Stem cells and basic fibroblast growth factor failed to improve tendon healing: an in vivo study using lentiviral gene transfer in a rat model

Growth factors in the treatment of Achilles tendon injury

Achilles tendon (AT) injury is one of the most common tendon injuries, especially in athletes, the elderly, and working-age people. In AT injury, the biomechanical properties of the tendon are severely affected, leading to abnormal function. In recent years, many efforts have been underway to develop effective treatments for AT injuries to enable patients to return to sports faster. For instance, several new techniques for tissue-engineered biological augmentation for tendon healing, growth factors (GFs), gene therapy, and mesenchymal stem cells were introduced. Increasing evidence has suggested that GFs can reduce inflammation, promote extracellular matrix production, and accelerate AT repair. In this review, we highlighted some recent investigations regarding the role of GFs, such as transforming GF-β(TGF-β), bone morphogenetic proteins (BMP), fibroblast GF (FGF), vascular endothelial GF (VEGF), platelet-derived GF (PDGF), and insulin-like GF (IGF), in tendon healing. In addition, we summarized the clinical trials and animal experiments on the efficacy of GFs in AT repair. We also highlighted the advantages and disadvantages of the different isoforms of TGF-β and BMPs, including GFs combined with stem cells, scaffolds, or other GFs. The strategies discussed in this review are currently in the early stages of development. It is noteworthy that although these emerging technologies may potentially develop into substantial clinical treatment options for AT injury, definitive conclusions on the use of these techniques for routine management of tendon ailments could not be drawn due to the lack of data.

Programmable DNA Hydrogel Provides Suitable Microenvironment for Enhancing TSPCS Therapy in Healing of Tendinopathy

Tendon stem/progenitor cells (TSPCs) therapy is a promising strategy for enhancing cell matrix and collagen synthesis, and regulating the metabolism of the tendon microenvironment during tendon injury repair. Nevertheless, the barren microenvironment and gliding shear of tendon cause insufficient nutrition supply, damage, and aggregation of injected TSPCs around tendon tissues, which severely hinders their clinical application in tendinopathy. In this study, a TSPCs delivery system is developed by encapsulating TSPCs within a DNA hydrogel (TSPCs-Gel) as the DNA hydrogel offers an excellent artificial extracellular matrix (ECM) microenvironment by providing nutrition for proliferation and protection against shear forces. This delivery method restricts TSPCs to the tendons, significantly extending their retention time. It is also found that TSPCs-Gel injections can promote the healing of rat tendinopathy in vivo, where cross-sectional area and load to failure of injured tendons in rats are significantly improved compared to the free TSPCs treatment group at 8 weeks. Furthermore, the potential healing mechanism of TSPCs-Gel is investigated by RNA-sequencing to identify a series of potential gene and signaling pathway targets for further clinical treatment strategies. These findings suggest the potential pathways of using DNA hydrogels as artificial ECMs to promote cell proliferation and protect TSPCs in TSPC therapy.

Insulin-Functionalized Bioactive Fiber Matrices with Bone Marrow-Derived Stem Cells in Rat Achilles Tendon Regeneration

Approximately half of annual musculoskeletal injuries in the US involve tendon tears. The naturally hypocellular and hypovascular tendon environment makes tendons injury-prone and heal slowly. Tendon tissue engineering strategies often use biomimetic scaffolds combined with bioactive factors and/or cells to enhance healing. FDA-approved growth factors to promote tendon healing are lacking, which highlights the need for safe and effective bioactive factors. Our previous work evaluated insulin as a bioactive factor and identified an optimal dose to promote in vitro mesenchymal stem cell survival, division, and tenogenesis. The present work evaluates the ability of insulin-functionalized electrospun nanofiber matrices with or without mesenchymal stem cells to enhance tendon repair in a rat Achilles injury model. Electrospun nanofiber matrices were functionalized with insulin, cultured with or without mesenchymal stem cells, and sutured to transected Achilles tendons in rats. We analyzed rat tendons 4 and 8 weeks after surgery for the tendon morphology, collagen production, and mechanical properties. Bioactive insulin-functionalized fiber matrices with mesenchymal stem cells resulted in significantly increased collagen I and III at 4 and 8 weeks postsurgery. Additionally, these matrices supported highly aligned collagen fibrils in the regenerated tendon tissue at 8 weeks. However, treatment- and control-regenerated tissues had similar tensile properties at 8 weeks, which were less than that of the native Achilles tendon. Our preliminary results establish the benefits of insulin-functionalized fiber matrices in promoting higher levels of collagen synthesis and alignment needed for functional recovery of tendon repair.

