The role of macrophages in early healing of a tendon graft in a bone tunnel

Remodeling Process of the Tendon Graft After Anterior Cruciate Ligament Reconstruction: Comprehensive Analysis With RNA Sequencing in a Murine Model

The tendon graft is known to undergo a remodeling process after anterior cruciate ligament (ACL) reconstruction. However, little is known about the transcriptional profile of this process. The aim of the present study is to identify differentially expressed genes inside the remodeling ACL graft in the early phase after ACL reconstruction in our murine model using RNA sequencing (RNAseq). Fifty four male C57BL/6 mice were used in this study. The mice were euthanized at 1, 2, and 4 weeks after surgery and used for histological evaluations and RNAseq of the tendon graft. Histologically, there was a progressive decrease in the tendon-bone interface gap space and increased tissue continuity between the grafted tendon and the bone tunnel over time. At 1 and 2 weeks after surgery, cell increase and loss of collagen fiber organization inside the tendon graft were observed. RNAseq showed that genes related to inflammation, matrix metalloproteinases, bone metabolism, chemokines and signaling pathways were upregulated at 1 and 2 weeks after surgery compared to the control group (p < 0.0001). Our transcriptional profiling data suggests that expression of inflammatory mediators and bone remodeling genes may play an important role in the early events in graft-to-bone healing. Further validation at the protein level is necessary to draw firm conclusions about the role of these mediators in graft remodeling and healing. Understanding the remodeling process of the grafted tendons may lead to the identification of new approaches to improve clinical outcomes after ACL reconstruction.

Adipose derived stem cell activation by macrophages and tendon fibroblasts

AIMS

Tendon injuries are common, and healing often fails due to an over-exuberant inflammatory response and a lack of regeneration. Inflammatory cells play key roles in these processes, with a balance between classically activated pro-inflammatory M1 macrophages and alternatively activated inflammatory resolving M2 macrophages. Adipose-derived mesenchymal stem cells (ASCs) can dampen the pro-inflammatory effectsof macrophages, promote a regenerative environment, and enhance healing. Therefore, the goal of the study was to understand how ASCs are activated by macrophages in vitro.

METHODS

In vitro co-culture experiments were carried out with ASCs, macrophages, and tendon fibroblasts. RNA-seq and qRT-PCR were performed to determine expression patterns of activated ASCs.

RESULTS

M1 macrophages prompted ASCs to upregulate pro-inflammatory signaling, matrix remodeling, and cytokine production pathways, while downregulating those related to cell adhesion and cell cycle. Conversely, TFs prompted ASCs to upregulate pathways involved in cell cycle and cytoskeleton remodeling, and to downregulate pathways associated with immune cell adhesion, inflammatory mediator production, and protein metabolism.

CONCLUSIONS

The cell-specific activation profiles indicate a possible switch in ASC paracrine signaling depending on the context, from a pro-inflammatory pattern in response to M1 macrophages to a proliferative pattern in response to TFs. Understanding crosstalk between ASCs, TFs, and macrophages is essential for developing stem cell-based therapeutic strategies.

Improved tendon repair with optimized chemically modified mRNAs: Combined delivery of Pdgf-BB and IL-1Ra using injectable nanoparticles

Tendon injuries, common in both athletic and non-athletic populations, present significant challenges due to their slow healing and the formation of scar tissue, which impairs function and potentially increases the risk of (re-)rupture. Conventional treatments often yield suboptimal functional and structural repair. This study investigates the potential of mRNA-based therapeutics to enhance tendon healing by targeting 2 distinct pathways via the delivery of chemically modified ARCA-capped mRNAs (cmRNAs) encoding Interleukin-1 receptor antagonist (IL1RA) and Platelet-Derived Growth Factor-BB (PDGF-BB) using injectable nanoparticle (NP) carriers. In vitro experiments demonstrate successful cmRNA delivery and translation, resulting in increased tendon cell proliferation, migration, and anti-inflammatory responses. In vivo, cmRNA treatment notably enhances tendon repair in a rat patellar tendon defect model, by reducing pro-inflammatory cytokines and fibrotic markers while enhancing repair tissue structure. These findings suggest that NP-based cmRNA delivery represents a promising therapeutic strategy for improving tendon healing, offering better outcomes over existing treatments by targeting both inflammatory and regenerative pathways. STATEMENT OF SIGNIFICANCE: In this study, we investigate an mRNA-based therapeutic approach aimed at enhancing tendon healing in a small animal model. Utilizing bioreducible poly(amidoamine)-based polymeric nanoparticles (PAA PNPs) for the delivery of cmRNAs encoding Interleukin-1 receptor antagonist (IL1RA) and Platelet-Derived Growth Factor-BB (PDGF-BB), we demonstrate effective delivery and protein translation in vitro and ex vivo, resulting in enhanced tendon cell proliferation, migration, and robust anti-inflammatory responses. By combining these therapeutic cmRNAs, we show improved tendon repair in vivo, with accelerated tissue regeneration, better collagen fiber organization, and signs of reduced fibrotic scarring. These findings highlight the potential of nanoparticle-mediated cmRNA delivery targeting two distinct pathways to improve tendon healing, offering a promising alternative to current treatments that often yield suboptimal results.

3D-Printed PCL Scaffolds Loaded with bFGF and BMSCs Enhance Tendon-Bone Healing in Rat Rotator Cuff Tears by Immunomodulation and Osteogenesis Promotion

Rotator cuff tears are the most common conditions in sports medicine and attract increasing attention. Scar tissue healing at the tendon-bone interface results in a high rate of retears, making it a major challenge to enhance the healing of the rotator cuff tendon-bone interface. Biomaterials currently employed for tendon-bone healing in rotator cuff tears still exhibit limited efficacy. As a promising technology, 3D printing enables the customization of scaffold shapes and properties. Bone marrow mesenchymal stem cells (BMSCs) have multidifferentiation potential and valuable immunomodulatory effects. The basic fibroblast growth factor (bFGF), known for its role in proliferation, has been reported to promote osteogenesis. These properties make them applicable in tissue engineering. In this study, we developed a 3D-printed polycaprolactone (PCL) scaffold loaded with bFGF and BMSCs (PCLMF) to restore the tendon-bone interface and regulate the local inflammatory microenvironment. The PCLMF scaffolds significantly improved the biomechanical strength, histological score, and local bone mineral density at regenerated entheses at 2 weeks postsurgery and achieved optimal performance at 8 weeks. Furthermore, PCLMF scaffolds facilitated BMSC osteogenic differentiation and suppressed adipogenic differentiation both in vivo and in vitro. In addition, RNA-seq showed that PCLMF scaffolds could regulate macrophage polarization and inflammation through the MAPK pathway. The implanted scaffold demonstrated excellent biocompatibility and biosafety. Therefore, this study proposes a promising and practical strategy for enhancing tendon-bone healing in rotator cuff tears.

Pharmacological antagonism of Ccr2+ cell recruitment to facilitate regenerative tendon healing

Successful tendon healing requires sufficient deposition and remodeling of new extracellular matrix at the site of injury, with this process mediating in part through fibroblast activation via communication with macrophages. Moreover, resolution of healing requires clearance or reversion of activated cells, with chronic interactions with persistent macrophages impairing resolution and facilitating the conversion to fibrotic healing. As such, modulation of the macrophage environment represents an important translational target to improve the tendon healing process. Circulating monocytes are recruited to sites of tissue injury, including the tendon, via upregulation of cytokines including Ccl2, which facilitates recruitment of Ccr2+ macrophages to the healing tendon. Our prior work has demonstrated that Ccr2-/- can modulate fibroblast activation and myofibroblast differentiation. However, this approach lacked temporal control and resulted in healing impairments. Thus, in the current study we have leveraged a Ccr2 antagonist to blunt macrophage recruitment to the healing tendon in a time-dependent manner. We first tested the effects of Ccr2 antagonism during the acute inflammatory phase and found that this had no effect on the healing process. In contrast, Ccr2 antagonism during the early proliferative/granulation tissue period resulted in significant improvements in mechanical properties of the healing tendon. Collectively, these data demonstrate the temporally distinct impacts of modulating Ccr2+ cell recruitment and Ccr2 antagonism during tendon healing and highlight the translational potential of transient Ccr2 antagonism to improve the tendon healing process.

Mechanisms and new advances in the efficacy of plant active ingredients in tendon-bone healing

The tendon-bone interface, known as the tenosynovial union or attachment, can be easily damaged by excessive exercise or trauma. Tendon-bone healing is a significant research topic in orthopedics, encompassing various aspects of sports injuries and postoperative recovery. Surgery is the most common treatment; however, it has limited efficacy in promoting tendon-bone healing and carries a risk of postoperative recurrence, necessitating the search for more effective treatments. Recently, plant-active ingredients such as tanshinone IIA, astragaloside, ginsenoside Rb1, and resveratrol have garnered significant attention due to their unique advantages in promoting tendon-bone healing. This review outlines the various mechanisms and research progress of these four plant-active ingredients, as well as compound ingredients, in promoting tendon-bone healing. For instance, tanshinone IIA significantly accelerates the healing rate and improves healing quality through anti-inflammatory, antioxidant, and cell proliferation-promoting mechanisms. Astragaloside expedites tendon-bone healing and enhances the mechanical strength of healing tissues primarily through anti-inflammatory, antioxidant, and immunoregulatory effects. Ginsenoside Rb1 enhances local blood supply and facilitates tendon-bone tissue repair through angiogenesis, anti-inflammatory, and antioxidant pathways. Resveratrol protects cellular function and accelerates tissue healing due to its potent antioxidant and anti-inflammatory effects. Additionally, the mechanisms and progress of certain Chinese herbal compound components in tendon-bone healing are outlined. This review concludes that these four plant-active ingredients and herbal compound components promote tendon-bone healing through various mechanisms. The efficacy mechanisms and research progress of these plant-active ingredients are summarized to provide references for clinical treatment and related research.

