Optimization of Human Tendon Tissue Engineering: Peracetic Acid Oxidation for Enhanced Reseeding of Acellularized Intrasynovial Tendon:

The trend of allogeneic tendon decellularization: literature review

Tendon injuries repair is a significant burden for orthopaedic surgeons. Finding a proper graft material to repair tendon is one of the main challenges in orthopaedics, for which the requirement of substitute for tendon repair would be different for each clinical application. Among biological scaffolds, the use of decellularized tendon increasingly represents an interesting approach to treat tendon injuries and several articles have investigated the approaches of tendon decellularization. To understand the outcomes of the the approaches of tendon decellularization on effect of tendon transplantation, a literature review was performed. This review was conducted by searching in Pubmed and Embase and 64 studies were included in this study. The findings revealed that the common approaches to decellularize tendon include chemical, physical, and enzymatic decellularization methods or their combination. With the development of tissue engineering, researchers also put forward new theories such as automatic acellular machine, 3D printing technology to manufacture acellular scaffold.

Exploring polysaccharide and protein-enriched decellularized matrix scaffolds for tendon and ligament repair: A review

Tissue engineering (TE) has become a primary research topic for the treatment of diseased or damaged tendon/ligament (T/L) tissue. T/L injuries pose a severe clinical burden worldwide, necessitating the development of effective strategies for T/L repair and tissue regeneration. TE has emerged as a promising strategy for restoring T/L function using decellularized extracellular matrix (dECM)-based scaffolds. dECM scaffolds have gained significant prominence because of their native structure, relatively high bioactivity, low immunogenicity, and ability to function as scaffolds for cell attachment, proliferation, and differentiation, which are difficult to imitate using synthetic materials. Here, we review the recent advances and possible future prospects for the advancement of dECM scaffolds for T/L tissue regeneration. We focus on crucial scaffold properties and functions, as well as various engineering strategies employed for biomaterial design in T/L regeneration. dECM provides both the physical and mechanical microenvironments required by cells to survive and proliferate. Various decellularization methods and sources of allogeneic and xenogeneic dECM in T/L repair and regeneration are critically discussed. Additionally, dECM hydrogels, bio-inks in 3D bioprinting, and nanofibers are briefly explored. Understanding the opportunities and challenges associated with dECM-based scaffold development is crucial for advancing T/L repairs in tissue engineering and regenerative medicine.

Development of three-layer collagen scaffolds to spatially direct tissue-specific cell differentiation for enthesis repair

Enthesis repair remains a challenging clinical indication. Herein, a three-layer scaffold composed of a tendon-like layer of collagen type I, a fibrocartilage-like layer of collagen type II and a bone-like layer of collagen type I and hydroxyapatite, was designed to recapitulate the matrix composition of the enthesis. To aid tenogenic and fibrochondrogenic differentiation, bioactive molecules were loaded in the tendon-like layer or the fibrocartilage-like layer and their effect was assessed in in vitro setting using human bone marrow derived mesenchymal stromal cells and in an ex vivo model. Seeded human bone marrow mesenchymal stromal cells infiltrated and homogeneously spread throughout the scaffold. As a response to the composition of the scaffold, cells differentiated in a localised manner towards the osteogenic lineage and, in combination with differentiation medium, towards the fibrocartilage lineage. Whilst functionalisation of the tendon-like layer did not improve tenogenic cell commitment within the time frame of this work, relevant fibrochondrogenic markers were detected in the fibrocartilage-like layer when scaffolds were functionalised with bone morphogenetic protein 2 or non-functionalised at all, in vitro and ex vivo, respectively. Altogether, our data advocate the use of compartmentalised scaffolds for the repair and regeneration of interfacial tissues, such as enthesis.

Influence of peracetic acid-ethanol sterilisation on the biomechanical properties of human meniscus transplants

Purpose

Meniscus allograft transplantation (MAT) is a possible treatment for patients suffering with pain after meniscectomy. Here, peracetic acid (PAA) sterilised meniscus transplants were investigated on whether they would provide an adequate alternative to fresh-frozen transplants in their viscoelastic and mechanical properties.

Methods

In this analysis, 31 menisci donors (26 male and 5 female) were included. The average donor age was 49.87 years, ranging from 32 to 65 years. Menisci of matched pairs of knees underwent chemical sterilisation while counterparts were left fresh-frozen. Stiffness and load to failure were determined via suture retention. Further menisci were analysed while attached to the tibial bone block using a novel test device to mimic physiological load distribution. Meniscus relaxation, stiffness and failure loads were determined. Histology and biphasic properties of the menisci were examined and results were analysed using paired t-tests.

Results

A novel custom built test device allowed the application of physiological loads for suture retention testing and revealed no significant differences between PAA sterilised (14.85 ± 4.46 N/mm, 50.49 ± 17.01 N) and fresh-frozen (18.26 ± 4.46 N/mm, 59.49 ± 21.07 N) regarding stiffness and failure load, respectively. Furthermore, initial 200 N loading showed significantly higher strain in sterilised menisci (18.87 ± 1.56) compared to fresh frozen (13.81 ± 1.04). Load relaxation experiments demonstrated significantly lower relaxation for sterilised menisci (77.71 ± 1.62) compared to fresh-frozen (89.11 ± 1.00, p-value < 0.0001).

