Tissue-engineering strategies for the tendon/ligament-to-bone insertion

Exosomes derived from tendon stem/progenitor cells enhance tendon-bone interface healing after rotator cuff repair in a rat model

The rate of retear after surgical repair remains high. Mesenchymal stem cells (MSCs) have been extensively employed in regenerative medicine for several decades. However, safety and ethical concerns constrain their clinical application. Tendon Stem/Progenitor Cells (TSPCs)-derived exosomes have emerged as promising cell-free therapeutic agents. Therefore, urgent studies are needed to investigate whether TSPC-Exos could enhance tendon-bone healing and elucidate the underlying mechanisms. In this study, TSPC-Exos were found to promote the proliferation, migration, and expression of fibrogenesis markers in BMSCs. Furthermore, TSPC-Exos demonstrated an ability to suppress the polarization of M1 macrophages while promoting M2 macrophage polarization. In a rat model of rotator cuff repair, TSPC-Exos modulated inflammation and improved the histological structure of the tendon-bone interface, the biomechanical properties of the repaired tendon, and the function of the joint. Mechanistically, TSPC-Exos exhibited high expression of miR-21a-5p, which regulated the expression of PDCD4. The PDCD4/AKT/mTOR axis was implicated in the therapeutic effects of TSPC-Exos on proliferation, migration, and fibrogenesis in BMSCs. This study introduces a novel approach utilizing TSPC-Exos therapy as a promising strategy for cell-free therapies, potentially benefiting patients with rotator cuff tear in the future.

Electrospinning technology: a promising approach for tendon-bone interface tissue engineering

The regeneration of tendon-bone interface tissue has become a topic of great interest in recent years. However, the complex nature of this interface has posed challenges in finding suitable solutions. Tissue engineering, with its potential to improve clinical outcomes and play a crucial role in musculoskeletal function, has been increasingly explored for tendon-bone interface regeneration. This review focuses on the research advancements of electrospinning technology in interface tissue engineering. By utilizing electrospinning, researchers have been able to fabricate scaffolds with tailored properties to promote the regeneration and integration of tendon and bone tissues. The review discusses the unique structure and function of the tendon-bone interface, the mechanisms involved in its healing, and the limitations currently faced in achieving successful regeneration. Additionally, it highlights the potential of electrospinning technology in scaffold fabrication and its role in facilitating the development of functional and integrated tendon-bone interface tissues. Overall, this review provides valuable insights into the application of electrospinning technology for tendon-bone interface tissue engineering, emphasizing its significance in addressing the challenges associated with regeneration in this complex interface.

Achilles tendon enthesis behavior under cyclic compressive loading: Consequences of unloading and early remobilization

The Achilles tendon enthesis (ATE) anchors the Achilles tendon into the calcaneus through fibrocartilaginous tissue. The latter is enriched in type II collagen and proteoglycans (PGs), both of which give the enthesis its capacity to withstand compressive stress. Because unloading and reloading induce remodeling of the ATE fibrocartilage (Camy et al., 2022), chronic changes in the mechanical load could modify the mechanical response under compressive stress. Therefore, we investigated the ATE fatigue behavior in mice, under cyclic compressive loading, after 14 days of hindlimb suspension and 6 days of reloading. In addition, we performed a qualitative histological study of PGs in ATE fibrocartilage. The mechanical behavior of ATE was impaired in unloaded mice. A significant loss of 27 % in Δd (difference between the maximum and minimum displacements) was observed at the end of the test. In addition, the hysteresis area decreased by approximately 27 % and the stiffness increased by over 45 %. The increased stiffness and loss of viscosity were thrice and almost twice those of the control, respectively. In the reloaded entheses, where the loss of Δd was not significant, we found a significant 28 % decrease in the hysteresis area and a 26 % increase in stiffness, both of which were higher regarding the control condition. These load-dependent changes in the mechanical response seem partly related to changes in PGs in the uncalficied part of the ATE. These findings highlight the importance of managing compressive loading on ATE when performing prophylactic and rehabilitation exercises.

Enhancing tendon-bone integration and healing with advanced multi-layer nanofiber-reinforced 3D scaffolds for acellular tendon complexes

Advancements in tissue engineering are crucial for successfully healing tendon-bone connections, especially in situations like anterior cruciate ligament (ACL) restoration. This study presents a new and innovative three-dimensional scaffold, reinforced with nanofibers, that is specifically intended for acellular tendon complexes. The scaffold consists of a distinct layered arrangement comprising an acellular tendon core, a middle layer of polyurethane/type I collagen (PU/Col I) yarn, and an outside layer of poly (L-lactic acid)/bioactive glass (PLLA/BG) nanofiber membrane. Every layer is designed to fulfill specific yet harmonious purposes. The acellular tendon core is a solid structural base and a favorable environment for tendon cell functions, resulting in considerable tensile strength. The central PU/Col I yarn layer is vital in promoting the tendinogenic differentiation of stem cells derived from tendons and increasing the expression of critical tendinogenic factors. The external PLLA/BG nanofiber membrane fosters the process of bone marrow mesenchymal stem cells differentiating into bone cells and enhances the expression of markers associated with bone formation. Our scaffold's biocompatibility and multi-functional design were confirmed through extensive in vivo evaluations, such as histological staining and biomechanical analyses. These assessments combined showed notable enhancements in ACL repair and healing. This study emphasizes the promise of multi-layered nanofiber scaffolds in orthopedic tissue engineering and also introduces new possibilities for the creation of improved materials for regenerating the tendon-bone interface.

Effect of tetrahedral framework nucleic acids on the reconstruction of tendon-to-bone injuries after rotator cuff tears

Clinicians and researchers have always faced challenges in performing surgery for rotator cuff tears (RCT) due to the intricate nature of the tendon-bone gradient and the limited long-term effectiveness. At the same time, the occurrence of an inflammatory microenvironment further aggravates tissue damage, which has a negative impact on the regeneration process of mesenchymal stem cells (MSCs) and eventually leads to the production of scar tissue. Tetrahedral framework nucleic acids (tFNAs), novel nanomaterials, have shown great potential in biomedicine due to their strong biocompatibility, excellent cellular internalisation ability, and unparalleled programmability. The objective of this research was to examine if tFNAs have a positive effect on regeneration after RCTs. Experiments conducted in a controlled environment demonstrated that tFNAs hindered the assembly of inflammasomes in macrophages, resulting in a decrease in the release of inflammatory factors. Next, tFNAs were shown to exert a protective effect on the osteogenic and chondrogenic differentiation of bone marrow MSCs under inflammatory conditions. The in vitro results also demonstrated the regulatory effect of tFNAs on tendon-related protein expression levels in tenocytes after inflammatory stimulation. Finally, intra-articular injection of tFNAs into a rat RCT model showed that tFNAs improved tendon-to-bone healing, suggesting that tFNAs may be promising tendon-to-bone protective agents for the treatment of RCTs.

Anatomical study of the sacrotuberous ligament and the hamstring muscles: A histomorphological analysis

The sacrotuberous ligament (STL) and the hamstrings are important structures that are mutually connected and influenced by the pelvis. However, the anatomical connectivity and histological characteristics of these structures remain unclear. The present study aimed to comprehensively investigate the relationship between the STL and the proximal hamstrings through histological analysis. Sixteen specimens were obtained from eight fresh cadavers (mean age at death, 73.4 years). Verhoeff Van Gieson, Masson's trichrome, and immunohistochemical staining were used to analyze the connectivity between the STL and the hamstrings and to verify the ratios of collagen and elastic fibers. Dense connective tissue that overlapped tightly between the STL and hamstrings was observed. The relative ratios of collagen and elastic fibers between the STL and hamstrings characteristically identified regional differences. The ratio of elastic fibers to collagen in the biceps femoris (BF) was ~38.6 ± 4.7%, and the lowest ratio was 5.9 ± 2.6% observed in the semimembranosus (SM). In the case of the BF, contractibility is well-regulated due to a high content of elastic fibers; however, the muscular structure of the BF is relatively fragile due to the low content of collagen. In the SM, collagen content is higher than that in the STL. This ratio of elastic fibers in the collagen analysis could provide crucial information for understanding the differences in hamstring contractility and maintaining the morphology of these structures.

