What are the validated animal models for tendinopathy?

Treatment options for Achilles tendinopathy: a scoping review of preclinical studies

Background

Achilles tendinopathy (AT) management can be difficult, given the paucity of effective treatment options and the degenerative nature of the condition. Innovative therapies for Achilles tendinopathy are therefore direly needed. New therapeutic developments predominantly begin with preclinical animal and in vitro studies to understand the effects at the molecular level and to evaluate toxicity. Despite the publication of many preclinical studies, a comprehensive, quality-assessed review of the basic molecular mechanisms in Achilles tendinopathy is lacking.

Objectives

This scoping review aims to summarize the literature regarding in vitro and in vivo animal studies examining AT treatments and evaluate their effect on tendon properties. Also, a quality assessment of the included animal studies is done. We provide a comprehensive insight into the current state of preclinical AT treatment research which may guide preclinical researchers in future research.

Eligibility criteria

Treatment options of Achilles tendinopathy in chemically or mechanically induced in vivo or in vitro Achilles tendinopathy models, reporting biomechanical, histological, and/or biochemical outcomes were included.

Sources of evidence

A systematically conducted scoping review was performed in PubMed, Embase.com, Clarivate Analytics/Web of Science, and the Wiley/Cochrane Library. Studies up to May 4, 2023 were included.

Charting Methods

Data from the included articles were extracted and categorized inductively in tables by one reviewer. The risk-of-bias quality assessment of the included animal studies is done with Systematic Review Centre for Laboratory Animal Experimentation risk-of-bias tool.

Results

A total of 98 studies is included, which investigated 65 different treatment options. 80% of studies reported significant improvement in the Achilles tendon characteristics after treatment. The main results were; maximum load and stiffness improvement; fibre structure recovered and less inflammation was observed; collagen I fibrils increased, collagen III fibrils decreased, and fewer inflammatory cells were observed after treatment. However, 65.4% to 92.5% of the studies had an uncertain to high risk of bias according to the risk-of-bias tool of the Systematic Review Centre for Laboratory Animal Experimentation.

Conclusions

Despite promising preclinical treatment outcomes, translation to clinical practice lags behind. This may be due to the poor face validity of animal models, heterogeneity in Achilles tendinopathy induction, and low quality of the included studies. Preclinical treatments that improved the biomechanical, histological, and biochemical tendon properties may be interesting for clinical trial investigation. Future efforts should focus on developing standardized preclinical Achilles tendinopathy models, improving reporting standards to minimize risk of bias, and facilitating translation to clinical practice.

High-fat diet-induced obesity exacerbated collagenase-induced tendon injury with upregulation of interleukin-1beta and matrix metalloproteinase-1

AIMS

Obesity increases tendinopathy's risk, but its mechanisms remain unclear. This study examined the effect of high-fat diet (HFD)-induced obesity on the outcomes and inflammation of collagenase-induced (CI) tendon injury.

METHODS

Mice were fed with standard chow (SC) or HFD for 12 weeks. Bacterial collagenase I or saline was injected over the patellar tendons of each mouse. At weeks 2 and 8 post-injection, the patellar tendons were harvested for histology, immunohistochemical staining, and gait analysis. The difference (Δ) of limb-idleness index (LII) at the time of post-injury and pre-injury states was calculated. Biomechanical test of tendons was also performed at week 8 post-injection.

RESULTS

HFD aggravated CI tendon injury with an increase in vascularity and cellularity compared to SC treatment. The histopathological score (week 2: p = 0.025; week 8: p = 0.013) and ΔLII (week 2: p = 0.012; week 8: p = 0.005) were significantly higher in the HFD group compared to those in the SC group after CI tendon injury. Stiffness (saline: p = 0.003; CI: p = 0.010), ultimate stress (saline: p < 0.001; CI: p = 0.006), and Young's modulus (saline: p = 0.017; CI: p = 0.007) were significantly lower in the HFD group compared to the SC group at week 8 after saline or collagenase injection. HFD induced higher expression of IL-1β (week 2: p = 0.010; week 8: p = 0.025) and MMP-1 (week 2: p = 0.010; week 8: p = 0.004) compared to SC treatment after CI tendon injury at both time points.

CONCLUSIONS

HFD-induced obesity exacerbated histopathological, functional, and biomechanical changes in the CI tendon injury model, which was associated with an upregulation of IL-1β and MMP-1.

Direct muscle neurotization: Previous advancements in animal models

Peripheral nerve repair is daily activity for several microsurgeons. Numerous nerve repair techniques are applied, including neurorrhaphy, nerve grafting and nerve transfer, depending on the nature and extent of the injury. However, these techniques become unfeasible when the distal nerve end is not preserved during the peripheral nerve injury or a segment of the muscle is transferred without the nerve supplying it. However, direct muscle neurotization (DMN) achieves muscle reinnervation by suturing the nerve directly into the muscle tissue, without requiring a distal nerve end for coaptation. Despite promising results in the literature, DMN is not widely adopted in clinical practice. Animal models may help in advancing novel therapeutic approaches, due to their anatomic and physiologic similarities to humans. This review highlights the current scientific understanding and recent advancements in DMN as well as the animal models and target muscle that have been used in the past to investigate the basic principles behind this surgical technique. The presented information should aid in establishing the clinical importance of DMN in peripheral nerve injury.

Extracellular Vesicle-Contained Thrombospondin 1 Retards Age-Related Degenerative Tendinopathy by Rejuvenating Tendon Stem/Progenitor Cell Senescence

Advanced age is a major risk factor for age-related degenerative tendinopathy. During aging, tendon stem/progenitor cell (TSPC) function declines owing to the transition from a normal quiescent state to a senescent state. Extracellular vesicles (EVs) from young stem cells are reported to possess anti-aging functions. However, it remains unclear whether EVs from young TSPCs (TSPC-EVs) can rejuvenate senescent TSPCs to delay age-related degeneration. Here, this study finds that TSPC-EVs can mitigate the aging phenotypes of senescent TSPCs and maintain their tenogenic capacity. In vitro studies reveal that TSPC-EVs can reinstall autophagy in senescent TSPCs to alleviate cellular senescence, and that the re-establishment of autophagy is mediated by the PI3K/AKT pathway. Mechanistically, this study finds that thrombospondin 1, a negative regulator of the PI3K/AKT pathway, is enriched in TSPC-EVs and can be transported to senescent TSPCs. Moreover, in vivo studies show that the local delivery of TSPC-EVs can rejuvenate senescent TSPCs and promote their tenogenic differentiation, thereby rescuing tendon regeneration in aged rats. Taken together, TSPC-EVs as a novel cell-free approach have promising therapeutic potential for aging-related degenerative tendinopathy.

Controlled Stiffness of Direct-Write, Near-Field Electrospun Gelatin Fibers Generates Differences in Tenocyte Morphology and Gene Expression

Tendinopathy is a leading cause of mobility issues. Currently, the cell-matrix interactions involved in the development of tendinopathy are not fully understood. In vitro tendon models provide a unique tool for addressing this knowledge gap as they permit fine control over biochemical, micromechanical, and structural aspects of the local environment to explore cell-matrix interactions. In this study, direct-write, near-field electrospinning of gelatin solution was implemented to fabricate micron-scale fibrous scaffolds that mimic native collagen fiber size and orientation. The stiffness of these fibrous scaffolds was found to be controllable between 1 MPa and 8 MPa using different crosslinking methods (EDC, DHT, DHT+EDC) or through altering the duration of crosslinking with EDC (1 h to 24 h). EDC crosslinking provided the greatest fiber stability, surviving up to 3 weeks in vitro. Differences in stiffness resulted in phenotypic changes for equine tenocytes with low stiffness fibers (∼1 MPa) promoting an elongated nuclear aspect ratio while those on high stiffness fibers (∼8 MPa) were rounded. High stiffness fibers resulted in the upregulation of matrix metalloproteinase (MMPs) and proteoglycans (possible indicators for tendinopathy) relative to low stiffness fibers. These results demonstrate the feasibility of direct-written gelatin scaffolds as tendon in vitro models and provide evidence that matrix mechanical properties may be crucial factors in cell-matrix interactions during tendinopathy formation.

Tissue-engineered mesenchymal stem cell constructs alleviate tendinopathy by suppressing vascularization

Tendinopathy leads to low-grade tissue inflammation and chronic damage, which progresses due to pathological imbalance in angiogenesis. Reducing early pathological vascularization may be a new approach in helping to regenerate tendon tissue. Conventional stem cell therapy and tissue engineering scaffolds have not been highly effective at treating tendinopathy. In this study, tissue engineered stem cells (TSCs) generated using human umbilical cord mesenchymal stem cells (hUC-MSCs) were combined with microcarrier scaffolds to limit excessive vascularization in tendinopathy. By preventing VEGF receptor activation through their paracrine function, TSCs reduced in vitro angiogenesis and the proliferation of vascular endothelial cells. TSCs also decreased the inflammatory expression of tenocytes while promoting their anabolic and tenogenic characteristics. Furthermore, local injection of TSCs into rats with collagenase-induced tendinopathy substantially reduced early inflammation and vascularization. Mechanistically, transcriptome sequencing revealed that TSCs could reduce the progression of pathological angiogenesis in tendon tissue, attributed to Rap1-mediated vascular inhibition. TSCs may serve as a novel and practical approach for suppressing tendon vascularization, and provide a promising therapeutic agent for early-stage clinical tendinopathy.