Biologic therapies for foot and ankle injuries

Introduction: The use of orthobiologics as supplemental treatment for foot and ankle pathologies have increased in the past decades. They have been used to improve the healing of bone and soft tissue injuries. There have been several studies that examined the use of biologics for knee and hip pathologies but the foot and ankle construct has unique features that must be considered.Areas covered: The biologics for foot and ankle injuries that are covered in this review are platelet-rich plasma (PRP), stem cells, growth factors, hyaluronic acid, bone grafts, bone substitutes, and scaffolds. These modalities are used in the treatment of pathologies related to tendon and soft tissue as well as cartilage.Expert opinion: The utilization of biological adjuncts for improved repair and regeneration of ankle injuries represents a promising future in our efforts to address difficult clinical problems. The application of concentrated bone marrow and PRP each represents the most widely studied and commonly used injection therapies with early clinical studies demonstrating promising results, research is also being done using other potential therapies such as stem cells and growth factors; further investigation and outcome data are still needed.

In Vitro Innovation of Tendon Tissue Engineering Strategies

Tendinopathy is the term used to refer to tendon disorders. Spontaneous adult tendon healing results in scar tissue formation and fibrosis with suboptimal biomechanical properties, often resulting in poor and painful mobility. The biomechanical properties of the tissue are negatively affected. Adult tendons have a limited natural healing capacity, and often respond poorly to current treatments that frequently are focused on exercise, drug delivery, and surgical procedures. Therefore, it is of great importance to identify key molecular and cellular processes involved in the progression of tendinopathies to develop effective therapeutic strategies and drive the tissue toward regeneration. To treat tendon diseases and support tendon regeneration, cell-based therapy as well as tissue engineering approaches are considered options, though none can yet be considered conclusive in their reproduction of a safe and successful long-term solution for full microarchitecture and biomechanical tissue recovery. In vitro differentiation techniques are not yet fully validated. This review aims to compare different available tendon in vitro differentiation strategies to clarify the state of art regarding the differentiation process.

Tendon and Ligament Healing and Current Approaches to Tendon and Ligament Regeneration

Ligament and tendon injuries are common problems in orthopedics. There is a need for treatments that can expedite nonoperative healing or improve the efficacy of surgical repair or reconstruction of ligaments and tendons. Successful biologically-based attempts at repair and reconstruction would require a thorough understanding of normal tendon and ligament healing. The inflammatory, proliferative, and remodeling phases, and the cells involved in tendon and ligament healing will be reviewed. Then, current research efforts focusing on biologically-based treatments of ligament and tendon injuries will be summarized, with a focus on stem cells endogenous to tendons and ligaments. Statement of clinical significance: This paper details mechanisms of ligament and tendon healing, as well as attempts to apply stem cells to ligament and tendon healing. Understanding of these topics could lead to more efficacious therapies to treat ligament and tendon injuries. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:7-12, 2020.

Use of Human Placenta-Derived Cells in a Preclinical Model of Tendon Injury

BACKGROUND

Emerging data suggest that human cells derived from extraembryonic tissues may have favorable musculoskeletal repair properties. The purpose of this study was to determine whether the injection of human placenta-derived mesenchymal-like stromal cells, termed placental expanded cells (PLX-PAD), would improve tendon healing in a preclinical model of tendinopathy.

METHODS

Sixty male Sprague-Dawley rats underwent bilateral patellar tendon injection with either saline solution (control) or PLX-PAD cells (2 × 10 cells/100 µL) 6 days after collagenase injection to induce tendon degeneration. Animals were killed at specific time points for biomechanical, histological, and gene expression analyses of the healing patellar tendons.