Characterization of the mechanical properties of the mouse Achilles tendon enthesis by microindentation. Effects of unloading and subsequent reloading

The fibrocartilaginous tendon enthesis, i.e. the site where a tendon is attached to bone through a fibrocartilaginous tissue, is considered as a functionally graded interface. However, at local scale, a very limited number of studies have characterized micromechanical properties of this transitional tissue. The first goal of this work was to characterize the micromechanical properties of the mineralized part of the healthy Achilles tendon enthesis (ATE) through microindentation testing and to assess the degree of mineralization and of carbonation of mineral crystals by Raman spectroscopy. Since little is known about enthesis biological plasticity, our second objective was to examine the effects of unloading and reloading, using a mouse hindlimb-unloading model, on both the micromechanical properties and the mineral phase of the ATE. Elastic modulus, hardness, degree of mineralization, and degree of carbonation were assessed after 14 days of hindlimb suspension and again after a subsequent 6 days of reloading. The elastic modulus gradually increased along the mineralized part of the ATE from the tidemark to the subchondral bone, with the same trend being found for hardness. Whereas the degree of carbonation did not differ according to zone of measurement, the degree of mineralization increased by >70 % from tidemark to subchondral bone. Thus, the gradient in micromechanical properties is in part explained by a mineralization gradient. A 14-day unloading period did not appear to affect the gradient of micromechanical properties of the ATE, nor the degree of mineralization or carbonation. However, contrary to a short period of unloading, early return to normal mechanical load reduced the micromechanical properties gradient, regardless of carbonate-to-phosphate ratios, likely due to the more homogeneous degree of mineralization. These findings provide valuable data not only for tissue bioengineering, but also for musculoskeletal clinical studies and microgravity studies focusing on long-term space travel by astronauts.

Biomimetic triphasic silk fibroin scaffolds seeded with tendon-derived stem cells for tendon-bone junction regeneration

The regeneration of tendon and bone junctions (TBJs), a fibrocartilage transition zone between tendons and bones, is a challenge due to the special triphasic structure. In our study, a silk fibroin (SF)-based triphasic scaffold consisting of aligned type I collagen (Col I), transforming growth factor β (TGF-β), and hydroxyapatite (HA) was fabricated to mimic the compositional gradient feature of the native tendon-bone architecture. Rat tendon-derived stem cells (rTDSCs) were loaded on the triphasic SF scaffold, and the high cell viability suggested that the scaffold presents good biocompatibility. Meanwhile, increased expressions of tenogenic-, chondrogenic-, and osteogenic-related genes in the TBJs were observed. The in vivo studies of the rTDSC-seeded scaffold in a rat TBJ rupture model showed tendon tissue regeneration with a clear transition zone within 8 weeks of implantation. These results indicated that the biomimetic triphasic SF scaffolds seeded with rTDSCs have great potential to be applied in TBJ regeneration.

Anatomical design and production of a novel three-dimensional co-culture system replicating the human flexor digitorum profundus enthesis

The enthesis, the specialized junction between tendon and bone, is a common site of injury. Although notoriously difficult to repair, advances in interfacial tissue engineering techniques are being developed for restorative function. Most notably are 3D in vitro co-culture models, built to recreate the complex heterogeneity of the native enthesis. While cell and matrix properties are often considered, there has been little attention given to native enthesis anatomical morphometrics and replicating these to enhance clinical relevance. This study focuses on the flexor digitorum profundus (FDP) tendon enthesis and, by combining anatomical morphometrics with computer-aided design, demonstrates the design and construction of an accurate and scalable model of the FDP enthesis. Bespoke 3D-printed mould inserts were fabricated based on the size, shape and insertion angle of the FDP enthesis. Then, silicone culture moulds were created, enabling the production of bespoke anatomical culture zones for an in vitro FDP enthesis model. The validity of the model has been confirmed using brushite cement scaffolds seeded with osteoblasts (bone) and fibrin hydrogel scaffolds seeded with fibroblasts (tendon) in individual studies with cells from either human or rat origin. This novel approach allows a bespoke anatomical design for enthesis repair and should be applied to future studies in this area.

Tendon-derived matrix crosslinking techniques for electrospun multi-layered scaffolds

Tendon tears are common and healing often occurs incompletely and by fibrosis. Tissue engineering seeks to improve repair, and one approach under investigation uses cell-seeded scaffolds containing biomimetic factors. Retention of biomimetic factors on the scaffolds is likely critical to maximize their benefit, while minimizing the risk of adverse effects, and without losing the beneficial effects of the biomimetic factors. The aim of the current study was to evaluate cross-linking methods to enhance the retention of tendon-derived matrix (TDM) on electrospun poly(ε-caprolactone) (PCL) scaffolds. We tested the effects of ultraviolet (UV) or carbodiimide (EDC:NHS:COOH) crosslinking methods to better retain TDM to the scaffolds and stimulate tendon-like matrix synthesis. Initially, we tested various crosslinking configurations of carbodiimide (2.5:1:1, 5:2:1, and 10:4:1 EDC:NHS:COOH ratios) and UV (30 s 1 J/cm2 , 60 s 1 J/cm2 , and 60 s 4 J/cm2 ) on PCL films compared to un-crosslinked TDM. We found that no crosslinking tested retained more TDM than coating alone (Kruskal-Wallis: p > .05), but that human adipose stem cells (hASCs) spread most on the 60 s 1 J/cm2 UV- and 2.5:1:1 EDC-crosslinked films (Kruskal-Wallis: p < .05). Next, we compared the effects of 60 s 1 J/cm2 UV- and 2.5:1:1 EDC-crosslinked to TDM-coated and untreated PCL scaffolds on hASC-induced tendon-like differentiation. UV-crosslinked scaffolds had greater modulus and stiffness than PCL or TDM scaffolds, and hASCs spread more on UV-crosslinked scaffolds (ANOVA: p < .05). Fourier transform infrared spectra revealed that UV- or EDC-crosslinking TDM did not affect the peaks at wavenumbers characteristic of tendon. Crosslinking TDM to electrospun scaffolds improves tendon-like matrix synthesis, providing a viable strategy for improving retention of TDM on electrospun PCL scaffolds.

Biologically-coupled bisphosphonate chaperones effectively deliver molecules to the site of soft tissue-bone healing

Tendon injuries are common and often treated surgically, however, current tendon repair healing results in poorly organized fibrotic tissue. While certain growth factors have been reported to improve both the strength and organization of the repaired enthesis, their clinical applicability is severely limited due to a lack of appropriate delivery strategies. In this study, we evaluated a recently developed fluorescent probe, Osteoadsorptive Fluorogenic Sentinel-3 that is composed of a bone-targeting bisphosphonate (BP) moiety linked to fluorochrome and quencher molecules joined via a cathepsin K-sensitive peptide sequence. Using a murine Achilles tendon-to-bone repair model, BP-based and/or Ctsk-coupled imaging probes were applied either locally or systemically. Fluorescence imaging was used to quantify the resultant signal in vivo. After tendon-bone repair, animals that received either local or systemic administration of imaging probes demonstrated significantly higher fluorescence signal at the repair site compared to the sham surgery group at all time points (p < 0.001), with signal peaking at 7-10 days after surgery. Our findings demonstrate the feasibility of using a novel BP-based targeting and Ctsk-activated delivery of molecules to the site of tendon-to-bone repair and creates a foundation for further development of this platform as an effective strategy to deliver bioactive molecules to sites of musculoskeletal injury.

