Tendon healing: a concise review on cellular and molecular mechanisms with a particular focus on the Achilles tendon

Suppression of experimental knee osteoarthritis by combination therapy of cross-linked hyaluronate and corticosteroids via anti-senescent effects

Osteoarthritis (OA) mainly affects the knee joint. Senescence and inflammation are key factors in knee OA pathogenesis, suggesting a potential therapeutic target. This study aims to explore the therapeutic effects of the optimized cross-linked hyaluronate (cHA) combined with corticosteroids formulation in mitigating OA progression by targeting anti-senescence. Human OA chondrocytes underwent treatment with various cHA formulations along with DEX, and assessments were made by cell viability, senescence phenotypes, and gene expression, including inflammatory cytokines, and matrix metalloproteinases (MMPs). Furthermore, in a rat OA model, the therapeutic effects of the targeted cHA + DEX formulations were evaluated via dynamic weight-bearing tests, micro-CT scans, histopathological and immunohistochemical examinations, and qRT-PCR analysis. Formulations of cHA(50:50) + DEX and cHA(20:80) + DEX effectively shielded chondrocytes from DEX-induced cytotoxicity and senescence, concurrently reducing inflammatory and matrix-degrading enzyme expressions. In the rat OA model, cHA(50:50) + DEX significantly ameliorated OA features, including histological scores and dynamic weight bearing ratio (p < 0.05, both), while suppressing senescence and inflammation marker expressions. Our findings underscore the effects of cHA(50:50) + DEX combination in mitigating OA progression by addressing senescence and inflammatory responses, so called inflammaging.

Biocompatible and stretchable chitosan piezoelectric gel with antibacterial capability and motion monitoring function for Achilles tendon rupture treatment

Achilles tendon rupture is a common and serious condition that remains a challenge in the restoration of tendon structure and function. The design and use of high-performance piezoelectric materials serve as an effective solution to enhance repair outcomes, shorten recovery times, and reduce the risk of recurrence. In this study, we prepared a chitosan piezoelectric gel (CSPG) as an organic polymer with excellent biocompatibility, stretchability, and piezoelectric properties as well as excellent antibacterial properties. In vitro experiments showed that CSPG, which induces a piezoelectric effect, can inhibit bacterial growth, promote cell proliferation and migration, upregulate the expression of tendon-related genes, and inhibit the expression of inflammation-related genes. In vivo experiments showed improved outcomes for Achilles tendon repair following CSPG intervention, as evidenced by enhanced animal mobility and improved mechanical test results. In addition, the CSPG exhibited sensory functions capable of monitoring temperature and motion, providing timely feedback on repair efficacy. In summary, this study not only successfully prepared a multifunctional piezoelectric material that can effectively promote Achilles tendon rupture repair and regeneration and control inflammatory response, it also possesses antibacterial and sensing functions, thus offering a new strategy for Achilles tendon rupture repair.

Epigenetic mechanisms in stem cell therapies for achilles tendinopathy

Achilles tendinopathy (AT) is a chronic degenerative tendinopathy that affects people's daily lives. Multiple clinical studies have found that current conservative treatments fail to promote quality tendon healing. Recent studies have found that stem cell therapy can target pathophysiological changes in the tendon by replenishing tendon-derived cells, promoting extracellular matrix (ECM) remodeling, and modulating the inflammatory response to improve the microenvironment of Achilles tendon regeneration. And epigenetic modifications play an important role in stem cell fate determination and function. In this review, we provided a brief overview of the biological properties of relevant stem cells. The influence of epigenetic modifications on stem cell proliferation, differentiation, and immune regulatory function in the treatment of AT was also explored. We focused on gene regulatory mechanisms controlled by DNA methylation, histones and non-coding RNAs including microRNAs, circRNAs and long non-coding RNAs. We also discuss the current challenges faced by stem cell therapies in treating AT and their potential solutions. Further research in this area will provide a more comprehensive epigenetic explanation for stem cell therapy for AT, leading to the development of stable, safe and effective stem cell therapies.

The fundamentals of WNT10A

Human wingless-type MMTV integration site family member 10A (WNT10A) is a secreted glycoprotein that is involved in signaling pathways essential to ectodermal organogenesis and tissue regeneration. WNT10A was first linked to human disorders in 2006, demonstrating a WNT10a variant to be associated with cleft lip with/without cleft palate. Numerous publications have since then identified the importance of WNT10A in the development of ectodermal appendages and beyond. In this review, we provide information on the structure of the WNT10A gene and protein, summarize its expression patterns in different animal models and in human, and describe the identified roles in tissue and organ development and repair in the different animal model organisms. We then correlate such identified functions and working mechanisms to the pathophysiology of a spectrum of human diseases and disorders that result from germline loss-of-function mutations in WNT10A, including ectodermal dysplasia (ED) syndromes Odonto-oncho-dermal dysplasia (OODD), Schöpf-Schulz-Passarge syndrome (SSPS), and selective tooth agenesis, as well as pathological conditions like fibrosis and carcinogenesis that can be correlated with increased WNT10A activity (Section 5).

Fabrication and Characterization of Electrospun DegraPol® Tubes Releasing TIMP-1 Protein to Modulate Tendon Healing

BACKGROUND

Tendon rupture repair can result from fibrotic scar formation through imbalanced ECM deposition during remodeling. The tissue inhibitors of matrix metalloprotease (TIMPs) not only decrease ECM degradation, regulated by matrix metalloproteases (MMPs), but also restrict TGF-β1 activation and thus diminish fibrosis.

