Fascicles and the interfascicular matrix show adaptation for fatigue resistance in energy storing tendons

The anisotropic and region-dependent mechanical response of wrap-around tendons under tensile, compressive and combined multiaxial loads

The present work reports on the multiaxial region and orientation-dependent mechanical properties of two porcine wrap-around tendons under tensile, compressive and combined loads based on an extensive study with n=175 samples. The results provide a detailed dataset of the anisotropic tensile and compressive longitudinal properties and document a pronounced tension-compression asymmetry. Motivated by the physiological loading conditions of these tendons, which include transversal compression at bony abutments in addition to longitudinal tension, we systematically investigated the change in axial tension when the tendon is compressed transversally along one or both perpendicular directions. The results reveal that the transversal compression can increase axial tension (proximal-distal direction) in both cases to orders of 30%, yet by a larger amount in the first case (transversal compression in anterior-posterior direction), which seems to be more relevant for wrap-around tendons in-vivo. These quantitative measurements are in line with earlier findings on auxetic properties of tendon tissue, but show for the first time the influence of this property on the stress response of the tendon, and may thus reveal an important functional principle within these essential elements of force transmission in the body. STATEMENT OF SIGNIFICANCE: The work reports for the first time on multiaxial region and orientation-dependent mechanical properties of wrap-around tendons under various loads. The results indicate that differences in the mechanical properties exist between zones that are predominantly in a uniaxial tensile state and those that experience complex load states. The observed counterintuitive increase of the axial tension upon lateral compression points at auxetic properties of the tendon tissue which may be pivotal for the function of the tendon as an element of the musculoskeletal system. It suggests that the tendon's performance in transmitting forces is not diminished but enhanced when the action line is deflected by a bony pulley around which the tendon wraps, representing an important functional principle of tendon tissue.

The Interfascicular Matrix of Energy Storing Tendons Houses Heterogenous Cell Populations Disproportionately Affected by Aging

Energy storing tendons such as the human Achilles and equine superficial digital flexor tendon (SDFT) are prone to injury, with incidence increasing with aging, peaking in the 5th decade of life in the human Achilles tendon. The interfascicular matrix (IFM), which binds tendon fascicles, plays a key role in energy storing tendon mechanics, and aging alterations to the IFM negatively impact tendon function. While the mechanical role of the IFM in tendon function is well-established, the biological role of IFM-resident cell populations remains to be elucidated. Therefore, the aim of this study was to identify IFM-resident cell populations and establish how these populations are affected by aging. Cells from young and old SDFTs were subjected to single cell RNA-sequencing, and immunolabelling for markers of each resulting population used to localise cell clusters. Eleven cell clusters were identified, including tenocytes, endothelial cells, mural cells, and immune cells. One tenocyte cluster localised to the fascicular matrix, whereas nine clusters localised to the IFM. Interfascicular tenocytes and mural cells were preferentially affected by aging, with differential expression of genes related to senescence, dysregulated proteostasis and inflammation. This is the first study to establish heterogeneity in IFM cell populations, and to identify age-related alterations specific to IFM-localised cells.

Guidelines for ex vivo mechanical testing of tendon

Tendons are critical for the biomechanical function of joints. Tendons connect muscles to bones and allow for the transmission of muscle forces to facilitate joint motion. Therefore, characterizing the tensile mechanical properties of tendons is important for the assessment of functional tendon health and efficacy of treatments for acute and chronic injuries. In this guidelines paper, we review methodological considerations, testing protocols, and key outcome measures for mechanical testing of tendons. The goal of the paper is to present a simple set of guidelines to the nonexpert seeking to perform tendon mechanical tests. The suggested approaches provide rigorous and consistent methodologies for standardized biomechanical characterization of tendon and reporting requirements across laboratories.

Challenging the assumption of uniformity in patellar tendon structure: Regional patellar tendon morphology and mechanical properties in vivo

Patellar tendons are assumed to be uniform in morphology and mechanical properties despite a higher prevalence of tendinopathies observed in the medial region. The purpose of this study was to compare the thickness, length, viscosity, and shear modulus of the medial, central, and lateral regions of healthy patellar tendons of young males and females in vivo. B-mode ultrasound and continuous shear wave elastography were performed on 35 patellar tendons (17 females, 18 males) over three regions of interest. A linear mixed-effects model (α = 0.05) was used to determine differences between the three regions and sexes followed by pairwise comparisons for significant findings. The lateral region (mean [95% confidence interval] = 0.34 [0.31-0.37] cm) was thinner compared with the medial (0.41 [0.39-0.44] cm, p < 0.001), and central (0.41 [0.39-0.44] cm, p < 0.001) regions regardless of sex. Viscosity was lower in the lateral (19.8 [16.9-22.7] Pa-s) versus medial region (27.4 [24.7-30.2] Pa-s, p = 0.001). Length had a region-by-sex interaction (p = 0.003) characterized by a longer lateral (4.83 [4.54-5.13] cm) versus medial (4.42 [4.12-4.72] cm) region in males (p < 0.001), but not females (p = 0.992). Shear modulus was uniform between regions and sexes. The thinner, and less viscous lateral patellar tendon may reflect the lower load the tendon experiences explaining the differences in regional prevalence of developing tendon pathology. Statement of Clinical Significance: Healthy patellar tendons are not uniform in morphology or mechanical properties. Considering regional tendon properties may help guide targeted interventions for patellar tendon pathologies.

Equine tendon mechanical behaviour: Prospects for repair and regeneration applications

Tendons are dense connective tissues that play an important role in the biomechanical function of the musculoskeletal system. The mechanical forces have been implicated in every aspect of tendon biology. Tendon injuries are frequently occurring and their response to treatments is often unsatisfactory. A better understanding of tendon biomechanics and mechanobiology can help develop treatment options to improve clinical outcomes. Recently, tendon tissue engineering has gained more attention as an alternative treatment due to its potential to overcome the limitations of current treatments. This review first provides a summary of tendon mechanical properties, focusing on recent findings of tendon mechanobiological responses. In the next step, we highlight the biomechanical parameters of equine energy-storing and positional tendons. The final section is devoted to how mechanical loading contributes to tenogenic differentiation using bioreactor systems. This study may help develop novel strategies for tendon injury prevention or accelerate and improve tendon healing.

