Bone Abstracts

Mineralized tissues like bone, articular calcified cartilage or mineralized turkey leg tendon (MTLT) are build by a composite of hydroxyapatite nano-particles and organic matrix. In bone and MTLT the matrix is formed by collagen type-I, but in contrast to bone in MTLT the collagen is uniaxial orientated, while in cartilage the matrix consists of collagen type-II and proteoglycans.

Composition/orientation differences were investigated by a new scanning acoustic microscopy method (SAM-TOF). Time-of-flight differences of ultrasound pulses obtained from human femoral head and distal MTLT samples with known thickness (30 microns) were determined with 0.125 ns time resolution to obtain sound velocities maps with 2 μm pixel resolution using a 330 MHz lens (Kibero GmbH). The velocity maps were combined with calcium content maps obtained by quantitative backscattered electron imaging to extract dynamic elastic moduli (E) maps.

Bone was found to require a lower mass density (−4.3%) than cartilage to achieve similar velocity (range 3700–4300 m/s) or elastic modulus (range 22–30 GPa), which is qualitatively in line with nanoindentation results. In circumferential compartment of MTLT, an axial/transversal velocity ratio of 1.13 and E ratio of 1.28 and in the interstitial compartment 1.16 and 1.32 ratios, respectively, were found. This anisotropy is clearly due to the preferred orientation of collagen. However, the higher E in cartilage-bone and lower ratio in MTLT compared with what is typically measured with (quasi-)static mechanical test such as uniaxial tension or nanoindentation could indicate an influence of relaxation processes.

These first results suggest that TOF scanning acoustic microscopy may be able not only to provide mechanical maps of mineralized tissues but to extend our understanding of the mechanical properties of bone and cartilage to the region of high loading rates, which may be highly relevant for the fracture resistance under an impact, e.g., during a fall.