From repair to regeneration: biomaterials to reprogram the meniscus wound microenvironment

Cyclic compressive loading induces a mature meniscal cell phenotype in mesenchymal stem cells with an atelocollagen-based scaffold

Introduction: Biomechanical stimulation is reportedly pivotal in meniscal regeneration, although its effect on mesenchymal stem cell (MSC) meniscal differentiation remains elusive. In this study, we investigated how cyclic compressive loading (CCL) could impact MSCs using three-dimensional cultures in atelocollagen-based meniscal substitute (ACMS). Methods: We extracted MSCs from the meniscus, synovium, and articular cartilage, cultured them in three-dimensional cultures, and exposed them to CCL for 7 days. We then compared the transcriptomes of MSCs treated with and without CCL. Results: Our RNA-seq analysis revealed that CCL induced significant transcriptome changes, significantly affecting chondrocyte-related genes, including SOX9, TGFB1, and PRG4 upregulation. CCL induced transcriptional differentiation of meniscus progenitors toward mature meniscal cells. Conclusion: This study unveils the potential of mechanical stress in promoting MSC meniscal differentiation within ACMS. Our investigations provide new insights for mechanisms underlying meniscal regeneration with ACMS.

Microinterfaces in biopolymer-based bicontinuous hydrogels guide rapid 3D cell migration

Cell migration is critical for tissue development and regeneration but requires extracellular environments that are conducive to motion. Cells may actively generate migratory routes in vivo by degrading or remodeling their environments or instead utilize existing extracellular matrix microstructures or microtracks as innate pathways for migration. While hydrogels in general are valuable tools for probing the extracellular regulators of 3-dimensional migration, few recapitulate these natural migration paths. Here, we develop a biopolymer-based bicontinuous hydrogel system that comprises a covalent hydrogel of enzymatically crosslinked gelatin and a physical hydrogel of guest and host moieties bonded to hyaluronic acid. Bicontinuous hydrogels form through controlled solution immiscibility, and their continuous subdomains and high micro-interfacial surface area enable rapid 3D migration, particularly when compared to homogeneous hydrogels. Migratory behavior is mesenchymal in nature and regulated by biochemical and biophysical signals from the hydrogel, which is shown across various cell types and physiologically relevant contexts (e.g., cell spheroids, ex vivo tissues, in vivo tissues). Our findings introduce a design that leverages important local interfaces to guide rapid cell migration.

Microinterfaces in bicontinuous hydrogels guide rapid 3D cell migration

Cell migration is critical for tissue development and regeneration but requires extracellular environments that are conducive to motion. Cells may actively generate migratory routes in vivo by degrading or remodeling their environments or may instead utilize existing ECM microstructures or microtracks as innate pathways for migration. While hydrogels in general are valuable tools for probing the extracellular regulators of 3D migration, few have recapitulated these natural migration paths. Here, we developed a biopolymer-based (i.e., gelatin and hyaluronic acid) bicontinuous hydrogel system formed through controlled solution immiscibility whose continuous subdomains and high micro-interfacial surface area enabled rapid 3D migration, particularly when compared to homogeneous hydrogels. Migratory behavior was mesenchymal in nature and regulated by biochemical and biophysical signals from the hydrogel, which was shown across various cell types and physiologically relevant contexts (e.g., cell spheroids, ex vivo tissues, in vivo tissues). Our findings introduce a new design that leverages important local interfaces to guide rapid cell migration.

Engineered biochemical cues of regenerative biomaterials to enhance endogenous stem/progenitor cells (ESPCs)-mediated articular cartilage repair

As a highly specialized shock-absorbing connective tissue, articular cartilage (AC) has very limited self-repair capacity after traumatic injuries, posing a heavy socioeconomic burden. Common clinical therapies for small- to medium-size focal AC defects are well-developed endogenous repair and cell-based strategies, including microfracture, mosaicplasty, autologous chondrocyte implantation (ACI), and matrix-induced ACI (MACI). However, these treatments frequently result in mechanically inferior fibrocartilage, low cost-effectiveness, donor site morbidity, and short-term durability. It prompts an urgent need for innovative approaches to pattern a pro-regenerative microenvironment and yield hyaline-like cartilage with similar biomechanical and biochemical properties as healthy native AC. Acellular regenerative biomaterials can create a favorable local environment for AC repair without causing relevant regulatory and scientific concerns from cell-based treatments. A deeper understanding of the mechanism of endogenous cartilage healing is furthering the (bio)design and application of these scaffolds. Currently, the utilization of regenerative biomaterials to magnify the repairing effect of joint-resident endogenous stem/progenitor cells (ESPCs) presents an evolving improvement for cartilage repair. This review starts by briefly summarizing the current understanding of endogenous AC repair and the vital roles of ESPCs and chemoattractants for cartilage regeneration. Then several intrinsic hurdles for regenerative biomaterials-based AC repair are discussed. The recent advances in novel (bio)design and application regarding regenerative biomaterials with favorable biochemical cues to provide an instructive extracellular microenvironment and to guide the ESPCs (e.g. adhesion, migration, proliferation, differentiation, matrix production, and remodeling) for cartilage repair are summarized. Finally, this review outlines the future directions of engineering the next-generation regenerative biomaterials toward ultimate clinical translation.

Suspension bath bioprinting and maturation of anisotropic meniscal constructs

Due to limited intrinsic healing capacity of the meniscus, meniscal injuries pose a significant clinical challenge. The most common method for treatment of damaged meniscal tissues, meniscectomy, leads to improper loading within the knee joint, which can increase the risk of osteoarthritis. Thus, there is a clinical need for the development of constructs for meniscal repair that better replicate meniscal tissue organization to improve load distributions and function over time. Advanced three-dimensional bioprinting technologies such as suspension bath bioprinting provide some key advantages, such as the ability to support the fabrication of complex structures using non-viscous bioinks. In this work, the suspension bath printing process is utilized to print anisotropic constructs with a unique bioink that contains embedded hydrogel fibers that align via shear stresses during printing. Constructs with and without fibers are printed and then cultured for up to 56 din vitroin a custom clamping system. Printed constructs with fibers demonstrate increased cell and collagen alignment, as well as enhanced tensile moduli when compared to constructs printed without fibers. This work advances the use of biofabrication to develop anisotropic constructs that can be utilized for the repair of meniscal tissue.

Biomaterials releasing drug responsively to promote wound healing via regulation of pathological microenvironment

Wound healing is characterized by complex, orchestrated, spatiotemporal dynamic processes. Recent findings demonstrated suitable local microenvironments were necessities for wound healing. Wound microenvironments include various biological, biochemical and physical factors, which are produced and regulated by endogenous biomediators, exogenous drugs, and external environment. Successful drug delivery to wound is complicated, and need to overcome the destroyed blood supply, persistent inflammation and enzymes, spatiotemporal requirements of special supplements, and easy deactivation of drugs. Triggered by various factors from wound microenvironment itself or external elements, stimuli-responsive biomaterials have tremendous advantages of precise drug delivery and release. Here, we discuss recent advances of stimuli-responsive biomaterials to regulate local microenvironments during wound healing, emphasizing on the design and application of different biomaterials which respond to wound biological/biochemical microenvironments (ROS, pH, enzymes, glucose and glutathione), physical microenvironments (mechanical force, temperature, light, ultrasound, magnetic and electric field), and the combination modes. Moreover, several novel promising drug carriers (microbiota, metal-organic frameworks and microneedles) are also discussed.

