Temporal expression of growth factors and matrix molecules in healing tendon lesions

The mechanisms and functions of TGF-β1 in tendon healing

Tendon injury accounts for 30% of musculoskeletal diseases and often leads to disability, pain, healthcare cost, and lost productivity. Following injury to tendon, tendon healing proceeds via three overlapping healing processes. However, due to the structural defects of the tendon itself, the tendon healing process is characterized by the formation of excessive fibrotic scar tissue, and injured tendons rarely return to native tendons, which can easily contribute to tendon reinjury. Moreover, the resulting fibrous scar is considered to be a precipitating factor for subsequent degenerative tendinopathy. Despite this, therapies are almost limited because underlying molecular mechanisms during tendon healing are still unknown. Transforming Growth Factor-β1 (TGF-β1) is known as one of most potent profibrogenic factors during tendon healing process. However, blockage TGF-β1 fails to effectively enhance tendon healing. A detailed understanding of real abilities of TGF-β1 involved in tendon healing can bring promising perspectives for therapeutic value that improve the tendon healing process. Thus, in this review, we describe recent efforts to identify and characterize the roles and mechanisms of TGF-β1 involved at each stage of the tendon healing and highlight potential roles of TGF-β1 leading to the fibrotic response to tendon injury.

Bio-Enhanced Neoligaments Graft Bearing FE002 Primary Progenitor Tenocytes: Allogeneic Tissue Engineering & Surgical Proofs-of-Concept for Hand Ligament Regenerative Medicine

Hand tendon/ligament structural ruptures (tears, lacerations) often require surgical reconstruction and grafting, for the restauration of finger mechanical functions. Clinical-grade human primary progenitor tenocytes (FE002 cryopreserved progenitor cell source) have been previously proposed for diversified therapeutic uses within allogeneic tissue engineering and regenerative medicine applications. The aim of this study was to establish bioengineering and surgical proofs-of-concept for an artificial graft (Neoligaments Infinity-Lock 3 device) bearing cultured and viable FE002 primary progenitor tenocytes. Technical optimization and in vitro validation work showed that the combined preparations could be rapidly obtained (dynamic cell seeding of 10⁵ cells/cm of scaffold, 7 days of co-culture). The studied standardized transplants presented homogeneous cellular colonization in vitro (cellular alignment/coating along the scaffold fibers) and other critical functional attributes (tendon extracellular matrix component such as collagen I and aggrecan synthesis/deposition along the scaffold fibers). Notably, major safety- and functionality-related parameters/attributes of the FE002 cells/finished combination products were compiled and set forth (telomerase activity, adhesion and biological coating potentials). A two-part human cadaveric study enabled to establish clinical protocols for hand ligament cell-assisted surgery (ligamento-suspension plasty after trapeziectomy, thumb metacarpo-phalangeal ulnar collateral ligamentoplasty). Importantly, the aggregated experimental results clearly confirmed that functional and clinically usable allogeneic cell-scaffold combination products could be rapidly and robustly prepared for bio-enhanced hand ligament reconstruction. Major advantages of the considered bioengineered graft were discussed in light of existing clinical protocols based on autologous tenocyte transplantation. Overall, this study established proofs-of-concept for the translational development of a functional tissue engineering protocol in allogeneic musculoskeletal regenerative medicine, in view of a pilot clinical trial.

Bioactive Scaffold With Spatially Embedded Growth Factors Promotes Bone-to-Tendon Interface Healing of Chronic Rotator Cuff Tear in Rabbit Model

BACKGROUND

Functional restoration of the bone-to-tendon interface (BTI) after rotator cuff repair is a challenge. Therefore, numerous biocompatible biomaterials for promoting BTI healing have been investigated.

PURPOSE

To determine the efficacy of scaffolds with spatiotemporal delivery of growth factors (GFs) to accelerate BTI healing after rotator cuff repair.

STUDY DESIGN

Controlled laboratory study.

METHODS

An advanced 3-dimensional printing technique was used to fabricate bioactive scaffolds with spatiotemporal delivery of multiple GFs targeting the tendon, fibrocartilage, and bone regions. In total, 50 rabbits were used: 2 nonoperated controls and 48 rabbits with induced chronic rotator cuff tears (RCTs). The animals with RCTs were divided into 3 groups: (A) saline injection, (B) scaffold without GF, and (C) scaffold with GF. To induce chronic models, RCTs were left unrepaired for 6 weeks; then, surgical repairs with or without bioactive scaffolds were performed. For groups B and C, each scaffold was implanted between the bony footprint and the supraspinatus tendon. Four weeks after repair, quantitative real-time polymerase chain reaction and immunofluorescence analyses were performed to evaluate early signs of regenerative healing. Histological, biomechanical, and micro-computed tomography analyses were performed 12 weeks after repair.

RESULTS

Group C had the highest mRNA expression of collagen type I alpha 1, collagen type III alpha 1, and aggrecan. Immunofluorescence analysis showed the formation of an aggrecan+/collagen II+ fibrocartilaginous matrix at the BTI when repaired with scaffold with GFs. Histologic analysis revealed greater collagen fiber continuity, denser collagen fibers, and a more mature tendon-to-bone junction in GF-embedded scaffolds than those in the other groups. Group C demonstrated the highest load-to-failure ratio, and modulus mapping showed that the distribution of the micromechanical properties of the BTI repaired with GF-embedded scaffolds was comparable with that of the native BTI. Micro-computed tomography analysis identified the highest bone mineral density and bone volume/total volume ratio in group C.

CONCLUSION

Bioactive scaffolds with spatially embedded GFs have significant potential to promote the BTI healing of chronic RCTs in a rabbit model.

CLINICAL RELEVANCE

The scaffolds with spatiotemporal delivery of GF may serve as an off-the-shelf biomaterial graft to promote the healing of RCTs.

Principles of Tendon Structure, Healing, and the Microenvironment

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

In Vitro and In Vivo Effects of IGF-1 Delivery Strategies on Tendon Healing: A Review

Tendon injuries suffer from a slow healing, often ending up in fibrovascular scar formation, leading to inferior mechanical properties and even re-rupture upon resumption of daily work or sports. Strategies including the application of growth factors have been under view for decades. Insulin-like growth factor-1 (IGF-1) is one of the used growth factors and has been applied to tenocyte in vitro cultures as well as in animal preclinical models and to human patients due to its anabolic and matrix stimulating effects. In this narrative review, we cover the current literature on IGF-1, its mechanism of action, in vitro cell cultures (tenocytes and mesenchymal stem cells), as well as in vivo experiments. We conclude from this overview that IGF-1 is a potent stimulus for improving tendon healing due to its inherent support of cell proliferation, DNA and matrix synthesis, particularly collagen I, which is the main component of tendon tissue. Nevertheless, more in vivo studies have to be performed in order to pave the way for an IGF-1 application in orthopedic clinics.

A pilot study to demonstrate the paracrine effect of equine, adult allogenic mesenchymal stem cells in vitro, with a potential for healing of experimentally-created, equine thoracic wounds in vivo

Regenerative biological therapies using mesenchymal stem cells (MSCs) are being studied and used extensively in equine veterinary medicine. One of the important properties of MSCs is the cells' reparative effect, which is brought about by paracrine signaling, which results in the release of biologically active molecules, which in turn, can affect cellular migration and proliferation, thus a huge potential in wound healing. The objective of the current study was to demonstrate the in vitro and in vivo potentials of equine allogenic bone marrow-derived MSCs for wound healing. Equine bone marrow-derived MSCs from one allogenic donor horse were used. Equine MSCs were previously characterized for their in vitro proliferation, expression of cluster-of-differentiation markers, and trilineage differentiation. MSCs were first evaluated for their migration using an in vitro wound healing scratch assay, and subsequently, the conditioned medium was evaluated for their effect on human fibroblast proliferation. Subsequently, allogenic cells were intradermally injected into full-thickness, cutaneous thoracic wounds of 4 horses. Wound healing was assessed by using 3-D digital imaging and by measuring mRNA expression of pro-and anti-inflammatory markers for 30 days. Using human fibroblasts in an in vitro wound healing assay, we demonstrate a significantly higher healing in the presence of conditioned medium collected from proliferating MSCs than in the presence of medium containing fetal bovine serum. The in vitro effect of MSCs did not translate into a detectable effect in vivo. Nonetheless, we proved that molecularly characterized equine allogenic MSCs do not illicit an immunologic response. Investigations using MSCs derived from other sources (adipose tissue, umbilical cord), or a higher number of MSCs or a compromised animal model may be required to prove the efficacy of equine MSCs in wound healing in vivo.

Intralesional bone marrow and superior check desmotomy is superior to conservative treatment of equine superficial digital flexor tendonitis

BACKGROUND

Superficial digital flexor tendon (SDFT) injury is common in racehorses and a significant cause of lost training days and wastage in the industry.

OBJECTIVES

To compare the post-injury performance of Thoroughbred and Standardbred racehorses diagnosed with SDFT tendonitis treated with intralesional bone marrow and superior check desmotomy or managed conservatively and further to compare this performance with that of uninjured racehorses.

STUDY DESIGN

Retrospective and case-controlled.

METHODS

Medical and race records of racehorses treated surgically or managed conservatively for SDFT tendonitis were collated. Signalment, lesion severity and treatment were identified and performance post-injury compared. Performance of the treatment groups was further assessed by comparison with uninjured racehorses matched for age, sex, number of starts pre-injury and randomly selected from the cases' last races.