Anisotropic wood-hydrogel composites: Extending mechanical properties of wood towards soft materials' applications

Delignified wood (DW) offers a versatile platform for the manufacturing of composites, with material properties ranging from stiff to soft and flexible by preserving the preferential fiber directionality of natural wood through a structure-retaining production process. This study presents a facile method for fabricating anisotropic and mechanically tunable DW-hydrogel composites. These composites were produced by infiltrating delignified spruce wood with an aqueous gelatin solution followed by chemical crosslinking. The mechanical properties could be modulated across a broad strength and stiffness range (1.2-18.3 MPa and 170-1455 MPa, respectively) by varying the crosslinking time. The diffusion-led crosslinking further allowed to manufacture mechanically graded structures. The resulting uniaxial, tubular structure of the anisotropic DW-hydrogel composite enabled the alignment of murine fibroblasts in vitro, which could be utilized in future studies on potential applications in tissue engineering.

Additive Manufacturing of Bioactive Glass and Its Polymer Composites as Bone Tissue Engineering Scaffolds: A Review

Bioactive glass (BG) and its polymer composites have demonstrated great potential as scaffolds for bone defect healing. Nonetheless, processing these materials into complex geometry to achieve either anatomy-fitting designs or the desired degradation behavior remains challenging. Additive manufacturing (AM) enables the fabrication of BG and BG/polymer objects with well-defined shapes and intricate porous structures. This work reviewed the recent advancements made in the AM of BG and BG/polymer composite scaffolds intended for bone tissue engineering. A literature search was performed using the Scopus database to include publications relevant to this topic. The properties of BG based on different inorganic glass formers, as well as BG/polymer composites, are first introduced. Melt extrusion, direct ink writing, powder bed fusion, and vat photopolymerization are AM technologies that are compatible with BG or BG/polymer processing and were reviewed in terms of their recent advances. The value of AM in the fabrication of BG or BG/polymer composites lies in its ability to produce scaffolds with patient-specific designs and the on-demand spatial distribution of biomaterials, both contributing to effective bone defect healing, as demonstrated by in vivo studies. Based on the relationships among structure, physiochemical properties, and biological function, AM-fabricated BG or BG/polymer composite scaffolds are valuable for achieving safer and more efficient bone defect healing in the future.

Strategies to promote tendon-bone healing after anterior cruciate ligament reconstruction: Present and future

At present, anterior cruciate ligament (ACL) reconstruction still has a high failure rate. Tendon graft and bone tunnel surface angiogenesis and bony ingrowth are the main physiological processes of tendon-bone healing, and also the main reasons for the postoperative efficacy of ACL reconstruction. Poor tendon-bone healing has been also identified as one of the main causes of unsatisfactory treatment outcomes. The physiological process of tendon-bone healing is complicated because the tendon-bone junction requires the organic fusion of the tendon graft with the bone tissue. The failure of the operation is often caused by tendon dislocation or scar healing. Therefore, it is important to study the possible risk factors for tendon-bone healing and strategies to promote it. This review comprehensively analyzed the risk factors contributing to tendon-bone healing failure after ACL reconstruction. Additionally, we discuss the current strategies used to promote tendon-bone healing following ACL reconstruction.

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.

Design and bioprinting for tissue interfaces

Tissue interfaces include complex gradient structures formed by transitioning of biochemical and mechanical properties in micro-scale. This characteristic allows the communication and synchronistic functioning of two adjacent but distinct tissues. It is particularly challenging to restore the function of these complex structures by transplantation of scaffolds exclusively produced by conventional tissue engineering methods. Three-dimensional (3D) bioprinting technology has opened an unprecedented approach for precise and graded patterning of chemical, biological and mechanical cues in a single construct mimicking natural tissue interfaces. This paper reviews and highlights biochemical and biomechanical design for 3D bioprinting of various tissue interfaces, including cartilage-bone, muscle-tendon, tendon/ligament-bone, skin, and neuro-vascular/muscular interfaces. Future directions and translational challenges are also provided at the end of the paper.

Canine ACL reconstruction with an injectable hydroxyapatite/collagen paste for accelerated healing of tendon-bone interface

Healing of an anterior cruciate ligament (ACL) autologous graft in a bone tunnel occurs through the formation of fibrovascular scar tissue, which is structurally and compositionally inferior to normal fibrocartilaginous insertion and thus may increase the reconstruction failure and the rate of failure recurrence. In this study, an injectable hydroxyapatite/type I collagen (HAp/Col Ⅰ) paste was developed to construct a suitable local microenvironment to accelerate the healing of bone-tendon interface. Physicochemical characterization demonstrated that the HAp/Col Ⅰ paste was injectable, uniform and stable. The in vitro cell culture illustrated that the paste could promote MC3T3-E1 cells proliferation and osteogenic expression. The results of a canine ACL reconstruction study showed that the reconstructive ACL had similar texture and color as the native ACL. The average width of the tunnel, total bone volume, bone volume/tissue volume and trabecular number acquired from micro-CT analysis suggested that the healing of tendon-bone interface in experimental group was better than that in control group. The biomechanical test showed the maximal loads in experimental group achieved approximately half of native ACL's maximal load at 24 weeks. According to histological examination, Sharpey fibers could be observed as early as 12 weeks postoperatively while a typical four-layer transitional structure of insertion site was regenerated at 48 weeks in the experimental group. The injectable HAp/Col Ⅰ paste provided a biomimetic scaffold and microenvironment for early cell attachment and proliferation, further osteogenic expression and extracellular matrix deposition, and in vivo structural and functional regeneration of the tendon-bone interface.

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A stable and injectable HAp/Col I paste was designed, optimized and characterized.

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The paste was applied in ACL reconstruction with an established standard operation procedure.

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Provided the safety and efficacy evidence for ACL reconstruction, and healing of tendon-bone interface was accelerated.

Rational Design of Soft-Hard Interfaces through Bioinspired Engineering

Soft-hard tissue interfaces in nature present a diversity of hierarchical transitions in composition and structure to address the challenge of stress concentrations that would otherwise arise at their interface. The translation of these into engineered materials holds promise for improved function of biomedical interfaces. Here, soft-hard tissue interfaces found in the body in health and disease, and the application of the diverse, functionally graded, and hierarchical structures that they present to bioinspired engineering materials are reviewed. A range of such bioinspired engineering materials and associated manufacturing technologies that are on the horizon in interfacial tissue engineering, hydrogel bioadhesion at the interfaces, and healthcare and medical devices are described.

Advanced strategies for constructing interfacial tissues of bone and tendon/ligament

Enthesis, the interfacial tissue between a tendon/ligament and bone, exhibits a complex histological transition from soft to hard tissue, which significantly complicates its repair and regeneration after injury. Because traditional surgical treatments for enthesis injury are not satisfactory, tissue engineering has emerged as a strategy for improving treatment success. Rapid advances in enthesis tissue engineering have led to the development of several strategies for promoting enthesis tissue regeneration, including biological scaffolds, cells, growth factors, and biophysical modulation. In this review, we discuss recent advances in enthesis tissue engineering, particularly the use of biological scaffolds, as well as perspectives on the future directions in enthesis tissue engineering.

