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Tendons have a uniaxially aligned structure with a hierarchical organization of collagen fibrils crucial for tendon function. Collagen XII is expressed in tendons and has been implicated in the regulation of fibrillogenesis. It is a non-fibrillar collagen belonging to the Fibril-Associated Collagens with Interrupted Triple Helices (FACIT) family. Mutations in COL12A1 cause myopathic Ehlers Danlos Syndrome with a clinical phenotype involving both joints and tendons supporting critical role(s) for collagen XII in tendon development and function. Here we demonstrate the molecular function of collagen XII during tendon development using a Col12a1 null mouse model. Col12a1 deficiency altered tenocyte shape, formation of interacting cell processes, and organization resulting in impaired cell-cell communication and disruption of hierarchal structure as well as decreased tissue stiffness. Immuno-localization revealed that collagen XII accumulated on the tenocyte surface and connected adjacent tenocytes by building matrix bridges between the cells, suggesting that collagen XII regulates intercellular communication. In addition, there was a decrease in fibrillar collagen I in collagen XII deficient tenocyte cultures compared with controls suggesting collagen XII signaling specifically alters tenocyte biosynthesis. This suggests that collagen XII provides feedback to tenocytes regulating extracellular collagen I. Together, the data indicate dual roles for collagen XII in determination of tendon structure and function. Through association with fibrils it functions in fibril packing, fiber assembly and stability. In addition, collagen XII influences tenocyte organization required for assembly of higher order structure; intercellular communication necessary to coordinate long range order and feedback on tenocytes influencing collagen synthesis. Integration of both regulatory roles is required for the acquisition of hierarchal structure and mechanical properties.

In his Lissner Award medal lecture in 2000, Stephen Cowin asked the question: "How is a tissue built?" It is not a new question, but it remains as relevant today as it did when it was asked 20 years ago. In fact, research on the organization and development of tissue structure has been a primary focus of tendon and ligament research for over two centuries. The tendon extracellular matrix (ECM) is critical to overall tissue function; it gives the tissue its unique mechanical properties, exhibiting complex non-linear responses, viscoelasticity and flow mechanisms, excellent energy storage and fatigue resistance. This matrix also creates a unique microenvironment for resident cells, allowing cells to maintain their phenotype and translate mechanical and chemical signals into biological responses. Importantly, this architecture is constantly remodeled by local cell populations in response to changing biochemical (systemic and local disease or injury) and mechanical (exercise, disuse, and overuse) stimuli. Here, we review the current understanding of matrix remodeling throughout life, focusing on formation and assembly during the postnatal period, maintenance and homeostasis during adulthood, and changes to homeostasis in natural aging. We also discuss advances in model systems and novel tools for studying collagen and non-collagenous matrix remodeling throughout life, and finally conclude by identifying key questions that have yet to be answered.

In his Lissner Award medal lecture in 2000, Stephen Cowin asked the question: "How is a tissue built?" It is not a new question, but it remains as relevant today as it did when it was asked 20 years ago. In fact, research on the organization and development of tissue structure has been a primary focus of tendon and ligament research for over two centuries. The tendon extracellular matrix (ECM) is critical to overall tissue function; it gives the tissue its unique mechanical properties, exhibiting complex non-linear responses, viscoelasticity and flow mechanisms, excellent energy storage and fatigue resistance. This matrix also creates a unique microenvironment for resident cells, allowing cells to maintain their phenotype and translate mechanical and chemical signals into biological responses. Importantly, this architecture is constantly remodeled by local cell populations in response to changing biochemical (systemic and local disease or injury) and mechanical (exercise, disuse, and overuse) stimuli. Here, we review the current understanding of matrix remodeling throughout life, focusing on formation and assembly during the postnatal period, maintenance and homeostasis during adulthood, and changes to homeostasis in natural aging. We also discuss advances in model systems and novel tools for studying collagen and non-collagenous matrix remodeling throughout life, and finally conclude by identifying key questions that have yet to be answered.

The periodontal ligament (PDL) is a critical player in the maintenance of tooth health, acting as the primary stabilizer of tooth position. Recent studies have identified two unique regions within the PDL, the 'dense collar' region and the 'furcation' region, which exhibit distinct structural and compositional differences. However, specific functional differences between these regions have yet to be investigated. We adapted an AFM-based nanoscale rheology method to regionally assess mechanical properties and poroelasticity in the mouse PDL while minimizing the disruption of the 3-dimensional native boundary conditions, and then explored tissue mechanical function in four different regions within the dense collar as well as in the furcation region. We found significant differences between the collar and furcation regions, with the collar acting as a stabilizing ligamentous structure and the furcation acting as both a compressive cushion for vertical forces and a conduit for nutrient transport. While this finding supports our hypothesis, based on previous studies investigating structural and compositional differences, we also found surprising inhomogeneity within the collar region itself. This inhomogeneity supports previous findings of a tilting movement in the buccal direction of mandibular molar teeth and the structural adaptation to prevent lingual movement. Future work will aim to understand how different regions of the PDL change functionally during biological or mechanical perturbations, such as orthodontic tooth movement, development, or aging, with the ultimate goal of better understanding the mechanobiology of the PDL function in health and disease.

