Session: 2G

Scanning Acoustic Microscopy (SAM) can deliver information on mechanical properties. For the investigation of bone microstructure we found the acoustic impedance as the most suitable parameter.A spatial resolution of approximately 1 um at 1 GHz allows morphological studies at the level of cellular structures. For quantitative analysis at high frequencies a smooth and plane surface is essential. In bone remains an unavoidable surface roughness sufficient to influence the reflected sound waves. To exclude these disturbing effects we used the Multi Layer Analysis technique (MLA). Methods: Specimens from human cadaver femora were cut in small pieces of cortical bone. With the acoustic microscope SAM 2000 measurements were made at 900 MHz. For correct data acquisition and device control we used a custom software (ELIPS) for automatic grabbing of images with decreasing z-positions. MLA calculates focussed output images by finding the V(z)-maxima for every x,y position. Mechanical data were acquired with the Nano-Indenter XP. At a maximum load of 500 mN elastic modulus was determined from the displacement-curve. The impression marks were identified by videomicroscopy and correlated with the acoustic impedance at the same location. Results: Quantitative SAM displays elastic properties of bone in two dimensions. At high frequencies the spatial resolution is comparable to light micropcopy and delivers morphological information. Thus regions lacking bone matrix, e.g. Haversian canals can be excluded from measurement. SAM is nondestructive and leaves the sample unchanged. Because of dispersion the acoustic impedance depends on the frequency. In our study at 900 MHz the acoustic impedance was varying between 2.6 Mrayl in younger, low mineralized osteons and 9.8 Mrayl in older regions with an higher mineral content. The transition of mineral-rich regions and mainly organic structures became clearly visible, contrasted without any staining. Corresponding with previous findings the elastic modulus in our nanoindention experiments ranged from 12.5 to 24 GPa. Correlation of acoustic impedance with Young’s modulus at the same location shows good correlation significant linear correlation (R=0.748, p < 0.001) and allows a deduction of common material parameters.