Age-dependent changes in matrix composition and organization at the ligament-to-bone insertion

The Structure, Biology, and Mechanical Function of Tendon/Ligament-Bone Interfaces

After tendon or ligament reconstruction, the interface between the hard bone and soft connective tissue is considerably weakened and is difficult to restore through healing. The tendon/ligament-bone interface is mechanically the weakest point under tensile loading and is often the source of various postoperative complications, such as bone resorption and graft laxity. A comprehensive understanding of the macro- and microfeatures of the native tendon/ligament-bone interface would be beneficial for developing strategies for regenerating the tissue. This article discusses the structural, biological, and mechanical features of the tendon/ligament-bone interfaces and how these can be affected by aging and loading conditions.

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.

Biomimetic gradient scaffolds for the tissue engineering and regeneration of rotator cuff enthesis

Rotator cuff tear is one of the most common musculoskeletal disorders, which often results in recurrent shoulder pain and limited movement. Enthesis is a structurally complex and functionally critical interface connecting tendon and bone that plays an essential role in maintaining integrity of the shoulder joint. Despite the availability of advanced surgical procedures for rotator cuff repair, there is a high rate of failure following surgery due to suboptimal enthesis healing and regeneration. Novel strategies based on tissue engineering are gaining popularity in improving tendon-bone interface (TBI) regeneration. Through incorporating physical and biochemical cues into scaffold design which mimics the structure and composition of native enthesis is advantageous to guide specific differentiation of seeding cells and facilitate the formation of functional tissues. In this review, we summarize the current state of research in enthesis tissue engineering highlighting the development and application of biomimetic scaffolds that replicate the gradient TBI. We also discuss the latest techniques for fabricating potential translatable scaffolds such as 3D bioprinting and microfluidic device. While preclinical studies have demonstrated encouraging results of biomimetic gradient scaffolds, the translation of these findings into clinical applications necessitates a comprehensive understanding of their safety and long-term efficacy.

Enthesis repair - State of play

The fibrocartilaginous enthesis is a highly specialised tissue interface that ensures a smooth mechanical transfer between tendon or ligament and bone through a fibrocartilage area. This tissue is prone to injury and often does not heal, even after surgical intervention. Enthesis augmentation approaches are challenging due to the complexity of the tissue that is characterised by the coexistence of a range of cellular and extracellular components, architectural features and mechanical properties within only hundreds of micrometres. Herein, we discuss enthesis repair and regeneration strategies, with particular focus on elegant interfacial and functionalised scaffold-based designs.

Enthesis maturation in engineered ligaments is differentially driven by loads that mimic slow growth elongation and rapid cyclic muscle movement

Entheses are complex attachments that translate load between elastic-ligaments and stiff-bone via organizational and compositional gradients. Neither natural healing, repair, nor engineered replacements restore these gradients, contributing to high re-tear rates. Previously, we developed a culture system which guides ligament fibroblasts in high-density collagen gels to develop early postnatal-like entheses, however further maturation is needed. Mechanical cues, including slow growth elongation and cyclic muscle activity, are critical to enthesis development in vivo but these cues have not been widely explored in engineered entheses and their individual contribution to maturation is largely unknown. Our objective here was to investigate how slow stretch, mimicking ACL growth rates, and intermittent cyclic loading, mimicking muscle activity, individually drive enthesis maturation in our system so to shed light on the cues governing enthesis development, while further developing our tissue engineered replacements. Interestingly, we found these loads differentially drive organizational maturation, with slow stretch driving improvements in the interface/enthesis region, and cyclic load improving the ligament region. However, despite differentially affecting organization, both loads produced improvements to interface mechanics and zonal composition. This study provides insight into how mechanical cues differentially affect enthesis development, while producing some of the most organized engineered enthesis to date. STATEMENT OF SIGNIFICANCE: Entheses attach ligaments to bone and are critical to load transfer; however, entheses do not regenerate with repair or replacement, contributing to high re-tear rates. Mechanical cues are critical to enthesis development in vivo but their individual contribution to maturation is largely unknown and they have not been widely explored in engineered replacements. Here, using a novel culture system, we provide new insight into how slow stretch, mimicking ACL growth rates, and intermittent cyclic loading, mimicking muscle activity, differentially affect enthesis maturation in engineered ligament-to-bone tissues, ultimately producing some of the most organized entheses to date. This system is a promising platform to explore cues regulating enthesis formation so to produce functional engineered replacements and better drive regeneration following repair.

