Growth of Trabecular Bone Damage Due to Off-axis Loads

描述（由申请人提供）：股骨近端和桡骨远端骨折很常见，这两个区域的骨小梁具有主要的承载功能。这种骨折通常与异常应力有关，例如跌倒期间发生的相对于小梁骨主轴的剪切力。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离轴和多轴载荷条件下骨小梁的机械行为。骨矿物质密度和小梁结构一直是小梁骨质量的主要指标，但微损伤的积累和传播也可能很重要。骨小梁的微损伤水平随着体内年龄的增长而增加，并且目前用于治疗骨质疏松症的抗再吸收疗法进一步增加。日常生活活动中积累的微损伤可能会成为进一步损伤传播的起始点，特别是在离轴负载期间，从而增加骨折风险。离轴加载过程中损伤发生和传播对骨强度的影响尚未得到解决，微损伤负荷和骨矿物质密度对骨小梁强度的相对影响也尚未得到解决。该提案的总体目标是确定微损伤和损伤传播对轴上和离轴载荷下骨小梁断裂韧性的影响，从而提供预测整个骨骼骨折敏感性所需的数据。假设是：1）正常载荷引起的微裂纹具有自限长度，但当骨骼随后受到离轴载荷时，可能会导致微裂纹进一步生长； 2) 裂纹扩展会导致已有损伤的骨小梁的破坏能量降低，从而增加整个骨的骨折风险； 3）由于小梁结构的变化，骨质疏松骨中轴上载荷的微损伤累积和随后的离轴载荷期间的微裂纹传播比正常骨更大。

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.