Growth of Trabecular Bone Damage Due to Off-axis Loads

描述(申请人提供)：股骨近端和桡骨远端骨折是常见的，这两个区域的松质骨具有主要的承重功能。这类骨折最常与异常应力有关，例如相对于骨小梁主轴的剪切，就像在跌倒时发生的那样。因此，了解并最终预测整个骨骼的骨折风险需要了解材料水平上的松质骨在离轴和多轴加载条件下的力学行为。骨密度和骨小梁结构一直是骨小梁质量的主要指标，但微损伤的积累和传播也可能是重要的。在体内，骨小梁的微损伤水平随着年龄的增长而增加，目前用于治疗骨质疏松症的抗吸收疗法进一步增加了这一水平。在日常生活活动中积累的微损伤可能成为进一步损伤扩展的起点，特别是在离轴载荷期间，从而增加骨折的风险。在离轴加载过程中损伤的起始和扩展对骨强度的影响，以及微损伤负荷和骨密度对骨小梁强度的相对影响还没有被讨论过。本研究的总体目标是确定在轴上和轴外载荷作用下，骨小梁的微损伤和损伤扩展对其断裂韧性的影响，从而为预测整个骨的断裂敏感性提供必要的数据。假设1)正常载荷引起的微裂纹有一个自我限制的长度，但在随后的离轴载荷下可以诱导微裂纹进一步扩展；2)裂纹扩展导致具有预先存在的损伤的松质骨的破坏能量较低，从而增加整个骨的骨折风险；3)由于骨小梁结构的改变，骨质疏松骨在轴上载荷中的微损伤积累和随后的离轴载荷下的微裂纹扩展比正常骨更大。

DESCRIPTION (provided by applicant): Fractures of the proximal femur and distal radius, both regions where trabecular bone has a major load bearing function, are common. Such fractures are most often associated with abnormal stresses, such as shear with respect to the principal axes of the trabecular bone, as occurs during a fall. As such, understanding and ultimately predicting fracture risk in whole bones requires knowledge of the mechanical behavior of trabecular bone subjected to off-axis and multiaxial loading conditions at the material level. Bone mineral density and trabecular architecture have been the primary indices of trabecular bone quality, but the accumulation and propagation of microdamage may also be important. The level of microdamage in trabecular bone increases with age in vivo, and is further increased by anti-resorptive therapies currently used to treat osteoporosis. Microdamage accumulated during activities of daily living may serve as initiation sites for further damage propagation, particularly during off-axis loads, thereby increasing fracture risk. The effects of damage initiation and propagation on bone strength during off-axis loading has not been addressed, nor have the relative affects of microdamage burden and bone mineral density on trabecular bone strength. The overall goal of this proposal is to determine the effect of microdamage and damage propagation on the fracture toughness of trabecular bone subjected to on-axis and off-axis loads, thereby providing data necessary to predict the fracture susceptibility of whole bones. The hypotheses are 1) microcracks caused by normal loading have a self limiting length, but can be induced to grow further when the bone is subsequently subjected to off-axis loads; 2) crack propagation results in lower energy to failure for trabecular bone with pre-existing damage which would increase the fracture risk in the whole bone; and 3) the microdamage accumulation in on-axis loading and microcrack propagation during subsequent off-axis loading are greater in osteoporotic bone than in normal bone due to changes in the trabecular architecture.