Purpose Tendinopathy is a significant clinical problem thought to be associated with altered mechanical loading. Explant culture models allow researchers to alter mechanical loading in a controlled in vitro environment while maintaining tenocytes in their native matrix. However, current models do not accurately represent commonly injured tendons, ignoring contributions of associated musculature and bone, as well as regional collagen structure. This study details the characterization of amouse rotator cuff explant culture model, including bone, tendon, and muscle (BTM). Materials and methods Following harvest, BTM explants were maintained in stress-deprived culture for one week and tendon was then assessed for changes in cell viability, metabolism, matrix structure and content. Results Matrix turnover occurred throughout culture as manifested in both gene expression and biosynthesis, but this did not translate to net changes in total collagen or sulfated glycosaminoglycan content. Furthermore, tendon structure was not significantly altered throughout culture. However, we found significant cell death in BTM tendons after 3 days in culture, which we hypothesize is cytokine-induced. Using a targeted multiplex assay, we found high levels of pro-inflammatory cytokines released to the culture medium from muscle and bone, levels that did cause cell deathin tendon-alone controls. Conclusions Overall, this model presents an innovative approach to understandingrotator cuff injury and tenocyte mechanobiology in a clinically-relevant tendon structure. Our model can be a powerful tool to investigate how mechanical and biological stimuli can alter normal tendon health and lead to tendon degeneration, and may provide a testbed for therapeutics for tendon repair.

Classic Ehlers-Danlos syndrome (EDS) patients suffer from connective tissue hyperelasticity, joint instability, skin hyperextensibility, tissue fragility, and poor wound healing due to heterozygous mutations in COL5a1 or COL5a2 genes. This study investigated the roles of collagen V in establishing structure and function in uninjured patellar tendons as well as in the injury response using a Col5a1+/- mouse, a model for classic EDS. These analyses were done comparing tendons from a classic EDS model (Col5a1+/- ) with wild-type controls. Tendons were subjected to mechanical testing, histological, and fibril analysis before injury as well as 3 and 6 weeks after injury. We found that Col5a1+/- tendons demonstrated diminished recovery of mechanical competency after injury as compared to normal wild-type tendons, which recovered their pre-injury values by 6 weeks post injury. Additionally, the Col5a1+/- tendons demonstrated altered fibril morphology and diameter distributions compared to the wild-type tendons. This study indicates that collagen V plays an important role in regulating collagen fibrillogenesis and the associated recovery of mechanical integrity in tendons after injury. In addition, the dysregulation with decreased collagen V expression in EDS is associated with a diminished injury response. The results presented herein have the potential to direct future targeted therapeutics for classic EDS patients. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2707-2715, 2017.

Tendons transmit load from muscle to bone by utilizing their unique static and viscoelastic tensile properties. These properties are highly dependent on the composition and structure of the tissue matrix, including the collagen I hierarchy, proteoglycans, and water. While the role of matrix constituents in the tensile response has been studied, their role in compression, particularly in matrix pressurization via regulation of fluid flow, is not well understood. Injured or diseased tendons and tendon regions that naturally experience compression are known to have alterations in glycosaminoglycan content, which could modulate fluid flow and ultimately mechanical function. While recent theoretical studies have predicted tendon mechanics using poroelastic theory, no experimental data have directly demonstrated such behavior. In this study, we use high-bandwidth AFM-based rheology to determine the dynamic response of tendons to compressive loading at the nanoscale and to determine the presence of poroelastic behavior. Tendons are found to have significant characteristic dynamic relaxation behavior occurring at both low and high frequencies. Classic poroelastic behavior is observed, although we hypothesize that the full dynamic response is caused by a combination of flow-dependent poroelasticity as well as flow-independent viscoelasticity. Tendons also demonstrate regional dependence in their dynamic response, particularly near the junction of tendon and bone, suggesting that the structural and compositional heterogeneity in tendon may be responsible for regional poroelastic behavior. Overall, these experiments provide the foundation for understanding fluid-flow-dependent poroelastic mechanics of tendon, and the methodology is valuable for assessing changes in tendon matrix compressive behavior at the nanoscale. Keywords: Poroelasticity; Viscoelasticity; Tendon; AFM; Nanomechanics, National Institutes of Health (U.S.) (Grant F32-AG052284), National Science Foundation (U.S.) (Grant CMMI-1536233)

