In-vivo and in-vitro environments affect the storage and release of energy in tendons

Abstract

Understanding tendon mechanical properties, such as stiffness and hysteresis, can provide insights into injury mechanisms. This research addresses the inconsistency of previously reported in-vivo and in-vitro tendon hysteresis properties. Although limited, our preliminary findings suggest that in-vivo hystereses (Mean ± SD; 55% ± 9%) are greater than in-vitro hystereses (14% ± 1%) when directly comparing the same tendon for the same loading conditions in a sheep model in-vivo versus within 24 h post-mortem. Overall, it therefore appears that the tendon mechanical properties are affected by the testing environment, possibly related to differences in muscle-tendon interactions and fluid flow experienced in-vivo versus in-vitro. This communication advocates for more detailed investigations into the mechanisms resulting in the reported differences in tendon behaviour. Overall, such knowledge contributes to our understanding of tendon function towards improving modelling and clinical interventions, bridging the gap between in-vivo and in-vitro observations and enhancing the translational relevance of biomechanical studies.

Keywords: Achilles tendon; experimental -animal models; hysteresis; mechanical properties; musculoskeletal biomechanics; sheep model.

FIGURE 1

Overview of previously reported hysteresis values of free tendon human and animal in-vivo and in-vitro experiments. Presented experimental data within this study are the outliers for both in-vivo and in-vitro data. Note that results of the present study are the first to achieve synchronized measurement of both force and strain data in-vivo, but also directly compare the same tissue under comparable mechanical conditions in-vitro.

FIGURE 2

Exemplary tendon strain vs. tendon force obtained for comparable force-time histories under in-vivo (purple) and in-vitro (black) conditions. Note that the slack length for the in-vivo (shortest length measured during all trials) and in-vitro conditions (length at zero force, just prior to the onset of force application) were defined differently, and likely caused a shift in tendon strains between the two conditions. To control for this shift, in-vivo peak strains were aligned with in-vitro peak strains causing a shift of the in-vivo hysteresis curves to the left.