The exact effects of microdamage on bone strength in vivo are still not well understood, in part due to our limited capability for imaging and measuring microdamage nondestructively. Most current methods of imaging microdamage are inherently two-dimensional and provide minimal information regarding the three-dimensional distribution or orientation of cracks. The objective of this study was to develop a non-destructive technique for three-dimensional imaging and quantification of microdamage in cortical bone.
A new three-dimensional and nondestructive technique was developed to image microdamage in cortical bone using micro-computed tomography (micro-CT). In order to image microdamage in micro-CT, lead sulfide (PbS) and barium sulfate (BaSO4) were precipitated within tissue as contrast agents. Four-point bending fatigue was used to induce microdamage bovine cortical bone beams. Symmetric loading on the beam enabled paired specimens to be produced with similar microdamage accumulation. The new technique was validated by direct comparison with paired specimens stained by calcein and imaged using ultraviolet light microscopy. Microdamage in double-notched bovine cortical bone beams was also studied in order to investigate microdamage accumulation at a stress concentration. The new technique was validated qualitatively by comparison between the observed staining shape using micro-CT and finite element analysis, and quantitatively by positively correlating the amount of stain with the number of loading cycles. The combined studies elucidated microdamage mechanisms in bovine plexiform bone under relatively low load and high cycle number. Microcracks initiated at vasculature and lacunae, and propagated along the weak interface between lamellae in the plane of the vascular network.
A limitation of the precipitated contrast agents was incomplete stain penetration. Further investigation showed that the problem could be alleviated by decreasing the specimen size, albeit counter to the objective of this study. Furthermore, in vivo imaging would require that micro-CT contrast agents be deliverable. Therefore, barium sulfate nanoparticles less than 100 nm in size were synthesized. The effects of the reagent solution concentration, molar ratio, feeding order, feeding speed and pH were investigated. Finally, the feasibility of imaging the synthesized barium sulfate nanoparticles in bone using micro-CT was verified.