RESULTS

The study population comprised 114 racehorses divided into surgical (39/114), conservative (38/114) and control groups (37/114). Horses that had surgery were more likely to return to racing than those managed conservatively (OR 4.7 95% CI [1.6-14.3], P = .006). Standardbreds were more likely to return to race compared with Thoroughbreds (OR 4.0 95% CI [1.2-13.3], P = .03). There was no statistically significant difference in the average number of placings, average number of wins and post-injury earnings between the surgically treated and conservatively managed groups (P = .9, P = .9 and P = .7, respectively). The average number of lifetime starts post-injury/post-selection between surgery, conservative and control groups were not statistically significantly different (P = .2). Surgically treated horses had a statistically significantly shorter time to start post-injury compared with the conservative group (P = .04).

MAIN LIMITATIONS

The retrospective nature of the study precludes an actual fit as a nonrandomised clinical trial and the nonrandomised nature of the allocation of the treatment groups is biased.

CONCLUSIONS

Surgical intervention of superficial digital flexor tendonitis in racehorses suffering flexor tendon injury showed a higher likelihood of return to racing than conservative treatment.

BIOMECHANICAL EVALUATION OF AN INVERTED FIXATION FOR ACL RECONSTRUCTION WITH NONMETALLIC HARDWARE AND TIBIAL SUBCORTICAL SUPPORT TO INCREASE STRENGTH AT THE TIBIAL SITE

In this paper, we evaluate the initial biomechanical properties of an anterior cruciate ligament (ACL) reconstruction technique that inverts the anatomical location of the commonly used hardware and relies the tibial fixation on the subcortical bone to increase tibial site strength. Four 7-specimen groups were tested in a porcine model: for the control ACL reconstruction technique, the femur with a cross-pin fixation supported in the trabecular bone and the tibia with a biodegradable interference screw (BIS); for the new proposed technique, the femur with a BIS and the tibia with a cross-pin fixation leaned on the tibial subcortical bone. The specimens were subjected to cyclic and load-to-failure tests to compute their biomechanical performance. At the tibia, the cross-pin fixation revealed higher resistance than the BIS ([Formula: see text] for ultimate load and [Formula: see text] = 0.006 for yield load), additionally cyclic and total displacement at representative loads showed extremely high values with BIS fixation (in two specimens greater than 9[Formula: see text]mm for 250 N and greater than 10[Formula: see text]mm for 450[Formula: see text]N). At the femur, no differences between fixations were observed. The inverted ACL reconstruction improves resistance at the tibial site with respect to the control technique, with similar resistance at the femoral site and no differences in total displacement at representative loads. It offers a useful and robust solution when greater tibial resistance is required.

Injection of Leukocyte-Poor Platelet-Rich Plasma for Moderate-to-Large Rotator Cuff Tears Does Not Improve Clinical Outcomes but Reduces Retear Rates and Fatty Infiltration: A Prospective, Single-Blinded Randomized Study

OBJECTIVES

To determine whether leukocyte-poor platelet-rich plasma (Lp-PRP) reduced retear rates, reduced fatty infiltration, and improved functional outcomes in patients with degenerative moderate-to-large rotator cuff tears.

METHODS

This was a randomized controlled study at a single center. A consecutive series of 104 patients with moderate-to-large rotator cuff tears was enrolled and randomly allocated to a control group (double-row suture-bridge arthroscopic rotator cuff repair alone, n = 52) and a study group (double-row suture-bridge repair followed by 3 Lp-PRP injections at the tendon repair site during surgery, at days 7 and 14 after surgery, n = 52). All patients were followed up for 27.2 months (range 24-36 months), with University of California at Los Angeles (UCLA) shoulder rating scale, the Constant score, and a visual analog scale (VAS) evaluated respectively. The integrity and fatty infiltration of repaired tissue were assessed by magnetic resonance imaging using the Sugaya classification and Goutallier grade classification at 24 months after surgery. Statistical analysis was performed based on the t test, χ² test, and the Kendall tau-b correlation coefficient.

RESULTS

Four patients refused follow-up, and 11 patients had incomplete data. Eventually, a total of 89 patients were available for 24 months follow-up. The mean UCLA score increased from 14.80 ± 2.53 to 29.37 ± 2.06 in control group and from 13.74 ± 3.30 to 30.14 ± 2.32 in study group (P = .103). The mean Constant score increased from 46.56 ± 5.90 to 86.83 ± 4.94 in control group and from 44.37 ± 7.92 to 88.80 ± 4.92 in study group (P = .063). The VAS score decreased from 3.22 ± 1.24 to 0.97 ± 1.12 in control group and in 3.49 ± 1.52 to 1.16 ± 0.99 in study group (P = .41). All differences in UCLA score, Constant score, and VAS between pre- and postoperation achieved minimal clinically important differences proposed for arthroscopic rotator cuff repair. Of the 89 patients, 76 had magnetic resonance imaging performed at 24 months after surgery. The retear rate was 17.6% in study group, which was lower than that in control group (38.1%, P = .049). In addition, the Goutallier grade was found to be significant difference between groups postoperatively (Kendall tau-b -0.24, P = .03) but no significant difference preoperatively (Kendall tau-b -0.18, P = .11). There were no complications in the patients.

CONCLUSIONS

Our procedures involving repeated injections of Lp-PRP during surgery and at days 7 and 14, as described in this study, have positive effects on reducing retear rate and promoting Goutallier grade in patients following arthroscopic rotator cuff repair and could also provide substantial clinical outcomes that reach the minimal clinically important difference for surgical treatment. However, given the numbers available for analysis, it did not promote better clinical results when compared with the control group.

LEVEL OF EVIDENCE

II, randomized controlled study.

Dynamic Load Model Systems of Tendon Inflammation and Mechanobiology

Dynamic loading is a shared feature of tendon tissue homeostasis and pathology. Tendon cells have the inherent ability to sense mechanical loads that initiate molecular-level mechanotransduction pathways. While mature tendons require physiological mechanical loading in order to maintain and fine tune their extracellular matrix architecture, pathological loading initiates an inflammatory-mediated tissue repair pathway that may ultimately result in extracellular matrix dysregulation and tendon degeneration. The exact loading and inflammatory mechanisms involved in tendon healing and pathology is unclear although a precise understanding is imperative to improving therapeutic outcomes of tendon pathologies. Thus, various model systems have been designed to help elucidate the underlying mechanisms of tendon mechanobiology via mimicry of the in vivo tendon architecture and biomechanics. Recent development of model systems has focused on identifying mechanoresponses to various mechanical loading platforms. Less effort has been placed on identifying inflammatory pathways involved in tendon pathology etiology, though inflammation has been implicated in the onset of such chronic injuries. The focus of this work is to highlight the latest discoveries in tendon mechanobiology platforms and specifically identify the gaps for future work. An interdisciplinary approach is necessary to reveal the complex molecular interplay that leads to tendon pathologies and will ultimately identify potential regenerative therapeutic targets.

Slowed-Down Rehabilitation Following Percutaneous Repair of Achilles Tendon Rupture

BACKGROUND

Following percutaneous repair of acute Achilles tendon (AT) ruptures, early postoperative weightbearing is advocated; however, it is debatable how aggressive rehabilitation should be. We compared the clinical and functional outcomes in 2 groups of patients who followed either our "traditional" or a "slowed down" rehabilitation after percutaneous surgical repair.

METHODS

Sixty patients were prospectively recruited to a slowed down (29 patients) or a traditional (31 patients) rehabilitation program. Both groups were allowed immediate weightbearing postoperatively; a removable brace with 5 heel wedges was applied at 2 weeks. In the slowed-down group, 1 wedge was removed after 4 weeks. Gradual removal of the boot took place after 4 wedges were kept for 4 weeks. In the traditional group, 1 wedge was removed every 2 weeks, with removal of the boot after 2 wedges had been kept for 2 weeks. The AT Resting Angle (ATRA) evaluated tendon elongation. Patient reported functional outcomes were assessed using the AT Rupture Score (ATRS). Calf circumference difference and the isometric plantarflexion strength of the gastro-soleus complex were evaluated.

RESULTS

At the 12-month follow-up, both ATRA and ATRS were more favorable in the slowed-down group. The isometric strength and the calf circumference were more similar to the contralateral leg in the slowed-down group than in the traditional one.

CONCLUSION

Following percutaneous repair of acute Achilles tendon patients undergoing slowed down rehabilitation performed better than the traditional one. These conclusions must be considered within the limitations of the present study.

LEVEL OF EVIDENCE

Level II, prospective comparative study.

Cell autonomous TGFβ signaling is essential for stem/progenitor cell recruitment into degenerative tendons

Understanding cell recruitment in damaged tendons is critical for improvements in regenerative therapy. We recently reported that targeted disruption of transforming growth factor beta (TGFβ) type II receptor in the tendon cell lineage (Tgfbr2^ScxCre) resulted in resident tenocyte dedifferentiation and tendon deterioration in postnatal stages. Here we extend the analysis and identify direct recruitment of stem/progenitor cells into the degenerative mutant tendons. Cre-mediated lineage tracing indicates that these cells are not derived from tendon-ensheathing tissues or from a Scleraxis-expressing lineage, and they turned on tendon markers only upon entering the mutant tendons. Through immunohistochemistry and inducible gene deletion, we further find that the recruited cells originated from a Sox9-expressing lineage and their recruitment was dependent on cell autonomous TGFβ signaling. The cells identified in this study thus differ from previous reports of cell recruitment into injured tendons and suggest a critical role for TGFβ signaling in cell recruitment, providing insights that may support improvements in tendon repair.