Bioprinted living tissue constructs with layer-specific, growth factor-loaded microspheres for improved enthesis healing of a rotator cuff

Substantial challenges remain in constructing the native tendon-to-bone interface for rotator cuff healing owing to the enthesis tissues' highly organized structural and compositional gradients. Herein, we propose to bioprint living tissue constructs with layer-specific growth factors (GFs) to promote enthesis regeneration by guiding the zonal differentiation of the loaded stem cells in situ. The sustained release of tenogenic, chondrogenic, and osteogenic GFs was achieved via microsphere-based delivery carriers embedded in the bioprinted constructs. Compared to the basal construct without GFs, the layer-specific tissue analogs realized region-specific differentiation of stem cells in vitro. More importantly, bioprinted living tissue constructs with layer-specific GFs rapidly enhanced the enthesis regeneration in a rabbit rotator cuff tear model in terms of biomechanical restoration, collagen deposition, and alignment, showing gradient interface of fibrocartilage structures with aligned collagen fibrils and an ultimate load failure of 154.3 ± 9.5 N resembling those of native enthesis tissues in 12 weeks. This exploration provides a feasible strategy to engineer living tissue constructions with region-specific differentiation potentials for the functional repair of gradient enthesis tissues. STATEMENT OF SIGNIFICANCE: Previous studies that employed acellular layer-specific scaffolds or stem cells for the reconstruction of the rotator cuff faced challenges due to their insufficient capability to rebuild the anisotropic compositional and structural gradients of native enthesis tissues. This manuscript proposed a living tissue construct with layer-specific, GFs-loaded µS, which can direct in situ and region-specific differentiation of the embedded stem cells to tenogenic, chondrogenic, and osteogenic lineages for functional regeneration of the enthesis tissues. This bioprinted living tissue construct with the unique capability to reduce fibrovascular scar tissue formation and simultaneously facilitate enthesis tissue remodeling might provide a promising strategy to repair complex and gradient tissues in the future.

In situ regeneration of bone-to-tendon structures: Comparisons between costal-cartilage derived stem cells and BMSCs in the rat model

Bone-tendon interface (BTI), also called enthesis, is composed of the bone, fibrocartilage, and tendon/ligament with gradual structural characteristics. The unique gradient structure is particularly important for mechanical stress transfer between bone and soft tissues. However, BTI injuries result in fibrous scar repairs and high incidences of re-rupture, which is attributed to the lack of local stem cells with tenogenic and osteogenic potentials. In the rat model, we identified unique stem cells from costal cartilage (CDSCs) with a high in situ regeneration potential of BTI structures. Compared to bone-marrow mesenchymal stem cells (BMSCs), CDSCs exhibit higher self-renewal capacities, better adaptability to low-oxygen and low-nutrient post-transplantation environments, as well as strong bi-potent differentiation abilities of osteogenesis and tenogenesis. After transplantation, CDSCs can survive, proliferate, and in situ gradually regenerate BTI structures. Therefore, CDSCs have a great potential for tissue engineering regeneration in BTI injuries, and have future clinical application prospects. STATEMENT OF SIGNIFICANCE: Tissue engineering is a promising technique for bone-to-tendon interface (BTI) regeneration after injury, but it is still a long way from clinical application. One of the major reasons is the lack of suitable seed cells. This study found an ideal source of seed cells derived from costal cartilages (CDSCs). Compared to the traditional seed cell BMSCs, CDSCs have higher proliferation ability, strong chondrogenic and tenogenic differentiation potential, and better adaptability to low-oxygen and low nutrient conditions. CDSCs were able to survive, proliferate, and regenerate BTI structures in situ, in contrast to BMSCs. CDSCs transplantation showed strong BTI structures regeneration potential both histologically and biomechanically, making it a suitable seed cell for the tissue engineering regeneration of BTI.

The effects of maturation and aging on the rotator cuff tendon-to-bone interface

Rotator cuff tendon injuries often occur at the tendon-to-bone interface (i.e., enthesis) area, with a high prevalence for the elderly population, but the underlying reason for this phenomenon is still unknown. The objective of this study is to identify the histological, molecular, and biomechanical alterations of the rotator cuff enthesis with maturation and aging in a mouse model. Four different age groups of mice (newborn, young, adult, and old) were studied. Striking variations of the entheses were observed between the newborn and other matured groups, with collagen content, proteoglycan deposition, collagen fiber dispersion was significantly higher in the newborn group. The compositional and histological features of young, adult, and old groups did not show significant differences, except having increased proteoglycan deposition and thinner collagen fibers at the insertion sites in the old group. Nanoindentation testing showed that the old group had a smaller compressive modulus at the insertion site when compared with other groups. However, tensile mechanical testing reported that the old group demonstrated a significantly higher failure stress when compared with the young and adult groups. The proteomics analysis detected dramatic differences in protein content between newborn and young groups but minor changes among young, adult, and old groups. These results demonstrated: (1) the significant alterations of the enthesis composition and structure occur from the newborn to the young time period; (2) the increased risk of rotator cuff tendon injuries in the elderly population is not solely because of old age alone in the rodent model.

Advances in Field-Assisted 3D Printing of Bio-Inspired Composites: From Bioprototyping to Manufacturing

Biocomposite systems evolve to superior structural strategies in adapting to their living environments, using limited materials to form functionality superior to their inherent properties. The synergy of physical-field and Three-dimensional （3D） printing technologies creates unprecedented opportunities that overcome the limitations of traditional manufacturing methods and enable the precise replication of bio-enhanced structures. Here, an overview of typical structural designs in biocomposite systems, their functions and properties, are provided and the recent advances in bio-inspired composites using mechanical, electrical, magnetic, and ultrasound-field-assisted 3D printing techniques are highlighted. Finally, in order to realize the preparation of bionic functional devices and equipment with more superior functions, here an outlook on the development of field-assisted 3D printing technology from three aspects are provided: Materials, technology, and post-processing.

Poly(vinyl alcohol)/Gelatin Scaffolds Allow Regeneration of Nasal Tissues

Need for regeneration and repair of nasal tissues occurs as a consequence of several pathologies affecting the nose, including, but not limited to infective diseases, traumas and tumor resections. A platform for nasal tissue regeneration was set up using poly(vinyl alcohol)/gelatin sponges with 20%–30% (w/w) gelatin content to be used as scaffolds, for their intrinsic hydrophilic, cell adhesive and shape recovery properties. We propose mesodermal progenitor cells (MPCs) isolated from the bone marrow as a unique stem cell source for obtaining different connective tissues of the nose, including vascular tissue. Finally, epithelial cell immune response to these scaffolds was assessed in vitro in an environment containing inflammatory molecules. The results showed that mesenchymal stromal cells (MSCs) deriving from MPCs could be used to differentiate into cartilage and fibrous tissue; whereas, in combination with endothelial cells still deriving from MPCs, into pre-vascularized bone. Finally, the scaffold did not significantly alter the epithelial cell response to inflammatory insults derived from interaction with bacterial molecules.

Cruciate Ligament Cell Sheets Can Be Rapidly Produced on Thermoresponsive poly(glycidyl ether) Coating and Successfully Used for Colonization of Embroidered Scaffolds

Anterior cruciate ligament (ACL) cell sheets combined with biomechanically competent scaffolds might facilitate ACL tissue engineering. Since thermoresponsive polymers allow a rapid enzyme-free detachment of cell sheets, we evaluated the applicability of a thermoresponsive poly(glycidyl ether) (PGE) coating for cruciate ligamentocyte sheet formation and its influence on ligamentocyte phenotype during sheet-mediated colonization of embroidered scaffolds. Ligamentocytes were seeded on surfaces either coated with PGE or without coating. Detached ligamentocyte sheets were cultured separately or wrapped around an embroidered scaffold made of polylactide acid (PLA) and poly(lactic-co-ε-caprolactone) (P(LA-CL)) threads functionalized by gas-phase fluorination and with collagen foam. Ligamentocyte viability, protein and gene expression were determined in sheets detached from surfaces with or without PGE coating, scaffolds seeded with sheets from PGE-coated plates and the respective monolayers. Stable and vital ligamentocyte sheets could be produced within 24 h with both surfaces, but more rapidly with PGE coating. PGE did not affect ligamentocyte phenotype. Scaffolds could be colonized with sheets associated with high cell survival, stable gene expression of ligament-related type I collagen, decorin, tenascin C and Mohawk after 14 d and extracellular matrix (ECM) deposition. PGE coating facilitates ligamentocyte sheet formation, and sheets colonizing the scaffolds displayed a ligament-related phenotype.