Conclusion

Peracetic acid sterilised human menisci performed equally to fresh-frozen counterparts in a suture retention test and in physiological failure testing providing an adequate alternative. However, meniscus relaxation, biphasic properties and strain were shown to be significantly different between the groups. A common problem of MAT is graft extrusion or shrinkage, therefore the parameters measured here should be considered and may influence meniscus extrusion after transplantation.

Level of evidence

n/a (experimental study)

Comparison of Decellularization Protocols to Generate Peripheral Nerve Grafts: A Study on Rat Sciatic Nerves

In critical nerve gap repair, decellularized nerve allografts are considered a promising tissue engineering strategy that can provide superior regeneration results compared to nerve conduits. Decellularized nerves offer a well-conserved extracellular matrix component that has proven to play an important role in supporting axonal guiding and peripheral nerve regeneration. Up to now, the known decellularized techniques are time and effort consuming. The present study, performed on rat sciatic nerves, aims at investigating a novel nerve decellularization protocol able to combine an effective decellularization in short time with a good preservation of the extracellular matrix component. To do this, a decellularization protocol proven to be efficient for tendons (DN-P1) was compared with a decellularization protocol specifically developed for nerves (DN-P2). The outcomes of both the decellularization protocols were assessed by a series of in vitro evaluations, including qualitative and quantitative histological and immunohistochemical analyses, DNA quantification, SEM and TEM ultrastructural analyses, mechanical testing, and viability assay. The overall results showed that DN-P1 could provide promising results if tested in vivo, as the in vitro characterization demonstrated that DN-P1 conserved a better ultrastructure and ECM components compared to DN-P2. Most importantly, DN-P1 was shown to be highly biocompatible, supporting a greater number of viable metabolically active cells.

Lateral slit delivery of bone marrow stromal cells enhances regeneration in the decellularized allograft flexor tendon

BACKGROUND/OBJECTIVE

Stem cell-based therapy has been applied to accelerate the revitalization of allograft tendon into a viable and functional tendon. Although many authors have proposed different methods to help the seeded stem cell distribution in the decellularized allograft, limited success has been achieved as tendon is a high dense connective tissue. We hypothesized that bone marrow stromal cells (BMSCs), seeded through the lateral slit, can regenerate the decellularized tendon (DCT) graft. The cell proliferation, cell viability, and tendon-specific gene expression are increased with the seeded cell density.

METHODS

Eighty-seven flexor digitorum profundus tendons were equally and randomly divided into 6 treatment groups that were seeded with low-density (2 × 107 cells/mL) and high-density (5 × 107 cells/mL) BMSCs through lateral slits cultured for 2 and 4 weeks, DCT without cells, and fresh live tendons. Tendons were evaluated for cell distribution, cell proliferation, cell viability, gene expression of Collagen I and Collagen III, tenogenic markers, and MMPs.

RESULTS

Histologic evaluation revealed BMSCs distributed from the lateral slit to the whole DCT. BMSCs were proliferated and kept viable in lateral slit decellularized tendon (LSDCT) in both seeded cell density groups after 2 and 4 weeks of culture. However, no significant differences in the cell proliferation between both cell density groups at 2 and 4 weeks of culture were observed. The lowest cell viability was found in the high-density group after 4 weeks of culture. BMSCs in LSDCT showed a significant tendency of higher gene expression of Collagen I, Collagen III, tenascin C, MMP2, MMP9, and MMP13 compared to normal tendons in both cell density groups at 2 and 4 weeks of culture.

CONCLUSION

BMSCs proliferated and remained viable after 2 and 4 weeks of culture with distribution throughout the lateral slits. Lateral slit preparation allows for the effective delivery and maintenance of mesenchymal cells with proliferation and generating a tenogenic behaviour of DCT in both the low and high cell densities in an in vitro model.

THE TRANSLATION POTENTIAL OF THIS ARTICLE

Revitalizing the implanted decellularized allograft is important for clinical application. In this study, we demonstrated that the DCT, with lateral slits, could harbour the seeded stem cell and stimulate proliferation with collagen synthesis. This evidence was presented for clinical application of the lateral slit technique, in DCT grafts, which would repopulate the seeded BMSCs during tendon and ligament reconstruction.

Independent, Controllable Stretch-Perfusion Bioreactor Chambers to Functionalize Cell-Seeded Decellularized Tendons

Tissue-engineered decellularized matrices can progress clinical replacement of full-thickness ruptures or tendon defects. This study develops and validates a custom-made automated bioreactor, called oscillating stretch-perfusion bioreactor (OSPB), consisting of multiple, independent culture chambers able to combine a bidirectional perfusion with a programmable, uniaxial strain to functionalize cell-seeded decellularized tendons. Decellularized tendon matrices were seeded on their surfaces and within the tendon fibers with mesenchymal stem cells. Then, they were subjected to a bidirectional perfusion and programmed stretching cycles of 15–30–60 min on–off two times per day for 7 days of culture. In vitro analyses showed viable cells, homogenously distributed on the surface of the constructs. More importantly, cell-seeded decellularized tendon grafts undergoing cyclic load in our bioreactor had a superior production and organization of newly formed collagen matrix compared to static cultured constructs. The coherency and local alignment of the new collagen deposition within the inner injected channels quantitatively supported histological findings. The designed OSPB could be considered a unique, cost-effective system able to involve multiple independently controlled chambers in terms of biological and mechanical protocols. This system allows parallel processing of several customized tendon constructs to be used as grafts to enhance the surgical repair of large tendon defects.