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Targeted deletion of TGFβ signaling led to degenerative changes in mouse tendons

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Stem/progenitor cells were recruited into the degenerative mutant tendons

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The recruited cells are different from the ones so far reported in tendon injury

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Recruitment was dependent on cell autonomous TGFβ signaling in the recruited cells

Mesenchymal Stromal Cells Adapt to Chronic Tendon Disease Environment with an Initial Reduction in Matrix Remodeling

Tendon lesions are common sporting injuries in humans and horses alike. The healing process of acute tendon lesions frequently results in fibrosis and chronic disease. In horses, local mesenchymal stromal cell (MSC) injection is an accepted therapeutic strategy with positive influence on acute lesions. Concerning the use of MSCs in chronic tendon disease, data are scarce but suggest less therapeutic benefit. However, it has been shown that MSCs can have a positive effect on fibrotic tissue. Therefore, we aimed to elucidate the interplay of MSCs and healthy or chronically diseased tendon matrix. Equine MSCs were cultured either as cell aggregates or on scaffolds from healthy or diseased equine tendons. Higher expression of tendon-related matrix genes and tissue inhibitors of metalloproteinases (TIMPs) was found in aggregate cultures. However, the tenogenic transcription factor scleraxis was upregulated on healthy and diseased tendon scaffolds. Matrix metalloproteinase (MMPs) expression and activity were highest in healthy scaffold cultures but showed a strong transient decrease in diseased scaffold cultures. The release of glycosaminoglycan and collagen was also higher in scaffold cultures, even more so in those with tendon disease. This study points to an early suppression of MSC matrix remodeling activity by diseased tendon matrix, while tenogenic differentiation remained unaffected.

Growth Factor Roles in Soft Tissue Physiology and Pathophysiology

Repair and healing of injured and diseased tendons has been traditionally fraught with apprehension and difficulties, and often led to rather unsatisfactory results. The burgeoning research field of growth factors has opened new venues for treatment of tendon disorders and injuries, and possibly for treatment of disorders of the aorta and major arteries as well. Several chapters in this volume elucidate the role of transforming growth factor β (TGFß) in pathogenesis of several heritable disorders affecting soft tissues, such as aorta, cardiac valves, and tendons and ligaments. Several members of the bone morphogenetic group either have been approved by the FDA for treatment of non-healing fractures or have been undergoing intensive clinical and experimental testing for use of healing bone fractures and tendon injuries. Because fibroblast growth factors (FGFs) are involved in embryonic development of tendons and muscles among other tissues and organs, the hope is that applied research on FGF biological effects will lead to the development of some new treatment strategies providing that we can control angiogenicity of these growth factors. The problem, or rather question, regarding practical use of imsulin-like growth factor I (IGF-I) in tendon repair is whether IGF-I acts independently or under the guidance of growth hormone. FGF2 or platelet-derived growth factor (PDGF) alone or in combination with IGF-I stimulates regeneration of periodontal ligament: a matter of importance in Marfan patients with periodontitis. In contrast, vascular endothelial growth factor (VEGF) appears to have rather deleterious effects on experimental tendon healing, perhaps because of its angiogenic activity and stimulation of matrix metalloproteinases-proteases whose increased expression has been documented in a variety of ruptured tendons. Other modalities, such as local administration of platelet-rich plasma (PRP) and/or of mesenchymal stem cells have been explored extensively in tendon healing. Though treatment with PRP and mesenchymal stem cells has met with some success in horses (who experience a lot of tendon injuries and other tendon problems), the use of PRP and mesenchymal stem cells in people has been more problematic and requires more studies before PRP and mesenchymal stem cells can become reliable tools in management of soft tissue injuries and disorders.

Biomaterials strategies to balance inflammation and tenogenesis for tendon repair

Adult tendon tissue demonstrates a limited regenerative capacity, and the natural repair process leaves fibrotic scar tissue with inferior mechanical properties. Surgical treatment is insufficient to provide the mechanical, structural, and biochemical environment necessary to restore functional tissue. While numerous strategies including biodegradable scaffolds, bioactive factor delivery, and cell-based therapies have been investigated, most studies have focused exclusively on either suppressing inflammation or promoting tenogenesis, which includes tenocyte proliferation, ECM production, and tissue formation. New biomaterials-based approaches represent an opportunity to more effectively balance the two processes and improve regenerative outcomes from tendon injuries. Biomaterials applications that have been explored for tendon regeneration include formation of biodegradable scaffolds presenting topographical, mechanical, and/or immunomodulatory cues conducive to tendon repair; delivery of immunomodulatory or tenogenic biomolecules; and delivery of therapeutic cells such as tenocytes and stem cells. In this review, we provide the biological context for the challenges in tendon repair, discuss biomaterials approaches to modulate the immune and regenerative environment during the healing process, and consider the future development of comprehensive biomaterials-based strategies that can better restore the function of injured tendon. STATEMENT OF SIGNIFICANCE: Current strategies for tendon repair focus on suppressing inflammation or enhancing tenogenesis. Evidence indicates that regulated inflammation is beneficial to tendon healing and that excessive tissue remodeling can cause fibrosis. Thus, it is necessary to adopt an approach that balances the benefits of regulated inflammation and tenogenesis. By reviewing potential treatments involving biodegradable scaffolds, biological cues, and therapeutic cells, we contrast how each strategy promotes or suppresses specific repair steps to improve the healing outcome, and highlight the advantages of a comprehensive approach that facilitates the clearance of necrotic tissue and recruitment of cells during the inflammatory stage, followed by ECM synthesis and organization in the proliferative and remodeling stages with the goal of restoring function to the tendon.

Evaluation of Autologous Protein Solution Injection for Treatment of Superficial Digital Flexor Tendonitis in an Equine Model

Autologous protein solution (APS) has been used anecdotally for intralesional treatment of tendon and ligament injuries, however, its use in these injuries has never been studied in vivo. Our objective was to evaluate the effect of APS on tendon healing in an equine superficial digital flexor (SDF) tendonitis model. We hypothesized intralesional injection of APS would result in superior structural and biomechanical healing. SDF tendonitis was induced in both forelimbs of eight horses using collagenase injection. One forelimb was randomly assigned to receive an intralesional injection of APS, while the other was injected with saline. Ultrasonographic examinations were performed at weeks −1, 0, 2, 4, 8, and 12 following treatment. At 12 weeks, horses were euthanized and SDF samples harvested. Histologic evaluation, biomechanical testing, gene expression analysis, total glycosaminoglycan (GAG) and total DNA quantification were performed. Collagen type III (COL3A1) expression was significantly higher (p = 0.028) in saline treated tendon than in normal tendon. Otherwise, there were no significant differences in gene expression. There were no significant differences in histologic or ultrasonographic scores between groups. Mean total DNA content was significantly higher (p = 0.024) in saline treated tendons than normal tendons, whereas total DNA content was not significantly different between APS treated tendon and normal tendon. Elastic modulus was higher in APS treated than saline treated tendon, but the difference was not significant. Reduced expression of COL3A1 in APS treated tendon may indicate superior healing. Increased total DNA content in saline treated tendon may indicate ongoing healing processes, vs. APS treated tendons which may be in the later stages of healing. Limitations include a relatively short study period and inconsistency in size and severity of induced lesions. Intralesional injection of APS resulted in some improvements in healing characteristics.

Comparison of Cellular Responses to TGF-β1 and BMP-2 Between Healthy and Torn Tendons

BACKGROUND

Tendons heal by fibrotic repair, increasing the likelihood of reinjury. Animal tendon injury and overuse models have identified transforming growth factor beta (TGF-β) and bone morphogenetic proteins (BMPs) as growth factors actively involved in the development of fibrosis, by mediating extracellular matrix synthesis and cell differentiation.

PURPOSE

To understand how TGF-β and BMPs contribute to fibrotic processes using tendon-derived cells isolated from healthy and diseased human tendons.

STUDY DESIGN

Controlled laboratory study.

METHODS

Tendon-derived cells were isolated from patients with a chronic rotator cuff tendon tear (large to massive, diseased) and healthy hamstring tendons of patients undergoing anterior cruciate ligament repair. Isolated cells were incubated with TGF-β1 (10 ng/mL) or BMP-2 (100 ng/mL) for 3 days. Gene expression was measured by real-time quantitative polymerase chain reaction. Cell signaling pathway activation was determined by Western blotting.

RESULTS

TGF-β1 treatment induced ACAN mRNA expression in both cell types but less in the diseased compared with healthy cells (P < .05). BMP-2 treatment induced BGN mRNA expression in healthy but not diseased cells (P < .01). In the diseased cells, TGF-β1 treatment induced increased ACTA2 mRNA expression (P < .01) and increased small mothers against decapentaplegic (SMAD) signaling (P < .05) compared with those of healthy cells. Moreover, BMP-2 treatment induced ACTA2 mRNA expression in the diseased cells only (P < .05).