© 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. Secondary joint damage is the process by which a single injury can lead to detrimental changes in adjacent tissue structures, typically through the spread of inflammatory responses. We recently developed an in vitro model of secondary joint damage using a murine rotator cuff explant system, in which injuries to muscle and bone cause massive cell death in otherwise uninjured tendon. The purpose of the present study was to test the ability cytokine-targeted and broad-spectrum therapeutics to prevent cell death and tissue degeneration associated with secondary joint damage. We treated injured bone-tendon-muscle explants with either interleukin-1 receptor antagonist, etanercept, or dexamethasone (DEX) for up to 7 days in culture. Only the low-dose DEX treatment was able to prevent cell death and tissue degeneration. We then identified a critical window between 24 and 72 h following injury for maximal benefit of DEX treatment through timed administration experiments. Finally, we performed two tendon-only explant studies to identify mechanistic effects on tendon health. Interestingly, DEX did not prevent cell death and degeneration in a model of cytokine-induced damage, suggesting other targets of DEX activity. Future studies will aim to identify factors in joint inflammation that may be targeted by DEX treatment, as well as to investigate novel delivery strategies. Statement of clinical significance: Overall, this work demonstrates beneficial effects of DEX administration on preventing tenocyte death and extracellular matrix degeneration in an explant model of secondary joint damage, supporting the clinical use of low-dose glucocorticoids for short-term treatment of joint inflammation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:139–149, 2020.

Recent advances in technology have allowed for the measurement of dynamic processes (re-alignment, crimp, deformation, sliding), but only a limited number of studies have investigated their relationship with mechanical properties. The overall objective of this study was to investigate the role of composition, structure, and the dynamic response to load in predicting tendon mechanical properties in a multi-level fashion mimicking native hierarchical collagen structure. Multiple linear regression models were investigated to determine the relationships between composition/structure, dynamic processes, and mechanical properties. Mediation was then used to determine if dynamic processes mediated structure-function relationships. Dynamic processes were strong predictors of mechanical properties. These predictions were location-dependent, with the insertion site utilizing all four dynamic responses and the midsubstance responding primarily with fibril deformation and sliding. In addition, dynamic processes were moderately predicted by composition and structure in a regionally-dependent manner. Finally, dynamic processes were partial mediators of the relationship between composition/structure and mechanical function, and results suggested that mediation is likely shared between multiple dynamic processes. In conclusion, the mechanical properties at the midsubstance of the tendon are controlled primarily by fibril structure and this region responds to load via fibril deformation and sliding. Conversely, the mechanical function at the insertion site is controlled by many other important parameters and the region responds to load via all four dynamic mechanisms. Overall, this study presents a strong foundation on which to design future experimental and modeling efforts in order to fully understand the complex structure-function relationships present in tendon.

Manipulations in cell culture and mouse models have demonstrated that reduction of collagen V results in altered fibril structure and matrix assembly. A tissue-dependent role for collagen V in determining mechanical function was recently established, but its role in determining regional properties has not been addressed. The objective of this study was to define the role(s) of collagen V expression in establishing the site-specific properties of the supraspinatus tendon. The insertion and midsubstance of tendons from wild type, heterozygous and tendon/ligament-specific null mice were assessed for crimp morphology, fibril morphology, cell morphology, as well as total collagen and pyridinoline cross-link (PYD) content. Fibril morphology was altered at the midsubstance of both groups with larger, but fewer, fibrils and no change in cell morphology or collagen compared to the wild type controls. In contrast, a significant disruption of fibril assembly was observed at the insertion site of the null group with the presence of structurally aberrant fibrils. Alterations were also present in cell density and PYD content. Altogether, these results demonstrate that collagen V plays a crucial role in determining region-specific differences in mouse supraspinatus tendon structure. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2154-2161, 2016.

Rotator cuff disorders are one of the most common causes of shoulder pain and disability in the aging population but, unfortunately, the etiology is still unknown. One factor thought to contribute to the progression of disease is the external compression of the rotator cuff tendons, which can be significantly increased by age-related changes such as muscle weakness and poor posture. The objective of this study was to investigate the baseline compressive response of tendon and determine how this response is altered during maturation and aging. We did this by characterizing the compressive mechanical, viscoelastic, and poroelastic properties of young, mature, and aged mouse supraspinatus tendons using macroscale indentation testing and nanoscale high-frequency AFM-based rheology testing. Using these multiscale techniques, we found that aged tendons were stiffer than their mature counterparts and that both young and aged tendons exhibited increased hydraulic permeability and energy dissipation. We hypothesize that regional and age-related variations in collagen morphology and organization are likely responsible for changes in the multiscale compressive response as these structural parameters may affect fluid flow. Importantly, these results suggest a role for age-related changes in the progression of tendon degeneration, and we hypothesize that decreased ability to resist compressive loading via fluid pressurization may result in damage to the extracellular matrix (ECM) and ultimately tendon degeneration. These studies provide insight into the regional multiscale compressive response of tendons and indicate that altered compressive properties in aging tendons may be a major contributor to overall tendon degeneration.

