Substantial creep in healing human Achilles tendons

The early tendon regenerate after an Achilles tendon rupture is a watery tissue, likely to show time-dependent deformation during load. To our knowledge the extent of such deformation has not previously been estimated in humans. Estimations of mechanical properties of intact tendons have usually applied ultrasonography1. This method measures the entire length of the tendon, and is therefore unlikely to capture the properties of the injured region. Methods are currently under development that probably will enable localized measurements, based on elastography. Using this method, the viscous properties of intact tendons have recently been described for the first time2, but so far there is not much known about the mechanics of healing human Achilles tendons. We have previously used tantalum beads implanted into the tendon during surgery for Achilles tendon rupture. This allowed use of the radiosterometric analysis (RSA) technique3, which can measure the distance between the beads with a precision around 0.1 mm. Strain during loading will increase the distance between the beads. By measuring tendon strain using two different weights for loading, we achieved 2 points of the load-deformation curve. Assuming a purely elastic behavior, we then estimated what we thought to be tendon stiffness and modulus of elasticity. These measurements were sufficiently accurate to allow prognostic statements3-6 and allowed us to show a statistically significant improvement due to early controlled loading in a randomized trial4. The estimated modulus seemed to remain constant from 7 to 19 weeks after injury, and then increased to 52 weeks after injury. Similarly, tissue density remained unchanged until 19 weeks and then increased dramatically to 52 weeks7. In contrast, the cross-sectional area increased from 7 to 19 weeks and then receded. Together, these data seem to suggest that tissue properties do not really change during the regenerative phase of healing, and that remodeling is not sufficiently effective to influence tissue characteristics until after 19 weeks. Our previous studies have assumed the tissue to be mainly elastic and disregarded the viscous component of deformation. By using pre-loading and standardized time points, a time-dependent component of Original article