycan contents, collagen orientation and biochemical analyses of water content. From the finite element simulations, collagen orientation and the void ratio (fluid-to-solid ratio) as a function of time were assessed and, together with the experimentally determined ultrasound speed, a linear model predicting variation of the ultrasound speed in human articular cartilage under mechanical compression was established. The model predicted compression-related ultrasound speed with an error of <0.3% at each time point. The effect of strain rate on variation of ultrasound speed was tested in bovine cartilage samples. The decrease in ultrasound speed was found to be proportional to the strain rate. The results suggest that ultrasound speed in articular cartilage is controlled mainly by collagen orientation and the void ratio and depends on the imposed strain rate. A numerical simulation revealed that the compression-related decrease in ultrasound speed induces notable errors in mechano-acoustically determined strain. A method to eliminate the compression-related errors in measured strain and elastic properties may be needed in mechano-acoustic measurements of articular cartilage.
超声弹性成像术是一种可用于确定软组织弹性特性的方法，最近已应用于关节软骨的研究。虽然超声弹性成像技术假定在机械压缩下组织中的超声速度恒定，但已发现关节软骨中的超声速度会根据加载条件而变化。这可能会限制该技术在沿超声传播轴确定关节软骨弹性特性方面的定量应用。本研究的目的是探究与负荷相关的超声速度变化的根源。在无侧限压缩过程中，对人和牛的关节软骨样本（n = 82）进行了力学和声学测试。在应力松弛试验中，发现软骨中的超声速度存在统计学上显著的（p < 0.05）变化。利用显微镜确定的胶原蛋白和蛋白聚糖含量、胶原蛋白取向以及含水量的生化分析构建了有限元模型。通过有限元模拟，评估了胶原蛋白取向和孔隙率（流体与固体的比率）随时间的变化，并结合实验确定的超声速度，建立了一个预测人体关节软骨在机械压缩下超声速度变化的线性模型。该模型在每个时间点预测与压缩相关的超声速度时，误差<0.3%。在牛软骨样本中测试了应变率对超声速度变化的影响。发现超声速度的降低与应变率成正比。结果表明，关节软骨中的超声速度主要由胶原蛋白取向和孔隙率控制，并取决于所施加的应变率。数值模拟表明，与压缩相关的超声速度降低会在机械声学确定的应变中引起显著误差。在关节软骨的机械声学测量中，可能需要一种方法来消除测量应变和弹性特性中与压缩相关的误差。
