Growing C57B1/6 mice increase whole bone mechanical properties by increasing geometric and material properties

In vivo murine models are becoming increasingly important in bone research. To establish baseline data for researchers using these models, we studied the long bones from C57BL/6 female mice, a strain that is widely used in bo ne research, We determined the femoral structural and material properties i n both torsion and four-point bending for mice at ages 4-24 weeks. Measurem ents of femoral cross-sectional geometry and tibial densitometric propertie s were also obtained. Results indicated that all structural properties (exc ept ultimate energy), changed significantly with age (p < 0.001). Ultimate torque, ultimate moment, torsional rigidity, and bending rigidity all incre ased to peak values at 20 weeks, whereas ultimate rotation and ultimate dis placement decreased to minimum values at 20 weeks. Our data indicate that i ncreases in the material properties contributed more than increases in cros s-sectional geometry to the changes in structural rigidity and ultimate loa d. For example, from 4-20 weeks torsional rigidity increased 1030%, while s hear modulus increased 610% and the polar moment of inertia (a measure of t he geometric resistance to rotation) increased by only 85%, Changes in the cross-sectional geometry with age were due to increases in periosteal diame ter and decreases in endosteal diameter, In general, both torsion and bendi ng techniques revealed large changes in structural and material properties with age. We conclude that peak bone strength is not achieved before 20 wee ks in C57BL/6 female mice, and that torsion and four-point bending tests ar e equally well suited for evaluating mechanical properties of murine long b ones.