Meniscus Matrix Structural and Biomechanical Evaluation: Age-Dependent Properties in a Swine Model

The analysis of the morphological, structural, biochemical, and mechanical changes of the Extracellular Matrix (ECM), which occur during meniscus development, represents the goal of the present study. Medial fully developed menisci (FD, 9-month-old pigs), partially developed menisci (PD, 1-month-old piglets), and not developed menisci (ND, from stillbirths) were collected. Cellularity and glycosaminoglycans (GAGs) deposition were evaluated by ELISA, while Collagen 1 and aggrecan were investigated by immunohistochemistry and Western blot analyses in order to be compared to the biomechanical properties of traction and compression tensile forces, respectively. Cellularity decreased from ND to FD and GAGs showed the opposite trend (p < 0.01 both). Collagen 1 decreased from ND to FD, as well as the ability to resist to tensile traction forces (p < 0.01), while aggrecan showed the opposite trend, in accordance with the biomechanics: compression test showed that FD meniscus greatly resists to deformation (p < 0.01). This study demonstrated that in swine meniscus, clear morphological and biomechanical changes follow the meniscal maturation and specialization during growth, starting with an immature pattern (ND) to the mature organized meniscus of the FD, and they could be useful to understand the behavior of this structure in the light of its tissue bioengineering.

Prospective Application of Partially Digested Autologous Chondrocyte for Meniscus Tissue Engineering

Background: Meniscus tissue engineering has yet to achieve clinical application because it requires chondrogenic induction and in vitro cell expansion. Contrarily, cartilage engineering from autologous chondrocytes has been successfully applied in one-stage surgery. If the natural chondrogenic potential of meniscus cells can be demonstrated, meniscus tissue engineering would have more value in clinical settings. Materials and Methods: In total, 10 menisci and pieces of cartilage were obtained during total knee replacements. The tissues were collected for cell isolation and expansion. Their chondrogenic properties were examined by immunohistofluorescence and gene expression analyses. Results: In native cartilage, immunofluorescence demonstrated the presence of collagen I, aggrecan, and traces of collagen I, whereas comparable staining was seen in the inner and middle meniscus. The presence of collagen I but the absence of collagen II and aggrecan were observed in the outer meniscus. In passage 2, chondrocytes showed the presence of collagen II and aggrecan, and the absence of vimentin. The vimentin and aggrecan staining were comparable in the inner and middle meniscus cells, whereas the outer cells showed only vimentin staining. In the gene expression analyses, the expressions of collagen II and aggrecan in the native chondrocyte and the inner and middle meniscus were higher than those of the cells from the outer meniscus, but they were not different in collagen I. In the passage 2 culture, chondrocytes had a higher expression of collagen II and aggrecan than the meniscus cells. Cells from the inner and middle areas had higher collagen II and aggrecan expression than those from the outer meniscus. Conclusion: Without chondrogenic induction, inner and middle meniscus cells possess a chondrogenic phenotype. Specifically, native meniscus cells exhibited more robust chondrogenic potential compared with those of the passage 2 monolayer culture.

Perspective: Challenges Presented for Regeneration of Heterogeneous Musculoskeletal Tissues that Normally Develop in Unique Biomechanical Environments

Perspective: Musculoskeletal (MSK) tissues such as articular cartilage, menisci, tendons, and ligaments are often injured throughout life as a consequence of accidents. Joints can also become compromised due to the presence of inflammatory diseases such as rheumatoid arthritis. Thus, there is a need to develop regenerative approaches to address such injuries to heterogeneous tissues and ones that occur in heterogeneous environments. Such injuries can compromise both the biomechanical integrity and functional capability of these tissues. Thus, there are several challenges to overcome in order to enhance success of efforts to repair and regenerate damaged MSK tissues.

Challenges: 1. MSK tissues arise during development in very different biological and biomechanical environments. These early tissues serve as a template to address the biomechanical requirements evolving during growth and maturation towards skeletal maturity. Many of these tissues are heterogeneous and have transition points in their matrix. The heterogeneity of environments thus presents a challenge to replicate with regard to both the cells and the ECM. 2. Growth and maturation of musculoskeletal tissues occurs in the presence of anabolic mediators such as growth hormone and the IGF-1 family of proteins which decline with age and are low when there is a greater need for the repair and regeneration of injured or damaged tissues with advancing age. Thus, there is the challenge of re-creating an anabolic environment to enhance incorporation of implanted constructs. 3. The environments associated with injury or chronic degeneration of tissues are often catabolic or inflammatory. Thus, there is the challenge of creating a more favorable in vivo environment to facilitate the successful implantation of in vitro engineered constructs to regenerate damaged tissues.

Conclusions: The goal of regenerating MSK tissues has to be to meet not only the biological requirements (components and structure) but also the heterogeneity of function (biomechanics) in vivo. Furthermore, for many of these tissues, the regenerative approach has to overcome the site of injury being influenced by catabolism/inflammation. Attempts to date using both endogenous cells, exogenous cells and scaffolds of various types have been limited in achieving long term outcomes, but progress is being made.

Advances in electrospun scaffolds for meniscus tissue engineering and regeneration

The meniscus plays a critical role in maintaining the homeostasis, biomechanics, and structural stability of the knee joint. Unfortunately, it is predisposed to damages either from sports-related trauma or age-related degeneration. The meniscus has an inherently limited capacity for tissue regeneration. Self-healing of injured adult menisci only occurs in the peripheral vascularized portion, while the spontaneous repair of the inner avascular region seems never happens. Repair, replacement, and regeneration of menisci through tissue engineering strategies are promising to address this problem. Recently, many scaffolds for meniscus tissue engineering have been proposed for both experimental and preclinical investigations. Electrospinning is a feasible and versatile technique to produce nano- to micro-scale fibers that mimic the microarchitecture of native extracellular matrix and is an effective approach to prepare nanofibrous scaffolds for constructing engineered meniscus. Electrospun scaffolds are reported to be capable of inducing colonization of meniscus cells by modulating local extracellular density and stimulating endogenous regeneration by driving reprogramming of meniscus wound microenvironment. Electrospun nanofibrous scaffolds with tunable mechanical properties, controllable anisotropy, and various porosities have shown promises for meniscus repair and regeneration and will undoubtedly inspire more efforts in exploring effective therapeutic approaches towards clinical applications. In this article, we review the current advances in the use of electrospun nanofibrous scaffolds for meniscus tissue engineering and repair and discuss prospects for future studies.

Extracellular vesicles derived from mesenchymal stromal cells mediate endogenous cell growth and migration via the CXCL5 and CXCL6/CXCR2 axes and repair menisci

Background

Mesenchymal stromal cell-derived extracellular vesicles (MSC-EVs) are promising candidates for tissue regeneration therapy. However, the therapeutic efficacy of MSC-EVs for meniscus regeneration is uncertain, and the mechanisms underlying MSC-EV-mediated tissue regeneration have not been fully elucidated. The aims of this study were to evaluate the therapeutic efficacy of intra-articular MSC-EV injection in a meniscus defect model and elucidate the mechanism underlying MSC-EV-mediated tissue regeneration via combined bioinformatic analyses.