Vascular Endothelial Growth Factor Enhances Proliferation of Human Tenocytes and Promotes Tenogenic Gene Expression

BACKGROUND

In obtaining human tenocytes for tendon tissue engineering, a low proliferation rate and phenotype loss during passaging is a problem. It was the authors' aim to evaluate the influence of vascular endothelial growth factor (VEGF) on human tenocyte growth and gene expression.

METHODS

Human tenocytes were exposed to human VEGF in various concentrations (5, 10, and 20 ng/ml) for 5 days. Cell proliferation was counted and expression of tendon-related genes was analyzed.

RESULTS

Tenocyte count was 1.4 × 10(5)/ml, 2.7 × 10(5)/ml, 2.3 × 10(5)/ml, and 3.7 × 10(5)/ml for 0, 5, 10, and 20 ng/ml VEGF, respectively. Expression of Col1 was up-regulated 6.4 ± 4.2-fold, 60.1 ± 21.6-fold, and 15.8 ± 10.2-fold for 5, 10, and 20 ng/ml VEGF; all differences were significant with p < 0.05. Col3 was down-regulated to 0.2 ± 0.1-fold, 0.3 ± 0.1-fold, and 0.1 ± 0.03-fold for 5, 10, and 20 ng/ml VEGF; all differences were significant. Eln was up-regulated 2.3 ± 1.7-fold, 25.5 ± 10.9-fold, and 16.6 ± 9.0-fold for 5, 10, and 20 ng/ml VEGF; differences were significant for 10 and 20 ng/ml VEGF. TSC was down-regulated to 0.3 ± 0.1-fold and 0.3 ± 0.1-fold for 5 and 20 ng/ml VEGF; differences were significant for 5 and 20 ng/ml. SCX was up-regulated to 31.3 ± 8.5-fold, 49.1 ± 23.4-fold, and 20.9 ± 9.5-fold for 5, 10, and 20 ng/ml VEGF; all changes were significant.

CONCLUSIONS

VEGF enhances proliferation and expression of tendon-related genes in human tenocytes. It could therefore be a useful addition for tenocyte cultivation.

Biologic and mechanical aspects of tendon fibrosis after injury and repair

Tendon injuries of the hand that require surgical repair often heal with excess scarring and adhesions to adjacent tissues. This can compromise the natural gliding mechanics of the flexor tendons in particular, which operate within a fibro-osseous tunnel system similar to a set of pulleys. Even combining the finest suture repair techniques with optimal hand therapy protocols cannot ensure predictable restoration of hand function in these cases. To date, the majority of research regarding tendon injuries has revolved around the mechanical aspects of the surgical repair (i.e. suture techniques) and postoperative rehabilitation. The central principles of treatment gleaned from this literature include using a combination of core and epitendinous sutures during repair and initiating motion early on in hand therapy to improve tensile strength and limit adhesion formation. However, it is likely that the best clinical solution will utilize optimal biological modulation of the healing response in addition to these core strategies and, recently, the research in this area has expanded considerably. While there are no proven additive biological agents that can be used in clinical practice currently, in this review, we analyze the recent literature surrounding cytokine modulation, gene and cell-based therapies, and tissue engineering, which may ultimately lead to improved clinical outcomes following tendon injury in the future.

Tissue-Engineered Grafts from Human Decellularized Extracellular Matrices: A Systematic Review and Future Perspectives

Tissue engineering and regenerative medicine involve many different artificial and biologic materials, frequently integrated in composite scaffolds, which can be repopulated with various cell types. One of the most promising scaffolds is decellularized allogeneic extracellular matrix (ECM) then recellularized by autologous or stem cells, in order to develop fully personalized clinical approaches. Decellularization protocols have to efficiently remove immunogenic cellular materials, maintaining the nonimmunogenic ECM, which is endowed with specific inductive/differentiating actions due to its architecture and bioactive factors. In the present paper, we review the available literature about the development of grafts from decellularized human tissues/organs. Human tissues may be obtained not only from surgery but also from cadavers, suggesting possible development of Human Tissue BioBanks from body donation programs. Many human tissues/organs have been decellularized for tissue engineering purposes, such as cartilage, bone, skeletal muscle, tendons, adipose tissue, heart, vessels, lung, dental pulp, intestine, liver, pancreas, kidney, gonads, uterus, childbirth products, cornea, and peripheral nerves. In vitro recellularizations have been reported with various cell types and procedures (seeding, injection, and perfusion). Conversely, studies about in vivo behaviour are poorly represented. Actually, the future challenge will be the development of human grafts to be implanted fully restored in all their structural/functional aspects.

A sterilization method for decellularized xenogeneic cardiovascular scaffolds

Decellularized xenogeneic scaffolds have shown promise to be employed as compatible and functional cardiovascular biomaterials. However, one of the main barriers to their clinical exploitation is the lack of appropriate sterilization procedures. This study investigated the efficiency of a two-step sterilization method, antibiotics/antimycotic (AA) cocktail and peracetic acid (PAA), on porcine and bovine decellularized pericardium. In order to assess the efficiency of the method, a sterilization assessment protocol was specifically designed, comprising: i) controlled contamination with a known amount of bacteria; ii) sterility test; iii) identification of contaminants through MALDI-TOF (Matrix-Assisted Laser Desorption/Ionization Time-of-Flight) mass spectrometry and iv) quantification by the Most Probable Number (MPN) method. This sterilization assessment protocol proved to be a successful tool to monitor and optimize the proposed sterilization method. The treatment with AA + PAA method provided sterile scaffolds while preserving the structural integrity and biocompatibility of the decellularized porcine and bovine tissues. However, surface properties and cellular adhesion resulted slightly impaired on porcine pericardium. This work developed a sterilization method suitable for decellularized pericardial scaffolds that could be adopted for in vivo tissue engineering. Together with the proposed sterilization assessment protocol, this decontamination method will foster the clinical translation of decellularized xenogeneic substitutes.