Autologous bone marrow derived mesenchymal stem cells are safe for the treatment of Achilles tendinopathy

Achilles tendinopathy is a disabling condition that affects more than 50% of runners. Pre-clinical studies in a large animal model of naturally-occurring tendinopathy similar to human Achilles tendinopathy has shown benefits of autologous bone marrow-derived mesenchymal stem cell (MSC) implantation. However, MSCs are advanced therapies medicinal products (ATMPs), with strict regulatory requirements. Guided by the regulator we carried out a first in man study to assess the safety and efficacy of autologous MSC injection in human patients with non-insertional Achilles tendinopathy. Ten patients, mean age 47 with mid-portion Achilles tendon pain and swelling for more than 6 months, underwent autologous cultured cell injections (median 12.2 × 106, range 5-19 × 106 cells) into their Achilles tendon. At 24 weeks follow-up, no serious adverse reactions or important medical events were observed. MOXFQ, EQ-5D-5L, and VISA-A scores improved clinically at 12 and 24 weeks. VAS pain improved increasingly at 6, 12 and 24 weeks. MOXFQ Pain and VISA-A Scores improved > 12 points from baseline to 24 weeks in 8 patients. Maximum anteroposterior tendon thickness as measured by greyscale US decreased by mean 0.8 mm at 24 weeks. This phase IIa study demonstrated the safety of autologous MSC injection for non-insertional Achilles tendinopathy and provides proof-of-concept of the technique in patients, all of whom had previously failed conservative treatments for chronic disease and leads the way for a larger randomised controlled trial.

Establishment of a Mouse Degenerative Model of Patellar Tendinopathy with Upregulation of Inflammation

There is no mouse model of patellar tendinopathy. This study aimed to establish a mouse inflammatory and degenerative patellar tendon injury model, which will facilitate research on patellar tendinopathy using advanced molecular tools including transgenic models. Collagenase at different doses (low dose (LD), medium dose (MD), high dose (HD)) or saline was injected over the mouse patellar tendon. At weeks 1, 2, 4, and 8 post-injection, the tendons were harvested for histology and further examined by micro-computed tomography (microCT) imaging at week 8. The optimal dose group and the saline group were further evaluated by immunohistochemical staining, gait pattern, and biomechanical properties. The histopathological score increased dose-dependently post-collagenase injection. Ectopic mineralization was observed and increased with collagenase dose. The LD group was selected for further analysis. The expression of IL-10, TNF-α, and MMP-1 significantly increased post-injection. The changes of limb idleness index (ΔLII) compared to preinjury state were significantly higher, while the ultimate load, stiffness, ultimate stress, and maximum Young's modulus were significantly lower in the LD group compared to the saline group. A mouse inflammatory degenerative model of patellar tendon injury resembling tendinopathy was established as indicated by the dose-dependent increase in tendon histopathology, ectopic calcification, decrease in biomechanical properties, and pain-associated gait changes.

The rat as a novel model for chronic rotator cuff injuries

Chronic rotator cuff injuries (CRCIs) still present a great challenge for orthopaedics surgeons. Many new therapeutic strategies are developed to facilitate repair and improve the healing process. However, there is no reliable animal model for chronic rotator cuff injury research. To present a new valuable rat model for future chronic rotator cuff injuries (CRCIs) repair studies, and describe the changes of CRCIs on the perspectives of histology, behavior and MRI. Sixty male Wistar rats were enrolled and underwent surgery of the left shoulder joint for persistent subacromial impingement. They were randomly divided into experimental group (n = 30, a 3D printed PEEK implant shuttled into the lower surface of the acromion) and sham operation group (n = 30, insert the same implant, but remove it immediately). Analyses of histology, behavior, MRI and inflammatory pain-related genes expression profiles were performed to evaluate the changes of CRCIs. After 2-weeks running, the rats in the experimental group exhibited compensatory gait patterns to protect the injured forelimb from loading after 2-weeks running. After 8-weeks running, the rats in the experimental group showed obvious CRCIs pathological changes: (1) acromion bone hyperplasia and thickening of the cortical bone; (2) supraspinatus muscle tendon of the humeral head: the bursal-side tendon was torn and layered with disordered structure, forming obvious gaps; the humeral-side tendon is partially broken, and has a neatly arranged collagen. Partial fat infiltration is found. The coronal T2-weighted images showed that abnormal tendon-to-bone junctions of the supraspinatus tendon. The signal intensity and continuity were destroyed with contracted tendon. At the nighttime, compared with the sham operation group, the expression level of IL-1β and COX-2 increased significantly (P = 0063, 0.0005) in the experimental group. The expression of COX-2 in experimental group is up-regulated about 1.5 times than that of daytime (P = 0.0011), but the expression of IL-1β, TNF-a, and NGF are all down-regulated (P = 0.0146, 0.0232, 0.0161). This novel rat model of chronic rotator cuff injuries has the similar characteristics with that of human shoulders. And it supplies a cost-effective, reliable animal model for advanced tissue engineered strategies and future therapeutic strategies.

Therapeutic potential of ginsenoside compound K in managing tenocyte apoptosis and extracellular matrix damage in diabetic tendinopathy

The prevalence of tendinopathy in patients with diabetes is well documented. Despite efforts to improve diabetes management, there is a lack of research on therapeutic agents targeting the core features of tendinopathy, namely, tenocyte apoptosis and extracellular matrix (ECM) damage. In this study, we investigated the potential of ginsenoside compound K (CK), known for its antidiabetic properties, to mitigate tenocyte apoptosis, inflammation, oxidative stress, and the metalloproteinase (MMP) system under hyperglycemic conditions. Our research also aimed to unravel the molecular mechanism underlying the effects of CK. The assessment of apoptosis involved observing intracellular chromatin condensation and measuring caspase 3 activity. To gauge oxidative stress, we examined cellular ROS levels and hydrogen peroxide and malondialdehyde concentrations. Western blotting was employed to determine the expression of various proteins. Our findings indicate that CK treatment effectively countered high glucose-induced apoptosis, inflammation, and oxidative stress in cultured tenocytes. Furthermore, CK normalized the expression of MMP-9, MMP-13, and TIMP-1. Notably, CK treatment boosted the expression of PPARγ and antioxidant enzymes. We conducted small interfering (si) RNA experiments targeting PPARγ, revealing its role in mediating CK's effects on tendinopathy features in hyperglycemic tenocytes. In conclusion, these in vitro results offer valuable insights into the potential therapeutic role of CK in managing tendinopathy among individuals with diabetes. By addressing crucial aspects of tendinopathy, CK presents itself as a promising avenue for future research and treatment development in this domain.

Treatment of Naturally Occurring Tendon Disease with Allogeneic Multipotent Mesenchymal Stromal Cells: A Randomized, Controlled, Triple-Blinded Pilot Study in Horses

The treatment of tendinopathies with multipotent mesenchymal stromal cells (MSCs) is a promising option in equine and human medicine. However, conclusive clinical evidence is lacking. The purpose of this study was to gain insight into clinical treatment efficacy and to identify suitable outcome measures for larger clinical studies. Fifteen horses with early naturally occurring tendon disease were assigned to intralesional treatment with allogeneic adipose-derived MSCs suspended in serum or with serum alone through block randomization (dosage adapted to lesion size). Clinicians and horse owners remained blinded to the treatment during 12 months (seven horses per group) and 18 months (seven MSC-group and five control-group horses) of follow-up including clinical examinations and diagnostic imaging. Clinical inflammation, lameness, and ultrasonography scores improved more over time in the MSC group. The lameness score difference significantly improved in the MSC group compared with the control group after 6 months. In the MSC group, five out of the seven horses were free of re-injuries and back to training until 12 and 18 months. In the control group, three out of the seven horses were free of re-injuries until 12 months. These results suggest that MSCs are effective for the treatment of early-phase tendon disease and provide a basis for a larger controlled study.

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.

Biomaterials for Tissue-Engineered Treatment of Tendinopathy in Animal Models: A Systematic Review

To conduct a systematic review of studies reporting the treatment of tendon injury using biomaterials in animal models. A systematic search was conducted to retrieve studies involving animal models of tendon repair using biomaterials, in PubMed (database construction to August 2022) and Ovid-Embase (1946 to August 2022). Data related to tendon repair with biomaterials were extracted by two researchers, respectively. Risk of bias was assessed following the Cochrane Handbook for Systematic Reviews of Interventions. A statistical analysis was performed based on the classification of tendon repair biomaterials included in our study. A total of 8413 articles were retrieved, with 78 studies included in our analysis. For tendon repair in animal models using biomaterials, the most commonly seen characteristics were as follows: naturally derived biomaterials, rabbits and rats as animal models, surgery as the injury model, and the Achilles tendon as the injury site. The histology and biomechanical recovery of tendon injury following repair are affected by different biomaterials. Studies of tendon repair in animal models indicate that biomaterials can significantly improve repair outcomes, including tendon structure and biomechanics. Among effective biomaterial strategies are the use of new composites and incorporation of cells or growth factors into the material, both of which provide obvious benefits for tendon healing. More high-quality preclinical studies are required to encourage the translation of biomaterials into clinical practice for tendon repair.