Osteochondral Interface: Regenerative Engineering and Challenges

Osteochondral (OC) defects are debilitating for patients and represent a significant clinical problem for orthopedic surgeons as well as regenerative engineers due to their potential complications, which are likely to lead to osteoarthritis and related diseases. If they remain untreated or are treated suboptimally, OC lesions are known to impact the articular cartilage and the transition from cartilage to bone, that is, the cartilage-bone interface. An important component of the OC interface, that is, a selectively permeable membrane, the tidemark, still remains unaddressed in more than 90% of the published research in the past decade. This review focuses on the structure, composition, and function of the OC interface, regenerative engineering attempts with different scaffolding strategies and challenges ahead of us in recapitulating the native OC interface. There are different schools of thought regarding the structure of the native OC interface: stratified and graded. The former assumes the cartilage-to-bone interface to be hierarchically divided into distinct yet continuous zones of uncalcified cartilage-calcified cartilage-subchondral bone. The latter assumes the interface is continuously graded, that is, formed by an infinite number of layers. The cellular composition of the interface, either in respective layers or continuously changing in a graded manner, is chondrocytes, hypertrophic chondrocytes, and osteoblasts as moved from cartilage to bone. Functionally, the interface is assumed to play a role in enabling a smooth transition of loads exerted on the cartilage surface to the bone underneath. Regenerative engineering involves, first, a characterization of the native OC interface in terms of the composition, structure, and function, and, then, proposes the appropriate biomaterials, cells, and biomolecules either alone or in combination to eventually form a structure that mimics and functionally behaves similar to the native interface. The major challenge regarding regeneration of the OC interface appears to lie, in addition to others, in the formation of tidemark, which is a thin membrane separating the OC interface into two distinct zones: the avascular OC interface and the vascular OC interface. There is a significant amount of literature on regenerative approaches to the OC interface; however, only a small portion of them consider the importance of tidemark. Therefore, this review aims at highlighting the significance of the structural organization of the components of the OC interface and increasing the awareness of the orthopedics community regarding the importance of tidemark formation after clinical interventions or regenerative engineering attempts.

Mesenchymal stem cells and macrophages and their interactions in tendon-bone healing

Tendon-bone insertion injuries (TBI), such as anterior cruciate ligament (ACL) and rotator cuff injuries, are common degenerative or traumatic pathologies with a negative impact on the patient's daily life, and they cause huge economic losses every year. The healing process after an injury is complex and is dependent on the surrounding environment. Macrophages accumulate during the entire process of tendon and bone healing and their phenotypes progressively transform as they regenerate. As the "sensor and switch of the immune system", mesenchymal stem cells (MSCs) respond to the inflammatory environment and exert immunomodulatory effects during the tendon-bone healing process. When exposed to appropriate stimuli, they can differentiate into different tissues, including chondrocytes, osteocytes, and epithelial cells, promoting reconstruction of the complex transitional structure of the enthesis. It is well known that MSCs and macrophages communicate with each other during tissue repair. In this review, we discuss the roles of macrophages and MSCs in TBI injury and healing. Reciprocal interactions between MSCs and macrophages and some biological processes utilizing their mutual relations in tendon-bone healing are also described. Additionally, we discuss the limitations in our understanding of tendon-bone healing and propose feasible ways to exploit MSC-macrophage interplay to develop an effective therapeutic strategy for TBI injuries.

The Translational potential of this article

This paper reviewed the important functions of macrophages and mesenchymal stem cells in tendon-bone healing and described the reciprocal interactions between them during the healing process. By managing macrophage phenotypes, mesenchymal stem cells and the interactions between them, some possible novel therapies for tendon-bone injury may be proposed to promote tendon-bone healing after restoration surgery.

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.

Regulating Macrophages through Immunomodulatory Biomaterials Is a Promising Strategy for Promoting Tendon-Bone Healing

The tendon-to-bone interface is a special structure connecting the tendon and bone and is crucial for mechanical load transfer between dissimilar tissues. After an injury, fibrous scar tissues replace the native tendon-to-bone interface, creating a weak spot that needs to endure extra loading, significantly decreasing the mechanical properties of the motor system. Macrophages play a critical role in tendon-bone healing and can be divided into various phenotypes, according to their inducing stimuli and function. During the early stages of tendon-bone healing, M1 macrophages are predominant, while during the later stages, M2 macrophages replace the M1 macrophages. The two macrophage phenotypes play a significant, yet distinct, role in tendon-bone healing. Growing evidence shows that regulating the macrophage phenotypes is able to promote tendon-bone healing. This review aims to summarize the impact of different macrophages on tendon-bone healing and the current immunomodulatory biomaterials for regulating macrophages, which are used to promote tendon-bone healing. Although macrophages are a promising target for tendon-bone healing, the challenges and limitations of macrophages in tendon-bone healing research are discussed, along with directions for further research.

