The effect of yield damage on the viscoelastic properties of cortical bone tissue as measured by dynamic mechanical analysis.

We have previously shown, using Dynamic Mechanical Analysis (DMA), that the presence of a defect in cortical bone tissue affects the apparent viscoelastic properties of that bone. However, mechanically induced damage is more complex than a machined defect making it difficult to predict its effect on bone viscoelasticity. We performed DMA measurements before and after introduction of yield damage into cortical bone beams from sheep radii. The specimens were placed in a DMA machine and baseline measurements of storage modulus (E1) and loss factor (tandelta) were performed using a 3-point bending configuration for a frequency range of 1-10 Hz. Measurements were done in all four bending directions (cranial, caudal, medial, and lateral) in random order. After subjecting the specimens to monotonic yield damage in a servohydraulic testing machine with the load applied to the cranial surface, oscillatory tests were repeated. To supplement results from the current experiment, additional analyses were performed on data from experiments where bone was either cut or fatigue-loaded between viscoelasticity measurements. Introduction of mechanical damage increased tan delta and frequency sensitivity of E1, consistent with the assertion that increased energy dissipation in damaged bone might contribute to its increased resistance to fatigue and fracture.