Overload in a Rat In Vivo Model of Synergist Ablation Induces Tendon Multiscale Structural and Functional Degeneration

Tendon degeneration is typically described as an overuse injury with little distinction made between magnitude of load (overload) and number of cycles (overuse). Further, in vivo, animal models of tendon degeneration are mostly overuse models, where tendon damage is caused by a high number of load cycles. As a result, there is a lack of knowledge of how isolated overload leads to degeneration in tendons. A surgical model of synergist ablation (SynAb) overloads the target tendon, plantaris, by ablating its synergist tendon, Achilles. The objective of this study was to evaluate the structural and functional changes that occur following overload of plantaris tendon in a rat SynAb model. Tendon cross-sectional area (CSA) and shape changes were evaluated by longitudinal MR imaging up to 8 weeks postsurgery. Tissue-scale structural changes were evaluated by semiquantified histology and second harmonic generation microscopy. Fibril level changes were evaluated with serial block face scanning electron microscopy (SBF-SEM). Functional changes were evaluated using tension tests at the tissue and microscale using a custom testing system allowing both video and microscopy imaging. At 8 weeks, overloaded plantaris tendons exhibited degenerative changes including increases in CSA, cell density, collagen damage area fraction (DAF), and fibril diameter, and decreases in collagen alignment, modulus, and yield stress. To interpret the differences between overload and overuse in tendon, we introduce a new framework for tendon remodeling and degeneration that differentiates between the inputs of overload and overuse. In summary, isolated overload induces multiscale degenerative structural and functional changes in plantaris tendon.

A pilot study testing an Achilles tendinopathy human cadaver model using intratendinous injection of collagenase

BACKGROUND

Achilles tendinopathy is one of the most frequently occurring soft-tissue injuries. Despite decades of research, there is still much that is unknown about the progression of tendinopathy. Animal models, such as collagenase injection, allow researchers to gain insight into disease progression and investigate clinical interventions, yet are limited in their direct application to humans. Establishment of a cadaver model of tendinopathy would provide another method of investigating clinical interventions in human tissues. The purpose of this study is to develop such a model and evaluate biomechanical changes in cadaveric Achilles tendons using ultrasound elastography.

METHODS

Achilles tendons of five female foot/ankle cadavers were injected with two different concentrations (three with 10 mg/mL, two 20 mg/mL) of collagenase and incubated for 24 h. Ultrasound elastography images were collected at baseline, 16 and 24 h post-injection. Elasticity of tendons was calculated using a custom image analysis program.

FINDINGS

Elasticity decreased over time in both dosage groups. In the 10 mg/mL group, mean elasticity decreased from 642 ± 246 kPa at baseline to 392 ± 38.3 kPa at 16 h and 263 ± 87.3 kPa at 24 h. In the 20 mg/mL group, mean elasticity decreased from 628 ± 206 kPa at baseline to 176 ± 152 kPa at 16 h and 188 ± 120 kPa at 24 h.

INTERPRETATION

Injection of collagenase into cadaveric Achilles tendons resulted in decreases in elasticity. Decreases were observed in tendons that received injections with both 10 and 20 mg/mL collagenase dosages. Further biomechanical and histological testing is needed to evaluate this cadaveric tendinopathy.

Ex Vivo Biomechanical Assessment of Various Repair Techniques in a Rabbit Calcaneal Tendon Avulsion Model: Application of Polycaprolactone Plate

This study was aimed at evaluating the biomechanical properties and gapping characteristics of tendon repair methods using a combination of a three-loop pulley (3LP) pattern, a titanium plate, and a polycaprolactone (PCL) plate in a rabbit gastrocnemius tendon (GT) model (n = 50). GTs were randomly assigned to five groups (n = 10/group). Transected GTs were repaired with a 3LP pattern alone or in conjunction with an epitendinous suture (ES), a 5-hole 1 mm PCL plate, a 5-hole 2 mm PCL plate, or a 5-hole 1.5 mm titanium plate. The yield, peak, and failure force, as well as the occurrence and force of 1-mm and 3-mm gapping were examined. The mean yield, peak, and failure force of the 3LP + titanium plate group were higher than that of other groups. The biomechanical properties of a 3LP + a 2 mm PCL plate were similar to 3LP + ES constructs in this model. In all specimens in all groups, 1 mm gap formation was observed. The frequency of 3 mm gap formation was 70% and 90% in the 3LP + 2 mm PCL plate group and the 3LP + 1.5 mm titanium plate group, respectively. Additional studies evaluating PCL plates to determine the effect on the healing and blood supply of tendon are needed.

Allogenic Synovia-Derived Mesenchymal Stem Cells for Treatment of Equine Tendinopathies and Desmopathies-Proof of Concept

Tendon and ligament injuries are frequent in sport horses and humans, and such injuries represent a significant therapeutic challenge. Tissue regeneration and function recovery are the paramount goals of tendon and ligament lesion management. Nowadays, several regenerative treatments are being developed, based on the use of stem cell and stem cell-based therapies. In the present study, the preparation of equine synovial membrane mesenchymal stem cells (eSM-MSCs) is described for clinical use, collection, transport, isolation, differentiation, characterization, and application. These cells are fibroblast-like and grow in clusters. They retain osteogenic, chondrogenic, and adipogenic differentiation potential. We present 16 clinical cases of tendonitis and desmitis, treated with allogenic eSM-MSCs and autologous serum, and we also include their evaluation, treatment, and follow-up. The concerns associated with the use of autologous serum as a vehicle are related to a reduced immunogenic response after the administration of this therapeutic combination, as well as the pro-regenerative effects from the growth factors and immunoglobulins that are part of its constitution. Most of the cases (14/16) healed in 30 days and presented good outcomes. Treatment of tendon and ligament lesions with a mixture of eSM-MSCs and autologous serum appears to be a promising clinical option for this category of lesions in equine patients.

Large animal models for the study of tendinopathy

Tendinopathy has a high incidence in athletes and the aging population. It can cause pain and movement disorders, and is one of the most difficult problems in orthopedics. Animal models of tendinopathy provide potentially efficient and effective means to develop understanding of human tendinopathy and its underlying pathological mechanisms and treatments. The selection of preclinical models is essential to ensure the successful translation of effective and innovative treatments into clinical practice. Large animals can be used in both micro- and macro-level research owing to their similarity to humans in size, structure, and function. This article reviews the application of large animal models in tendinopathy regarding injuries to four tendons: rotator cuff, patellar ligament, Achilles tendon, and flexor tendon. The advantages and disadvantages of studying tendinopathy with large animal models are summarized. It is hoped that, with further development of animal models of tendinopathy, new strategies for the prevention and treatment of tendinopathy in humans will be developed.

Quantitative analysis of tendon histopathology using digital pathology in rat models with Achilles tendon injury

Although digital image analysis methods that quantify histopathologic features have emerged, no validated quantitative methods are available to evaluate tendon injury. This study aimed to propose and validate a quantitative analysis method to identify the histopathologic features of tendon injuries. The histopathologic features of two Achilles tendon injury models (a partial full-thickness defect model and a collagenase injection model) using Sprague-Dawley rats were evaluated by semiquantitative grading and a quantitative analysis method using a digital pathology software at weeks 1 and 4 after tendon injury (six tendons per group at each time point). The outcome variables between tendon injury models and between time points were compared, and the correlation between semiquantitative scores and the results of the quantitative analysis was investigated. The proposed analysis method quantified the severity of the histopathological features after tendon injury. Quantitative analysis differentiated the cell morphology between tendon injury models and time points better than semiquantitative scoring. The results from quantitative measurements correlated significantly with the semiquantitative scores. The proposed quantitative method can be effective in evaluating the histopathology of tendon injuries.

Comparative Effects of Exosomes and Ectosomes Isolated From Adipose-Derived Mesenchymal Stem Cells on Achilles Tendinopathy in a Rat Model

BACKGROUND

Extracellular vesicles derived from mesenchymal stem cells (MSC-EVs) have gained momentum as a treatment for tendinopathy. Multiple studies have demonstrated significant differences in cargo composition between the 2 subtypes of MSC-EVs (ie, exosomes and ectosomes), which may result in different therapeutic effects. However, the effects of the 2 EV subtypes on tendinopathy have not yet been compared.

PURPOSE

To compare the effects of adipose stem cell-derived exosomes (ASC-Exos) and ectosomes (ASC-Ectos) on Achilles tendinopathy.

STUDY DESIGN

Controlled laboratory study.

METHODS

Rats were administered collagenase injections to generate a model of Achilles tendinopathy. A week later, 36 rats were randomly assigned to 3 groups. In each group, Achilles tendons were injected with equal volumes of ASC-Exos, ASC-Ectos, or saline (12 legs/group). The healing outcomes were evaluated by magnetic resonance imaging, histology, immunohistochemistry, transmission electron microscopy, and biomechanical testing at 3 and 5 weeks after collagenase injection.