Surgical outcomes in patients with Achilles tendon rupture-a retrospective study

Background

There are two main methods used to treat Achilles tendon rupture (ATR): conservative treatment and surgical intervention. Surgical techniques are divided into three main categories: open surgical repair, mini-open surgical repair, and percutaneous repair (PR). We aimed to compare clinical outcomes in individuals with ATR who were treated with PR, primary repair, and flexor hallucis longus augmentation (FHL-A) with those treated with V-Y plasty and FHL-A.

Methods

The study involved 54 patients who underwent ATR surgical intervention retrospectively. Thirty-two of these were identified as acute and 22 were chronic rupture patients. PR was performed in 32 patients, primary repair and FHL-A in 14 patients, and V-Y plasty and FHL-A in eight patients.

Results

The mean forward jump was 142.69 ± 7.14 cm in individuals who received PR, 137.71 ± 4.51 cm in those who received primary repair + FHL-A, and 123.88 ± 3.09 cm in those who received V-Y plasty + FHL-A (p < 0.001). The decrease in the mean vertical jump distance on the operated side compared to the contralateral extremity was 0.97 ± 0.93, 2.07 ± 0.99, and 3.00 ± 1.69 cm in individuals who underwent PR, primary repair + FHL-A, and V-Y plasty + FHL-A, respectively (p < 0.001). The decrease in the mean dorsiflexion of the operated side ankle compared to the contralateral extremity was found to be 4.34 ± 1.18, 1.93 ± 1.07, and 2.38 ± 0.92 degrees in individuals who underwent PR, primary repair + FHL-A, and V-Y plasty + FHL-A, respectively (p < 0.001).

Conclusion

Although no surgical technique is completely superior to another, better performance tests were observed after PR repair compared to open surgery in individuals involved in sports, but rerupture, dorsiflexion restriction, and painful ankle were disadvantages. FHL-A, which has gained popularity in recent years, showed better performance in tests by contributing to more stable and stronger ankles in sporting individuals with chronic ruptures who had undergone open surgery. FHL-A can be utilized in addition to primary surgical intervention in individuals with high performance expectations before and after injury.

Downregulation of the PI3K/AKT/mTOR/MMP-13 pathway for promoting interface healing via lubricating microspheres

Interface friction impedes tissue healing and stimulates interface cells to produce matrix metalloproteinases (MMPs); however, the precise mechanisms underlying matrix degradation, and the formation of fibrous scars remain unclear. This research involved the development of interface lubricating microspheres that inhibit the PI3K/AKT/mTOR signaling pathway in tenocytes. This inhibition significantly decreased MMP-13 expression and increased COL-1 production, thereby facilitating interface repair and regeneration. In vitro experiments demonstrated that interface friction activates the PI3K/AKT/mTOR/MMP-13 signaling pathway, while the use of interface lubricating microspheres reduced friction by 78 %, resulting in a threefold decrease in MMP-13 expression through pathway inhibition. Animal studies showed that the application of interface lubricating microspheres reduced friction at the tendon-bone interface, mitigating MMP-13-mediated matrix degradation and effectively reducing fibrous scar formation (as evidenced by decreased α-SMA expression), thus promoting interface healing following ACLR surgery in rats. Consequently, this study suggests that interface friction can trigger the PI3K/AKT/mTOR signaling pathway in tenocytes, leading to increased MMP-13 expression, matrix degradation, and fibrous scar formation. The use of interface lubricating microspheres can enhance interface healing by inhibiting this pathway, offering strategies for improving interface healing and minimizing fibrous scar formation. STATEMENT OF SIGNIFICANCE: Interface healing plays a crucial role following tendon-bone surgeries, yet it is often hindered by challenges such as interface friction and scar formation. In this study, we propose a combined approach in which lubricating microspheres and an anti-matrix degradation drug are used to enhance interface healing. We fabricated novel lubricating microspheres that exhibit outstanding biocompatibility and degradability; these microspheres serve as lubricants for the tendon-bone interface and facilitate the delivery of doxycycline to reduce excessive matrix metalloproteinase (MMP) secretion. The experimental results demonstrated that this method could enhance tendon-bone interface healing in rats, resulting in increased bone formation and higher histological scores than those of the control group. This study represents a preliminary effort to integrate lubrication and anti-matrix degradation in interface healing, potentially offering new insights into the mechanism between interface friction and fibrous scar healing, while promoting interface healing by reducing interfacial friction.

Inflammation-Responsive Functional Core-Shell Micro-Hydrogels Promote Rotator Cuff Tendon-To-Bone Healing by Recruiting MSCs and Immuno-Modulating Macrophages in Rats

Rotator cuff injuries often necessitate surgical intervention, but the outcomes are often unsatisfactory. The underlying reasons can be attributed to multiple factors, with the intricate inflammatory activities and insufficient presence of stem cells being particularly significant. In this study, an innovative inflammation-responsive core-shell micro-hydrogel is designed for independent release of SDF-1 and IL-4 within a single delivery system to promote tendon-to-bone healing by recruiting MSCs and modulating M2 macrophages polarization. First, a MMP-2 responsive hydrogel loaded with IL-4 (GelMA-MMP/IL-4) is synthesized by cross-linking gelatin methacrylate (GelMA) with MMP-2 substrate peptide. Then, the resulting core particles are coated with a shell of chitosan /SDF-1/hyaluronic acid (CS/HA/SDF-1) using the layer-by-layer electrostatic deposition method to form a core-shell micro-hydrogel composite. The core-shell micro-hydrogel shows sustained release of SDF-1 and MMP-2-responsive release of IL-4 associated in situ MSCs homing and smart inflammation regulation by promoting M2 macrophages polarization. Additionally, by injecting these micro-hydrogels into a rat rotator cuff tear and repair model, notable improvements of fibrocartilage layer are observed between tendon and bone. Notably, this study presents a new and potentially powerful environment-responsive drug delivery strategy that offers valuable insights for regulating the intricate micro-environment associated with tissue regeneration.

Challenges in tendon-bone healing: emphasizing inflammatory modulation mechanisms and treatment

Tendons are fibrous connective tissues that transmit force from muscles to bones. Despite their ability to withstand various loads, tendons are susceptible to significant damage. The healing process of tendons and ligaments connected to bone surfaces after injury presents a clinical challenge due to the intricate structure, composition, cellular populations, and mechanics of the interface. Inflammation plays a pivotal role in tendon healing, creating an inflammatory microenvironment through cytokines and immune cells that aid in debris clearance, tendon cell proliferation, and collagen fiber formation. However, uncontrolled inflammation can lead to tissue damage, and adhesions, and impede proper tendon healing, culminating in scar tissue formation. Therefore, precise regulation of inflammation is crucial. This review offers insights into the impact of inflammation on tendon-bone healing and its underlying mechanisms. Understanding the inflammatory microenvironment, cellular interactions, and extracellular matrix dynamics is essential for promoting optimal healing of tendon-bone injuries. The roles of fibroblasts, inflammatory cytokines, chemokines, and growth factors in promoting healing, inhibiting scar formation, and facilitating tissue regeneration are discussed, highlighting the necessity of balancing the suppression of detrimental inflammatory responses with the promotion of beneficial aspects to enhance tendon healing outcomes. Additionally, the review explores the significant implications and translational potential of targeted inflammatory modulation therapies in refining strategies for tendon-bone healing treatments.

Exosomes derived from tendon stem/progenitor cells enhance tendon-bone interface healing after rotator cuff repair in a rat model

The rate of retear after surgical repair remains high. Mesenchymal stem cells (MSCs) have been extensively employed in regenerative medicine for several decades. However, safety and ethical concerns constrain their clinical application. Tendon Stem/Progenitor Cells (TSPCs)-derived exosomes have emerged as promising cell-free therapeutic agents. Therefore, urgent studies are needed to investigate whether TSPC-Exos could enhance tendon-bone healing and elucidate the underlying mechanisms. In this study, TSPC-Exos were found to promote the proliferation, migration, and expression of fibrogenesis markers in BMSCs. Furthermore, TSPC-Exos demonstrated an ability to suppress the polarization of M1 macrophages while promoting M2 macrophage polarization. In a rat model of rotator cuff repair, TSPC-Exos modulated inflammation and improved the histological structure of the tendon-bone interface, the biomechanical properties of the repaired tendon, and the function of the joint. Mechanistically, TSPC-Exos exhibited high expression of miR-21a-5p, which regulated the expression of PDCD4. The PDCD4/AKT/mTOR axis was implicated in the therapeutic effects of TSPC-Exos on proliferation, migration, and fibrogenesis in BMSCs. This study introduces a novel approach utilizing TSPC-Exos therapy as a promising strategy for cell-free therapies, potentially benefiting patients with rotator cuff tear in the future.