CONCLUSION

Diseased tendon-derived cells show reduced expression of the proteoglycans aggrecan and biglycan in response to TGF-β1 and BMP-2 treatments. These same treatments induced enhanced fibrotic differentiation and canonical SMAD cell signaling in diseased compared with healthy cells.

CLINICAL RELEVANCE

Findings from this study suggest that diseased tendon-derived cells respond differently than healthy cells in the presence of TGF-β1 and BMP-2. The altered responses of diseased cells may influence fibrotic repair processes during tendon healing.

Industrial Development of Standardized Fetal Progenitor Cell Therapy for Tendon Regenerative Medicine: Preliminary Safety in Xenogeneic Transplantation

Tendon defects require multimodal therapeutic management over extensive periods and incur high collateral burden with frequent functional losses. Specific cell therapies have recently been developed in parallel to surgical techniques for managing acute and degenerative tendon tissue affections, to optimally stimulate resurgence of structure and function. Cultured primary human fetal progenitor tenocytes (hFPT) have been preliminarily considered for allogeneic homologous cell therapies, and have been characterized as stable, consistent, and sustainable cell sources in vitro. Herein, optimized therapeutic cell sourcing from a single organ donation, industrial transposition of multi-tiered progenitor cell banking, and preliminary preclinical safety of an established hFPT cell source (i.e., FE002-Ten cell type) were investigated. Results underlined high robustness of FE002-Ten hFPTs and suitability for sustainable manufacturing upscaling within optimized biobanking workflows. Absence of toxicity or tumorigenicity of hFPTs was demonstrated in ovo and in vitro, respectively. Furthermore, a 6-week pilot good laboratory practice (GLP) safety study using a rabbit patellar tendon partial-thickness defect model preliminarily confirmed preclinical safety of hFPT-based standardized transplants, wherein no immune reactions, product rejection, or tumour formation were observed. Such results strengthen the rationale of the multimodal Swiss fetal progenitor cell transplantation program and prompt further investigation around such cell sources in preclinical and clinical settings for musculoskeletal regenerative medicine.

Amniotic membrane-mesenchymal stromal cells secreted factors and extracellular vesicle-miRNAs: Anti-inflammatory and regenerative features for musculoskeletal tissues

Human amniotic membrane‐derived mesenchymal stromal cells (hAMSCs) are easily obtained in large quantities and free from ethical concerns. Promising therapeutic results for both hAMSCs and their secreted factors (secretome) were described by several in vitro and preclinical studies, often for treatment of orthopedic disorders such as osteoarthritis (OA) and tendinopathy. For clinical translation of the hAMSC secretome as cell‐free therapy, a detailed characterization of hAMSC‐secreted factors is mandatory. Herein, we tested the presence of 200 secreted factors and 754 miRNAs in extracellular vesicles (EVs). Thirty‐seven cytokines/chemokines were identified at varying abundance, some of which involved in both chemotaxis and homeostasis of inflammatory cells and in positive remodeling of extracellular matrix, often damaged in tendinopathy and OA. We also found 336 EV‐miRNAs, 51 of which accounted for more than 95% of the genetic message. A focused analysis based on miRNAs related to OA and tendinopathy showed that most abundant EV‐miRNAs are teno‐ and chondro‐protective, able to induce M2 macrophage polarization, inhibit inflammatory T cells, and promote Treg. Functional analysis on IL‐1β treated tenocytes and chondrocytes resulted in downregulation of inflammation‐associated genes. Overall, presence of key regulatory molecules and miRNAs explain the promising therapeutic results of hAMSCs and their secretome for treatment of musculoskeletal conditions and are a groundwork for similar studies in other pathologies. Furthermore, identified molecules will pave the way for future studies aimed at more sharply predicting disease‐targeted clinical efficacy, as well as setting up potency and release assays to fingerprint clinical‐grade batches of whole secretome or purified components.

A novel murine muscle loading model to investigate Achilles musculotendinous adaptation

Achilles tendinopathy is a debilitating condition affecting the entire spectrum of society and a condition that increases the risk of tendon rupture. Effective therapies remain elusive, as anti-inflammatory drugs and surgical interventions show poor long-term outcomes. Eccentric loading of the Achilles muscle-tendon unit is an effective physical therapy for treatment of symptomatic human tendinopathy. Here, we introduce a novel mouse model of hindlimb muscle loading designed to achieve a tissue-targeted therapeutic exercise. This model includes the application of tissue (muscle and tendon)-loading "doses," coupled with ankle dorsiflexion and plantarflexion, inspired by human clinical protocols. Under computer control, the foot was rotated through the entire ankle joint range of motion while the plantar flexors simultaneously contracted to simulate body mass loading, consistent with human therapeutic exercises. This approach achieved two key components of the heel drop and raise movement: ankle range of motion coupled with body mass loading. Model development entailed the tuning of parameters such as footplate speed, number of repetitions, number of sets of repetitions, treatment frequency, treatment duration, and treatment timing. Initial model development was carried out on uninjured mice to define a protocol that was well tolerated and nondeleterious to tendon biomechanical function. When applied to a murine Achilles tendinopathy model, muscle loading led to a significant improvement in biomechanical outcome measures, with a decrease in cross-sectional area and an increase in material properties, compared with untreated animals. Our model facilitates the future investigation of mechanisms whereby rehabilitative muscle loading promotes healing of Achilles tendon injuries.NEW & NOTEWORTHY We introduce a novel mouse model of hindlimb muscle loading designed to achieve a tissue-targeted therapeutic exercise. This innovative model allows for application of muscle loading "doses," coupled with ankle dorsiflexion and plantarflexion, inspired by human loading clinical treatment. Our model facilitates future investigation of mechanisms whereby rehabilitative muscle loading promotes healing of Achilles tendon injuries.

Evaluation of Secretome Tenogenic Potential from Adipose Stem Cells (ACS) in Hypoxic Condition with Fresh Frozen Tendon Scaffold Using Scleraxis (Scx), Insulin-Like Growth Factor 1 (IGF-1) and Collagen Type 1

Various studies have been conducted to see the scaffold that supports the regeneration of tendon. This study aims to analyze thein vitrosecretome tenogenic potential produced by ASCs culture with fresh frozen tendon scaffold in hypoxic conditions. ELISA tests for Scx and IGF-1 levels in secretome were obtained from ASC culture with fresh frozen tendon scaffold under normoxic (21%) and hypoxia (2%) conditions. The immunohistochemical examination of COL-1 was also carried out on the 2ndand 6thdays of cell culture. The secretion of Scx and IGF-1 was increased in secretome from ASC cultures using a fresh frozen tendon scaffold compared with those which did not (p <0.05). In the normoxia condition, Scx and IGF-1 in secretome with fresh frozen tendons had better results than hypoxic conditions (p <0.05). The highest Scx levels were obtained in culture on the 6thday (p <0.05), while the highest IGF-1 levels were obtained in the culture on the 2ndday (p <0.05). There was an increase in the secretion of Scx and IGF-1 from ASC cultures with fresh frozen tendon scaffold under the hypoxic condition of 2%.

Localized delivery of ibuprofen via a bilayer delivery system (BiLDS) for supraspinatus tendon healing in a rat model

The high prevalence of tendon retear following rotator cuff repair motivates the development of new therapeutics to promote improved tendon healing. Controlled delivery of non-steroidal anti-inflammatory drugs to the repair site via an implanted scaffold is a promising option for modulating inflammation in the healing environment. Furthermore, biodegradable nanofibrous delivery systems offer an optimized architecture and surface area for cellular attachment, proliferation, and infiltration while releasing soluble factors to promote tendon regeneration. To this end, we developed a bilayer delivery system (BiLDS) for localized and controlled release of ibuprofen (IBP) to temporally mitigate inflammation and enhance tendon remodeling following surgical repair by promoting organized tissue formation. In vitro evaluation confirmed the delayed and sustained release of IBP from Labrafil-modified poly(lactic-co-glycolic) acid microspheres within sintered poly(ε-caprolactone) electrospun scaffolds. Biocompatibility of the BiLDS was demonstrated with primary Achilles tendon cells in vitro. Implantation of the IBP-releasing BiLDS at the repair site in a rat rotator cuff injury and repair model led to decreased expression of proinflammatory cytokine, tumor necrotic factor-α, and increased anti-inflammatory cytokine, transforming growth factor-β1. The BiLDS remained intact for mechanical reinforcement and recovered the tendon structural properties by 8 weeks. These results suggest the therapeutic potential of a novel biocompatible nanofibrous BiLDS for localized and tailored delivery of IBP to mitigate tendon inflammation and improve repair outcomes. Future studies are required to define the mechanical implications of an optimized BiLDS in a rat model beyond 8 weeks or in a larger animal model.