Biomimetic strategies for tendon/ligament-to-bone interface regeneration

Tendon/ligament-to-bone healing poses a formidable clinical challenge due to the complex structure, composition, cell population and mechanics of the interface. With rapid advances in tissue engineering, a variety of strategies including advanced biomaterials, bioactive growth factors and multiple stem cell lineages have been developed to facilitate the healing of this tissue interface. Given the important role of structure-function relationship, the review begins with a brief description of enthesis structure and composition. Next, the biomimetic biomaterials including decellularized extracellular matrix scaffolds and synthetic-/natural-origin scaffolds are critically examined. Then, the key roles of the combination, concentration and location of various growth factors in biomimetic application are emphasized. After that, the various stem cell sources and culture systems are described. At last, we discuss unmet needs and existing challenges in the ideal strategies for tendon/ligament-to-bone regeneration and highlight emerging strategies in the field.

Magnetic Nanocomposite Hydrogels for Tissue Engineering: Design Concepts and Remote Actuation Strategies to Control Cell Fate

Most tissues of the human body are characterized by highly anisotropic physical properties and biological organization. Hydrogels have been proposed as scaffolding materials to construct artificial tissues due to their water-rich composition, biocompatibility, and tunable properties. However, unmodified hydrogels are typically composed of randomly oriented polymer networks, resulting in homogeneous structures with isotropic properties different from those observed in biological systems. Magnetic materials have been proposed as potential agents to provide hydrogels with the anisotropy required for their use on tissue engineering. Moreover, the intrinsic properties of magnetic nanoparticles enable their use as magnetomechanic remote actuators to control the behavior of the cells encapsulated within the hydrogels under the application of external magnetic fields. In this review, we combine a detailed summary of the main strategies to prepare magnetic nanoparticles showing controlled properties with an analysis of the different approaches available to their incorporation into hydrogels. The application of magnetically responsive nanocomposite hydrogels in the engineering of different tissues is also reviewed.

Mineral Distribution Spatially Patterns Bone Marrow Stromal Cell Behavior on Monolithic Bone Scaffolds

Interfaces between soft tissue and bone are characterized by transitional gradients in composition and structure that mediate substantial changes in mechanical properties. For interfacial tissue engineering, scaffolds with mineral gradients have shown promise in controlling osteogenic behavior of seeded bone marrow stromal cells (bMSCs). Previously, we have demonstrated a 'top-down' method for creating monolithic bone-derived scaffolds with patterned mineral distributions similar to native tissue. In the present work, we evaluated the ability of these scaffolds to pattern osteogenic behavior in bMSCs in basic, osteogenic, and chondrogenic biochemical environments. Immunohistochemical (IHC) and histological stains were used to characterize cellular behavior as a function of local mineral content. Alkaline phosphatase, an early marker of osteogenesis, and osteocalcin, a late marker of osteogenesis, were positively correlated with mineral content in basic, osteogenic, and chondrogenic media. The difference in bMSC behavior between the mineralized and demineralized regions was most pronounced in an basic biochemical environment. In the mineralized regions of the scaffold, osteogenic markers were clearly present as early as 4 days in culture. In osteogenic media, osteogenic behavior was observed across the entire scaffold, whereas in chondrogenic media, there was an overall reduction in osteogenic biomarkers. Overall, these results indicate local mineral content of the scaffold plays a key role in spatially patterning bMSC behavior. Our results can be utilized for the development of interfacial tissue engineered scaffolds and understanding the role of local environment in determining bMSC behavior. STATEMENT OF SIGNIFICANCE: Soft tissue-to-bone interfaces, such as tendon-bone, ligament-bone, and cartilage-bone, are ubiquitous in mammalian musculoskeletal systems. These interfacial tissues have distinct, hierarchically-structured gradients of cellular, biochemical, and materials components. Given the complexity of the biological structures, interfacial tissues present unique challenges for tissue engineering. Here, we demonstrate that material-derived cues can spatially pattern osteogenic behavior in bone marrow stromal cells (bMSCs). Specifically, we observed that when the bMSCs are cultured on bone-derived scaffolds with mineral gradients, cells in contact with higher mineral content display osteogenic behavior at earlier times than those on the unmineralized substrate. The ability to pattern the cellular complexity found in native interfaces while maintaining biologically relevant structures is a key step towards creating engineered tissue interfaces.

Role of Biomaterials and Controlled Architecture on Tendon/Ligament Repair and Regeneration

Engineering synthetic scaffolds to repair and regenerate ruptured native tendon and ligament (T/L) tissues is a significant engineering challenge due to the need to satisfy both the unique biological and biomechanical properties of these tissues. Long-term clinical outcomes of synthetic scaffolds relying solely on high uniaxial tensile strength are poor with high rates of implant rupture and synovitis. Ideal biomaterials for T/L repair and regeneration need to possess the appropriate biological and biomechanical properties necessary for the successful repair and regeneration of ruptured tendon and ligament tissues.

Three-dimensional printed multiphasic scaffolds with stratified cell-laden gelatin methacrylate hydrogels for biomimetic tendon-to-bone interface engineering

Background

The anatomical properties of the enthesis of the rotator cuff are hardly regained during the process of healing. The tendon-to-bone interface is normally replaced by fibrovascular tissue instead of interposition fibrocartilage, which impairs biomechanics in the shoulder and causes dysfunction. Tissue engineering offers a promising strategy to regenerate a biomimetic interface. Here, we report heterogeneous tendon-to-bone interface engineering based on a 3D-printed multiphasic scaffold.

Methods

A multiphasic poly(ε-caprolactone) (PCL)–PCL/tricalcium phosphate (TCP)–PCL/TCP porous scaffold was manufactured using 3D printing technology. The three phases of the scaffold were designed to mimic the graded tissue regions in the tendon-to-bone interface—tendon, fibrocartilage, and bone. Fibroblasts, bone marrow–derived mesenchymal stem cells, and osteoblasts were separately encapsulated in gelatin methacrylate (GelMA) and loaded seriatim on the relevant phases of the scaffold, by which a cells/GelMA-multiphasic scaffold (C/G-MS) construct, replicating the native interface, was fabricated. Cell proliferation, viability, and chondrogenic differentiation were evaluated in vitro. The C/G-MS constructs were further examined to determine the potential of regenerating a continuous interface with gradual transition of teno-, fibrocartilage- and osteo-like tissues in vivo.

Results

In vitro tests confirmed the good cytocompatibility of the constructs. After seven days in culture, cellular microfilament staining indicated that not only could cells well proliferate in GelMA hydrogels ​but also efficiently attach to and grow on scaffold fibres. Moreover, by immunolocalizing collagen type II, chondrogenesis was identified in the intermediate phase of the C/G-MS construct that had been treated with transforming growth factor β3 for 21 days. After subcutaneous implantation in mice, the C/G-MS construct exhibited a heterogeneous and graded structure up to eight weeks, with distinguished matrix distribution observed at one week. Overall, gene expression of tenogenic, chondrogenic, and osteogenic markers showed increasing patterns for eight weeks. Among them, expression of collagen type X gene was found drastically increasing during eight weeks, indicating progressive formation of calcifying cartilage within the constructs.

Conclusion

Our findings demonstrate that the stratified manner of fabrication based on the 3D-printed multiphasic scaffold is an effective strategy for tendon-to-bone interface engineering in terms of efficient cell seeding, chondrogenic potential, and distinct matrix deposition in varying phases.

The translational potential of this article

We fabricated a biomimetic tendon-to-bone interface by using a 3D-printed multiphasic scaffold and adopting a stratified cell-seeding manner with GelMA. The biomimetic interface might have applications in tendon-to-bone repair in the rotator cuff. It can not only be an alternative to a biological fixation device ​but also offer an ex vivo living graft to replace the damaged enthesis.