RESULTS

At 3 and 5 weeks, the ASC-Exo group had better histological scores (P = .0036 and P = .0276, respectively), a lower fibril density (P < .0001 and P = .0310, respectively), and a larger collagen diameter (P = .0052 and P < .0001, respectively) than the ASC-Ecto group. At 5 weeks, the expression of collagen type 1 and CD206 in the ASC-Exo group was significantly higher than that in the ASC-Ecto group (P = .0025 and P = .0010, respectively). Regarding biomechanical testing, the ASC-Exo group showed higher failure load (P = .0005), tensile stress (P < .0001), and elastic modulus (P < .0001) than the ASC-Ecto group.

CONCLUSION

ASC-Exos had more beneficial effects on tendon repair than ASC-Ectos in a rat model of Achilles tendinopathy.

CLINICAL RELEVANCE

Administration of ASC-EVs may have the potential to treat Achilles tendinopathy, and delivery of ASC-Exos could provide additional benefits. It is necessary to compare the healing responses caused by different EV subtypes to further understand their effects on tendinopathy and to aid clinical decision making.

mRNA and long non-coding RNA expression profiles of rotator cuff tear patients reveal inflammatory features in long head of biceps tendon

Background

This study aimed to identify the differentially expressed mRNAs and lncRNAs in inflammatory long head of biceps tendon (LHBT) of rotator cuff tear (RCT) patients and further explore the function and potential targets of differentially expressed lncRNAs in biceps tendon pathology.

Methods

Human gene expression microarray was made between 3 inflammatory LHBT samples and 3 normal LHBT samples from RCT patients. GO analysis and KEGG pathway analysis were performed to annotate the function of differentially expressed mRNAs. The real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR) was admitted to verify their expression. LncRNA-mRNA co-expression network, cis-acting element, trans-acting element and transcription factor (TF) regulation analysis were constructed to predict the potential molecular regulatory mechanisms and targets for LHB tendinitis.

Results

103 differentially expressed lncRNAs and mRNAs, of which 75 were up-regulated and 28 were down-regulated, were detected to be differentially expressed in LHBT. The expressions of 4 most differentially expressed lncRNAs (A2MP1, LOC100996671, COL6A4P, lnc-LRCH1-5) were confirmed by qRT-PCR. GO functional analysis indicated that related lncRNAs and mRNAs were involved in the biological processes of regulation of innate immune response, neutrophil chemotaxis, interleukin-1 cell response and others. KEGG pathway analysis indicated that related lncRNAs and mRNAs were involved in MAPK signaling pathway, NF-kappa B signaling pathway, cAMP signaling pathway and others. TF regulation analysis revealed that COL6A4P2, A2MP1 and LOC100996671 target NFKB2.

Conclusions

LlncRNA-COL6A4P2, A2MP1 and LOC100996671 may regulate the inflammation of LHBT in RCT patients through NFKB2/NF-kappa B signaling pathway, and preliminarily revealed the pathological molecular mechanism of tendinitis of LHBT.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12920-022-01292-y.

Inflammatory mechanisms linking obesity and tendinopathy

Chronic tendinopathy is a debilitating tendon disorder with disappointing treatment outcomes. This review focuses on the potential roles of chronic low-grade inflammation in promoting tendinopathy in obesity. A systematic literature search was performed to identify all clinical studies supporting the actions of obesity-associated inflammatory mediators in the development of tendinopathy. The mechanisms of obesity-induced chronic inflammation in adipose tissue are firstly reviewed. Common inflammatory mediators potentially linking obesity and the development of tendinopathy, and their association with mechanical overuse, are discussed, along with pre-clinical evidences and a systematic literature search on clinical studies. The potential contribution of local adipose tissues in the promotion of inflammation, pain and tendon degeneration is then discussed. The future research directions are proposed.

Translational potential statement

Better understanding of the roles of obesity-associated inflammatory mediators on tendons will clarify the pathophysiological drivers of tendinopathy in patients with obesity and identify possible treatment targets. Further studies on the mechanisms of obesity-induced chronic inflammation on tendon are a promising direction for the treatment of tendinopathy.

CTRP3 exacerbates tendinopathy by dysregulating tendon stem cell differentiation and altering extracellular matrix composition

Tendinopathy, the most common disorder affecting tendons, is characterized by chronic disorganization of the tendon matrix, which leads to tendon tear and rupture. The goal was to identify a rational molecular target whose blockade can serve as a potential therapeutic intervention for tendinopathy. We identified C1q/TNF-related protein-3 (CTRP3) as a markedly up-regulated cytokine in human and rodent tendinopathy. Overexpression of CTRP3 enhanced the progression of tendinopathy by accumulating cartilaginous proteoglycans and degenerating collagenous fibers in the mouse tendon, whereas CTRP3 knockdown suppressed the tendinopathy pathogenesis. Functional blockade of CTRP3 using a neutralizing antibody ameliorated overuse-induced tendinopathy of the Achilles and rotator cuff tendons. Mechanistically, CTRP3 elicited a transcriptomic pattern that stimulates abnormal differentiation of tendon stem/progenitor cells and ectopic chondrification as an effect linked to activation of Akt signaling. Collectively, we reveal an essential role for CTRP3 in tendinopathy and propose a potential therapeutic strategy for the treatment of tendinopathy.

Screening of key biomarkers of tendinopathy based on bioinformatics and machine learning algorithms

Tendinopathy is a complex multifaceted tendinopathy often associated with overuse and with its high prevalence resulting in significant health care costs. At present, the pathogenesis and effective treatment of tendinopathy are still not sufficiently elucidated. The purpose of this research is to intensely explore the genes, functional pathways, and immune infiltration characteristics of the occurrence and development of tendinopathy. The gene expression profile of GSE106292, GSE26051 and GSE167226 are downloaded from GEO (NCBI comprehensive gene expression database) and analyzed by WGCNA software bag using R software, GSE26051, GSE167226 data set is combined to screen the differential gene analysis. We subsequently performed gene enrichment analysis of Gene Ontology (GO) and "Kyoto Encyclopedia of Genes and Genomes" (KEGG), and immune cell infiltration analysis. By constructing the LASSO regression model, Support vector machine (SVM-REF) and Gaussian mixture model (GMMs) algorithms are used to screen, to identify early diagnostic genes. We have obtained a total of 171 DEGs through WGCNA analysis and differentially expressed genes (DEGs) screening. By GO and KEGG enrichment analysis, it is found that these dysregulated genes were related to mTOR, HIF-1, MAPK, NF-κB and VEGF signaling pathways. Immune infiltration analysis showed that M1 macrophages, activated mast cells and activated NK cells had infiltration significance. After analysis of THE LASSO SVM-REF and GMMs algorithms, we found that the gene MACROD1 may be a gene for early diagnosis. We identified the potential of tendon disease early diagnosis way and immune gene regulation MACROD1 key infiltration characteristics based on comprehensive bioinformatics analysis. These hub genes and functional pathways may as early biomarkers of tendon injuries and molecular therapy level target is used to guide drug and basic research.

Early-Stage Primary Anti-inflammatory Therapy Enhances the Regenerative Efficacy of Platelet-Rich Plasma in a Rabbit Achilles Tendinopathy Model

BACKGROUND

Tendinopathy is a pervasive clinical problem that afflicts both athletes and the general public. Although the inflammatory changes in tendinopathy are well characterized, how the therapeutic effects of platelet-rich plasma (PRP) on tendinopathy are being modulated by the inflammatory environment is not well defined.

PURPOSE/HYPOTHESIS

In this study, we aimed to compare the therapeutic effects of PRP alone versus a combination of PRP with a primary glucocorticoid (GC) injection at the early stage of tendinopathy. We hypothesized that PRP treatment could promote better tendon regeneration through the suppression of inflammation with GC.

STUDY DESIGN

Controlled laboratory study.

METHODS

The gene expression profile of tendon stem/progenitor cells (TSPCs) cultured with PRP was analyzed with RNA sequencing. To evaluate the cell viability, senescence, and apoptosis of TSPCs under different conditions, TSPCs were treated with 0.1 mg/mL triamcinolone acetonide (TA) and/or 10% PRP in an IL1B-induced inflammatory environment. To further verify the effects of the sequential therapy of GCs and PRP, an early tendinopathy animal model was established through a local injection of collagenase in the rabbit Achilles tendon. The tendinopathy model was then treated with isopycnic normal saline (NS group), TA (TA group), PRP (PRP group), or TA and PRP successively (TA+PRP group). At 8 weeks after treatment, the tendons were assessed with magnetic resonance imaging (MRI), histological examination, transmission electron microscopy (TEM), and mechanical testing.

RESULTS

Gene Ontology enrichment analysis indicated that PRP treatment of TPSCs induced an inflammatory response, regulated cell migration, and remodeled the extracellular matrix. Compared with the sole use of PRP, successive treatment with TA followed by PRP yielded similar results in cell viability and senescence but less cell apoptosis in vitro. In vivo experiments demonstrated that the TA+PRP group achieved significantly better tendon regeneration, as confirmed by MRI, histological examination, TEM, and mechanical testing.

CONCLUSION

This study showed that the primary use of GCs did not exert any obvious deleterious side effects on the treated tendon but instead enhanced the regenerative effects of PRP in early inflammatory tendinopathy.