METHODS

Rabbit tenocytes (rbTenocytes) and rabbit adipose-derived stem cells (rbASCs) were cultivated under different TIMP-1 concentrations. Proliferation and gene expression were assessed. TIMP-1 was incorporated into emulsion electrospun DegraPol® (DP) tubes that were characterized by SEM for fiber thickness, pore size, and wall thickness. Static and dynamic water contact angles, FTIR spectra, and TIMP-1 release kinetics were determined.

RESULTS

While the proliferation of rbTenocytes and rbACS was not affected by TIMP-1 supplementation in vitro, the gene expression of Col1A1 was increased in rbTenocytes, the gene expression of ki67 was increased in both cell types, the gene expression of tenomodulin was increased in both cell types at 100 ng/mL TIMP-1, and alkaline phosphatase expression ALP rose significantly in rbASCs. Electrospun TIMP-1/DP fibers had a ~5 μm diameter, a ~10 μm pore size, and a mesh thickness of ~200 μm. TIMP-1/DP meshes were more hydrophilic than pure DP meshes. TIMP-1 was released from the meshes with a sustained release of up to 7 days.

CONCLUSIONS

TIMP-1/DP tubes may be used to modulate the fibrotic tissue reaction when applied around conventionally sutured tendon ruptures.

No significant impact of platelet-rich plasma on recovery after Achilles tendon surgery: A double-blind randomized controlled trial

Purpose

Double-blind, randomized, placebo-controlled trials evaluating the efficacy and safety of Platelet-rich plasma (PRP) in the treatment of Achilles tendon rupture (ATR) have been scant. This study examines the therapeutic impact of PRP injection 3 weeks after surgery in middle-aged males.

Methods

This double-blind, randomized, placebo-controlled trial included consecutive ATR patients who satisfied the inclusion criteria and was conducted from 5 September 2018 to 24 June 2021. Three weeks after surgery using the side-locking loop technique, PRP or saline was injected at the suture site under ultrasound guidance. Evaluations were conducted at predetermined intervals (6, 10, 12, 16 and 24 weeks and 1 and 2 years) after surgery. The primary outcome was the period needed to perform a bilateral heel raise, and the important secondary outcomes were the periods needed to perform a single heel raise and 20 unilateral heel raises, respectively.

Results

There were seven participants in the PRP group and seven in the saline group. Demographically, both groups exhibited comparable characteristics. No complications were reported. At 6 weeks after surgery, all participants achieved bilateral heel raise. The PRP and saline groups averaged 12.3 ± 2.7 and 15.7 ± 5.9 weeks to achieve a single heel raise and 14.3 ± 2.7 and 17.7 ± 4.5 weeks to achieve 20 unilateral heel raises, respectively, with no significant differences between both groups. Moreover, no substantial disparities in clinical scores, period of jogging initiation and magnetic resonance imaging tendon assessments were noted.

Conclusions

PRP did not offer a distinct advantage over saline in terms of recovery from ATR in middle-aged males. This finding underscores the need to reassess the post-operative significance of PRP and highlights the importance of further research to determine its potential advantages and risks.

Level of Evidence

Level I.

Adhesion extent data of repaired rabbit Achilles tendons three weeks post-surgery and characterization data of different implant materials used for these surgeries

As one major problem after tendon rupture repair, surgeons are confronted with fibrotic adhesion formation of the healing tendon to the surrounding tissue. Although early active motion is recommended during rehabilitation, adhesions may lead to joint stiffness and a restricted range of motion. One viable option to counteract adhesion formation is to add a thin elastic tube that is placed over the conventionally sutured repair site. Such a tube reduces adhesion formation because it acts as a physical barrier. Additionally, such barriers can be optimized by adding a biolubricant. We here present adhesion data of rabbit Achilles tendons that were fully transsected, repaired with a 4-strand suture and received a) no implant; b) an electrospun DegraPol tube and c) a bi-layered tube with one electrospun DegraPol layer and one high molecular weight hyaluronic acid (HA)/polyethylene oxide (PEO) electrospun layer. Based on Picrosirius red stained tendon cross-sections three weeks post-operation, the percentage of adhesion data is presented. Moreover, mechanical data of the implant materials are presented as a further dataset, with the following readouts: fracture strain [%], ultimate tensile stress [MPa] and Young's modulus [MPa]. They are presented in axial and transverse stretching directions, respectively. The adhesion data can be reused for comparison to other implant materials, drugs or anti-adhesive strategies that are applied in similar pre-clinical models like the rabbit Achilles tendon model. The mechanical data of the implant materials offer the possibility to compare electrospun meshes based on other polymers to the materials used here or for computational models of such materials.

The current status of various preclinical therapeutic approaches for tendon repair

Tendons are fibroblastic structures that link muscle and bone. There are two kinds of tendon injuries, including acute and chronic. Each form of injury or deterioration can result in significant pain and loss of tendon function. The recovery of tendon damage is a complex and time-consuming recovery process. Depending on the anatomical location of the tendon tissue, the clinical outcomes are not the same. The healing of the wound process is divided into three stages that overlap: inflammation, proliferation, and tissue remodeling. Furthermore, the curing tendon has a high re-tear rate. Faced with the challenges, tendon injury management is still a clinical issue that must be resolved as soon as possible. Several newer directions and breakthroughs in tendon recovery have emerged in recent years. This article describes tendon injury and summarizes recent advances in tendon recovery, along with stem cell therapy, gene therapy, Platelet-rich plasma remedy, growth factors, drug treatment, and tissue engineering. Despite the recent fast-growing research in tendon recovery treatment, still, none of them translated to the clinical setting. This review provides a detailed overview of tendon injuries and potential preclinical approaches for treating tendon injuries.