RESULTS

Biomechanical testing 2 weeks after the collagenase injury demonstrated better biomechanical properties in the tendons treated with PLX-PAD cells. The load to failure of the PLX-PAD-treated tendons was higher than that of the saline-solution-treated controls at 2 weeks (77.01 ± 10.51 versus 58.87 ± 11.97 N, p = 0.01). There was no significant difference between the 2 groups at 4 weeks. There were no differences in stiffness at either time point. Semiquantitative histological analysis demonstrated no significant differences in collagen organization or cellularity between the PLX-PAD and saline-solution-treated tendons. Gene expression analysis demonstrated higher levels of interleukin-1β (IL-1β) and IL-6 early in the healing process in the PLX-PAD-treated tendons.

CONCLUSIONS

Human placenta-derived cell therapy induced an early inflammatory response and a transient beneficial effect on tendon failure load in a model of collagenase-induced tendon degeneration.

CLINICAL RELEVANCE

Human extraembryonic tissues, such as the placenta, are an emerging source of cells for musculoskeletal repair and may hold promise as a point-of-care cell therapy for tendon injuries.

Chemically Modified Messenger RNA: Modified RNA Application for Treatment of Achilles Tendon Defects

Different regenerative medicine approaches for tendon healing exist. Recently, especially gene therapy gained popularity. However, potential mutagenic and immunologic effects might prevent its translation to clinical research. Chemically modified mRNA (cmRNA) might bypass these limitations of gene therapy. Therefore, the purpose of this study was to evaluate the early healing properties of Achilles tendon defects in rats treated with basic fibroblast growth factor (bFGF) cmRNA. Forty male Lewis rats were used for the study and randomly assigned to two study groups: (1) treatment with cmRNA coding for bFGF and (2) noncoding cmRNA control. Protein expression was measured using in vivo bioluminescence imaging at 24, 48, and 72 h, as well as 14 days. Animals were euthanized 2 weeks following surgery. Biomechanical, histological, and immunohistological analyses were performed with the significance level set at p < 0.05. Protein expression was evident for 3 days. At 14 days, bioluminescence imaging revealed only little protein expression. Biomechanically, tendons treated with bFGF cmRNA showed a construct stiffness closer to the healthy contralateral side when compared with the control group (p = 0.034), without any significant differences in terms of load to failure. Hematoxylin and eosin staining detected no side effects of the treatment, as signs of inflammation, or necrosis. Furthermore, it revealed the shape of the nuclei to be more oval in the bFGF group in the tendon midsubstance (p = 0.043) with a reduced cell count (p = 0.035). Immunohistological staining for type I, II, III, and IV collagen did not differ significantly between the two groups. In conclusion, this pilot study demonstrates the feasibility of a novel messenger RNA (mRNA)-based therapy for Achilles tendon defects using chemically modified mRNA coding for bFGF.

Rescue plan for Achilles: Therapeutics steering the fate and functions of stem cells in tendon wound healing

Due to the increasing age of our society and a rise in engagement of young people in extreme and/or competitive sports, both tendinopathies and tendon ruptures present a clinical and financial challenge. Tendon has limited natural healing capacity and often responds poorly to treatments, hence it requires prolonged rehabilitation in most cases. Till today, none of the therapeutic options has provided successful long-term solutions, meaning that repaired tendons do not recover their complete strength and functionality. Our understanding of tendon biology and healing increases only slowly and the development of new treatment options is insufficient. In this review, following discussion on tendon structure, healing and the clinical relevance of tendon injury, we aim to elucidate the role of stem cells in tendon healing and discuss new possibilities to enhance stem cell treatment of injured tendon. To date, studies mainly apply stem cells, often in combination with scaffolds or growth factors, to surgically created tendon defects. Deeper understanding of how stem cells and vasculature in the healing tendon react to growth factors, common drugs used to treat injured tendons and promising cellular boosters could help to develop new and more efficient ways to manage tendon injuries.