FGF-2-Induced Human Amniotic Mesenchymal Stem Cells Seeded on a Human Acellular Amniotic Membrane Scaffold Accelerated Tendon-to-Bone Healing in a Rabbit Extra-Articular Model

Background

FGF-2 (basic fibroblast growth factor) has a positive effect on the proliferation and differentiation of many kinds of MSCs. Therefore, it represents an ideal molecule to facilitate tendon-to-bone healing. Nonetheless, no studies have investigated the application of FGF-2-induced human amniotic mesenchymal stem cells (hAMSCs) to accelerate tendon-to-bone healing in vivo.

Objective

The purpose of this study was to explore the effect of FGF-2 on chondrogenic differentiation of hAMSCs in vitro and the effect of FGF-2-induced hAMSCs combined with a human acellular amniotic membrane (HAAM) scaffold on tendon-to-bone healing in vivo.

Methods

In vitro, hAMSCs were transfected with a lentivirus carrying the FGF-2 gene, and the potential for chondrogenic differentiation of hAMSCs induced by the FGF-2 gene was assessed using immunofluorescence and toluidine blue (TB) staining. HAAM scaffold was prepared, and hematoxylin and eosin (HE) staining and scanning electron microscopy (SEM) were used to observe the microstructure of the HAAM scaffold. hAMSCs transfected with and without FGF-2 were seeded on the HAAM scaffold at a density of 3 × 10⁵ cells/well. Immunofluorescence staining of vimentin and phalloidin staining were used to confirm cell adherence and growth on the HAAM scaffold. In vivo, the rabbit extra-articular tendon-to-bone healing model was created using the right hind limb of 40 New Zealand White rabbits. Grafts mimicking tendon-to-bone interface (TBI) injury were created and subjected to treatment with the HAAM scaffold loaded with FGF-2-induced hAMSCs, HAAM scaffold loaded with hAMSCs only, HAAM scaffold, and no special treatment. Macroscopic observation, imageological analysis, histological assessment, and biomechanical analysis were conducted to evaluate tendon-to-bone healing after 3 months.

Results

In vitro, cartilage-specific marker staining was positive for the FGF-2 overexpression group. The HAAM scaffold displayed a netted structure and mass extracellular matrix structure. hAMSCs or hAMSCs transfected with FGF-2 survived on the HAAM scaffold and grew well. In vivo, the group treated with HAAM scaffold loaded with FGF-2-induced hAMSCs had the narrowest bone tunnel after three months as compared with other groups. In addition, macroscopic and histological scores were higher for this group than for the other groups, along with the best mechanical strength.

Conclusion

hAMSCs transfected with FGF-2 combined with the HAAM scaffold could accelerate tendon-to-bone healing in a rabbit extra-articular model.

Stem cells in the periodontal ligament differentiated into osteogenic, fibrogenic and cementogenic lineages for the regeneration of the periodontal complex

OBJECTIVE

Human periodontal ligament stem cells (hPDLSCs) are promising for periodontal regeneration. However, to date, there has been no report of hPDLSC differentiation into the fibrogenic lineage. There has been no report demonstrating hPDLSC differentiation into all three (osteogenic, fibrogenic and cementogenic fibrogenic) lineages in the same report. The objectives of this study were to harvest hPDLSCs from the periodontal ligaments (PDL) of the extracted human teeth, and use the same vial of hPDLSCs to differentiate into all three (osteogenic, fibrogenic and cementogenic) lineages for the first time.

METHODS

hPDLSCs were harvested from PDL tissues of the extracted premolars. The ability of hPDLSCs to form bone, cementum and collagen fibers was tested in culture mediums. Gene expressions were analyzed using quantitative real-time polymerase chain reaction (qRT-PCR). Immunofluorescence, alizarin red (ARS), Xylenol orange, picro sirius red staining (PSRS), alcian blue staining (ABS) and alkaline phosphatase (ALP) staining were evaluated.

RESULTS

In osteogenic medium, hPDLSCs had high expressions of osteogenic genes (RUNX2, ALP, OPN and COL1) at 14 and 21 days (15-20 folds of that of control), and produced mineral nodules and ALP activity (5 and 10 folds those of the control). hPDLSCs in fibrogenic medium expressed high levels of PDL fibrogenic genes (COL1, COL3, FSP-1, PLAP-1 and Elastin) at 28 days (20-70 folds of control). They were stained strongly with F-actin and fibronection, and secreted PDL collagen fibers (5 folds of control). hPDLSCs in cementogenic medium showed high expressions of cementum genes (CAP, CEMP1 and BSP) at 21 days (10-15 folds of control) and synthesized mineralized cementum (50 folds via ABS, and 40 folds via ALP staining, compared to those of control).

CONCLUSIONS

hPDLSCs differentiated into bone-, fiber- and cementum-forming cells, with potential for regeneration of periodontium to form the bone-PDL-cementum complex.

[Research progress of structured repair of tendon-bone interface]

In sports system, the tendon-bone interface has the effect of tensile and bearing load, so the effect of healing plays a crucial role in restoring joint function. The process of repair is the formation of scar tissue, so it is difficult to achieve the ideal effect for morphology and biomechanical strength. The tissue engineering method can promote the tendon-bone interface healing from the seed cells, growth factors, and scaffolds, and is a new direction in the field of development of the tendon-bone interface healing.

Regulation of Cell Behavior by Hydrostatic Pressure

Hydrostatic pressure (HP) regulates diverse cell behaviors including differentiation, migration, apoptosis, and proliferation. Abnormal HP is associated with pathologies including glaucoma and hypertensive fibrotic remodeling. In this review, recent advances in quantifying and predicting how cells respond to HP across several tissue systems are presented, including tissues of the brain, eye, vasculature and bladder, as well as articular cartilage. Finally, some promising directions on the study of cell behaviors regulated by HP are proposed.

Tissue engineering a tendon-bone junction with biodegradable braided scaffolds

Background

Tendons play an important role in transferring stress between muscles and bones and in maintaining the stability of joints. Tendon tears are difficult to heal and are associated with high recurrence rates. So, the objective of this study was to develop a biodegradable scaffold for tendon-bone junction regeneration.

Methods

Two types of polylactic acid (PLA) yarns, having fibers with round and four deep grooved cross-sections, were braided into tubular scaffolds and cultured with murine Transforming growth factor beta type II receptor (Tgfbr2)-expressing joint progenitor cells. The scaffolds were designed to mimic the mechanical, immuno-chemical and biological properties of natural mouse tendon-bone junctions. Three different tubular scaffolds measuring 2 mm in diameter were braided on a Steeger 16-spindle braiding machine and biological and mechanical performance of the three scaffolds were evaluated.

Results

The mechanical test results indicated that three different braided scaffold structures provided a wide range of mechanical properties that mimic the components of tendon bone junction and results of the biological tests confirmed cell viability, active cell attachment and proliferation throughout all three scaffolds.

Conclusions

This study has identified that the three proposed types of braided scaffolds with some improvement in their structures have the potential to be used as scaffolds for the regeneration of a tendon bone tissue junction.

Enhancement of rotator cuff tendon-bone healing using combined aligned electrospun fibrous membranes and kartogenin

Rotator cuff tear (RCT) is a major challenging shoulder disease because the fibrocartilage zone is hard to regenerate in the enthesis. Electrospun membranes with aligned nanofibers can guide the ordered tissue regeneration and kartogenin (KGN) is able to stimulate chondrocyte differentiation of mesenchymal stem cells. In this study, we fabricated a functional engineered scaffold for regenerating tendon-bone enthesis in RCTs by taking advantage of both the structural guiding ability of aligned nanofibers and the biology effects of KGN. Polycaprolactone (PCL) fibrous membranes with aligned nanofibers loaded with or without KGN were fabricated using electrospinning and characterized using scanning electron microscopy (SEM). The release of KGN from PCL membranes and the effects of KGN on differentiation of mesenchymal stem cells were investigated. Results indicated that 100 μM KGN-loaded PCL (KGN-PCL) membranes significantly stimulated chondrogenic and tenogenic differentiation of rat bone marrow stromal cells. In addition, after PCL and 100 μM KGN-PCL membranes were applied to an acute rat RCT model, KGN-PCL membranes promoted fibrocartilage formation and collagen organization as well as increased cross-sectional area and load failure. In conclusion, PCL electrospun fibrous membranes with aligned nanofibers and KGN could be an effective tissue engineering scaffold to enhance tendon-bone healing in RCTs.