Rotator Cuff Injury: Pathogenesis, Biomechanics, and Repair

Anterior Rotator cuff tears are one of the most common surgically addressed disorders, as the tears in the tendon can affect anyone regardless of risk factors or activity level. The rotator cuff is responsible for most of the shoulder motion, hence the tendon-bone interface experiences immense stress making it incredibly prone to failure. Rotator cuff tendon tears can either occur due to trauma or natural degeneration of the rotator cuff. To help mitigate effects of high stress on the rotator cuff tendon-bone interface, the rotator cuff is intrinsically designed to redistribute stress through protective mechanisms, such as the rotator crescent or coronal-transverse force. But when the tear goes through the intrinsic protective mechanisms, the glenohumeral joint is left unstable and thus is no longer capable of its normal range of motion. Location, size, and type of rotator cuff tendon tears are the strongest indicators for interventional therapy. Surgical therapies demonstrate low success rates, as seen by the significantly high recurrence rate of rotator cuff reinjury following initial repair. This is due to extrinsically healing of rotator cuff tendons, instead of the more intrinsic healing, which causes rotator cuff tendons to not undergo the necessary biomechanical remodeling to prevent reinjury leading to a mechanically and functionally inferior healed tendon. In this article, we thoroughly discussed the underlying pathophysiology of rotator cuff tears from onset to repair to healing, demonstrating that rotator cuff tendon healing is an intrinsically flawed process, irrespective of the risk factors, occurrence of rotator cuff tears, or surgical treatment. Rotator cuff healing can only be successful if rotator cuff tendon repair surgery is augmented with biologics to promote a successful intrinsic healing environment.

Endothelial cells-derived exosomes-based hydrogel improved tendinous repair via anti-inflammatory and tissue regeneration-promoting properties

Tendon injuries are common orthopedic ailments with a challenging healing trajectory, especially in cases like the Achilles tendon afflictions. The healing trajectory of tendon injuries is often suboptimal, leading to scar formation and functional impairment due to the inherent low metabolic activity and vascularization of tendon tissue. As pressing is needed for effective interventions, efforts are made to explore biomaterials to augment tendon healing. However, tissue engineering approaches face hurdles in optimizing tissue scaffolds and nanomedical strategies. To navigate these challenges, an injectable hydrogel amalgamated with human umbilical vein endothelial cells-derived exosomes (HUVECs-Exos) was prepared and named H-Exos-gel in this study, aiming to enhance tendon repair. In our research involving a model of Achilles tendon injuries in 60 rats, we investigated the efficacy of H-Exos-gel through histological assessments performed at 2 and 4 weeks and behavioral assessments conducted at the 4-week mark revealed its ability to enhance the Achilles tendon's mechanical strength, regulate inflammation and facilitate tendon regeneration and functional recovery. Mechanically, the H-Exos-gel modulated the cellular behaviors of macrophages and tendon-derived stem cells (TDSCs) by inhibiting inflammation-related pathways and promoting proliferation-related pathways. Our findings delineate that the H-Exos-gel epitomizes a viable bioactive medium for tendon healing, heralding a promising avenue for the clinical amelioration of tendon injuries.

Factors associated with Achilles tendon re-rupture following operative fixation

Aims

Achilles tendon re-rupture (ATRR) poses a significant risk of postoperative complication, even after a successful initial surgical repair. This study aimed to identify risk factors associated with Achilles tendon re-rupture following operative fixation.

Methods

This retrospective cohort study analyzed a total of 43,287 patients from national health claims data spanning 2008 to 2018, focusing on patients who underwent surgical treatment for primary Achilles tendon rupture. Short-term ATRR was defined as cases that required revision surgery occurring between six weeks and one year after the initial surgical repair, while omitting cases with simultaneous infection or skin necrosis. Variables such as age, sex, the presence of Achilles tendinopathy, and comorbidities were systematically collected for the analysis. We employed multivariate stepwise logistic regression to identify potential risk factors associated with short-term ATRR.

Results

From 2009 to 2018, the short-term re-rupture rate for Achilles tendon surgeries was 2.14%. Risk factors included male sex, younger age, and the presence of Achilles tendinopathy.

Conclusion

This large-scale, big-data study reaffirmed known risk factors for short-term Achilles tendon re-rupture, specifically identifying male sex and younger age. Moreover, this study discovered that a prior history of Achilles tendinopathy emerges as an independent risk factor for re-rupture, even following initial operative fixation.