Engineering the Second Generation of Therapeutic Cells with Enhanced Targeting of Injured Tissues

Experimental approaches to improving tissue repair utilize cells and growth factors needed to restore the architecture and function of damaged tissues and organs. Key limitations of these approaches include poor delivery of therapeutic cells and growth factors into injury sites, as well as their short-term retention in target areas. In our earlier studies, we demonstrated that artificial collagen-specific anchor (ACSA) expressed on the surface of therapeutic cells directs them into collagen-rich sites of injury. Moreover, we demonstrated that the ACSA improves the retention of these cells in target sites, thereby promoting tissue repair. To advance the ACSA-based technology, we engineered the second generation of the ACSA-expressing cells able to deliver growth factors to target sites. In this study, we specifically focused on insulin growth factor 1 (IGF1), which enhances the repair of a number of collagen-rich connective tissues, including ligament and tendon. Utilizing gene engineering, we produced IGF1 in the ACSA-expressing cells. Using relevant experimental models, we demonstrated that recombinant IGF1 secreted by these cells maintains its specificity and biological activity. Moreover, our studies show that IGF1 produced by the ACSA-expressing cells cultured in three-dimensional environment promotes the formation of the collagen-rich fibrillar matrix. Furthermore, the engineered cells integrated well with the native collagen-rich tendon tissue. Our study provides strong evidence for the great potential of cells with rationally engineered target-specific receptors to restore damaged connective tissues. Future studies in relevant animal models will determine the utility of these cells in vivo.

rhPDGF-BB combined with ADSCs in the treatment of Achilles tendinitis via miR-363/PI3 K/Akt pathway

To investigate the mechanism of recombinant human platelet-derived growth factor-BB (rhPDGF-BB) and human adipose-derived stem cells (hADSCs) in the treatment of Achilles tendinitis. Biomechanical indices of stiffness, stress, and maximum load-to-failure were detected by biomechanical test. mRNA and protein levels of miR-363, p-PI3K/AKT, tendon-related genes Collagen I, Scleraxis (Scx), and Tenascin C (TNC) were measured by qRT-PCR and western blot. The proliferation of hADSCs was accessed by MTT assay. Biomechanical indices of stiffness, stress, and maximum load-to-failure, and mRNA and protein levels of tendon-related genes could be improved by rhPDGF-BB or hADSCs alone, and could be further improved by rhPDGF-BB + hADSCs. rhPDGF-BB and hADSCs downregulated the expression of miR-363 and upregulated the levels of p-PI3K/Akt, and rhPDGF-BB + hADSCs further strengthened these effects. In addition, rhPDGF-BB promoted the proliferation of hADSCs in vitro and upregulated the expression of tendon-related genes. miR-363 mimic downregulated the levels of p-PI3K/Akt, miR-363 inhibitor upregulated the levels of p-PI3K/Akt, and miR-363 mimic and PI3K/Akt pathway inhibitor LY294002 reversed the positive effect of rhPDGF-BB on the proliferation of hADSCs, which suggested that rhPDGF-BB promoted the proliferation of hADSCs via miR-363/PI3K/Akt pathway. Biomechanical indices and tendon-related genes could be improved by rhPDGF-BB and hADSCs. Moreover, rhPDGF-BB promoted the proliferation of hADSCs via miR-363/PI3K/Akt pathway, indicating that rhPDGF-BB combined with ADSCs could treat Achilles tendinitis via miR-363/PI3K/Akt pathway.

Tendon injuries: Basic science and new repair proposals

Tendons connect muscles to bones, ensuring joint movement. With advanced age, tendons become more prone to degeneration followed by injuries. Tendon repair often requires lengthy periods of rehabilitation, especially in elderly patients. Existing medical and surgical treatments often fail to regain full tendon function.

The development of novel treatment methods has been hampered due to limited understanding of basic tendon biology. Recently, it was discovered that tendons, similar to other mesenchymal tissues, contain tendon stem/progenitor cells (TSPCs) which possess the common stem cell properties.

The current strategies for enhancing tendon repair consist mainly of applying stem cells, growth factors, natural and artificial biomaterials alone or in combination. In this review, we summarise the basic biology of tendon tissues and provide an update on the latest repair proposals for tendon tears.