Biarticular mechanisms of the gastrocnemii muscles enhance ankle mechanical power and work during running

The objective of the study was to explore how biarticular mechanisms of the gastrocnemii muscles may provide an important energy source for power and work at the ankle joint with increasing running speed. Achilles tendon force was quantified as a proxy of the triceps surae muscle force and the contribution of the monoarticular soleus and the biarticular gastrocnemii to the mechanical power and work performed at the ankle joint was investigated in three running speeds (transition 2.0 m s-1, slow 2.5 m s-1, fast 3.5 m s-1). Although the contribution of the soleus was higher, biarticular mechanisms of the gastrocnemii accounted for a relevant part of the performed mechanical power and work at the ankle joint. There was an ankle-to-knee joint energy transfer in the first part of the stance phase and a knee-to-ankle joint energy transfer during push-off via the gastrocnemii muscles, which made up 16% of the total positive ankle joint work. The rate of knee-to-ankle joint energy transfer increased with speed, indicating a speed-related participation of biarticular mechanisms in running. This energy transfer via the gastrocnemii seems to occur with negligible energy absorption/production from the quadriceps vasti contractile elements and is rather an energy exchange between elastic structures.

Structure-function characterization of the transition zone in the intervertebral disc

Understanding the structure-function relationship in the intervertebral disk (IVD) is crucial for the development of novel tissue engineering strategies to regenerate IVD and the establishment of accurate computational models for low back pain research. A large number of studies have improved our knowledge of the mechanical and structural properties of the nucleus pulposus (NP) and annulus fibrosus (AF), two of the main regions in the IVD. However, few studies have focused on the AF-NP interface (transition zone; TZ). Therefore, the current study aims to, for the first time, characterize the cyclic and failure mechanical properties of the TZ region under physiological loading (1, 3, and 5%s^-1 strain rates) and investigate the structural integration mechanisms between the NP, TZ, and AF regions. The results of the current study reveal significant effects of region (NP, TZ, and AF) and strain rates (1, 3, and 5%s^-1) on stiffness (p < 0.001). In addition, energy absorption is significantly higher for the AF compared to the TZ and NP (p <0.001) as well as between the TZ and NP (p <0.001). The current research finds adaptation, direct penetration, and entanglement between TZ and AF fibers as three common mechanisms for structural integration between the TZ and AF regions. STATEMENT OF SIGNIFICANCE: Despite a large number of studies that have mechanically, structurally, and biologically characterized the nucleus pulposus (NP) and annulus fibrosus (AF) regions, few studies have focused on the NP-AF interface region (known as Transition Zone; TZ) in the IVD; hence, our understanding of the TZ structure-function relationship is still incomplete. Of particular importance, the cyclic mechanical properties of the TZ, compared to the adjacent regions (NP and AF), are yet to be explored and the precise nature of the structural integration between the NP and AF via the TZ region is not yet known. The current study explores both the mechanical and structural properties of the TZ region to ultimately identify the mechanism of integration between the NP and AF.

Prospects of magnetically based approaches addressing inflammation in tendon tissues

Tendon afflictions constitute a significant share of musculoskeletal diseases and represent a primary cause of incapacity worldwide. Unresolved/chronic inflammatory states have been associated with the onset and progression of tendon disorders, contributing to undesirable immune stimulation and detrimental tissue effects. Thus, targeting persistent inflammatory events could assist important developments to solve pathophysiological processes and innovative therapeutics to address impaired healing and accomplish complete tendon regeneration. This review overviews the impact of inflammation and inflammatory mediators in tendon niches, unveiling the importance of tendon cell populations and their signature features, and the influence of microenvironmental factors on inflamed and injured tendons. The demand for non-invasive instructive strategies to manage persistent inflammatory mediators, guide inflammatory pathways, and modulate cellular responses will also be approached by exploring the role of pulsed electromagnetic field (PEMF). PEMF alone or combined with more sophisticated systems triggered by magnetic fields will be considered in the design of successful therapies to control inflammation in tendinopathic conditions.

The tendon microenvironment: Engineered in vitro models to study cellular crosstalk

Tendinopathy is a multi-faceted pathology characterized by alterations in tendon microstructure, cellularity and collagen composition. Challenged by the possibility of regenerating pathological or ruptured tendons, the healing mechanisms of this tissue have been widely researched over the past decades. However, so far, most of the cellular players and processes influencing tendon repair remain unknown, which emphasizes the need for developing relevant in vitro models enabling to study the complex multicellular crosstalk occurring in tendon microenvironments. In this review, we critically discuss the insights on the interaction between tenocytes and the other tendon resident cells that have been devised through different types of existing in vitro models. Building on the generated knowledge, we stress the need for advanced models able to mimic the hierarchical architecture, cellularity and physiological signaling of tendon niche under dynamic culture conditions, along with the recreation of the integrated gradients of its tissue interfaces. In a forward-looking vision of the field, we discuss how the convergence of multiple bioengineering technologies can be leveraged as potential platforms to develop the next generation of relevant in vitro models that can contribute for a deeper fundamental knowledge to develop more effective treatments.

The Role of the Non-Collagenous Extracellular Matrix in Tendon and Ligament Mechanical Behavior: A Review

Tendon is a connective tissue that transmits loads from muscle to bone, while ligament is a similar tissue that stabilizes joint articulation by connecting bone to bone. The 70-90% of tendon and ligament's extracellular matrix (ECM) is composed of a hierarchical collagen structure that provides resistance to deformation primarily in the fiber direction, and the remaining fraction consists of a variety of non-collagenous proteins, proteoglycans, and glycosaminoglycans (GAGs) whose mechanical roles are not well characterized. ECM constituents such as elastin, the proteoglycans decorin, biglycan, lumican, fibromodulin, lubricin, and aggrecan and their associated GAGs, and cartilage oligomeric matrix protein (COMP) have been suggested to contribute to tendon and ligament's characteristic quasi-static and viscoelastic mechanical behavior in tension, shear, and compression. The purpose of this review is to summarize existing literature regarding the contribution of the non-collagenous ECM to tendon and ligament mechanics, and to highlight key gaps in knowledge that future studies may address. Using insights from theoretical mechanics and biology, we discuss the role of the non-collagenous ECM in quasi-static and viscoelastic tensile, compressive, and shear behavior in the fiber direction and orthogonal to the fiber direction. We also address the efficacy of tools that are commonly used to assess these relationships, including enzymatic degradation, mouse knockout models, and computational models. Further work in this field will foster a better understanding of tendon and ligament damage and healing as well as inform strategies for tissue repair and regeneration.