CLINICAL RELEVANCE

The sequential therapy of GCs followed by PRP provides a promising treatment strategy for tendinopathy in clinical practice. PRP combined with the primary use of GCs appears to promote tendon regeneration in early inflammatory tendinopathy.

Autoregulation in Resistance Training for Lower Limb Tendinopathy: A Potential Method for Addressing Individual Factors, Intervention Issues, and Inadequate Outcomes

Musculoskeletal disorders, such as tendinopathy, are placing an increasing burden on society and health systems. Tendinopathy accounts for up to 30% of musculoskeletal disorders, with a high incidence in athletes and the general population. Although resistance training has shown short-term effectiveness in the treatment of lower limb tendinopathy, more comprehensive exercise protocols and progression methods are required due to poor long-term outcomes. The most common resistance training protocols are predetermined and standardized, which presents significant limitations. Current standardized protocols do not adhere to scientific resistance training principles, consider individual factors, or take the importance of individualized training into account. Resistance training programs in case of tendinopathy are currently not achieving the required intensity and dosage, leading to high recurrence rates. Therefore, better methods for individualizing and progressing resistance training are required to improve outcomes. One potential method is autoregulation, which allows individuals to progress training at their own rate, taking individual factors into account. Despite the finding of their effectiveness in increasing the strength of healthy athletes, autoregulation methods have not been investigated in case of tendinopathy. The purpose of this narrative review was 3-fold: firstly, to give an overview and a critical analysis of the individual factors involved in tendinopathy and current resistance training protocols and their limitations. Secondly, to give an overview of the history, methods, and application of autoregulation strategies both in sports performance and physiotherapy. Finally, a theoretical adaptation of a current tendinopathy resistance training protocol using autoregulation methods is presented, providing an example of how the method could be implemented in clinical practice or future research.

"One Health" in tendinopathy research: Current concepts

Tendinopathy remains one of the most common musculoskeletal disorders affecting both human and equine athletes and presents a considerable therapeutic challenge. The following workshop report comes from the third Dorothy Havemeyer Symposium of Tendinopathy which provided a unique overview of our current understanding of both the basic science and the clinical challenges for diagnosing and treating tendinopathy in both species. Pathologically, tendon demonstrates alterations in both cellular, molecular, structural, and biomechanical features, leading to a spectrum of pathological endotypes. To develop novel interventions to manage, treat or prevent tendinopathies it is vital to understand the underlying mechanisms that lead to both tendon failure, and also regeneration and resolution of inflammation. The horse shows analogous pathology with both human Achilles tendinopathy (superficial digital flexor tendon) and intrathecal rotator cuff tears (deep digital flexor tendon tears) enabling scientists and clinicians from both medical and veterinary fields to work jointly on matching naturally occurring disease models. The experience in human medicine on the design, conduct, and impact of clinical trials has much to inform clinical trials in horses. There is a need to design appropriate studies to address clear questions, socialize the study to achieve good enrollment, and consider the significance and impact of the clinical question as well as the cost of addressing it. Because economics is often a limitation in equine medicine the use of observational studies, and specifically registries, should be given careful consideration.

Effect of Exercise Intensity on the Healing of the Bone-Tendon Interface: A Mouse Rotator Cuff Injury Model Study

BACKGROUND

Injuries at the bone-tendon interface (BTI) are common findings in clinical practice. Rehabilitation procedures after BTI surgery are important but are controversial.

PURPOSE

To investigate the effects of different exercise intensities on BTI healing by means of an established mouse rotator cuff injury model.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 150 specific pathogen free male C57BL/6 mice, with supraspinatus insertion injury, were randomly assigned to 1 of 5 groups according to postoperative rehabilitation of different exercise intensities: (1) control group, (2) low-intensity exercise group, (3) moderate-intensity exercise group, (4) high-intensity exercise group, and (5) increasing-intensity exercise group (IG). The specimens were harvested 4 or 8 weeks postoperatively for microarchitectural, histological, molecular biological, and mechanical evaluations.

RESULTS

Histological test results showed that the degrees of tissue fusion and polysaccharide protein distribution at the healing interface at 4 and 8 weeks after surgery were significantly better in the IG than in the other 4 groups. Synchrotron radiation micro-computed tomography showed that the quantity of subchondral bone at the enthesis (bone volume/total volume fraction, trabecular thickness, trabecular number) was higher and trabecular separation was lower in the IG than in the other 4 groups. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis revealed that the healing interface in the IG expressed more transcription factors, such as sox 9, runx 2, and scleraxis, than the interfaces in the other groups. Although no significant difference was seen in the cross-sectional area between the groups at postoperative weeks 4 and 8 (P > .05), the tensile load, ultimate strength, and stiffness of the specimens in the IG were significantly better than those in the other 4 groups (P < .05).

CONCLUSION

The rehabilitation program with increasing-intensity exercise was beneficial for BTI healing.

CLINICAL RELEVANCE

The results of this study provide evidence supporting the use of a simple and progressive exercise rehabilitation program after rotator cuff surgery.

Species variations in tenocytes' response to inflammation require careful selection of animal models for tendon research

For research on tendon injury, many different animal models are utilized; however, the extent to which these species simulate the clinical condition and disease pathophysiology has not yet been critically evaluated. Considering the importance of inflammation in tendon disease, this study compared the cellular and molecular features of inflammation in tenocytes of humans and four common model species (mouse, rat, sheep, and horse). While mouse and rat tenocytes most closely equalled human tenocytes’ low proliferation capacity and the negligible effect of inflammation on proliferation, the wound closure speed of humans was best approximated by rats and horses. The overall gene expression of human tenocytes was most similar to mice under healthy, to horses under transient and to sheep under constant inflammatory conditions. Humans were best matched by mice and horses in their tendon marker and collagen expression, by horses in extracellular matrix remodelling genes, and by rats in inflammatory mediators. As no single animal model perfectly replicates the clinical condition and sufficiently emulates human tenocytes, fit-for-purpose selection of the model species for each specific research question and combination of data from multiple species will be essential to optimize translational predictive validity.

Bringing tendon biology to heel: Leveraging mechanisms of tendon development, healing, and regeneration to advance therapeutic strategies

Tendons are specialized matrix-rich connective tissues that transmit forces from muscle to bone and are essential for movement. As tissues that frequently transfer large mechanical loads, tendons are commonly injured in patients of all ages. Following injury, mammalian tendons heal poorly through a slow process that forms disorganized fibrotic scar tissue with inferior biomechanical function. Current treatments are limited and patients can be left with a weaker tendon that is likely to rerupture and an increased chance of developing degenerative conditions. More effective, alternative treatments are needed. However, our current understanding of tendon biology remains limited. Here, we emphasize why expanding our knowledge of tendon development, healing, and regeneration is imperative for advancing tendon regenerative medicine. We provide a comprehensive review of the current mechanisms governing tendon development and healing and further highlight recent work in regenerative tendon models including the neonatal mouse and zebrafish. Importantly, we discuss how present and future discoveries can be applied to both augment current treatments and design novel strategies to treat tendon injuries.

Molecular and Structural Effects of Percutaneous Interventions in Chronic Achilles Tendinopathy

Achilles tendinopathy (AT) is a common problem, especially in people of working age, as well as in the elderly. Although the pathogenesis of tendinopathy is better known, therapeutic management of AT remains challenging. Various percutaneous treatments have been applied to tendon lesions: e.g., injectable treatments, platelet-rich plasma (PRP), corticosteroids, stem cells, MMP inhibitors, and anti-angiogenic agents), as well as percutaneous procedures without any injection (percutaneous soft tissue release and dry needling). In this review, we will describe and comment on data about the molecular and structural effects of these treatments obtained in vitro and in vivo and report their efficacy in clinical trials. Local treatments have some impact on neovascularization, inflammation or tissue remodeling in animal models, but evidence from clinical trials remains too weak to establish an accurate management plan, and further studies will be necessary to evaluate their value.

One health in regenerative medicine: report on the second Havemeyer symposium on regenerative medicine in horses

Regenerative medicine is commonly used in human and equine athletes. Potential therapies include culture expanded stem cells, stromal vascular fraction of adipose tissue, platelet-rich plasma, bone marrow concentrate, or autologous conditioned serum. The purpose of this manuscript is to disseminate findings from a workshop on the development of translational regenerative medicine in the equine field. Five themes emerged: stem cell characterization and tenogenic differentiation; interactions between mesenchymal stem cells, other cells and the environment; scaffolds and cell packaging; blood- and bone marrow-based regenerative medicines; clinical use of regenerative therapies. Evidence gained through the use of regenerative medicine applications in the horse should continue to translate to the human patient, bringing novel regenerative therapies to both humans and horses.

Sequential inflammation model for Achilles tendinopathy by elastin degradation with treadmill exercise

Background & objective

Tendinopathy is a tendon disease with abnormal mechanical loading to induce chronic repetitive injury. However, lack of a comparable animal model to demonstrate clinical progressions has hindered the understanding of anatomical and pathological changes. The major extracellular matrix (ECM) in the tendon consists of abundant type I collagen (COL) and minimal amount of elastin (ELN).

Methods

To study the ECM breakdown and inflammation, rat Achilles tendon was harvested and ex vivo incubated with specific enzymes of elastase (ELNase) or collagenase (COLase).