Methods

MSC-EVs were isolated from human synovial MSC culture supernatants via ultrafiltration. To evaluate the meniscus regeneration ability, MSC-EVs were injected intra-articularly in the mouse meniscus defect model immediately after meniscus resection and weekly thereafter. After 1 and 3 weeks, their knees were excised for histological and immunohistochemical evaluations. To investigate the mechanisms through which MSC-EVs accelerate meniscus regeneration, cell growth, migration, and chondrogenesis assays were performed using treated and untreated chondrocytes and synovial MSCs with or without MSC-EVs. RNA sequencing assessed the gene expression profile of chondrocytes stimulated by MSC-EVs. Antagonists of the human chemokine CXCR2 receptor (SB265610) were used to determine the role of CXCR2 on chondrocyte cell growth and migration induced by MSC-EVs.

Results

In the meniscus defect model, MSC-EV injection accelerated meniscus regeneration and normalized the morphology and composition of the repaired tissue. MSC-EVs stimulated chondrocyte and synovial MSC cell growth and migration. RNA sequencing revealed that MSC-EVs induced 168 differentially expressed genes in the chondrocytes and significantly upregulated CXCL5 and CXCL6 in chondrocytes and synovial MSCs. Suppression of CXCL5 and CXCL6 and antagonism of the CXCR2 receptor binding CXCL5 and CXCL6 negated the influence of MSC-EVs on chondrocyte cell growth and migration.

Conclusions

Intra-articular MSC-EV administration repaired meniscus defects and augmented chondrocyte and synovial MSC cell growth and migration. Comprehensive transcriptome/RNA sequencing data confirmed that MSC-EVs upregulated CXCL5 and CXCL6 in chondrocytes and mediated the cell growth and migration of these cells via the CXCR2 axis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02481-9.

The regenerative effect of different growth factors and platelet lysate on meniscus cells and mesenchymal stromal cells and proof of concept with a functionalized meniscus implant

Meniscus regeneration could be enhanced by targeting meniscus cells and mesenchymal stromal cells (MSCs) with the right growth factors. Combining these growth factors with the Collagen Meniscus Implant (CMI®) could accelerate cell ingrowth and tissue formation in the implant and thereby improve clinical outcomes. Using a transwell migration assay and a micro-wound assay, the effect of insulin-like growth factor-1, platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), transforming growth factor beta 1 (TGF-β1), fibroblast growth factor, and platelet lysate (PL) on migration and proliferation of meniscus cells and MSCs was assessed. The formation of extracellular matrix under influence of the above-mentioned growth factors was assessed after 28 days of culture of both MSCs and meniscus cells. As a proof of concept, the CMI® was functionalized with a VEGF binding peptide and coated with platelet-rich plasma (PRP) for clinical application. Our results demonstrate that PDGF, TGF-β1, and PL stimulate migration, proliferation, and/or extracellular matrix production of meniscus cells and MSCs. Additionally, the CMI® was successfully functionalized with a VEGF binding peptide and PRP which increased migration of meniscus cell and MSC into the implant. This study demonstrates proof of concept of functionalizing the CMI® with growth factor binding peptides. A CMI® functionalized with the right growth factors holds great potential for meniscus replacement after partial meniscectomy.

Strontium-doped gelatin scaffolds promote M2 macrophage switch and angiogenesis through modulating the polarization of neutrophils

The immune system mediates inflammation, vascularization and the first response to injuries or implanted biomaterials. Although the function of neutrophils in tissue repair has been extensively studied, its complete role in the tissue regeneration of biomaterials, specifically the resolution of inflammation and promotion of angiogenesis, is unclear. Here, we fabricate nanofibrous gelatin scaffolds containing 10% (w/w) strontium-hydroxyapatite (SrHA) via phase-separation methods to investigate Sr-mediated regulation of neutrophil polarization and, subsequently, the effects on angiogenesis and macrophage polarization. Compared with neutrophils cultured on pure gelatin or HA-incorporated gelatin scaffolds, neutrophils on SrHA-incorporated gelatin scaffolds show more N2 polarization in vitro and in vivo and significantly greater production of immunomodulatory and angiogenic factors. The Sr-induced immunomodulatory and proangiogenic functions of neutrophils are mediated through NF-κB pathway downregulation and increased STAT3 phosphorylation. Thus, neutrophils play a vital role in tissue engineering, and Sr-incorporated scaffolds efficiently promote neutrophil polarization to the N2 phenotype, enhancing resolution of inflammation and ultimately promoting angiogenesis and tissue regeneration. Thus, incorporation of neutrophils in analyses of the immune characteristics of scaffolds and the development of immunomodulatory biomaterials that can regulate neutrophils are novel and promising strategies in tissue engineering.

N-Acetyl-L-Cysteine Reduces Fibrosis and Improves Muscle Function After Acute Compartment Syndrome Injury

INTRODUCTION

Upon injury, skeletal muscle undergoes a multiphase process beginning with degeneration of the damaged tissue, which is accompanied by inflammation and finally regeneration. One consequence of an injured microenvironment is excessive production of reactive oxygen species, which results in attenuated regeneration and recovery of function ultimately leading to fibrosis and disability. The objective of this research was to test the potential of the antioxidant, N-Acetyl-L-Cysteine (NAC), as a mediator of reactive oxygen species damage that results from traumatic muscle injury in order to support repair and regeneration of wounded muscle tissue and improve function recovery.

MATERIALS AND METHODS

Adult female Lewis rats were subjected to compartment syndrome injury as previously published by our group. Rats received intramuscular injections of NAC or vehicle at 24, 48, and 72 hours postinjury. Muscle function, tissue fibrosis, and the expression of myogenic and angiogenic markers were measured.

RESULTS

Muscle function was significantly improved, and tissue fibrosis was significantly decreased in NAC-treated muscles.

CONCLUSIONS

These results suggest that NAC treatment of skeletal muscle after injury may be a viable option for the prevention of long-term fibrosis and scar formation, facilitating recovery of muscle function.

Potential of Soluble Decellularized Extracellular Matrix for Musculoskeletal Tissue Engineering - Comparison of Various Mesenchymal Tissues

Background

It is well studied that preparations of decellularized extracellular matrix (ECM) obtained from mesenchymal tissues can function as biological scaffolds to regenerate injured musculoskeletal tissues. Previously, we reported that soluble decellularized ECMs derived from meniscal tissue demonstrated excellent biocompatibility and produced meniscal regenerate with native meniscal anatomy and biochemical characteristics. We therefore hypothesized that decellularized mesenchymal tissue ECMs from various mesenchymal tissues should exhibit tissue-specific bioactivity. The purpose of this study was to test this hypothesis using porcine tissues, for potential applications in musculoskeletal tissue engineering.