Design-Build-Validate Strategy to 3D Print Bioglass Gradients for Anterior Cruciate Ligament Enthesis Reconstruction

A rupture of the anterior cruciate ligament (ACL) is one of the most common knee ligament injuries affecting the young and active population. Tissue engineering strategies to reconstruct the damaged ACL have met with significant challenges mainly associated with poor graft integration at the bone-ligament interface (i.e., enthesis). In this study, a "design-build-validate" strategy was employed by combining 3D Raman spectral mapping and 3D printing to develop a tissue engineered scaffold that is compositionally similar to the ACL bone-ligament interface and can provide the essential biochemical cues to promote interface regeneration and facilitate functional graft to bone integration. Results showed that Raman spectroscopy is a highly efficient nondestructive technique to determine the biochemical composition of native ACL enthesis. 3D printing using combinatory inks consisting of different compositions of methacrylated collagen (CMA) and Bioglass (BG) allowed for the fabrication of BG gradient-incorporated collagen matrices (BioGIMs) with a transition region confirmed by Alizarin red S staining. Furthermore, Raman spectroscopy validated replication of ACL enthesis composition in BioGIMs. In addition, human mesenchymal stem cells (hMSCs) cultured on BioGIMs showed morphological differences along the length of the BioGIMs as evidenced by confocal microscopy of cell cytoskeleton-stained images indicating that the cells can sense the underlying differences in matrix composition. Overall, the "design-build-validate" strategy developed in this study has significant potential to generate biomimetic tissue constructs for use at the interface regions of synthetic grafts to promote better host integration and achieve full reconstruction of the ACL. Impact statement Poor graft integration at the bone-ligament interface (i.e., enthesis) is a significant clinical problem in anterior cruciate ligament (ACL) repair and reconstruction. In this study, Raman spectroscopy and 3D printing technologies were used in combination for the first time in a design-build-validate strategy to develop a continuous biomimetic Bioglass gradient-incorporated collagen matrix (BioGIM) that compositionally emulates the native ACL enthesis. These BioGIMs can be fused onto the ends of synthetic ACL grafts and have significant potential to provide the essential biochemical cues to guide tissue-specific cell differentiation, augment functional matrix reorganization, promote better graft integration, and achieve full reconstruction of damaged ACL.

Driving native-like zonal enthesis formation in engineered ligaments using mechanical boundary conditions and β-tricalcium phosphate

Fibrocartilaginous entheses are structurally complex tissues that translate load from elastic ligaments to stiff bone via complex zonal gradients in the organization, mineralization, and cell phenotype. Currently, these complex gradients necessary for long-term mechanical function are not recreated in soft tissue-to-bone healing or engineered replacements, contributing to high failure rates. Previously, we developed a culture system that guides ligament fibroblasts to develop aligned native-sized collagen fibers using high-density collagen gels and mechanical boundary conditions. These constructs are promising ligament replacements, however functional ligament-to-bone attachments, or entheses, are required for long-term function in vivo. The objective of this study was to investigate the effect of compressive mechanical boundary conditions and the addition of beta-tricalcium phosphate (βTCP), a known osteoconductive agent, on the development of zonal ligament-to-bone entheses. We found that compressive boundary clamps, that restrict cellular contraction and produce a zonal tensile-compressive environment, guide ligament fibroblasts to produce 3 unique zones of collagen organization and zonal accumulation of glycosaminoglycans (GAGs), type II, and type X collagen. Ultimately, by 6 weeks of culture these constructs had similar organization and composition as immature bovine entheses. Further, βTCP applied under the clamp enhanced maturation of these entheses, leading to significantly increased tensile moduli, and zonal GAG accumulation, ALP activity, and calcium-phosphate accumulation, suggesting the initiation of endochondral ossification. This culture system produced some of the most organized entheses to date, closely mirroring early postnatal enthesis development, and provides an in vitro platform to better understand the cues that drive enthesis maturation in vivo. STATEMENT OF SIGNIFICANCE: Ligaments are attached to bone via entheses. Entheses are complex tissues with gradients in organization, composition, and cell phenotype. Entheses are necessary for proper transfer of load from ligament-to-bone, but currently are not restored with healing or replacements. Here, we provide new insight into how tensile-compressive boundary conditions and βTCP drive zonal gradients in collagen organization, mineralization, and matrix composition, producing tissues similar to immature ligament-to-bone attachments. Collectively, this culture system uses a bottom-up approach with mechanical and biochemical cues to produce engineered replacements which closely mirror postnatal enthesis development. This culture system is a promising platform to better understanding the cues that regulate enthesis formation so to better drive enthesis regeneration following graft repair and in engineered replacements.