Results

The ELNase broke down ELN, loosened the tendon structure, and increased the COL composition. Increases in cyclooxygenase-2 expression levels in tenocytes were revealed to induce inflammation with either ELNase or COLase. However, incubation of COLase for 12 hours severely digested the tendon. To create a proper ELN degradation in rats, the present study used high-frequency ultrasound to guide the injection of ELNase at the paratendon tissue of the Achilles tendon. The effect of mechanically triggered inflammatory responses was investigated by applying treadmill exercise (15 m/min for 20 min per day). After ELNase injection for 14 and 28 days, a significant loss of ELN was observed, and exercise further facilitated the pathological transition of COL. The dynamics of inflammatory cell recruitments was revealed by specific staining of CD-11b (neutrophils) and CD-68 (macrophage) after in vivo injection of ELNase or COLase for 1, 3, 7, 14, and 28 days. The combination of ELNase and exercise caused early recruitment of neutrophil on day 1 and sequential expression of macrophage on day 7 in peritendinous tissue.

Conclusion

These results suggested that ELN degradation with repetitive mechanical loading may present a suitable model for the pathogenesis of tendinopathy.

The Translational potential of this article

This discover the role of elastin degradation in tendinopathy and the interaction of exercise in the histological changes. The established the pathological model mimicking the pathogenesis to the human disease by injecting the elastase using ultrasound guidance and then applying treadmill exercise. The loss of elastin and change of collagen composition in clinical tendinopathy samples were observed in the rats. In addition, the sequential inflammation cascades were observed in the histological outcomes with combination of elastase injection and treadmill exercise. Thus, this model may be used to test the clinical treatment of tendinopathy in different stages.

Development of a Cartilage Oligomeric Matrix Protein Neo-Epitope Assay for the Detection of Intra-Thecal Tendon Disease

The diagnosis of tendon injury relies on clinical signs and diagnostic imaging but imaging is subjective and does not always correlate with clinical signs. A molecular marker would potentially offer a sensitive and specific diagnostic tool that could also provide objective assessment of healing for the comparison of different treatments. Cartilage Oligomeric Matrix Protein (COMP) has been used as a molecular marker for osteoarthritis in humans and horses but assays for the protein in tendon sheath synovial fluids have shown overlap between horses affected by tendinopathy and controls. We hypothesized that quantifying a COMP neoepitope would be more discriminatory of injury. COMP fragments were purified from synovial fluids of horses with intra-thecal tendon injuries and media from equine tendon explants, and mass spectrometry of a consistent and abundant fragment revealed a ~100 kDa COMP fragment with a new N-terminus at the 78th amino-acid (NH₂-TPRVSVRP) located just outside the junctional region of the protein. A competitive inhibition ELISA based on a polyclonal antibody raised to this sequence yielded more than a 10-fold rise in the mean neoepitope levels for tendinopathy cases compared to controls (5.3 ± 1.3 µg/mL (n = 7) versus 58.8 ± 64.3 µg/mL (n = 13); p = 0.002). However, there was some cross-reactivity of the neoepitope polyclonal antiserum with intact COMP, which could be blocked by a peptide spanning the neoepitope. The modified assay demonstrated a lower concentration but a significant > 500-fold average rise with tendon injury (2.5 ± 2.2 ng/mL (n = 6) versus 1029.8 ± 2188.8 ng/ml (n = 14); p = 0.013). This neo-epitope assay therefore offers a potentially useful marker for clinical use.

Inhibition of CD44 induces apoptosis, inflammation, and matrix metalloproteinase expression in tendinopathy

Apoptosis has emerged as a primary cause of tendinopathy. CD44 signaling pathways exert anti-apoptotic and -inflammatory effects on tumor cells, chondrocytes, and fibroblast-like synoviocytes. The aim of this study was to examine the association among CD44, apoptosis, and inflammation in tendinopathy. Expression of CD44 and apoptotic cell numbers in tendon tissue from patients with long head of biceps (LHB) tendinopathy were determined according to the histological grades of tendinopathy. Primary tenocytes from Achilles tendon of Sprague-Dawley rats 1 week after collagenase injection were cultured with an antagonizing antibody against CD44. Treatment responses were determined by evaluating cell viability and expression of tendon-related proliferation markers, inflammatory mediators, and apoptosis. The expression of CD44 and apoptosis were positively correlated with the severity of tendinopathy in the human LHB tendinopathy. Furthermore, CD44 expression and apoptotic cells were co-stained in tendinopathic tendon. Blocking the CD44 signaling pathways in rat primary tenocytes by OX-50 induced cell apoptosis and the elevated levels of cleaved caspase-3. Furthermore, they had decreased cell viability and expression of collagen type I, type III, tenomodulin, and phosphorylated AKT. In contrast, there were elevated levels of inflammatory mediators, including interleukin (IL)-1β, IL-6, tumor necrosis factor-α, cyclooxygenase-2, and phosphorylated NF-κB, as well as matrix metalloproteinase (MMP) family members including MMP-1, -3, -9, and -13 in tenocytes upon OX-50 treatment. This study is the first to demonstrate the association of CD44 and apoptosis in tendinopathy. Our data imply that CD44 may play a role in tendinopathy via regulating apoptosis, inflammation, and extracellular matrix homeostasis.

Treatment with Human Amniotic Suspension Allograft Improves Tendon Healing in a Rat Model of Collagenase-Induced Tendinopathy

Treatment of tendon injuries is challenging, with neither conservative nor surgical approaches providing full recovery. Placental-derived tissues represent a promising tool for the treatment of tendon injuries. In this study, human amniotic suspension allograft (ASA) was investigated in a pre-clinical model of Achilles tendinopathy. Collagenase type I was injected in the right hind limb of Sprague Dawley rats to induce disease. Contralateral tendons were either left untreated or injected with saline as controls. Seven days following induction, tendons were injected with saline, ASA, or left untreated. Rats were sacrificed 14 and 28 days post-treatment. Histological and biomechanical analysis of tendons was completed. Fourteen days after ASA injection, improved fiber alignment and reduced cell density demonstrated improvement in degenerated tendons. Twenty-eight days post-treatment, tendons in all treatment groups showed fewer signs of degeneration, which is consistent with normal tendon healing. No statistically significant differences in histological or biomechanical analyses were observed between treatment groups at 28 days independent of the treatment they received. In this study, ASA treatment was safe, well-tolerated, and resulted in a widespread improvement of the tissue. The results of this study provide preliminary insights regarding the potential use of ASA for the treatment of Achilles tendinopathy.

Aspirin inhibits adipogenesis of tendon stem cells and lipids accumulation in rat injury tendon through regulating PTEN/PI3K/AKT signalling

Tendon injury repairs are big challenges in sports medicine, and fatty infiltration after tendon injury is very common and hampers tendon injury healing process. Tendon stem cells (TSCs), as precursors of tendon cells, have shown promising effect on injury tendon repair for their tenogenesis and tendon extracellular matrix formation. Adipocytes and lipids accumulation is a landmark event in pathological process of tendon injury, and this may induce tendon rupture in clinical practice. Based on this, it is important to inhibit TSCs adipogenesis and lipids infiltration to restore structure and function of injury tendon. Aspirin, as the representative of non‐steroidal anti‐inflammatory drugs (NSAIDs), has been widely used in tendon injury for its anti‐inflammatory and analgesic actions, but effect of aspirin on TSCs adipogenesis and fatty infiltration is still unclear. Under adipogenesis conditions, TSCs were treated with concentration gradient of aspirin. Oil red O staining was performed to observe changes of lipids accumulation. Next, we used RNA sequencing to compare profile changes of gene expression between induction group and aspirin‐treated group. Then, we verified the effect of filtrated signalling on TSCs adipogenesis. At last, we established rat tendon injury model and compared changes of biomechanical properties after aspirin treatment. The results showed that aspirin decreased lipids accumulation in injury tendon and inhibited TSCs adipogenesis. RNA sequencing filtrated PTEN/PI3K/AKT signalling as our target. After adding the signalling activators of VO‐Ohpic and IGF‐1, inhibited adipogenesis of TSCs was reversed. Still, aspirin promoted maximum loading, ultimate stress and breaking elongation of injury tendon. In conclusion, by down‐regulating PTEN/PI3K/AKT signalling, aspirin inhibited adipogenesis of TSCs and fatty infiltration in injury tendon, promoted biomechanical properties and decreased rupture risk of injury tendon. All these provided new therapeutic potential and medicine evidence of aspirin in treating tendon injury and tendinopathy.