Methods

Nine types of porcine tissue, including cartilage, meniscus, ligament, tendon, muscle, synovium, fat pad, fat, and bone, were decellularized using established methods and solubilized. Although the current trend is to develop tissue specific decellularization protocols, we selected a simple standard protocol across all tissues using Triton X-100 and DNase/RNase after mincing to compare the outcome. The content of sulfated glycosaminoglycan (sGAG) and hydroxyproline were quantified to determine the biochemical composition of each tissue. Along with the concentration of several growth factors, known to be involved in tissue repair and/or maturation, including bFGF, IGF-1, VEGF, and TGF-β1. The effect of soluble ECMs on cell differentiation was explored by combining them with 3D collagen scaffold culturing human synovium derived mesenchymal stem cells (hSMSCs).

Results

The decellularization of each tissue was performed and confirmed both histologically [hematoxylin and eosin (H&E) and 4’,6-diamidino-2-phenylindole (DAPI) staining] and on the basis of dsDNA quantification. The content of hydroxyproline of each tissue was relatively unchanged during the decellularization process when comparing the native and decellularized tissue. Cartilage and meniscus exhibited a significant decrease in sGAG content. The content of hydroxyproline in meniscus-derived ECM was the highest when compared with other tissues, while sGAG content in cartilage was the highest. Interestingly, a tissue-specific composition of most of the growth factors was measured in each soluble decellularized ECM and specific differentiation potential was particularly evident in cartilage, ligament and bone derived ECMs.

Conclusion

In this study, soluble decellularized ECMs exhibited differences based on their tissue of origin and the present results are important going forward in the field of musculoskeletal regeneration therapy.

Advanced Polymer-Based Drug Delivery Strategies for Meniscal Regeneration

The meniscus plays a critical role in maintaining knee joint homeostasis. Injuries to the meniscus, especially considering the limited self-healing capacity of the avascular region, continue to be a challenge and are often treated by (partial) meniscectomy, which has been identified to cause osteoarthritis. Currently, meniscus tissue engineering focuses on providing extracellular matrix (ECM)-mimicking scaffolds to direct the inherent meniscal regeneration process, and it has been found that various stimuli are essential. Numerous bioactive factors present benefits in regulating cell fate, tissue development, and healing, but lack an optimal delivery system. More recently, bioengineers have developed various polymer-based drug delivery systems (PDDSs), which are beneficial in terms of the favorable properties of polymers as well as novel delivery strategies. Engineered PDDSs aim to provide not only an ECM-mimicking microenvironment but also the controlled release of bioactive factors with release profiles tailored according to the biological concerns and properties of the factors. In this review, both different polymers and bioactive factors involved in meniscal regeneration are discussed, as well as potential candidate systems, with examples of recent progress. This article aims to summarize drug delivery strategies in meniscal regeneration, with a focus on novel delivery strategies rather than on specific delivery carriers. The current challenges and future prospects for the structural and functional regeneration of the meniscus are also discussed. Impact statement Meniscal injury remains a clinical Gordian knot owing to the limited healing potential of the region, restricted surgical approaches, and risk of inducing osteoarthritis. Existing tissue engineering scaffolds that provide mechanical support and a favorable microenvironment also lack biological cues. Advanced polymer-based delivery strategies consisting of polymers incorporating bioactive factors have emerged as a promising direction. This article primarily reviews the types and applications of biopolymers and bioactive factors in meniscal regeneration. Importantly, various carrier systems and drug delivery strategies are discussed with the hope of inspiring further advancements in this field.

Overcoming negatively charged tissue barriers: Drug delivery using cationic peptides and proteins

Negatively charged tissues are ubiquitous in the human body and are associated with a number of common diseases yet remain an outstanding challenge for targeted drug delivery. While the anionic proteoglycans are critical for tissue structure and function, they make tissue matrix dense, conferring a high negative fixed charge density (FCD) that makes drug penetration through the tissue deep zones and drug delivery to resident cells extremely challenging. The high negative FCD of these tissues is now being utilized by taking advantage of electrostatic interactions to create positively charged multi-stage delivery methods that can sequentially penetrate through the full thickness of tissues, create a drug depot and target cells. After decades of work on attempting delivery using strong binding interactions, significant advances have recently been made using weak and reversible electrostatic interactions, a characteristic now considered essential to drug penetration and retention in negatively charged tissues. Here we discuss these advances using examples of negatively charged tissues (cartilage, meniscus, tendons and ligaments, nucleus pulposus, vitreous of eye, mucin, skin), and delve into how each of their structures, tissue matrix compositions and high negative FCDs create barriers to drug entry and explore how charge interactions are being used to overcome these barriers. We review work on tissue targeting cationic peptide and protein-based drug delivery, compare and contrast drug delivery designs, and also present examples of technologies that are entering clinical trials. We also present strategies on further enhancing drug retention within diseased tissues of lower FCD by using synergistic effects of short-range binding interactions like hydrophobic and H-bonds that stabilize long-range charge interactions. As electrostatic interactions are incorporated into design of drug delivery materials and used as a strategy to create properties that are reversible, tunable and dynamic, bio-electroceuticals are becoming an exciting new direction of research and clinical work.

Engineering Anisotropic Meniscus: Zonal Functionality and Spatiotemporal Drug Delivery

Human meniscus is a fibrocartilaginous structure that is crucial for an adequate performance of the human knee joint. Degeneration of the meniscus is often followed by partial or total meniscectomy, which enhances the risk of developing knee osteoarthritis. The lack of a satisfactory treatment for this condition has triggered a major interest in drug delivery (DD) and tissue engineering (TE) strategies intended to restore a bioactive and fully functional meniscal tissue. The aim of this review is to critically discuss the most relevant studies on spatiotemporal DD and TE, aiming for a multizonal meniscal reconstruction. Indeed, the development of meniscal tissue implants should involve a provision for adequate active molecules and scaffold features that take into account the anisotropic ultrastructure of human meniscus. This zonal differentiation is reflected in the meniscus biochemical composition, collagen fiber arrangement, and cell distribution. In this sense, it is expected that a proper combination of advanced DD and zonal TE strategies will play a key role in the future trends in meniscus regeneration. Impact statement Meniscus degeneration is one of the main causes of knee pain, inflammation, and reduced mobility. Currently used suturing procedures and meniscectomy are far from being ideal solutions to the loss of meniscal function. Therefore, drug delivery (DD) and tissue engineering (TE) strategies are currently under investigation. DD systems aim at an in situ controlled release of growth factors, whereas TE strategies aim at mimicking the anisotropy of native meniscus. The goal of this review is to discuss these two main approaches, as well as synergies between them that are expected to lead to a real breakthrough in the field.