Supraspinatus tears often result in the setting of chronic tendinopathy. However, the typical repair model utilizes an acute injury. In recognition of that distinction, our laboratory developed an overuse animal model; however it is unclear whether induced overuse is necessary in the repair model. We studied the repair properties of overuse-induced tendons compared to normal tendons. We hypothesized that histological and mechanical properties would not be altered between the overuse-induced and normal tendons 1 and 4 weeks after repair. Thirty-one adult male Sprague-Dawley rats were subjected to either overuse or cage activity for 4 weeks prior to bilateral supraspinatus tendon repair surgery. Rats were sacrificed at 1 and 4 weeks post-surgery and evaluated for histology and mechanics. Results at 1 week showed no clear histologic changes, but increased inflammatory protein expression in overuse tendons. At 4 weeks, percent relaxation was slightly increased in the overuse group. No other alterations in mechanics or histology were observed. Our results suggest that the effects of the surgical injury overshadow the changes evoked by overuse. Because clinically relevant mechanical parameters were not altered in the overuse group, we conclude that when examining tendons 4 weeks after repair in the classic rat supraspinatus model, inducing overuse prior to surgery is likely to be unnecessary.

Collagen V mutations underlie classic Ehlers-Danlos syndrome, and joint hypermobility is an important clinical manifestation. We define the function of collagen V in tendons and ligaments, as well as the role of alterations in collagen V expression in the pathobiology in classic Ehlers-Danlos syndrome. A conditional Col5a1(flox/flox) mouse model was bred with Scleraxis-Cre mice to create a targeted tendon and ligament Col5a1-null mouse model, Col5a1(Δten/Δten). Targeting was specific, resulting in collagen V-null tendons and ligaments. Col5a1(Δten/Δten) mice demonstrated decreased body size, grip weakness, abnormal gait, joint laxity, and early-onset osteoarthritis. These gross changes were associated with abnormal fiber organization, as well as altered collagen fibril structure with increased fibril diameters and decreased fibril number that was more severe in a major joint stabilizing ligament, the anterior cruciate ligament (ACL), than in the flexor digitorum longus tendon. The ACL also had a higher collagen V content than did the flexor digitorum longus tendon. The collagen V-null ACL and flexor digitorum longus tendon both had significant alterations in mechanical properties, with ACL exhibiting more severe changes. The data demonstrate critical differential regulatory roles for collagen V in tendon and ligament structure and function and suggest that collagen V regulatory dysfunction is associated with an abnormal joint phenotype, similar to the hypermobility phenotype in classic Ehlers-Danlos syndrome.

Patients with classic (type I) Ehlers-Danlos syndrome (EDS), characterized by heterozygous mutations in the Col5a1 and Col5a2 genes, exhibit connective tissue hyperelasticity and recurrent joint dislocations, indicating a potential regulatory role for collagen V in joint stabilizing soft tissues. This study asked whether the contribution of collagen V to the establishment of mechanical properties is tissue dependent. We mechanically tested four different tissues from wild type and targeted collagen V-null mice: the flexor digitorum longus (FDL) tendon, Achilles tendon (ACH), the anterior cruciate ligament (ACL), and the supraspinatus tendon (SST). Area was significantly reduced in the Col5a1(ΔTen/ΔTen) group in the FDL, ACH, and SST. Maximum load and stiffness were reduced in the Col5a1(ΔTen/ΔTen) group for all tissues. However, insertion site and midsubstance modulus were reduced only for the ACL and SST. This study provides evidence that the regulatory role of collagen V in extracellular matrix assembly is tissue dependent and that joint instability in classic EDS may be caused in part by insufficient mechanical properties of the tendons and ligaments surrounding each joint.

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 ( 10 2 to 10 4 Hz) whereas viscoelasticity was typically dominant at lower frequencies ( 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.

The cervix is a unique organ able to dramatically change its shape and function by serving as a physical barrier for the growing fetus and then undergoing dramatic dilation allowing for delivery of a term infant. As a result, the cervix endures changing mechanical forces from the growing fetus. There is an emerging concept that the cervix may change or remodel “early” in many cases of spontaneous preterm birth (sPTB). However, the mechanical role of the cervix in both normal and preterm birth remains unclear. Therefore, the primary objective of this study was to determine the mechanical and structural responses of murine cervical tissue throughout a normal gestational time course. In this study, both tissue structural and material properties were determined via a quasi-static tensile load-to-failure test, while simultaneously obtaining dynamic collagen fiber re-alignment via cross-polarization imaging. This study demonstrated that the majority of the mechanical properties evaluated decreased at midgestation and not just at term, while collagen fiber re-alignment occurred earlier in the loading curve for cervices at term. This suggests that although structural changes in the cervix occur throughout gestation, the differences in material properties function in combination with collagen fiber re-alignment as mechanical precursors to regulate term gestation. This work lays a foundation for investigating cervical biomechanics and the role of the cervix in preterm birth.

