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

Research progress in decellularized extracellular matrix hydrogels for intervertebral disc degeneration

As one of the most common clinical disorders, low back pain (LBP) influences patient quality of life and causes substantial social and economic burdens. Many factors can result in LBP, the most common of which is intervertebral disc degeneration (IDD). The progression of IDD cannot be alleviated by conservative or surgical treatments, and gene therapy, growth factor therapy, and cell therapy have their own limitations. Recently, research on the use of hydrogel biomaterials for the treatment of IDD has garnered great interest, and satisfactory treatment results have been achieved. This article describes the classification of hydrogels, the methods of decellularized extracellular matrix (dECM) production and the various types of gel formation. The current research on dECM hydrogels for the treatment of IDD is described in detail in this article. First, an overview of the material sources, decellularization methods, and gel formation methods is given. The focus is on research performed over the last three years, which mainly consists of bovine and porcine NP tissues, while for decellularization methods, combinations of several approaches are primarily used. dECM hydrogels have significantly improved mechanical properties after the polymers are cross-linked. The main effects of these gels include induction of stem cell differentiation to intervertebral disc (IVD) cells, good mechanical properties to restore IVD height after polymer cross-linking, and slow release of exosomes. Finally, the challenges and problems still faced by dECM hydrogels for the treatment of IDD are summarised, and potential solutions are proposed. This paper is the first to summarise the research on dECM hydrogels for the treatment of IDD and aims to provide a theoretical reference for subsequent studies.

Aponeurosis discission, a low-detergent method for tissue-engineered acellular ligament scaffolds

Detergent treatment is the most commonly used method for the decellularization of ligaments and tendon grafts. However, it is well recognized that detergent treatment can also adversely affect the extracellular matrix. This study found that discission into the aponeurosis layer of the patellar tendon (PT) before decellularization is conducive to extracting cells from the PT using a low quantity of detergent in a short time period. The acellular aponeurosis discission ligament (AADL) retains its native collagen fibril structure and mechanical properties. Moreover, the PT retained cell and tissue compatibility in vitro and in vivo. After implantation into a defective allogeneic PT, we found that the AADL healed well in the host, and its collagen structure exhibited gradual improvement 12 months after implantation with satisfactory reconstruction. IMPACT: The aponeurosis of tendons/ligaments is the main barrier to achieving complete decellularization, and it thus prevents complete recellularization for applications in tissue engineering. Aponeurosis can obstruct the removal of cell components. We found that excising the aponeurosis before decellularization allows for the removal of cellular components with a reduced amount of detergent, thus improving the biological properties of the acellular ligament. To the best of our knowledge, no similar studies have been performed. Graphical abstract.

Decellularized scaffold and its elicited immune response towards the host: the underlying mechanism and means of immunomodulatory modification

The immune response of the host towards a decellularized scaffold is complex. Not only can a number of immune cells influence this process, but also the characteristics, preparation and modification of the decellularized scaffold can significantly impact this reaction. Such factors can, together or alone, trigger immune cells to polarize towards either a pro-healing or pro-inflammatory direction. In this article, we have comprehensively reviewed factors which may influence the immune response of the host towards a decellularized scaffold, including the source of the biomaterial, biophysical properties or modifications of the scaffolds with bioactive peptides, drugs and cytokines. Furthermore, the underlying mechanism has also been recapitulated.

Relation between micro- and nanostructure features and biological properties of the decellularized rat liver

Organ decellularization is one of the promising technologies of regenerative medicine, which allows obtaining cell-free extracellular matrix (ECM), which provide preservation of the composition, architecture, vascular network and biological activity of the ECM. The method of decellularization opens up wide prospects for its practical application not only in the field of creating full-scale bioengineered structures, but also in the manufacture of vessels, microcarriers, hydrogels, and coatings. The main goal of our work was the investigation of structure and biological properties of lyophilized decellularized Wistar rat liver fragments (LDLFs), as well as we assessed the regenerative potential of the obtained ECM. We obtained decellularized liver of a Wistar rat, the vascular network and the main components of the ECM of tissue were preserved. H&E staining of histological sections confirmed the removal of cells. DNA content of ECM is equal to 0.7% of native tissue DNA content. Utilizing scanning probe nanotomogrphy method, we showed sinuous, rough topography and highly nanoporous structure of ECM, which provide high level of mouse 3T3 fibroblast and Hep-G₂cells biocompatibility. Obtained LDLF had a high regenerative potential, which we studied in an experimental model of a full-thickness rat skin wound healing: we observed the acceleration of wound healing by 2.2 times in comparison with the control.

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.

Post-decellularization techniques ameliorate cartilage decellularization process for tissue engineering applications

Due to the current lack of innovative and effective therapeutic approaches, tissue engineering (TE) has attracted much attention during the last decades providing new hopes for the treatment of several degenerative disorders. Tissue engineering is a complex procedure, which includes processes of decellularization and recellularization of biological tissues or functionalization of artificial scaffolds by active cells. In this review, we have first discussed those conventional steps, which have led to great advancements during the last several years. Moreover, we have paid special attention to the new methods of post-decellularization that can significantly ameliorate the efficiency of decellularized cartilage extracellular matrix (ECM) for the treatment of osteoarthritis (OA). We propose a series of post-decellularization procedures to overcome the current shortcomings such as low mechanical strength and poor bioactivity to improve decellularized ECM scaffold towards much more efficient and higher integration.