Pharmacological Antagonism of Ccr2+ Cell Recruitment to Facilitate Regenerative Tendon Healing

Successful tendon healing requires sufficient deposition and remodeling of new extracellular matrix at the site of injury, with this process mediating in part through fibroblast activation via communication with macrophages. Moreover, resolution of healing requires clearance or reversion of activated cells, with chronic interactions with persistent macrophages impairing resolution and facilitating the conversion the conversion to fibrotic healing. As such, modulation of the macrophage environment represents an important translational target to improve the tendon healing process. Circulating monocytes are recruited to sites of tissue injury, including the tendon, via upregulation of cytokines including Ccl2, which facilitates recruitment of Ccr2+ macrophages to the healing tendon. Our prior work has demonstrated that Ccr2-/- can modulate fibroblast activation and myofibroblast differentiation. However, this approach lacked temporal control and resulted in healing impairments. Thus, in the current study we have leveraged a Ccr2 antagonist to blunt macrophage recruitment to the healing tendon in a time-dependent manner. We first tested the effects of Ccr2 antagonism during the acute inflammatory phase and found that this had no effect on the healing process. In contrast, Ccr2 antagonism during the late inflammatory/ early proliferative period resulted in significant improvements in mechanical properties of the healing tendon. Collectively, these data demonstrate the temporally distinct impacts of modulating Ccr2+ cell recruitment and Ccr2 antagonism during tendon healing and highlight the translational potential of transient Ccr2 antagonism to improve the tendon healing process.

Macrophage Mechano-Responsiveness Within Three-Dimensional Tissue Matrix upon Mechanotherapy-Associated Strains

Mechano-rehabilitation, also known as mechanotherapy, represents the forefront of noninvasive treatment for musculoskeletal (MSK) tissue disorders, encompassing conditions affecting tendons, cartilage, ligaments, and muscles. Recent emphasis has underscored the significance of macrophage presence in the healing of MSK tissues. However, a considerable gap still exists in comprehending how mechanical strains associated with mechanotherapy impact both the naïve and pro-inflammatory macrophage phenotypes within the three-dimensional (3D) tissue matrix, as well as whether the shift in macrophage phenotype is contingent on the mechanical strains inherent to mechanotherapy. In this study, we delineated alterations in mechano-adaptation and polarization of both naive and M1 macrophages within 3D matrices, elucidating their response to varying degrees of mechanical strain exposure (3%, 6%, and 12%). To evaluate macrophage mechano-adaptation and mechano-sensitivity within 3D collagen matrices under mechanical loading, we employed structural techniques (scanning electron microscopy, histology), quantitative morphological measures for phenotypic assessment, and genotypic methods such as quantitative real-time polymerase chain reaction. Our data reveal that the response of macrophages to mechanical loading is not only contingent on their specific sub-phenotype but also varies with the amplitude of mechanical strain. Notably, although supra-mechanical loading (12% strain) was requisite to induce a phenotypic shift in naive (M0) macrophages, as little as 3% mechanical strain proved sufficient to prompt phenotypic alterations in pro-inflammatory (M1) macrophages. These findings pave the way for leveraging the macrophage mechanome in customized and targeted applications of mechanical strain within the mechano-therapeutic framework. Considering the prevalence of MSK tissue injuries and their profound societal and economic implications, the development of well-informed and effective clinical mechanotherapy modalities for MSK tissue healing becomes an imperative endeavor. Impact statement Mechanotherapy is a primary noninvasive treatment for musculoskeletal (MSK) tissue injuries, but the effect of mechanical strain on macrophage phenotypes is not fully understood. A recent study found that macrophage response to mechanical loading is both sub-phenotype specific and amplitude-dependent, with even small strains enough to induce phenotypic changes in pro-inflammatory macrophages. These findings could pave the way for using macrophage mechanome in targeted mechanotherapy applications for better MSK tissue healing.

Mechanome-Guided Strategies in Regenerative Rehabilitation

Regenerative Rehabilitation represents a multifaceted approach that merges mechanobiology with therapeutic intervention to harness the body's intrinsic tissue repair and regeneration capacity. This review delves into the intricate interplay between mechanical loading and cellular responses in the context of musculoskeletal tissue healing. It emphasizes the importance of understanding the phases involved in translating mechanical forces into biochemical responses at the cellular level. The review paper also covers the mechanosensitivity of macrophages, fibroblasts, and mesenchymal stem cells, which play a crucial role during regenerative rehabilitation since these cells exhibit unique mechanoresponsiveness during different stages of the tissue healing process. Understanding how mechanical loading amplitude and frequency applied during regenerative rehabilitation influences macrophage polarization, fibroblast-to-myofibroblast transition (FMT), and mesenchymal stem cell differentiation is crucial for developing effective therapies for musculoskeletal tissues. In conclusion, this review underscores the significance of mechanome-guided strategies in regenerative rehabilitation. By exploring the mechanosensitivity of different cell types and their responses to mechanical loading, this field offers promising avenues for accelerating tissue healing and functional recovery, ultimately enhancing the quality of life for individuals with musculoskeletal injuries and degenerative diseases.

Cartilage fragments combined with BMSCs-Derived exosomes can promote tendon-bone healing after ACL reconstruction

Anterior cruciate ligament reconstruction (ACLR) often fails due to the inability of tendon-bone integration to regenerate normal tissues and formation of fibrous scar tissues in the tendon-bone interface. Cartilage fragments and exosomes derived from bone mesenchymal stromal cells (BMSCs-Exos) can enhance enthesis healing. Nevertheless, the effects on the tendon-bone healing of ACLR remain unknown. This study found that BMSCs-Exos can promote the proliferation of chondrocytes in cartilage fragments, and activated the expression of chondro-related genes SOX9 and Aggrecan. The optimal effect concentration was 1012 events/uL. Besides, BMSCs-Exos could significantly upregulated the expression of BMP7 and Smad5 in cartilage fragments, and further enhanced the expression of chondrogenic genes. Moreover, this study established a rat model of ACLR and implanted the BMSCs-Exos/cartilage fragment complex into the femoral bone tunnel. Results demonstrated that the mean diameters of the femoral bone tunnels were significantly smaller in the BE-CF group than those in the CF group (p = 0.038) and control group (p = 0.007) at 8 weeks after surgery. Besides, more new bone formation was observed in the femoral tunnels in the BE-CF group, as demonstrated by a larger BV/TV ratio based on the reconstructed CT scans. Histological results also revealed the regeneration of tendon-bone structures, especially fibrocartilage. Thus, these findings provide a promising result that BMSCs-Exos/cartilage fragment complex can prevent the enlargement of bone tunnel and promote tendon-bone healing after ACLR, which may have resulted from the regulation of the BMP7/Smad5 signaling axis.

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.

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.

Release of TGF-β3 from Surface-Modified PCL Fiber Mats Triggers a Dose-Dependent Chondrogenic Differentiation of Human Mesenchymal Stromal Cells

The design of implants for tissue transitions remains a major scientific challenge. This is due to gradients in characteristics that need to be restored. The rotator cuff in the shoulder, with its direct osteo-tendinous junction (enthesis), is a prime example of such a transition. Our approach towards an optimized implant for entheses is based on electrospun fiber mats of poly(ε-caprolactone) (PCL) as biodegradable scaffold material, loaded with biologically active factors. Chitosan/tripolyphosphate (CS/TPP) nanoparticles were used to load transforming growth factor-β₃ (TGF-β₃) with increasing loading concentrations for the regeneration of the cartilage zone within direct entheses. Release experiments were performed, and the concentration of TGF-β₃ in the release medium was determined by ELISA. Chondrogenic differentiation of human mesenchymal stromal cells (MSCs) was analyzed in the presence of released TGF-β₃. The amount of released TGF-β₃ increased with the use of higher loading concentrations. This correlated with larger cell pellets and an increase in chondrogenic marker genes (SOX9, COL2A1, COMP). These data were further supported by an increase in the glycosaminoglycan (GAG)-to-DNA ratio of the cell pellets. The results demonstrate an increase in the total release of TGF-β₃ by loading higher concentrations to the implant, which led to the desired biological effect.

Effects of Routine Antithrombotic-Adjusted Dose of Rivaroxaban and Nadroparin Calcium on Tendon Healing of Rats: An Experimental Study

Introduction  Achilles tendon injury necessitates thromboembolism prophylaxis after repair. This study aimed to investigate the effects of antithrombotic-adjusted prophylactic doses of nadroparin calcium and rivaroxaban on Achilles tendon healing. Materials and Methods  Twenty-four young adult male Wistar Albino type rats were randomly divided into three groups. All rats underwent a full-thickness surgical incision of the Achilles tendon, followed by primary repair. After the procedure, group 1 was determined as the control group and received no medication. Group 2 received 2.03 mg/kg rivaroxaban daily via gastric lavage once daily, and group 3 was given subcutaneous 114 IU AXa nadroparin calcium once daily for 28 days. After euthanization, the degrees of inflammation, neovascularization, fibroblastic activity, and collagen fiber sequencing were examined and scored for histopathological evaluation. The Statistical Package for Social Science (SPSS) version 21.0 for Windows software (SPSS, Inc., Chicago, Illinois, United States) was used for all statistical analyses. The number of inflammatory cells, capillary vessels, and fibroblasts, which met the parametric tests' assumptions, were compared between three independent groups by one-way analysis of variance. The significance level was set at p- value < 0.05. Results  Histological examination of the group 1 sample showed the presence of inflammatory cells, an increase in the number of fibroblasts, and sequencing of collagen fibers scattered. The presence of inflammatory cells, remarkable increases in the number of fibroblasts, the presence of mature collagen fibers, and regular sequencing of collagen fibers regular were shown in groups 2 and 3. There were statistically significant differences between the groups regarding the number of inflammatory cells and fibroblasts. In group 2, the number of inflammatory cells was lower than in groups 1 and 3. Elsewhere, the number of fibroblasts was higher in group 1 compared than in groups 2 and 3. Conclusion  Both rivaroxaban and nadroparin calcium in their daily dosage have a beneficial effect on Achilles tendon healing.

