Stiffness Dynamics of Rabbit's Achilles Tendons Evaluated by Shear Wave Elastography in vivo

The physiological function of tendon is to withstand the tension generated by muscles during various type of joint movement, and thus prevent muscle from violent damage. Hence, the dynamic change of tendon stiffness for adapting various external forces is highly related to its functional performance. In other words, monitoring the dynamic tendon stiffness at various stretching conditions can be used for assessment of tendon functions. Recently, a relatively new imaging technique called shear wave elastography (SWE) has emerged as a promising tool for estimation of tissue stiffness. However, because tendons are anisotropic materials with the highest stiffness along the longitudinal direction, the general relation between the shear wave speed and Young's modulus derived from isotropic materials (i.e., E= 3 rho c(2)) does not hold. To precisely estimate the mechanical properties of tendon, it is necessary to analyze the velocity dispersion of the guided waves propagating inside tendons. The aim of this study is thus to evaluate SWE combined with dispersion analysis as a diagnostic tool for tendon functionality by monitoring the dynamics of tendon stiffness at various stretching conditions. SWE was in vivo applied to six Achilles tendons of three New Zealand rabbits. Assuming the tendons as a transverse isotropic material, the tendons were passively stretched at four ankle joint angles and the dispersion of shear waves running in parallel with the longitudinal direction of the tendons were analyzed to yield the elastic constants C-55 in Christoffel's tensor. The measured mean value of C-55 at joint angles of 125 degrees,110 degrees,95 degrees and 80 degrees were 0.42MPa, 0.95MPa, 1.49MPa and 2.27MPa, respectively. Our results show that the change of C-55 is highly correlated with the stretching conditions. This suggests that the dynamic stiffness of tendon at various stretching conditions can be monitored by elastic constant C-55. In summary, SWE combined with dispersion analysis is a powerful tool to non-invasively monitor the stiffness dynamic changes of tendons and highly potential for diagnosis of tendon injury and monitoring of the treatment efficiency.