## 3 Bone Pathologies: Alterations to Bone Ultrastruct and Mechanics
### Osteogenesis Imperfecta  
Osteogenesis imperfecta (OI), more commonly known as brittle bone disease, is a skeletal disease with a genetic origin, affecting one in 15,000 to 20,000 people at birth. It is a heterogeneous disease in that there are more than ten types of OI, some of which are yet to be clinically classified [141]. In almost all OI pathologies, the Type I collagen encoding genes _COLIA1_ and _COLIA2_ undergo autosomal mutations, giving rise to defective collagen in terms of its quality--including mutations in the amino acid sequence, over-modification of the collagen chain, and its protein folding--and quantity [141]. Because OI is a collagen-related disease, most studies have focused on the ultrastructural examination of OI-affected bone, especially the organic collagen phase. Osteogenesis imperfecta murine (OIM) models are widely used in the literature to mimic OI pathologies of varying severities, where homozygous _oim_/_oim_ best recapitulate the characteristics of moderate-to-severe human OI and heterozygous _oim_/+ recapitulate those of mild human OI [32; 141; 142; 143; 144]. For the mineral phase, smaller bone apatite crystal size and decreased crystal thickness were established in both human and mice models [142; 143; 145]. Moreover, the reduction in crystal thickness correlated well with OI severity, where the homozygous OI phenotype possessed a crystal thickness approximately 10% smaller compared to the heterozygous phenotype [145]. As in osteoporosis pathology, OI pathology led to a change in the aspect ratio and thus the morphology of the bone mineral at the nanoscale. Increased crystal misorientation was confirmed using SAXS, TEM imaging, and SAED pattern analysis [32; 142; 143; 146]. By analyzing a large number of high-resolution TEM images, the angular distribution of the (intrafibrillar) crystal orientation for the OI sample (Figure 5(ai)) showed a spread that was twice as large as that of healthy bone [146], which corroborated similar quantitative comparison using SAED data [32]. It was also interesting to note that, through quantifying their distributions, the minerals were less misoriented in bone with spondyloepiphyseal dysplasia congenia, another pathology that is caused by defective collagen due to a mutation in the _COL2A1_ gene, than in bone with OI. This suggests that nanoscale changes to bone structure, whilst similar in qualitative terms, can be highly differential [146]. In terms of mineralization, there were incidences of localized patchy biomineralized clusters in Type II OI, the most lethal form of OI [147]. For the collagen phase, starting at the molecular scale, the trimeric tropocollagen can comprise a mix of normal and mutant \(\alpha\)1 Type I chains in OI pathology [148]. Such molecular defects can then propagate to higher structural hierarchies. Indeed, fibril assemblies of tropocollagen were seen to have an altered morphology in OI patient bone [147]. Specifically, the collagen fibril diameter was reduced, and the diameter distribution appeared to be more variable based on TEM observation [147]. Cross-sectional TEM images also showed angular-like features of the collagen fibril edges, which can result in the loss of the typical quasi-hexagonal packing (Figure 5(aii)) [147]. Both patient (type III OI) and OIM samples showed that the collagen fibrils are less aligned with increased tortuosity and sparseness in its organization [32; 143; 147; 149]. Using TEM, [PERSON] et al. were also successful in directly visualizing structural disorders in the collagen in OI pathology [146]. However, disparate results on the effect of OI on the collagen banding pattern were recorded based on TEM image analysis: D banding periodicity remained constant, \(\sim\)65 nm, in both WT and the OIM model [32], whereas this value was significantly reduced to 62 nm in a type IV OI specimen from an eight-year-old child [146]. Figure 5: Altered bone structure, chemistry, and micromechanics in _osteogenesis imperfecta_ (OI) pathology.