Cite this article: EFORT Open Rev 2017;2:332-342. DOI: 10.1302/2058-5241.2.160075

Therapeutic Mechanisms of Human Adipose-Derived Mesenchymal Stem Cells in a Rat Tendon Injury Model

BACKGROUND

Although survival of transplanted stem cells in vivo and differentiation of stem cells into tenocytes in vitro have been reported, there have been no in vivo studies demonstrating that mesenchymal stem cells (MSCs) could secrete their own proteins as differentiated tenogenic cells. Purpose/Hypothesis: Using a xenogeneic MSC transplantation model, we aimed to investigate whether MSCs could differentiate into the tenogenic lineage and secrete their own proteins. The hypothesis was that human MSCs would differentiate into the human tenogenic lineage and the cells would be able to secrete human-specific proteins in a rat tendon injury model.

STUDY DESIGN

Controlled laboratory study.

METHODS

The Achilles tendons of 57 Sprague Dawley rats received full-thickness rectangular defects. After the modeling, the defective tendons were randomly assigned to 3 groups: (1) cell group, implantation with human adipose-derived mesenchymal stem cells (hASCs) and fibrin glue (10⁶ cells in 60 μL); (2) fibrin group, implantation with fibrin glue and same volume of cell media; and (3) sham group, identical surgical procedure without any treatment. Gross observation and biomechanical, histopathological, immunohistochemistry, and Western blot analyses were performed at 2 and 4 weeks after modeling.

RESULTS

hASCs implanted into the defective rat tendons were viable for 4 weeks as detected by immunofluorescence staining. Tendons treated with hASCs showed better gross morphological and biomechanical recovery than those in the fibrin and sham groups. Furthermore, the expression of both human-specific collagen type I and tenascin-C was significantly higher in the cell group than in the other 2 groups.

CONCLUSION

Transplantation of hASCs enhanced rat tendon healing biomechanically. hASCs implanted into the rat tendon defect model survived for at least 4 weeks and secreted human-specific collagen type I and tenascin-C. These findings suggest that transplanted MSCs may be able to differentiate into the tenogenic lineage and contribute their own proteins to tendon healing.

CLINICAL RELEVANCE

In tendon injury, MSCs can enhance tendon healing by secreting their own protein and have potential as a therapeutic option in human tendinopathy.

Mesenchymal stem cells for tendon healing: what is on the horizon?

Tendon injuries are a noteworthy morbidity but at present there are few effective scientifically proven treatments. In recent decades, a number of new strategies including tissue engineering with mesenchymal stem cells (MSCs) have been proposed to enhance tendon healing. Although MSCs are an interesting and promising approach, many questions regarding their use in tendon repair remain unanswered. This descriptive overview of the literature of the last decade explores the in vivo studies on tendon healing, in small and large animal models, which used MSCs harvested from different tissues, and the state of the art in clinical applications. It was observed that there are still doubts about the optimum amount of MSCs to use and their source and the type of scaffolds to deliver the cells. Thus, further studies are needed to determine the best protocol for MSC use in tendon healing. Copyright © 2016 John Wiley & Sons, Ltd.

Bone Marrow Derived Mesenchymal Stem Cell Augmentation of Rabbit Flexor Tendon Healing

BACKGROUND

This study investigated the effect of mesenchymal stem cell implantation on flexor tendon healing using a rabbit model of flexor tendon repair. Specifically, we compared the difference between autologous and allogeneic stem cells. The influence of cell number on the outcome of flexor tendon healing was also investigated.

METHODS

Repaired tendons on the rear paws of rabbits were randomly assigned into four groups: control group, 1 million autologous cells, 1 million allogeneic cells, and 4 million allogeneic cells. Rabbits were sacrificed at 3 or 8 weeks after surgery.

RESULTS

Implantation of 4 million stem cells resulted in a significant increase in range of motion compared with control group at three weeks after surgery. The positive staining of collagen I in healing tendons was enhanced in stem cell treated groups three weeks after surgery. However, stem cells did not improve biomechanical properties of flexor tendons.

