Tensile and compressive mechanical property of human bone tissue

doi:10.3969/j.issn.2095-4344.2013.07.008

Abstract

Abstract:

BACKGROUND: The mechanical parameters of human bone tissue materials are not only the fundamental data for the stress calculation, fracture damage analysis and stress-related bone remodeling, but also one of the main indispensable factors for the further investigation of the characteristics of biomechanical behavior of human bone. However, no international standards for the bone mechanical property tests have been reported.
OBJECTIVE: To summarize the research progress on the topics of specimen preparation, specimen constraints and fixation, test data processing and experiment error of tension-compression testing.
METHODS: Under the environment of no specific standard for the tensile and compression testing of bone tissue to follow, the verified test methods and ways were introduced. The error and its decrease during the tension-compression testing were concluded and emphasized; the standardized bone tissue macro tension and compression test method was also introduced with the purpose of reducing the systematic errors.
RESULTS AND CONCLUSION: By performing the tension-compression testing, key parameters such as elastic modulus, Poisson’s ratio, tensile strength and compress strength of the bone tissue could be acquired. Important testing procedures involving designing and machining of standard specimen, specimen gripping, data processing and mechanical parameter calculation were tested. During each procedure, the systematic errors need to be reduced according to the latest researches.

Key words: tissue construction, tissue construction and biomechanics, cortical bone, cancellous bone, mechanical property, testing, biomechanics, the National Natural Science Foundation of China

CLC Number:

R318

Wang Shang-cheng, Wang Dong-mei, Wang Fang, Wang Qiu-gen, Wang Chun-hui, Chen Shan-guang. Tensile and compressive mechanical property of human bone tissue[J]. Chinese Journal of Tissue Engineering Research, 2013, 17(7): 1180-1184.

Figures/Tables 5

1 试样制备 1.1 骨试样的取材方向旧规范对取材方向不重视。实际上骨表现为各向异性，测试轴线应与材料主方向重合，以形成在轴加载，否则将出现偏轴效应[1，10]。松质骨的力学各向异性方向和结构主方向几乎重合，因此松质骨组织根据骨小梁的结构主方向来确定试样轴向，皮质骨则根据哈弗氏系统的结构方向来确定，偏轴角需减小到10°以下[11-12]。 1.2 试样尺寸 1.2.1 符合连续体假设骨组织的宏观力学性能测试基于连续性和均匀假设。皮质骨试样尺寸至少应含有几个基本的加强单元和定向结构即哈弗氏系统[2]。松质骨是骨小梁相互交织组成的网状固体，试样则须包含至少5个骨小梁及其间距时才符合连续体假设。 1.2.2 试样尺寸的选择立方块或长方块骨试样适合测量骨的单轴或多轴压缩模量及各向异性，且易加工，但只能用压盘方法压缩。压盘法所用试样几何尺寸的不同，带来的端部效应和摩擦误差就不同，因此实验结果是依赖于尺寸的[14-16]。圆柱试样适用端帽法压缩和拉伸，端帽法下的摩擦和端部效应已经锐减，Lievers等[17]的研究表明试验结果已独立于纵横比，但由于边界效应的影响，结果仍受直径影响，且大直径带来误差较小[18]。考虑到骨实际尺寸，推荐直径为5-10 mm、测试长度-直径比为2∶1(不应小于1∶1)并且测试长度最小为10 mm的规格[1，17]。哑铃状试样也叫狗骨头状试样，是理想的拉伸试样形状，但往往只能在大骨头上获得[19-20]，中段截面积减小即为缩颈，横截面可为圆形或长方形。缩颈设计使标距端的应力大于其他节段，有利于保证断裂优先发生在标距段而不是端部。圆弧式缩颈较斜线式缩颈更能减小夹持区附近的应力集中，防止在过渡处发生断裂[19]。 1.3 试样加工在选定取材方向和设计好试样最终尺寸后，即可规划试样加工流程。对于哑铃试样，宜用切片机切出皮质骨平片，平整后再用端铣刀铣出圆弧或斜线缩颈。对圆柱试样，用钻床上的空心钻刀即可钻取松质骨芯。加工骨头时用金刚石涂层的刀具，采用低切削速率和不断的液体冷却来防止骨过热。失败的加工包括试样瘢痕、过热烧灼或者取材的过度偏轴。 1.4 骨髓的处理与否骨髓在孔隙内的黏性流动会对骨力学性能的测量产生影响。当应变率低于1应变每秒时，骨髓的流动不会显著影响骨的刚度和强度[21]。准静态测试是以低应变率加载试样，所取试样中的骨髓不必去除，但高速测试则需考虑髓脂的影响。 1.5 冰冻保存骨组织离体后就开始发生组织自溶，对其力学性能产生不可逆转的破坏。在-20 ℃下冰冻保存并且确保骨内水分的情况下，骨组织的力学性能没有较明显变化[22]。冰冻过的试样在拉压测试前在磷酸化生理盐水中常温解冻数小时以重新水合。 2 拉压测试 2.1 测试温度和保水一般认为温度和水合将影响骨的力学性能。理论上骨试样温度维持在37 ℃。实际上将温度设定在24 ℃左右，此时弹性模量只比在37 ℃时高出2%-4%[23]，可见误差是非常小的。然而室温下脱水将使松质骨的表观弹性模量下降约14%[24]，故需采取措施减缓骨组织的脱水，如包裹湿纱布等。 2.2 试样装夹试样尺寸的设计决定了采用何种装夹方式，如长方块试样采用压盘法压缩，圆柱试样可采用端帽法拉压或压盘法，哑铃试样则直接夹具夹持端部以拉压。传统压缩测试多采用压盘方法，即把试样直接置于两个平行压盘之间压缩[6]。这种方法简单易行，对长方块试样很实用。但端部效应及摩擦会带来20%-40%的误差。因此尽量不用压盘法压缩，或者采取措施减少误差，见表1。"

5 总结骨组织的拉压测试看似简单和直接，却很容易受试样制备、测试方法、环境条件和数据处理的影响。如果按标准操作，不同研究之间便易对比，或许模量-密度关系能得到更准确的反映，而不是非常离散地分布。本文回答了骨组织的拉压试验准备与执行、数据处理和误差评估等关键问题，对误差来源和误差减少技术进行了重点介绍。以往骨组织力学性能测试的研究较多是简单地照抄材料力学中的典型方法，殊不知旧方法存在很大缺陷和系统误差，急需整理和解决争议，在减少误差的宗旨下尝试解决骨力学性能测试的标准问题[33]。本文潜在的误差来源和误差大小。要知道当要求高精度分析(如材料敏感性分析)和研究对比时，试验误差的减小和规范就变得格外重要[34-35]。因此，本文论述了骨组织拉压测试的最新进展，对少误差的方法做了重点介绍，有助于此类试验的正确开展。致谢：感谢上海市第一人民医院骨科刘锐和王倞对试验标准问题和作者的讨论。基金资助：国家重点基础研究发展计划(973计划)(2011CB711000)；国家自然科学基金资助项目(81071234)；上海市松江区卫生局经费资助项目(2011LX14)。作者贡献：第一作者构思并设计文章，同时分析并解析相关数据，经导师王冬梅副教授修改，所有作者共同起草，第一作者对本文负责。利益冲突：课题未涉及任何厂家及相关雇主货其他经济组织直接或间接的经济或利益的赞助。伦理要求：无涉及伦理冲突的内容。作者声明：文章为原创作品，数据准确，内容不涉及泄密，无一稿两投，无抄袭，无内容剽窃，无作者署名争议，无与他人课题以及专利技术的争执，内容真实，文责自负。"

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