Fascicular Elastin Modulates Recovery of Mechanical Properties Following Sub-Failure Repetitive Loading

While fascicular elastic fibers have been shown to significantly affect mechanical properties of tendon in stress relaxation and ramp to failure testing, the contribution of elastin to fatigue properties has only recently been investigated. This study expanded upon recent fatigue-to-failure data in wild-type and elastin knockdown mice (Prx1Cre+;Elnfl/fl) by halting tests at 50% of cyclic fatigue (based on normalized strain) instead of completing tests to tissue failure. Following 50% fatigue loading, Achilles (AT) and tibialis anterior (TB) tendons were subjected to subsequent stress relaxation and ramp to failure testing, enabling comparison to prior properties of non-damaged tendons to determine the effects of sub-failure fatigue. Indeed, multiple properties (e.g., ultimate stress, linear modulus) were decreased following fatigue loading, especially in elastin-deficient tendons, and genotype-dependent differences in stress relaxation properties were observed. Quantitative metrics of damage (i.e., collagen denaturation and fiber kinking) were not different between wild-type and elastin knockdown tendons as observed previously following fatigue-induced failure, suggesting that tendon damage develops later in the fatigue lifecycle. In addition, results suggest that elastin mediates collagen fiber alignment more in AT than TB, providing evidence that the different effects of elastin on tendon mechanics rely on microstructural mechanisms that vary by tendon type. Results suggest that individuals with deficient or depleted elastin may experience impaired recovery following repetitive tendon loading, which could have downstream effects on subsequent damage accumulation and tissue remodeling that should be investigated further in future studies.