GAG depletion increases the stress-relaxation response of tendon fascicles, but does not influence recovery

Abstract

Cyclic and static loading regimes are commonly used to study tenocyte metabolism in vitro and to improve our understanding of exercise-associated tendon pathologies. The aims of our study were to investigate if cyclic and static stress relaxation affected the mechanical properties of tendon fascicles differently, if this effect was reversible after a recovery period, and if the removal of glycosaminoglycans (GAGs) affected sample recovery. Tendon fascicles were dissected frombovine-foot extensors and subjected to 14% cyclic (1 Hz) or static tensile strain for 30 min. Additional fascicles were incubated overnight in buffer with 0.5 U chondroitinase ABC or in buffer alone prior to the static stress-relaxation regime. To assess the effect of different stress-relaxation regimes, a quasi-static test to failure was carried out, either directly post loading or after a 2 h recovery period, and compared with unloaded control fascicles. Both stress-relaxation regimes led to a significant reduction in fascicle failure stress and strain, but this was more pronounced in the cyclically loaded specimens. Removal of GAGs led to more stress relaxation and greater reductions in failure stress after static loading compared to controls. The reduction in mechanical properties was partially reversible in all samples, given a recovery period of 2 h. This has implications for mechanical testing protocols, as a time delay between fatiguing specimens and characterization of mechanical properties will affect the results. GAGs appear to protect tendon fascicles from fatigue effects, possibly by enabling sample hydration.

Collagen network strengthening following cyclic tensile loading

“…Failure to permit an adequate viscoelastic recovery time between testing events can lead to apparent increases in the zero-load strain and apparent increases in the tangent modulus. Previous investigations in the literature have used a range of recovery times, from the order of hundreds of seconds [14,67] to hours [23] to days [68]. In this investigation, a 5-min hold at zero displacement was prescribed between events.…”

Time-dependent mechanical behavior of human amnion: Macroscopic and microscopic characterization

“…In addition to fiber recruitment and uncrimping, global fiber alignment towards the direction of loading needs to be considered as an important mechanism that contributes to the time-dependent response. Creep [54] and relaxation [41] behavior are also affected by tissue hydration, due to the fact that fibers move in a viscous hydrated matrix. The present data indicate a pronounced volume decrease during loading and holding phases, related to water outflow from the collagenous network.…”

“…Stress-relaxation and cyclic experiments on human amnion showed a stresslevel-dependent response and, surprisingly, lower dissipation at higher strain levels, which could indicate an intrinsic coupling of strain-and time-dependency [15,16]. Stress-relaxation in soft biological tissues arises from microstructural mechanisms, such as relaxation of single collagen fibrils [33,34], global rearrangement of collagen microstructure [34,35,36], progressive failures of crosslinks [37,38,39], liquid phase rearrangement or dehydration [40,41], and may depend on the stress level reached [42]. The specific mechanisms determining the mechanical time-dependence of amnion have not yet been identified.…”

Modelling approaches for evaluating multiscale tendon mechanics

“…It is well known that tendon under quasi-static tensile loading exhibits three stages of the stress-strain curve: an initial low-strain toe region, a higher-strain linear region, followed by failure (figure 2c) [40,77]. Because waviness (or crimp) of collagen fibrils and a helical superstructure of tendon are thought to contribute to the toe region [78,79], a few models have sought to address the influence of these Hurschler et al proposed that the straightening stretch ratio l S (i.e.…”

“…Towards this end, experimental studies that evaluate distinct tendons using consistent testing methods and protocols [26,31,[35][36][37][38][39] are beginning to provide data necessary to develop appropriate model formulations. Furthermore, the utilization of multiscale modelling could help explore physical mechanisms regulating tendon behaviour, including controversial issues such as whether PGs mediate load share between fibrils [22,40,41].…”