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.

Stem cell therapies in tendon-bone healing

Tendon-bone insertion injuries such as rotator cuff and anterior cruciate ligament injuries are currently highly common and severe. The key method of treating this kind of injury is the reconstruction operation. The success of this reconstructive process depends on the ability of the graft to incorporate into the bone. Recently, there has been substantial discussion about how to enhance the integration of tendon and bone through biological methods. Stem cells like bone marrow mesenchymal stem cells (MSCs), tendon stem/progenitor cells, synovium-derived MSCs, adipose-derived stem cells, or periosteum-derived periosteal stem cells can self-regenerate and potentially differentiate into different cell types, which have been widely used in tissue repair and regeneration. Thus, we concentrate in this review on the current circumstances of tendon-bone healing using stem cell therapy.

Resistance Training Modulates the Matrix Metalloproteinase-2 Activity in Different Trabecular Bones in Aged Rats

Background

Aging decreases osteogenic ability, inducing harmful effects on the bone extracellular matrix (ECM), while exercise training has been indicated as a tool to counteract bone disorders related to advancing age. The modulation of bone ECM is regulated by several types of matrix metalloproteinase (MMP); however, MMP-2 activity in different trabecular bones in response to resistance training (RT) has been neglected. Remodeling differs in different bones under the application of the same mechanical loading. Thus, we investigated the effects of 12 weeks of RT on MMP-2 activity in the lumbar vertebra (L6), tibia, and femur of young (3 months) and older rats (21 months).

Methods

Twenty Wistar rats were divided into four groups (five animals per group): young sedentary or trained and older sedentary or trained. The 12-week RT consisted of climbing a 1.1-m vertical ladder three times per week with progressive weights secured to the animals’ tails. The animals were killed 48 h after the end of the experimental period. The MMP-2 activity was assessed by the zymography method.

Results

The aging process induced lower MMP-2 activity in the lumbar vertebrae and tibia (p=0.01). RT upregulated pro, intermediate, and active MMP-2 activity in the tibia of young rats (p=0.001). RT also upregulated pro and active MMP-2 activity in the lumbar vertebrae and tibia with advancing age (p=0.01). There was no significant difference (p>0.05) between groups for MMP-2 of the femur, regardless of age and RT.

Conclusion

The aging process impairs MMP-2 activity, but RT is a potential therapeutic approach to minimize the deleterious effects of ECM degeneration in different aged bones. Distinct MMP-2 responses to exercise training may result in specific remodeling processes.

Spatial Presentation of Tissue-Specific Extracellular Matrix Components along Electrospun Scaffolds for Tissue Engineering the Bone-Ligament Interface

The bone-ligament interface transitions from a highly organized type I collagen rich matrix to a nonmineralized fibrocartilage region and finally to a mineralized fibrocartilage region that interfaces with the bone. Therefore, engineering the bone-ligament interface requires a biomaterial substrate capable of maintaining or directing the spatially defined differentiation of multiple cell phenotypes. To date the appropriate combination of biophysical and biochemical factors that can be used to engineer such a biomaterial substrate remain unknown. Here we show that microfiber scaffolds functionalized with tissue-specific extracellular matrix (ECM) components can direct the differentiation of MSCs toward the phenotypes seen at the bone-ligament interface. Ligament-ECM (L-ECM) promoted the expression of the ligament-marker gene tenomodulin (TNMD) and higher levels of type I and III collagen expression compared to functionalization with commercially available type I collagen. Functionalization of microfiber scaffolds with cartilage-ECM (C-ECM) promoted chondrogenesis of MSCs, as evidenced by adoption of a round cell morphology and increased SRY-box 9 (SOX9) expression in the absence of exogenous growth factors. Next, we fabricated a multiphasic scaffold by controlling the spatial presentation of L-ECM and C-ECM along the length of a single electrospun microfiber construct, with the distal region of the C-ECM coated fibers additionally functionalized with an apatite layer (using simulated body fluid) to promote endochondral ossification. These ECM functionalized scaffolds promoted spatially defined differentiation of MSCs, with higher expression of TNMD observed in the region functionalized with L-ECM, and higher expression of type X collagen and osteopontin (markers of endochondral ossification) observed at the end of the scaffold functionalized with C-ECM and the apatite coating. Our results demonstrate the utility of tissue-specific ECM derived components as a cue for directing MSC differentiation when engineering complex multiphasic interfaces such as the bone-ligament enthesis.