Neutrophils in Biomaterial-Guided Tissue Regeneration: Matrix Reprogramming for Angiogenesis

Biomaterial-guided in situ tissue regeneration uses biomaterials to stimulate and guide the body's endogenous, regenerative processes to drive functional tissue repair and regeneration. To be successful, cell migration into the biomaterials is essential, which requires angiogenesis to maintain cell viability. Neutrophils, the first cells responding to an implanted biomaterial, are now known to play an integral part in angiogenesis in multiple tissues and exhibit considerable potential for driving angiogenesis in the context of tissue regeneration. In terms of biomaterial-guided in situ tissue regeneration, harnessing the proangiogenic potential of the neutrophil through its robust secretion of matrix metalloproteinase 9 (MMP-9) may provide a mechanism to improve biomaterial performance by initiating matrix reprogramming. This review will discuss neutrophils as matrix reprogrammers and what is currently known about their ability to create a microenvironment that is more conducive for angiogenesis and tissue regeneration through the secretion of MMP-9. It will first review a set of ground-breaking studies in tumor biology and then present an overview of what is currently known about neutrophils and MMP-9 in biomaterial vascularization. Finally, it will conclude with potential strategies and considerations to engage neutrophils in biomaterial-guided angiogenesis and in situ tissue regeneration. Impact statement This review draws attention to a highly neglected topic in tissue engineering, the role of neutrophils in biomaterial-guided tissue regeneration and angiogenesis. Moreover, it highlights their abundant secretion of matrix metalloproteinase 9 (MMP-9) for matrix reprogramming, a topic with great potential yet to be vetted in the literature. It presents strategies and considerations for designing the next generation of immunomodulatory biomaterials. While there is literature discussing the overall role of neutrophils in angiogenesis, there are a limited number of review articles focused on this highly relevant topic in the context of biomaterial integration and tissue regeneration, making this a necessary and impactful article.

Influence of Fiber Stiffness on Meniscal Cell Migration into Dense Fibrous Networks

Fibrous scaffolds fabricated via electrospinning are being explored to repair injuries within dense connective tissues. However, there is still much to be understood regarding the appropriate scaffold properties that best support tissue repair. In this study, the influence of the stiffness of electrospun fibers on cell invasion into fibrous scaffolds is investigated. Specifically, soft and stiff electrospun fibrous networks are fabricated from crosslinked methacrylated hyaluronic acid (MeHA), where the stiffness is altered via the extent of MeHA crosslinking. Meniscal fibrochondrocyte (MFC) adhesion and migration into fibrous networks are investigated, where the softer MeHA fibrous networks are easily deformed and densified through cellular tractions and the stiffer MeHA fibrous networks support ≈50% greater MFC invasion over weeks when placed adjacent to meniscal tissue. When the scaffolds are sandwiched between meniscal tissues and implanted subcutaneously, the stiffer MeHA fibrous networks again support enhanced cellular invasion and greater collagen deposition after 4 weeks when compared to the softer MeHA fibrous networks. These results indicate that the mechanics and deformability of fibrous networks likely alter cellular interactions and invasion, providing an important design parameter toward the engineering of scaffolds for tissue repair.

The Meniscus Tear: A Review of Stem Cell Therapies

Meniscal injuries have posed a challenging problem for many years, especially considering that historically the meniscus was considered to be a structure with no important role in the knee joint. This led to earlier treatments aiming at the removal of the entire structure in a procedure known as a meniscectomy. However, with the current understanding of the function and roles of the meniscus, meniscectomy has been identified to accelerate joint degradation significantly and is no longer a preferred treatment option in meniscal tears. Current therapies are now focused to regenerate, repair, or replace the injured meniscus to restore its native function. Repairs have improved in technique and materials over time, with various implant devices being utilized and developed. More recently, strategies have applied stem cells, tissue engineering, and their combination to potentiate healing to achieve superior quality repair tissue and retard the joint degeneration associated with an injured or inadequately functioning meniscus. Accordingly, the purpose of this current review is to summarize the current available pre-clinical and clinical literature using stem cells and tissue engineering for meniscal repair and regeneration.

Meniscus-Derived Matrix Bioscaffolds: Effects of Concentration and Cross-Linking on Meniscus Cellular Responses and Tissue Repair

Meniscal injuries, particularly in the avascular zone, have a low propensity for healing and are associated with the development of osteoarthritis. Current meniscal repair techniques are limited to specific tear types and have significant risk for failure. In previous work, we demonstrated the ability of meniscus-derived matrix (MDM) scaffolds to augment the integration and repair of an in vitro meniscus defect. The objective of this study was to determine the effects of percent composition and dehydrothermal (DHT) or genipin cross-linking of MDM bioscaffolds on primary meniscus cellular responses and integrative meniscus repair. In all scaffolds, the porous microenvironment allowed for exogenous cell infiltration and proliferation, as well as endogenous meniscus cell migration. The genipin cross-linked scaffolds promoted extracellular matrix (ECM) deposition and/or retention. The shear strength of integrative meniscus repair was improved with increasing percentages of MDM and genipin cross-linking. Overall, the 16% genipin cross-linked scaffolds were most effective at enhancing integrative meniscus repair. The ability of the genipin cross-linked scaffolds to attract endogenous meniscus cells, promote glycosaminoglycan and collagen deposition, and enhance integrative meniscus repair reveals that these MDM scaffolds are promising tools to augment meniscus healing.

Core-Shell Nanofibrous Scaffolds for Repair of Meniscus Tears

Electrospinning is an attractive method of fabricating nanofibers that replicate the ultrastructure of the human meniscus. However, it is challenging to approximate the mechanical properties of meniscal tissue while maintaining the biocompatibility of collagen fibers. Our objective was to determine if functionalizing polylactic acid (PLA) nanofibers with collagen would enhance their biocompatibility. We therefore used coaxial electrospinning to generate core-shell nanofibers with a core of PLA for mechanical strength and a shell of collagen to enhance cell attachment and matrix synthesis. We characterized the nanostructure of the engineered scaffolds and measured the hydrophilic and mechanical properties. We assessed the performance of human meniscal cells seeded on coaxial electrospun scaffolds to produce meniscal tissue by gene expression and histology. Finally, we investigated whether these cell-seeded scaffolds could repair surgical tears created ex vivo in avascular meniscal explants. Histology, immunohistochemistry, and mechanical testing of ex vivo repair provided evidence of neotissue that was significantly better integrated with the native tissue than with the acellular coaxial electrospun scaffolds. Human meniscal cell-seeded coaxial electrospun scaffolds may have potential in enhancing repair of avascular meniscus tears. Impact Statement The success of any tissue-engineered meniscus graft relies on its ability to mimic native three-dimensional microstructure, support cell growth, produce tissue-specific matrix, and enhance graft integration into the repair site. Polylactic acid scaffolds possess the desired mechanical properties, whereas collagen scaffolds induce better cell attachment and enhanced tissue regeneration. We therefore fabricated nanofibrous scaffolds that combined the properties of two biomaterials. These novel coaxial scaffolds more closely emulated the structure, mechanical properties, and biochemical composition of native meniscal tissue. Our findings of meniscogenic tissue generation and integration in meniscus defects have the potential to be translated to clinical use.

Meniscus-Derived Matrix Scaffolds Promote the Integrative Repair of Meniscal Defects

Meniscal tears have a poor healing capacity, and damage to the meniscus is associated with significant pain, disability, and progressive degenerative changes in the knee joint that lead to osteoarthritis. Therefore, strategies to promote meniscus repair and improve meniscus function are needed. The objective of this study was to generate porcine meniscus-derived matrix (MDM) scaffolds and test their effectiveness in promoting meniscus repair via migration of endogenous meniscus cells from the surrounding meniscus or exogenously seeded human bone marrow-derived mesenchymal stem cells (MSCs). Both endogenous meniscal cells and MSCs infiltrated the MDM scaffolds. In the absence of exogenous cells, the 8% MDM scaffolds promoted the integrative repair of an in vitro meniscal defect. Dehydrothermal crosslinking and concentration of the MDM influenced the biochemical content and shear strength of repair, demonstrating that the MDM can be tailored to promote tissue repair. These findings indicate that native meniscus cells can enhance meniscus healing if a scaffold is provided that promotes cellular infiltration and tissue growth. The high affinity of cells for the MDM and the ability to remodel the scaffold reveals the potential of MDM to integrate with native meniscal tissue to promote long-term repair without necessarily requiring exogenous cells.