The role of the tendon ECM in mechanotransduction: disruption and repair following overuse

Purpose: Tendon overuse injuries are prevalent conditions with limited therapeutic options to halt disease progression. The specialized extracellular matrix (ECM) both enables joint function and mediates mechanical signals to tendon cells, driving biological responses to exercise or injury. With overuse, tendon ECM composition and structure changes at multiple scales, disrupting mechanotransduction and resulting in inadequate repair and disease progression. This review highlights the multiscale ECM changes that occur with tendon overuse and corresponding effects on cell-matrix interactions and cellular response to load.Results: Different functional joint requirements and tendon types experience a wide range of loading profiles, creating varied downstream mechanical stimuli. Distinct ECM structure and mechanical properties within the fascicle matrix, interfascicle matrix, and enthesis and their varied disruption with overuse are considered. The pericellular matrix (PCM) comprising the microscale tendon cell environment has a unique composition that changes with overuse injury and exercise, suggesting an important role in mechanotransduction and promoting repair. Cell-matrix interactions are mediated by structures including cilia, integrins, connexins and cytoskeleton that signal downstream homeostasis, adaptation, or repair. ECM disruption with tendon overuse may cause altered mechanical loading and cell-matrix interactions, resulting in mechanobiological understimulation, apoptosis, and ineffective repair. Current interventions to promote repair of tendon overuse injuries including exercise, targeting cell signaling, and modulating inflammation are considered.Conclusion: Future therapeutics should be assessed with regard of their effects on multiscale mechanotransduction in addition to joint function, with consideration of the central role of ECM.

Mechanisms of Achilles Tendon Rupture in National Basketball Association Players

A systematic search was performed of online databases for any Achilles tendon (AT) injuries occurring within the National Basketball Association (NBA). Video was obtained of injuries occurring during competition and downloaded for analysis in Dartfish. NBA athletes (n = 27) were identified with AT rupture over a 30-year period (1991–2021). Of the 27 NBA athletes found to have AT ruptures (mean age: 29.3 [3.3] y; average time in the NBA: 8.5 [3.8] y), 15 in-game videos were obtained for analysis. Noncontact rupture was presumed to have occurred in 12/13 cases. Eight of the 13 athletes had possession of the ball during time of injury. The ankle joint of the injured limb for all 13 athletes was in a dorsiflexed position during the time of injury (47.9° [6.5°]). All 13 athletes performed a false-step mechanism at time of injury where they initiated the movement by taking a rearward step posterior to their center of mass with the injured limb before translating forward. NBA basketball players that suffered AT ruptures appeared to present with a distinct sequence of events, including initiating a false step with ankle dorsiflexion of the injured limb at the time of injury.

Basic Structure, Physiology, and Biochemistry of Connective Tissues and Extracellular Matrix Collagens

The physiology of connective tissues like tendons and ligaments is highly dependent upon the collagens and other such extracellular matrix molecules hierarchically organized within the tissues. By dry weight, connective tissues are mostly composed of fibrillar collagens. However, several other forms of collagens play essential roles in the regulation of fibrillar collagen organization and assembly, in the establishment of basement membrane networks that provide support for vasculature for connective tissues, and in the formation of extensive filamentous networks that allow for cell-extracellular matrix interactions as well as maintain connective tissue integrity. The structures and functions of these collagens are discussed in this chapter. Furthermore, collagen synthesis is a multi-step process that includes gene transcription, translation, post-translational modifications within the cell, triple helix formation, extracellular secretion, extracellular modifications, and then fibril assembly, fibril modifications, and fiber formation. Each step of collagen synthesis and fibril assembly is highly dependent upon the biochemical structure of the collagen molecules created and how they are modified in the cases of development and maturation. Likewise, when the biochemical structures of collagens or are compromised or these molecules are deficient in the tissues - in developmental diseases, degenerative conditions, or injuries - then the ultimate form and function of the connective tissues are impaired. In this chapter, we also review how biochemistry plays a role in each of the processes involved in collagen synthesis and assembly, and we describe differences seen by anatomical location and region within tendons. Moreover, we discuss how the structures of the molecules, fibrils, and fibers contribute to connective tissue physiology in health, and in pathology with injury and repair.

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

Musculoskeletal soft connective tissues are commonly injured due to repetitive use, but the evolution of mechanical damage to the tissue structure during repeated loading is poorly understood. We investigated the strain-rate dependence of mechanical denaturation of collagen as a form of structural microdamage accumulation during creep fatigue loading of rat tail tendon fascicles. We cycled tendons at three strain rates to the same maximum stress relative to their rate-dependent tensile strength. Collagen denaturation at distinct points during the fatigue process was measured by fluorescence quantification of collagen hybridizing peptide binding. The amount of collagen denaturation was significantly correlated with fascicle creep strain, independent of the cyclic strain rate, supporting our hypothesis that tissue level creep is caused by collagen triple-helix unfolding. Samples that were loaded faster experienced more creep strain and denaturation as a function of the number of loading cycles relative to failure. Although this increased damage capacity at faster rates may serve as a protective measure during high-rate loading events, it may also predispose these tissues to subsequent injury and indicate a mechanism of overuse injury development. These results build on evidence that molecular-level collagen denaturation is the fundamental mechanism of structural damage to tendons during tensile loading. STATEMENT OF SIGNIFICANCE: This study is the first to investigate the accumulation of denatured collagen in tendons throughout fatigue loading when the maximum stress is scaled with the applied strain rate. The amount of denatured collagen was correlated with creep strain, independent of strain rate, but samples that were cycled faster withstood greater amounts of denaturation before failure. Differential accumulation of collagen damage between fast and slow repetitive loading has relevance toward understanding the prevalence of overuse musculoskeletal injuries following sudden changes in activity level. Since collagen is a ubiquitous biological structural component, the basic patterns and mechanisms of loading-induced collagen damage in connective tissues are relevant for understanding injury and disease in other tissues, including those from the cardiovascular and pulmonary systems.

