Stress mapping of undamaged, strained, and failed regions of bone using Raman spectroscopy

Stress differences via spectral shifts that arise among failed, strained, and undamaged regions of bone can be determined using Raman spectroscopy and double-notch specimens. A double-notch specimen is a model in which the early stages of fracture can be examined. At four-point bending, fracture occurs at one of the notches. Tissue near each notch is representative of bone in a state either directly before or after bone failure. Raman images are acquired among three regions: control, strained (root of unbroken notch), and failed (root of fractured notch). The center of gravities (CGs), a way to monitor wavenumber shifts, of the phosphate v ₁ band are calculated. A PO ₄ ^-3 v ₁ band shift most likely corresponds to a change in spacing between phosphate cations and anions. This spectral shift is converted into stress values using the dv/dP coefficient, determined by applying known pressures/stresses and measuring the change in position of the PO ₄ ^-3 v ₁ band. In comparison to control regions, the residual stress in strained and failed regions is significantly higher (p=0.0425 and p=0.0169, respectively). In strained regions, residual stress is concentrated near the corners of the unbroken notch, whereas in failed regions the high stresses are confined near the edge of the fracture.