Standing wave-assisted acoustic droplet vaporization for single and dual payload release in acoustically-responsive scaffolds

An ultrasound standing wave field (SWF) has been utilized in many biomedical applications. Here, we demonstrate how a SWF can enhance drug release using acoustic droplet vaporization (ADV) in an acoustically-responsive scaffold (ARS). ARSs are composite fibrin hydrogels containing payload-carrying, monodispersed perfluorocarbon (PFC) emulsions and have been used to stimulate regenerative processes such as angiogenesis. Elevated amplitudes in the SWF significantly enhanced payload release from ARSs containing dextran-loaded emulsions (nominal diameter: 6 μm) compared to the -SWF condition, both at sub- and suprathreshold excitation pressures. At 2.5 MHz and 4 MPa peak rarefactional pressure, the cumulative percentage of payload released from ARSs reached 84.1 ± 5.4% and 66.1 ± 4.4% under + SWF and -SWF conditions, respectively, on day 10. A strategy for generating a SWF for an in situ ARS is also presented. For dual-payload release studies, bi-layer ARSs containing a different payload within each layer were exposed to temporally staggered ADV at 3.25 MHz (day 0) and 8.6 MHz (day 4). Sequential payload release was demonstrated using dextran payloads as well as two growth factors relevant to angiogenesis: basic fibroblast growth factor (bFGF) and platelet-derived growth factor BB (PDGF-BB). In addition, bubble growth and fibrin degradation were characterized in the ARSs under +SWF and -SWF conditions. These results highlight the utility of a SWF for modulating single and dual payload release from an ARS and can be used in future therapeutic studies.

Mesenchymal Stem Cells Seeded Decellularized Tendon Scaffold for Tissue Engineering

Tendon is a collagenous tissue to connect bone and muscle. Healing of damaged/injured tendon is the primary clinical challenge in musculoskeletal regeneration because they often react poorly to treatment. Tissue engineering (a triad strategy of scaffolds, cells and growth factors) may have the potential to improve the quality of tendon tissue healing under such impaired situations. Tendon tissue engineering aims to synthesize graft alternatives to repair the injured tendon. Biological scaffolds derived from decellularized tissue may be a better option as their biomechanical properties are similar to the native tissue. This review is designed to provide background information on the current challenges in curing torn/worn out the tendon and the clinical relevance of decellularized scaffolds for such applications.

Histologic changes and gene expression patterns in biopsy specimens from bacteria-inoculated and noninoculated excisional body and limb wounds in horses healing by second intention

OBJECTIVE

To evaluate histologic changes and gene expression patterns in body and limb wounds in horses in response to bacterial inoculation.

SAMPLE

Wound biopsy specimens from 6 horses collected on days 7, 14, 21, and 27 after excisional wounds (20 wounds/horse) were created over the metacarpal and metatarsal region and lateral thoracic region (body) and then inoculated or not inoculated on day 4 with Staphylococcus aureus and Pseudomonas aeruginosa.

PROCEDURES

Specimens were histologically scored for the amount of inflammation, edema, angiogenesis, fibrosis organization, and epithelialization. Quantitative PCR assays were performed to quantify gene expression of 10 inflammatory, proteolytic, fibrotic, and hypoxia-related markers involved in wound healing.

RESULTS

Except for gene expression of interleukin-6 on day 27 and tumor necrosis factor-α on day 14, bacterial inoculation had no significant effect on histologic scores and gene expression. Gene expression of interleukin-1β and -6, serum amyloid A, and matrix metalloproteinase-9 was higher in limb wounds versus body wounds by day 27. Gene expression of cellular communication network factor 1 was higher in limb wounds versus body wounds throughout the observation period.

CONCLUSIONS AND CLINICAL RELEVANCE

The lack of clear markers of wound infection in this study reflected well-known difficulties in detecting wound infections in horses. Changes consistent with protracted inflammation were evident in limb wounds, and gene expression patterns of limb wounds shared similarities with those of chronic wounds in humans. Cellular communication network factor warrants further investigation and may be useful in elucidating the mechanisms underlying poor limb wound healing in horses.

Is clinical effect of autologous conditioned serum in spontaneously occurring equine articular lameness related to ACS cytokine profile?

Background

Biologic’ therapies, such as autologous conditioned serum (ACS), are gaining popularity in treating orthopaedic conditions in equine veterinary medicine. Evidence is scarce regarding ACS constituents, and large inter-individual differences in cytokine and growth factor content have been demonstrated. The objective of the current study was to investigate the potential association between cytokine and growth factor content of ACS and clinical effect in harness racehorses with spontaneously occurring low-grade articular lameness. Horses received 3 intra-articular injections of ACS administered at approximately 2-week intervals. Lameness evaluation consisting of a trot-up with subsequent flexions tests was performed at inclusion and approximately 2 weeks after the last treatment (re-evaluation); horses were classified as responders when there was no detectable lameness on trot-up and a minimum of 50% reduction in flexion test scores at re-evaluation. Association between clinical outcome (responders vs. non-responders) and age, lameness grades at inclusion (both initial trot-up and after flexion tests), treatment interval, follow-up time and the ACS content of IL-1Ra, IGF-1 and TGF-β was determined by regression modelling.

Results

Outcome analysis was available for 19 of 20 included horses; 11 responded to treatment whereas 8 did not. There was considerable inter-individual variability in cytokine/growth factor content of ACS, and in the majority of the horses, the level of IL-10, IL-1β and TNF-α was below the detection limit. In the final multivariate logistic regression model, ACS content of IGF-1 and IL-1Ra was significantly associated with clinical response (P = 0.01 and P = 0.03, respectively). No association with clinical response was found for the other tested variables.

Conclusions

The therapeutic benefit of ACS may be related to higher levels of IL-1Ra and IGF-1. Our study corroborates previous findings of considerable inter-individual variability of cytokine- and growth factor content in ACS.

Quantitative Assessment of Tendon Hierarchical Structure by Combined Second Harmonic Generation and Immunofluorescence Microscopy

Histological evaluation of healing tendons is primarily focused on monitoring restoration of longitudinal collagen alignment, although the elastic property of energy-storing flexor tendons is largely attributed to interfascicular sliding facilitated by the interfascicular matrix (IFM). The objectives of this study were to explore the utility of second harmonic generation (SHG) imaging to objectively assess cross-sectional tendon fascicle architecture, to combine SHG microscopy with elastin immunofluorescence to assess the ultrastructure of collagen and elastin in longitudinal and transverse sections, and lastly, to quantify changes in IFM elastin and fascicle collagen alignment of normal and collagenase-injured flexor tendons. Paraffin-embedded transverse and longitudinal histological sections (10-μm thickness) derived from normal and collagenase-injured (6- and 16-week time-points) equine superficial digital flexor tendons were de-paraffinized, treated with Tris EDTA at 80°C for epitope retrieval, and incubated with mouse monoclonal anti-elastin antibody (1:100 dilution) overnight. Anti-mouse IgG Alexa Flour 546 secondary antibody was applied, and sections were mounted with ProLong Gold reagent with 4',6-diamidino-2-phenylindole (DAPI). Nuclei (DAPI) and elastin (Alexa Fluor 546) signals were captured by using standard confocal imaging with 405 and 543 nm excitation wavelengths, respectively. The SHG signal was captured by using a tunable Ti:Sapphire laser tuned to 950 nm to visualize type I collagen. Quantitative measurements of fascicle cross-sectional area (CSA), IFM thickness in transverse SHG-DAPI merged z-stacks, fascicle/IFM elastin area fraction (%), and elastin-collagen alignment in longitudinal SHG-elastin merged z-stacks were conducted by using ImageJ software. Using this methodology, fascicle CSA, IFM thickness, and IFM elastin area fraction (%) at 6 weeks (∼2.25-fold; ∼2.8-fold; 60% decrease; p < 0.001) and 16 weeks (∼2-fold; ∼1.5-fold; 70% decrease; p < 0.001) after collagenase injection, respectively, were found to be significantly different from normal tendon. IFM elastin and fascicle collagen alignment characterized via fast Fourier transform (FFT) frequency plots at 16 weeks demonstrated that collagen re-alignment was more advanced than that of elastin. The integration of SHG-derived quantitative measurements in transverse and longitudinal tendon sections supports comprehensive assessment of tendon structure. Our findings demonstrate the importance of including IFM and non-collagenous proteins in tendon histological evaluations, tasks that can be effectively carried out by using SHG and immunofluorescence microscopy. Impact statement This work demonstrated that second harmonic generation microscopy in conjunction with elastin immunofluorescence provided a comprehensive assessment of multiscale structural re-organization in healing tendon than when restricted to longitudinal collagen fiber alignment alone. Utilizing this approach for tendon histomorphometry is ideal not only to improve our understanding of hierarchical structural changes that occur after tendon injury and during remodeling but also to monitor the efficacy of therapeutic approaches.

Flexor Tendon: Development, Healing, Adhesion Formation, and Contributing Growth Factors

Management of flexor tendon injuries of the hand remains a major clinical problem. Even with intricate repair, adhesion formation remains a common complication. Significant progress has been made to better understand the mechanisms of healing and adhesion formation. However, there has been slow progress in the clinical prevention and reversal of flexor tendon adhesions. The goal of this article is to discuss recent literature relating to tendon development, tendon healing, and adhesion formation to identify areas in need of further research. Additional research is needed to understand and compare the molecular, cellular, and genetic mechanisms involved in flexor tendon morphogenesis, postoperative healing, and mechanical loading. Such knowledge is critical to determine how to improve repair outcomes and identify new therapeutic strategies to promote tissue regeneration and prevent adhesion formation.

