Biomechanical and structural response of healing Achilles tendon to fatigue loading following acute injury

Abstract

Achilles tendon injuries affect both athletes and the general population, and their incidence is rising. In particular, the Achilles tendon is subject to dynamic loading at or near failure loads during activity, and fatigue induced damage is likely a contributing factor to ultimate tendon failure. Unfortunately, little is known about how injured Achilles tendons respond mechanically and structurally to fatigue loading during healing. Knowledge of these properties remains critical to best evaluate tendon damage induction and the ability of the tendon to maintain mechanical properties with repeated loading. Thus, this study investigated the mechanical and structural changes in healing mouse Achilles tendons during fatigue loading. Twenty four mice received bilateral full thickness, partial width excisional injuries to their Achilles tendons (IACUC approved) and twelve tendons from six mice were used as controls. Tendons were fatigue loaded to assess mechanical and structural properties simultaneously after 0, 1, 3, and 6 weeks of healing using an integrated polarized light system. Results showed that the number of cycles to failure decreased dramatically (37-fold, p<0.005) due to injury, but increased throughout healing, ultimately recovering after 6 weeks. The tangent stiffness, hysteresis, and dynamic modulus did not improve with healing (p<0.005). Linear regression analysis was used to determine relationships between mechanical and structural properties. Of tendon structural properties, the apparent birefringence was able to best predict dynamic modulus (R2=0.88–0.92) throughout healing and fatigue life. This study reinforces the concept that fatigue loading is a sensitive metric to assess tendon healing and demonstrates potential structural metrics to predict mechanical properties.

Collagen V-heterozygous and -null supraspinatus tendons exhibit altered dynamic mechanical behaviour at multiple hierarchical scales

“…Damage was significantly decreased only in the initial phase (5%) of fatigue life in the null group (figure 4b). Peak cyclic strain and damage increased while tangent stiffness and hysteresis decreased throughout fatigue life, as previously reported [20,43]. The null group had a significantly increased peak cyclic strain than the heterozygous group at 50% and both groups at 95% fatigue life (figure 4d).…”

“…An additional twenty specimens from each group were subjected to a fatigue loading protocol, as described previously [20,43]. Preliminary studies determined that the mean failure loads were significantly different between groups and as a result, all tendons were fatigue tested at 1 Hz between 20 and 75% of their ultimate failure load (0.75 -2.75 N for wild-type and heterozygous groups, 0.25-1.0 N for the null group).…”

“…In addition to their anisotropic and nonlinear behaviour, tendons exhibit several viscoelastic properties, such as stress relaxation, hysteresis and creep [2,[13][14][15][16][17][18][19]. This process is both static and dynamic, as demonstrated by a similar decrease in peak stress over time with repetitive, cyclic tensile loading [20]. Fibril sliding has also been cited as a contributor to dynamic viscoelastic behaviour during this initial response to load [19].…”

The effect of strain and age on the mechanical properties of rat Achilles tendons

“…The clamp was secured to the AT itself during experiment in order to eliminate any cofounding artifacts from muscle tissue. Specimens were placed in saline soaked gauze and frozen at -20°F 25 . Specimens were retrieved for testing and thawed slowly over a twentyfour hour period.…”

Rabbit common calcanean tendon as an animal model: ultrasonographic anatomy and morphometry

“…Considering that the tissue specimens of injured tendons are not available for histopathological examination in clinical practice, the ultrasound and magnetic resonance imaging are the gold standard in diagnosing tendon disorders, with ultrasonography being cost-effective, non-invasive and generally accessible [10,11,21]. Several animal models are currently used in tendinopathy research including laboratory animals, like rabbits [5,23,25,26,29,33], rats [9,15,17] and mice [8] as well as companion animals, like horses [2,3,31] and dogs [20]. Despite the fact that veterinary patients are increasingly recognised as translational models of human tendinopathies [19,30] their use in experimental protocols seems controversial and is not accepted in several countries due to ethical issues.…”