Department of Biomedical Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA.
Proc Natl Acad Sci U S A. 2012 Nov 20;109(47):19178-83. doi: 10.1073/pnas.1201513109. Epub 2012 Nov 5.
Toughening in hierarchically structured materials like bone arises from the arrangement of constituent material elements and their interactions. Unlike microcracking, which entails micrometer-level separation, there is no known evidence of fracture at the level of bone's nanostructure. Here, we show that the initiation of fracture occurs in bone at the nanometer scale by dilatational bands. Through fatigue and indentation tests and laser confocal, scanning electron, and atomic force microscopies on human and bovine bone specimens, we established that dilatational bands of the order of 100 nm form as ellipsoidal voids in between fused mineral aggregates and two adjacent proteins, osteocalcin (OC) and osteopontin (OPN). Laser microdissection and ELISA of bone microdamage support our claim that OC and OPN colocalize with dilatational bands. Fracture tests on bones from OC and/or OPN knockout mice (OC(-/-), OPN(-/-), OC-OPN(-/-;-/-)) confirm that these two proteins regulate dilatational band formation and bone matrix toughness. On the basis of these observations, we propose molecular deformation and fracture mechanics models, illustrating the role of OC and OPN in dilatational band formation, and predict that the nanometer scale of tissue organization, associated with dilatational bands, affects fracture at higher scales and determines fracture toughness of bone.
分层结构材料（如骨骼）的增强源于组成材料元素的排列及其相互作用。与涉及微米级分离的微裂纹不同，在骨骼的纳米结构水平上没有已知的断裂证据。在这里，我们通过膨胀带证明了断裂是从骨骼的纳米尺度开始的。通过对人骨和牛骨标本进行疲劳和压痕试验以及激光共聚焦、扫描电子和原子力显微镜分析，我们确定了 100nm 左右的膨胀带在融合的矿物质聚集体和两个相邻的蛋白质（骨钙素（OC）和骨桥蛋白（OPN））之间形成椭圆形空隙。骨微损伤的激光微切割和 ELISA 支持我们的观点，即 OC 和 OPN 与膨胀带共定位。对 OC 和/或 OPN 敲除小鼠（OC(-/-)、OPN(-/-)、OC-OPN(-/-;-/-)）骨骼的断裂试验证实，这两种蛋白质调节膨胀带的形成和骨基质的韧性。基于这些观察结果，我们提出了分子变形和断裂力学模型，说明了 OC 和 OPN 在膨胀带形成中的作用，并预测与膨胀带相关的组织纳米尺度会影响更高尺度的断裂，并决定骨骼的断裂韧性。
