HISTOLOGIC MICROCRACKS AND FATIGUE OF CORTICAL BONE

结构在多次加载时会发生疲劳断裂。 如果只施加一次就不会导致骨折的力。每个周期 产生微小的伤害，一直累积到骨折发生。在骨子里， 疲劳损伤是通过重塑修复的。当重塑不能保持 可能导致步速、疲劳或“应力性”骨折；这些骨折在 赛马，军事实习生，芭蕾舞演员，还有娱乐节目 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人的骨脆性，包括 患有骨质疏松症和其他代谢性骨病的人。因此， 疲劳的临床意义可能非常大。实验研究 骨骼疲劳的症状有两种。有些人做了疲劳测试 试件失效，研究断裂所需的循环次数 在给定载荷下的试件。其他调查人员也研究了 组织学上可观察到的骨微裂纹的存在 这样的裂缝是疲劳损伤。虽然事实证明， 重复加载后，微裂纹的数量增加， 组织学微裂纹、骨脆性与疲劳的关系 故障目前尚不清楚。显然，微裂缝的假设 如果不修复会导致脆性或疲劳断裂需要进行 直接测试。微裂纹、疲劳损伤、 脆弱性和疲劳失效需要从现象上解决 而且是机械的。拟议的研究将通过以下方式解决这一问题 四点弯曲疲劳加载的两个体外实验 马掌骨。实验1将测试关联性 在微裂纹和弹性模量的减小之间(另一种 疲劳损伤的量度)。它还将测试这两种形式是否 作为应力大小和应力强度的可预测函数累积 施加的载荷循环次数，直到疲劳断裂点。 实验2将确定微裂纹和僵硬损伤 影响骨骼对创伤性骨折的抵抗力。此外，理论上， 建模将探索组织损伤改变的机制 强度和硬度。

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.