Correlations between nanostructure and micromechanical properties of healing bone

All hierarchical levels in bone are known to contribute to its mechanical behavior. The basic building block is the mineralized collagen fibril which is assembled into larger structures with varying fibrillar organization. The collagen organization increases from unordered woven bone in the callus which is gradually replaced by higher ordered lamellar bone during bone development and healing and finally results in cortical lamellar bone with highest degree of organization. The structural and mechanical description of these organizational motifs is not yet complete. We investigated a femoral osteotomy mouse model and analyzed newly formed callus tissue and mature lamellar bone in the cortex. This model exhibits three bone types with different fibrillar organization: (i) woven, (ii) moderate lamellar and (iii) lamellar. Using high resolution synchrotron small angle X-ray scattering in combination with back-scattered electron imaging we characterized the ultrastructure of the different regions in terms of degree of mineralization, averaged mineral particle thickness and mineral particle orientation. We further used microindentation to correlate hardness, induced crack lengths and crack patterns with the bone ultrastructure. The newly formed callus tissue contains highly mineralized woven bone islands, featuring thick but poorly ordered mineral particles. Such islands are surrounded by layers of lamellar bone with a low mineralization level and thin but well aligned particles. Callus tissue shows lower hardness values and longer cracks than the cortex. Callus woven bone exhibits shorter cracks than callus lamellar bone. However, the poorly mineralized callus lamellar bone shows crack propagation mechanisms similar to cortical bone due to its very similar lamellar organization and high degree of mineral particle orientation. In conclusion we demonstrate that woven and increasingly higher oriented lamellar bone do not only differ in collagen fibril organization, but also that the amount, orientation and different shape of mineral particles are also likely to contribute to the reduced mechanical competence of woven as compared to lamellar bone. This may explain why many organisms replace less organized bone types with higher organized ones.