Decellularized Extracellular Matrix-based Bioinks for Engineering Tissue- and Organ-specific Microenvironments

Biomaterials-based biofabrication methods have gained much attention in recent years. Among them, 3D cell printing is a pioneering technology to facilitate the recapitulation of unique features of complex human tissues and organs with high process flexibility and versatility. Bioinks, combinations of printable hydrogel and cells, can be utilized to create 3D cell-printed constructs. The bioactive cues of bioinks directly trigger cells to induce tissue morphogenesis. Among the various printable hydrogels, the tissue- and organ-specific decellularized extracellular matrix (dECM) can exert synergistic effects in supporting various cells at any component by facilitating specific physiological properties. In this review, we aim to discuss a new paradigm of dECM-based bioinks able to recapitulate the inherent microenvironmental niche in 3D cell-printed constructs. This review can serve as a toolbox for biomedical engineers who want to understand the beneficial characteristics of the dECM-based bioinks and a basic set of fundamental criteria for printing functional human tissues and organs.

Tenocyte-imprinted substrate: a topography-based inducer for tenogenic differentiation in adipose tissue-derived mesenchymal stem cells

Tendon tissue engineering based on stem cell differentiation has attracted a great deal of attention in recent years. Previous studies have examined the effect of cell-imprinted polydimethylsiloxane (PDMS) substrate on induction differentiation in stem cells. In this study, we used tenocyte morphology as a positive mold to create a tenocyte-imprinted substrate on PDMS. The morphology and topography of this tenocyte replica on PDMS was evaluated with scanning electron microscopy (SEM) and atomic force microscopy. The tenogenic differentiation induction capacity of the tenocyte replica in adipose tissue-derived mesenchymal stem cells (ADSCs) was then investigated and compared with other groups, including tissue replica (which was produced similarly to the tenocyte replica and was evaluated by SEM), decellularized tendon, and bone morphogenic protein (BMP)-12, as other potential inducers. This comparison gives us an estimate of the ability of tenocyte-imprinted PDMS (called cell replica in the present study) to induce differentiation compared to other inducers. For this reason, ADSCs were divided into five groups, including control, cell replica, tissue replica, decellularized tendon and BMP-12. ADSCs were seeded on each group separately and investigated by the real-time reverse transcription polymerase chain reaction (RT-PCR) technique after seven and 14 days. Our results showed that in spite of the higher effect of the growth factor on tenogenic differentiation, the cell replica can also induce tenocyte marker expression (scleraxis and tenomodulin) in ADSCs. Moreover, the tenogenic differentiation induction capacity of the cell replica was greater than tissue replica. Immunocytochemistry analysis revealed that ADSCs seeding on the cell replica for 14 days led to scleraxis and tenomodulin expression at the protein level. In addition, immunohistochemistry indicated that contrary to the promising results in vitro, there was little difference between ADSCs cultured on tenocyte-imprinted PDMS and untreated ADSCs. The results of such studies could lead to the production of inexpensive cell culture plates or biomaterials that can induce differentiation in stem cells without growth factors or other supplements.

Method to decellularize the human chorion membrane

The human placenta is considered a biological waste, thus it is a great source of extracellular matrix (ECM) proteins. The human chorion membrane (HCM) is a membrane that composes the human placenta and is constituted by collagens type I, II, IV, V and VI, fibronectin and laminin. To the best of our knowledge, the potential of HCM alone is largely unexplored as a substrate to be used in tissue engineering and regenerative medicine. In this work, we describe, for the first time, the process and method to decellularize the chorion membrane alone. To verify the success of the decellularization protocol, the presence and distribution of cell nuclei and double-stranded DNA were quantified and analyzed by DAPI staining, PicoGreen and electrophoresis. After the decellularization protocol an ECM compact and handleably membrane is obtained, the decellularized human chorion membrane (dHCM).

Efficient decellularization of equine tendon with preserved biomechanical properties and cytocompatibility for human tendon surgery indications

Chronic and acute tendon injuries are frequent afflictions, for which treatment is often long and unsatisfactory. When facing extended injuries, matrices and scaffolds with sufficient biomechanical properties are required for surgical repair and could additionally serve as supports for cellular therapies to improve healing. In this study, protocols of either commonly used detergents only (SDS 1%, Triton 1%, TBP 1%, and Tween‐20 1%) or a combination of freeze/thaw (F/T) cycles with decellularization agents (NaCl 1M, ddH₂O) were evaluated for the decellularization of horse equine superficial digital flexor tendon (SDFT) for hand flexor or extensor tendon reconstruction. Decellularization efficiency was assessed microscopically by histological staining (HE, DAPI) and DNA quantification. Macroscopical structure and biomechanical integrity of the tendon matrices were further assessed by gross observation, histological staining (SR), and mechanical testing (ultimate strain and stress, Young’s modulus, energy to failure) for select protocols. Decellularization with hypertonic NaCl 1M in association with F/T cycles produced the most robust tendon matrices, which were nontoxic after 10 days for subsequent recellularization with human fetal progenitor tendon cells (hFPTs). This standardized protocol uses a less aggressive decellularization agent than current practice, which allows subsequent reseeding with allogenic cells, therefore making them very suitable and bioengineered tendon matrices for human tendon reconstruction in the clinic.