CONCLUSIONS

High dose stem cells attenuated adhesions in the early time point following flexor tendon repair. Further work is needed determine the value of stem cell therapy in flexor tendon healing in humans.

Stem cells and bFGF in tendon healing: Effects of lentiviral gene transfer and long-term follow-up in a rat Achilles tendon defect model

BACKGROUND

The influence of stem cells and lentiviral expression of basic fibroblastic growth factor (bFGF) on tendon healing and remodelling was investigated in an in-vivo long-term (12 weeks) rat Achilles tendon defect model.

METHODS

In sixty male Lewis rats, complete tendon defects (2.4 mm) were created and either left untreated (PBS) or treated by injection of stem cells lentivirally expressing the enhanced green fluorescence marker gene eGFP (MSC-LV-eGFP) or basic fibroblast growth factor bFGF (MSC-LV-bFGF). Tendons were harvested after 12 weeks and underwent biomechanical and (immuno)-histological analysis.

RESULTS

After 12 weeks the mean ultimate load to failure ratio (treated side to contralateral side) in biomechanical testing reached 97 % in the bFGF-group, 103 % in the eGFP-group and 112 % in the PBS-group. Also in the stiffness testing both MSC groups did not reach the results of the PBS group. Histologically, the MSC groups did not show better results than the control group. There were clusters of ossifications found in all groups. In immunohistology, only the staining collagen-type-I was strongly increased in both MSC groups in comparison to PBS control group. However, there were no significant differences in the (immuno)-histological results between both stem cell groups.

CONCLUSION

The biomechanical and (immuno)-histological results did not show positive effects of the MSC groups on tendon remodelling in a long-term follow-up. Interestingly, in later stages stem cells had hardly any effects on biomechanical results. This study inspires a critical and reflected use of stem cells in tendon healing.

Stem cell technology for tendon regeneration: current status, challenges, and future research directions

Tendon injuries are a common cause of physical disability. They present a clinical challenge to orthopedic surgeons because injured tendons respond poorly to current treatments without tissue regeneration and the time required for rehabilitation is long. New treatment options are required. Stem cell-based therapies offer great potential to promote tendon regeneration due to their high proliferative, synthetic, and immunomodulatory activities as well as their potential to differentiate to the target cell types and undergo genetic modification. In this review, I first recapped the challenges of tendon repair by reviewing the anatomy of tendon. Next, I discussed the advantages and limitations of using different types of stem cells compared to terminally differentiated cells for tendon tissue engineering. The safety and efficacy of application of stem cells and their modified counterparts for tendon tissue engineering were then summarized after a systematic literature search in PubMed. The challenges and future research directions to enhance, optimize, and standardize stem cell-based therapies for augmenting tendon repair were then discussed.

Tendon basic science: Development, repair, regeneration, and healing

Tendinopathy and tendon rupture are common and disabling musculoskeletal conditions. Despite the prevalence of these injuries, a limited number of investigators are conducting fundamental, basic science studies focused on understanding processes governing tendinopathies and tendon healing. Development of effective therapeutics is hindered by the lack of fundamental guiding data on the biology of tendon development, signal transduction, mechanotransduction, and basic mechanisms underlying tendon pathogenesis and healing. To propel much needed progress, the New Frontiers in Tendon Research Conference, co-sponsored by NIAMS/NIH, the Orthopaedic Research Society, and the Icahn School of Medicine at Mount Sinai, was held to promote exchange of ideas between tendon researchers and basic science experts from outside the tendon field. Discussed research areas that are underdeveloped and represent major hurdles to the progress of the field will be presented in this review. To address some of these outstanding questions, conference discussions and breakout sessions focused on six topic areas (Cell Biology and Mechanics, Functional Extracellular Matrix, Development, Mechano-biology, Scarless Healing, and Mechanisms of Injury and Repair), which are reviewed in this special issue and briefly presented in this review. Review articles in this special issue summarize the progress in the field and identify essential new research directions.