CD146 Delineates an Interfascicular Cell Sub-Population in Tendon That Is Recruited during Injury through Its Ligand Laminin-α4

The interfascicular matrix (IFM) binds tendon fascicles and contains a population of morphologically distinct cells. However, the role of IFM-localised cell populations in tendon repair remains to be determined. The basement membrane protein laminin-α4 also localises to the IFM. Laminin-α4 is a ligand for several cell surface receptors, including CD146, a marker of pericyte and progenitor cells. We used a needle injury model in the rat Achilles tendon to test the hypothesis that the IFM is a niche for CD146+ cells that are mobilised in response to tendon damage. We also aimed to establish how expression patterns of circulating non-coding RNAs alter with tendon injury and identify potential RNA-based markers of tendon disease. The results demonstrate the formation of a focal lesion at the injury site, which increased in size and cellularity for up to 21 days post injury. In healthy tendon, CD146+ cells localised to the IFM, compared with injury, where CD146+ cells migrated towards the lesion at days 4 and 7, and populated the lesion 21 days post injury. This was accompanied by increased laminin-α4, suggesting that laminin-α4 facilitates CD146+ cell recruitment at injury sites. We also identified a panel of circulating microRNAs that are dysregulated with tendon injury. We propose that the IFM cell niche mediates the intrinsic response to injury, whereby an injury stimulus induces CD146+ cell migration. Further work is required to fully characterise CD146+ subpopulations within the IFM and establish their precise roles during tendon healing.

Structure-function specialisation of the interfascicular matrix in the human achilles tendon

Tendon consists of highly aligned collagen-rich fascicles surrounded by interfascicular matrix (IFM). Some tendons act as energy stores to improve locomotion efficiency, but such tendons commonly obtain debilitating injuries. In equine tendons, energy storing is achieved primarily through specialisation of the IFM. However, no studies have investigated IFM structure-function specialisation in human tendons. Here, we compare the human positional anterior tibial tendon and energy storing Achilles tendons, testing the hypothesis that the Achilles tendon IFM has specialised composition and mechanical properties, which are lost with ageing. Data demonstrate IFM specialisation in the energy storing Achilles, with greater elasticity and fatigue resistance than in the positional anterior tibial tendon. With ageing, alterations occur predominantly to the proteome of the Achilles IFM, which are likely responsible for the observed trends towards decreased fatigue resistance. Knowledge of these key energy storing specialisations and their changes with ageing offers crucial insight towards developing treatments for tendinopathy.

Statement of significance

Developing effective therapeutics or preventative measures for tendon injury necessitates the understanding of healthy tendon function and mechanics. By establishing structure-function relationships in human tendon and determining how these are affected by ageing, potential targets for therapeutics can be identified. In this study, we have used a combination of mechanical testing, immunolabelling and proteomics analysis to study structure-function specialisations in human tendon. We demonstrate that the interfascicular matrix is specialised for energy storing in the Achilles tendon, and that its proteome is altered with ageing, which is likely responsible for the observed trends towards decreased fatigue resistance. Knowledge of these key energy storing specialisations and their changes with ageing offers crucial insight towards developing treatments and preventative approaches for tendinopathy.

The effects of triceps surae muscle stimulation on localized Achilles subtendon tissue displacements

The triceps surae muscle-tendon unit is composed of the lateral and medial gastrocnemius (MG) and soleus (SOL) muscles and three in-series elastic 'subtendons' that form the Achilles tendon. Comparative literature and our own in vivo evidence suggest that sliding between adjacent subtendons may facilitate independent muscle actuation. We aim to more clearly define the relationship between individual muscle activation and subtendon tissue displacements. Here, during fixed-end contractions, electrical muscle stimulation controlled the magnitude of force transmitted via individual triceps surae muscles while ultrasound imaging recorded resultant subtendon tissue displacements. We hypothesized that MG and SOL stimulation would elicit larger displacements in their associated subtendon. Ten young adults completed four experimental activations at three ankle angles (-20, 0 and 20 deg) with the knee flexed to approximately 20 deg: MG stimulation (STIMMG), SOL stimulation (STIMSOL), combined stimulation, and volitional contraction. At 20 deg plantarflexion, STIMSOL elicited 49% larger tendon non-uniformity (SOL-MG subtendon tissue displacement) than that of STIMMG (P=0.004). For STIMSOL, a one-way post hoc ANOVA revealed a significant main effect of ankle angle (P=0.009) on Achilles tendon non-uniformity. However, peak tendon non-uniformity decreased by an average of 61% from plantarflexion to dorsiflexion, likely due to an increase in passive tension. Our results suggest that localized tissue displacements within the Achilles tendon respond in anatomically consistent ways to differential patterns of triceps surae muscle activation, but these relations are highly susceptible to ankle angle. This in vivo evidence points to at least some mechanical independence in actuation between the human triceps surae muscle-subtendon units.

Cyclic tensile tests of Shetland pony superficial digital flexor tendons (SDFTs) with an optimized cryo-clamp combined with biplanar high-speed fluoroscopy

Background

Long-term cyclic tensile testing with equine palmar/plantar tendons have not yet been performed due to problems in fixing equine tendons securely and loading them cyclically. It is well established that the biomechanical response of tendons varies during cyclic loading over time. The aim of this study was to develop a clamping device that enables repetitive cyclic tensile testing of equine superficial digital flexor tendon for at least 60 loading cycles and for 5 min.

Results

A novel cryo-clamp was developed and built. Healthy and collagenase-treated pony SDFTs were mounted in the custom-made cryo-clamp for the proximal tendon end and a special clamping device for the short pastern bone (os coronale). Simultaneously with tensile testing, we used a biplanar high-speed fluoroscopy system (FluoKin) to track tendon movement. The FluoKin system was additionally validated in precision measurements. During the cyclic tensile tests of the SDFTs, the average maximal force measured was 325 N and 953 N for a length variation of 2 and 4 % respectively. The resulting stress averaged 16 MPa and 48 MPa respectively, while the modulus of elasticity was 828 MPa and 1212 MPa respectively. Length variation of the metacarpal region was, on average, 4.87 % higher after incubation with collagenase. The precision of the FluoKin tracking was 0.0377 mm, defined as the standard deviation of pairwise intermarker distances embedded in rigid bodies. The systems accuracy was 0.0287 mm, which is the difference between the machined and mean measured distance.

Conclusion

In this study, a good performing clamping technique for equine tendons under repetitive cyclic loading conditions is described. The presented cryo-clamps were tested up to 50 min duration and up to the machine maximal capacity of 10 kN. With the possibility of repetitive loading a stabilization of the time-force-curve and changes of hysteresis and creep became obvious after a dozen cycles, which underlines the necessity of repetitive cyclical testing. Furthermore, biplanar high-speed fluoroscopy seems an appropriate and highly precise measurement tool for analysis of tendon behaviour under repetitive load in equine SDFTs.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-021-02914-w.