The influence of ruptured scar pattern of human anterior cruciate ligament remnant tissue on tendon-bone healing in vivo

The purpose of this study was to determine whether the transplantation of human cells from a non-reattached injured anterior cruciate ligament (ACL) remnant could enhance tendon-bone healing. Human ACL remnant tissue was classified into two groups based on the morphologic pattern as per Crain's classification: (1) non-reattachment group (Crain Ⅳ) and (2) reattachment group (Crain Ⅰ-Ⅲ). Seventy-five 10-week-old immunodeficient rats underwent ACL reconstruction followed by intracapsular administration of one of the following: (1) ACL-derived cells from the non-reattached remnant (non-reattachment group) (n = 5), (2) ACL-derived cells from the reattached tissue (reattachment group) (n = 5), or (3) phosphate-buffered saline (PBS) only (PBS group) (n = 5). Histological (Weeks 2, 4, and 8), immunohistochemical (Week 2), radiographic (Weeks 0, 2, 4, and 8), and biomechanical (Week 8) assessments were performed. Histological evaluation showed high and early healing, induction of endochondral ossification-like integration, and mature bone ingrowth at Week 4 in the non-reattachment group. Microcomputed tomography at Week 4 showed that the tibial bone tunnels in the non-reattachment group were significantly reduced compared to those in the reattachment and PBS groups. Moreover, biomechanical testing showed that ultimate load-to-failure in the non-reattachment group tended to be larger than that in the reattachment group, though not statistically significant. The enhanced healing potential in the non-reattachment group was explained by the increase in intrinsic angiogenesis/osteogenesis. In the subacute phase, the ACL-derived cells with the non-reattached morphologic pattern showed greater and earlier tendon bone healing compared with the cells obtained from the reattached morphologic pattern.

Mesenchymal stem cells and macrophages and their interactions in tendon-bone healing

Tendon-bone insertion injuries (TBI), such as anterior cruciate ligament (ACL) and rotator cuff injuries, are common degenerative or traumatic pathologies with a negative impact on the patient's daily life, and they cause huge economic losses every year. The healing process after an injury is complex and is dependent on the surrounding environment. Macrophages accumulate during the entire process of tendon and bone healing and their phenotypes progressively transform as they regenerate. As the "sensor and switch of the immune system", mesenchymal stem cells (MSCs) respond to the inflammatory environment and exert immunomodulatory effects during the tendon-bone healing process. When exposed to appropriate stimuli, they can differentiate into different tissues, including chondrocytes, osteocytes, and epithelial cells, promoting reconstruction of the complex transitional structure of the enthesis. It is well known that MSCs and macrophages communicate with each other during tissue repair. In this review, we discuss the roles of macrophages and MSCs in TBI injury and healing. Reciprocal interactions between MSCs and macrophages and some biological processes utilizing their mutual relations in tendon-bone healing are also described. Additionally, we discuss the limitations in our understanding of tendon-bone healing and propose feasible ways to exploit MSC-macrophage interplay to develop an effective therapeutic strategy for TBI injuries.

The Translational potential of this article

This paper reviewed the important functions of macrophages and mesenchymal stem cells in tendon-bone healing and described the reciprocal interactions between them during the healing process. By managing macrophage phenotypes, mesenchymal stem cells and the interactions between them, some possible novel therapies for tendon-bone injury may be proposed to promote tendon-bone healing after restoration surgery.

Therapeutic potential and mechanisms of mesenchymal stem cell-derived exosomes as bioactive materials in tendon-bone healing

Tendon-bone insertion (TBI) injuries, such as anterior cruciate ligament injury and rotator cuff injury, are the most common soft tissue injuries. In most situations, surgical tendon/ligament reconstruction is necessary for treating such injuries. However, a significant number of cases failed because healing of the enthesis occurs through scar tissue formation rather than the regeneration of transitional tissue. In recent years, the therapeutic potential of mesenchymal stem cells (MSCs) has been well documented in animal and clinical studies, such as chronic paraplegia, non-ischemic heart failure, and osteoarthritis of the knee. MSCs are multipotent stem cells, which have self-renewability and the ability to differentiate into a wide variety of cells such as chondrocytes, osteoblasts, and adipocytes. Numerous studies have suggested that MSCs could promote angiogenesis and cell proliferation, reduce inflammation, and produce a large number of bioactive molecules involved in the repair. These effects are likely mediated by the paracrine mechanisms of MSCs, particularly through the release of exosomes. Exosomes, nano-sized extracellular vesicles (EVs) with a lipid bilayer and a membrane structure, are naturally released by various cell types. They play an essential role in intercellular communication by transferring bioactive lipids, proteins, and nucleic acids, such as mRNAs and miRNAs, between cells to influence the physiological and pathological processes of recipient cells. Exosomes have been shown to facilitate tissue repair and regeneration. Herein, we discuss the prospective applications of MSC-derived exosomes in TBI injuries. We also review the roles of MSC-EVs and the underlying mechanisms of their effects on promoting tendon-bone healing. At last, we discuss the present challenges and future research directions.

Comparison of cortical versus cancellous bone fixation in tendon-to-bone healing with a rat trans-calcaneal suture model for Achilles tendon sleeve avulsion

BACKGROUND

Trans-calcaneal suture technique is an economical and effective method for repairing Achilles tendon sleeve avulsion. Whether cancellous bone fixation upon this technique could accelerate tendon-to-bone healing is unknown. The purpose of this study is to compare the effect of cortical versus cancellous bone fixation on tendon-bone healing with a novel rat trans-calcaneal suture model.

METHODS

Trans-calcaneal suture treatment was carried out on the right hindlimb in male Sprague-Dawley rats (N = 80). They were randomly divided into the cortical group (Achilles fixed to the calcaneal cortical bone, n = 40) and the cancellous group (Achilles fixed to the calcaneal cancellous bone, n = 40). Gait analysis and immunohistochemistry were performed 1, 4, 7, and 14 days after the operation. Gross observation, biomechanical analysis, micro-CT, and histological analysis were performed 4 and 8 weeks after surgery. Independent-samples t tests were used for comparison between groups.

RESULTS

At 1, 4, and 7 days, the swing time of the affected limb in the cancellous group decreased, while the duty cycle, the maximum contact area, the print area, and the mean intensity increased significantly. The cross-sectional area of the tendon-bone junction in the cancellous group was smaller, and the failure load and stiffness were higher 4 weeks after the operation. The cancellous group showed more proportion of new bone and a relatively well-organized and dense connective tissue interface with better fibrocartilage-like tissue at 4 weeks after the operation. The ratio of ED2 + macrophages in the cancellous group was significantly higher than in the cortical group on 1, 4, 7, and 14 days. There were no significant differences in gait at 2 weeks, in appearance, biomechanics, new bone formation, and histology at 8 weeks after surgery between the two groups.

CONCLUSION

In the new rat trans-calcaneal suture model, cancellous fixation can accelerate tendon-to-bone healing in the early stage, which perhaps is related to the abundant bone marrow tissue in the cancellous bone that modulates the inflammatory processes.

Injectable bioactive glass/sodium alginate hydrogel with immunomodulatory and angiogenic properties for enhanced tendon healing

Tendon healing is a complex process involving inflammation, proliferation, and remodeling, eventually achieving a state of hypocellularity and hypovascularity. Currently, few treatments can satisfactorily restore the structure and function of native tendon. Bioactive glass (BG) has been shown to possess immunomodulatory and angiogenic properties. In this study, we investigated whether an injectable hydrogel fabricated of BG and sodium alginate (SA) could be applied to enhance tenogenesis following suture repair of injured tendon. We demonstrated that BG/SA hydrogel significantly accelerated tenogenesis without inducing heterotopic ossification based on histological analysis. The therapeutic effect could attribute to increased angiogenesis and M1 to M2 phenotypic switch of macrophages within 7 days post-surgery. Morphological characterization demonstrated that BG/SA hydrogel partially reverted the pathological changes of Achilles tendon, including increased length and cross-sectional area (CSA). Finally, biomechanical test showed that BG/SA hydrogel significantly improved ultimate load, failure stress, and tensile modulus of the repaired tendon. In conclusion, administration of an injectable BG/SA hydrogel can be a novel and promising therapeutic approach to augment Achilles tendon healing in conjunction with surgical intervention.

Vitamin D Metabolic Pathway Components in Orthopedic Patientes-Systematic Review

Vitamin D takes part in the functioning of many processes that ensure the homeostasis of the body. In orthopedics, it is indicated as an inseparable element ensuring proper bone growth and functioning, and its deficiencies are indicated in various diseases, mainly in the proper structure and function of the skeleton. In this review, we focus on the most important components of the vitamin D metabolic pathway, in correlation with selected orthopedic conditions. Records were obtained from the PubMed database in a timeline of 2010-2022. The keywords were as follows: vitamin D/cholesterol/vitamin D binding protein/ VDBP/Cytochrome/CYP24A1/CYP 27B1/Vitamin D receptor/VDR/ + diseases (ACL reconstruction, rotator cuff, arthroplasty knee/hip/shoulder). The recent original studies were analyzed, discussed, and the most important data were shown. The vast majority of articles concern the metabolite of vitamin D (25(OH)D), which is measured as a standard in diagnostic laboratories. Even though there is a lot of valuable information in the literature, we believe that the other elements of the vitamin D pathway also deserve attention and suggest their research in correlation with orthopedic disorders to supplement the missing knowledge on this topic.

