The local mechanical properties of bone are influenced not only by the material chemical composition but also by the spatial arrangement of the components e.g. orientation of collagen matrix. However, not much is known about local elastic modulus variations in cortical bone. Our goal was to use acoustic imaging to map elastic properties of murine bone with a several microns resolution. Rodent long bones exhibit a permanent growth with endosteal/periosteal bone formation and early bone formation residual may remain.
The local microstructure was characterized with a new scanning acoustic microscopy method (SAM-TOF) using tibia from normal mice. Time-of-flight differences of ultrasound pulses across thin transversal cortical sections with known thickness (~30 microns) were determined with 0.125 ns time resolution to obtain sound velocities maps (2 μm pixel resolution) using a 330 MHz lens (kibero Gmbh). Velocity maps were combined with density maps derived from calcium content obtained by quantitative backscattered electron imaging to extract dynamic elastic moduli maps. Based on polarized light microscopy, we distinguished bone areas of different collagen fibril arrangement/orientation: bone with predominantly longitudinal collagen orientation (dark) (LB), with transverse collagen orientation (bright)(TB) and poorly ordered bone (PB) found as an asymmetrically band in the middle of the cortical cross-section.
The mean material density did not differ significantly between the three areas. However the velocity was found significantly lower in TB (−14.5%) and PB (−12.4%) compared to LB (4343 m/s). The elastic modulus was found lower in TB (−26%) and PB (−22%) compared to LB (33 GPa). No difference between TB and PB was found. No correlation was found between density and velocity (r2=0.074) or elastic modulus (r2=0.055).
These results show that TOF scanning acoustic microscopy reveals elastic moduli depending on collagen orientation and mineral content and emphasizes the importance of collagen orientation in determining the local mechanical properties of bone.
17 May 2014 - 20 May 2014