The future application of nanomedicine and biomimicry in plastic and reconstructive surgery

Plastic surgery encompasses a broad spectrum of reconstructive challenges and prides itself upon developing and adopting new innovations. Practice has transitioned from microsurgery to supermicrosurgery with a possible future role in even smaller surgical frontiers. Exploiting materials on a nanoscale has enabled better visualization and enhancement of biological processes toward better wound healing, tumor identification and viability of tissues, all cornerstones of plastic surgery practice. Recent advances in nanomedicine and biomimicry herald further reconstructive progress facilitating soft and hard tissue, nerve and vascular engineering. These lay the foundation for improved biocompatibility and tissue integration by the optimization of engineered implants or tissues. This review will broadly examine each of these technologies, highlighting areas of progress that reconstructive surgeons may not be familiar with, which could see adoption into our armamentarium in the not-so-distant future.

Electrospun polycaprolactone/collagen nanofibers cross-linked with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide/N-hydroxysuccinimide and genipin facilitate endothelial cell regeneration and may be a promising candidate for vascular scaffolds

Purpose

A promising vascular scaffold must possess satisfying mechanical properties, great hemocompatibility, and favorable tissue regeneration. Combining natural with synthetic materials is a popular method of creating/enhancing such scaffolds. However, the effect of additional modification on the materials requires further exploration.

Materials and methods

We selected polycaprolactone (PCL), which has excellent mechanical properties and biocompatibility and can be combined with collagen. Electrospun fibers created using a PCL/collagen solution were used to fashion mixed nanofibers, while separate syringes of PCL and collagen were used to create separated nanofibers, resulting in different pore sizes. Mixed and separated nanofibers were cross-linked with glutaraldehyde (GA), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC), and genipin; hence, we named them as mixed GA, mixed EDC (ME), mixed genipin (MG), separated GA, separated EDC (SE), and separated genipin (SG).

Results

Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction showed that cross-linking did not affect the main functional groups of fibers in all groups. ME, MG, SE, and SG met the requisite mechanical properties, and they also resisted collagenase degradation. In hemocompatibility assays, only ME and MG demonstrated ideal safety. Furthermore, ME and MG presented the greatest cytocompatibility. For vascular scaffolds, rapid endothelialization helps to prevent thrombosis. According to human umbilical vein endothelial cell migration on different nanofibers, ME and MG are also successful in promoting cell migration.

Conclusion

ME and MG may be promising candidates for vascular tissue engineering. The study suggests that collagen cross-linked by EDC/N-hydroxysuccinimide or genipin facilitates endothelial cell regeneration, which could be of great benefit in tissue engineering of vascular scaffolds.

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.

Optimal number of chemical extraction treatments for maintaining the biological properties of an allogeneic tendon

The aim of this study was to explore the biological effects of the amount of chemical extraction treatments performed on an allogeneic tendon through histomorphology, biological mechanics testing, and an immunogenicity assay. Sixteen New Zealand rabbits (body weight 2.5-3.0 kg) were randomly divided into four groups: group A (chemical extraction once), group B (chemical extraction twice), group C (chemical extraction three times), and group D (blank control group), with four rabbits in each group. The Achilles tendons of each rabbit were separated and subjected to a chemical extraction process with Triton X-100 and sodium deoxycholate, followed by hematoxylin and eosin staining, electron microscopy observation, biomechanical testing, and mixed lymphocyte culture. There were no significant differences in the surface color and fiber bundles between groups A and B and the blank control group, whereas group C showed clear differences from the blank control group with a rough surface, loose fibers, and poor tension. There were no significant differences in the biomechanics among the four groups. The four groups showed significant differences in the lymphocyte conversion ratio, with reduced rates of lymphocyte conversion along with increasing treatment numbers. Two chemical extractions of the tendon allowed for retaining most of the integrity of the original tendon fiber while removing immunogenicity with good biological properties. These findings lay a foundation for application of this method to human tendons so as to provide a good tissue source for tendon transplantation.

Development of a Micronized Meniscus Extracellular Matrix Scaffold for Potential Augmentation of Meniscal Repair and Regeneration

Decellularized scaffolds composed of extracellular matrix (ECM) hold promise for repair and regeneration of the meniscus, given the potential for ECM-based biomaterials to aid in stem cell recruitment, infiltration, and differentiation. The objectives of this study were to decellularize canine menisci to fabricate a micronized, ECM-derived scaffold and to determine the cytocompatibility and repair potential of the scaffold ex vivo. Menisci were decellularized with a combination of physical agitation and chemical treatments. For scaffold fabrication, decellularized menisci were cryoground into a powder and the size and morphology of the ECM particles were evaluated using scanning electron microscopy. Histologic and biochemical analyses of the scaffold confirmed effective decellularization with loss of proteoglycan from the tissue but no significant reduction in collagen content. When washed effectively, the decellularized scaffold was cytocompatible to meniscal fibrochondrocytes, synoviocytes, and whole meniscal tissue based on the resazurin reduction assay and histologic evaluation. In an ex vivo model for meniscal repair, radial tears were augmented with the scaffold delivered with platelet-rich plasma as a carrier, and compared to nonaugmented (standard-of-care) suture techniques. Histologically, there was no evidence of cellular migration or proliferation noted in any of the untreated or standard-of-care treatment groups after 40 days of culture. Conversely, cellular infiltration and proliferation were noted in scaffold-augmented repairs. These data suggest the potential for the scaffold to promote cellular survival, migration, and proliferation ex vivo. Further investigations are necessary to examine the potential for the scaffold to induce cellular differentiation and functional meniscal fibrochondrogenesis.