Current clinical treatment after tendon repairs often includes prescribing NSAIDs to limit pain and inflammation. The negative influence of NSAIDs on bone repair is well documented, but their effects on tendon healing are less clear. While NSAIDs may be detrimental to early tendon healing, some evidence suggests that they may improve healing if administered later in the repair process.We asked whether the biomechanical and histologic effects of systemic ibuprofen administration on tendon healing are influenced by either immediate or delayed drug administration.After bilateral supraspinatus detachment and repair surgeries, rats were divided into groups and given ibuprofen orally for either Days 0 to 7 (early) or Days 8 to 14 (delayed) after surgery; a control group did not receive ibuprofen. Healing was evaluated at 1, 2, and 4 weeks postsurgery through biomechanical testing and histologic assessment.Biomechanical evaluation resulted in decreased stiffness and modulus at 4 weeks postsurgery for early ibuprofen delivery (mean ± SD [95% CI]: 10.8 ± 6.4 N/mm [6.7-14.8] and 8.9 ± 5.9 MPa [5.4-12.3]) when compared to control repair (20.4 ± 8.6 N/mm [16.3-24.5] and 15.7 ± 7.5 MPa [12.3-19.2]) (p = 0.003 and 0.013); however, there were no differences between the delayed ibuprofen group (18.1 ± 7.4 N/mm [14.2-22.1] and 11.5 ± 5.6 MPa [8.2-14.9]) and the control group. Histology confirmed mechanical results with reduced fiber reorganization over time in the early ibuprofen group.Early administration of ibuprofen in the postoperative period was detrimental to tendon healing, while delayed administration did not affect tendon healing.Historically, clinicians have often prescribed ibuprofen after tendon repair, but this study suggests that the timing of ibuprofen administration is critical to adequate tendon healing. This research necessitates future clinical studies investigating the use of ibuprofen for pain control after rotator cuff repair and other tendon injuries.

Classic Ehlers-Danlos syndrome (EDS) patients suffer from connective tissue hyperelasticity, joint instability, skin hyperextensibility, tissue fragility, and poor wound healing due to heterozygous mutations in COL5a1 or COL5a2 genes. This study investigated the roles of collagen V in establishing structure and function in uninjured patellar tendons as well as in the injury response using a Col5a1+/− mouse, a model for classic EDS. These analyses were done comparing tendons from a classic EDS model (Col5a1+/−) with wild type controls. Tendons were subjected to mechanical testing, histological and fibril analysis before injury as well as 3 weeks and 6 weeks after injury. We found that Col5a1+/− tendons demonstrated diminished recovery of mechanical competency after injury as compared to normal wild type tendons, which recovered their pre-injury values by 6 weeks post injury. Additionally, the Col5a1+/−tendons demonstrated altered fibril morphology and diameter distributions compared to the wild type tendons. This study indicates that collagen V plays an important role in regulating collagen fibrillogenesis and the associated recovery of mechanical integrity in tendons after injury. In addition, the dysregulation with decreased collagen V expression in EDS is associated with a diminished injury response. The results presented herein have the potential to direct future targeted therapeutics for classic EDS patients.

Collagen fiber re-alignment is one postulated mechanism of tendon structural response to load. While collagen fiber distribution has been shown to vary by tendon location in the supraspinatus tendon, changes in local re-alignment behavior have not been examined throughout postnatal development. Postnatal tendons, with immature collagen fibrils, may respond to load in a much different manner than collagen fibers with mature fiber-fiber and fiber-matrix connections. Local collagen fiber re-alignment is quantified throughout tensile mechanical testing in a developmental mouse supraspinatus tendon model and corresponding mechanical properties measured. Collagen fiber re-alignment occurred during preconditioning for 28 day old tendons, at the toe-region for 10 day tendons and at the linear-region for 4 day tendon midsubstance. Mechanical properties increased with developmental age. Linear modulus was lower at the insertion site compared to the midsubstance location at all time points. Local differences in collagen fiber distributions were found at 10 and 28 days for all mechanical testing points (except the 10 day transition point). This study found that collagen fiber re-alignment depends on developmental age and suggests that collagen fibrillogenesis may influence the tendon’s ability to structurally respond to load. Additionally, results indicate that the insertion site and tendon midsubstance locations develop differently.

