Measurements of elastic properties of tendons: comparison of two approaches

Ultrasonic elasticity imaging has great potential in rehabilitative medicine due to its ability to quantitatively measure mechanical properties of tendon. In this paper, two different approaches for assessing elastic properties of tendon were employed. Six bovine Achilles tendon specimens were used. The first approach directly measured Young´s moduli along the transverse direction (i.e., E_⊥) and the longitudinal direction (i.e., E_|| ) using an electronic balance, a compressor positioned by a 3-axis stepping motor system and a personal computer for central control and data acquisition. Young´s moduli were derived based on the measured strain and stress values. The second approach was based on the assumption that tendons are transversely isotropic. Three observable second-order elastic stiffness constants (c₁₁, c₁₃ and c₃₃) were obtained by measuring the speed of sound along different directions of propagation using a transmission mode method. Sound velocity along 0, 30, 45 and 90 degrees relative to the fiber axis were measured for all six specimens. The measured elastic stiffness constants were also compared to the average E_⊥ and E_|| obtained from the first set of experiments. It was found that measurements in first approach were influenced by viscoelasticity, thus resulting in significant discrepancy between the two approaches. In this paper, details of the two approaches are presented. Potential sources of the discrepancy are also discussed