Regeneration of Damaged Tendon-Bone Junctions (Entheses)-TAK1 as a Potential Node Factor

Musculoskeletal dysfunctions are highly prevalent due to increasing life expectancy. Consequently, novel solutions to optimize treatment of patients are required. The current major research focus is to develop innovative concepts for single tissues. However, interest is also emerging to generate applications for tissue transitions where highly divergent properties need to work together, as in bone-cartilage or bone-tendon transitions. Finding medical solutions for dysfunctions of such tissue transitions presents an added challenge, both in research and in clinics. This review aims to provide an overview of the anatomical structure of healthy adult entheses and their development during embryogenesis. Subsequently, important scientific progress in restoration of damaged entheses is presented. With respect to enthesis dysfunction, the review further focuses on inflammation. Although molecular, cellular and tissue mechanisms during inflammation are well understood, tissue regeneration in context of inflammation still presents an unmet clinical need and goes along with unresolved biological questions. Furthermore, this review gives particular attention to the potential role of a signaling mediator protein, transforming growth factor beta-activated kinase-1 (TAK1), which is at the node of regenerative and inflammatory signaling and is one example for a less regarded aspect and potential important link between tissue regeneration and inflammation.

Gradient Biomineralized Silk Fibroin Nanofibrous Scaffold with Osteochondral Inductivity for Integration of Tendon to Bone

Enthesis injury repair remains a huge challenge because of the unique biomolecular composition, microstructure, and mechanics in the interfacial region. Surgical reconstruction often creates new bone-scaffold interfaces with mismatched properties, resulting in poor osseointegration. To mimic the natural interface tissue structures and properties, we fabricated a nanofibrous scaffold with gradient mineral coating based on 10 × simulated body fluid (SBF) and silk fibroin (SF). We then characterized the physicochemical properties of the scaffold and evaluated its biological functions both in vitro and in vivo. The results showed that different areas of SF nanofibrous scaffold had varying levels of mineralization with disparate mechanical properties and had different effects on bone marrow mesenchymal stem cell growth and differentiation. Furthermore, the gradient scaffolds exhibited an enhancement of integration in the tendon-to-bone interface with a higher ultimate load and more fibrocartilage-like tissue formation. These findings demonstrate that the silk-based nanofibrous scaffold with gradient mineral coating can regulate the formation of interfacial tissue and has the potential to be applied in interface tissue engineering.

Cruciate ligament healing and injury prevention in the age of regenerative medicine and technostress: homeostasis revisited

PURPOSE

This clinical concepts paper discusses the essential elements of cruciate ligament recuperation, micro-trauma repair, and remodeling.

METHODS

Cruciate ligament mechanobiology and tissue heterogeneity, anatomy and vascularity, and synovial membrane and fluid functions are discussed in relationship to deficiency-induced inflammatory responses, nervous and immune system function, recuperation, repair and remodeling, and modern threats to homeostasis.

RESULTS

Cruciate ligament surgical procedures do not appreciate the vital linked functions of the central, peripheral, and autonomic nervous systems and immune system function on knee ligament injury recuperation, micro-trauma repair, and remodeling. Enhanced knowledge of these systems could provide innovative ways to decrease primary non-contact knee injury rates and improve outcomes following reconstruction or primary repair.

CONCLUSIONS

Restoration of knee joint homeostasis is essential to cruciate ligament recuperation, micro-trauma repair, and remodeling. The nervous and immune systems are intricately involved in this process. Varying combinations of high-intensity training, under-recovery, technostress, and environmental pollutants (including noise) regularly expose many athletically active individuals to factors that abrogate the environment needed for cruciate ligament recuperation, micro-trauma repair, and remodeling. Current sports training practice, lifestyle psychobehaviors, and environmental factors combine to increase both primary non-contact knee injury risk and the nervous and immune system dysregulation that lead to poor sleep, increased anxiety, and poorly regulated hormone and cytokine levels. These factors may create a worst-case scenario leading to poor ligament recuperation, micro-trauma repair, and remodeling. Early recognition and modification of these factors may decrease knee ligament injury rates and improve cruciate ligament repair or reconstruction outcomes.

LEVEL OF EVIDENCE

V.