Microdamage in the equine superficial digital flexor tendon

The forelimb superficial digital flexor tendon (SDFT) is an energy-storing tendon that is highly susceptible to injury during activities such as galloping and jumping, such that it is one of the most commonly reported causes of lameness in the performance horse. This review outlines the biomechanical and biothermal effects of strain on the SDFT and how these contribute to the accumulation of microdamage. The effect of age-related alterations on strain response and subsequent injury risk is also considered. Given that tendon is a slowly healing and poorly regenerative tissue, prompt detection of early stages of pathology in vivo and timely adaptations to training protocols are likely to have a greater outcome than advances in treatment. Early screening tools and detection protocols could subsequently be of benefit in identifying subclinical signs of degeneration during the training programme. This provides an opportunity for preventative strategies to be implemented to minimise incidences of SDFT injury and reduce recovery periods in elite performance horses. Therefore, this review will focus on the modalities available to implement early screening and prevention protocols as opposed to methods to diagnose and treat injuries.

Elastase treatment of tendon specifically impacts the mechanical properties of the interfascicular matrix

The tendon interfascicular matrix (IFM) binds tendon fascicles together. As a result of its low stiffness behaviour under small loads, it enables non-uniform loading and increased overall extensibility of tendon by facilitating fascicle sliding. This function is particularly important in energy storing tendons, with previous studies demonstrating enhanced extensibility, recovery and fatigue resistance in the IFM of energy storing compared to positional tendons. However, the compositional specialisations within the IFM that confer this behaviour remain to be elucidated. It is well established that the IFM is rich in elastin, therefore we sought to test the hypothesis that elastin depletion (following elastase treatment) will significantly impact IFM, but not fascicle, mechanical properties, reducing IFM resilience in all samples, but to a greater extent in younger tendons, which have a higher elastin content. Using a combination of quasi-static and fatigue testing, and optical imaging, we confirmed our hypothesis, demonstrating that elastin depletion resulted in significant decreases in IFM viscoelasticity, fatigue resistance and recoverability compared to untreated samples, with no significant changes to fascicle mechanics. Ageing had little effect on fascicle or IFM response to elastase treatment.

This study offers a first insight into the functional importance of elastin in regional specific tendon mechanics. It highlights the important contribution of elastin to IFM mechanical properties, demonstrating that maintenance of a functional elastin network within the IFM is essential to maintain IFM and thus tendon integrity.

Statement of significance

Developing effective treatments or preventative measures for musculoskeletal tissue injuries necessitates the understanding of healthy tissue function and mechanics. By establishing the contribution of specific proteins to tissue mechanical behaviour, key targets for therapeutics can be identified. Tendon injury is increasingly prevalent and chronically debilitating, with no effective treatments available.

Here, we investigate how elastin modulates tendon mechanical behaviour, using enzymatic digestion combined with local mechanical characterisation, and demonstrate for the first time that removing elastin from tendon affects the mechanical properties of the interfascicular matrix specifically, resulting in decreased recoverability and fatigue resistance. These findings provide a new level of insight into tendon hierarchical mechanics, important for directing development of novel therapeutics for tendon injury.

Characterization of the structure, vascularity, and stem/progenitor cell populations in porcine Achilles tendon (PAT)

This study aimed to characterize porcine Achilles tendon (PAT) in terms of its structural components, vascularity, and resident tendon cells. We found that PAT is composed of a paratenon sheath, a core of fascicles, and an endotenon/interfascicular matrix (IFM) that encases the fascicle bundles. We analyzed each of these three tendon components structurally using tissue sections and by isolating cells from each component and analyzing in vitro. Many blood vessel-like tissues were present in the paratenon and IFM but not in fascicles, and the vessels in the paratenon and IFM appeared to be inter-connected. Cells isolated from the paratenon and IFM displayed characteristics of vascular stem/progenitor cells expressing the markers CD105, CD31, with α-smooth muscle actin (α-SMA) localized surrounding blood vessels. The isolated cells from paratenon and IFM also harbored abundant stem/progenitor cells as evidenced by their ability to form colonies and express stem cell markers including CD73 and CD146. Furthermore, we demonstrate that both paratenon and IFM-isolated cells were capable of undergoing multi-differentiation. In addition, both paratenon and IFM cells expressed elastin, osteocalcin, tubulin polymerization promoting protein (TPPP), and collagen IV, whereas fascicle cells expressed none of these markers, except collagen I. The neurotransmitter substance P (SP) was also found in the paratenon and IFM-localized surrounding blood vessels. The findings of this study will help us to better understand the vascular and cellular mechanisms of tendon homeostasis, injury, healing, and regeneration.

Collagen denaturation is initiated upon tissue yield in both positional and energy-storing tendons

Tendons are collagenous soft tissues that transmit loads between muscles and bones. Depending on their anatomical function, tendons are classified as positional or energy-storing with differing biomechanical and biochemical properties. We recently demonstrated that during monotonic stretch of positional tendons, permanent denatured collagen begins accumulating upon departing the linear region of the stress-strain curve. However, it is unknown if this observation is true during mechanical overload of other types of tendons. Therefore, the purpose of this study was to investigate the onset of collagen denaturation relative to applied strain, and whether it differs between the two tendon types. Rat tail tendon (RTT) fascicles and rat flexor digitorum longus (FDL) tendons represented positional and energy-storing tendons, respectively. The samples were stretched to incremental levels of strain, then stained with fluorescently labeled collagen hybridizing peptides (CHPs); the CHP fluorescence was measured to quantify denatured collagen. Denatured collagen in both positional and energy-storing tendons began to increase at the yield strain, upon leaving the linear region of the stress-strain curve as the sample started to permanently deform. Despite significant differences between the two tendon types, it appears that collagen denaturation is initiated at tissue yield during monotonic stretch, and the fundamental mechanism of failure is the same for the two types of tendons. At tissue failure, positional tendons had double the percentage of denatured collagen compared to energy-storing tendons, with no difference between 0% control groups. These results help to elucidate the etiology of subfailure injury and rupture in functionally distinct tendons.