Hyaluronic acid/PEO electrospun tube reduces tendon adhesion to levels comparable to native tendons - An in vitro and in vivo study

A major problem after tendon injury is adhesion formation to the surrounding tissue leading to a limited range of motion. A viable strategy to reduce adhesion extent is the use of physical barriers that limit the contact between the tendon and the adjacent tissue. The purpose of this study was to fabricate an electrospun bilayered tube of hyaluronic acid/polyethylene oxide (HA/PEO) and biodegradable DegraPol® (DP) to improve the anti-adhesive effect of the implant in a rabbit Achilles tendon full laceration model compared to a pure DP tube. Additionally, the attachment of rabbit tenocytes on pure DP and HA/PEO containing scaffolds was tested and Scanning Electron Microscopy, Fourier-transform Infrared Spectroscopy, Differential Scanning Calorimetry, Water Contact Angle measurements, and testing of mechanical properties were used to characterize the scaffolds. In vivo assessment after three weeks showed that the implant containing a second HA/PEO layer significantly reduced adhesion extent reaching levels comparable to native tendons, compared with a pure DP implant that reduced adhesion formation only by 20 %. Tenocytes were able to attach to and migrate into every scaffold, but cell number was reduced over two weeks. Implants containing HA/PEO showed better mechanical properties than pure DP tubes and with the ability to entirely reduce adhesion extent makes this implant a promising candidate for clinical application in tendon repair.

In vivo study on the repair of rat Achilles tendon injury treated with non-thermal atmospheric-pressure helium microplasma jet

Non-thermal atmospheric-pressure plasma (NTAPP) has been widely studied for clinical applications, e.g., disinfection, wound healing, cancer therapy, hemostasis, and bone regeneration. It is being revealed that the physical and chemical actions of plasma have enabled these clinical applications. Based on our previous report regarding plasma-stimulated bone regeneration, this study focused on Achilles tendon repair by NTAPP. This is the first study to reveal that exposure to NTAPP can accelerate Achilles tendon repair using a well-established Achilles tendon injury rat model. Histological evaluation using the Stoll's and histological scores showed a significant improvement at 2 and 4 weeks, with type I collagen content being substantial at the early time point of 2 weeks post-surgery. Notably, the replacement of type III collagen with type I collagen occurred more frequently in the plasma-treated groups at the early stage of repair. Tensile strength test results showed that the maximum breaking strength in the plasma-treated group at two weeks was significantly higher than that in the untreated group. Overall, our results indicate that a single event of NTAPP treatment during the surgery can contribute to an early recovery of an injured tendon.

Ultrasound assessment of acute Achilles tendon rupture and measurement of the tendon gap

Achilles tendon rupture is a common sports-related injury which can carry significant morbidity to patients. Ultrasound remains the workhorse of imaging as it can confirm and localise the extent of Achilles tendon injury. The sonographic anatomy, both normal and ruptured sonographic appearances, as well as sonographic technique must be appreciated to accurately image and report findings, critical to patient management. Particular attention should be applied to the measurement of the diastasis between acutely ruptured tendon ends as this information can assist with informing the decision of conservative vs. operative management. Further work is necessary to standardise the measurement technique including correlating the degree of plantarflexion of the foot with the sonographic tendon gap measures.

The Use of Extracellular Vesicles in Achilles Tendon Repair: A Systematic Review

Achilles tendon (AT) pathologies are common musculoskeletal conditions that can significantly impair function. Despite various traditional treatments, recovery is often slow and may not restore full functionality. The use of extracellular vesicles (EVs) has emerged as a promising therapeutic option due to their role in cell signaling and tissue regeneration. This systematic review aims to consolidate current in vivo animal study findings on the therapeutic effects of EVs on AT injuries. An extensive literature search was conducted using the PubMed, Scopus, and Embase databases for in vivo animal studies examining the effects of EVs on AT pathologies. The extracted variables included but were not limited to the study design, type of EVs used, administration methods, efficacy of treatment, and proposed therapeutic mechanisms. After screening, 18 studies comprising 800 subjects were included. All but one study reported that EVs augmented wound healing processes in the AT. The most proposed mechanisms through which this occurred were gene regulation of the extracellular matrix (ECM), the enhancement of macrophage polarization, and the delivery of therapeutic microRNAs to the injury site. Further research is warranted to not only explore the therapeutic potential of EVs in the context of AT pathologies, but also to establish protocols for their clinical application.

Achilles tendon thickening does not affect elasticity and functional outcome after surgical repair of Achilles rupture: A retrospective study

PURPOSE

Previous studies have confirmed that Achilles tendon occurs Achilles thickening after repair surgery of the rupture. Although this mechanism has been elucidated in the laboratory, there are few reports on its impact on clinical function. We designed a retrospective study to investigate the Achilles thickening after Achilles tendon rupture repair and its correlation between the elasticity and postoperative function.

METHODS

In this retrospective analysis, patients who underwent surgical treatment for acute Achilles tendon rupture from April 2016 to April 2020 were included. All the patients were regularly followed up at 3 months, 1 year, and 2 years after surgery. American Orthopaedic Foot Ankle Surgeon (AOFAS) scale and Leppilahti score were used to evaluate functional outcomes. Achilles elasticity was measured by ultrasound shear wave of elasticity. Achilles thickening was calculated as maximal transverse and longitudinal diameter in cross-sectional plane of magnetic resonance scan. Sample t-tests was used for different follow-up periods. Correlation between Achilles thickening and other factors were analyzed using Pearson's method. p < 0.05 indicates a statistically significant difference.

