Role of Hysteresis Energy in Bone Response to Cyclic Loading

Mechanical stimulation is critical for the homeostasis and architecture of bone. Objectives: The goals of the present study were to identify the physical properties of bone that are important in the response of bone to loading using a micromechanical-testing device. Methods: The study used tibial bones dissected from 7 to 8 day old CD-1 mice. Accurate measurements of the displacement and loads delivered to the bone were obtained along with the calculated physical parameters stiffness and hysteresis energy (HE). HE is characteristic of viscoelastic materials like bone tissue and is the energy dissipated by anelastic processes such as fluid flow. A range of cyclic strain magnitudes and frequencies were applied to tibial bones and total HE (HE integrated over all the cycles) for each experiment was determined. Previous studies in our lab had established that DNA synthesis and protein formation, indicators of bone formation, were dependent on cyclic strain magnitude and frequency. These anabolic responses were maximal at a peak load of 100 mN at 1 Hz/1000 micro strain (p<0.05, paired t-test vs. control). Results: Incremental cyclic compression tests showed that bone stiffness and hysteresis energy dissipation were dependent on the maximum load magnitude. Hysteresis energy per cycle was greatest at the range of loads (100 mN) that caused maximum anabolic responses. The results also showed the existence of a range of 300 to 1500 ergs of total HE within which there was a significant increase in DNA synthesis (p<0.05, paired t-test vs. control) for cyclically loaded bones compared to non-loaded control bones. Conclusions: These findings suggest that hysteresis energy is an important physical parameter in determining the anabolic response of bones to mechanical stimulation, and support the concept that fluid flow is a major mechanism involved in the response of bone to cyclic loading. (NIDCR grant K08 DE00446)