The future: rehabilitation, gene therapy, optimization of healing

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.

From fetal tendon regeneration to adult therapeutic modalities: TGF-β3 in scarless healing

Tendon injuries are common disorders that can significantly impact people's lives. Unfortunately, the limited regenerative ability of tendons results in tissue healing in a scar-mediated manner. The current therapeutic strategies fail to fully recover the functions of the injured tendons, and as such, the conception of 'scarless healing' has gained prominent attention in the field of regenerative medicine. Interestingly, injured fetal tendons possess the capability to heal through regeneration, which builds an ideal blueprint for adult tendon regeneration. Studies have shown that fetal biochemical cues have the potential to improve adult tendon healing. Here we review the biological factors that contribute to fetal tendon regeneration and how manipulation of these biochemical cues in the adult tendon healing process could achieve regeneration.

The tendon unit: biochemical, biomechanical, hormonal influences

The current literature has mainly focused on the biology of tendons and on the characterization of the biological properties of tenocytes and tenoblasts. It is still not understood how these cells can work together in homeostatic equilibrium. We put forward the concept of the "tendon unit" as a morpho-functional unit that can be influenced by a variety of external stimuli such as mechanical stimuli, hormonal influence, or pathological states. We describe how this unit can modify itself to respond to such stimuli. We evidence the capability of the tendon unit of healing itself through the production of collagen following different mechanical stimuli and hypothesize that restoration of the homeostatic balance of the tendon unit should be a therapeutic target.

Picrosirius Red Staining: Revisiting Its Application to the Qualitative and Quantitative Assessment of Collagen Type I and Type III in Tendon

Collagen has a major role in the structural organization of tendons. Picrosirius red (PSR) staining viewed under polarized light microscopy is the standard method to evaluate the organization of collagen fibers in tissues. It is also used to distinguish between type I and type III collagen in tissue sections. However, accurate analysis and interpretation of PSR images are challenging because of technical factors and historical misconceptions. The aim of this study was to clarify whether collagen types I and III can be distinguished by PSR staining in rat Achilles tendons, using double immunohistochemistry as the positive control. Our findings showed that PSR staining viewed with polarized light microscopy was suitable for qualitative and quantitative assessment of total collagen but was not able to distinguish collagen types. We found it critical to use a polarizing microscope equipped with a rotating stage; tendon section orientation at 45° with respect to crossed polarizers was optimal for the qualitative and quantitative assessment of collagen organization. Immunohistochemistry was superior to PSR staining for detection of collagen type III. We also compared formalin and Bouin solution as fixatives. Both produced similar birefringence, but formalin-fixed tendons provided higher quality histological detail with both hematoxylin-eosin and immunostaining.

Using tools in mechanobiology to repair tendons

Purpose of review

The purpose of this review is to describe the mechanobiological mechanisms of tendon repair as well as outline current and emerging tools in mechanobiology that might be useful for improving tendon healing and regeneration. Over 30 million musculoskeletal injuries are reported in the US per year and nearly 50% involve soft tissue injuries to tendons and ligaments. Yet current therapeutic strategies for treating tendon injuries are not always successful in regenerating and returning function of the healing tendon.

Recent findings

The use of rehabilitative strategies to control the motion and transmission of mechanical loads to repairing tendons following surgical reattachment is beneficial for some, but not all, tendon repairs. Scaffolds that are designed to recapitulate properties of developing tissues show potential to guide the mechanical and biological healing of tendon following rupture. The incorporation of biomaterials to control alignment and reintegration, as well as promote scar-less healing, are also promising. Improving our understanding of damage thresholds for resident cells and how these cells respond to bioelectrical cues may offer promising steps forward in the field of tendon regeneration.

Summary

The field of orthopaedics continues to advance and improve with the development of regenerative approaches for musculoskeletal injuries, especially for tendon, and deeper exploration in this area will lead to improved clinical outcomes.