Age-related alterations of articular cartilage in pituitary adenylate cyclase-activating polypeptide (PACAP) gene-deficient mice

Pituitary adenylate cyclase activating polypeptide (PACAP) is an evolutionarly conserved neuropeptide which is produced by various neuronal and non-neuronal cells, including cartilage and bone cells. PACAP has trophic functions in tissue development, and it also plays a role in cellular and tissue aging. PACAP takes part in the regulation of chondrogenesis, which prevents insufficient cartilage formation caused by oxidative and mechanical stress. PACAP knockout (KO) mice have been shown to display early aging signs affecting several organs. In the present work, we investigated articular cartilage of knee joints in young and aged wild-type (WT) and PACAP KO mice. A significant increase in the thickness of articular cartilage was detected in aged PACAP gene-deficient mice. Amongst PACAP receptors, dominantly PAC1 receptor was expressed in WT knee joints and a remarkable decrease was found in aged PACAP KO mice. Expression of PKA-regulated transcription factors, Sox5, Sox9 and CREB, decreased both in young and aged gene deficient mice, while Sox6, collagen type II and aggrecan expressions were elevated in young but were reduced in aged PACAP KO animals. Increased expression of hyaluronan (HA) synthases and HA-binding proteins was detected parallel with an elevated presence of HA in aged PACAP KO mice. Expression of bone related collagens (I and X) was augmented in young and aged animals. These results suggest that loss of PACAP signaling results in dysregulation of cartilage matrix composition and may transform articular cartilage in a way that it becomes more prone to degenerate.

Nanofiber-based transforming growth factor-β3 release induces fibrochondrogenic differentiation of stem cells

Fibrocartilage is typically found in regions subject to complex, multi-axial loads and plays a critical role in musculoskeletal function. Mesenchymal stem cell (MSC)-mediated fibrocartilage regeneration may be guided by administration of appropriate chemical and/or physical cues, such as by culturing cells on polymer nanofibers in the presence of the chondrogenic growth factor TGF-β3. However, targeted delivery and maintenance of effective local factor concentrations remain challenges for implementation of growth factor-based regeneration strategies in clinical settings. Thus, the objective of this study was to develop and optimize the bioactivity of a biomimetic nanofiber scaffold system that enables localized delivery of TGF-β3. To this end, we fabricated TGF-β3-releasing nanofiber meshes that provide sustained growth factor delivery and demonstrated their potential for guiding synovium-derived stem cell (SDSC)-mediated fibrocartilage regeneration. TGF-β3 delivery enhanced cell proliferation and synthesis of relevant fibrocartilaginous matrix in a dose-dependent manner. By designing a scaffold that eliminates the need for exogenous or systemic growth factor administration and demonstrating that fibrochondrogenesis requires a lower growth factor dose compared to previously reported, this study represents a critical step towards developing a clinical solution for regeneration of fibrocartilaginous tissues. STATEMENT OF SIGNIFICANCE: Fibrocartilage is a tissue that plays a critical role throughout the musculoskeletal system. However, due to its limited self-healing capacity, there is a significant unmet clinical need for more effective approaches for fibrocartilage regeneration. We have developed a nanofiber-based scaffold that provides both the biomimetic physical cues, as well as localized delivery of the chemical factors needed to guide stem cell-mediated fibrocartilage formation. Specifically, methods for fabricating TGF-β3-releasing nanofibers were optimized, and scaffold-mediated TGF-β3 delivery enhanced cell proliferation and synthesis of fibrocartilaginous matrix, demonstrating for the first time, the potential for nanofiber-based TGF-β3 delivery to guide stem cell-mediated fibrocartilage regeneration. This nanoscale delivery platform represents an exciting new strategy for fibrocartilage regeneration.

Fibril deformation under load of the rabbit Achilles tendon and medial collateral ligament femoral entheses

Microscopic visualization under load of the region connecting ligaments/tendons to bone, the enthesis, has been performed previously; however, specific investigation of individual fibril deformation may add insight to such studies. Detailed visualization of fibril deformation would inform on the mechanical strategies employed by this tissue in connecting two mechanically disparate materials. Clinically, an improved understanding of enthesis mechanics may help guide future restorative efforts for torn or injured ligaments/tendons, where the enthesis is often a point of weakness. In this study, a custom ligament/tendon enthesis loading device was designed and built, a unique method of sample preparation was devised, and second harmonic and two-photon fluorescence microscopy were used to capture the fibril-level load response of the rabbit Achilles tendon and medial collateral ligament femoral entheses. A focus was given to investigation of the mechanical problem of fibril embedment. Resultant images indicate a rapid (occurring over approximately 60 μm) change in fibril orientation at the interface of ligament/tendon and calcified fibrocartilage early in the loading regime, before becoming relatively constant. Such a change in fibril angle helps confirm the materially graded region demonstrated by others, while, in this case, providing additional insight into fibril bending. We speculate that the scale of the mechanical problem (i.e., fibril diameters being on the order of 250 nm) allows fibrils to bend over the small (relative to the imaging field of view, but large relative to fibril diameter) distances observed; thus, potentially lessening required embedment lengths. Nevertheless, this behavior merits further investigation to be confirmed. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2506-2515, 2018.

