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Multiscale elastic imaging of bone

Bone consists of a hierarchically structured heterogeneous tissue with continuously changing properties. The hierarchical highly ordered structure results in different anisotropic elastic properties at each structural level. Pathologies, e.g. osteoporosis are characterized by a loss of mechanical function and a higher risk of fracture. An individual increase of fracture risk is not only caused by a reduction of bone mass, but also by multiple compositional and ultrastructural alterations of the mineralized bone matrix.

The aim of this work was the development of novel methods that are suitable to assess the heterogeneous anisotropic structural and material properties of cortical bone. High resolution acoustic impedance imaging has been established for the derivation of elastic tissue properties at several length scales. The impact of tissue mineralization on tissue elasticity has been analyzed by site-matched acoustic measurements in combination with Raman spectroscopy, nanoindentation and synchrotron radiation microand nanocomputed tomography.

We have developed models, which describe the relation between tissue degree of mineralization, acoustic impedance, sound velocity and anisotropic elastic coefficients (Tiburtius et al. 2014). The potential of the combined assessment of structural and elastic tissue properties has been demonstrated in several studies. Moreover, the combination of experimental microelastic data with finite element models provided the basis for realistic numerical micromechanical deformation analyses.

Acoustic impedance images of a native human cortical bone cross-section from the femoral mid-diaphysis

Acoustic impedance images, measured at 50 MHz (Fig. 7a). The Haversian canals can be distinguished from the mineralized tissue. The indicated rectangular area was measured again with a 200-MHz transducer (Fig. 7b). Remnants of circumferential tissue in the upper left part of the image can be well distinguished osteonal and interstitial tissue. The big dark spots are Haversian canals and the small spots corresponds osteocyte lacunae. The latter are not resolved at this frequency and the size of the spots is larger than the actual size of the lacunae. An osteon of an embedded human femoral bone cross section, measured at 1223 MHz is shown in (Fig. 7c). The Haversian canal system is surrounded by several lamellar units exhibiting a characteristic impedance alteration. The osteocyte lacunae appear as small dark elliptically shaped spots.