Platelet-derived growth factor-coated decellularized meniscus scaffold for integrative healing of meniscus tears

The aim of this study was to examine the potential of platelet-derived growth factor (PDGF)-coated decellularized meniscus scaffold in mediating integrative healing of meniscus tears by inducing endogenous cell migration. Fresh bovine meniscus was chemically decellularized and covalently conjugated with heparin and PDGF-BB. In vitro PDGF release kinetics was measured. The scaffold was transplanted into experimental tears in avascular bovine meniscus explants and cultured for 2 and 4 weeks. The number migrating and proliferating cells at the borderline between the scaffold and injured explant and PDGF receptor-β (PDGFRβ) expressing cells were counted. The alignment of the newly produced ECM and collagen was analyzed by Safranin-O, picrosirius red staining, and differential interference contrast (DIC). Tensile testing of the explants was performed after culture for 2 and 4 weeks. Heparin conjugated scaffold showed immobilization of high levels of PDGF-BB, with sustained release over 2 weeks. Insertion of the PDGF-BB treated scaffold in defects in avascular meniscus led to increased PDGFRβ expression, cell migration and proliferation into the defect zone. Safranin-O, picrosirius red staining and DIC showed tissue integration between the scaffold and injured explants. Tensile properties of injured explants treated with PDGF-BB coated scaffold were significantly higher than in the scaffold without PDGF. In conclusion, PDGF-BB-coated scaffold increased PDGFRβ expression and promoted migration of endogenous meniscus cells to the defect area. New matrix was formed that bridged the space between the native meniscus and the scaffold and this was associated with improved biomechanical properties. The PDGF-BB-coated scaffold will be promising for clinical translation to healing of meniscus tears.

STATEMENT OF SIGNIFICANCE

Meniscus tears are the most common injury of the knee joint. The most prevalent forms that occur in the inner third typically do not spontaneously heal and represent a major risk factor for the development of knee osteoarthritis. The goal of this project was to develop an approach that is readily applicable for clinical use. We selected a natural and readily available decellularized meniscus scaffold and conjugated it with PDGF, which we had previously found to have strong chemotactic activity for chondrocytes and progenitor cells. The present results show that insertion of the PDGF-conjugated scaffold in defects in avascular meniscus led to endogenous cell migration and proliferation into the defect zone with tissue integration between the scaffold and injured explants and improved tensile properties. This PDGF-conjugated scaffold will be promising for a translational approach to healing of meniscus tears.

Engineered Healing of Avascular Meniscus Tears by Stem Cell Recruitment

Meniscus injuries are extremely common with approximately one million patients undergoing surgical treatment annually in the U.S. alone. Upon injury, the outer zone of the meniscus can be repaired and expected to functionally heal but tears in the inner avascular region are unlikely to heal. To date, no regenerative therapy has been proven successful for consistently promoting healing in inner-zone meniscus tears. Here, we show that controlled applications of connective tissue growth factor (CTGF) and transforming growth factor beta 3 (TGFβ3) can induce seamless healing of avascular meniscus tears by inducing recruitment and step-wise differentiation of synovial mesenchymal stem/progenitor cells (syMSCs). A short-term release of CTGF, a selected chemotactic and profibrogenic cue, successfully recruited syMSCs into the incision site and formed an integrated fibrous matrix. Sustain-released TGFβ3 then led to a remodeling of the intermediate fibrous matrix into fibrocartilaginous matrix, fully integrating incised meniscal tissues with improved functional properties. Our data may represent a novel clinically relevant strategy to improve healing of avascular meniscus tears by recruiting endogenous stem/progenitor cells.

Impacts of maturation on the micromechanics of the meniscus extracellular matrix

To elucidate how maturation impacts the structure and mechanics of meniscus extracellular matrix (ECM) at the length scale of collagen fibrils and fibers, we tested the micromechanical properties of fetal and adult bovine menisci via atomic force microscopy (AFM)-nanoindentation. For circumferential fibers, we detected significant increase in the effective indentation modulus, E_ind, with age. Such impact is in agreement with the increase in collagen fibril diameter and alignment during maturation, and is more pronounced in the outer zone, where collagen fibrils are more aligned and packed. Meanwhile, maturation also markedly increases the E_ind of radial tie fibers, but not those of intact surface or superficial layer. These results provide new insights into the effect of maturation on the assembly of meniscus ECM, and enable the design of new meniscus repair strategies by modulating local ECM structure and mechanical behaviors.

The role of biomaterials in the treatment of meniscal tears

Extensive investigations over the recent decades have established the anatomical, biomechanical and functional importance of the meniscus in the knee joint. As a functioning part of the joint, it serves to prevent the deterioration of articular cartilage and subsequent osteoarthritis. To this end, meniscus repair and regeneration is of particular interest from the biomaterial, bioengineering and orthopaedic research community. Even though meniscal research is previously of a considerable volume, the research community with evolving material science, biology and medical advances are all pushing toward emerging novel solutions and approaches to the successful treatment of meniscal difficulties. This review presents a tactical evaluation of the latest biomaterials, experiments to simulate meniscal tears and the state-of-the-art materials and strategies currently used to treat tears.

Large Animal Models of Meniscus Repair and Regeneration: A Systematic Review of the State of the Field

Injury to the meniscus is common, but few viable strategies exist for its repair or regeneration. To address this, animal models have been developed to translate new treatment strategies toward the clinic. However, there is not yet a regulatory document guiding such studies. The purpose of this study was to carry out a systematic review of the literature on meniscus treatment methods and outcomes to define the state of the field. Public databases were queried by using search terms related to animal models and meniscus injury and/or repair over the years 1980-2015. Identified peer-reviewed manuscripts were screened by using Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. One of nine reviewers read each manuscript and scored them based on whether the publication described a series of predefined study descriptors and outcome measures. Additional data were extracted to identify common assays used. A total of 128 full-length peer-reviewed manuscripts were identified. The number of publications increased over the time frame analyzed, with 48% focused on augmented repair. Rabbit was, by far, the most prevalent species utilized (46%), with dog (21%) and sheep (20%) being the next most common. Analysis of study descriptors revealed that most studies appropriately documented details of the animal used, the surgical approach, and defect and implant characteristics (e.g., 63% of studies identified clearly the defect size). In terms of outcome parameters, most studies carried out macroscopic (85%), histologic (90%), and healing/integration (83%) analyses of the meniscus. However, many studies did not provide further analysis beyond these fundamental measures, and less than 40% reported on the adjacent cartilage and synovium, as well as joint function. There is intense interest in the field of meniscus repair. However, given the current lack of guidance documentation in this area, preclinical animal models are not performed in a standardized fashion. The development of a "Best Practices" document would increase reproducibility and external validity of experiments, while accelerating advancements in translational research. Advancement is of paramount importance given the high prevalence of meniscal injuries and the paucity of effective repair or regenerative strategies.