Tissue Engineering in Hand Surgery: A Technology Update

The field of hand surgery is constantly evolving to meet the challenges of repairing intricate anatomical structures with limited availability of donor tissue. The past 10 years have seen an exponential growth in tissue engineering, which has broadened the perspectives of tackling these age-old problems. Various fabrication techniques such as melt electrospinning and fused deposition modelling have been employed to synthesize 3-dimensional bioscaffolds that can be used to replace lost tissue. These bioscaffolds with strategic biomimicry have been shown to allow for integrative and functional repair of tissue injuries. This review article summarizes the most current advances in tissue engineering and its applications in the field of hand surgery. It outlines the current tissue engineering techniques commonly used for tackling musculoskeletal problems and highlights the most promising approaches according to clinical evidence. In particular, the paper explores regenerative medicine concepts applied to specific tissues including nerve, bone, cartilage, tendon, ligament, and vessels. In the face of innovative and pioneering research, tissue engineering will undoubtedly play a key role in reconstructive hand surgery in the not too distant future.

Collagenous Extracellular Matrix Biomaterials for Tissue Engineering: Lessons from the Common Sea Urchin Tissue

Scaffolds for tissue engineering application may be made from a collagenous extracellular matrix (ECM) of connective tissues because the ECM can mimic the functions of the target tissue. The primary sources of collagenous ECM material are calf skin and bone. However, these sources are associated with the risk of having bovine spongiform encephalopathy or transmissible spongiform encephalopathy. Alternative sources for collagenous ECM materials may be derived from livestock, e.g., pigs, and from marine animals, e.g., sea urchins. Collagenous ECM of the sea urchin possesses structural features and mechanical properties that are similar to those of mammalian ones. However, even more intriguing is that some tissues such as the ligamentous catch apparatus can exhibit mutability, namely rapid reversible changes in the tissue mechanical properties. These tissues are known as mutable collagenous tissues (MCTs). The mutability of these tissues has been the subject of on-going investigations, covering the biochemistry, structural biology and mechanical properties of the collagenous components. Recent studies point to a nerve-control system for regulating the ECM macromolecules that are involved in the sliding action of collagen fibrils in the MCT. This review discusses the key attributes of the structure and function of the ECM of the sea urchin ligaments that are related to the fibril-fibril sliding action-the focus is on the respective components within the hierarchical architecture of the tissue. In this context, structure refers to size, shape and separation distance of the ECM components while function is associated with mechanical properties e.g., strength and stiffness. For simplicity, the components that address the different length scale from the largest to the smallest are as follows: collagen fibres, collagen fibrils, interfibrillar matrix and collagen molecules. Application of recent theories of stress transfer and fracture mechanisms in fibre reinforced composites to a wide variety of collagen reinforcing (non-mutable) connective tissue, has allowed us to draw general conclusions concerning the mechanical response of the MCT at specific mechanical states, namely the stiff and complaint states. The intent of this review is to provide the latest insights, as well as identify technical challenges and opportunities, that may be useful for developing methods for effective mechanical support when adapting decellularised connective tissues from the sea urchin for tissue engineering or for the design of a synthetic analogue.

Establishment of a Cytocompatible Cell-Free Intervertebral Disc Matrix for Chondrogenesis with Human Bone Marrow-Derived Mesenchymal Stromal Cells

Tissue-engineered intervertebral discs (IVDs) utilizing decellularized extracellular matrix (ECM) could be an option for the reconstruction of impaired IVDs due to degeneration or injury. The objective of this study was to prepare a cell-free decellularized human IVD scaffold and to compare neotissue formation in response to recellularization with human IVD cells (hIVDCs) or human bone marrow-derived (hBM) mesenchymal stromal cells (MSCs). IVDs were decellularized via freeze-thaw cycles, detergents and trypsin. Histological staining was performed to monitor cell removal and glycosaminoglycan (GAG) removal. The decellularized IVD was preconditioned using bovine serum albumin and fetal bovine serum before its cytocompatibility for dynamically cultured hBM-MSCs (chondrogenically induced or not) and hIVDCs was compared after 14 days. In addition, DNA, total collagen and GAG contents were assessed. The decellularization protocol achieved maximal cell removal, with only few remaining cell nuclei compared with native tissue, and low toxicity. The DNA content was significantly higher in scaffolds seeded with hIVDCs compared with native IVDs, cell-free and hBM-MSC-seeded scaffolds (p < 0.01). The GAG content in the native tissue was significantly higher compared to the others groups except for the scaffolds reseeded with chondrogenically induced hBM-MSCs (p < 0.05). In addition, there was a significantly increased total collagen content in the chondrogenically induced hBM-MSCs group (p < 0.01) compared with the native IVDs, cell-free and hIVDC-seeded scaffolds (p < 0.01); both recolonizing cell types were more evenly distributed on the scaffold surface, but only few cells penetrated the scaffold. The resulting decellularized ECM was cytocompatible and allowed hBM-MSCs/hIVDCs survival and ECM production.