High-purity magnesium screws modulate macrophage polarization during the tendon-bone healing process in the anterior cruciate ligament reconstruction rabbit model

Magnesium (Mg) screws perform clinical potential in anterior cruciate ligament (ACL) reconstruction, and promote fibrocartilaginous entheses regeneration at the femoral entrance. We aim to prove that high-purity Magnesium (HP Mg) screws modulate macrophage polarization in fibrocartilage interface regeneration both in vitro and in vivo. HP Mg extracts performed good cytocompatibility and significantly promoted M2 macrophage polarization in the flow cytometry and ELISA assays. M2 macrophages stimulated fibrochondrocyte differentiation of co-cultured hBMSCs, and HP Mg extracts had synergistic effect on the process. Then we applied HP Mg screws, with Ti screws as control, in the ACL reconstruction rabbit model. In the histological and immunofluorescence analysis, HP Mg screws inhibited M1 polarization at 2 weeks and highly promoted M2 polarization at 2 and 4 weeks at the tendon-bone interface. Furthermore, regeneration of fibrocartilaginous entheses, rather than the fibrovascular scar interface, was detected in the HP Mg group at 12 weeks. For further mechanism study via RNA-seq detection and WB assays, we found that AKT1 was highly activated in M2 polarization, and HP Mg could stimulate AKT1 expression, rather than AKT2, in the early phase of tendon-bone healing. Our study elucidated macrophage polarization during tendon-bone healing process and emphasized HP Mg on M2 polarization and fibrocartilage interface regeneration via the selective activation of AKT1 and PI3K/AKT pathway.

Joint effusion at 3 months after anterior cruciate ligament reconstruction is associated with reinjury

PURPOSE

To evaluate whether joint effusion at 3 months after anterior cruciate ligament (ACL) reconstruction is associated with ACL reinjury.

METHODS

The medical records of 227 consecutive patients who underwent single-bundle ACL reconstruction between 2015 and 2018 were reviewed in this retrospective single-center study. Demographic data such as sex and age at surgery, as well as data on preinjury Tegner activity scale score, time from injury to surgery, presence of meniscus and cartilage injuries, and the occurrence of ACL reinjury within 2 years, were collected. Joint effusion was defined as grade 3 (range 0-3) according to the ACL Osteoarthritis Score by magnetic resonance imaging at 3 months postoperatively. Multivariate logistic regression analysis was performed to control for potential confounders.

RESULTS

A total of 176 patients (mean age 22.5 ± 9.9 years) were included. Among these patients, 18 (10.2%) had ACL reinjury. At the multivariate logistic regression analysis, higher Tegner activity scale (odds ratio [OR] 3.12; 95% confidence interval [CI] 1.61-6.04; p < 0.001) and presence of joint effusion (OR 34.5; 95% CI 6.63-179.7; p < 0.001) increased the odds of ACL reinjury, and older age (OR 0.68; 95% CI 0.51-0.92; p = 0.012) decreased the odds of ACL reinjury.

CONCLUSIONS

Joint effusion with a larger fluid volume at 3 months postoperatively was one of the risk factors for ACL reinjury independent of confounders, such as age and activity level. This result suggests the possibility of postoperative intervention for ACL reinjury.

LEVEL OF EVIDENCE

III.

BMSC-derived exosomes promote tendon-bone healing after anterior cruciate ligament reconstruction by regulating M1/M2 macrophage polarization in rats

Background

Recent studies have shown that bone marrow stromal cell-derived exosomes (BMSC-Exos) can be used for tissue repair. However, whether the BMSC-Exos can promote tendon-bone healing after anterior cruciate ligament reconstruction (ACLR) is still unclear. In this study, we observed in vivo and in vitro the effect of rat BMSC-Exos on tendon-bone healing after ACLR and its possible mechanism.

Methods

Highly expressed miRNAs in rat BMSC-Exos were selected by bioinformatics and verified in vitro. The effect of overexpressed miRNA in BMSC-Exos on M2 macrophage polarization was observed. A rat model of ACLR was established. The experimental components were divided into three groups: the control group, the BMSC-Exos group, and the BMSC-Exos with miR-23a-3p overexpression (BMSC-Exos mimic) group. Biomechanical tests, micro-CT, and histological staining were performed for analysis.

Results

Bioinformatics analysis showed that miR-23a-3p was highly expressed in rat BMSC-Exos and could target interferon regulatory factor 1 (IRF1, a crucial regulator in M1 macrophage polarization). In vitro, compared with the control group or the BMSC-Exos group, the BMSC-Exos mimic more significantly promoted the polarization of macrophages from M1 to M2. In vivo, at 2 weeks, the number of M2 macrophages in the early local stage of ACLR was significantly increased in the BMSC-Exos mimic group; at 4 and 8 weeks, compared with the control group or the BMSC-Exos group, the bone tunnels of the tibia and femur sides of the rats in the BMSC-Exos mimic group were significantly smaller, the interface between the graft and the bone was narrowed, the bone volume/total volume ratio (BV/TV) increased, the collagen type II alpha 1 level increased, and the mechanical strength increased.

Conclusions

BMSC-Exos promoted M1 macrophage to M2 macrophage polarization via miR-23a-3p, reduced the early inflammatory reaction at the tendon-bone interface, and promoted early healing after ACLR.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02975-0.

Exosomes derived from magnetically actuated bone mesenchymal stem cells promote tendon-bone healing through the miR-21-5p/SMAD7 pathway

Graft healing after anterior cruciate ligament reconstruction (ACLR) involves slow biological processes, and various types of biological modulations have been explored to promote tendon-to-bone integration. Exosomes have been extensively studied as a promising new cell-free strategy for tissue regeneration, but few studies have reported their potential in tendon-to-bone healing. In this study, a novel type of exosome derived from magnetically actuated (iron oxide nanoparticles (IONPs) combined with a magnetic field) bone mesenchymal stem cells (BMSCs) (IONP-Exos) was developed, and the primary purpose of this study was to determine whether IONP-Exos exert more significant effects on tendon-to-bone healing than normal BMSC-derived exosomes (BMSC-Exos). Here, we isolated and characterized the two types of exosomes, conducted in vitro experiments to measure their effects on fibroblasts (NIH3T3), and performed in vivo experiments to compare the effects on tendon-to-bone integration. Moreover, functional exploration of exosomal miRNAs was further performed by utilizing a series of gain- and loss-of-function experiments. Experimental results showed that both BMSC-Exos and IONP-Exos could be shuttled intercellularly into NIH3T3 fibroblasts and enhanced fibroblast activity, including proliferation, migration, and fibrogenesis. In vivo, we found that IONP-Exos significantly prevented peri-tunnel bone loss, promoted more osseous ingrowth into the tendon graft, increased fibrocartilage formation at the tendon-bone tunnel interface, and induced a higher maximum load to failure than BMSC-Exos. Furthermore, overexpression of miR-21-5p remarkably enhanced fibrogenesis in vitro, and SMAD7 was shown to be involved in the promotive effect of IONP-Exos on tendon-to-bone healing. Our findings may provide new insights into the regulatory roles of IONPs in IONP-Exos communication via stimulating exosomal miR-21-5p secretion and the SMAD7 signaling pathway in the fibrogenic process of tendon-to-bone integration. This work could provide a new strategy to promote tendon-to-bone healing for tissue engineering in the future.

Changes in Macrophage Polarization During Tendon-to-Bone Healing After ACL Reconstruction With Insertion-Preserved Hamstring Tendon: Results in a Rabbit Model

BACKGROUND

Decreasing the proinflammatory M1 macrophages or shifting the polarization status from M1 to M2 phenotype is thought to be beneficial for tendon-to-bone healing. In anterior cruciate ligament reconstruction (ACLR), using an insertion-preserved hamstring tendon (IP-HT) graft compared with a free hamstring tendon (FHT) graft has been shown to reduce graft necrosis and improve healing. However, the role of macrophage polarization at the tendon-to-bone interface is unclear.

HYPOTHESIS

ACLR using IP-HT graft would facilitate the phenotype shift from M1 to M2 macrophages at the tendon-to-bone interface.

STUDY DESIGN

Controlled laboratory study.

METHODS

Unilateral ACLR was performed on 42 healthy New Zealand White rabbits (study group, 21 rabbits with IP-HT graft; control group, 21 rabbits with FHT graft). At days 1, 3, and 7 and weeks 3, 6, 12, and 24 postoperatively, 3 rabbits in each group were sacrificed to investigate and compare the expression of surrogate markers for M1 macrophages (inducible nitric oxide synthase [iNOS] and tumor necrosis factor α [TNF-α]) and M2 macrophages (CD206 and transforming growth factor β [TGF-β]) via immunohistochemical staining and evaluation.

RESULTS

In the control group, the percentage of iNOS- and TNF-α-positive cells from postoperative day 7 and week 3 increased then decreased by week 6; positive expression of CD206 and TGF-β was weaker and peaked at 3 weeks postoperatively. In the study group, high CD206- and TGF-β-positive expression was observed from weeks 3 to 12 and peaked at week 6, and positive expression of iNOS- and TNF-α was weaker and peaked on day 7. At both 7 days and 3 weeks, the percentages of iNOS- and TNF-α-positive cells in the control group were both significantly higher than in the study group (P ≤ .04 for all). At 6 weeks, the percentages of CD206- and TGF-β-positive cells in the study group were both significantly higher than in the control group (P = .02 and P = .04, respectively).