Efficacy of Combining PRP and MMP Inhibitors in Treating Moderately Damaged Tendons Ex Vivo

Platelet-rich plasma (PRP) and broad-spectrum matrix metalloproteinase inhibitors (MMPIs) have been used as therapeutic options for tendinopathy. However, mixed results have been reported regarding their efficacy. We posited that the combination of these two treatment strategies would be more beneficial for healing tendons than each treatment alone. Rat tail tendons were harvested and cultured without mechanical stress for 0, 4, or 10 days. Single and combination treatment with PRP and MMPIs with either broad- or narrow-spectrum (MMP-13 selective), was administered to 4-day stress-deprived (SD) tendons, an ex vivo model for moderate tendinopathy. This treatment was applied to the damaged tendons over 6 days. At the end of their culture time, the tendons were subjected to traction testing and pathohistology, immunohistochemistry, and viability assays. The results showed better histological features for the PRP + narrow-spectrum MMPI group compared with all individual treatment modalities. Moreover, higher fiber density, more elongated nucleus shape, smaller space between fibers, and a trend toward higher mechanical strength were noted for PRP + narrow-spectrum MMPI group compared with 10-day SD tendons. This study shows that the combination of PRP + narrow-spectrum MMPI is a potentially effective treatment approach for tendinopathy. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1838-1847, 2019.

Anatomy and histology of the rabbit common calcanean tendon

Despite the great progress in the field of tendon injuries and chronic tendinopathies in recent years, treatment of these conditions is still challenging. Research utilising animal models is crucial for further advancement in tendon research, with the rabbit being a commonly used species in this field. The objective of the present study was to comprehensively describe the macro- and microanatomy of the common calcanean tendon (tendo Achillei), together with associated structures. Eight female New Zealand rabbits were subjected to anatomical dissection and histologic analysis, revealing significant species-specific features. The soleus muscle presented a thin, spindle shape with no tendon of insertion and attached directly to the lateral gastrocnemius muscle. Therefore, it does not contribute to the formation of the common calcanean tendon. The calcaneal tendon of the semitendinosus muscle was identified as a strong tendinous band at the medial side of the medial gastrocnemius muscle and the superficial digital flexor muscle and tendon distally. The saphenous artery was accompanied by the medial saphenous vein and characteristic accessory medial saphenous vein. The complex structure of the paratendinous connective tissue is described, with the paratenon being contiguous with the epitenon of the individual tendons and epimysium of associated muscles. At the level of the calcaneal tuber, the retromalleolar fat pad was identified, adhering to the cranial surface of the conjoint gastrocnemius tendon. Histologic studies confirmed the presence of the subtendinous calcaneal bursa of superficial digital flexor tendon and the bursa of calcaneal tendon.

In vitro tissue culture model validation-the influence of tissue culture components on IPL energy output

Intense pulsed light (IPL) has been used therapeutically in a number of clinical settings and has been shown to have a photobiomodulatory effect on connective tissue cells, such as those derived from skin and tendon. In vitro cell culture models are essential tools preclinically in investigating such treatment modalities, as they help in optimising parameters for successful treatment. However, as culture system components have been reported to absorb part of the irradiated energy, which in turn has a bearing on the amount of light reaching the cells, it is important to establish specific parameters for the particular in vitro model used. This study, therefore, investigates the effect of our tissue culture system components on the IPL energy delivered. Individual wells of multi-well plates were irradiated with IPL at different device settings and under variable culture conditions (e.g. in the absence or presence of cell culture media with or without the pH indicator dye, phenol red), and the energy lost through the culture system determined. Our data demonstrated that the IPL device delivered significantly lower outputs than those published, and energy absorption by the culture equipment would further reduce fluencies delivered to the cell monolayer. Furthermore, energy absorption by media containing phenol red was marginally greater than clear media and resulted in only a small increase in temperature, which would not be harmful to cells. The use of phenol red-containing media therefore is valid and physiologically relevant when examining light-culture system interactions.

Strategies of tenogenic differentiation of equine stem cells for tendon repair: current status and challenges

Tendon injuries, as one of the most common orthopedic disorders, are the major cause of early retirement or wastage among sport horses which mainly affect the superficial digital flexor tendon (SDFT). Tendon repair is a slow process, and tendon tissue is often replaced by scar tissue. The current treatment options are often followed by an incomplete recovery that increases the susceptibility to re-injury. Recently, cell therapy has been used in veterinary medicine to treat tendon injuries, although the risk of ectopic bone formation after cell injection is possible in some cases. In vitro tenogenic induction may overcome the mentioned risk in clinical application. Moreover, a better understanding of treatment strategies for musculoskeletal injuries in horse may have future applications for human and vice versa. This comprehensive review outlines the current strategies of stem cell therapy in equine tendon injury and in vitro tenogenic induction of equine stem cell.

Prevalence and incidence rate of lower-extremity tendinopathies in a Danish general practice: a registry-based study

Background

Tendinopathies of the lower extremity (e.g. Achilles, patellar, and plantar heel pain) are common in both general and sporting populations. However, the prevalence and incidence in Danish general practice is unknown. The aim was to determine the prevalence and incidence rate of lower-extremity tendinopathies in a Danish general practice.

Methods

In this registry-based study, we extracted data from the electronic patient files of all patients in a single Danish general practice. The practice included 8836 patients. We searched ICPC-2 codes to identify patients with either of the following lower-extremity tendinopathies: plantar heel pain; Achilles tendinopathy; patellar tendinopathy; greater trochanteric pain syndrome or adductor tendinopathy. We defined an incident and prevalent case as a patient with a consultation because of tendinopathy in 2016 only. A prevalent, but not incident case was a patient with consultations in both 2015 and 2016. Incidence and prevalence were expressed as the number of patients with a tendinopathy per 1000 registered patients.

Results

The prevalence and incidence rate were 16.6 and 7.9 per 1000 registered patients, respectively. Plantar heel pain was the most prevalent tendinopathy and accounted for 39% of lower-extremity tendinopathies. Patients with tendinopathies were significantly older than all registered patients (46.0 years (95%CI: 43.3;48.7) versus 38.8 years (95%CI: 38.4;39.3), respectively).

Conclusions

Lower-extremity tendinopathies, especially plantar heel pain, had a high prevalence and incidence rate in a Danish general practice. In a typical general practice with 5000 patients, general practitioners should expect to see more than 80 patients with a lower-extremity tendinopathy every year.

Histopathological study of the infrapatellar fat pad in the rat model of patellar tendinopathy: A basic study

BACKGROUND

Patellar tendinopathy is difficult to successfully treat. This study aimed to characterize the pathological changes of the infrapatellar fat pad (IPFP) in patellar tendinopathy (PT), and to investigate the influence of PT on the development of fibrotic changes in the IPFP.

METHODS

Forty male Wistar rats were randomly divided into PT (n = 20) and control groups (n = 20). Bacterial collagenase I (patellar tendinopathy group) or saline (control) was injected, intratendinous, into the patellar tendon. Rats were sacrificed at week 12. The whole knee joint was sagittally sectioned and stained with hematoxylin-eosin and Masson's trichrome. The IPFP samples were graded according to cellularity, fibrosis, and vascularity. The whole IPFP and blue-stained area was measured. Mann-Whitney U tests were used to compare the between-group differences of each score and quantitative value.

RESULTS

Scores for cellularity were three (2-3) and 0 (0-1) in the PT and control groups, respectively (P < 0.01). Mean scores for fibrosis were two (1-3) and 0 (0-1) in the PT and control groups, respectively (P < 0.01). Mean scores for vascularity were two (2-3) and one (1-1) in the PT and control groups, respectively (P < 0.01). There was a significant difference in the total score between the PT and control groups (seven (5-8) and two (1-3), respectively) (P < 0.01). Average percentages of the fibrous area of the IPFP were 38.2 ± 26.5% and 11.2 ± 3.9% in the patellar tendinopathy and control groups, respectively (P < 0.01).

CONCLUSION

The IPFP in the patellar tendinopathy group showed greater cellularity, fibrosis, and vascularity than the control group.

Hydrogen peroxide induced tendinopathic changes in a rat model of patellar tendon injury

Tendinopathy includes cases with chronic tendon pain and spontaneous tendon ruptures, which is putatively resulted from failed tendon healing. Overuse is a major risk factor of tendinopathy, which can impose mechanical and oxidative stress to tendons. Previous studies investigated the influences of mechanical stress, but the direct impact of oxidative stress on tendon healing remains unclear. We hypothesized that imposed oxidative stress can impair tendon healing and lead to tendinopathic changes. Thirty-nine rats were operated for patellar tendon window injury. From weeks 3-5 post-operation, the rats received three weekly subcutaneous injections of saline, 50 or 500 μM H₂ O₂ (n = 13) over patellar tendon. Gait analysis for pain assessment and 3D ultrasound imaging for detection of tendinopathic changes were performed at pre-injury and 6-week post-operation. At week 6, knee specimens were harvested for histology or tensile mechanical test. Elastic modulus of the healing patellar tendons was significantly lower in 50 μM but not 500 μM H₂ O₂ group, while ultimate mechanical stress was not significantly different across groups. Similarly, only the 50 μM H₂ O₂ group exhibited pain-associated gait asymmetry. Significant tendon swelling with increased tendon volume was observed in the 50 μM H₂ O₂ group. There were hypoechogenic changes in the tendon wound, but there was no significant difference in percentage vascularity. H₂ O₂ impaired tendon healing and elicited tendinopathic changes, with respect to pain and structural abnormalities. Oxidative stress plays a role in the failed tendon healing of tendinopathies, and H₂ O₂ -induced failed tendon healing may serve as a good animal model to study tendinopathy. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:3268-3274, 2018.

