Ligament creep recruits fibres at low stresses and can lead to modulus‐reducing fibre damage at higher creep stresses: a study in rabbit medial collateral ligament model

Abstract

Ligaments are subjected to a range of loads during different activities in vivo, suggesting that they must resist creep at various stresses. Cyclic and static creep tests of rabbit medial collateral ligament were used as a model to examine creep over a range of stresses in the toe-and linear-regions of the stress-strain curve: 4.1 MPa (n = 7), 7.1 MPa ( n = 6), 14 MPa ( n = 9) and 28 MPa ( n = 6). We quantified ligament creep behaviour to determine if, at low stresses, modulus would increase in a cyclic creep test and collagen fibres would be recruited in a static creep test. At higher creep stresses, a decrease in measured modulus was expected to be a potential marker of damage. The increase in modulus during cyclic creep and the increase in strain during static creep were similar between the three toe-region stresses (4.1, 7.1, 14 MPa). However, at the linear-region stress (28 MPa), both these parameters increased significantly compared to the increases at the three toe-region stresses. A concurrent crimp analysis revealed that collagen fibres were recruited during creep, evidenced by decreased area of crimped fibres at the end of the static creep test. Interestingly, a predominance of straightened fibres was observed at the end of the 28 MPa creep test, suggesting a limited potential for fibre recruitment at higher, linear-region stresses. An additional 28 MPa ( n = 6) group had mechanically detectable discontinuities in their stress-strain curves during creep that were related to reductions in modulus and suggested fibre damage. These data support the concept that collagen fibre recruitment is a mechanism by which ligaments resist creep at low stresses. At a higher creep stress, which was still only about a third of the failure capacity, damage to some ligaments occurred and was marked by a sudden reduction in modulus. In the cyclic tests, with continued cycling, the modulus increased back to original values obtained before the discontinuity suggesting that other fibres were being recruited to bear load. These results have important implications for our understanding of how fibre recruitment and stress redistribution act in normal ligament to minimize creep and restore modulus after fibre damage.

Material Properties of the Human Lumbar Facet Joint Capsule

“…This is analogous to predictions of the model by Maksym and Bates [42] for lung viscoelasticity, which was shown to depend on the interactions between elastin, and collagen waviness, and recruitment, and as suggested earlier by Viidik [43][44][45]. In composite tissues, such as facet joint capsule, elastin tends to dominate load bearing at low strain magnitudes, while the collagen fibers are still being straightened out; whereas, at higher strains, the collagen fibers are largely straight [46] and, being orders of magnitude stiffer than elastin [47,48], bear more of the load. Tissues with low variability in collagen fiber waviness and/or length would have increased recruitment of the fiber bundles over a relatively smaller span of tensile strains compared to tissues with high variability [42], resulting in an increase in the developed stress at large tensile strains.…”

Behaviour of the human gastrocnemius muscle architecture during submaximal isometric fatigue

“…Therefore, the results of this study indicate that there might be an additional mechanism contributing to the changes in the force-generation potential of the triceps surae muscle and hence to the observed increase in EMG activity during the isometric fatiguing task, namely the creep of the tendon and aponeurosis. The fact that the tendon illustrates viscoelastic behaviour such as creep even at lower strains (toe-region) has been shown at in vitro studies (Rigby 1964;Cohen et al 1976;Thornton et al 2002). During fatigue, the fascicle length decreased about 11% of its initial value.…”

“…Another well-known phenomenon in the literature is that in vitro the tendon exhibits viscoelastic behaviour such as creep even at low strains (Rigby 1964;Cohen et al 1976; Thornton et al 2002). A possible tendon creep during a long-lasting isometric fatiguing contraction in vivo could influence the geometry of the muscle.…”

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

“…This model was able to predict a sufficiently different response in creep and relaxation tests. According to the model, the progressive fiber recruitment minimizes creep strain accumulation at the physiological stress level [42] and can be interpreted as the same microstructural mechanism that induces the stress-strain nonlinearity in this range [53]. Additionally, the collagenous crimp pattern visualized from frozen histological sections was shown to change significantly after creep, and was larger in straightened fibers with higher loading stresses [35].…”

“…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.…”