Bone hierarchical structure and mechanics through 3D X-ray imaging techniques*

Bone is an anisotropic hierarchical composite material that is special amongst biominerals in that it contains living cells called osteocytes that acts as sensors of damage. The multi-length-scale structures of bone remain far from understood. We have harnessed recent developments in X-ray imaging including X-ray nanotomography and X-ray multimodal imaging to shed new light on bone structure and mechanics.

Nanotomography reveals the osteocyte cellular network. We found that the network contains junctions, seen as voids in the bone matrix, where connections between several cells cross. Their presence raises questions about current models of stress sensing in bone.
Bone has anisotropic mechanical properties. Its constituent mineral and collagen phases each carry a part of the applied load. However, it is not clear how a macroscopic load is distributed within bone. To address this problem we have developed in situ loading diffraction scattering computed tomography (DSCT, [1]) with 30 µm resolution. DSCT combines diffraction and scattering with tomography and allows reconstructing diffractograms from inside a specimen. In in situ loading DSCT, diffraction information is obtained under load allowing us to unravel how macroscopic mechanical load distributes across a bone noninvasively. Extending DSCT to smaller X-ray beams, we unravel the spatial distribution of bone nanocrystal properties across a human osteon using 400 nm diameter X-ray beams.
[1] M. E. Birkbak, H. Leemreize, S. Frølich, S. R. Stock, H. Birkedal Nanoscale 2015, 7, 18402-18410