Mechanisms of tendon injury and repair

Tendon disorders are common and lead to significant disability, pain, healthcare cost, and lost productivity. A wide range of injury mechanisms exist leading to tendinopathy or tendon rupture. Tears can occur in healthy tendons that are acutely overloaded (e.g., during a high speed or high impact event) or lacerated (e.g., a knife injury). Tendinitis or tendinosis can occur in tendons exposed to overuse conditions (e.g., an elite swimmer's training regimen) or intrinsic tissue degeneration (e.g., age-related degeneration). The healing potential of a torn or pathologic tendon varies depending on anatomic location (e.g., Achilles vs. rotator cuff) and local environment (e.g., intrasynovial vs. extrasynovial). Although healing occurs to varying degrees, in general healing of repaired tendons follows the typical wound healing course, including an early inflammatory phase, followed by proliferative and remodeling phases. Numerous treatment approaches have been attempted to improve tendon healing, including growth factor- and cell-based therapies and rehabilitation protocols. This review will describe the current state of knowledge of injury and repair of the three most common tendinopathies--flexor tendon lacerations, Achilles tendon rupture, and rotator cuff disorders--with a particular focus on the use of animal models for understanding tendon healing.

The role of stem cells and tissue engineering in orthopaedic sports medicine: current evidence and future directions

The use of stem cell therapies for the treatment of orthopaedic injuries continues to advance. The purpose of this review was to provide an update of the current role and future directions of stem cell strategies in sports medicine. The application of cell-based treatments in the sports medicine arena has expanded in recent years. Promising preclinical results have led to translation of these novel therapies into the clinical setting. Early well-designed comparative clinical studies have also shown positive outcomes. Despite significant advances in this arena, there remains a need for additional high-powered and well-designed clinical trials to confirm the safety and efficacy of treatment.

Progress in cell-based therapies for tendon repair

The last decade has seen significant developments in cell therapies, based on permanently differentiated, reprogrammed or engineered stem cells, for tendon injuries and degenerative conditions. In vitro studies assess the influence of biophysical, biochemical and biological signals on tenogenic phenotype maintenance and/or differentiation towards tenogenic lineage. However, the ideal culture environment has yet to be identified due to the lack of standardised experimental setup and readout system. Bone marrow mesenchymal stem cells and tenocytes/dermal fibroblasts appear to be the cell populations of choice for clinical translation in equine and human patients respectively based on circumstantial, rather than on hard evidence. Collaborative, inter- and multi-disciplinary efforts are expected to provide clinically relevant and commercially viable cell-based therapies for tendon repair and regeneration in the years to come.

Biologics in Achilles tendon healing and repair: a review

Injuries of the Achilles tendon are relatively common with potentially devastating outcomes. Healing Achilles tendons form a fibrovascular scar resulting in a tendon which may be mechanically weaker than the native tendon. The resulting strength deficit causes a high risk for reinjury and other complications. Treatments using biologics aim to restore the normal properties of the native tendon and reduce the risk of rerupture and maximize tendon function. The purpose of this review was to summarize the current findings of various therapies using biologics in an attempt to improve the prognosis of Achilles tendon ruptures and tendinopathies. A PubMed search was performed using specific search terms. The search was open for original manuscripts and review papers limited to publication within the last 10 years. From these searches, papers were included in the review if they investigated the effects of biological augmentation on Achilles tendon repair or healing. Platelet-rich plasma may assist in the healing process of Achilles tendon ruptures, while the evidence to support its use in the treatment of chronic Achilles tendinopathies remains insufficient. The use of growth factors such as hepatocyte growth factor, recombinant human platelet-derived growth factor-BB, interleukin-6, and transforming growth factor beta as well as several bone morphogenetic proteins have shown promising results for Achilles tendon repair. In vitro and preclinical studies have indicated the potential effectiveness of bone marrow aspirate as well. Stem cells also have positive effects on Achilles tendon healing, particularly during the early phases. Polyhydroxyalkanoates (PHA), decellularized tendon tissue, and porcine small intestinal submucosa (SIS) are biomaterials which have shown promising results as scaffolds used in Achilles tendon repair. The application of biological augmentation techniques in Achilles tendon repair appears promising; however, several techniques require further investigation to evaluate their clinical application.