HISTOLOGIC MICROCRACKS AND FATIGUE OF CORTICAL BONE

疲劳断裂发生在当结构多次加载时， 仅施加一次不会导致断裂的力。 每个周期 会产生微小的损伤，这些损伤会累积直至发生断裂。 在骨头里， 疲劳损伤通过重塑修复。 当重塑不能保持 可能会导致疲劳或“应力”骨折;这些是常见的 赛马，军事训练，芭蕾舞演员，以及娱乐 作为有竞争力的跑步者。 然而，也有人假设，疲劳 损伤会增加许多其他个体的骨脆弱性，包括 骨质疏松症和其他代谢性骨病患者。 因此 疲劳的临床意义可能非常大。 实验研究 有两种类型。 有些人进行了疲劳测试 试件失效，研究断裂所需的循环次数 在给定载荷下的试样。 其他研究人员研究了 骨中存在组织学上可观察到的微裂纹， 这种裂纹是疲劳损伤。 虽然已经表明， 微裂纹的数量增加后，重复加载， 组织学微裂纹、骨脆性和疲劳之间的关系 目前，失败是未知的。 很明显，微裂纹 如果不进行修复， 直接测试。 微裂纹、疲劳损伤、 脆弱性和疲劳破坏需要从现象学的角度来解决 和机械性。 拟议的研究将通过以下方式解决这一问题： 两项体外实验采用4点弯曲疲劳载荷， 马的掌骨 实验1将测试一种关联 之间的微裂纹和弹性模量的减少（另一个 疲劳损伤）。 它还将测试这两种形式是否 损伤的累积是应力大小的可预测函数， 施加到疲劳断裂点的载荷循环次数。 实验2将确定是否微裂纹和刚度损伤 影响骨骼对创伤性骨折的抵抗力。 此外，理论上 模型将探索组织损伤改变的机制， 强度和刚度。

Fatigue fracture occurs when a structure is loaded many times with a force which would not cause fracture if applied only once. Each cycle produces minute damage which accumulates until fracture occurs. In bone, fatigue damage is repaired by remodeling. When remodeling does not keep pace, fatigue or "stress" fractures may result; these are common among race-horses, military trainees, ballet dancers, and recreational as well as competitive runners. However, it is also hypothesized that fatigue damage increases bone fragility in many other individuals, including those with osteoporosis and other metabolic bone diseases. Thus, the clinical significance of fatigue may be very great. Experimental studies of fatigue in bone have been of two kinds. Some have fatigue tested specimens to failure, studying the number of cycles required to fracture a specimen under a given load. Other investigators have studied the existence of histologically observable microcracks in bone with the view that such cracks are fatigue damage. While it has been shown that the number of microcracks increases following repetitive loading, the relationships between histologic microcracks, bone fragility, and fatigue failure are presently unknown. Clearly, the hypothesis that microcracks cause fragility or fatigue fracture if they are not repaired needs to be directly tested. The relationships between microcracks, fatigue damage, fragility, and fatigue failure need to be worked out phenomenologically and mechanistically. The proposed research will address this problem via two in vitro experiments employing fatigue loading in 4 point bending of the equine metacarpal bone. Experiment 1 will test for an association between microcracks and diminishment of the elastic modulus (another measure of fatigue damage). It will also test whether these two forms of damage accumulate as predictable functions of stress magnitude and the number of load cycles applied, to the point of fatigue fracture. Experiment 2 will determine whether microcracks and stiffness damage affect bone's resistance to traumatic fracture. In addition, theoretical modeling will explore the mechanisms by which histologic damage alters strength and stiffness.