AFM-Nanomechanical Test: An Interdisciplinary Tool That Links the Understanding of Cartilage and Meniscus Biomechanics, Osteoarthritis Degeneration, and Tissue Engineering

Our objective is to provide an in-depth review of the recent technical advances of atomic force microscopy (AFM)-based nanomechanical tests and their contribution to a better understanding and diagnosis of osteoarthritis (OA), as well as the repair of tissues undergoing degeneration during OA progression. We first summarize a range of technical approaches for AFM-based nanoindentation, including considerations in both experimental design and data analysis. We then provide a more detailed description of two recently developed modes of AFM-nanoindentation, a high-bandwidth nanorheometer system for studying poroviscoelasticity and an immunofluorescence-guided nanomechanical mapping technique for delineating the pericellular matrix (PCM) and territorial/interterritorial matrix (T/IT-ECM) of surrounding cells in connective tissues. Next, we summarize recent applications of these approaches to three aspects of joint-related healthcare and disease: cartilage aging and OA, developmental biology and OA pathogenesis in murine models, and nanomechanics of the meniscus. These studies were performed over a hierarchy of length scales, from the molecular, cellular to the whole tissue level. The advances described here have contributed greatly to advancing the fundamental knowledge base for improved understanding, detection, and treatment of OA.

Micromechanical anisotropy and heterogeneity of the meniscus extracellular matrix

To understand how the complex biomechanical functions of the meniscus are endowed by the nanostructure of its extracellular matrix (ECM), we studied the anisotropy and heterogeneity in the micromechanical properties of the meniscus ECM. We used atomic force microscopy (AFM) to quantify the time-dependent mechanical properties of juvenile bovine meniscus at deformation length scales corresponding to the diameters of collagen fibrils. At this scale, anisotropy in the elastic modulus of the circumferential fibers, the major ECM structural unit, can be attributed to differences in fibril deformation modes: uncrimping when normal to the fiber axis, and laterally constrained compression when parallel to the fiber axis. Heterogeneity among different structural units is mainly associated with their variations in microscale fiber orientation, while heterogeneity across anatomical zones is due to alterations in collagen fibril diameter and alignment at the nanoscale. Unlike the elastic modulus, the time-dependent properties are more homogeneous and isotropic throughout the ECM. These results enable a detailed understanding of the meniscus structure-mechanics at the nanoscale, and can serve as a benchmark for understanding meniscus biomechanical functions, documenting disease progression and designing tissue repair strategies.

STATEMENT OF SIGNIFICANCE

Meniscal damage is a common cause of joint injury, which can lead to the development of post-traumatic osteoarthritis among young adults. Restoration of meniscus function requires repairing its highly heterogeneous and complex extracellular matrix. Employing AFM, this study quantifies the anisotropic and heterogeneous features of the meniscus ECM structure and mechanics. The micromechanical properties are interpreted within the context of the collagen fibril nanostructure and its variation with tissue anatomical locations. These results provide a fundamental structure-mechanics knowledge benchmark, against which, repair and regeneration strategies can be developed and evaluated with respect to the specialized structural and functional complexity of the native tissue.

Cell-Based Meniscus Repair and Regeneration: At the Brink of Clinical Translation?: A Systematic Review of Preclinical Studies

Background:

Meniscus damage can be caused by trauma or degeneration and is therefore common among patients of all ages. Repair or regeneration of the menisci could be of great importance not only for pain relief or regaining function but also to prevent degenerative disease and osteoarthritis. Current treatment does not offer consistent long-term improvement. Although preclinical research focusing on augmentation of meniscal tear repair and regeneration after meniscectomy is encouraging, clinical translation remains difficult.

Purpose:

To systematically evaluate the literature on in vivo meniscus regeneration and explore the optimal cell sources and conditions for clinical translation. We aimed at thorough evaluation of current evidence as well as clarifying the challenges for future preclinical and clinical studies.

Study Design:

Systematic review.

Methods:

A search was conducted using the electronic databases of MEDLINE, Embase, and the Cochrane Collaboration. Search terms included meniscus, regeneration, and cell-based.

Results:

After screening 81 articles based on title and abstract, 51 articles on in vivo meniscus regeneration could be included; 2 additional articles were identified from the references. Repair and regeneration of the meniscus has been described by intra-articular injection of multipotent mesenchymal stromal (stem) cells from adipose tissue, bone marrow, synovium, or meniscus or the use of these cell types in combination with implantable or injectable scaffolds. The use of fibrochondrocytes, chondrocytes, and transfected myoblasts for meniscus repair and regeneration is limited to the combination with different scaffolds. The comparative in vitro and in vivo studies mentioned in this review indicate that the use of allogeneic cells is as successful as the use of autologous cells. In addition, the implantation or injection of cell-seeded scaffolds increased tissue regeneration and led to better structural organization compared with scaffold implantation or injection of a scaffold alone. None of the studies mentioned in this review compare the effectiveness of different (cell-seeded) scaffolds.

Conclusion:

There is heterogeneity in animal models, cell types, and scaffolds used, and limited comparative studies are available. The comparative in vivo research that is currently available is insufficient to draw strong conclusions as to which cell type is the most promising. However, there is a vast amount of in vivo research on the use of different types of multipotent mesenchymal stromal (stem) cells in different experimental settings, and good results are reported in terms of tissue formation. None of these studies compare the effectiveness of different cell-scaffold combinations, making it hard to conclude which scaffold has the greatest potential.

An overview of the role of neutrophils in innate immunity, inflammation and host-biomaterial integration

Despite considerable recent progress in defining neutrophil functions and behaviors in tissue repair, much remains to be determined with regards to its overall role in the tissue integration of biomaterials. This article provides an overview of the neutrophil’s numerous, important roles in both inflammation and resolution, and subsequently, their role in biomaterial integration. Neutrophils function in three primary capacities: generation of oxidative bursts, release of granules and formation of neutrophil extracellular traps (NETs); these combined functions enable neutrophil involvement in inflammation, macrophage recruitment, M2 macrophage differentiation, resolution of inflammation, angiogenesis, tumor formation and immune system activation. Neutrophils exhibit great flexibility to adjust to the prevalent microenvironmental conditions in the tissue; thus, the biomaterial composition and fabrication will potentially influence neutrophil behavior following confrontation. This review serves to highlight the neutrophil’s plasticity, reiterating that neutrophils are not just simple suicidal killers, but the true maestros of resolution and regeneration.