Hypoxia-Induced Mesenchymal Stem Cells Exhibit Stronger Tenogenic Differentiation Capacities and Promote Patellar Tendon Repair in Rabbits

Tendon injury is a common but tough medical problem. Unsatisfactory clinical results have been reported in tendon repair using mesenchymal stem cell (MSC) therapy, creating a need for a better strategy to induce MSCs to tenogenic differentiation. This study was designed to examine the effect of hypoxia on the tenogenic differentiation of different MSCs and their tenogenic differentiation capacities under hypoxia condition in vitro and to investigate the in vivo inductility of hypoxia in tenogenesis. Adipose tissue-derived MSCs (AMSCs) and bone marrow-derived MSCs (BMSCs) were isolated and characterized. The expression of hypoxia-induced factor-1 alpha (Hif-1α) was examined to confirm the establishment of hypoxia condition. qRT-PCR, western blot, and immunofluorescence staining were used to evaluate the expression of tendon-associated marker Col-1a1, Col-3a1, Dcn, and Tnmd in AMSCs and BMSCs under hypoxia condition, compared with Tgf-β1 induction. In vivo, a patellar tendon injury model was established. Normoxic and hypoxic BMSCs were cultured and implanted. Histological, biomechanical, and transmission electron microscopy analyses were performed to assess the improved healing effect of hypoxic BMSCs on tendon injury. Our in vitro results showed that hypoxia remarkably increased the expression of Hif-1α and that hypoxia not only promoted a significant increase in tenogenic markers in both AMSCs and BMSCs compared with the normoxia group but also showed higher inductility compared with Tgf-β1. In addition, hypoxic BMSCs exhibited higher potential of tenogenic differentiation than hypoxic AMSCs. Our in vivo results demonstrated that hypoxic BMSCs possessed better histological and biomechanical properties than normoxic BMSCs, as evidenced by histological scores, patellar tendon biomechanical parameters, and the range and average of collagen fibril diameters. These findings suggested that hypoxia may be a practical and reliable strategy to induce tenogenic differentiation of BMSCs for tendon repair and could enhance the effectiveness of MSCs therapy in treating tendon injury.

Cell Therapies in Tendon, Ligament, and Musculoskeletal System Repair

In the last few decades, several techniques have been used to optimize tendon, ligament, and musculoskeletal healing. The evidence in favor of these techniques is still not proven, and level I studies are lacking. We performed an analysis of the therapeutic strategies and tissue engineering projects recently published in this field. Here, we try to give an insight into the current status of cell therapies and the latest techniques of bioengineering applied to the field of orthopedic surgery. The future areas for research in the management of musculoskeletal injuries are outlined. There are emerging technologies developing into substantial clinical treatment options that need to be critically evaluated. Mechanical stimulation of the constructs reproduces a more propitious environment for effective healing.

Achilles Tendinopathy

Achilles tendinopathy is a common cause of disability. Despite the economic and social relevance of the problem, the causes and mechanisms of Achilles tendinopathy remain unclear. Tendon vascularity, gastrocnemius-soleus dysfunction, age, sex, body weight and height, pes cavus, and lateral ankle instability are considered common intrinsic factors. The essence of Achilles tendinopathy is a failed healing response, with haphazard proliferation of tenocytes, some evidence of degeneration in tendon cells and disruption of collagen fibers, and subsequent increase in noncollagenous matrix. Tendinopathic tendons have an increased rate of matrix remodeling, leading to a mechanically less stable tendon which is more susceptible to damage. The diagnosis of Achilles tendinopathy is mainly based on a careful history and detailed clinical examination. The latter remains the best diagnostic tool. Over the past few years, various new therapeutic options have been proposed for the management of Achilles tendinopathy. Despite the morbidity associated with Achilles tendinopathy, many of the therapeutic options described and in common use are far from scientifically based. New minimally invasive techniques of stripping of neovessels from the Kager's triangle of the tendo Achillis have been described, and seem to allow faster recovery and accelerated return to sports, rather than open surgery. A genetic component has been implicated in tendinopathies of the Achilles tendon, but these studies are still at their infancy.

Evaluation of the Effect of Platelet-Rich Fibrin on Long Bone Healing: An Experimental Rat Model

Pseudoarthrosis, or nonunion, of the long bones is a challenging medical condition for orthopedic surgeons to treat. Therefore, healing enhancer materials are commonly used. The authors investigated whether platelet-rich fibrin accelerates long bone healing by comparing radiological and histological findings in a rat model of open femoral fracture. Platelet-rich fibrin is a current biomaterial that contains many growth factors and platelets. There are no studies in the literature investigating the effects of platelet-rich fibrin on fracture healing. Sixteen mature male rats were divided into 2 groups. In both groups, an open femoral fracture was created. The platelet-rich fibrin was obtained by centrifuging blood collected from the rats. Rats in the study group were treated with sterile platelet-rich fibrin, and those in the control group were administered saline. The rats were killed at the end of 4 weeks and examined histologically and radiologically. The radiographic and histological scores of the 2 groups differed significantly (P<.05). These results indicate that platelet-rich fibrin is an efficient biomaterial in fracture healing and that it increases the amount of osseous tissue formation. Platelet-rich fibrin does not cause an allergic reaction, is cost-effective, and is easy to obtain. Additional studies are necessary to determine whether platelet-rich fibrin accelerates the fracture healing process or induces a better quality of fracture healing. [Orthopedics. 2017; 40(3):e479-e484.].