Synthesis and Formulation of Four-Arm PolyDMAEA-siRNA Polyplex for Transient Downregulation of Collagen Type III Gene Expression in TGF-β1 Stimulated Tenocyte Culture

The natural healing process for tendon repair is associated with high upregulation of collagen type III, leading to scar tissue and tendon adhesions with functionally deficient tendons. Gene delivery systems are widely reported as potential nanotherapeutics to treat diseases, providing a promising approach to modulate collagen type III synthesis. This work investigates a proof-of-concept four-arm cationic polymer-siRNA polyplex to mediate a transient downregulation of collagen type III expression in a tendon cell culture system. The tendon culture system was first supplemented with TGF-β1 to stimulate the upregulation of collagen type III prior to silencing experiments. The four-arm poly[2-(dimethylamino) ethyl acrylate] (PDMAEA) polymer was successfully synthesized via RAFT polymerization and then mixed with siRNA to formulate the PDMAEA-siRNA polyplexes. The formation of the polyplex was optimized for the N:P ratio (10:1) and confirmed by agarose gel electrophoresis. The size and solution behavior of the polyplex were analyzed by dynamic light scattering and zeta potential, showing a hydrodynamic diameter of 155 ± 21 nm and overall positive charge of +30 mV at physiological pH. All the polyplex concentrations used had a minimal effect on the metabolic activity of cultured cells, indicating good biocompatibility. The dose and time effects of the TGF-β1 on collagen type III gene expressions were analyzed by qPCR, showing an optimal dose of 10 ng mL^-1 TGF-β1 and 3-fold increase of COL3α1 expression at 48 h in cultured tenocytes. The PDMAEA-siRNA polyplex concept observed a limited yet successful and promising efficiency in silencing collagen type III at 48 h compared to PEI-siRNA. Therefore, this concept is a promising approach to reduce tissue scarring and adhesion following injuries.

The "other" 15-40%: The Role of Non-Collagenous Extracellular Matrix Proteins and Minor Collagens in Tendon

Extracellular matrix (ECM) determines the physiological function of all tissues, including musculoskeletal tissues. In tendon, ECM provides overall tissue architecture, which is tailored to match the biomechanical requirements of their physiological function, that is, force transmission from muscle to bone. Tendon ECM also constitutes the microenvironment that allows tendon-resident cells to maintain their phenotype and that transmits biomechanical forces from the macro-level to the micro-level. The structure and function of adult tendons is largely determined by the hierarchical organization of collagen type I fibrils. However, non-collagenous ECM proteins such as small leucine-rich proteoglycans (SLRPs), ADAMTS proteases, and cross-linking enzymes play critical roles in collagen fibrillogenesis and guide the hierarchical bundling of collagen fibrils into tendon fascicles. Other non-collagenous ECM proteins such as the less abundant collagens, fibrillins, or elastin, contribute to tendon formation or determine some of their biomechanical properties. The interfascicular matrix or endotenon and the outer layer of tendons, the epi- and paratenon, includes collagens and non-collagenous ECM proteins, but their function is less well understood. The ECM proteins in the epi- and paratenon may provide the appropriate microenvironment to maintain the identity of distinct tendon cell populations that are thought to play a role during repair processes after injury. The aim of this review is to provide an overview of the role of non-collagenous ECM proteins and less abundant collagens in tendon development and homeostasis. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:23-35, 2020.

Augmenting endogenous repair of soft tissues with nanofibre scaffolds

As our ability to engineer nanoscale materials has developed we can now influence endogenous cellular processes with increasing precision. Consequently, the use of biomaterials to induce and guide the repair and regeneration of tissues is a rapidly developing area. This review focuses on soft tissue engineering, it will discuss the types of biomaterial scaffolds available before exploring physical, chemical and biological modifications to synthetic scaffolds. We will consider how these properties, in combination, can provide a precise design process, with the potential to meet the requirements of the injured and diseased soft tissue niche. Finally, we frame our discussions within clinical trial design and the regulatory framework, the consideration of which is fundamental to the successful translation of new biomaterials.

Stem Cell Therapy for Tendon Regeneration: Current Status and Future Directions

In adults the healing tendon generates fibrovascular scar tissue and recovers never histologically, mechanically, and functionally which leads to chronic and to degenerative diseases. In this review, the processes and mechanisms of tendon development and fetal regeneration in comparison to adult defect repair and degeneration are discussed in relation to regenerative therapeutic options. We focused on the application of stem cells, growth factors, transcription factors, and gene therapy in tendon injury therapies in order to intervene the scarring process and to induce functional regeneration of the lesioned tissue. Outlines for future therapeutic approaches for tendon injuries will be provided.

Parametric Study of Acoustic Droplet Vaporization Thresholds and Payload Release From Acoustically-Responsive Scaffolds

Hydrogels are commonly used for the delivery of bioactive molecules, especially growth factors and cytokines capable of stimulating tissue regeneration. Regenerative processes are regulated by the concentrations and spatiotemporal presentations of these molecules. With conventional hydrogels, these critical delivery parameters cannot be actively modulated after implantation. We have developed composite hydrogel scaffolds where payload release is non-invasively modulated, in an on-demand manner, using ultrasound (US). These acoustically-responsive scaffolds (ARSs) consist of a fibrin matrix doped with a payload-carrying, perfluorocarbon (PFC) double emulsion. Previously, acoustic droplet vaporization (ADV) was used to trigger release of a pro-angiogenic growth factor, encapsulated in the ARS, which stimulated blood vessel formation in vivo. In the present study, we assess how characteristics of the monodispersed emulsion, fibrin matrix, and US impact ADV thresholds and the release efficiency of a dextran payload. ADV thresholds increased with the molecular weight of the PFC in the emulsion and inversely with the volume fraction of emulsion in the ARS. Payload release from ARSs with perfluoroheptane (C7) or perfluorooctane (C8) emulsions was dependent on the number of z-planes of US used to generate ADV and inversely dependent on the lateral spacing. Conversely, release from ARSs with perfluoropentane (C5) or perfluorohexane (C6) emulsions was less dependent on these US exposure parameters. After ADV, payload diffusion decreased significantly in ARSs with C5 or C6 emulsions compared with ARSs with C7 or C8 emulsions. The expansion of the ARS after ADV decreased with the molecular weight of the PFC. Non-selective release increased with the molecular weight of the PFC and thrombin concentration. Overall, these findings can be used for optimization of ARS properties and US parameters in future therapeutic applications.

The cellular basis of fibrotic tendon healing: challenges and opportunities

Tendon injuries are common and can dramatically impair patient mobility and productivity, resulting in a significant socioeconomic burden and reduced quality of life. Because the tendon healing process results in the formation of a fibrotic scar, injured tendons never regain the mechanical strength of the uninjured tendon, leading to frequent reinjury. Many tendons are also prone to the development of peritendinous adhesions and excess scar formation, which further reduce tendon function and lead to chronic complications. Despite this, there are currently no treatments that adequately improve the tendon healing process due in part to a lack of information regarding the contributions of various cell types to tendon healing and how their activity may be modulated for therapeutic value. In this review, we summarize recent efforts to identify and characterize the distinct cell populations involved at each stage of tendon healing. In addition, we examine the mechanisms through which different cell populations contribute to the fibrotic response to tendon injury, and how these responses can be affected by systemic factors and comorbidities. We then discuss gaps in our current understanding of tendon fibrosis and highlight how new technologies and research areas are shedding light on this clinically important and intractable challenge. A better understanding of the complex cellular environment during tendon healing is crucial to the development of new therapies to prevent fibrosis and promote tissue regeneration.

Adipose-Derived Stromal Vascular Fraction and Cultured Stromal Cells as Trophic Mediators for Tendon Healing

Adipose-derived stromal vascular fraction (SVF) is a heterogeneous population of cells that yields a homogeneous population of plastic-adherent adipose tissue-derived stromal cells (ASC) when culture-expanded. SVF and ASC have been used clinically to improve tendon healing, yet their mechanism of action is not fully elucidated. The objective of this study was to investigate the potential for ASC to act as trophic mediators for tendon healing. Flexor digitorum superficialis tendons and adipose tissue were harvested from adult horses to obtain SVF, ASC, and tenocytes. Growth factor gene expression was quantified in SVF and ASC in serial passages and growth factors were quantified in ASC-conditioned medium (CM). Microchemotaxis assays were performed using ASC-CM. Tenocytes were grown in co-culture with autologous ASC or allogeneic SVF. Gene expression for insulin-like growth factor 1 (IGF-1), stromal cell-derived factor-1α (SDF-1α), transforming growth factor-β1 (TGF-β1) and TGF-β3 was significantly higher in SVF compared to ASC. Concentrations were significantly increased in ASC-CM compared to controls for IGF-1 (4-fold) and SDF-1α (6-fold). Medium conditioned by ASC induced significant cell migration in a dose-dependent manner. Gene expression for collagen types I and III, decorin, and cartilage oligomeric matrix protein was modestly, but significantly increased following co-culture of tenocytes with autologous ASC. Our findings support the ability of SVF and ASC to act as trophic mediators in tendon healing, particularly through chemotaxis, which stands to critically impact the intrinsic healing response. In vivo studies to further delineate the potential for SVF and/or ASC to improve tendon healing are warranted. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1429-1439, 2019.