Postnatal mechanical loading drives adaptation of tissues primarily through modulation of the non-collagenous matrix

Mature connective tissues demonstrate highly specialised properties, remarkably adapted to meet their functional requirements. Tissue adaptation to environmental cues can occur throughout life and poor adaptation commonly results in injury. However, the temporal nature and drivers of functional adaptation remain undefined. Here, we explore functional adaptation and specialisation of mechanically loaded tissues using tendon; a simple aligned biological composite, in which the collagen (fascicle) and surrounding predominantly non-collagenous matrix (interfascicular matrix) can be interrogated independently. Using an equine model of late development, we report the first phase-specific analysis of biomechanical, structural, and compositional changes seen in functional adaptation, demonstrating adaptation occurs postnatally, following mechanical loading, and is almost exclusively localised to the non-collagenous interfascicular matrix. These novel data redefine adaptation in connective tissue, highlighting the fundamental importance of non-collagenous matrix and suggesting that regenerative medicine strategies should change focus from the fibrous to the non-collagenous matrix of tissue.

Heterogeneity of proteome dynamics between connective tissue phases of adult tendon

Maintenance of connective tissue integrity is fundamental to sustain function, requiring protein turnover to repair damaged tissue. However, connective tissue proteome dynamics remain largely undefined, as do differences in turnover rates of individual proteins in the collagen and glycoprotein phases of connective tissue extracellular matrix (ECM). Here, we investigate proteome dynamics in the collagen and glycoprotein phases of connective tissues by exploiting the spatially distinct fascicular (collagen-rich) and interfascicular (glycoprotein-rich) ECM phases of tendon. Using isotope labelling, mass spectrometry and bioinformatics, we calculate turnover rates of individual proteins within rat Achilles tendon and its ECM phases. Our results demonstrate complex proteome dynamics in tendon, with ~1000 fold differences in protein turnover rates, and overall faster protein turnover within the glycoprotein-rich interfascicular matrix compared to the collagen-rich fascicular matrix. These data provide insights into the complexity of proteome dynamics in tendon, likely required to maintain tissue homeostasis.

Loading Rate Has Little Influence on Tendon Fascicle Mechanics

Mechanically, tendons behave like springs and store energy by stretching in proportion to applied stress. This relationship is potentially modified by the rate at which stress is applied, a phenomenon known as viscosity. Viscoelasticity, the combined effects of elasticity and viscosity, can affect maximum strain, the amount of stored energy, and the proportion of energy recovered (resilience). Previous studies of tendons have investigated the functional effects of viscoelasticity, but not at the intermediate durations of loading that are known to occur in fast locomotor events. In this study, we isolated tendon fascicles from rat tails and performed force-controlled tensile tests at rates between ∼10 MPa s^–1 to ∼80 MPa s^–1. At high rates of applied stress, we found that tendon fascicles strained less, stored less energy, and were more resilient than at low rates of stress (p = 0.007, p = 0.040, and p = 0.004, respectively). The measured changes, however, were very small across the range of strain rates studied. For example, the average strain for the slowest loading rate was 0.637% while it was 0.614% for the fastest loading. We conclude that although there is a measurable effect of loading rate on tendon mechanics, the effect is small and can be largely ignored in the context of muscle-actuated locomotion, with the possible exception of extreme muscle-tendon morphologies.

Distinguishing Quadriceps Tendinopathy and Patellar Tendinopathy: Semantics or Significant?

Jumper's knee is not synonymous with patellar tendinopathy. The term includes patellar tendinopathy and quadriceps tendinopathy. Although the patellar and quadriceps tendons work in tandem as part of the extensor mechanism of the knee, they have distinct anatomy and functional roles. As a result, there are probable differences in risk factors, etiology, and response to treatment. It is time to clinically separate patellar tendinopathy and quadriceps tendinopathy and design more specific rehabilitation programs. In this Viewpoint, the authors will (1) provide a rationale for distinguishing the 2 clinical entities-patellar tendinopathy and quadriceps tendinopathy-for treatment decision making, and (2) identify areas of research priority in quadriceps tendinopathy. J Orthop Sports Phys Ther 2019;49(9):627-630. doi:10.2519/jospt.2019.0611.

Load transfer, damage, and failure in ligaments and tendons

The function of ligaments and tendons is to support and transmit loads applied to the musculoskeletal system. These tissues are often able to perform their function for many decades; however, connective tissue disease and injury can compromise ligament and tendon integrity. A range of protein and non-protein constituents, combined in a complex structural hierarchy from the collagen molecule to the tissue and covering nanometer to centimeter length scales, govern tissue function, and impart characteristic non-linear material behavior. This review summarizes the structure of ligaments and tendons, the roles of their constituent components for load transfer across the hierarchy of structure, and the current understanding of how damage occurs in these tissues. Disease and injury can alter the constituent make-up and structural organization of ligaments and tendons, affecting tissue function, while also providing insight to the role and interactions of individual constituents. The studies and techniques presented here have helped to understand the relationship between tissue constituents and the physical mechanisms (e.g., stretching, sliding) that govern material behavior at and between length scales. In recent years, new techniques have been developed to probe ever smaller length scales and may help to elucidate mechanisms of load transfer and damage and the molecular constituents involved in the in the earliest stages of ligament and tendon damage. A detailed understanding of load transfer and damage from the molecular to the tissue level may elucidate targets for the treatment of connective tissue diseases and inform practice to prevent and rehabilitate ligament and tendon injuries. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3093-3104, 2018.

Ultrastructure of tendon rupture depends on strain rate and tendon type

Previous research has shown that both the mechanics and elongation mechanisms of tendon and ligament vary with strain rate during tensile loading. In this study, we sought to determine if the ultrastructural damage created during tendon rupture also varies with strain rate. A bovine forelimb model was used, allowing two anatomically proximate but physiologically distinct tendons to be studies: the positional common digital extensor tendon, and the energy storing superficial digital flexor tendon. Samples from the two tendon types were ruptured at rates of either 1%/s or 10%/s. Relative to unruptured control samples, changes to collagen fibril structure were assessed using scanning electron microscopy (SEM), and changes to collagen molecule packing were studied using differential scanning calorimetry (DSC). Rupture at 1%/s caused discrete plasticity damage that extended along the length of collagen fibrils in both the extensor and flexor tendons. Consistent with this, DSC showed molecular packing disruption relative to control samples. Both SEM and DSC showed that extensor tendon fibrils sustained more severe damage than the more highly crosslinked flexor tendon fibrils. Increasing strain rate during rupture decreased the level of longitudinal disruption experienced by the collagen fibrils of both tendon types. Disruption to D-banding was no longer seen in the extensor tendon fibrils, and discrete plasticity damage was completely eliminated in the flexor tendon fibrils, indicating a transition to localized point failure. Ultrastructural damage resulting from tendon rupture depends on both strain rate and tendon type. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2842-2850, 2018.