Amniotic Epithelial Stem Cells: Salient Features and Possible Therapeutic Role

This is a study of amniotic epithelial cells, which form the innermost layer of the amniotic membrane. These cells can be easily isolated and display peculiar and unique properties, such as plasticity and differentiation potential toward the 3 germinal layers, that may aid regeneration and/or repair of damaged or diseased tissues and organs. A robust literature based on in vitro, experimental, and clinical studies in large animals demonstrates that these cells can enhance the regeneration of tendons, bone, and articular cartilage. On the basis of these considerations, allotransplantation of human amniotic epithelial cells could be proposed for clinical trials in human orthopedic conditions.

Bone marrow mesenchymal stem cells do not enhance intra-synovial tendon healing despite engraftment and homing to niches within the synovium

Background

Intra-synovial tendon injuries display poor healing, which often results in reduced functionality and pain. A lack of effective therapeutic options has led to experimental approaches to augment natural tendon repair with autologous mesenchymal stem cells (MSCs) although the effects of the intra-synovial environment on the distribution, engraftment and functionality of implanted MSCs is not known. This study utilised a novel sheep model which, although in an anatomically different location, more accurately mimics the mechanical and synovial environment of the human rotator cuff, to determine the effects of intra-synovial implantation of MSCs.

Methods

A lesion was made in the lateral border of the lateral branch of the ovine deep digital flexor tendon within the digital sheath and 2 weeks later 5 million autologous bone marrow MSCs were injected under ultrasound guidance into the digital sheath. Tendons were recovered post mortem at 1 day, and 1–2, 4, 12 and 24 weeks after MSC injection. For the 1-day and 1–2-week groups, MSCs labelled with fluorescent-conjugated magnetic iron-oxide nanoparticles (MIONs) were tracked with MRI, histology and flow cytometry. The 4, 12 and 24-week groups were implanted with non-labelled cells and compared with saline-injected controls for healing.

Results

The MSCs displayed no reduced viability in vitro to an uptake of 20.0 ± 4.6 pg MIONs per cell, which was detectable by MRI at minimal density of ~ 3 × 10⁴ cells. Treated limbs indicated cellular distribution throughout the tendon synovial sheath but restricted to the synovial tissues, with no MSCs detected in the tendon or surgical lesion.

The lesion was associated with negligible morbidity with minimal inflammation post surgery. Evaluation of both treated and control lesions showed no evidence of healing of the lesion at 4, 12 and 24 weeks on gross and histological examination.

Conclusions

Unlike other laboratory animal models of tendon injury, this novel model mimics the failed tendon healing seen clinically intra-synovially. Importantly, however, implanted stem cells exhibited homing to synovium niches where they survived for at least 14 days. This phenomenon could be utilised in the development of novel physical or biological approaches to enhance localisation of cells in augmenting intra-synovial tendon repair.

A Protocol to Acquire the Degenerative Tenocyte from Humans

Tendinopathy, a painful condition that develops in response to tendon degeneration, is on the rise in the developed world due to increasing physical activity and longer life expectancy. Despite its increasing prevalence, the underlying pathogenesis still remains unclear, and treatment is generally symptomatic. Recently, numerous therapeutic options, including growth factors, stem cells, and gene therapy, were investigated in hopes of enhancing the healing potency of the degenerative tendon. However, the majority of these research studies were conducted only on animal models or healthy human tenocytes. Despite some studies using pathological tenocytes, to the best of our knowledge there is currently no protocol describing how to obtain human degenerative tenocytes. The aim of this study is to describe a standard protocol for acquiring human degenerative tenocytes. Initially, the tendon tissue was harvested from a patient with lateral epicondylitis during surgery. Then biopsy samples were taken from the extensor carpi radialis brevis tendon corresponding to structural changes observed at the time of surgery. All of the harvested tendons appeared to be dull, gray, friable, and edematous, which made them visually distinct from the healthy ones. Tenocytes were cultured and used for experiments. Meanwhile, half of the harvested tissues were analyzed histologically, and it was shown that they shared the same key features of tendinopathy (angiofibroblastic dysplasia or hyperplasia). A secondary analysis by immunocytochemistry confirmed that the cultured cells were tenocytes with the majority of the cells having positive stains for mohawk and tenomodulin proteins. The qualities of the degenerative nature of tenocytes were then determined by comparing the cells with the healthy control using a proliferation assay or qRT-PCR. The degenerative tenocyte displayed a higher proliferation rate and similar gene expression patterns of tendinopathy that matched previous reports. Overall, this new protocol might provide a useful tool for future studies of tendinopathy.

Randomized controlled trial demonstrates the benefit of RGTA® based matrix therapy to treat tendinopathies in racing horses

A randomized controlled trial was performed on racing horses, to evaluate the efficacy of a new class of therapeutic agents in regenerative medicine—ReGeneraTing Agents^® (RGTA^®), to treat tendinopathies. Preliminary uncontrolled studies on tendon healing in racing horses with RGTA^® (OTR4131)—Equitend^® showed encouraging results, justifying performing a randomized, controlled, multicenter study with a two-year racing performance follow up. The objective of this study was to evaluate the effect of Equitend^® versus placebo on acute superficial digital flexor tendonitis in racing French Standardbred Trotters (ST). Twenty-two ST were randomly and blindly assigned to receive with a ratio of 2 to 1, a single Equitend^® (n = 14) or placebo (n = 8) intralesional injection under ultrasonographic guidance. Horses were evaluated over 4 months, by clinical and ultrasonographic evaluations (day 0, months 1, 2, 4), and their racing performances followed up over the 2 years after treatment. During the first month of treatment, a significant decrease in the cross-sectional area (CSA) was found in the Equitend^® group (p = 0.04). After 4 months, the number of Equitend^® treated horses with an improved CSA was significantly higher than the placebo-treated horses (p = 0.03571). The Equitend^® group returned to their pre-injury performance level, racing in, and winning, significantly more races than the placebo group (p = 0.01399 and 0.0421, respectively). Furthermore, recurrence was significantly higher in the placebo group than in the Equitend^® group (71.4% vs 16.6%, p = 0.02442). In conclusion, we measured a significant, short-term, reduction effect on CSA and demonstrated a long-term beneficial effect of intralesional injection of Equitend^® for the treatment of superficial digital flexor tendonitis on racing ST, racing 2. 3 times more often than placebo, with 3.3 times fewer recurrences maintaining pre-injury performance level. This study may open the way for the development of a human treatment of tendonitis.

Novel animal model for Achilles tendinopathy: Controlled experimental study of serial injections of collagenase in rabbits

Our goal was to develop a novel technique for inducing Achilles tendinopathy in animal models which more accurately represents the progressive histological and biomechanical characteristic of chronic Achilles tendinopathy in humans. In this animal research study, forty-five rabbits were randomly assigned to three groups and given bilateral Achilles injections. Low dose (LD group) (n = 18) underwent a novel technique with three low-dose (0.1mg) injections of collagenase that were separated by two weeks, the high dose group (HD) (n = 18) underwent traditional single high-dose (0.3mg) injections, and the third group were controls (n = 9). Six rabbits were sacrificed from each experimental group (LD and HD) at 10, 12 and 16 weeks. Control animals were sacrificed after 16 weeks. Histological and biomechanical properties were then compared in all three groups. At 10 weeks, Bonar score and tendon cross sectional area was highest in HD group, with impaired biomechanical properties compared to LD group. At 12 weeks, Bonar score was higher in LD group, with similar biomechanical findings when compared to HD group. After 16 weeks, Bonar score was significantly increased for both LD group (11,8±2,28) and HD group (5,6±2,51), when compared to controls (2±0,76). LD group showed more pronounced histological and biomechanical findings, including cross sectional area of the tendon, Young’s modulus, yield stress and ultimate tensile strength. In conclusion, Achilles tendinopathy in animal models that were induced by serial injections of low-dose collagenase showed more pronounced histological and biomechanical findings after 16 weeks than traditional techniques, mimicking better the progressive and chronic characteristic of the tendinopathy in humans.

Vitamin C Improves Therapeutic Effects of Adipose-derived Stem Cell Transplantation in Mouse Tendonitis Model

BACKGROUND/AIM

We hypothesized that combined therapy using adipose-derived stem cells (ASCs) and vitamin C might improve tendon regeneration in tendonitis. To determine combined effects of ASC transplantation with vitamin C, we used senescence marker protein 30 (SMP30)-knockout (KO) mice that cannot biosynthesize vitamin C by themselves.

MATERIALS AND METHODS

SMP30-KO were divided into four groups: Control, vitamin C, ASCs, and vitamin C plus ASCs. Tendonitis was induced in the achilles tendons via injection of collagenase type I. After 1 week, ASCs were injected into the intratendonal region. After 30 days, all mice were sacrificed and Achilles tendons were isolated.

RESULTS

Gross and microscopic findings showed mice treated with combination of ASC transplantation and vitamin C showed better tendon regeneration than those in other groups. This combination led to higher serum vitamin C levels than use of vitamin C alone. This indicates that the vitamin C-treated group used more vitamin C as a precursor to collagen synthesis, whereas vitamin C was in excess in the combination group because of the added effect of ASCs on tendon healing.

CONCLUSION

This study showed that vitamin C improved the effect of ASC transplantation on tendonitis by inducing a better stem cell niche.