STATEMENT OF SIGNIFICANCE

Clinical application of functional and compatible xenogeneic decellularized scaffolds has been delayed due to the lack of appropriate sterilization methodologies. In this study, it was investigated an effective sterilization method optimized for porcine and bovine decellularized pericardia, based on the use of antibiotics/antimycotics followed by peracetic acid treatment. This treatment effectively sterilizes both species scaffolds, proves to maintain tissue overall structure and components, preserves biocompatibility and biomechanical properties. Furthermore, it was also developed a sterilization assessment protocol used to monitor and validate the previous method, consisting in three main parts: i) controlled contamination; ii) sterility test, and iii) identification and quantification of contaminants. Both methodologies were optimized for the tissues in study but can be applied to other scaffolds and accelerate their clinical translation.

Boosting tendon repair: interplay of cells, growth factors and scaffold-free and gel-based carriers

Background

Tendons are dense connective tissues and critical components for the integrity and function of the musculoskeletal system. Tendons connect bone to muscle and transmit forces on which locomotion entirely depends. Due to trauma, overuse and age-related degeneration, many people suffer from acute or chronic tendon injuries. Owing to their hypovascularity and hypocellularity, tendinopathies remain a substantial challenge for both clinicians and researchers. Surgical treatment includes suture or transplantation of autograft, allograft or xenograft, and these serve as the most common technique for rescuing tendon injuries. However, the therapeutic efficacies are limited by drawbacks including inevitable donor site morbidity, poor graft integration, adhesion formations and high rates of recurrent tearing. This review summarizes the literature of the past 10 y concerning scaffold-free and gel-based approaches for treating tendon injuries, with emphasis on specific advantages of such modes of application, as well as the obtained results regarding in vitro and in vivo tenogenesis.

Results

The search was focused on publications released after 2006 and 83 articles have been analysed. The main results are summarizing and discussing the clear advantages of scaffold-free and hydrogels carriers that can be functionalized with cells alone or in combination with growth factors.

Conclusion

The improved understanding of tissue resident adult stem cells has made a significant progress in recent years as well as strategies to steer their fate toward tendon lineage, with the help of growth factors, have been identified. The field of tendon tissue engineering is exploring diverse models spanning from hard scaffolds to gel-based and scaffold-free approaches seeking easier cell delivery and integration in the site of injury. Still, the field needs to consider a multifactorial approach that is based on the combination and fine-tuning of chemical and biomechanical stimuli. Taken together, tendon tissue engineering has now excellent foundations and enters the period of precision and translation to models with clinical relevance on which better treatment options of tendon injuries can be shaped up.

Intratendinous Injection of Hydrogel for Reseeding Decellularized Human Flexor Tendons

BACKGROUND

Decellularized cadaveric tendons are a potential source for reconstruction. Reseeding to enhance healing is ideal; however, cells placed on the tendon surface result in inadequate delivery. The authors used an injection technique to evaluate intratendinous cell delivery.

METHODS

Decellularized tendons were reseeded with adipose-derived stem cells in culture, and injected with fetal bovine serum or hydrogel. PKH26-stained cells in cross-section were quantified. To evaluate cell viability, the authors delivered luciferase-labeled cells and performed bioluminescent imaging. To evaluate synthetic ability, the authors performed immunohistochemistry of procollagen. Adipose-derived stem cells' ability to attract tenocytes was assessed using transwell inserts. Cell-to-cell interaction was assessed by co-culturing, measuring proliferation and collagen production, and quantifying synergy. Finally, tensile strength was tested.

RESULTS

Both fetal bovine serum (p < 0.001) and hydrogel (p < 0.001) injection led to more cells inside the tendon compared with culturing. Hydrogel injection initially demonstrated greater bioluminescence than culturing (p < 0.005) and fetal bovine serum injection (p < 0.05). Injection groups demonstrated intratendinous procollagen staining correlating with the cells' location. Co-culture led to greater tenocyte migration (p < 0.05). Interaction index of proliferation and collagen production assays were greater than 1 for all co-culture ratios, demonstrating synergistic proliferation and collagen production compared with controls (p < 0.05). There were no differences in tensile strength.

CONCLUSIONS

Hydrogel injection demonstrated the greatest intratendinous seeding efficiency and consistency, without compromising tensile strength. Intratendinous cells demonstrated synthetic capabilities and can potentially attract tenocytes inside the tendon, where synergy would promote intrinsic tendon healing.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, V.

Automated freeze-thaw cycles for decellularization of tendon tissue - a pilot study

Background

Decellularization of tendon tissue plays a pivotal role in current tissue engineering approaches for in vitro research as well as for translation of graft-based tendon restoration into clinics. Automation of essential decellularization steps like freeze-thawing is crucial for the development of more standardized decellularization protocols and commercial graft production under good manufacturing practice (GMP) conditions in the future.

