Effects of age-related ultra-structural level changes in bone on microdamage mechanisms

The objective of this thesis was to study the effect of age-related changes in microstructure and composition on microdamage formation and growth in cortical bone. A continuum of microdamage exists in bone between diffuse ultra-structural level damage (<10μm) and microcracking (>10μm). It is hypothesized that diffuse ultra-structural level damage zones contributing to fatigue processes are reduced with age due to changes in the mechanism by which mineral phase composition and distribution respond to fatigue at an ultra-structural level. Ultimately, alterations in the mechanism of microdamage formation and growth could contribute significantly to age-related skeletal fragility.; The specific aims were to: (1) quantify and compare microdamage in femoral cortical bone associated with 2 different age groups of mice (3–4 month and 22–24 month C57BL6); (2) determine predominant locations of microdamage formation and growth using laser scanning confocal microscopy (LSCM) techniques; (3) relate changes in damage mode and distribution to local mineral content using Raman spectroscopic techniques; and (4) relate changes in damage mode and distribution to the spatial distribution of local mineral using quantitative x-ray scattering techniques.; The results of this research established a damage mechanism, where diffuse ultra-structural damage is a precursor to the formation and growth of linear microcracks. Raman spectroscopic analyses showed the presence of distinct mineral species (i.e. “phase transformations”) associated with damage. Regions surrounding fatigue microcracks and diffuse damage contained less-carbonated apatite. With monotonic loading, older specimens contained one unique mineral species and younger specimens contained two unique mineral species localized to fracture sites, suggesting that the dissipation of mechanical energy was achieved by age-dependent mechanisms. X-ray data showed that mineral orientation increased with age and significantly greater disorientation was associated with monotonic loading.; The primary result of this work is the characterization of a dynamic damage process that occurs at the ultra-structural level in bone, where damage can be characterized collectively by age-related changes in morphology, local mineral composition and regional mineral orientation. These findings are important to understanding the evolution of age-related skeletal fragility.