Naturally derived biomaterials for addressing inflammation in tissue regeneration

Tissue regeneration strategies have traditionally relied on designing biomaterials that closely mimic features of the native extracellular matrix (ECM) as a means to potentially promote site-specific cellular behaviors. However, inflammation, while a necessary component of wound healing, can alter processes associated with successful tissue regeneration following an initial injury. These processes can be further magnified by the implantation of a biomaterial within the wound site. In addition to designing biomaterials to satisfy biocompatibility concerns as well as to replicate elements of the composition, structure, and mechanics of native tissue, we propose that ECM analogs should also include features that modulate the inflammatory response. Indeed, strategies that enhance, reduce, or even change the temporal phenotype of inflammatory processes have unique potential as future pro-regenerative analogs. Here, we review derivatives of three natural materials with intrinsic anti-inflammatory properties and discuss their potential to address the challenges of inflammation in tissue engineering and chronic wounds.

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.

Engineering Tendon: Scaffolds, Bioreactors, and Models of Regeneration

Tendons bridge muscle and bone, translating forces to the skeleton and increasing the safety and efficiency of locomotion. When tendons fail or degenerate, there are no effective pharmacological interventions. The lack of available options to treat damaged tendons has created a need to better understand and improve the repair process, particularly when suitable autologous donor tissue is unavailable for transplantation. Cells within tendon dynamically react to loading conditions and undergo phenotypic changes in response to mechanobiological stimuli. Tenocytes respond to ultrastructural topography and mechanical deformation via a complex set of behaviors involving force-sensitive membrane receptor activity, changes in cytoskeletal contractility, and transcriptional regulation. Effective ex vivo model systems are needed to emulate the native environment of a tissue and to translate cell-matrix forces with high fidelity. While early bioreactor designs have greatly expanded our knowledge of mechanotransduction, traditional scaffolds do not fully model the topography, composition, and mechanical properties of native tendon. Decellularized tendon is an ideal scaffold for cultivating replacement tissue and modeling tendon regeneration. Decellularized tendon scaffolds (DTS) possess high clinical relevance, faithfully translate forces to the cellular scale, and have bulk material properties that match natural tissue. This review summarizes progress in tendon tissue engineering, with a focus on DTS and bioreactor systems.

Shear lag sutures: Improved suture repair through the use of adhesives

Suture materials and surgical knot tying techniques have improved dramatically since their first use over five millennia ago. However, the approach remains limited by the ability of the suture to transfer load to tissue at suture anchor points. Here, we predict that adhesive-coated sutures can improve mechanical load transfer beyond the range of performance of existing suture methods, thereby strengthening repairs and decreasing the risk of failure. The mechanical properties of suitable adhesives were identified using a shear lag model. Examination of the design space for an optimal adhesive demonstrated requirements for strong adhesion and low stiffness to maximize the strength of the adhesive-coated suture repair construct. To experimentally assess the model, we evaluated single strands of sutures coated with highly flexible cyanoacrylates (Loctite 4903 and 4902), cyanoacrylate (Loctite QuickTite Instant Adhesive Gel), rubber cement, rubber/gasket adhesive (1300 Scotch-Weld Neoprene High Performance Rubber & Gasket Adhesive), an albumin-glutaraldehyde adhesive (BioGlue), or poly(dopamine). As a clinically relevant proof-of-concept, cyanoacrylate-coated sutures were then used to perform a clinically relevant flexor digitorum tendon repair in cadaver tissue. The repair performed with adhesive-coated suture had significantly higher strength compared to the standard repair without adhesive. Notably, cyanoacrylate provides strong adhesion with high stiffness and brittle behavior, and is therefore not an ideal adhesive for enhancing suture repair. Nevertheless, the improvement in repair properties in a clinically relevant setting, even using a non-ideal adhesive, demonstrates the potential for the proposed approach to improve outcomes for treatments requiring suture fixation. Further study is necessary to develop a strongly adherent, compliant adhesive within the optimal design space described by the model.

Engineered composite tissue as a bioartificial limb graft

The loss of an extremity is a disastrous injury with tremendous impact on a patient's life. Current mechanical prostheses are technically highly sophisticated, but only partially replace physiologic function and aesthetic appearance. As a biologic alternative, approximately 70 patients have undergone allogeneic hand transplantation to date worldwide. While outcomes are favorable, risks and side effects of transplantation and long-term immunosuppression pose a significant ethical dilemma. An autologous, bio-artificial graft based on native extracellular matrix and patient derived cells could be produced on demand and would not require immunosuppression after transplantation. To create such a graft, we decellularized rat and primate forearms by detergent perfusion and yielded acellular scaffolds with preserved composite architecture. We then repopulated muscle and vasculature with cells of appropriate phenotypes, and matured the composite tissue in a perfusion bioreactor under electrical stimulation in vitro. After confirmation of composite tissue formation, we transplanted the resulting bio-composite grafts to confirm perfusion in vivo.

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.

[Use of tissue engineering in the reconstruction of flexor tendon injuries of the hand]

In his literary analysis, the author describes a novel method applied in the reconstruction of flexor tendon injuries of the hand. This procedure is named tissue engineering, and it is examined mainly under experimental circumstances. After definition of the method and descriptions of literary preliminaries the author discusses the healing process of the normal tendon tissue, then development of the scaffold, an important step of tissue engineering is described. After these topics the introduction of the pluripotent mesenchymal stem cells into the scaffold, and proliferation of these cells and development of the sliding systems are presented. The mechanical resisting ability of the formed tendon tissue is also discussed. Finally, the author concludes that as long as results of experimental research cannot be successfully applied into clinical practice, well-tried tendon reconstruction operations and high quality postoperative rehabilitation are needed.