Methods

In this study, a liquid nitrogen-based controlled rate freezer was utilized for automation of repeated freeze-thawing for decellularization of equine superficial digital flexor tendons. Additional tendon specimens underwent manually performed freeze-thaw cycles based on an established procedure. Tendon decellularization was completed by using non-ionic detergent treatment (Triton X-100). Effectiveness of decellularization was assessed by residual nuclei count and calculation of DNA content. Cytocompatibility was evaluated by culturing allogeneic adipose tissue-derived mesenchymal stromal cells on the tendon scaffolds.

Results

There were no significant differences in decellularization effectiveness between samples decellularized by the automated freeze-thaw procedure and samples that underwent manual freeze-thaw cycles. Further, we inferred no significant differences in the effectiveness of decellularization between two different cooling and heating rates applied in the automated freeze-thaw process. Both the automated protocols and the manually performed protocol resulted in roughly 2% residual nuclei and 13% residual DNA content. Successful cell culture was achieved with samples decellularized by automated freeze-thawing as well as with tendon samples decellularized by manually performed freeze-thaw cycles.

Conclusions

Automated freeze-thaw cycles performed by using a liquid nitrogen-based controlled rate freezer were as effective as previously described manual freeze-thaw procedures for decellularization of equine superficial digital flexor tendons. The automation of this key procedure in decellularization of large tendon samples is an important step towards the processing of large sample quantities under standardized conditions. Furthermore, with a view to the production of commercially available tendon graft-based materials for application in human and veterinary medicine, the automation of key procedural steps is highly required to develop manufacturing processes under GMP conditions.

The impact of decellularization agents on renal tissue extracellular matrix

The combination of patient-specific cells with scaffolds obtained from natural sources may result in improved regeneration of human tissues. Decellularization of the native tissue is the first step in this technology. Effective decellularization uses agents that lyse cells and remove all cellular materials, leaving intact collagenous extracellular matrices (ECMs). Removing cellular remnants prevents an immune response while preserving the underlying structure. In this study, the impact of five decellularization agents (0.1 N NaOH, 1% peracetic acid, 3% Triton X-100, 1% sodium dodecyl sulfate (SDS), and 0.05% trypsin/EDTA) on renal tissue was examined using slices of porcine kidneys. The NaOH solution induced the most efficient cell removal, and resulted in the highest amount of cell viability and proliferation after recellularization, although it also produced the most significant damage to collagenous fiber networks, glycosaminoglycans (GAGs) and fibroblast growth factor (FGF). The SDS solution led to less severe damage to the ECM structure but it resulted in lower metabolic activity and less proliferation. Peracetic acid and Triton X-100 resulted in minimum disruption of ECMs and the most preserved GAGs and FGF. However, these last two agents were not as efficient in removing cellular materials as NaOH and SDS, especially peracetic acid, which left more than 80% of cellular material within the ECM. As a proof of principle, after completing the comparison studies using slices of renal ECM, the NaOH process was used to decellularize a whole kidney, with good results. The overall results demonstrate the significant effect of cell lysing agents and the importance of developing an optimized protocol to avoid extensive damage to the ECM while retaining the ability to support cell growth.

Decellularized and Engineered Tendons as Biological Substitutes: A Critical Review

Tendon ruptures are a great burden in clinics. Finding a proper graft material as a substitute for tendon repair is one of the main challenges in orthopaedics, for which the requirement of a biological scaffold would be different for each clinical application. Among biological scaffolds, the use of decellularized tendon-derived matrix increasingly represents an interesting approach to treat tendon ruptures. We analyzed in vitro and in vivo studies focused on the development of efficient protocols for the decellularization and for the cell reseeding of the tendon matrix to obtain medical devices for tendon substitution. Our review considered also the proper tendon source and preclinical animal models with the aim of entering into clinical trials. The results highlight a wide panorama in terms of allogenic or xenogeneic tendon sources, specimen dimensions, physical or chemical decellularization techniques, and the cell type variety for reseeding from terminally differentiated to undifferentiated mesenchymal stem cells and their static or dynamic culture employed to generate implantable constructs tested in different animal models. We try to identify the most efficient approach to achieve an optimal biological scaffold for biomechanics and intrinsic properties, resembling the native tendon and being applicable in clinics in the near future, with particular attention to the Achilles tendon substitution.

Principles of tendon reconstruction following complex trauma of the upper limb

Reconstruction of tendons following complex trauma to the upper limb presents unique clinical and research challenges. In this article, the authors review the principles guiding preoperative assessment, surgical reconstruction, and postoperative rehabilitation and management of the upper extremity. Tissue engineering approaches to address tissue shortages for tendon reconstruction are also discussed.

The effect of terminal sterilization on structural and biophysical properties of a decellularized collagen-based scaffold; implications for stem cell adhesion

Terminal sterilization induces physical and chemical changes in the extracellular matrix (ECM) of ex vivo-derived biomaterials due to their aggressive mechanism of action. Prior studies have focused on how sterilization affects the mechanical integrity of tissue-based biomaterials but have rarely characterized effects on early cellular interaction, which is indicative of the biological response. Using a model fibrocartilage disc scaffold, these investigations compare the effect of three common sterilization methods [peracetic acid (PAA), gamma irradiation (GI), and ethylene oxide (EtO)] on a range of material properties and characterized early cellular interactions. GI and EtO produced unfavorable structural damage that contributed to inferior cell adhesion. Conversely, exposure to PAA resulted in limited structural alterations while inducing chemical modifications that favored cell attachment. Results suggest that the sterilization approach can be selected to modulate biomaterial properties to favor cellular adhesion and has relevance in tissue engineering and regenerative medicine applications. Furthermore, the study of cellular interactions with modified biomaterials in vitro provides information of how materials may react in subsequent clinical applications.