Management of Anterior Cruciate Ligament Injuries in Adults Aged >40 Years

Management of anterior cruciate ligament (ACL) injuries in adults aged >40 years has received increased attention in the literature because of an increase in the functional demands of aging athletes. Multiple structural and biomechanical age-dependent changes exist in the ACL, for example, fewer mesenchymal stem cells, decreased healing potential, decreased structural organization, decreased stiffness, and a decreased load to failure with age. As in younger patients, ACL insufficiency can predispose an older patient to the same risks of recurrent instability, meniscal and chondral injury, and osteoarthritis. The role of nonsurgical versus surgical management in these patients remains controversial. Lower-demand patients may be able to cope with ACL deficiency. Higher-demand patients may have functional instability, and the limited studies available suggest good functional outcomes with surgical reconstruction of the ACL in this population.

Improving knowledge on the activation of bone marrow fibroblasts in MGUS and MM disease through the automatic extraction of genes via a nonnegative matrix factorization approach on gene expression profiles

Background

Multiple myeloma (MM) is a cancer of terminally differentiated plasma that is part of a spectrum of blood diseases. The role of the micro-environment is crucial for MM clonal evolution.

Methods

This paper describes the analysis carried out on a limited number of genes automatically extracted by a nonnegative matrix factorization (NMF) based approach from gene expression profiles of bone marrow fibroblasts of patients with monoclonal gammopathy of undetermined significance (MGUS) and MM.

Results

Automatic exploration through NMF, combined with a motivated post-processing procedure and a pathways analysis of extracted genes, allowed to infer that a functional switch is required to lead fibroblasts to acquire pro-tumorigenic activity in the progression of the disease from MGUS to MM.

Conclusion

The extracted biologically relevant genes may be representative of the considered clinical conditions and may contribute to a deeper understanding of tumor behavior.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1589-1) contains supplementary material, which is available to authorized users.

Integrating soft and hard tissues via interface tissue engineering

The enthesis, or interface between bone and soft tissues such as ligament and tendon, is prone to injury and often does not heal, even post surgical intervention. Interface tissue engineering represents an integrative strategy for regenerating the native enthesis by functionally connecting soft and hard tissues and thereby improving clinical outcome. This review focuses on integrative and cell-instructive scaffold designs that target the healing of the two most commonly injured soft tissue-bone junctions: tendon-bone interface (e.g., rotator cuff) and ligament-bone interface (e.g., anterior cruciate ligament). The inherent connectivity between soft and hard tissues is instrumental for musculoskeletal motion and is therefore a key design criterion for soft tissue regeneration. To this end, scaffold design for soft tissue regeneration have progressed from single tissue systems to the emerging focus on pre-integrated and functional composite tissue units. Specifically, a multifaceted, bioinspired approach has been pursued wherein scaffolds are tailored to stimulate relevant cell responses using spatially patterned structural and chemical cues, growth factors, and/or mechanical stimulation. Moreover, current efforts to elucidate the essential scaffold design criteria via strategic biomimicry are emphasized as these will reduce complexity in composite tissue regeneration and ease the related burden for clinical translation. These innovative studies underscore the clinical relevance of engineering connective tissue integration and have broader impact in the formation of complex tissues and total joint regeneration. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1069-1077, 2018.

Development of Tissue-Engineered Ligaments: Elastin Promotes Regeneration of the Rabbit Medial Collateral Ligament

When ligaments are injured, reconstructive surgery is sometimes required to restore function. Methods of reconstructive surgery include transplantation of an artificial ligament and autotransplantation of a tendon. However, these methods have limitations related to the strength of the bone-ligament insertion and biocompatibility of the transplanted tissue after surgery. Therefore, it is necessary to develop new reconstruction methods and pursue the development of artificial ligaments. Elastin is a major component of elastic fibers and ligaments. However, the role of elastin in ligament regeneration has not been described. Here, we developed a rabbit model of a medial collateral ligament (MCL) rupture and treated animal knees with exogenous elastin [100 µg/(0.5 mL·week)] for 6 or 12 weeks. Elastin treatment increased gene expression and protein content of collagen and elastin (gene expression, 6-fold and 42-fold, respectively; protein content, 1.6-fold and 1.9-fold, respectively), and also increased the elastic modulus of MCL increased with elastin treatment (2-fold) compared with the controls. Our data suggest that elastin is involved in the regeneration of damaged ligaments.

