Chattah Netta Lev-Tov, Sharir Amnon, Weiner Steve, Shahar Ron
Department of Structural Biology, Weizmann Institute of Science, Rehovot, Israel.
Bone. 2009 Jul;45(1):84-90. doi: 10.1016/j.bone.2009.03.664. Epub 2009 Mar 28.
Mice phenotypes are invaluable for understanding bone formation and function, as well as bone disease. The elastic modulus is an important property of bones that can provide insights into bone quality. The determination of the elastic modulus of mouse cortical bone is complicated by the small dimensions of the bones. Whole bone bending tests are known to under estimate the elastic modulus compared to nanoindentation tests. The latter however provides information on extremely localized areas that do not necessarily correspond to the bulk elastic modulus under compression. This study presents a novel method for determining the bulk or effective elastic modulus of mouse cortical bone using the femur. We use Electronic Speckle Pattern Interferometry (ESPI), an optical method that enables the measurement of displacements on the bone surface, as it is compressed under water. This data is combined with geometric information obtained from micro-CT to calculate the elastic modulus. Roughly tubular cortical bone segments (2 mm) were cut from the diaphyses of femora of four week old C57BL/6 (B6) female mice and compressed axially using a mechanical tension-compression device. Displacements in the loading direction were mapped on the bone surface after loading the specimen. A linear regression of the displacement vs. axial-position enabled the calculation of the effective strain. Effective stress was calculated using force (N) data from the system's load cell and the mean cross-sectional area of the sample as determined by micro-CT. The effective elastic modulus (E) was calculated from the stress to strain ratio. The method was shown to be accurate and precise using a standard material machined to similar dimensions as those of the mouse femoral segments. Diaphyses of mouse femora were shown to have mean elastic moduli of 10.4+/-0.9 GPa for femora frozen for eight months, 8.6+/-1.4 GPa for femora frozen for two weeks and 8.9+/-1.1 GPa for the fresh femora. These values are much higher than those measured using three-point bending, and lower than values reported in the literature based on nanoindentation tests from mice bones of the same age. We show that this method can be used to accurately and precisely measure the effective elastic modulus of mouse cortical bone.
小鼠表型对于理解骨形成、功能以及骨疾病而言具有极高的价值。弹性模量是骨骼的一项重要属性，能够为骨质提供相关见解。小鼠皮质骨弹性模量的测定因骨骼尺寸较小而变得复杂。众所周知，与纳米压痕测试相比，全骨弯曲测试会低估弹性模量。然而，后者提供的是关于极其局部区域的信息，这些区域不一定与压缩状态下的整体弹性模量相对应。本研究提出了一种利用股骨测定小鼠皮质骨整体或有效弹性模量的新方法。我们使用电子散斑图案干涉术（ESPI），这是一种光学方法，能够在骨骼在水下受压时测量其表面的位移。该数据与从微型计算机断层扫描（micro-CT）获得的几何信息相结合，以计算弹性模量。从四周龄C57BL/6（B6）雌性小鼠的股骨骨干切下大致呈管状的皮质骨段（2毫米），并使用机械拉伸 - 压缩装置进行轴向压缩。在加载标本后，将加载方向上的位移映射到骨表面。位移与轴向位置的线性回归能够计算有效应变。有效应力通过系统称重传感器的力（N）数据以及由micro-CT确定的样品平均横截面积来计算。有效弹性模量（E）由应力与应变的比值计算得出。使用加工成与小鼠股骨段尺寸相似的标准材料表明该方法准确且精确。对于冷冻八个月的股骨，小鼠股骨骨干的平均弹性模量为10.4±0.9吉帕（GPa）；对于冷冻两周的股骨，为8.6±1.4 GPa；对于新鲜股骨，为8.9±1.1 GPa。这些值远高于使用三点弯曲法测得的值，且低于基于相同年龄小鼠骨骼的纳米压痕测试在文献中报道的值。我们表明该方法可用于准确且精确地测量小鼠皮质骨的有效弹性模量。
