In vitro mechanical and cellular responses of neonatal mouse bones to loading using a novel micromechanical-testing device

Mechanical stimulation is critical for the maintenance of bone architecture and bone mass. These effects are dependent on the magnitude, duration, and rate of the mechanical stimuli. The goals of the present study were to develop and optimize a micromechanical-testing device for in vitro mechanical stimulation of whole viable bones, and to identify the physical parameters of loading that elicit maximal anabolic responses. The model was the 7-8-day-old neonatal CD-1 mouse tibia. A range of cyclic strain magnitudes [500-7000 microstrain (microstrain)] and frequencies [0.2-30 hertz (Hz)] were applied to the neonatal bones. Incremental cyclic compression tests showed that the bones were nonlinearly viscoelastic. Bone stiffness and hysteresis energy dissipation were dependent on the maximum load magnitude. DNA and protein synthesis were significantly enhanced in bones that were cyclically loaded at 0.5 Hz/1000 microstrain, 0.5 Hz/2000 microstrain, or l Hz/1000 microstrain, compared to nonloaded controls. Anabolic responses were maximal at a peak load of 100 mN at l Hz/1000 microstrain. Autoradiography of the bones loaded under these conditions showed proliferation of cells at periosteal surfaces. Hysteresis energy per cycle was greatest at loads that caused the largest anabolic responses. The parameters of strain and load that elicit optimal effects on the neonatal bones are comparable to those in other systems, validating the use of the instrumentation for studying the mechanisms of the anabolic responses. The findings also suggest that hysteresis energy per cycle may be a determinant of the anabolic response of bones to mechanical stimulation.