Cellular, matrix, and mechano-biological differences in load-bearing versus positional tendons throughout development and aging: a narrative review

PURPOSE

Summarise available evidence comparing the cellular, biochemical, structural and biomechanical properties, and the changes that occur in these parameters in response to stimuli, in differentially loaded tendons across different stages of life.

METHODS

The PubMed database was searched for literature pertaining to differences between tendons using the term "tendon" or "tendinopathy", plus one or more of the following descriptors: "loading", "positional", "weight- or load-bearing", and "energy-storing". The abstracts were reviewed and relevant full-length articles retrieved and used to assemble a narrative review.

RESULTS

The incidence and prevalence of tendon disorders ("tendinopathies") is increasing in Western societies, with limited evidence that currently available treatments have any significant long-term effect on the disease course. A key emerging hypothesis is that disease in different tendons and even different regions within a tendon may be distinct. The available literature indicates that there are phenotypic differences, not only in the constitutive compositional and material properties but also in resident cells of positional compared with load-bearing tendons. Evident during early tendon growth, such differences have become well established by adulthood.

CONCLUSIONS

The pheno-endotype of tendinopathy may be distinct between load-bearing tendons compared to positional tendons, which has translational implications with regard to preventing and managing tendinopathy. Better understanding of the molecular, cellular, and biomechanical pathophysiology underlying disease phenotypes, will allow more targeted/personalised treatment and therefore improve outcomes.

Functionally Distinct Tendons From Elastin Haploinsufficient Mice Exhibit Mild Stiffening and Tendon-Specific Structural Alteration

Elastic fibers are present in low quantities in tendon, where they are located both within fascicles near tenocytes and more broadly in the interfascicular matrix (IFM). While elastic fibers have long been known to be significant in the mechanics of elastin-rich tissue (i.e., vasculature, skin, lungs), recent studies have suggested a mechanical role for elastic fibers in tendons that is dependent on specific tendon function. However, the exact contribution of elastin to properties of different types of tendons (e.g., positional, energy-storing) remains unknown. Therefore, this study purposed to evaluate the role of elastin in the mechanical properties and collagen alignment of functionally distinct supraspinatus tendons (SSTs) and Achilles tendons (ATs) from elastin haploinsufficient (HET) and wild type (WT) mice. Despite the significant decrease in elastin in HET tendons, a slight increase in linear stiffness of both tendons was the only significant mechanical effect of elastin haploinsufficiency. Additionally, there were significant changes in collagen nanostructure and subtle alteration to collagen alignment in the AT but not the SST. Hence, elastin may play only a minor role in tendon mechanical properties. Alternatively, larger changes to tendon mechanics may have been mitigated by developmental compensation of HET tendons and/or the role of elastic fibers may be less prominent in smaller mouse tendons compared to the larger bovine and human tendons evaluated in previous studies. Further research will be necessary to fully elucidate the influence of various elastic fiber components on structure-function relationships in functionally distinct tendons.

Fatigue of soft fibrous tissues: Multi-scale mechanics and constitutive modeling

In recent experimental studies a possible damage mechanism of collagenous tissues mainly caused by fatigue was disclosed. In this contribution, a multi-scale constitutive model ranging from the tropocollagen (TC) molecule level up to bundles of collagen fibers is proposed and utilized to predict the elastic and inelastic long-term tissue response. Material failure of collagen fibrils is elucidated by a permanent opening of the triple helical collagen molecule conformation, triggered either by overstretching or reaction kinetics of non-covalent bonds. This kinetics is described within a probabilistic framework of adhesive detachments of molecular linkages providing collagen fiber integrity. Both intramolecular and interfibrillar linkages are considered. The final constitutive equations are validated against recent experimental data available in literature for both uniaxial tension to failure and the evolution of fatigue in subsequent loading cycles. All material parameters of the proposed model have a clear physical interpretation.

STATEMENT OF SIGNIFICANCE

Irreversible changes take place at different length scales of soft fibrous tissues under supra-physiological loading and alter their macroscopic mechanical properties. Understanding the evolution of those histologic pathologies under loading and incorporating them into a continuum mechanical framework appears to be crucial in order to predict long-term evolution of various diseases and to support the development of tissue engineering.

Biological connective tissues exhibit viscoelastic and poroelastic behavior at different frequency regimes: Application to tendon and skin biophysics

In this study, a poroviscoelastic finite element model (FEM) was developed and used in conjunction with an AFM-based wide-bandwidth nanorheology system to predict the frequency-dependent mechanical behavior of tendon and dermis subjected to compression via nanoindentation. The aim was to distinguish between loading rates that are dominated by either poroelasticity, viscoelasticity, or the superposition of these processes. Using spherical probe tips having different radii, the force and tip displacement were measured and the magnitude, E^∗, and phase angle, ϕ, of the dynamic complex modulus were evaluated for mouse supraspinatus tendon and mouse dermis. The peak frequencies of the phase angle were associated with the characteristic time constants of poroelastic and viscoelastic material behavior. The developed FE model could predict the separate poroelastic and viscoelastic responses of these soft tissues over a 4 decade frequency range, showing good agreement with experimental results. We observed that poroelasticity was the dominant energy dissipation mechanism for mouse dermis and supraspinatus tendon at higher indentation frequencies (10² to 10⁴ Hz) whereas viscoelasticity was typically dominant at lower frequencies (<10² Hz). These findings show the underlying mechanical behavior of biological connective tissues and give insight into the role played by these different energy dissipation mechanisms in governing the function of these tissues at nanoscale.

STATEMENT OF SIGNIFICANCE

Soft biological tissues exhibit complex, load- and time-dependent mechanical behavior. Evaluating their mechanical behavior requires sophisticated experimental tools and numerical models that can capture the fundamental mechanisms governing tissue function. Using an Atomic-force-microscopy-based rheology system and finite element models, the roles of the two most dominant time-dependent mechanisms (poroelasticity and viscoelasticity) that govern the dynamic loading behavior of mouse skin and tendon have been investigated. FE models were able to predict and quantify the contribution of each mechanism to the overall dynamic response and confirming the presence of these two distinct mechanisms in the mechanical response. Overall, these results provide novel insight into the viscoelastic and poroelastic properties of mouse skin and tendon and promote better understanding of the underlying origins of each mechanism.

