Detection of Fatigue Microdamage in Whole Rat Femora Using Contrast-Enhanced Micro-Computed Tomography

Microdamage in bone tissue results from repetitive mechanical loading, and the accumulation of microdamage has been implicated with increased fracture susceptibility, including stress fractures in active individuals and fragility fractures in the elderly [1–3]. Conventional methods used to detect microdamage in bone are limited to thin histological sections, which are inherently invasive, destructive, tedious and two-dimensional [3]. A non-destructive, three-dimensional (3-D) method would enable correlation of the spatial location and accumulation of microdamage with variations in the mechanical loading and morphology of whole bones.

Fatigue Microdamage in Bovine Trabecular Bone

Microdamage, in the form of small cracks, may accumulate in trabecular bone loaded in fatigue. Specimens of bovine trabecular bone were loaded in compressive fatigue at one of four normalized stresses and loading was stopped after the specimens reached one of six maximum strains. Microdamage was identified using a fluorochrome staining technique, and microdamage parameters, including the number of damaged trabeculae and the damaged area fraction, were measured. No microdamage was observed during loading to strains below the yield strain; at higher strains, all microdamage parameters increased with increasing maximum compressive strain. Few significant differences were observed in the type or amount of microdamage accumulation between specimens loaded to the same maximum strain at different normalized stresses; however, more trabecular fractures were observed at high numbers of cycles, which corresponded to low normalized stresses.

The Effect of Number of Cycles on Microdamage Accumulation in Bovine Trabecular Bone

Microdamage, in the form of small cracks, exists in healthy bone. Microdamage can be created by an overload or by repetitive motion (fatigue) during daily activities. Usually, microdamage is repaired during bone remodeling and a steady state is maintained. However, in cases of excessive microdamage creation or slowed bone remodeling, microdamage can coalesce to create a fracture. Our previous work [1,2] has investigated microdamage accumulation with increasing strain in bovine trabecular bone loaded in monotonic compression and compressive fatigue. Specimens fatigued at relatively high load levels fail after a few loading cycles, while specimens fatigued at lower load levels may undergo thousands of cycles before failure. During high cycle fatigue, microdamage may accumulate by the growth of pre-existing microcracks, as well as by the crack initiation seen in low cycle fatigue.