CONCLUSION

More expression of surrogate markers for M2 macrophages was observed in the tendon-to-bone healing process after ACLR using IP-HT versus FTP graft.

CLINICAL RELEVANCE

Using IP-HT grafts in ACLR may facilitate postoperative healing by shifting the local status of macrophage polarization at the tendon-to-bone interface.

Distinct inflammatory macrophage populations sequentially infiltrate bone‐to‐tendon interface tissue after ACL reconstruction surgery in mice

Macrophages are important for repair of injured tissues, but their role in healing after surgical repair of musculoskeletal tissues is not well understood. We used single‐cell RNA sequencing (RNA‐seq), flow cytometry, and transcriptomics to characterize functional phenotypes of macrophages in a mouse anterior cruciate ligament reconstruction (ACLR) model that involves bone injury followed by a healing phase of bone and fibrovascular interface tissue formation that results in bone‐to‐tendon attachment. We identified a novel “surgery‐induced” highly inflammatory CD9+ IL1+ macrophage population that expresses neutrophil‐related genes, peaks 1 day after surgery, and slowly resolves while transitioning to a more homeostatic phenotype. In contrast, CX3CR1+ CCR2+ macrophages accumulated more slowly and unexpectedly expressed an interferon signature, which can suppress bone formation. Deletion of Ccr2 resulted in an increased amount of bone in the surgical bone tunnel at the tendon interface, suggestive of improved healing. The “surgery‐induced macrophages” identify a new cell type in the early phase of inflammation related to bone injury, which in other tissues is dominated by blood‐derived neutrophils. The complex patterns of macrophage and inflammatory pathway activation after ACLR set the stage for developing therapeutic strategies to target specific cell populations and inflammatory pathways to improve surgical outcomes. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Time-series biological responses toward decellularized bovine tendon graft and autograft for 52 consecutive weeks after rat anterior cruciate ligament reconstruction

There is an essential demand for developing biocompatible grafts for knee anterior cruciate ligament reconstruction (ACLR). This study investigated cell infiltration into decellularized bovine tendon xenografts using a rat knee ACLR model. Twelve-week-old Sprague–Dawley rats were used. At weeks 1, 2, 4, 8, 16, 26, and 52 (each period, n = 6) after ACLR, rats receiving decellularized bovine tendon (group D, n = 42) or autologous tendon (group A, n = 42) as grafts underwent peritibial bone tunnel bone mineral density (BMD), histological, and immunohistological assessments. BMD increased over time in both the groups until week 16 and then remained unchanged without exhibiting significant differences between the groups. Initially, cellularity in group D was lower than that in group A; however, by weeks 4–8, both the groups were comparable to the native anterior cruciate ligament group and cellularity remained unchanged until week 52. Initially, group A had more M1 macrophages, indicating inflammation, whereas group D had more M2 macrophages, indicating tissue regeneration. Nonetheless, the M1 and M2 macrophage counts of both the groups were comparable at most times. This study revealed the excellent recellularization and tendon–bone integration abilities of decellularized tendons using a cross-species model.

Small Leucine-Rich Proteoglycans in Tendon Wound Healing

Significance: Tendon injury possesses a high morbidity rate and is difficult to achieve a satisfying prognosis with currently available treatment strategies. Current approaches used for tendon healing always lead to the formation of fibrovascular scar tissue, which significantly compromises the biomechanics of the healed tendon. Moreover, the related functional deficiency deteriorates over time with an increased injury recurrence risk. Small leucine-rich proteoglycans (SLRPs) link and interact with collagen fibrils to regulate tendon structure and biomechanics, which can provide a new and promising method in the field of tendon injury management. Recent Advances: The effect of SLRPs on tendon development has been extensively investigated. SLRP deficiency impairs tendon collagen fibril structure and biomechanic properties, while administration of SLRPs generally benefits tendon wound healing and regains better mechanical properties. Critical Issues: Current knowledge on the role of SLRPs in tendon development and regeneration mostly comes from uninjured knockout mice, and mainly focuses on the morphology description of collagen fibril profile and mechanical properties. Little is known about the regulatory mechanism on the molecular level. Future Directions: This article reviews the current knowledge in this highly translational topic and provides an evidence-based conclusion, thereby encouraging in-depth investigations of SLRPs in tendons and the development of SLRP-based treatments for desired tendon healing.

Infrapatellar Fat Pad Mesenchymal Stromal Cell-Derived Exosomes Accelerate Tendon-Bone Healing and Intra-articular Graft Remodeling After Anterior Cruciate Ligament Reconstruction

BACKGROUND

Exosomes derived from mesenchymal stromal cells (MSCs) reportedly enhance the healing process. However, no studies have investigated the effect of exosomes from infrapatellar fat pad (IPFP) MSCs on tendon-bone healing and intra-articular graft remodeling after anterior cruciate ligament reconstruction (ACLR).

PURPOSE

To evaluate the in vivo effect of exosomes from IPFP MSCs on tendon-bone healing and intra-articular graft remodeling in a rat model of ACLR.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 90 skeletally mature male Sprague Dawley rats underwent unilateral ACLR using an autograft. All rats were randomly divided into 3 groups: sham injection (SI) group (n = 30), control injection (CI) group (n = 30), and IPFP MSC-derived exosome injection (IMEI) group (n = 30). At 2, 4, and 8 weeks postoperatively, tendon-bone healing and intra-articular graft remodeling were evaluated via biomechanical testing, micro-computed tomography, and histological analysis; macrophage polarization was evaluated using immunohistochemical staining.

RESULTS

Biomechanical testing demonstrated a significantly higher failure load and stiffness in the IMEI group than in the SI and CI groups at 4 and 8 weeks postoperatively. Moreover, a thinner graft-to-bone healing interface with more fibrocartilage was observed in the IMEI group at both time points. Micro-computed tomography revealed greater new bone ingrowth in the IMEI group than in the other groups, as demonstrated by smaller mean bone tunnel areas and a larger bone volume/total volume ratio. Additionally, more cellular infiltration was observed in the intra-articular graft in the IMEI group than in the other groups at 4 weeks, followed by more regularly organized fibers with mature collagen at 8 weeks. Notably, similar trends of macrophage polarization were found at both the graft-to-bone interface and the intra-articular graft in the IMEI group, with significantly fewer proinflammatory M1 macrophages and larger numbers of reparative M2 macrophages than in the SI and CI groups.

CONCLUSION

IPFP MSC-derived exosomes accelerated tendon-bone healing and intra-articular graft remodeling after ACLR, which may have resulted from the immunomodulation of macrophage polarization.

CLINICAL RELEVANCE

The IPFP can be easily harvested by most orthopaedic surgeons. Exosomes from IPFP MSCs, constituting a newly emerging cell-free approach, may represent a treatment option for improving tendon-bone healing and intra-articular graft remodeling after ACLR.

Tackling the Challenges of Graft Healing After Anterior Cruciate Ligament Reconstruction-Thinking From the Endpoint

Anterior cruciate ligament (ACL) tear is common in sports and accidents, and accounts for over 50% of all knee injuries. ACL reconstruction (ACLR) is commonly indicated to restore the knee stability, prevent anterior–posterior translation, and reduce the risk of developing post-traumatic osteoarthritis. However, the outcome of biological graft healing is not satisfactory with graft failure after ACLR. Tendon graft-to-bone tunnel healing and graft mid-substance remodeling are two key challenges of biological graft healing after ACLR. Mounting evidence supports excessive inflammation due to ACL injury and ACLR, and tendon graft-to-bone tunnel motion negatively influences these two key processes. To tackle the problem of biological graft healing, we believe that an inductive approach should be adopted, starting from the endpoint that we expected after ACLR, even though the results may not be achievable at present, followed by developing clinically practical strategies to achieve this ultimate goal. We believe that mineralization of tunnel graft and ligamentization of graft mid-substance to restore the ultrastructure and anatomy of the original ACL are the ultimate targets of ACLR. Hence, strategies that are osteoinductive, angiogenic, or anti-inflammatory should drive graft healing toward the targets. This paper reviews pre-clinical and clinical literature supporting this claim and the role of inflammation in negatively influencing graft healing. The practical considerations when developing a biological therapy to promote ACLR for future clinical translation are also discussed.

Ligamentous healing potential of the acromioclavicular ligament following acute anatomical reconstruction

BACKGROUND

Horizontal instability following acute acromioclavicular joint (ACJ) reconstruction still occurs with a high prevalence. Although the human acromioclavicular ligament complex (ACLC) represents the major horizontal ACJ stabilizer, experimental studies on healing characteristics are lacking. Therefore, the purpose of this histological study was to investigate the healing potential of the ACLC following acute anatomical reconstruction METHODS: In this prospective clinical-experimental study, 28 ACLC biopsies were performed in patients with complete ACJ dislocations (Rockwood type 4 or 5) during acute hook plate stabilization (IG: implantation group; n = 14) and hook plate removal (EG: explantation group; n = 14). Histological analyses included Giemsa staining, polarized light microscopy and immunostaining against CD68, αSMA and collagen type I and type III. Histomorphological evaluation entailed cell counts, collagen expression score, ligament tissue maturity index (LTMI) and descriptive analysis of specific ligamentous structures. Statistics consisted of nonparametric Mann-Whitney U tests and a level of significance of P < .05.