Mechanical Coupling Between Muscle-Tendon Units Reduces Peak Stresses

The presence of mechanical linkages between synergistic muscles and their common tendons may distribute forces among the involved structures. We review studies, using humans and other animals, examining muscle and tendon interactions and discuss the hypothesis that connections between muscle bellies and within tendons may serve as a mechanism to distribute forces and mitigate peak stresses.

Development of overuse tendinopathy: A new descriptive model for the initiation of tendon damage during cyclic loading

Tendinopathic tissue has long been characterized by changes to collagen microstructure. However, initial tendon damage from excessive mechanical loading-a hallmark of tendinopathy development-could occur at the nanoscale level of collagen fibrils. Indeed, it is on this scale that tenocytes interact directly with tendon matrix, and excessive collagen fibril damage not visible at the microscale could trigger a degenerative cascade. In this study, we explored whether initiation of tendon damage during cyclic loading occurs via a longitudinal compression-induced buckling mechanism of collagen fibrils leading to nanoscale kinkband development. Two groups of tendons were cyclically loaded to equivalent peak stresses. In each loading cycle, tendons in one group were unloaded to the zero displacement mark, while those in the other group were unloaded to a nominal level of tension, minimizing the potential for fibril buckling. Tendons that were unloaded to the zero displacement mark ruptured significantly sooner during cyclic loading (1,446 ± 737 vs. 4,069 ± 1,129 cycles), indicating that significant fatigue damage is accrued in the low stress, toe region of the load-deformation response. Ultrastructural analysis using scanning electron microscopy of tendons stopped after 1,000 cycles showed that maintaining a nominal tension slowed the accumulation of kinkbands, supporting a longitudinal compression-induced buckling mechanism as the basis for kinkband development. Based on our results, we present a new descriptive model for the initiation of tendon damage during cyclic loading. The so-called Compression of Unrecovered Elongation or CUE Model may provide useful insight into the development of tendinopathy. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:467-476, 2018.

Elastin is Localised to the Interfascicular Matrix of Energy Storing Tendons and Becomes Increasingly Disorganised With Ageing

Tendon is composed of fascicles bound together by the interfascicular matrix (IFM). Energy storing tendons are more elastic and extensible than positional tendons; behaviour provided by specialisation of the IFM to enable repeated interfascicular sliding and recoil. With ageing, the IFM becomes stiffer and less fatigue resistant, potentially explaining why older tendons become more injury-prone. Recent data indicates enrichment of elastin within the IFM, but this has yet to be quantified. We hypothesised that elastin is more prevalent in energy storing than positional tendons, and is mainly localised to the IFM. Further, we hypothesised that elastin becomes disorganised and fragmented, and decreases in amount with ageing, especially in energy storing tendons. Biochemical analyses and immunohistochemical techniques were used to determine elastin content and organisation, in young and old equine energy storing and positional tendons. Supporting the hypothesis, elastin localises to the IFM of energy storing tendons, reducing in quantity and becoming more disorganised with ageing. These changes may contribute to the increased injury risk in aged energy storing tendons. Full understanding of the processes leading to loss of elastin and its disorganisation with ageing may aid in the development of treatments to prevent age related tendinopathy.

Fascicles and the interfascicular matrix show decreased fatigue life with ageing in energy storing tendons

Tendon is composed of rope-like fascicles bound together by interfascicular matrix (IFM). The IFM is critical for the function of energy storing tendons, facilitating sliding between fascicles to allow these tendons to cyclically stretch and recoil. This capacity is required to a lesser degree in positional tendons. We have previously demonstrated that both fascicles and IFM in energy storing tendons have superior fatigue resistance compared with positional tendons, but the effect of ageing on the fatigue properties of these different tendon subunits has not been determined. Energy storing tendons become more injury-prone with ageing, indicating reduced fatigue resistance, hence we tested the hypothesis that the decline in fatigue life with ageing in energy storing tendons would be more pronounced in the IFM than in fascicles. We further hypothesised that tendon subunit fatigue resistance would not alter with ageing in positional tendons. Fascicles and IFM from young and old energy storing and positional tendons were subjected to cyclic fatigue testing until failure, and mechanical properties were calculated. The results show that both IFM and fascicles from the SDFT exhibit a similar magnitude of reduced fatigue life with ageing. By contrast, the fatigue life of positional tendon subunits was unaffected by ageing. The age-related decline in fatigue life of tendon subunits in energy storing tendons is likely to contribute to the increased risk of injury in aged tendons. Full understanding of the mechanisms resulting in this reduced fatigue life will aid in the development of treatments and interventions to prevent age-related tendinopathy.

Statement of Significance

Understanding the effect of ageing on tendon-structure function relationships is crucial for the development of effective preventative measures and treatments for age-related tendon injury. In this study, we demonstrate for the first time that the fatigue resistance of the interfascicular matrix decreases with ageing in energy storing tendons. This is likely to contribute to the increased risk of injury in aged tendons. Full understanding of the mechanisms that result in this reduced fatigue resistance will aid in the development of treatments and interventions to prevent age-related tendinopathy.

Fatigue life of bovine meniscus under longitudinal and transverse tensile loading

The knee meniscus is composed of a fibrous extracellular matrix that is subjected to large and repeated loads. Consequently, the meniscus is frequently torn, and a potential mechanism for failure is fatigue. The objective of this study was to measure the fatigue life of bovine meniscus when applying cyclic tensile loads either longitudinal or transverse to the principal fiber direction. Fatigue experiments consisted of cyclic loads to 60%, 70%, 80% or 90% of the predicted ultimate tensile strength until failure occurred or 20,000 cycles was reached. The fatigue data in each group was fit with a Weibull distribution to generate plots of stress level vs. cycles to failure (S-N curve). Results showed that loading transverse to the principal fiber direction gave a two-fold increase in failure strain, a three-fold increase in creep, and a nearly four-fold increase in cycles to failure (not significant), compared to loading longitudinal to the principal fiber direction. The S-N curves had strong negative correlations between the stress level and the mean cycles to failure for both loading directions, where the slope of the transverse S-N curve was 11% less than the longitudinal S-N curve (longitudinal: S=108-5.9ln(N); transverse: S=112-5.2ln(N)). Collectively, these results suggest that the non-fibrillar matrix is more resistant to fatigue failure than the collagen fibers. Results from this study are relevant to understanding the etiology of atraumatic radial and horizontal meniscal tears, and can be utilized by research groups that are working to develop meniscus implants with fatigue properties that mimic healthy tissue.