Tendons exhibit greater resistance to tissue and molecular-level damage with increasing strain rate during cyclic fatigue

Targeting damaged collagen for intra-articular delivery of therapeutics using collagen hybridizing peptides

The objective of this study was to investigate the potential of collagen hybridizing peptides (CHPs), which bind to denatured collagen, to extend the retention time of near-infrared fluorophores (NIRF) following intra-articular (IA) injection in rat knee joints. CHPs were synthesized with a NIRF conjugated to the N-terminus. Male Sprague-Dawley rats were assigned to one of four experimental groups: healthy, CHP; osteoarthritis (OA), CHP; healthy, scrambled-sequence CHP (sCHP), which has no collagen binding affinity; or OA, sCHP. Animals in the OA groups received an IA injection of monosodium iodoacetate to induce OA. All animals then received the corresponding CHP injection. Animals were imaged repeatedly over 2 weeks using an in vivo fluorescence imaging system. Joint components were isolated and imaged to determine CHP binding distribution. Safranin-O and Fast Green histological staining was performed to confirm the development of OA. CHPs were found to be retained within the joint following IA injection in both healthy and OA animals for the full study period. In contrast, sCHP signal was negligible by 24-48 h. CHP signal was significantly greater (p < 0.05) in OA joints when compared to healthy joints. At the 2-week end point, multiple joint components retained CHPs, including cartilage, meniscus, and synovium. CHPs dramatically extended the retention time of NIRFs following IA injection in healthy and OA knee joints by binding to multiple collagenous tissues in the joint. These results support the pursuit of further research to develop CHP based therapeutics for IA treatment of OA.

Effects of extreme cyclic loading on the cushioning performance of human heel pads under engineering test condition

Human heel pads commonly undergo cyclic loading during daily activities. Low cyclic loadings such as daily human walking tend to have less effect on the mechanical properties of heel pads. However, the impact of cyclic loading on cushion performance, a vital biomechanical property of heel pads, under engineering test condition remains unexplored. Herein, dynamic mechanical measurements and finite element (FE) simulations were employed to explore this phenomenon. It was found that the wavy collagen fibers in the heel pad will be straightened under cycle compression loading, which resulted in increased stiffness of the heel pad. The stiffness of the heel pads demonstrated an inclination to escalate over a span of 50,000 loading cycles, consequently resulting in a corresponding increase in peak impact force over the same loading cycles. Sustained cyclic loading has the potential to result in the fracturing of the straightened collagen fibers, this collagen breakage may diminish the stiffness of the heel pad, leading to a reduction in peak impact force. This work enhances understanding of the biomechanical functions of human heel pad and may provide potential inspirations for the innovative development of healthcare devices for foot complex.

Tendon cell and tissue culture: Perspectives and recommendations

The wide variety of cell and tissue culture systems used to study and engineer tendons can make it difficult to choose the best approach and "optimal" culture conditions to test a given hypothesis. Therefore, a breakout session was organized at the 2022 ORS Tendon Section Meeting that focused on establishing a set of guidelines for conducting cell and tissue culture studies of tendon. This paper summarizes the outcomes of that discussion and presents recommendations for future studies. In the case of studying tendon cell behavior, cell and tissue culture systems are reductionist models in which the culture conditions should be strictly defined to approximate the in vivo condition as closely as possible. In contrast, for tissue engineering tendon replacements, the culture conditions do not need to replicate native tendon, but the outcome measures for success should be narrowly defined for the specific clinical application. Common recommendations for both applications are that researchers should perform a baseline phenotypic characterization of the cells that are ultimately used for experimentation. For models of tendon cell behavior, culture conditions should be well justified by existing literature and meticulously reported, tissue explant viability should be assessed, and comparisons to in vivo conditions should be made to determine baseline physiological relevance. For tissue engineering applications, the functional/structural/compositional outcome targets should be defined by the specific tendons they seek to replace, with key biologic and material properties prioritized for construct assessment. Lastly, when engineering tendon replacements, researchers should utilize clinically approved cGMP materials to facilitate clinical translation.

Modeling fatigue failure in soft tissue using a visco-hyperelastic model with discontinuous damage

Soft tissue is susceptible to injury from single high-magnitude static loads and from repetitive low-magnitude fatigue loads. While many constitutive formulations have been developed and validated to model static failure in soft tissue, a modeling framework is not well-established for fatigue failure. Here we determined the feasibility of using a visco-hyperelastic damage model with discontinuous damage (strain energy-based damage criterion) to simulate low- and high-cycle fatigue failure in soft fibrous tissue. Cyclic creep data from six uniaxial tensile fatigue experiments of human medial meniscus were used to calibrate the specimen-specific material parameters. The model was able to successfully simulate all three characteristic stages of cyclic creep, and predict the number of cycles until tissue rupture. Mathematically, damage propagated under constant cyclic stress due to time-dependent viscoelastic increases in tensile stretch that in turn increased strain energy. Our results implicate solid viscoelasticity as a fundamental regulator of fatigue failure in soft tissue, where tissue with slow stress relaxation times will be more resistant to fatigue injury. In a validation study, the visco-hyperelastic damage model was able to simulate characteristic stress-strain curves of pull to failure experiments (static failure) when using material parameters curve fit to the fatigue experiments. For the first time, we've shown that a visco-hyperelastic discontinuous damage framework can model cyclic creep and predict material rupture in soft tissue, and may enable the reliable simulation of both fatigue and static failure behavior from a single constitutive formulation.

