Multi-scale Loading and Damage Mechanisms of Rat Tail, Plantaris, and Achilles Tendons

Tendinopathy, degeneration of tendon that leads to pain and dysfunction, is common in both sports and occupational settings, but multi-scale mechanisms for tendinopathy are still unknown. We recently showed that micro-scale sliding (shear) is responsible for both load transfer and damage mechanisms in rat tail tendon; however, rat tail tendon is a specialized non-load bearing tendon, and thus the load transfer and damage mechanisms are still unknown for load-bearing tendons. The objective of this study was to investigate the load transfer and damage mechanisms of load-bearing tendons using rat plantaris and Achilles tendons. We demonstrated that the micro-scale sliding is a key component for both mechanisms in plantaris tendon, similar to tail tendon. Namely, the micro-scale sliding was correlated with applied strain, demonstrating that load was transferred via micro-scale sliding in the plantaris and tail tendons. In addition, while the micro-scale strain fully recovered, the micro-scale sliding was non-recoverable and strain-dependent, and correlated with a tissue-scale mechanical parameters. When the applied strain was normalized, the % magnitudes of non-recoverable sliding was similar between the plantaris and tail tendons. Achilles tendon demonstrated some of the mechanical responses observed in plantaris and tail tendons, yet the results were inconclusive due to its complex structure. Statement of Clinical Significance: Understanding the mechanisms responsible for the pathogenesis and progression of tendinopathy can improve prevention and rehabilitation strategies and guide therapies and design of engineered constructs.