Tissue engineering in flexor tendon surgery: current state and future advances

Tissue engineering of flexor tendons addresses a challenge often faced by hand surgeons: the restoration of function and improvement of healing with a limited supply of donor tendons. Creating an engineered tendon construct is dependent upon understanding the normal healing mechanisms of the tendon and tendon sheath. The production of a tendon construct includes: creating a three-dimensional scaffold; seeding cells within the scaffold; encouraging cellular growth within the scaffold while maintaining a gliding surface; and finally ensuring mechanical strength. An effective construct incorporates these factors in its design, with the ultimate goal of creating tendon substitutes that are readily available to the reconstructive hand surgeon.

Human flexor tendon tissue engineering: in vivo effects of stem cell reseeding

BACKGROUND

Tissue-engineered human flexor tendons may be an option to aid in reconstruction of complex upper extremity injuries with significant tendon loss. The authors hypothesize that human adipose-derived stem cells remain viable following reseeding on human tendon scaffolds in vivo and aid in graft integration.

METHODS

Decellularized human flexor tendons harvested from fresh-frozen cadavers and reseeded with green fluorescent protein-labeled pooled human adipose-derived stem cells were examined with bioluminescent imaging and immunohistochemistry. Reseeded repaired tendons were compared biomechanically with unseeded controls following implantation in athymic rats at 2 and 4 weeks. The ratio of collagen I to collagen III at the repair site was examined using Sirius red staining. To confirm cell migration, reseeded and unseeded tendons were placed either in contact or with a 1-mm gap for 12 days. Green fluorescent protein signal was then detected.

RESULTS

Following reseeding, viable cells were visualized at 12 days in vitro and 4 weeks in vivo. Biomechanical testing revealed no significant difference in ultimate load to failure and 2-mm gap force. Histologic evaluation showed host cell invasion and proliferation of the repair sites. No increase in collagen III was noted in reseeded constructs. Cell migration was confirmed from reseeded constructs to unseeded tendon scaffolds with tendon contact.

CONCLUSIONS

Human adipose-derived stem cells reseeded onto decellularized allograft scaffolds are viable over 4 weeks in vivo. The movement of host cells into the scaffold and movement of adipose-derived stem cells along and into the scaffold suggests biointegration of the allograft.

Repopulation of intrasynovial flexor tendon allograft with bone marrow stromal cells: an ex vivo model

PURPOSE

Delayed healing is a common problem whenever tendon allografts are used for tendon or ligament reconstruction. Repopulating the allograft with host cells may accelerate tendon regeneration, but cell penetration into the allograft tendon is limited. Processing the tendon surface with slits that guide cells into the allograft substrate may improve healing. The purpose of this study was to describe a surface modification of allograft tendon that includes slits to aid cell repopulation and lubrication to enhance tendon gliding.

METHODS

Canine flexor digitorum profundus tendons were used for this study. Cyclic gliding resistance was measured over 1000 cycles. Tensile stiffness was assessed for normal tendon, tendon decellularized with trypsin and Triton X-100 (decellularized group), tendon decellularized and perforated with multiple slits (MS group) and tendon decellularized, perforated with slits and treated with a carbodiimide-derivatized hyaluronic acid and gelatin (cd-HA-gelatin) surface modification (MS-SM group). To assess tendon repopulation, bone marrow stromal cells (BMSCs) were used in the decellularized and MS groups. DNA concentration and histology were evaluated and compared to normal tendons and nonseeded decellularized tendons.

RESULTS

The gliding resistance of the decellularized and MS groups was significantly higher compared with the normal group. There was no significant difference in gliding resistance between the decellularized and MS group. Gliding resistance of the normal group and MS-SM group was not significantly different. The Young's modulus was not significantly different among the four groups. The DNA concentration in the MS group was significantly lower than in normal tendons, but significantly higher than in decellularized tendons, with or without BMSCs. Viable BMSCs were found in the slits after 2 weeks in tissue culture.

CONCLUSIONS

Tendon slits can successfully harbor BMSCs without compromising their survival and without changing tendon stiffness. Surface modification restores normal gliding function to the slit tendon.

CLINICAL RELEVANCE

A multislit tendon reseeded with BMSCs, with a surface treatment applied to restore gliding properties, may potentially promote tendon revitalization and accelerate healing for tendon or ligament reconstruction applications.

Physicochemical decellularization of composite flexor tendon-bone interface grafts

BACKGROUND

Extremity injuries involving tendon attachment to bone are difficult to address. Clinically, tendon-bone interface allografts must be decellularized to reduce immunogenicity. Composite grafts are difficult to decellularize because chemical agents cannot reach cells between tissues. In this study, the authors attempted to optimize tendon-bone interface graft decellularization.

METHODS

Human flexor digitorum profundus tendons with attached distal phalanx were harvested from cadavers and divided into four groups. Group 1 (control) was untreated. Group 2 (chemical) was chemically treated with 5% peracetic acid, 0.1% ethylenediaminetetraacetic acid, and 0.1% sodium dodecyl sulfate. Group 3 (low-power) underwent targeted ultrasonication for 3 minutes (22,274 J, 126W) followed by chemical decellularization. Group 4 (high-power) underwent targeted ultrasonication for 10 minutes (88,490 J, 155W) followed by chemical decellularization. Decellularization was assessed histologically with hematoxylin and eosin stain and stains for major histocompatibility complex I stains. Cell counts were performed. The ultimate tensile load of decellularized grafts (group 4) were compared with pair-matched untreated grafts (group 1).

