## 3 Bone Pathologies: Alterations to Bone Ultrastruct and Mechanics  
### Osteoporosis  
Osteoporosis, a chronic metabolic bone disorder, is clinically diagnosed using BMD values, where a BMD greater than or equal to 2.5 standard deviations below the normative reference is indicative of an osteoporoptic bone [127]. However, osteoporosis affects bone quality starting at the ultrastructural scale, at least five or six orders of magnitude below the centimeter scale at which an averaged BMD is evaluated. In addition, osteoporosis is a complex disease in that not all osteoporoptic bones are the same because there exist multiple types of osteoporosis: senile, postmenopausal, and steroid-induced (more specifically glucocorticoid-induced). For instance, senile osteoporosis exhibits more significant cortical bone loss, whereas, for postmenopausal women, the bone loss mainly takes place in the trabecular region, giving rise to differential fracture risks [127]. Such complication necessitates an understanding of the disease starting at the nanoscale. There appear to be plenty of works on the nanoscale imaging of osteoporotic bone, but the underlying differences in the sample types render comparison and cross-examination difficult, especially considering that the findings are non-uniform. Some employ human patient samples for which idiopathic/general osteoporosis predominates, while others use animals to model a more specific type of osteoporosis. In what follows, we will draw commonalities in results and discuss case-by-case ultrastructural and chemical changes in osteoporotic bones. Examining bones from osteoporosis patients, [PERSON] et al. concluded that there were no discernible ultrastructural differences between healthy and osteoporotic bone for both the mineral and collagen phases of bone [128]. However, this is not the case in animal models of specific subtypes of osteoporosis. Consider first the mineral phase of hydroxyapatite, where a suite of studies has inspected crystal dimensions, morphology, mineral arrangement and packing, orientation patterns, and the degree of mineralization [128; 129; 130; 131]. Using small-angle X-ray scattering (SAXS), [PERSON] et al. measured shorter apatite crystals with increased thickness in steroid-induced osteoporotic mouse bone, and an overall change in the crystal morphology was ascertained [129]. In the same study, the authors established a positive correlation between the length and thickness of the apatite nanocrystals [129], suggesting that the apatite volume was not conserved in steroid-induced osteoporotic bone as in normal WT bone. Thus, there may be fundamental changes to the mineralization process in osteoporosis pathology, which is sensible, given that both OPN and ON deficiencies correlate well with an osteoporotic phenotype (Section 2.2). In an ovariectomized mouse model of postmenopausal osteoporosis, the nanocrystallites were found to be more spatially dispersed compared to their tightly-packed counterpart in WT bone (Figure 4(ai)) [130]. For mineralized collagen fibrils, the mineralization pattern can be quantified by the crystal orientation angles--though such information is, in reality, three-dimensional due to the cross-fibrillar nature of the minerals deposited. [PERSON] et al. did not observe any qualitative change in crystal orientations in human osteoporotic samples, but incidences of slight increase in mineral misorientation were visualized in high-resolution TEM for bone from a postmenopausal osteoporotic mouse [128; 129; 130]. Only SAXS has been used to directly quantify and map misorientation angles of the nanocrystals in a steroid-induced osteoporotic mouse (Figure 4(aiii)) [129]. Given the above information, this may suggest that the mineralization processes are different for different types of osteoporosis. Nevertheless, one must remain cautious since the great disparity in the sample statistics and techniques employed to examine the same ultrastructural detail render any rational comparisons difficult. More systematic work is required to clarify this.