RESULTS

AOFAS scale and Leppilahti score at 1 year were significantly higher than at 3 months postoperatively (both p < 0.001). These functional scales were also improved at 2-year follow-up significantly (both p < 0.001). The dorsiflexion difference showed gradually recovery in each follow-up period (t = -17.907, p < 0.001). The elasticity of the Achilles appeared to continuously decreases during the postoperative follow-up period in all position sets (p < 0.001). In thickening evaluation, the cross-sectional area of the thickest plane of Achilles was significantly higher at 1 year postoperatively (310.5 ± 25.2) mm2 than that at 3 months postoperatively ((278.0 ± 26.2) mm2, t = -8.219, p < 0.001) and became thinner in 2-year magnetic resonance scan ((256.1 ± 15.1) mm2, t = 16.769, p < 0.001). The correlations between Achilles thickening, elasticity, and functional outcome did not show statistical significance (p > 0.05) in every follow-up period.

CONCLUSION

Achilles tendon thickens after surgery in the 1st year, but begins to gradually return to thinning about 2 years after surgery. There was no significant correlation between the increase and decrease of thickening and the patients' clinical function scores, Achilles elasticity, and bilateral ankle dorsiflexion difference.

Adiponectin inhibits fibrosis of the palmar aponeurosis in Dupuytren's contracture in male patients

Aims

Dupuytren's contracture is characterized by increased fibrosis of the palmar aponeurosis, with eventual replacement of the surrounding fatty tissue with palmar fascial fibromatosis. We hypothesized that adipocytokines produced by adipose tissue in contact with the palmar aponeurosis might promote fibrosis of the palmar aponeurosis.

Methods

We compared the expression of the adipocytokines adiponectin and leptin in the adipose tissue surrounding the palmar aponeurosis of male patients with Dupuytren's contracture, and of male patients with carpal tunnel syndrome (CTS) as the control group. We also examined the effects of adiponectin on fibrosis-related genes and proteins expressed by fibroblasts in the palmar aponeurosis of patients with Dupuytren's contracture.

Results

Adiponectin expression in the adipose tissue surrounding the palmar aponeurosis was significantly lower in patients with Dupuytren's contracture than in those with CTS. The expression of fibrosis-related genes and proteins, such as types 1 and 3 collagen and α-smooth muscle actin, was suppressed in a concentration-dependent manner by adding AdipoRon, an adiponectin receptor agonist. The expression of fibrosis-related genes and proteins was also suppressed by AdipoRon in the in vitro model of Dupuytren's contracture created by adding TGF-β to normal fibroblasts collected from patients with CTS.

Conclusion

Fibrosis of the palmar aponeurosis in Dupuytren's contracture in males may be associated with adiponectin expression in the adipose tissue surrounding the palmar aponeurosis. Although fibroblasts within the palmar aponeurosis are often the focus of attention when elucidating the pathogenesis of Dupuytren's contracture, adiponectin expression in adipose tissues warrants closer attention in future research.

A tri-layer decellularized, dehydrated human amniotic membrane scaffold supports the cellular functions of human tenocytes in vitro

Differences in scaffold design have the potential to influence cell-scaffold interactions. This study sought to determine whether a tri-layer design influences the cellular function of human tenocytes in vitro. The single-layer decellularized, dehydrated human amniotic membrane (DDHAM) and the tri-layer DDHAM (DDHAM-3L) similarly supported tenocyte function as evidenced by improved cell growth and migration, reduced dedifferentiation, and an attenuated inflammatory response. The tri-layer design provides a mechanically more robust scaffold without altering biological activity.

Angiogenesis and Tissue Repair Depend on Platelet Dosing and Bioformulation Strategies Following Orthobiological Platelet-Rich Plasma Procedures: A Narrative Review

Angiogenesis is the formation of new blood vessel from existing vessels and is a critical first step in tissue repair following chronic disturbances in healing and degenerative tissues. Chronic pathoanatomic tissues are characterized by a high number of inflammatory cells; an overexpression of inflammatory mediators; such as tumor necrosis factor-α (TNF-α) and interleukin-1 (IL-1); the presence of mast cells, T cells, reactive oxygen species, and matrix metalloproteinases; and a decreased angiogenic capacity. Multiple studies have demonstrated that autologous orthobiological cellular preparations (e.g., platelet-rich plasma (PRP)) improve tissue repair and regenerate tissues. There are many PRP devices on the market. Unfortunately, they differ greatly in platelet numbers, cellular composition, and bioformulation. PRP is a platelet concentrate consisting of a high concentration of platelets, with or without certain leukocytes, platelet-derived growth factors (PGFs), cytokines, molecules, and signaling cells. Several PRP products have immunomodulatory capacities that can influence resident cells in a diseased microenvironment, inducing tissue repair or regeneration. Generally, PRP is a blood-derived product, regardless of its platelet number and bioformulation, and the literature indicates both positive and negative patient treatment outcomes. Strangely, the literature does not designate specific PRP preparation qualifications that can potentially contribute to tissue repair. Moreover, the literature scarcely addresses the impact of platelets and leukocytes in PRP on (neo)angiogenesis, other than a general one-size-fits-all statement that "PRP has angiogenic capabilities". Here, we review the cellular composition of all PRP constituents, including leukocytes, and describe the importance of platelet dosing and bioformulation strategies in orthobiological applications to initiate angiogenic pathways that re-establish microvasculature networks, facilitating the supply of oxygen and nutrients to impaired tissues.

