Back-directional Gated Spectroscopic Imaging for Nanoscale Deformation Analysis in Bone

Although crack mechanisms in bone have been intensively studied to have a better understanding of bone fracture, exact prefailure damage mechanisms about how cracks or deformations at nanoscales occur still remain unexplored due to technical limitations. In this pilot study, we apply back-directional gated spectroscopic imaging (BGSI) to examine the exact spatial extent of such damage in in-situ mechanical testing of bovine cortical bone. Our imaging approach provides a relatively large field of view, while the wavelength dependence of light elastically backscattered from bone at each pixel can capture structural alterations in a few tens of nanometers. Thus, our imaging method can simultaneously examine various length scales. Using a notched bovine cortical bone wafer, we report that an altered field of a couple of square millimeters forms at the tip of the notch during tensile loading in the transverse orientation, and this field disappears upon unloading. We conducted simple pilot simulations of optical waves in one-dimensional layered media to gain an understanding of the potential mechanisms about the spectral dependence on nanostructure alterations. Our results imply that the bone nanostructure may allow the formation of nanoscale deformation over a relatively large area to prevent microcrack formation or fracture as an energy dissipating mechanism. We further envision that BGSI may facilitate understanding how the nanostructure of bone controls bone characteristics and properties.