Fatigue-driven compliance increase and collagen unravelling in mechanically tested anterior cruciate ligament

Approximately 300,000 anterior cruciate ligament (ACL) tears occur annually in the United States, half of which lead to the onset of knee osteoarthritis within 10 years of injury. Repetitive loading is known to result in fatigue damage of both ligament and tendon in the form of collagen unravelling, which can lead to structural failure. However, the relationship between tissue's structural, compositional, and mechanical changes are poorly understood. Herein we show that repetitive submaximal loading of cadaver knees causes an increase in co-localised induction of collagen unravelling and tissue compliance, especially in regions of greater mineralisation at the ACL femoral enthesis. Upon 100 cycles of 4× bodyweight knee loading, the ACL exhibited greater unravelled collagen in highly mineralized regions across varying levels of stiffness domains as compared to unloaded controls. A decrease in the total area of the most rigid domain, and an increase in the total area of the most compliant domain was also found. The results highlight fatigue-driven changes in both protein structure and mechanics in the more mineralized regions of the ACL enthesis, a known site of clinical ACL failure. The results provide a starting point for designing studies to limit ligament overuse injury.

Strain-induced collagen denaturation is rate dependent in failure of cerebral arteries

While soft tissues are commonly damaged by mechanical loading, the manifestation of this damage at the microstructural level is not fully understood. Specifically, while rate-induced stiffening has been previously observed in cerebral arteries, associated changes in microstructural damage patterns following high-rate loading are largely undefined. In this study, we stretched porcine middle cerebral arteries to failure at 0.01 and >150 s^-1, both axially and circumferentially, followed by probing for denatured tropocollagen using collagen hybridizing peptide (CHP). We found that collagen fibrils aligned with the loading direction experienced less denaturation following failure tests at high than low rates. Others have demonstrated similar rate dependence in tropocollagen denaturation during soft tissue failure, but this is the first study to quantify this behavior using CHP and to report it for cerebral arteries. These findings may have significant implications for traumatic brain injury and intracranial balloon angioplasty. We additionally observed possible tropocollagen denaturation in vessel layers primarily composed of fibrils transversely aligned to the loading axis. To our knowledge, this is the first observation of collagen denaturation due to transverse loading, but further research is needed to confirm this finding. STATEMENT OF SIGNIFICANCE: Previous work shows that collagen hybridizing peptide (CHP) can be used to identify collagen molecule unfolding and denaturation in mechanically overloaded soft tissues, including the cerebral arteries. But experiments have not explored collagen damage at rates relevant to traumatic brain injury. In this work, we quantified collagen damage in cerebral arteries stretched to failure at both high and low rates. We found that the collagen molecule is less damaged at high than at low rates, suggesting that damage mechanisms of either the collagen molecule or other elements of the collagen superstructure are rate dependent. This work implies that arteries failed at high rates, such as in traumatic brain injury, will have different molecular-level damage patterns than arteries failed at low rates. Consequently, improved understanding of damage characteristics may be expanded in the future to better inform clinically relevant cases of collagen damage such as angioplasty and injury healing.

Collagen fibrils from both positional and energy-storing tendons exhibit increased amounts of denatured collagen when stretched beyond the yield point

Collagen molecules are the base structural unit of tendons, which become denatured during mechanical overload. We recently demonstrated that during tendon stretch, collagen denaturation occurs at the yield point of the stress-strain curve in both positional and energy-storing tendons. We were interested in investigating how this load is transferred throughout the collagen hierarchy, and sought to determine the onset of collagen denaturation when collagen fibrils are stretched. Fibrils are one level above the collagen molecule in the collagen hierarchy, allowing more direct probing of the effect of strain on collagen molecules. We isolated collagen fibrils from both positional and energy-storing tendon types and stretched them using a microelectromechanical system device to various levels of strain. We stained the fibrils with fluorescently labeled collagen hybridizing peptides that specifically bind to denatured collagen, and examined whether samples stretched beyond the yield point of the stress-strain curve exhibited increased amounts of denatured collagen. We found that collagen denaturation in collagen fibrils from both tendon types occurs at the yield point. Greater amounts of denatured collagen were found in post-yield positional fibrils than in energy-storing fibrils. This is despite a greater yield strain and yield stress in fibrils from energy-storing tendons compared to positional tendons. Interestingly, the peak modulus of collagen fibrils from both tendon types was the same. These results are likely explained by the greater crosslink density found in energy-storing tendons compared to positional tendons. The insights gained from this study could help management of tendon and other musculoskeletal injuries by targeting collagen molecular damage at the fibril level. STATEMENT OF SIGNIFICANCE: When tendons are stretched or torn, this can lead to collagen denaturation (damage). Depending on their biomechanical function, tendons are considered positional or energy-storing with different crosslink profiles. By stretching collagen fibrils instead of fascicles from both tendon types, we can more directly examine the effect of tensile stretch on the collagen molecule in tendons. We found that regardless of tendon type, collagen denaturation in fibrils occurs when they are stretched beyond the yield point of the stress-strain curve. This provides insight into how load affects different tendon sub-structures during tendon injuries and failure, which will help clinicians and researchers understand mechanisms of injuries and potentially target collagen molecular damage as a treatment strategy, leading to improved clinical outcomes following injury.