Optimizing growth factor induction of tenogenesis in three-dimensional culture of mesenchymal stem cells

Adult tissue stem cells have shown promise for the treatment of debilitating tendon injuries. However, few comparisons of stem cells from different tissue sources have been made to determine the optimum stem cell source for treating tendon. Moreover, it is likely that the application of tenogenic growth factors will improve tendon stem cell treatments further, and a comprehensive comparison of a number of growth factors is needed. Thus far, different types of stem cells cannot be evaluated in a high-throughput manner. To this end, we have developed an approach to culture mesenchymal stem cells isolated from bone marrow in collagen type I hydrogels with tenogenic growth factors using economical, commercially available supplies. To optimize growth factors for this assay, FGF-2, TGF-β1, IGF-1, and/or BMP-12 were tested singly and in novel combinations of (1) BMP-12 and IGF-1, (2) TGF-β1 and IGF-1, and/or (3) BMP-12 and FGF-2 over 10 days. Our data suggest that BMP-12 supplementation alone results in the strongest expression of tendon marker genes, controlled contractility of constructs, a higher degree of cell alignment, and tendon-like tissue morphology. This easy-to-use benchtop assay can be used to screen novel sources of stem cells and cell lines for tissue engineering and tendon healing applications.

Evaluation of two distinct placental-derived membranes and their effect on tenocyte responses in vitro

Tendon healing is a complex, multiphase process that results in increased scar tissue formation, leading to weaker tendons. The purpose of this study was to evaluate the response of tenocytes to both hypothermically stored amniotic membrane (HSAM) and dehydrated amnion/chorion membrane (dACM). Composition and growth factor release from HSAM and dACM were evaluated using proteomics microarrays. HSAM and dACM releasate was used to assess tenocyte proliferation, migration, gene expression, extracellular matrix (ECM) protein deposition, and response to inflammation. Additionally, tenocyte-ECM interactions were evaluated. HSAM and dACM contain and release growth factors relevant to tendon healing, including insulin-like growth factor I, platelet-derived growth factor, and basic fibroblast growth factor. Both dACM and HSAM promoted increased tenocyte proliferation and migration; tenocytes treated with dACM proliferated more robustly, whereas treatment with HSAM resulted in higher migration. Both dACM and HSAM resulted in altered ECM gene expression; dACM grafts alone resulted in increases in collagen deposition. Furthermore, both allografts resulted in altered tenocyte responses to inflammation with reduced transforming growth factor beta levels. Additionally, dACM treatment resulted in increased expression and production of matrix metalloprotease-1 (MMP-1), whereas HSAM treatment resulted in decreased production of MMP-1. Tenocytes migrated into and remodeled HSAM only. These results indicate that both grafts have properties that support tendon healing; however, the results presented here suggest that the responses to each type of graft may be different. Due to the complex environment during tendon repair, additional work is needed to evaluate these effects using in vivo models.

Platelet-Rich Fibrin Scaffolds for Cartilage and Tendon Regenerative Medicine: From Bench to Bedside

Nowadays, research in Tissue Engineering and Regenerative Medicine is focusing on the identification of instructive scaffolds to address the requirements of both clinicians and patients to achieve prompt and adequate healing in case of injury. Among biomaterials, hemocomponents, and in particular Platelet-rich Fibrin matrices, have aroused widespread interest, acting as delivery platforms for growth factors, cytokines and immune/stem-like cells for immunomodulation; their autologous origin and ready availability are also noteworthy aspects, as safety- and cost-related factors and practical aspects make it possible to shorten surgical interventions. In fact, several authors have focused on the use of Platelet-rich Fibrin in cartilage and tendon tissue engineering, reporting an increasing number of in vitro, pre-clinical and clinical studies. This narrative review attempts to compare the relevant advances in the field, with particular reference being made to the regenerative role of platelet-derived growth factors, as well as the main pre-clinical and clinical research on Platelet-rich Fibrin in chondrogenesis and tenogenesis, thereby providing a basis for critical revision of the topic.

Placental Stem Cells from Domestic Animals: Translational Potential and Clinical Relevance

The field of regenerative medicine is moving toward clinical practice in veterinary science. In this context, placenta-derived stem cells isolated from domestic animals have covered a dual role, acting both as therapies for patients and as a valuable cell source for translational models. The biological properties of placenta-derived cells, comparable among mammals, make them attractive candidates for therapeutic approaches. In particular, stemness features, low immunogenicity, immunomodulatory activity, multilineage plasticity, and their successful capacity for long-term engraftment in different host tissues after autotransplantation, allo-transplantation, or xenotransplantation have been demonstrated. Their beneficial regenerative effects in domestic animals have been proven using preclinical studies as well as clinical trials starting to define the mechanisms involved. This is, in particular, for amniotic-derived cells that have been thoroughly studied to date. The regenerative role arises from a mutual tissue-specific cell differentiation and from the paracrine secretion of bioactive molecules that ultimately drive crucial repair processes in host tissues (e.g., anti-inflammatory, antifibrotic, angiogenic, and neurogenic factors). The knowledge acquired so far on the mechanisms of placenta-derived stem cells in animal models represent the proof of concept of their successful use in some therapeutic treatments such as for musculoskeletal disorders. In the next future, legislation in veterinary regenerative medicine will be a key element in order to certify those placenta-derived cell-based protocols that have already demonstrated their safety and efficacy using rigorous approaches and to improve the degree of standardization of cell-based treatments among veterinary clinicians.

Effect of metritis on endometrium tissue transcriptome during puerperium in Holstein lactating cows

The objective of this prospective cohort study was to evaluate the effect of parity and uterine health status postpartum on the gene expression profile of the endometrium early post-partum. Twenty-four Holstein cows were randomly selected (16 multiparous (MP) and 8 primiparous (PP)) and endometrium biopsies were collected on days 1, 3, and 6 after calving and clinically monitored for metritis. Rectal temperature was measured twice and fever was defined as a temperature ≥39.5 °C. A case of metritis was diagnosed with the presence of red-brown watery, foul-smelling uterine discharge or a purulent discharge with more than 50% pus and fever between days 1 and 6 postpartum. Cows were then retrospectively selected (cows diagnosed with metritis were paired with healthy ones) to analyze the expression of 66 genes measured on the NanoString nCounter Analysis System. The genes selected were related with adhesion, immune system, steroid and prostaglandin biosynthesis regulation, insulin metabolism and transcription factors, and nutrient transporters. The results indicated a different pattern on genes related to immune function by parity. PTX3, involved in antigen presentation, was increased in healthy MP compared with healthy PP whereas inflammatory cytokine TNFα and complement-related protein SERPING1 was upregulated in MP compared with PP (P < 0.05). As expected, presence of a metritis condition affected the expression of genes related to immune function. There was an increased expression of the antiviral factor MX2 and MYH10 gene, which is involved in macrophages recruitment, in metritic compared with healthy cows (P < 0.05). Differences in uterine involution from cows diagnosed with metritis were reflected by the downregulation of IGF1 (P < 0.10), involved in endometrium remodeling, and a possible compensatory upregulation of its receptor IGFR1 (P < 0.05). A greater expression of prostaglandins and oxytocin receptors (PGR and OXTR), involved in the involution process, were observed in metritic PP compared with healthy PP (P < 0.05). Overall, it seems that metritis significantly modulate processes closely tied with the physical involution of the uterus early post-partum (IGF1, IGFR1, PGR, OXTR), whereas both metritis and multiparous cows tended to upregulate genes related to immune response (PTX3, TNFα, SERPING1, MX2, MYH10).

Rotator cuff tear healing process with graft augmentation of fascia lata in a rabbit model

Background

Fascia lata augmentation of massive rotator cuff tears has shown good clinical results. However, its biological effect during the early healing process is not clearly understood. The purpose of the study was to evaluate the biological efficacy of fascia lata augmentation during the early healing process of rotator cuff tears using a rabbit rotator cuff defect model.

Methods

The infraspinatus tendon was resected from the greater tuberosity of a rabbit to create a rotator cuff tear. The tendon edge was directly sutured to the humeral head. The rotator cuff repaired site was augmented with a fascia lata autograft (augmentation group, group A). The rotator cuff defect in the contralateral shoulder was repaired without augmentation (reattachment group, group R). A group with intact rotator cuff was set as the control group. Histological examinations and mechanical analysis were conducted 4 and 8 weeks postoperatively.

Results

In the HE staining, the tendon maturing score of group A was higher than that of group R at 4 weeks postoperatively. In the safranin O staining, proteoglycan staining at the repaired enthesis in group A at 4 weeks postoperatively was stronger than that in group R. Picrosirius red staining showed that type III and type I collagen in group A was more strongly expressed than that in group R at 4 weeks postoperatively. The ultimate failure load of the infraspinatus tendon–humeral head complex in group A was statistically higher than that in group R at 4 weeks postoperatively. The ultimate failure load of group A was similar to that of the control group.

Conclusion

The biological and mechanical contribution of fascia lata augmentation for massive rotator cuff tears was analyzed in this study. Type III collagen was reported to be expressed during the tendon healing process. Although the biological action similar to natural ligament healing occurred around the fascia lata grafts, type III collagen was gradually replaced by type I collagen as the tissue matured. Our results suggest that fascia lata augmentation could stimulate biological healing and provide initial fixation strength of the repaired rotator cuff.

Effect of recombinant human growth hormone on rotator cuff healing after arthroscopic repair: preliminary result of a multicenter, prospective, randomized, open-label blinded end point clinical exploratory trial

BACKGROUND

This study evaluated the effect of systemic injection of recombinant human growth hormone (rhGH) on outcomes after arthroscopic rotator cuff repair.