RESULTS

Total cell counts (cells/mm² 1491 ± 296 vs. 635 ± 430; P < 0.001) and collagen III expression (3.22 ± 0.22 vs. 1.78 ± 0.41; P < 0.001) were higher in EG compared to IG. Inversely αSMA + (11 ± 9 vs. 179 ± 186; P < 0.001) and CD68 + cell counts (56 ± 20 vs. 100 ± 57; P 0.009) were significantly lower in the EG. The EG revealed a comparable reorientation of ligamentous structures. Consistently, ACLC samples of the EG (21.6 ± 2.4) displayed a high total but differently composed LTMI score (IG: 24.5 ± 1.2; P < 0.001).

CONCLUSIONS

This experimental study proved the ligamentous healing potential of the human ACLC following acute anatomical reconstruction. Histomorphologically, the ACLC reliably showed a ligamentous state of healing at a mean of about 12 weeks after surgery. However, processes of ligamentous remodeling were still evident. These experimental findings support recent clinical data showing superior horizontal ACJ stability with additional AC stabilization in the context of acute ACJ reconstruction. Though, prospective clinical and biomechanical studies are warranted to evaluate influencing factors on ACLC healing and potential impacts of acute ACLC repair on clinical outcome.

STUDY TYPE

Controlled Laboratory Study.

Immunologic Contributions Following Rotator Cuff Injury and Development of Cuff Tear Arthropathy

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Rotator cuff tear arthropathy (RCTA) describes a pattern of glenohumeral degenerative changes following chronic rotator cuff tears that is characterized by superior humeral head migration, erosion of the greater tuberosity of the humeral head, contouring of the coracoacromial arch to create a socket for the humeral head, and eventual glenohumeral arthritis.

»

Acute and chronic inflammatory changes following rotator cuff tears are thought to contribute to cartilage damage, muscle fibrosis, and fatty infiltration in the glenohumeral joint.

»

In vitro animal studies targeting various inflammatory modulators, including macrophages, insulin-like growth factor-I, and transforming growth factor-beta pathways, provide promising therapeutic targets to improve healing after rotator cuff tears.

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The role of platelet-rich plasma in the treatment and prevention of RCTA has been investigated, with conflicting results.

Preventing postoperative adhesions after hand tendon repair using acellular dermal matrix

AIMS

Postoperative tendon adhesions contribute to functional disability and reconstructive failure. In this study, we present the long-term outcomes of a prospective study in which acellular dermal matrix (ADM) was used to prevent postoperative adhesion after tendon injury.

METHODS

The study was conducted between March 2014 and February 2017. Patients, aged 19-65 years, with an acute single flexor tendon injury in zones 1 or 2, distal to the palmar digital crease were candidates for the study. Patients were allocated to either an ADM treatment group or a control group without ADM treatment.

RESULTS

A total of 37 patients were enrolled in the study: 21 patients in the ADM group and 16 patients in the control group. At six months after surgery, the range of motion in the proximal interphalangeal joint was 81.0±5.1 degrees in the ADM group and 75.8±6.9 degrees in the control group. The range of motion in the distal interphalangeal joint was 79.9±7.1 in the ADM group and 71.2±5.7 degrees in the control group, with significant difference (p=0.03 and p<0.05, respectively). In addition, the total active motion was higher in the ADM group than in the control group. The patients' scores on the Patient Satisfaction Questionnaire were also significantly different, with higher satisfaction scores in the ADM group (p=0.02). The minimal follow-up period was six months.

CONCLUSION

The use of ADM after tendon repair has the potential to significantly improve the outcome of tendon surgery in terms of range of motion.

DECLARATION OF INTEREST

None of the authors has any financial interest in the products, devices, or drugs mentioned in this article.

Single-Cell Integration Analysis of Heterotopic Ossification and Fibrocartilage Developmental Lineage: Endoplasmic Reticulum Stress Effector Xbp1 Transcriptionally Regulates the Notch Signaling Pathway to Mediate Fibrocartilage Differentiation

INTRODUCTION

Regeneration of fibrochondrocytes is essential for the healing of the tendon-bone interface (TBI), which is similar to the formation of neurogenic heterotopic ossification (HO). Through single-cell integrative analysis, this study explored the homogeneity of HO cells and fibrochondrocytes.

METHODS

This study integrated six datasets, namely, GSE94683, GSE144306, GSE168153, GSE138515, GSE102929, and GSE110993. The differentiation trajectory and key transcription factors (TFs) for HO occurrence were systematically analyzed by integrating single-cell RNA (scRNA) sequencing, bulk RNA sequencing, and assay of transposase accessible chromatin seq. The differential expression and enrichment pathways of TFs in heterotopically ossified tissues were identified.

RESULTS

HO that mimicked pathological cells was classified into HO1 and HO2 cell subsets. Results of the pseudo-temporal sequence analysis suggested that HO2 is a differentiated precursor cell of HO1. The analysis of integrated scRNA data revealed that ectopically ossified cells have similar transcriptional characteristics to cells in the fibrocartilaginous zone of tendons. The modified SCENIC method was used to identify specific transcriptional regulators associated with ectopic ossification. Xbp1 was defined as a common key transcriptional regulator of ectopically ossified tissues and the fibrocartilaginous zone of tendons. Subsequently, the CellPhoneDB database was completed for the cellular ligand-receptor analysis. With further pathway screening, this study is the first to propose that Xbp1 may upregulate the Notch signaling pathway through Jag1 transcription. Twenty-four microRNAs were screened and were found to be potentially associated with upregulation of XBP1 expression after acute ischemic stroke.

CONCLUSION

A systematic analysis of the differentiation landscape and cellular homogeneity facilitated a molecular understanding of the phenotypic similarities between cells in the fibrocartilaginous region of tendon and HO cells. Furthermore, by identifying Xbp1 as a hub regulator and by conducting a ligand-receptor analysis, we propose a potential Xbp1/Jag1/Notch signaling pathway.

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.

Exosomes From M2 Macrophage Promote Peritendinous Fibrosis Posterior Tendon Injury via the MiR-15b-5p/FGF-1/7/9 Pathway by Delivery of circRNA-Ep400

Achilles tendon rupture prognosis is usually unsatisfactory. After the tendon is injured, it may not function properly because of the fibrotic healing response, which restrains tendon motion. Inflammatory monocytes and tissue-resident macrophages are indispensable regulators in tissue repair, fibrosis, and regeneration. Exosomes from macrophages are crucial factors in tissue microenvironment regulation following tissue injury. This study therefore aimed to clarify the roles of macrophage exosomes in tendon injury (TI) repair. The results show that macrophages play a role after TI. M1 macrophages were increased relative to peritendinous fibrosis after TI. High-throughput sequencing showed abnormal expression of circular RNAs (circRNAs) between exosomes from M2 and M0 macrophages. Among the abnormal expressions of circRNA, circRNA-Ep400 was significantly increased in M2 macrophage exosomes. The results also show that M2 macrophage-derived circRNA-Ep400-containing exosomes are important for promoting peritendinous fibrosis after TI. Bioinformatics and dual-luciferase reporting experiments confirmed that miR-15b-5p and fibroblast growth factor (FGF)-1/7/9 were downstream targets of circRNA-Ep400. High circRNA-Ep400-containing exosome treatment inhibited miR-15b-5p, but promoted FGF1/7/9 expression in both fibroblasts and tenocytes. Furthermore, high circRNA-Ep400-containing exosome treatment promoted fibrosis, proliferation, and migration in both fibroblasts and tenocytes. Taken together, the results show that M2 macrophage-derived circRNA-Ep400-containing exosomes promote peritendinous fibrosis after TI via the miR-15b-5p/FGF-1/7/9 pathway, which suggests novel therapeutics for tendon injury treatment.

Effect of co-administration of atelocollagen and hyaluronic acid on rotator cuff healing

Background

This study aimed to evaluate the co-administration effect of atelocollagen combined with hyaluronic acid (HA) injections for treatment of full-thickness rotator cuff tear (RCT).

Methods

Eighty patients who underwent arthroscopic rotator cuff repair for full-thickness RCT from March 2018 to November 2019 were enrolled. The patients were randomly allocated to the following groups: combined atelocollagen and HA injection (group I, n=28), only HA injection (group II, n=26), and no injection (group III, n=26). Clinical outcomes were assessed at 3, 6, and 12 months after surgery using the American Shoulder and Elbow Surgeons score, visual analog scale pain score , functional scores (pain visual analog scale, function visual analog score), and range of motion. Magnetic resonance imaging was performed 12 months after surgery to evaluate rotator cuff integrity.

Results

Preoperative demographic data and postoperative clinical outcomes did not differ significantly among the three groups (p>0.05). However, in group I, the number of steroid injections after surgery was significantly lower than that in the other groups (p=0.011). The retear rate on follow-up magnetic resonance imaging was significantly higher in group II (9.5%, n=2) and group III (13.6%, n=3) than in group I (0%) (p=0.021).

Conclusions

Co-administration of atelocollagen and HA improves healing of the rotator cuff and increases the integrity of the rotator cuff repair site. This study provides encouraging evidence for use of combined atelocollagen-HA injections to treat patients with full-thickness RCT.