Nucleostemin- and Oct 3/4-positive stem/progenitor cells exhibit disparate anatomical and temporal expression during rat Achilles tendon healing

Background

The recent discovery of residing tendon stem/progenitor cells has triggered a growing interest in stem cells as a useful tool in tendon repair. Our knowledge of their involvement in naturally healing tendons is, however, sparse. The aim of this study was to identify and determine stem/progenitor cells in relation to different healing phases and regions in a rat model of Achilles tendon rupture.

Methods

Surgery was performed to create a mid-tendon rupture on the right Achilles tendon of 24 rats, whereas the left tendon was used as a control. Tendons were harvested at one, two, eight and 17 weeks post-rupture and stained with antibodies specific to stem/progenitor cells (Octamer-binding transcription factor 3/4 (Oct 3/4) and nucleostemin), migrating cells (Dynamin 2 (Dyn 2)) and leukocytes (CD45). A histological examination was performed on sections stained with Alcian blue.

Results

At one and two weeks post-rupture, a large number of stem/progenitor cells were discovered throughout the tendon. Most of these cells were nucleostemin positive, whereas only a few Oct 3/4-positive cells were found, mainly situated inside the injury region (I region). At eight and 17 weeks, the increment in stem/progenitor cells had diminished to equal that in the control tendons. At all time points, Oct 3/4-positive cells were also found in the connective tissue surrounding the tendon and at the muscle-tendon junction in both ruptured and control tendons and were often seen at the same location as the migration marker, Dyn 2.

Conclusions

The whole length of the Achilles tendon is infiltrated by stem/progenitor cells at early time points after a mid-tendon rupture. However, different stem/progenitor cell populations exhibit varying anatomical and temporal expressions during Achilles tendon healing, suggesting distinct reparative implications. Oct 3/4 may thus act as a more local, migrating stem/progenitor cell involved in injury-site-specific regenerative effects, as compared to the more general proliferative role of nucleostemin-positive stem/progenitor cells.

Tendon healing: an overview of physiology, biology, and pathology of tendon healing and systematic review of state of the art in tendon bioengineering

PURPOSE

Tendon injuries vary from acute rupture to chronic tendinopathy. For an optimal treatment of either condition, a profound knowledge is essential. Therefore, this article shall give an overview of physiology, biology, and pathology of tendon healing and state of the art in tendon bioengineering.

METHODS

For a preferably comprehensive survey, the current literature listed in PubMed and published in English peer-reviewed journals (March 2013) was systematically reviewed for tendon healing and tendon bioengineering including cytokine modulation, autologous sources of growth factors, biomaterials, gene therapy, and cell-based therapy. No differentiation was made between clinical and preclinical in vitro investigations.

RESULTS

Tendon healing happens in certain stadiums of inflammation, formation, and remodelling. An additional process of "collagen recycling" close to the healing site has been described recently. With increasing comprehension of physiology and pathology of tendon healing, several promising approaches in tendon bioengineering using growth factors, biomaterials, gene therapy, or cell-based therapy are described. However, only some of these are already used routinely in clinics.

CONCLUSION

Strong and resistant tendons are crucial for a healthy musculoskeletal system. The new approaches in tendon bioengineering are promising to aid physiological tendon healing and thus resulting in a stronger and more resistant tendon after injury. The growing knowledge in this field will need to be further taken into clinical studies so that especially those patients with prolonged courses, revision surgery, or chronic tendinopathy and high-demanding patients, i.e., professional athletes would benefit.

LEVEL OF EVIDENCE

II.

Effect of BMP-12, TGF-β1 and autologous conditioned serum on growth factor expression in Achilles tendon healing

PURPOSE

Achilles tendon ruptures are devastating and recover slowly and incompletely. There is a great demand for biomolecular therapies to improve recovery, yet little is understood about growth factors in a healing tendon. Here, the role of growth factors during tendon healing in a rat model and their reaction to single and multiple growth factor treatment are explored.

