Murine bone properties and their relationship to gait during growth

Abstract

Allometric relationships have been queried over orders of mammals to understand how bone accommodates the mechanical demands associated with increasing mass. However, less attention has been given to the scaling of bone within a single lifetime. We aimed to determine if bone morphology and apparent density is related to (1) bending and compressive strength, and (2) gait dynamics. Longitudinal in vivo computed tomography and gait data were collected from female rats (n=5, age 8 - 20 weeks). Cross sectional properties and apparent density were measured at the diaphysis, distal, and proximal regions of the tibia and scaling exponents were calculated. Finite-element models were used to simulate four-point bending and axial compression using time-specific ground reaction forces (GRF) to calculate the mean strain energy density (SED) at the midshaft. Second moment of area at the diaphysis followed strain similarity based allometry, while bone area was positively allometric. The average SED at the diaphysis decreased, especially after the age of 10 weeks (R2=0.99), while it increased in compression (R2=0.96). The apparent density in all regions initially increased and converged by 11 weeks of age and this was correlated with changes in joint angle. The scaling analyses implies that rodent tibia is (re)modeled in order to sustain bending at the midshaft during growth. The finite element results and relatively constant density after 10 weeks of age indicate that structural parameters may be the primary determinant of bone strength in the growing rodent tibia. The correlations between bone properties and joint angles imply that the changes in posture may affect bone growth in specific regions.