Combined decellularisation and dehydration improves the mechanical properties of tissue-engineered sinews

Novel sources of replacement sinews are needed to repair damaged tissue after injury. The current methods of repair ultilise autografts, allografts or xenografts, although each method has distinct disadvantages that limit their success. Decellularisation of harvested tissues has been previously investigated for sinew repair with the long-term aim of repopulating the structure with autologous cells. Although this procedure shows promise, the demand for donor scaffolds will always outweigh supply. Here, we report the fabrication of fibrin-based tissue-engineered sinews, which can be decellularised, dehydrated and stored. The sinews may then be rehydrated and repopulated with an autologous cell population. In addition to enabling production of patient-specific implants, interestingly, the process of combined decellularisation, dehydration and rehydration enhanced the mechanical properties of the sinew. The treated sinews exhibited a 2.6-fold increase in maximum load and 8-fold increase in ultimate tensile strength when compared with the control group (p < 0.05 in both cases).

Decellularized tissue and cell-derived extracellular matrices as scaffolds for orthopaedic tissue engineering

The reconstruction of musculoskeletal defects is a constant challenge for orthopaedic surgeons. Musculoskeletal injuries such as fractures, chondral lesions, infections and tumor debulking can often lead to large tissue voids requiring reconstruction with tissue grafts. Autografts are currently the gold standard in orthopaedic tissue reconstruction; however, there is a limit to the amount of tissue that can be harvested before compromising the donor site. Tissue engineering strategies using allogeneic or xenogeneic decellularized bone, cartilage, skeletal muscle, tendon and ligament have emerged as promising potential alternative treatment. The extracellular matrix provides a natural scaffold for cell attachment, proliferation and differentiation. Decellularization of in vitro cell-derived matrices can also enable the generation of autologous constructs from tissue specific cells or progenitor cells. Although decellularized bone tissue is widely used clinically in orthopaedic applications, the exciting potential of decellularized cartilage, skeletal muscle, tendon and ligament cell-derived matrices has only recently begun to be explored for ultimate translation to the orthopaedic clinic.

Tissue engineering for plastic surgeons: a primer

A central tenet of reconstructive surgery is the principle of "replacing like with like." However, due to limitations in the availability of autologous tissue or because of the complications that may ensue from harvesting it, autologous reconstruction may be impractical to perform or too costly in terms of patient donor-site morbidity. The field of tissue engineering has long held promise to alleviate these shortcomings. Scaffolds are the structural building blocks of tissue-engineered constructs, akin to the extracellular matrix within native tissues. Commonly used scaffolds include allogenic or xenogenic decellularized tissue, synthetic or naturally derived hydrogels, and synthetic biodegradable nonhydrogel polymeric scaffolds. Embryonic, induced pluripotent, and mesenchymal stem cells also hold immense potential for regenerative purposes. Chemical signals including growth factors and cytokines may be harnessed to augment wound healing and tissue regeneration. Tissue engineering is already clinically prevalent in the fields of breast augmentation and reconstruction, skin substitutes, wound healing, auricular reconstruction, and bone, cartilage, and nerve grafting. Future directions for tissue engineering in plastic surgery include the development of prevascularized constructs and rationally designed scaffolds, the use of stem cells to regenerate organs and tissues, and gene therapy.

Reconstruction of the tendon-bone insertion with decellularized tendon-bone composite grafts: comparison with conventional repair

PURPOSE

Injuries involving the tendon-bone interface (TBI) are difficult to address. Standard techniques typically lead to diminished strength of the healed insertion site. We hypothesized that these injuries would benefit from being reconstructed with decellularized composite grafts replacing both tendon and bone. To test this hypothesis, decellularized grafts were compared with conventional pullout repairs in an in vivo animal model.

METHODS

We harvested 48 Achilles TBI grafts from rats and decellularized them. Tendon-bone interface graft reconstruction and pullout repairs were compared using a pair-matched design. Biomechanical properties were evaluated at 2, 4, 8, and 12 weeks. We evaluated histological analysis of insertion morphology and collagen type I/III content.

RESULTS

There was a significant increase in ultimate failure load (35 ± 11 vs 24 ± 7 N) and ultimate tensile stress (1.5 ± 0.3 vs 1.0 ± 0.4 N/mm(2)) of the TBI grafts compared with pullout repairs at 2 weeks. These differences remained at 4 weeks. At 12 weeks, both TBI grafts and pullout repairs were as strong as native tissue and not significantly different from each other. Histology showed a more organized extracellular matrix in the TBI graft group at the early time points. Repopulation of the decellularized grafts increased over time. At 12 weeks, the insertion points of both groups were richly populated with cells that possessed morphologies similar to those found in native TBI.

CONCLUSIONS

This study showed that decellularized TBI grafts were stronger compared with conventional pullout repairs at 2 and 4 weeks but were comparable at 12 weeks. A more organized extracellular matrix and different collagen composition in the early time points may explain the observed differences in strength.

CLINICAL RELEVANCE

In the future, decellularized TBI grafts may be used to reconstruct tendon-bone insertion tears in multiple areas including the flexor tendon system.

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.