Supraspinatus tears often result in the setting of chronic tendinopathy. However, the typical repair model utilizes an acute injury. In recognition of that distinction, our laboratory developed an overuse animal model; however it is unclear whether induced overuse is necessary in the repair model. We studied the repair properties of overuse-induced tendons compared to normal tendons. We hypothesized that histological and mechanical properties would not be altered between the overuse-induced and normal tendons 1 and 4 weeks after repair. Thirty-one adult male Sprague-Dawley rats were subjected to either overuse or cage activity for 4 weeks prior to bilateral supraspinatus tendon repair surgery. Rats were sacrificed at 1 and 4 weeks post-surgery and evaluated for histology and mechanics. Results at 1 week showed no clear histologic changes, but increased inflammatory protein expression in overuse tendons. At 4 weeks, percent relaxation was slightly increased in the overuse group. No other alterations in mechanics or histology were observed. Our results suggest that the effects of the surgical injury overshadow the changes evoked by overuse. Because clinically relevant mechanical parameters were not altered in the overuse group, we conclude that when examining tendons 4 weeks after repair in the classic rat supraspinatus model, inducing overuse prior to surgery is likely to be unnecessary.

Patients with classic (type I) Ehlers-Danlos syndrome (EDS), characterized by heterozygous mutations in the Col5a1 and Col5a2 genes, exhibit connective tissue hyperelasticity and recurrent joint dislocations, indicating a potential regulatory role for collagen V in joint stabilizing soft tissues. This study asked whether the contribution of collagen V to the establishment of mechanical properties is tissue dependent. We mechanically tested four different tissues from wild type and targeted collagen V-null mice: the flexor digitorum longus (FDL) tendon, Achilles tendon (ACH), the anterior cruciate ligament (ACL), and the supraspinatus tendon (SST). Area was significantly reduced in the Col5a1(ΔTen/ΔTen) group in the FDL, ACH, and SST. Maximum load and stiffness were reduced in the Col5a1(ΔTen/ΔTen) group for all tissues. However, insertion site and midsubstance modulus were reduced only for the ACL and SST. This study provides evidence that the regulatory role of collagen V in extracellular matrix assembly is tissue dependent and that joint instability in classic EDS may be caused in part by insufficient mechanical properties of the tendons and ligaments surrounding each joint.

As tendons are loaded, they reduce in volume and exude fluid to the surrounding medium. Experimental studies have shown that tendon stretching results in a Poisson’s ratio greater than 0.5, with a maximum value at small strains followed by a nonlinear decay. Here we present a computational model that attributes this macroscopic observation to the microscopic mechanism of the load transfer between fibrils under stretch. We develop a finite element model based on the mechanical role of the interfibrillar-linking elements, such as thin fibrils that are bridging between the aligned fibrils or macromolecules such as glycosaminoglycans (GAGs) in the interfibrillar sliding and verify it with a theoretical shear-lag model. We showed the existence of a previously unappreciated structure-function mechanism whereby the Poisson’s ratio in tendon is affected by the strain applied and interfibrillar-linker properties, and together these features predict tendon volume shrinkage under tensile loading. During loading, the interfibrillar-linkers pulled fibrils towards each other and squeezed the matrix, leading to the Poisson’s ratio larger than 0.5 and fluid expulsion. In addition, the rotation of the interfibrillar-linkers with respect to the fibrils at large strains caused a reduction in the volume shrinkage and eventual nonlinear decay in Poisson’s ratio at large strains. Our model also predicts a fluid flow that has a radial pattern toward the surrounding medium, with the larger fluid velocities in proportion to the interfibrillar sliding.

Tendons are able to transmit high loads efficiently due to their finely optimized hierarchical collagen structure. Two mechanisms by which tendons respond to load are collagen fibril sliding and deformation (stretch). Although many studies have demonstrated that regional variations in tendon structure, composition, and organization contribute to the full tendon׳s mechanical response, the location-dependent response to loading at the fibril level has not been investigated. In addition, the instantaneous response of fibrils to loading, which is clinically relevant for repetitive stretch or fatigue injuries, has also not been studied. Therefore, the purpose of this study was to quantify the instantaneous response of collagen fibrils throughout a mechanical loading protocol, both in the insertion site and in the midsubstance of the mouse supraspinatus tendon. Utilizing a novel atomic force microscopy-based imaging technique, tendons at various strain levels were directly visualized and analyzed for changes in fibril d-period with increasing tendon strain. At the insertion site, d-period significantly increased from 0% to 1% tendon strain, increased again from 3% to 5% strain, and decreased after 5% strain. At the midsubstance, d-period increased from 0% to 1% strain and then decreased after 7% strain. In addition, fibril d-period heterogeneity (fibril sliding) was present, primarily at 3% strain with a large majority occurring in the tendon midsubstance. This study builds upon previous work by adding information on the instantaneous and regional-dependent fibrillar response to mechanical loading and presents data proposing that collagen fibril sliding and stretch are directly related to tissue organization and function.