Effect of Autologous Platelet-Rich Plasma and Gelatin Sponge for Tendon-to-Bone Healing After Rabbit Anterior Cruciate Ligament Reconstruction

PURPOSE

To investigate platelet-rich plasma (PRP) combined with gelatin sponge (GS) to improve tendon-bone interface healing and structure formation.

METHODS

Characterization of the GS scaffold was performed with a scanning electron microscope, and the release curve after loading with PRP was evaluated. A real-time reverse transcription quantitative polymerase chain reaction assay was performed to test the levels of tendon-to-bone healing-related gene expression. Finally, 18 New Zealand white rabbits were randomly divided into 3 groups and underwent semitendinosus autograft anterior cruciate ligament reconstruction: autograft group without PRP, PRP group, and PRP-GS group. All rabbits were killed 8 weeks after the operation. Magnetic resonance imaging scans, biomechanical testing, and histologic evaluation were performed.

RESULTS

An enzyme-linked immunosorbent assay and cell counting kit-8 assay showed that the GS could control the release of PRP and prolong its bioactivity time, as well as promote bone marrow mesenchymal stem cell proliferation. In the PRP-GS group, the levels of related genes were upregulated compared with the PRP group (P < .05). Lower signal in the magnetic resonance images indicated fibrocartilage formation in the 2 groups with PRP. In addition, histologic staining showed that the tendon-bone connection had a greater fibrocartilaginous transition region in the PRP-GS group, and the histologic scores were higher (vs the PRP group, P = .039). The maximum failure load and stiffness were higher in the PRP-GS group than in the other 2 groups.

CONCLUSIONS

GS loading with PRP could prolong the bioactivity time of PRP and promote bone marrow mesenchymal stem cell proliferation and osteogenic gene expression in vitro. It also promoted the early healing process at the tendon-bone junction in a rabbit anterior cruciate ligament reconstruction model.

CLINICAL RELEVANCE

GS is a natural material and offers satisfactory biocompatibility. Using GS as a scaffold to control the release of bioactive factors in bone tunnels may be useful, but additional studies in human subjects will be necessary to evaluate its clinical prospects.

Entropic Forces Drive Cellular Contact Guidance

Contact guidance-the widely known phenomenon of cell alignment induced by anisotropic environmental features-is an essential step in the organization of adherent cells, but the mechanisms by which cells achieve this orientational ordering remain unclear. Here, we seeded myofibroblasts on substrates micropatterned with stripes of fibronectin and observed that contact guidance emerges at stripe widths much greater than the cell size. To understand the origins of this surprising observation, we combined morphometric analysis of cells and their subcellular components with a, to our knowledge, novel statistical framework for modeling nonthermal fluctuations of living cells. This modeling framework is shown to predict not only the trends but also the statistical variability of a wide range of biological observables, including cell (and nucleus) shapes, sizes, and orientations, as well as stress-fiber arrangements within the cells with remarkable fidelity with a single set of cell parameters. By comparing observations and theory, we identified two regimes of contact guidance: 1) guidance on stripe widths smaller than the cell size (w ≤ 160 μm), which is accompanied by biochemical changes within the cells, including increasing stress-fiber polarization and cell elongation; and 2) entropic guidance on larger stripe widths, which is governed by fluctuations in the cell morphology. Overall, our findings suggest an entropy-mediated mechanism for contact guidance associated with the tendency of cells to maximize their morphological entropy through shape fluctuations.

Cellular and Chemical Gradients to Engineer the Meniscus-to-Bone Insertion

Tissue-engineered menisci hold promise as an alternative to allograft procedures but require a means of robust fixation to the native bone. The insertion of the meniscus into bone is critical for meniscal function and inclusion of a soft tissue-to-bone interface in a tissue engineered implant can aid in the fixation process. The native insertion is characterized by gradients in composition, tissue architecture, and cellular phenotype, which are all difficult to replicate. In this study, a soft tissue-to-bone interface is tissue engineered with a cellular gradient of fibrochondrocytes and mesenchymal stem cells and subjected to a biochemical gradient through a custom media diffusion bioreactor. These constructs, consisting of interpenetrating collagen and boney regions, display improved mechanical performance and collagen organization compared to controls without a cellular or chemical gradient. Media gradient exposure produces morphological features in the constructs that appear similar to the native tissue. Collectively, these data show that cellular and biochemical gradients improve integration between collagen and bone in a tissue engineered soft tissue-to-bone construct.

Hybrid Additive Microfabrication Scaffold Incorporated with Highly Aligned Nanofibers for Musculoskeletal Tissues

Background

Latest tissue engineering strategies for musculoskeletal tissues regeneration focus on creating a biomimetic microenvironment closely resembling the natural topology of extracellular matrix. This paper presents a novel musculoskeletal tissue scaffold fabricated by hybrid additive manufacturing method.

Methods

The skeleton of the scaffold was 3D printed by fused deposition modeling, and a layer of random or aligned polycaprolactone nanofibers were embedded between two frames. A parametric study was performed to investigate the effects of process parameters on nanofiber morphology. A compression test was performed to study the mechanical properties of the scaffold. Human fibroblast cells were cultured in the scaffold for 7 days to evaluate the effect of scaffold microstructure on cell growth.

Results

The tip-to-collector distance showed a positive correlation with the fiber alignment, and the electrospinning time showed a negative correlation with the fiber density. With reinforced nanofibers, the hybrid scaffold demonstrated superior compression strength compared to conventional 3D-printed scaffold. The hybrid scaffold with aligned nanofibers led to higher cell attachment and proliferation rates, and a directional cell organization. In addition, there was a nonlinear relationship between the fiber diameter/density and the cell actinfilament density.

Conclusion

This hybrid biofabrication process can be established as a highly efficient and scalable platform to fabricate biomimetic scaffolds with patterned fibrous microstructure, and will facilitate future development of clinical solutions for musculoskeletal tissue regeneration.

Tunable Enzymatically Cross-Linked Silk Fibroin Tubular Conduits for Guided Tissue Regeneration

Hollow tubular conduits (TCs) with tunable architecture and biological properties are in great need for modulating cell functions and drug delivery in guided tissue regeneration. Here, a new methodology to produce enzymatically cross-linked silk fibroin TCs is described, which takes advantage of the tyrosine groups present in silk structure that are known to allow the formation of a covalently cross-linked hydrogel. Three different processing methods are used as a final step to modulate the properties of the silk-based TCs. This approach allows to virtually adjust any characteristic of the final TCs. The final microstructure ranges from a nonporous to a highly porous network, allowing the TCs to be selectively porous to 4 kDa molecules, but not to human skin fibroblasts. Mechanical properties are dependent both on the processing method and thickness of the TCs. Bioactivity is observed after 30 days of immersion in simulated body fluid only for the TCs submitted to a drying processing method (50 °C). The in vivo study performed in mice demonstrates the good biocompatibility of the TCs. The enzymatically cross-linked silk fibroin TCs are versatile and have adjustable characteristics that can be exploited in a variety of biomedical applications, particularly in guidance of peripheral nerve regeneration.

Physiology and Engineering of the Graded Interfaces of Musculoskeletal Junctions

The connective tissues of the musculoskeletal system can be grouped into fibrous, cartilaginous, and calcified tissues. While each tissue type has a distinct composition and function, the intersections between these tissues result in the formation of complex, composite, and graded junctions. The complexity of these interfaces is a critical aspect of their healthy function, but poses a significant challenge for their repair. In this review, we describe the organization and structure of complex musculoskeletal interfaces, identify emerging technologies for engineering such structures, and outline the requirements for assessing the complex nature of these tissues in the context of recapitulating their function through tissue engineering.