Decellularized human tendon-bone grafts for composite flexor tendon reconstruction: a cadaveric model of initial mechanical properties

PURPOSE

After complex hand trauma, restoration of tendon strength is challenging. Tendon insertion tears typically heal as fibrous scars after surgical reconstruction and create a weak point at the tendon-bone interface. In addition, major tendon loss may overwhelm the amount of available autograft for reconstruction. An off-the-shelf product may help address these challenges. We hypothesized that decellularized human flexor digitorum profundus and distal phalanx tendon-bone composite grafts were a feasible option for flexor tendon reconstruction after complex hand trauma. By replacing the entire injured composite segment, the need for tendon repair within the tendon sheath, reconstruction of the tendon-bone interface, and use of limited autograft could be eliminated.

METHODS

Paired human cadaver forearms were dissected to obtain the flexor digitorum profundus tendon with an attached block of distal phalanx. Tendon-bone grafts were pair-matched and divided into 2 groups: decellularized grafts (n = 12) and untreated (control) grafts (n = 11). Grafts in the decellularized group were subjected to physiochemical decellularization. Pair-matched tendon-bone grafts (decellularized and untreated) were placed back into the flexor tendon sheath and secured distally using a tie-over button and proximally by weaving the graft into the flexor digitorum superficialis tendon in the distal forearm. The ultimate load, location of failure, and excursion were determined.

RESULTS

Decellularized tendon-bone composite grafts demonstrated no significant difference in ultimate failure load or stiffness compared with untreated grafts. Both groups eventually failed in varied locations along the repair. The most common site of failure in both groups was the tie-over button. The decellularized group failed at the tendon-bone insertion in 3 specimens (25%) compared with none in the untreated group. Both groups demonstrated an average tendon excursion of approximately 82 mm before failure.

CONCLUSIONS

Decellularization of human flexor tendon-distal phalanx tendon-bone constructs did not compromise initial strength despite chemical and mechanical decellularization in a cadaveric model. At the time of repair, decellularized flexor tendon-bone grafts can exceed the strength and excursion needed for hand therapy immediately after reconstruction.

CLINICAL RELEVANCE

These tendon-bone grafts may become an option for complex hand reconstruction at or near tendon-bone insertions and throughout the tendon sheath. Further work is required to assess the role of reseeding in an in vivo model.

Antigen removal for the production of biomechanically functional, xenogeneic tissue grafts

Xenogeneic tissues are derived from other animal species and provide a source of material for engineering mechanically functional tissue grafts, such as heart valves, tendons, ligaments, and cartilage. Xenogeneic tissues, however, contain molecules, known as antigens, which invoke an immune reaction following implantation into a patient. Therefore, it is necessary to remove the antigens from a xenogeneic tissue to prevent immune rejection of the graft. Antigen removal can be accomplished by treating a tissue with solutions and/or physical processes that disrupt cells and solubilize, degrade, or mask antigens. However, processes used for cell and antigen removal from tissues often have deleterious effects on the extracellular matrix (ECM) of the tissue, rendering the tissue unsuitable for implantation due to poor mechanical properties. Thus, the goal of an antigen removal process should be to reduce the antigen content of a xenogeneic tissue while preserving its mechanical functionality. To expand the clinical use of antigen-removed xenogeneic tissues as biomechanically functional grafts, it is essential that researchers examine tissue antigen content, ECM composition and architecture, and mechanical properties as new antigen removal processes are developed.

Freeze-thaw cycles enhance decellularization of large tendons

The use of decellularized tendon tissue as a scaffold for tendon tissue engineering provides great opportunities for future clinical and current research applications. The aim of this study was to assess the effect of repetitive freeze-thaw cycles and two different detergents, t-octyl-phenoxypolyethoxyethanol (Triton X-100) and sodium dodecyl sulfate (SDS), on decellularization effectiveness and cytocompatibility in large tendons. Freshly collected equine superficial and deep digital flexor tendons were subjected to decellularization according to four different protocols (1 and 2: freeze-thaw cycles combined with either Triton X-100 or SDS; 3 and 4: Triton X-100 or SDS). Decellularization effectiveness was assessed based on the reduction of vital cell counts, histologically visible nuclei, and DNA content. Transmission electron microscopy was performed to evaluate cellular and extracellular matrix integrity. Further, cytocompatibility of scaffolds that had been decellularized according to the protocols including freeze-thaw cycles (protocols 1 and 2) was assessed by seeding the scaffolds with superparamagnetic iron oxide labeled mesenchymal stromal cells and monitoring the cells histologically and by magnetic resonance imaging for two weeks. Decellularization was significantly more effective when using the protocols including freeze-thaw cycles, leaving only roughly 1% residual nuclei and 20% residual DNA, whereas samples that had not undergone additional freeze-thaw cycles contained roughly 20% residual nuclei and 40% residual DNA. No morphological extracellular matrix alterations due to decellularization could be observed. Scaffolds prepared by both protocols including freeze-thaw cycles were cytocompatible, but the cell distribution into the scaffold tended to be better in scaffolds that had been decellularized using freeze-thaw cycles combined with Triton X-100 instead of SDS.

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.

Factors influencing the long-term behavior of extracellular matrix-derived scaffolds for musculoskeletal soft tissue repair

Musculoskeletal connective tissues such as tendon, ligament, and cartilage possess a limited ability for self-repair. Tissue engineering seeks to use combinations of cells, bioactive molecules, and biomaterials to develop new treatment options for the repair or replacement of damaged tissues. The use of native extracellular matrix as scaffold material for tissue engineering has become increasingly attractive because such tissues can not only provide structural support, but also regulate cell behavior. Although demineralized bone matrix has long been recognized for its osteoinductive abilities, recent studies have identified the ability of cartilage and tendon extracellular matrices to stimulate the differentiation of mesenchymal or adipose-derived adult stem cells toward chondrogenic or tenogenic lineages, respectively. This review discusses the motivation for fabricating scaffolds from musculoskeletal tissues, the in vitro and in vivo efficacy of these tissue-derived scaffolds, and various processing techniques such as decellularization or cross-linking that can mitigate immunogenic responses, moderate the degradation profile, and enhance the mechanical properties of these constructs following long-term implantation in vivo.