Bioactive and Elastic Emulsion Electrospun DegraPol Tubes Delivering IGF-1 for Tendon Rupture Repair

Tendon injuries can result in two major drawbacks. Adhesions to the surrounding tissue may limit the range of motion, while fibrovascular scar formation can lead to poor biomechanical outcomes. Prosthetic devices may help to mitigate those problems. Emulsion electrospinning was used to develop a novel three-layer tube based on the polymer DegraPol (DP), with incorporated insulin-like growth factor-1 (IGF-1) in the middle layer. Scanning electron microscopy was utilized to assess the fiber diameter in IGF-1 containing pure DP meshes. Further characterization was performed with Fourier Transformed Infrared Spectroscopy, Differential Scanning Calorimetry, and water contact angle, as well as through the assessment of mechanical properties and release kinetics from ELISA, and the bioactivity of IGF-1 by qPCR of collagen I, ki67, and tenomodulin in rabbit Achilles tenocytes. The IGF-1-containing tubes exhibited a sustained release of the growth factor up to 4 days and showed bioactivity by significantly upregulated ki67 and tenomodulin gene expression. Moreover, they proved to be mechanically superior to pure DP tubes (significantly higher fracture strain, failure stress, and elastic modulus). The novel three-layer tubes intended to be applied over conventionally sutured tendons after a rupture may help accelerate the healing process. The release of IGF-1 stimulates proliferation and matrix synthesis of cells at the repair site. In addition, adhesion formation to surrounding tissue can be reduced due to the physical barrier.

Establishing in vivo and ex vivo chick embryo models to investigate fetal tendon healing

Injured adult tendons heal fibrotically and possess high re-injury rates, whereas fetal tendons appear to heal scarlessly. However, knowledge of fetal tendon wound healing is limited due in part to the need for an accessible animal model. Here, we developed and characterized an in vivo and ex vivo chick embryo tendon model to study fetal tendon healing. In both models, injury sites filled rapidly with cells and extracellular matrix during healing, with wound closure occurring faster in vivo. Tendons injured at an earlier embryonic stage improved mechanical properties to levels similar to non-injured controls, whereas tendons injured at a later embryonic stage did not. Expression levels of tendon phenotype markers, collagens, collagen crosslinking regulators, matrix metalloproteinases, and pro-inflammatory mediators exhibited embryonic stage-dependent trends during healing. Apoptosis occurred during healing, but ex vivo tendons exhibited higher levels of apoptosis than tendons in vivo. Future studies will use these in vivo and ex vivo chick embryo tendon injury models to elucidate mechanisms of stage-specific fetal tendon healing to inform the development of therapeutic approaches to regeneratively heal adult tendons.

Anatomical Tissue Engineering of the Anterior Cruciate Ligament Entheses

The firm integration of anterior cruciate ligament (ACL) grafts into bones remains the most demanding challenge in ACL reconstruction, since graft loosening means graft failure. For a functional-tissue-engineered ACL substitute to be realized in future, robust bone attachment sites (entheses) have to be re-established. The latter comprise four tissue compartments (ligament, non-calcified and calcified fibrocartilage, separated by the tidemark, bone) forming a histological and biomechanical gradient at the attachment interface between the ACL and bone. The ACL enthesis is surrounded by the synovium and exposed to the intra-articular micromilieu. This review will picture and explain the peculiarities of these synovioentheseal complexes at the femoral and tibial attachment sites based on published data. Using this, emerging tissue engineering (TE) strategies addressing them will be discussed. Several material composites (e.g., polycaprolactone and silk fibroin) and manufacturing techniques (e.g., three-dimensional-/bio-printing, electrospinning, braiding and embroidering) have been applied to create zonal cell carriers (bi- or triphasic scaffolds) mimicking the ACL enthesis tissue gradients with appropriate topological parameters for zones. Functionalized or bioactive materials (e.g., collagen, tricalcium phosphate, hydroxyapatite and bioactive glass (BG)) or growth factors (e.g., bone morphogenetic proteins [BMP]-2) have been integrated to achieve the zone-dependent differentiation of precursor cells. However, the ACL entheses comprise individual (loading history) asymmetric and polar histoarchitectures. They result from the unique biomechanical microenvironment of overlapping tensile, compressive and shear forces involved in enthesis formation, maturation and maintenance. This review should provide a road map of key parameters to be considered in future in ACL interface TE approaches.

Effect of treadmill training on fibrocartilage complex repair in tendon-bone insertion healing in the postinflammatory stage

Aims

Mechanical stimulation is a key factor in the development and healing of tendon-bone insertion. Treadmill training is an important rehabilitation treatment. This study aims to investigate the benefits of treadmill training initiated on postoperative day 7 for tendon-bone insertion healing.

Methods

A tendon-bone insertion injury healing model was established in 92 C57BL/6 male mice. All mice were divided into control and training groups by random digital table method. The control group mice had full free activity in the cage, and the training group mice started the treadmill training on postoperative day 7. The quality of tendon-bone insertion healing was evaluated by histology, immunohistochemistry, reverse transcription quantitative polymerase chain reaction, Western blotting, micro-CT, micro-MRI, open field tests, and CatWalk gait and biomechanical assessments.

Results

Our results showed a significantly higher tendon-bone insertion histomorphological score in the training group, and the messenger RNA and protein expression levels of type II collagen (COL2A1), SOX9, and type X collagen (COL10A1) were significantly elevated. Additionally, tendon-bone insertion resulted in less scar hyperplasia after treadmill training, the bone mineral density (BMD) and bone volume/tissue volume (BV/TV) were significantly improved, and the force required to induce failure became stronger in the training group. Functionally, the motor ability, limb stride length, and stride frequency of mice with tendon-bone insertion injuries were significantly improved in the training group compared with the control group.