Immune and repair responses in joint tissues and lymph nodes after knee arthroplasty surgery in mice

The importance of a local tissue immune response in healing injured tissues such as skin and lung is well established. Little is known about whether sterile wounds elicit lymph node (LN) responses and inflammatory responses after injury of musculoskeletal tissues that are mechanically loaded during the repair response. We investigated LN and tissue immune responses in a tibial implant model of joint replacement surgery where wounded tissue is subjected to movement and mechanical loading postoperatively. Draining inguinal and iliac LNs expanded postoperatively, including increases in regulatory T cells and activation of a subset of T cells. Thus, tissue injury was actively sensed in secondary lymphoid organs, with the potential to activate adaptive immunity. Joint tissues exhibited three temporally distinct immune response components, including a novel interferon (IFN) response with activation of signal transducer and activator of transcription (STAT) and interferon regulatory factor (IRF) pathways. Fibrovascular tissue formation was not associated with a macrophage type 2 (M2) reparative immune response, but instead with delayed induction of interleukin-1 family (IL-1β, IL-33, IL-36), IL-17, and prostaglandin pathway genes concomitant with transforming growth factor (TGF)-β and growth factor signaling, fibroblast activation, and tissue formation. Tissue remodeling was associated with activity of the HOX antisense intergenic RNA (HOTAIR) pathway. These results provide insights into immune responses and regulation of tissue healing after knee arthroplasty that potentially can be used to develop therapeutic strategies to improve healing, prevent arthrofibrosis, and improve surgical outcomes. © 2021 American Society for Bone and Mineral Research (ASBMR).

Application of Stem Cell Therapy for ACL Graft Regeneration

Graft regeneration after anterior cruciate ligament (ACL) reconstruction surgery is a complex three-stage process, which usually takes a long duration and often results in fibrous scar tissue formation that exerts a detrimental impact on the patients' prognosis. Hence, as a regeneration technique, stem cell transplantation has attracted increasing attention. Several different stem cell types have been utilized in animal experiments, and almost all of these have shown good capacity in improving tendon-bone regeneration. Various differentiation inducers have been widely applied together with stem cells to enhance specific lineage differentiation, such as recombinant gene transfection, growth factors, and biomaterials. Among the various different types of stem cells, bone marrow-derived mesenchymal stem cells (BMSCs) have been investigated the most, while ligament stem progenitor cells (LDSCs) have demonstrated the best potential in generating tendon/ligament lineage cells. In the clinic, 4 relevant completed trials have been reported, but only one trial with BMSCs showed improved outcomes, while 5 relevant trials are still in progress. This review describes the process of ACL graft regeneration after implantation and summarizes the current application of stem cells from bench to bedside, as well as discusses future perspectives in this field.

Macrophage depletion impairs neonatal tendon regeneration

Tendons are dense connective tissues that transmit muscle forces to the skeleton. After adult injury, healing potential is generally poor and dominated by scar formation. Although the immune response is a key feature of healing, the specific immune cells and signals that drive tendon healing have not been fully defined. In particular, the immune regulators underlying tendon regeneration are almost completely unknown due to a paucity of tendon regeneration models. Using a mouse model of neonatal tendon regeneration, we screened for immune-related markers and identified upregulation of several genes associated with inflammation, macrophage chemotaxis, and TGFβ signaling after injury. Depletion of macrophages using AP20187 treatment of MaFIA mice resulted in impaired functional healing, reduced cell proliferation, reduced ScxGFP+ neo-tendon formation, and altered tendon gene expression. Collectively, these results show that inflammation is a key component of neonatal tendon regeneration and demonstrate a requirement for macrophages in effective functional healing.

Adipose Stem Cell-Derived Exosomes Ameliorate Chronic Rotator Cuff Tendinopathy by Regulating Macrophage Polarization: From a Mouse Model to a Study in Human Tissue

BACKGROUND

Chronic rotator cuff (RC) tendinopathy is one of the most prevalent causes of shoulder pain. Growing evidence suggests that macrophages play a significant role in the proinflammatory response, resolution of inflammation, and tissue healing of tendinopathy. In particular, enhancement of M2 macrophage (M2φ) activity contributes to the accelerated healing of tendinopathy. Therefore, a treatment that enhances M2φ polarization would be useful for patients with this common musculoskeletal disorder.

PURPOSE

To investigate whether adipose stem cell-derived exosomes (ASC-Exos) enhance M2φ polarization and ameliorate chronic RC tendinopathy.

STUDY DESIGN

Controlled laboratory study.

METHODS

First, we compared the effects of ASC-Exos on polarization of mouse bone marrow-derived macrophages between a classically activated phenotype (M1φ) and an alternatively activated phenotype (M2φ) in vitro. In total, 72 C57BL/6 mice were assigned to normal cage activity (n = 24) or 5 weeks of treadmill overuse (n = 48). The supraspinatus tendon of each treadmill overuse mouse was treated with ASC-Exos (n = 24) or saline (n = 24). Histological and biomechanical outcomes were assessed 4 weeks after treatment. Finally, tissue samples from human patients with RC tendinopathy were obtained to assay the effect of ASC-Exos on the M1φ/M2φ balance in tissue-resident macrophages.

RESULTS

ASC-Exos inhibited M1φ polarization and augmented M2φ polarization in vitro and in vivo. Mice in the ASC-Exos group showed less severe pathological changes than those in the saline group, including less cellular infiltration, disorganization of collagen, and ground substance deposition. The modified Bonar score of the ASC-Exos group (mean ± SD, 7.68 ± 1.03) was significantly lower than that of the saline group (9.81 ± 0.96; P < .05). Furthermore, the maximum failure load was significantly higher in the ASC-Exos group than in the saline group (4.23 ± 0.66 N vs 3.86 ± 0.65 N; P < .05), as was stiffness (3.38 ± 0.34 N/m vs 2.68 ± 0.49 N/m; P < .05).

CONCLUSION

ASC-Exos-mediated polarization balance of M1φ/M2φ contributes to the amelioration of chronic RC tendinopathy. Regulation of the M1φ/M2φ balance could be a new target for the treatment of chronic RC tendinopathy.

CLINICAL RELEVANCE

Administration of ASC-Exos is a cell-free approach that may become a novel treatment option for chronic RC tendinopathy and should be explored further.

Platelet-Rich Plasma Releasate Promotes Early Healing in Tendon After Acute Injury

Background

Acute tendon injury can limit motion and thereby inhibit tendon healing. Positive results have been found after the use of platelet-rich plasma (PRP) to treat tendon injury; however, the early effects of PRP on tendon regeneration are not known.

Purpose/Hypothesis

The purpose of this study was to evaluate the effects of PRP releasate (PRPr) on the early stages of tendon healing in a rat partial tenotomy model. It was hypothesized that PRPr can promote early healing of an Achilles tendon in rats.

Study Design

Controlled laboratory study.

Methods

PRP was prepared by a 2-step method of manual platelet concentration from 10 rats. PRPr was isolated from the clotted preparation after activation by thrombin and was applied to an Achilles tendon on 1 side of 30 rats on the second day after partial tenotomy, with normal saline used as the control on the other side. Achilles tendon samples were harvested 5 and 10 days after tenotomy. At each time point, 15 Achilles tendon samples were obtained, of which 5 samples were evaluated by Masson trichrome staining, apoptosis, and cell proliferation, while the other 10 samples were tested for tensile strength using a material testing machine.

Results

Compared with saline-treated control tendons, the PRPr-treated tendons showed increased collagen synthesis near the cut edge and fewer apoptotic cells (P = .01). An immunohistochemical analysis revealed more Ki-67-positive cells but fewer cluster of differentiation (CD) 68⁺ (ED1⁺) macrophages in PRPr tendons compared with saline-treated tendons (P < .01). Tendons treated with PRPr also showed higher ultimate tensile strength than those treated with saline (P = .03).

Conclusion

PRPr treatment promotes tissue recovery in the early phase of tendon healing by stimulating tendon cell proliferation and collagen production while inhibiting cell apoptosis and CD68⁺ (ED1⁺) macrophage infiltration.

Clinical Relevance

These findings suggest that with PRPr treatment, higher loads can be applied to the healing tendon at an earlier time, which can help the patient resume activity earlier.

Hepatocyte Growth Factor-Induced Tendon Stem Cell Conditioned Medium Promotes Healing of Injured Achilles Tendon

Tendon repair is a medical challenge. Our present study investigated the effectiveness of acellular therapy consisting of conditioned medium (CM) of tendon stem cells (TSCs) induced with hepatocyte growth factor (HGF) in promoting the healing of injured Achilles tendon in a rat model. Proteomic analysis of soluble substances in the CM was performed using an array chip, and bioinformatic analysis was carried out to evaluate interactions among the factors. The effects of CM on viability and migratory capacity of tendon fibroblasts derived from rats with ruptured Achilles tendon were evaluated with the Cell Counting Kit 8 and wound healing assay, respectively. The expression of extracellular matrix (ECM)-related protein was assessed by western blotting. Rats with Achilles tendon injury were treated with CM by local injection for 2 weeks, and the organization of tendon fibers at the lesion site was evaluated by hematoxylin and eosin and Masson's trichrome staining of tissue samples. The deposition and degradation of ECM proteins and the expression of inflammatory factors at the lesion site were evaluated by immunohistochemistry and immunofluorescence. Biomechanical testing was carried out on the injured tendons to assess functional recovery. There were 12 bioactive molecules in the CM, with HGF as the hub of the protein-protein interaction network. CM treatment enhanced the viability and migration of tendon fibroblasts, altered the expression of ECM proteins, promoted the organization of tendon fibers, suppressed inflammation and improved the biomechanics of the injured Achilles tendon. These results suggest that HGF stimulates the secretion of soluble secretory products by TSCs and CM promotes the repair and functional recovery of ruptured Achilles tendon. Thus, HGF-induced TSC CM has therapeutic potential for the treatment of tendinopathy.