RESULTS

Average cell counts were 100 ± 41, 27 ± 10, 12 ± 11, and 6 ± 11 per high-power field for groups 1, 2, 3, and 4, respectively (p < 0.001). Decellularization using physical and chemical treatments (groups 3 and 4) resulted in substantial reduction of cells and major histocompatibility complex I molecules. There was no difference in ultimate tensile load between treated (group4) and untreated (group 1) samples (p > 0.5).

CONCLUSIONS

Physicochemical decellularization of tendon-bone interface grafts using targeted ultrasonication and chemical treatment resulted in near-complete reduction in cellularity and maintenance of tensile strength. In the future, these decellularized composite scaffolds may be used for reconstruction of tendon-bone injuries.

Human hamstring tenocytes survive when seeded into a decellularized porcine Achilles tendon extracellular matrix

Tendon ruptures and defects remain major orthopaedic challenges. Tendon healing is a time-consuming process, which results in scar tissue with an altered biomechanical competence. Using a xenogeneic tendon extracellular matrix (ECM) as a natural scaffold, which can be reseeded with autologous human tenocytes, might be a promising approach to reconstruct damaged tendons. For this purpose, the porcine Achilles (AS) tendons serving as a scaffold were histologically characterized in comparison to human cell donor tendons. AS tendons were decellularized and then reseeded with primary human hamstring tenocytes using cell centrifuging, rotating culture and cell injection techniques. Vitality testing, histology and glycosaminoglycan/DNA quantifications were performed to document the success of tendon reseeding. Porcine AS tendons were characterized by a higher cell and sulfated glycosaminoglycan content than human cell donor tendons. Complete decellularization could be achieved, but led to a wash out of sulfated glycosaminoglycans. Nevertheless, porcine tendon could be recellularized with vital human tenocytes. The recellularization led to a slight increase in cell number compared to the native tendon and some glycosaminoglycan recovery. This study indicates that porcine tendon can be de- and recellularized using adult human tenocytes. Future work should optimize cell distribution within the recellularized tendon ECM and consider tendon- and donor species-dependent differences.

Decellularized tendon extracellular matrix-a valuable approach for tendon reconstruction?

Tendon healing is generally a time-consuming process and often leads to a functionally altered reparative tissue. Using degradable scaffolds for tendon reconstruction still remains a compromise in view of the required high mechanical strength of tendons. Regenerative approaches based on natural decellularized allo- or xenogenic tendon extracellular matrix (ECM) have recently started to attract interest. This ECM combines the advantages of its intrinsic mechanical competence with that of providing tenogenic stimuli for immigrating cells mediated, for example, by the growth factors and other mediators entrapped within the natural ECM. A major restriction for their therapeutic application is the mainly cell-associated immunogenicity of xenogenic or allogenic tissues and, in the case of allogenic tissues, also the risk of disease transmission. A survey of approaches for tendon reconstruction using cell-free tendon ECM is presented here, whereby the problems associated with the decellularization procedures, the success of various recellularization strategies, and the applicable cell types will be thoroughly discussed. Encouraging in vivo results using cell-free ECM, as, for instance, in rabbit models, have already been reported. However, in comparison to native tendon, cells remain mostly inhomogeneously distributed in the reseeded ECM and do not align. Hence, future work should focus on the optimization of tendon ECM decellularization and recolonization strategies to restore tendon functionality.

Allograft tendon for second-stage tendon reconstruction

Tendons are made of compact dense collagen fibers with only sparse cellularity and naturally low immunogenicity. Allogenic tendons may be preserved through deep freezing methods and retain excellent mechanical properties after revitalization. Allogenic tendons were used in 22 patients (30 tendons) for second-stage tendon reconstruction in the hand. Preliminary results indicate no observable adverse tissue reactions, and functional recovery after tendon grafting does not seem different from reconstruction using tendon autografts. This type of allogenic graft does not seem to produce serious concern as a foreign tissue in the body, at least in the short term.

Successful recellularization of human tendon scaffolds using adipose-derived mesenchymal stem cells and collagen gel

The major goal of regenerative medicine is to determine experimental techniques that take maximal advantage of reparative processes that occur naturally in the animal body. Injection of mesenchymal stem cells into the core of a damaged tendon represents such an approach. Decellularization of native tendons as potential targets and seeding protocols are currently under investigation. The aim of our study was to manufacture a recellularized biocompatible scaffold from cadaveric tissue for use in total or partial tendon injuries. Results showed that it was possible to introduce proliferating cells into the core of a decellularized tendon to treat the scaffold with a collagen gel. The method was effective in maintaining scaffold extracellular matrix and for expressing collagen type I and cartilage oligomeric matrix protein by injecting mesenchymal stem cells.