Tendons function to transfer load from muscle to bone through their complex composition and hierarchical structure, consisting mainly of type I collagen. Recent evidence suggests that type II diabetes may cause alterations in collagen structure, such as irregular fibril morphology and density, which could play a role in the mechanical function of tendons. Using the db/db mouse model of type II diabetes, the diabetic skin was found to have impaired biomechanical properties when compared to the non-diabetic group. The purpose of this study was to assess the effect of diabetes on biomechanics, collagen fiber re-alignment, and biochemistry in three functionally different tendons (Achilles, supraspinatus, patellar) using the db/db mouse model. Results showed that cross-sectional area and stiffness, but not modulus, were significantly reduced in all three tendons. However, the tendon response to load (transition strain, collagen fiber re-alignment) occurred earlier in the mechanical test, contrary to expectations. In addition, the patellar tendon had an altered response to diabetes when compared to the other two tendons, with no changes in fiber re-alignment and decreased collagen content at the midsubstance of the tendon. Overall, type II diabetes alters tendon mechanical properties and the dynamic response to load.

Collagen fiber realignment is one mechanism by which tendon responds to load. Re-alignment is altered when the structure of tendon is altered, such as in the natural process of aging or with alterations of matrix proteins, such as proteoglycan expression. While changes in re-alignment and mechanical properties have been investigated recently during development, they have not been studied in (1) aged tendons, or (2) in the absence of key proteoglycans. Collagen fiber re-alignment and the corresponding mechanical properties are quantified throughout tensile mechanical testing in both the insertion site and the midsubstance of mouse supraspinatus tendons in wild type (WT), decorin-null (Dcn(-/-)), and biglycan-null (Bgn(-/-)) mice at three different ages (90 days, 300 days, and 570 days). Percent relaxation was significantly decreased with age in the WT and Dcn(-/-) tendons, but not in the Bgn(-/-) tendons. Changes with age were found in the linear modulus at the insertion site where the 300 day group was greater than the 90 day and 570 day group in the Bgn(-/-) tendons and the 90 day group was smaller than the 300 day and 570 day groups in the Dcn(-/-) tendons. However, no changes in modulus were found across age in WT tendons were found. The midsubstance fibers of the WT and Bgn(-/-) tendons were initially less aligned with increasing age. The re-alignment was significantly altered with age in the WT tendons, with older groups responding to load later in the mechanical test. This was also seen in the Dcn(-/-) midsubstance and the Bgn(-/-) insertion, but not in the other locations. Although some studies have found changes in the WT mechanical properties with age, this study did not support those findings. However, it did show fiber re-alignment changes at both locations with age, suggesting a breakdown of tendon's ability to respond to load in later ages. In the proteoglycan-null tendons however, there were changes in the mechanical properties, accompanied only by location-dependent re-alignment changes, suggesting a site-specific role for these molecules in loading. Finally, changes in the mechanical properties did not occur in concert with changes in re-alignment, suggesting that typical mechanical property measurements alone are insufficient to describe how structural alterations affect tendon's response to load.

This review highlights recent research on structure-function relationships in tendon and comments on the parallels between development and healing. The processes of tendon development and collagen fibrillogenesis are reviewed, but due to the abundance of information in this field, this work focuses primarily on characterizing the mechanical behavior of mature and developing tendon, and how the latter parallels healing tendon. The role that extracellular matrix components, mainly collagen, proteoglycans, and collagen cross-links, play in determining the mechanical behavior of tendon will be examined in this review. Specifically, collagen fiber re-alignment and collagen fibril uncrimping relate mechanical behavior to structural alterations during development and during healing. Finally, attention is paid to a number of recent efforts to augment injured tendon and how future efforts could focus on recreating the important structure-function relationships reviewed here.

One postulated mechanism of tendon structural response to mechanical load is collagen fiber re-alignment. Recently, where collagen fiber re-alignment occurs during a tensile mechanical test has been shown to vary by tendon age and location in a postnatal developmental mouse supraspinatus tendon (SST) model [1]. It is thought that as the tendon matures and its collagen fibril network, collagen cross-links and collagen-matrix interactions develop, its ability to respond quickly to mechanical stimuli hastens [1]. Additionally, the insertion site and midsubstance of postnatal SST may develop differently and at different rates, providing a potential explanation for differences in fiber re-alignment behaviors at the insertion site and midsubstance at postnatal developmental time points [1]. However, collagen fiber re-alignment behavior, in response to mechanical load at a mature age and in comparison to developmental ages, have not been examined. Therefore, the objectives of this study are to locally measure: 1) fiber re-alignment during preconditioning and tensile mechanical testing and 2) to compare local differences in collagen fiber alignment and corresponding mechanical properties to address tissue response to mechanical load in the mature and postnatal developmental mouse SST. We hypothesize that 1) 90 day tendons will demonstrate the largest shift in fiber re-alignment during preconditioning, but will also re-align during the toe- and linear-regions. Additionally, we hypothesize that 2) mechanical properties and initial collagen fiber alignment will be greater in the midsubstance of the tendon compared to the tendon-to-bone insertion site at 90 days, 3) that mechanical properties will increase with age, and that 4) collagen fiber organization at the insertion site will decrease with age.Copyright © 2012 by ASME

