Age-dependent mechanical properties of rat femur. Measured in vivo and in vitro.

Strain gauges were implanted on the anterior surface of the femoral diaphysis of rats aged 6, 12 or 52 weeks. Strain was recorded while the rats were running on a treadmill. The peak strain was of the same magnitude at different animal ages, although there was a somewhat lower value for 52-week-old animals. Stiffness calculated from in vivo strain measurements and from a 3-point bending test on excised femora was correlated: stiffness increased with age. Maximum bending stress increased from 6-12 weeks of age, but then there was no further increase. Ultimate load, on the contrary, increased steadily over the entire 52-week period. This indicates that higher load-bearing capacity with increased age in adult rats is due to increased dimensions of the bone rather than its material properties. The present study demonstrates that conventional in vitro measurements of mechanical properties of bone correspond to measurements of strain during physical activity, and that both are valid measurements of physical properties of bone.