Primary and Secondary Consequences of Rotator Cuff Injury on Joint Stabilizing Tissues in the Shoulder

Rotator cuff tears (RCTs) are one of the primary causes of shoulder pain and dysfunction in the upper extremity accounting over 4.5 million physician visits per year with 250,000 rotator cuff repairs being performed annually in the U.S. While the tear is often considered an injury to a specific tendon/tendons and consequently treated as such, there are secondary effects of RCTs that may have significant consequences for shoulder function. Specifically, RCTs have been shown to affect the joint cartilage, bone, the ligaments, as well as the remaining intact tendons of the shoulder joint. Injuries associated with the upper extremities account for the largest percent of workplace injuries. Unfortunately, the variable success rate related to RCTs motivates the need for a better understanding of the biomechanical consequences associated with the shoulder injuries. Understanding the timing of the injury and the secondary anatomic consequences that are likely to have occurred are also of great importance in treatment planning because the approach to the treatment algorithm is influenced by the functional and anatomic state of the rotator cuff and the shoulder complex in general. In this review, we summarized the contribution of RCTs to joint stability in terms of both primary (injured tendon) and secondary (remaining tissues) consequences including anatomic changes in the tissues surrounding the affected tendon/tendons. The mechanical basis of normal shoulder joint function depends on the balance between active muscle forces and passive stabilization from the joint surfaces, capsular ligaments, and labrum. Evaluating the role of all tissues working together as a system for maintaining joint stability during function is important to understand the effects of RCT, specifically in the working population, and may provide insight into root causes of shoulder injury.

Rehabilitation of symptomatic atraumatic degenerative rotator cuff tears: A clinical commentary on assessment and management

STUDY DESIGN

Clinical Commentary.

INTRODUCTION

Atraumatic rotator cuff (RC) disease, is one of the most common cause of shoulder pain, which encompasses a continuum from tendinopathy to full thickness cuff tears. Extrinsic, intrinsic and environmental factors have been implicated in the pathophysiology of this disorder, affecting the clinical presentation of symptoms including pain and irritability. Successful rehabilitation of symptomatic atraumatic degenerative rotator cuff (SADRC) tears must address the underlying mechanisms causing dysfunction and correct modifiable factors.

PURPOSE OF THE STUDY

The purpose of this paper is to review the shoulder complex anatomy, introduce atraumatic degenerative RC pathology, differentiate between symptomatic and asymptomatic degenerative RC tears, propose an assessment and introduce the Rotator Cuff Protocol 1 (RCP1) designed by the clinical reasoning of one of the lead authors (LW) as a rehabilitation management approach for those clients who present with SADRC tears.

METHODS/RESULTS/DISCUSSION

N/A for clinical commentary.

CONCLUSIONS

The ability to identify SADRC tears should consider shoulder anatomy, extrinsic, intrinsic and environmental factors, and the consideration for the natural history of atraumatic partial and full thickness tears in the general population. A thorough clinical history and examination, which includes shoulder symptom modification tests, allows the examiner to determine at what phase the patient may start their exercise program. The RCP1 is a program that has been used clinically by many therapists and clients over the years and research is underway to test this protocol in atraumatic rotator cuff disease including SADRC tears.

LEVEL OF EVIDENCE

5.

Ultrasound-Mediated Gene Delivery Enhances Tendon Allograft Integration in Mini-Pig Ligament Reconstruction

Ligament injuries occur frequently, substantially hindering routine daily activities and sports participation in patients. Surgical reconstruction using autogenous or allogeneic tissues is the gold standard treatment for ligament injuries. Although surgeons routinely perform ligament reconstructions, the integrity of these reconstructions largely depends on adequate biological healing of the interface between the ligament graft and the bone. We hypothesized that localized ultrasound-mediated, microbubble-enhanced therapeutic gene delivery to endogenous stem cells would lead to significantly improved ligament graft integration. To test this hypothesis, an anterior cruciate ligament reconstruction procedure was performed in Yucatan mini-pigs. A collagen scaffold was implanted in the reconstruction sites to facilitate recruitment of endogenous mesenchymal stem cells. Ultrasound-mediated reporter gene delivery successfully transfected 40% of cells recruited to the reconstruction sites. When BMP-6 encoding DNA was delivered, BMP-6 expression in the reconstruction sites was significantly enhanced. Micro-computed tomography and biomechanical analyses showed that ultrasound-mediated BMP-6 gene delivery led to significantly enhanced osteointegration in all animals 8 weeks after surgery. Collectively, these findings demonstrate that ultrasound-mediated gene delivery to endogenous mesenchymal progenitor cells can effectively improve ligament reconstruction in large animals, thereby addressing a major unmet orthopedic need and offering new possibilities for translation to the clinical setting.

Biological and Biomechanical Evaluation of Autologous Tendon Combined with Ligament Advanced Reinforcement System Artificial Ligament in a Rabbit Model of Anterior Cruciate Ligament Reconstruction

OBJECTIVE

To compare the biomechanical and histological changes in a rabbit model after reconstructing the anterior cruciate ligament (ACL) with solely autologous tendon and with autologous tendon combined with the ligament advanced reinforcement system (LARS) artificial ligament.

METHODS

Anterior cruciate ligament reconstruction was performed in 72 knees from 36 healthy New Zealand white rabbits (bodyweight, 2500-3000 g). The Achilles tendons were harvested bilaterally. The left ACL were reconstructed solely with autografts (autologous tendon group), while the right ACL were reconstructed with autografts combined with LARS ligaments (combined ligaments group). The gross observation, histological determination, and the tension failure loads in both groups were evaluated at 12 weeks (n = 18) and 24 weeks (n = 18) postoperatively.

RESULTS

Gross examination of the knee joints showed that all combined ligaments were obviously covered by a connective tissue layer at 12 weeks, and were completely covered at 24 weeks. Fibrous tissue ingrowth was observed between fascicles and individual fibers in the bone-artificial ligament interface at both time points; this fibrovascular tissue layer localized at the bone-artificial ligament interface tended to be denser in specimens obtained at 24 weeks compared with those obtained at 12 weeks. The tension failure loads of the knees were similar in the autologous tendon group and the combined ligaments group at 12 weeks (144.15 ± 3.92 N vs. 140.88 ± 2.75 N; P > 0.05), and at 24 weeks (184.15 ± 1.96 N vs. 180.88 ± 3.21 N; P > 0.05).

CONCLUSION

Reconstructing the ACL in rabbits using autologous tendon combined with the LARS artificial ligament results in satisfactory biointegration, with no obvious immunological rejection between the autologous tendon and the artificial ligament, and is, therefore, a promising ACL reconstruction method.

Multilineage Constructs for Scaffold-Based Tissue Engineering: A Review of Tissue-Specific Challenges

There is a growing interest in the regeneration of tissue in interfacial regions, where biological, physical, and chemical attributes vary across tissue type. The simultaneous use of distinct cell lineages can help in developing in vitro structures, analogous to native composite tissues. This literature review gathers the recent reports that have investigated multiple cell types of various sources and lineages in a coculture system for tissue-engineered constructs. Such studies aim at mimicking the native organization of tissues and their interfaces, and/or to improve the development of complex tissue substitutes. This paper thus distinguishes itself from those focusing on technical aspects of coculturing for a single specific tissue. The first part of this review is dedicated to variables of cocultured tissue engineering such as scaffold, cells, and in vitro culture environment. Next, tissue-specific coculture methods and approaches are covered for the most studied tissues. Finally, cross-analysis is performed to highlight emerging trends in coculture principles and to discuss how tissue-specific challenges can inspire new approaches for regeneration of different interfaces to improve the outcomes of various tissue engineering strategies.