The effect of decellularization of tracheal allografts on leukocyte infiltration and of recellularization on regulatory T cell recruitment

Tracheal transplantation without immunosuppressive therapy has been accomplished with a tissue-engineering approach using decellularized biological scaffolds in combination with recipient progenitor cells. The mechanisms of immune response directed towards these tracheal allografts have not been fully determined. In this study, we evaluated the immunogenicity of these grafts at the protein level, and functionally, in vitro and in vivo in a large animal model. Long-segment circumferential tracheal allografts were decellularized using two different protocols and recellularized using recipient mesenchymal stromal cells (MSC) and tracheal epithelial progenitor cells (TEC). Residual MHCI and MHCII immunostaining was found surrounding the submucosal glands despite cyclical decellularization. In an in vitro lymphocyte proliferation assay, CD4+ T cells continued to proliferate on decellularized pieces and this proliferation was inhibited by co-culture with autologous MSC. Allografts were heterotopically transplanted under a muscle flap in the neck of the recipients and decellularization was found to delay leukocyte infiltration but resulted in eventual cartilage degradation. Recellularization prevented this infiltration up to 3 weeks post-transplantation and allowed for preservation of the cartilage. The immune cells found within these grafts included a significant number of CD4+CD25+Foxp3+ regulatory T cells. Furthermore, gene expression of anti-inflammatory cytokines, such as IL-10 and TGF-β1, involved in proliferation, differentiation and function of regulatory T cells was found in these grafts. These results indicate that the immunological modification induced by recellularized tracheal scaffolds is an active process involving the recruitment of immunosuppressive cells, rather than simply the removal of donor-derived antigenic components.

Production of a sterilised decellularised tendon allograft for clinical use

Application of a high-level decontamination or sterilisation procedure and cell removal technique to tendon allograft can reduce the concerns of disease transmission, immune reaction, and may improve remodelling of the graft after implantation. The decellularised matrix can also be used as a matrix for tendon tissue engineering. One such sterilisation factor, Peracetic acid (PAA) has the advantage of not producing harmful reaction residues. The aim of this study was to evaluate the effects of PAA treatment and a cell removal procedure on the production of tendon matrix. Human patellar tendons, thawed from frozen were treated respectively as: Group 1, control with no treatment; Group 2, sterilised with PAA (0.1 % (w/v) PAA for 3 h) Group 3, decellularised (incubation successively in hypotonic buffer, 0.1 % (w/v) sodium dodecyl sulphate, and a nuclease solution); Group 4, decellularised and PAA sterilised. Histological analysis showed that no cells were visible after the decellularisation treatment. The integrity of tendon structure was maintained after decellularisation and PAA sterilisation, however, the collagen waveform was slightly loosened. No contact cytotoxicity was found in any of the groups. Determination of de-natured collagen showed no significant increase when compared with the control. This suggested that the decellularisation and sterilisation processing procedures did not compromise the major properties of the tendon. The sterilised, decellularised tendon could be suitable for clinical use.

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.

Tissue-engineered collateral ligament composite allografts for scapholunate ligament reconstruction: an experimental study

PURPOSE

In patients with chronic scapholunate (SL) dissociation or dynamic instability, ligament repair is often not possible, and surgical reconstruction is indicated. The ideal graft ligament would recreate both anatomical and biomechanical properties of the dorsal scapholunate ligament (dorsal SLIL). The finger proximal interphalangeal joint (PIP joint) collateral ligament could possibly be a substitute ligament.

METHODS

We harvested human PIP joint collateral ligaments and SL ligaments from 15 cadaveric limbs. We recorded ligament length, width, and thickness, and measured the biomechanical properties (ultimate load, stiffness, and displacement to failure) of native dorsal SLIL, untreated collateral ligaments, decellularized collateral ligaments, and SL repairs with bone-collateral ligament-bone composite collateral ligament grafts. As proof of concept, we then reseeded decellularized bone-collateral ligament-bone composite grafts with green fluorescent protein-labeled adipo-derived mesenchymal stem cells and evaluated them histologically.

RESULTS

There was no difference in ultimate load, stiffness, and displacement to failure among native dorsal SLIL, untreated and decellularized collateral ligaments, and SL repairs with tissue-engineered collateral ligament grafts. With pair-matched untreated and decellularized scaffolds, there was no difference in ultimate load or stiffness. However, decellularized ligaments revealed lower displacement to failure compared with untreated ligaments. There was no difference in displacement between decellularized ligaments and native dorsal SLIL. We successfully decellularized grafts with recently described techniques, and they could be similarly reseeded.

CONCLUSIONS

Proximal interphalangeal joint collateral ligament-based bone-collateral ligament-bone composite allografts had biomechanical properties similar to those of native dorsal SLIL. Decellularization did not adversely affect material properties.

CLINICAL RELEVANCE

These tissue-engineered grafts may offer surgeons another option for reconstruction of chronic SL instability.