Conclusion

Treadmill training initiated on postoperative day 7 is beneficial to tendon-bone insertion healing, promoting biomechanical strength and motor function. Our findings are expected to guide clinical rehabilitation training programmes.

Principles of Tendon Structure, Healing, and the Microenvironment

Tendon is a strong viscoelastic tissue, responsible for conducting force from muscle to bone. In the hand, flexor tendons course through fibro-osseous sheaths, composed of an intricate tenosynovium and fibrocartilaginous pulley system. After flexor tendon laceration, changes occur in tendon force transduction as well as vascularity, affecting tendon healing on a tissue and cellular level. Tendon healing occurs through intrinsic and extrinsic mechanisms, which in combination with local anatomy, can predispose to adhesion formation. Understanding the relationship between microenvironmental cues and tendon healing on the cellular and tissue level will improve our knowledge and treatment of flexor tendon injuries.

Co-Culture of Mesenchymal Stem Cells and Ligamentocytes on Triphasic Embroidered Poly(L-lactide-co-ε-caprolactone) and Polylactic Acid Scaffolds for Anterior Cruciate Ligament Enthesis Tissue Engineering

Successful anterior cruciate ligament (ACL) reconstructions strive for a firm bone-ligament integration. With the aim to establish an enthesis-like construct, embroidered functionalized scaffolds were colonized with spheroids of osteogenically differentiated human mesenchymal stem cells (hMSCs) and lapine (l) ACL fibroblasts in this study. These triphasic poly(L-lactide-co-ε-caprolactone) and polylactic acid (P(LA-CL)/PLA) scaffolds with a bone-, a fibrocartilage transition- and a ligament zone were colonized with spheroids directly after assembly (DC) or with 14-day pre-cultured lACL fibroblast and 14-day osteogenically differentiated hMSCs spheroids (=longer pre-cultivation, LC). The scaffolds with co-cultures were cultured for 14 days. Cell vitality, DNA and sulfated glycosaminoglycan (sGAG) contents were determined. The relative gene expressions of collagen types I and X, Mohawk, Tenascin C and runt-related protein (RUNX) 2 were analyzed. Compared to the lACL spheroids, those with hMSCs adhered more rapidly. Vimentin and collagen type I immunoreactivity were mainly detected in the hMSCs colonizing the bone zone. The DNA content was higher in the DC than in LC whereas the sGAG content was higher in LC. The gene expression of ECM components and transcription factors depended on cell type and pre-culturing condition. Zonal colonization of triphasic scaffolds using spheroids is possible, offering a novel approach for enthesis tissue engineering.

TGF-β1 derived from macrophages contributes to load-induced tendon-bone healing in the murine rotator cuff repair model by promoting chondrogenesis

It has been established that mechanical stimulation benefits tendon-bone (T-B) healing, and macrophage phenotype can be regulated by mechanical cues; moreover, the interaction between macrophages and mesenchymal stem cells (MSCs) plays a fundamental role in tissue repair. This study aimed to investigate the role of macrophage-mediated MSC chondrogenesis in load-induced T-B healing in depth. C57BL/6 mice rotator cuff (RC) repair model was established to explore the effects of mechanical stimulation on macrophage polarization, transforming growth factor (TGF)-β1 generation, and MSC chondrogenesis within T-B enthesis by immunofluorescence and enzyme-linked immunosorbent assay (ELISA). Macrophage depletion was performed by clodronate liposomes, and T-B healing quality was evaluated by histology and biomechanics. In vitro, bone marrow-derived macrophages (BMDMs) were stretched with CELLOAD-300 load system and macrophage polarization was identified by flow cytometry and quantitative real-time polymerase chain reaction (qRT-PCR). MSC chondrogenic differentiation was measured by histochemical analysis and qRT-PCR. ELISA and qRT-PCR were performed to screen the candidate molecules that mediated the pro-chondrogenic function of mechanical stimulated BMDMs. Mechanical stimulation promoted macrophage M2 polarization in vivo and in vitro. The conditioned media from mechanically stimulated BMDMs (MS-CM) enhanced MSC chondrogenic differentiation, and mechanically stimulated BMDMs generated more TGF-β1. Further, neutralizing TGF-β1 in MS-CM can attenuate its pro-chondrogenic effect. In vivo, mechanical stimulation promoted TGF-β1 generation, MSC chondrogenesis, and T-B healing, which were abolished following macrophage depletion. Macrophages subjected to appropriate mechanical stimulation could polarize toward the M2 phenotype and secrete TGF-β1 to promote MSC chondrogenesis, which subsequently augments T-B healing.

Mechanisms of skeletal muscle-tendon development and regeneration/healing as potential therapeutic targets

Skeletal muscle contraction is essential for the movement of our musculoskeletal system. Tendons and ligaments that connect the skeletal muscles to bones in the correct position at the appropriate time during development are also required for movement to occur. Since the musculoskeletal system is essential for maintaining basic bodily functions as well as enabling interactions with the environment, dysfunctions of these tissues due to disease can significantly reduce quality of life. Unfortunately, as people live longer, skeletal muscle and tendon/ligament diseases are becoming more common. Sarcopenia, a disease in which skeletal muscle function declines, and tendinopathy, which involves chronic tendon dysfunction, are particularly troublesome because there have been no significant advances in their treatment. In this review, we will summarize previous reports on the development and regeneration/healing of skeletal muscle and tendon tissues, including a discussion of the molecular and cellular mechanisms involved that may be used as potential therapeutic targets.

