Changes of residual stress, diaphyseal size, and micro-nano structure in bovine femurs during growth and maturation

Bone tissue is subjected to multiple forms of mechanical stress. Even in the absence of external loads, however, residual stress is measured, although the underlying mechanisms remain unknown. This study measured the changes in residual stresses, diaphyseal size, and the micro- and nanostructures of bone during growth and maturation, periods associated with different in vivo mechanical loads due to increasing body weight. Mid-diaphyses from bovine femurs in the following three age groups were examined: 1) less than one month old, 2) two years old, and 3) 8-9 years old. Residual stresses along the bone axis at anterior, posterior, lateral, and medial positions on the diaphyseal surface were measured by X-ray diffraction and averaged. Diaphyseal size, porosity, mineral contents, and degree of hydroxyapatite crystal orientation of transverse cross-sections were investigated for relations with residual stress. Residual stress increased significantly from less than one month old (83.7 ± 53.3 MPa) to two years old (125.5 ± 61.9 MPa) in parallel with expanding diaphyseal width and cortical thickness. Residual stress plateaued until 8-9 years old (114.6 ± 42.2 MPa) and was correlated with local cortical thickness (p < 0.05). At the stage, diaphyseal width was only slightly greater than at 2 years and cortical thickness was not significantly different. For all measurements across groups, residual stress statistically correlated with porosity (p < 0.05), mineral contents (p < 0.01), and degree of crystal orientation (p < 0.01). These observations suggest that residual stresses are generated due to bone formation and reconstruction under changing in vivo mechanical loads with age. In conclusion, residual stresses in bone are generated during development and maintained in maturation, and are indirectly related to diaphyseal size and both bone micro- and nanostructure. (author)