METHODS

Rat tendons were transected surgically and resutured. The expression of bFGF, BMP-12, VEGF and TGF-β1 was assessed by immunohistochemical analysis one to 8 weeks after surgery. Paracrine effects of TGF-β1 or BMP-12 added by adenoviral transfer, as well as the effect of autologous conditioned serum (ACS) on growth factor expression, were evaluated.

RESULTS

bFGF, BMP-12 and VEGF expression was highest 1 week after transection. bFGF and BMP-12 declined during the remaining period whereas VEGF expression persisted. TGF-β1 expression dramatically increased after 8 weeks. ACS treatment increased bFGF (P = 0.007) and BMP-12 (P = 0.004) expression significantly after 8 weeks. Also overall expression of bFGF, BMP-12 and TGF-β1 regardless of time point was significantly greater than controls with ACS treatment (P < 0.05). Both BMP-12 and TGF-β1 treatments had no significant effect. No effect was observed in VEGF with any treatment.

CONCLUSION

bFGF, BMP-12, VEGF and TGF-β1 are differentially expressed during tendon healing. Additional BMP-12 or TGF-β1 has no significant influence, whereas ACS generally increases expression of all factors except VEGF. Staged application of multiple growth factors may be the most promising biomolecular treatment.

Musculoskeletal diseases--tendon

INTRODUCTION

Tendons establish specific connections between muscles and the skeleton by transferring contraction forces from skeletal muscle to bone thereby allowing body movement. Tendon physiology and pathology are heavily dependent on mechanical stimuli. Tendon injuries clinically represent a serious and still unresolved problem since damaged tendon tissues heal very slowly and no surgical treatment can restore a damaged tendon to its normal structural integrity and mechanical strength. Understanding how mechanical stimuli regulate tendon tissue homeostasis and regeneration will improve the treatment of adult tendon injuries that still pose a great challenge in today's medicine.

SOURCE OF DATA

This review summarizes the current status of tendon treatment and discusses new directions from the point of view of cell-based therapy and regenerative medicine approach. We searched the available literature using PubMed for relevant original articles and reviews.

GROWING POINTS

Identification of tendon cell markers has enabled us to study precisely tendon healing and homeostasis. Clinically, tissue engineering for tendon injuries is an emerging technology comprising elements from the fields of cellular source, scaffold materials, growth factors/cytokines and gene delivering systems.

AREAS TIMELY FOR DEVELOPING RESEARCH

The clinical settings to establish appropriate microenvironment for injured tendons with the combination of these novel cellular- and molecular-based scaffolds will be critical for the treatment.

[Surgical treatment for pain syndromes of the Achilles tendon]

Pain syndromes of the Achilles tendon (AT) include both insertional and non-insertional tendinopathy, two distinct disorders with different underlying pathophysiologies and management options, characterized by pain, impaired performance and swelling in and around the tendon. This article gives an overview of the operative treatment of pain syndromes of the Achilles tendon, including both insertional tendinopathy of the AT and tendinopathy of the main body of the AT. New minimally invasive techniques for the management of this condition, including endoscopy are also reported.

Tissue engineered biological augmentation for tendon healing: a systematic review

INTRODUCTION

Tendon injuries give rise to significant morbidity. In the last few decades, several techniques have been increasingly used to optimize tendon healing.

SOURCES OF DATA

We performed a comprehensive search of PubMed, Medline, Cochrane, CINAHL and Embase databases using various combinations of the commercial names of each scaffold and the keywords 'tendon', 'rotator cuff', 'supraspinatus tendon', 'Achilles tendon', 'growth factors', 'cytokines', 'gene therapy', 'tissue engineering', 'mesenchymal' and 'stem cells' over the years 1966-2009. All articles relevant to the subject were retrieved, and their bibliographies were hand searched for further references in the context to tissue-engineered biological augmentation for tendon healing.

AREAS OF AGREEMENT

Several new techniques are available for tissue-engineered biological augmentation for tendon healing, growth factors, gene therapy and mesenchimal stem cells.

AREAS OF CONTROVERSY

Data are lacking to allow definitive conclusions on the use of these techniques for routine management of tendon ailments.

GROWING POINTS

The emerging field of tissue engineering holds the promise to use new techniques for tendon augmentation and repair. Preliminary studies support the idea that these techniques can provide an alternative for tendon augmentation with great therapeutic potential.

AREAS TIMELY FOR DEVELOPING RESEARCH

The optimization strategies discussed in this article are currently at an early stage of development. Although these emerging technologies may develop into substantial clinical treatment options, their full impact needs to be critically evaluated in a scientific fashion.