Tendons function to transfer load, maintain alignment and permit motion in joints. To perform these functions, tendons have complex mechanical behavior which is modulated by the tissue’s structure and composition, such as the collagen fibers and surrounding extracellular matrix. When these matrix proteins are altered, the mechanical properties are also altered, which could potentially lead to reduced loading and healing capacity. Diabetes is a metabolic disease which, among other co-morbidities, has been associated with Achilles tendon disorganization and tendinopathy, as well as increased overall joint stiffness in humans [1]. We have recently reported that the skin from the Db/Db diabetic mouse, a model of Type II Diabetes, as well as the skin from human diabetics, have impaired biomechanical properties compared to non-diabetic skin as the result of altered extracellular matrix composition. [2]. However, the mechanical properties of tendons from these animals have never been studied and could serve as a unique model of altered collagen structure as well as provide further understanding to the cause of tendinous injuries in the diabetic population. Therefore, the objective of this study is to measure the tensile mechanical properties and collagen fiber re-alignment in the db/db mouse model compared to non-diabetic controls. We hypothesize that tendon stiffness and modulus will be increased in the db/db group in the insertion site and midsubstance, and that db/db tendons will re-align earlier and faster during the testing protocol.Copyright © 2012 by ASME

Background Collagen fiber re-alignment and uncrimping are two postulated mechanisms of tendon structural response to load. Recent studies have examined structural changes in response to mechanical testing in a postnatal development mouse supraspinatus tendon model (SST), however, those changes in the mature mouse have not been characterized. The objective of this study was to characterize collagen fiber re-alignment and crimp behavior throughout mechanical testing in a mature mouse SST. Method of approach A tensile mechanical testing set-up integrated with a polarized light system was utilized for alignment and mechanical analysis. Local collagen fiber crimp frequency was quantified immediately following the designated loading protocol using a traditional tensile set up and a flash-freezing method. The effect of number of preconditioning cycles on collagen fiber re-alignment, crimp frequency and mechanical properties in midsubstance and insertion site locations were examined. Results Decreases in collagen fiber crimp frequency were identified at the toe-region of the mechanical test at both locations. The insertion site re-aligned throughout the entire test, while the midsubstance re-aligned during preconditioning and the test's linear-region. The insertion site demonstrated a more disorganized collagen fiber distribution, lower mechanical properties and a higher cross-sectional area compared to the midsubstance location. Conclusions Local collagen fiber re-alignment, crimp behavior and mechanical properties were characterized in a mature mouse SST model. The insertion site and midsubstance respond differently to mechanical load and have different mechanisms of structural response. Additionally, results support that collagen fiber crimp is a physiologic phenomenon that may explain the mechanical test toe-region.

Background Repair techniques of rotator cuff tendon tears have improved in recent years; nonetheless, the failure rate remains high. Despite the availability of various graft materials for repair augmentation, there has yet to be a biomechanical study using fiber-aligned scaffolds in vivo. The objective of this study was to evaluate the efficacy of fiber-aligned nanofibrous polymer scaffolds as a potential treatment-delivery vehicle in a rat rotator cuff injury model. Materials and methods Scaffolds with and without sacrificial fibers were fabricated via electrospinning and implanted to augment supraspinatus repair in rats. Repairs without scaffold augmentation were also performed to serve as controls. Rats were sacrificed at 4 and 8 weeks postoperatively, and repairs were evaluated histologically and biomechanically. Results Both scaffold formulations remained in place, with more noticeable cellular infiltration and colonization at 4 and 8 weeks after injury and repair for scaffolds lacking sacrificial fibers. Specimens with scaffolds were larger in cross-sectional area compared with controls. Biomechanical testing revealed no significant differences in structural properties between the groups. Some apparent material properties were significantly reduced in the scaffold groups. These reductions were due to increases in cross-sectional area, most likely caused by the extra thickness of the implanted scaffold material. No differences were observed between the 2 scaffold groups. Conclusions No adverse effect of surgical implantation of overlaid fiber-aligned scaffolds on structural properties of supraspinatus tendons in rat rotator cuff repair was demonstrated, validating this model as a platform for targeted delivery.