Strain dependence of ultrasound speed in bovine articular cartilage under compression in vitro

The change of the ultrasound (US) speed in articular cartilage (artC) under applied strain conditions may induce significant measurement errors of the mechanical properties of the artC during both indentation and compression tests using US. In this paper, the strain dependence of the US speed in bovine artC (n = 20) under compression in vitro was investigated by virtue of using a custom-made US compression testing system. The US speed of the artC at the instant after the compression and that after a period of stress-relaxation were estimated under the applied strain ranging from 0% to 20%. Moreover, the instantaneous modulus and the modulus after the stress-relaxation of the artC were measured and correlated with the US speed. There was no significant difference (p > 0.05) between the US speed at the instant after the compression and that after the stress-relaxation, although there was a discrepancy between the instantaneous modulus and the modulus after stress-relaxation. The US speed was found to be highly correlated to the applied strain (r(2) = 0.98, p < 0.001) in a quadratic relation and changed by 7.8% (from 1581 +/- 36 m/s to 1671 +/- 56 m/s) when the applied strain reached 20%. The results suggest that the strain-dependent effect on the US speed in artC should be considered when the US is deployed for the assessment of artC using the compression or indentation test.