Müller R, Gerber S C, Hayes W C
Orthopedic Biomechanics Laboratory, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, MA 02215, USA.
Technol Health Care. 1998 Dec;6(5-6):433-44.
Many bones within the axial and appendicular skeleton are subjected to repetitive, cyclic loading during the course of ordinary daily activities. If this repetitive loading is of sufficient magnitude or duration, fatigue failure of the bone tissue may result. In clinical orthopedics, trabecular fatigue fractures are observed as compressive stress fractures in the proximal femur, vertebrae, calcaneus and tibia, and are often preceded by buckling and bending of microstructural elements. However, the relative importance of bone density and architecture in the etiology of these fractures is poorly understood. The aim of the study was to investigate failure mechanisms of 3D trabecular bone using micro-computed tomography (microCT). Because of its nondestructive nature, microCT represents an ideal approach for performing not only static measurements of bone architecture but also dynamic measurements of failure initiation and propagation as well as damage accumulation. For the purpose of the study, a novel micro-compression device was devised to measure loaded trabecular bone specimens directly in a micro-tomographic system. The measurement window in the device was made of a radiolucent, highly stiff plastic to enable X-rays to penetrate the material. The micro-compressor has an outer diameter of 19 mm and a total length of 65 mm. The internal load chamber fits wet or dry bone specimens with maximal diameters of 9 mm and maximal lengths of 22 mm. For the actual measurement, first, the unloaded bone is measured in the microCT. Second, a load-displacement curve is recorded where the load is measured with an integrated mini-button load cell and the displacement is computed directly from the microCT scout-view. For each load case, a 3D snap-shot of the structure under load is taken providing 34 microm nominal resolution. Initial measurements included specimens from bovine tibiae and whale spine to investigate the influence of the structure type on the failure mechanism. In a rod-like type of architecture as seen in the whale spine, structural failure was described by an initial buckling and bending of structural elements followed by a collapse of the overloaded trabeculae. In the more plate-like bovine tibial architecture, buckling and bending could not be observed. Failure rather seemed to occur instantaneously. In conclusion, micro-compression in combination with 3D microCT allows visualization of failure initiation and propagation and monitoring of damage accumulation in a nondestructive way. We expect these findings to improve our understanding of the relative importance of density, architecture and load in the etiology of spontaneous fractures of the hip and the spine. Eventually, this improved understanding may lead to more successful approaches to the prevention of age-related fractures.
在日常活动过程中，中轴骨和附肢骨骼中的许多骨骼都会受到反复的周期性负荷。如果这种反复负荷的强度或持续时间足够，可能会导致骨组织疲劳失效。在临床骨科中，小梁疲劳骨折表现为股骨近端、椎骨、跟骨和胫骨的压缩应力骨折，并且通常在微观结构元件发生屈曲和弯曲之前出现。然而，人们对骨密度和结构在这些骨折病因中的相对重要性了解甚少。本研究的目的是使用微型计算机断层扫描（microCT）研究三维小梁骨的失效机制。由于其无损性质，microCT不仅是进行骨结构静态测量的理想方法，也是进行失效起始和扩展以及损伤积累动态测量的理想方法。为了本研究的目的，设计了一种新型微型压缩装置，用于在微型断层扫描系统中直接测量加载的小梁骨标本。该装置中的测量窗口由射线可透过的、高刚性塑料制成，以使X射线能够穿透该材料。微型压缩机的外径为19毫米，总长度为65毫米。内部加载腔可容纳最大直径为9毫米、最大长度为22毫米的湿或干骨标本。对于实际测量，首先，在microCT中测量未加载的骨。其次，记录载荷-位移曲线，其中载荷用集成的微型按钮式测力传感器测量，位移直接从microCT扫描视图计算得出。对于每个载荷情况，拍摄加载状态下结构的三维快照，提供34微米的标称分辨率。初始测量包括来自牛胫骨和鲸脊椎的标本，以研究结构类型对失效机制的影响。在鲸脊椎中看到的杆状结构类型中，结构失效表现为结构元件首先发生屈曲和弯曲，随后过载小梁坍塌。在更呈板状的牛胫骨结构中，未观察到屈曲和弯曲。失效似乎是瞬间发生的。总之，微型压缩与三维microCT相结合能够以无损方式可视化失效起始和扩展，并监测损伤积累。我们期望这些发现能增进我们对密度、结构和负荷在髋部和脊柱自发性骨折病因中的相对重要性的理解。最终，这种增进的理解可能会带来更成功的预防与年龄相关骨折的方法。