Studies in flexor tendon reconstruction: biomolecular modulation of tendon repair and tissue engineering

The Andrew J. Weiland Medal is presented each year by the American Society for Surgery of the Hand and the American Foundation for Surgery of the Hand for a body of work related to hand surgery research. This essay, awarded the Weiland Medal in 2011, focuses on the clinical need for flexor tendon reconstruction and on investigations into flexor tendon biology. Reconstruction of the upper extremity is limited by 2 major problems after injury or degeneration of the flexor tendons. First, adhesions formed after flexor tendon repair can cause decreased postoperative range of motion and hand function. Second, tendon losses can result from trauma and degenerative diseases, necessitating additional tendon graft material. Tendon adhesions are even more prevalent after tendon grafting; therefore these 2 problems are interrelated and lead to considerable disability. The total costs in terms of disability and inability to return to work are enormous. In this essay, published work from the past 12 years in our basic science laboratory is summarized and presented with the common theme of using molecular techniques to understand the cellular process of flexor tendon wound healing and to create substances and materials to improve tendon repair and regeneration. These are efforts to address 2 interrelated and clinically relevant problems that all hand surgeons face in their practice.

Human flexor tendon tissue engineering: decellularization of human flexor tendons reduces immunogenicity in vivo

BACKGROUND

In mutilating hand injuries, tissue engineered tendon grafts may provide a reconstructive solution. We have previously described a method to decellularize cadaveric human flexor tendons while preserving mechanical properties and biocompatibility. The purpose of this study is to evaluate the immunogenicity and strength of these grafts when implanted into an immunocompetent rat model.

METHODS

Cadaveric human flexor tendons were divided into two groups. Group 1 was untreated, and Group 2 was decellularized by treatment with sodium dodecyl sulfate (SDS), ethylenediaminetetraacetic acid (EDTA), and peracetic acid (PAA). Both groups were then analyzed for the presence of major histocompatibility complexes by immunohistochemistry (IHC). Pair-matched tendons from each group were then placed into the dorsal subcutaneous tissue and anchored to the spinal ligaments of Wistar rats for 2 or 4 weeks, and harvested. The infiltration of B-cells and macrophages was determined using IHC. The explants where then subjected to mechanical testing to determine the ultimate tensile stress (UTS) and elastic modulus (EM). Statistical analysis was performed using a paired Student's t-test.

RESULTS

The decellularization protocol successfully removed cells and MHC-1 complexes. At 2 weeks after implantation, there was increased infiltration of B-cells in Group 1 (untreated) compared with Group 2 (acellular), both in the capsule and tendon substance. There was improved ultimate tensile stress (UTS, 42.7 ± 8.3 vs. 22.8 ± 7.8 MPa, p<0.05) and EM (830.2 ± 206.7 vs. 421.2 ± 171.3 MPa, p<0.05) in tendons that were decellularized. At 4 weeks, there was continued B-cell infiltration in Group 1 (untreated) compared with Group 2 (acellular). There was no appreciable difference in macrophage infiltration at both time points. At 4 weeks Group 2 (acellular) demonstrated persistently greater UTS (40.5 ± 9.1 vs. 14.6 ± 4.2 MPa, p<0.05) and EM (454.05 ± 101.5 vs. 204.6 ± 91.3 MPa, p<0.05) compared with Group 1 (untreated).

CONCLUSIONS

Human flexor tendons that were decellularized with SDS, EDTA, and PAA resulted in removal of cellular antigens and a decreased immune response when placed into Wistar rats. These grafts showed better mechanical properties at 2 and 4 weeks when compared with control tendons. Decellularization is an important step toward the use of tissue engineered flexor tendons in upper extremity reconstruction.

Three-dimensional-construct bioreactor conditioning in human tendon tissue engineering

Human tendon tissue engineering attempts to address the shortage of autologous tendon material arising from mutilating injuries and diseases of the hand and forearm. It is important to maximize the tissue-engineered construct's (TEC's) biomechanical properties to ensure that the construct is in its strongest possible state before reimplantation. In this study, we sought to determine the bioreactor treatment parameters that affect these properties. Using small- and large-chamber three-dimensional-construct bioreactors (SCB and LCB, respectively), we applied cyclic axial load to TECs comprising reseeded human flexor and extensor tendons of the hand. First, small-sample pilot studies using the LCB were performed on matched-paired full-length flexor tendons to establish proof of concept. Next, large-sample studies using the SCB were performed on matched-paired extensor tendon segments to determine how reseeding, load duty cycle, load magnitude, conditioning duration, and testing delay affected ultimate tensile stress (UTS) and elastic modulus (EM). We found that compared with reseeded matched-paired controls under dynamic-loading at 1.25 N per TEC for 5 days, (1) acellular TECs had lower UTS (p=0.04) and EM (p<0.01), (2) unloaded TECs had lower UTS (p=0.01) and EM (p=0.02), (3) static-loaded TECs had lower UTS (p=0.01) and EM (p<0.01), (4) TECs conditioned for 3 days had lower UTS (p=0.03) and EM (p=0.04), and (5) TECs conditioned for 8 days had higher UTS (p=0.04) and EM (p=0.01). However, TECs conditioned at higher loads (2.5 N per TEC) and lower loads (0.625 N per TEC) possessed similar UTS (p=0.83 and p=0.89, respectively) and EM (p=0.48 and p=0.89, respectively) as controls stimulated with 1.25 N per TEC. After cycle completion, there is attrition of UTS (p=0.03) and EM (p=0.04) over a 2-day period. Our study showed that the material properties of human allograft TECs can be enhanced by reseeding and dynamic-conditioning. While conditioning duration has a significant effect on material properties, the load magnitude does not. The issue of attrition in biomechanical properties with time following cycle completion must be addressed before bioreactor preconditioning can be successfully introduced as a step in the processing of these constructs for clinical application.