Advances in Quantification of Meniscus Tensile Mechanics Including Nonlinearity, Yield, and Failure

The meniscus provides crucial knee function and damage to it leads to osteoarthritis of the articular cartilage. Accurate measurement of its mechanical properties is therefore important, but there is uncertainty about how the test procedure affects the results, and some key mechanical properties are reported using ad hoc criteria (modulus) or not reported at all (yield). This study quantifies the meniscus' stress-strain curve in circumferential and radial uniaxial tension. A fiber recruitment model was used to represent the toe region of the stress-strain curve, and new reproducible and objective procedures were implemented for identifying the yield point and measuring the elastic modulus. Patterns of strain heterogeneity were identified using strain field measurements. To resolve uncertainty regarding whether rupture location (i.e., midsubstance rupture versus at-grip rupture) influences the measured mechanical properties, types of rupture were classified in detail and compared. Dogbone (DB)-shaped specimens are often used to promote midsubstance rupture; to determine if this is effective, we compared DB and rectangle (R) specimens in both the radial and circumferential directions. In circumferential testing, we also compared expanded tab (ET) specimens under the hypothesis that this shape would more effectively secure the meniscus' curved fibers and thus produce a stiffer response. The fiber recruitment model produced excellent fits to the data. Full fiber recruitment occurred approximately at the yield point, strongly supporting the model's physical interpretation. The strain fields, especially shear and transverse strain, were extremely heterogeneous. The shear strain field was arranged in pronounced bands of alternating positive and negative strain in a pattern similar to the fascicle structure. The site and extent of failure showed great variation, but did not affect the measured mechanical properties. In circumferential tension, ET specimens underwent earlier and more rapid fiber recruitment, had less stretch at yield, and had greater elastic modulus and peak stress. No significant differences were observed between R and DB specimens in either circumferential or radial tension. Based on these results, ET specimens are recommended for circumferential tests and R specimens for radial tests. In addition to the data obtained, the procedural and modeling advances made in this study are a significant step forward for meniscus research and are applicable to other fibrous soft tissues.

AMECM/DCB scaffold prompts successful total meniscus reconstruction in a rabbit total meniscectomy model

Tissue-engineered meniscus regeneration is a very promising treatment strategy for meniscus lesions. However, generating the scaffold presents a huge challenge for meniscus engineering as this has to meet particular biomechanical and biocompatibility requirements. In this study, we utilized acellular meniscus extracellular matrix (AMECM) and demineralized cancellous bone (DCB) to construct three different types of three-dimensional porous meniscus scaffold: AMECM, DCB, and AMECM/DCB, respectively. We tested the scaffolds' physicochemical characteristics and observed their interactions with meniscus fibrochondrocytes to evaluate their cytocompatibility. We implanted the three different types of scaffold into the medial knee menisci of New Zealand rabbits that had undergone total meniscectomy; negative control rabbits received no implants. The reconstructed menisci and corresponding femoral condyle and tibial plateau cartilage were all evaluated at 3 and 6 months (n = 8). The in vitro study demonstrated that the AMECM/DCB scaffold had the most suitable biomechanical properties, as this produced the greatest compressive and tensile strength scores. The AMECM/DCB and AMECM scaffolds facilitated fibrochondrocyte proliferation and the secretion of collagen and glycosaminoglycans (GAGs) more effectively than did the DCB scaffold. The in vivo experiments demonstrated that both the AMECM/DCB and DCB groups had generated neomeniscus at both 3 and 6 months post-implantation, but there was no obvious meniscus regeneration in the AMECM or control groups, so the neomeniscus analysis could not perform on AMECM and control group. At both 3 and 6 months, histological scores were better for regenerated menisci in the AMECM/DCB than in the DCB group, and significantly better for articular cartilage in the AMECM/DCB group compared with the other three groups. Knee MRI scores (Whole-Organ Magnetic Resonance Imaging Scores (WORMS)) were better in the AMECM/DCB group than in the other three groups at both 3 and 6 months. At both 3 and 6 months, RT-PCR demonstrated that aggrecan, Sox9, and collagen II content was significantly higher, and mechanical testing demonstrated greater tensile strength, in the AMECM/DCB group neomenisci compared with the DCB group.

Meniscus tear surgery and meniscus replacement

OBJECTIVE

the menisci are easily injured and difficult to repair. The aim of this study was to analyze the current state of meniscal surgery aimed at preserving morphology and conserving the biomechanics of the knee to prevent joint degeneration.

METHODOLOGY

a search of the electronic medical literature database Medline was conducted, from http://www.ncbi.nlm.nih.gov/pubmed. The search was not limited by language. Candidate articles were identified by searching for those that included the keywords meniscus, surgery, suture, implant, allograft. The limits were included for clinical research and clinical trials. Basic research was not included. The studies selected were evaluated and classified in three different categories: basic science, reconstruction (suture and meniscectomy) and implants (scaffolds and allograft).

RESULTS

the consequences of meniscectomy performed at a young age can lead to a joint cartilage degeneration twenty years later. There are few surgical options for the repair of meniscal injuries in order both to preserve the meniscus and to ensure the long term survival of the knee joint, meniscectomy, repair, suturing the tear, or reconstruction, when a meniscal allograft or synthetic substitute is used to replace the meniscus, but the biomechanical properties of the native meniscus are not reproduced entirely by the scaffolds that exist today.

CONCLUSION

therapies that successfully repair or replace the meniscus are therefore likely to prevent or delay osteoarthritis progression.

Tissue adhesives for meniscus tear repair: an overview of current advances and prospects for future clinical solutions

Menisci are crucial structures in the knee joint as they play important functions in load transfer, maintaining joint stability and in homeostasis of articular cartilage. Unfortunately, ones of the most frequently occurring knee injuries are meniscal tears. Particularly tears in the avascular zone of the meniscus usually do not heal spontaneously and lead to pain, swelling and locking of the knee joint. Eventually, after a (partial) meniscectomy, they will lead to osteoarthritis. Current treatment modalities to repair tears and by that restore the integrity of the native meniscus still carry their drawbacks and a new robust solution is desired. A strong tissue adhesive could provide such a solution and could potentially improve on sutures, which are the current gold standard. Moreover, a glue could serve as a carrier for biological compounds known to enhance tissue healing. Only few tissue adhesives, e.g., Dermabond(®) and fibrin glue, are already successfully used in clinical practice for other applications, but are not considered suitable for gluing meniscus tissue due to their sub-optimal mechanical properties or toxicity. There is a growing interest and research field focusing on the development of novel polymer-based tissue adhesives, but up to now, there is no material specially designed for the repair of meniscal tears. In this review, we discuss the current clinical gold standard treatment of meniscal tears and present an overview of new developments in this field. Moreover, we discuss the properties of different tissue adhesives for their potential use in meniscal tear repair. Finally, we formulate recommendations regarding the design criteria of material properties and adhesive strength for clinically applicable glues for meniscal tears.

Multiple mechanisms of 3D migration: the origins of plasticity

Cells migrate through 3D environments using a surprisingly wide variety of molecular mechanisms. These distinct modes of migration often rely on the same intracellular components, which are used in different ways to achieve cell motility. Recent work reveals that how a cell moves can be dictated by the relative amounts of cell-matrix adhesion and actomyosin contractility. A current concept is that the level of difficulty in squeezing the nucleus through a confining 3D environment determines the amounts of adhesion and contractility required for cell motility. Ultimately, determining how the nucleus controls the mode of cell migration will be essential for understanding both physiological and pathological processes dependent on cell migration in the body.

Egg shell membrane – a potential natural scaffold for human meniscal tissue engineering: an in vitro study

Enhanced human primary meniscal cell proliferation in autoclaved egg shell membrane.