METHODS

This multicenter, prospective, randomized, comparative trial, randomized patients who underwent arthroscopic repair of large-sized rotator cuff tears into 3 groups: rhGH 4 mg group (n = 26), rhGH 8 mg group (n = 24) , and control group (n = 26). Sustained release rhGH was injected subcutaneously once weekly for 3 months postoperatively. The healing failure rate (primary end point), fatty infiltration, and atrophy of the supraspinatus muscle, and functional scores (Constant and American Shoulder and Elbow Surgeons scores) were evaluated at 6 months. Range of motion, pain visual analog scale, and serum insulin-like growth factor-1 level were measured at each follow-up.

RESULTS

The healing failure rate was similar between groups (rhGH 4 mg group, 30.8%; rhGH 8 mg group, 16.7%; and control group, 34.6%; all P > .05) The proportion of severe fatty infiltration (Goutallier grade ≥3) was 20.8% in the rhGH 8 mg group, 23.1% in the rhGH 4 mg group, and 34.6% in the control group (P > .05). Functional outcomes, range of motion, and pain visual analog scale were similar between groups (all P > .05). The rhGH 8 mg group showed more increased peak insulin-like growth factor-1 level (279.43 ng/mL) than the rhGH 4 mg group ((196.82 ng/mL) and control group (186.31 ng/mL), which was not statistically different (all P > .05). No rhGH injection-related major safety issues occurred.

CONCLUSIONS

This preliminary study showed no statistically significant improvement in healing or outcomes related to the treatment of rhGH after rotator cuff repair. However, further study with more enrolled patients after resetting the rhGH dose or daily administration protocol would be mandatory.

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.

Psychosocial Factors Impacting Workplace Injury Rehabilitation: Evaluation of a Concise Screening Tool

Purpose To determine whether the delayed recovery often observed in simple musculoskeletal injuries occurring at work is related to poor workplace and home social support. Method A four question psychosocial screening tool called the "How are you coping gauge?" (HCG) was developed. This tool was implemented as part of the initial assessment for all new musculoskeletal workplace injuries. Participants were excluded if they did not meet the strict criteria used to classify a musculoskeletal injury as simple. The HCG score was then compared to the participant's number of days until return to full capacity (DTFC). It was hypothesised that those workers indicating a poorer level of workplace and home support would take longer time to return to full capacity. Results A sample of 254 participants (316 excluded) were included in analysis. Significant correlation (p < 0.001) was observed between HCG scores for self-reported work and home support and DTFC thereby confirming the hypothesis. Path analysis found workplace support to be a significant moderate-to-strong predictor of DTFC (-0.46). Conclusion A correlation was observed between delayed workplace injury recovery and poor perceived workplace social support. The HCG may be an effective tool for identifying these factors in musculoskeletal workplace injuries of a minor pathophysiological nature. There may be merit in tailoring injury rehabilitation towards addressing psychosocial factors early in the injury recovery process to assist with a more expedient return to full work capacity following simple acute musculoskeletal injury.

Growth factor delivery strategies for rotator cuff repair and regeneration

The high incidence of degenerative tears and prevalence of retears (20-95%) after surgical repair makes rotator cuff injuries a significant health problem. This high retear rate is attributed to the failure of the repaired tissue to regenerate the native tendon-to-bone insertion (enthesis). Biological augmentation of surgical repair such as autografts, allografts, and xenografts are confounded by donor site morbidity, immunogenicity, and disease transmission, respectively. In contrast, these risks may be alleviated via growth factor therapy, which can actively influence the healing environment to promote functional repair. Several challenges have to be overcome before growth factor delivery can translate into clinical practice such as the selection of optimal growth factor(s) or combination, identification of the most efficient stage and duration of delivery, and the design considerations for the delivery device. Emerging insight into the injury-repair microenvironment and our understanding of growth factor mechanisms in healing are informing the design of advanced delivery scaffolds to effectively treat rotator cuff tears. Here, we review potential growth factor candidates, design parameters and material selection for growth factor delivery, innovative and dynamic delivery scaffolds, and novel therapeutic targets from tendon and developmental biology for the structural and functional healing of rotator cuff repair.

A Midterm Evaluation of Postoperative Platelet-Rich Plasma Injections on Arthroscopic Supraspinatus Repair: A Randomized Controlled Trial

BACKGROUND

Platelet-rich plasma (PRP) has been applied as an adjunct to rotator cuff repair to improve tendon-bone healing and potentially reduce the incidence of subsequent tendon retears.

PURPOSE

To investigate whether the midterm clinical and radiographic outcomes of arthroscopic supraspinatus repair are enhanced after repeated postoperative applications of PRP.

STUDY DESIGN

Randomized controlled trial; Level of evidence, 1.

METHODS

A total of 60 patients (30 control; 30 PRP) were initially randomized to receive 2 ultrasound-guided injections of PRP to the tendon repair site at 7 and 14 days after double-row arthroscopic supraspinatus repair or not. A total of 55 patients (91.7%) underwent a clinical review and magnetic resonance imaging (MRI) at a mean of 3.5 years after surgery (range, 36-51 months). Patient-reported outcome measures (PROMs) included the Constant score, Quick Disabilities of the Arm, Shoulder and Hand (QuickDASH) questionnaire, Oxford Shoulder Score (OSS), and visual analog scale (VAS) for pain. Global rating of change (GRC) scale and patient satisfaction scores were evaluated. Structural integrity of the surgical repair was assessed via MRI using the Sugaya classification system.

RESULTS

At the midterm review, there was no difference between the groups for any of the PROMs. No differences between the groups were demonstrated for the subjective and range of motion subscales of the Constant score, although a significantly higher Constant strength subscale score was observed in the PRP group (3.3 points; 95% CI, 1.0-5.7; P = .006). There was no evidence for any group differences in MRI scores or retear rates, with 66.7% of PRP patients and 64.3% of control patients rated as Sugaya grade 1. Two control patients had symptomatic retears (both full thickness) within the first 16 weeks after surgery compared with 2 PRP patients, who suffered symptomatic retears (both partial thickness) between 16 weeks and a mean 3.5-year follow-up.

CONCLUSION

Significant postoperative clinical improvements and high levels of patient satisfaction were observed in patients at the midterm review after supraspinatus repair. While pain-free, maximal abduction strength was greater in the midterm after PRP treatment, repeated applications of PRP delivered at 7 and 14 days after surgery provided no additional benefit to tendon integrity.

Cellular and Molecular Factors Influencing Tendon Repair

Tendons are complex connective tissues that transmit tensile forces between muscles and tendons. Tendon injuries are among the most common orthopedic problems with long-term disability as a frequent consequence due to prolonged healing time. Furthermore, the repair tissue is of inferior quality, predisposing patients to high rates of recurrence following initial injury. Coordinated cellular processes and biological factors under the influence of mechanical loading are involved in tendon healing and our understanding of these events lags behind other musculoskeletal tissues. Tendons are relatively hypocellular and hypovascular, with little or no intrinsic regenerative capacity. Studies have documented fatty degeneration, chondrogenic dysplasia, and ectopic ossification within tendon repair tissue. The underlying pathogenesis for these metaplastic changes that compromise the quality of tendon repair tissue is poorly understood. The purpose of this review is to compile literature reporting molecular processes that regulate/control the phenotype of cells responsible for abnormal matrix deposition at repair site. In addition, recent studies reporting the interplay of mechanotransduction and cellular responses during tendon repair are summarized. Identifying the links between cellular, biological, and mechanical parameters involved in tendon repair is paramount to develop successful therapies for tendon healing.

Effect of MLS(®) laser therapy with different dose regimes for the treatment of experimentally induced tendinopathy in sheep: pilot study

OBJECTIVE

The aim of this preliminary study was to investigate the effect of Multiwave Locked System (MLS(®)), a particular model of low-level laser, in the acute phase of collagenase-induced tendon lesions in six adult sheep randomly assigned to two groups.

BACKGROUND DATA

Tendon injuries are common among human athletes and in sport horses, require a long recovery time, and have a high risk of relapse. Many traditional treatments are not able to repair the injured tendon tissue correctly. In recent years, the use of low-level laser therapy (LLLT) produced interesting results in inflammatory modulation in different musculoskeletal disorders.

METHODS

Group 1 received 10 treatments of MLS laser therapy at a fluence of 5 J/cm(2) on the left hindlimb. Group 2 received 10 treatments of MLS laser therapy at a fluence of 2.5 J/cm(2) on the left hindlimb. In every subject in both groups, the right hindlimb was considered as the control leg.

RESULTS

Clinical follow-up and ultrasonography examinations were performed during the postoperative period, and histological examinations were performed at day 30 after the first application of laser therapy. In particular, results from histological examinations indicate that both treatments induced a statistically significant cell number decrease, although only in the second group did the values return to normal. Moreover, the MLS laser therapy dose of 2.5 J/cm(2) (group 2) caused a significant decrease of vessel area.

CONCLUSIONS

In this study, clinical and histological evaluation demonstrated that a therapeutic dose <5 J/cm(2) furnished an anti-inflammatory effect, and induced a decrease of fibroblasts and vessel area. Overall, our results suggest that MLS laser therapy was effective in improving collagen fiber organization in the deep digital flexor tendon.