In vivo ligamentogenesis in embroidered poly(lactic-co-ε-caprolactone) / polylactic acid scaffolds functionalized by fluorination and hexamethylene diisocyanate cross-linked collagen foams

Although autografts represent the gold standard for anterior cruciate ligament (ACL) reconstruction, tissue-engineered ACLs provide a prospect to minimize donor site morbidity and limited graft availability. This study characterizes the ligamentogenesis in embroidered poly(L-lactide-co-ε-caprolactone) (P(LA-CL)) / polylactic acid (PLA) constructs using a dynamic nude mice xenograft model. (P(LA-CL))/PLA scaffolds remained either untreated (co) or were functionalized by gas fluorination (F), collagen foam cross-linked with hexamethylene diisocyanate (HMDI) (coll), or F combined with the foam (F + coll). Cell-free constructs or those seeded for 1 week with lapine ACL ligamentocytes were implanted into nude mice for 12 weeks. Following explantation, cell vitality and content, histo(patho)logy of scaffolds (including organs: liver, kidney, spleen), sulphated glycosaminoglycan (sGAG) contents and biomechanical properties were assessed.Scaffolds did not affect mice weight development and organs, indicating no organ toxicity. Moreover, scaffolds maintained their size and shape and reflected a high cell viability prior to and following implantation. Coll or F + coll scaffolds seeded with cells yielded superior macroscopic properties compared to the controls. Mild signs of inflammation (foreign-body giant cells and hyperemia) were limited to scaffolds without collagen. Microscopical score values and sGAG content did not differ significantly. Although remaining stable after explantation, elastic modulus, maximum force, tensile strength and strain at F_max were significantly lower in explanted scaffolds compared to those before implantation, with no significant differences between scaffold subtypes, except for a higher maximum force in F + coll compared with F samples (in vivo). Scaffold functionalization with fluorinated collagen foam provides a promising approach for ACL tissue engineering. a Lapine anterior cruciate ligament (LACL): red arrow, posterior cruciate ligament: yellow arrow. Medial anterior meniscotibial ligament: black arrow. b Explant culture to isolate LACL fibroblasts. c Scaffold variants: co: controls; F: functionalization by gas-phase fluorination; coll: collagen foam cross-linked with hexamethylene diisocyanate (HMDI). c1-2 Embroidery pattern of the scaffolds. d Scaffolds were seeded with LACL fibroblasts using a dynamical culturing approach as depicted. e Scaffolds were implanted subnuchally into nude mice, fixed at the nuchal ligament and sacrospinal muscle tendons. f Two weeks after implantation. g Summary of analyses performed. Scale bars 1 cm (b, d), 0.5 cm (c). (sketches drawn by G.S.-T. using Krita 4.1.7 [Krita foundation, The Netherlands]).

Human Muscle-Derived Cells Are Capable of Tenogenic Differentiation and Contribution to Tendon Repair

BACKGROUND

It has been reported that the harvested hamstring tendon for autograft could be regenerated with well-oriented fibers and uniformly distributed spindle-shaped cells after removal. However, which cell type might participate in the repair process remains unknown.

PURPOSE

To investigate the tenogenic differentiation potential of human muscle-derived cells (MDCs) both in vitro and in vivo.

STUDY DESIGN

Controlled laboratory study.

METHODS

Primary human MDCs and tenocytes were isolated from discarded materials during a peroneus longus tendon-harvesting procedure. Expression of tenogenic genes were evaluated and compared among MDCs, MDCs with tenogenic induction, and tenocytes. RNA sequencing was performed to evaluate the expression profile of differentiated MDCs. Human MDCs were implanted in a tendon injury model to investigate the in vivo tenogenic differentiation potential. Histologic and functional analyses were performed to evaluate the function of MDCs for tendon repair.

RESULTS

The relative expression levels (in fold change) of tenogenic genes Col I, MKX, SCX, THBS4, and TNC in MDCs were significantly upregulated 11.5 ± 1.3, 957.1 ± 63.7, 19.1 ± 2.8, 61.9 ± 4.8, and 10.2 ± 2.8 after tenogenic induction, respectively. The expression profile of tenogenically differentiated MDCs was much closer to primary tenocytes. Activation of TGF-β/Smad3 signaling significantly promoted the tenogenic differentiation ability of MDCs. Transplanted human MDCs were identified in regenerated tendon and expressed tenogenic genes. As for biomechanical properties, the failure loads in the Matrigel, transplantation, and uninjured groups were 7.2 ± 0.5, 11.6 ± 0.3, and 13.9 ± 0.7 N, while the stiffness values were 4.4 ± 1.3 × 10³, 7.6 ± 0.8 × 10³, and 10.9 ± 1.1 × 10³ N/m. Plantarflexion force, histologic morphology, and motor function were also significantly improved after MDC transplantation in a tendon injury model.

CONCLUSION

There exist cells with tenogenic differentiation potential in human skeletal muscles. Activation of TGF-β/Smad3 signaling plays an important role in tenogenic differentiation for human MDCs. Human MDCs contribute to structural and functional repair for the injured tendon. MDCs are a potential cell source to participate in the repair process after tendon injury.

CLINICAL RELEVANCE

The MDCs could be a promising cell source to repair tendon injury.