Compositional mapping of the mature anterior cruciate ligament-to-bone insertion

The anterior cruciate ligament (ACL)-to-bone interface constitutes a complex, multi-tissue structure comprised of contiguous ligament, non-mineralized fibrocartilage, mineralized fibrocartilage, and bone regions. This composite structure enables load transfer between structurally and functionally dissimilar tissues and is critical for ligament homeostasis and joint stability. Presently, there is a lack of quantitative understanding of the matrix composition and organization across this junction, especially after the onset of skeletal maturity. The objective of this study is to characterize the adult bovine ACL-to-bone interface using Fourier transform infrared spectroscopic imaging (FTIRI), testing the hypothesis that regional changes in collagen, proteoglycan, and mineral distribution, as well as matrix organization, persist at the mature insertion. It was observed that while collagen content increases continuously across the adult interface, collagen alignment decreases between ligament and bone. Proteoglycans were primarily localized to the fibrocartilage region and an exponential increase in mineral content was observed between the non-mineralized and mineralized regions. These observations reveal significant changes in collagen distribution and alignment with maturity, and these trends underscore the role of physiologic loading in postnatal matrix remodeling. Findings from this study provide new insights into interface organization and serve as benchmark design criteria for interface regeneration and integrative soft tissue repair. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2513-2523, 2017.

Next Generation Tissue Engineering of Orthopedic Soft Tissue-to-Bone Interfaces

Soft tissue-to-bone interfaces are complex structures that consist of gradients of extracellular matrix materials, cell phenotypes, and biochemical signals. These interfaces, called entheses for ligaments, tendons, and the meniscus, are crucial to joint function, transferring mechanical loads and stabilizing orthopedic joints. When injuries occur to connected soft tissue, the enthesis must be re-established to restore function, but due to structural complexity, repair has proven challenging. Tissue engineering offers a promising solution for regenerating these tissues. This prospective review discusses methodologies for tissue engineering the enthesis, outlined in three key design inputs: materials processing methods, cellular contributions, and biochemical factors.

Variation in the shape of the tibial insertion site of the anterior cruciate ligament: classification is required

PURPOSE

To propose a classification system for the shape of the tibial insertion site (TIS) of the anterior cruciate ligament (ACL) and to demonstrate the intra- and inter-rater agreement of this system. Due to variation in shape and size, different surgical approaches may be feasible to improve reconstruction of the TIS.

METHODS

One hundred patients with a mean age of 26 ± 11 years were included. The ACL was cut arthroscopically at the base of the tibial insertion site. Arthroscopic images were taken from the lateral and medial portal. Images were de-identified and duplicated. Two blinded observers classified the tibial insertion site according to a classification system.

RESULTS

The tibial insertion site was classified as type I (elliptical) in 51 knees (51 %), type II (triangular) in 33 knees (33 %) and type III (C-shaped) in 16 knees (16 %). There was good agreement between raters when viewing the insertion site from the lateral portal (κ = 0.65) as well as from the medial portal (κ = 0.66). Intra-rater reliability was good to excellent. Agreement in the description of the insertion site between the medial and lateral portals was good for rater 1 and good for rater 2 (κ = 0.74 and 0.77, respectively).

CONCLUSION

There is variation in the shape of the ACL TIS. The classification system is a repeatable and reliable tool to summarize the shape of the TIS using three common patterns. For clinical relevance, different shapes may require different types of reconstruction to ensure proper footprint restoration. Consideration of the individual TIS shape is required to prevent iatrogenic damage of adjacent structures like the menisci.

LEVEL OF EVIDENCE

III.

Micro- and Ultrastructural Characterization of Age-Related Changes at the Anterior Cruciate Ligament-to-Bone Insertion

There remains a lack of understanding of the structural changes that occur across the complex, multitissue anterior cruciate ligament (ACL)-to-bone insertion as a function of aging. The objective of this study is to provide a multiscale comparison of matrix properties across the skeletally immature and mature ACL-to-bone insertion. Using complementary imaging methods, micro- and ultrastructural analysis of the insertion revealed that collagen fiber orientation at the interface changes with age, though the degree of collagen organization is maintained over time. These changes are accompanied by a decrease in collagen fibril density and are likely driven by physiological loading. Mineral crystal structure and crystallinity are conserved over time, despite regional differences in crystallinity between the interface and bone. This suggests that mineral chemistry is established early in development and underscores its important functional role. Collectively, these findings provide new insights into interface development and set critical design benchmarks for integrative soft tissue repair.

Orthopedic Interface Repair Strategies Based on Native Structural and Mechanical Features of the Multiscale Enthesis

The enthesis is an organ that connects a soft, aligned tissue (tendon/ligament) to a hard, amorphous tissue (bone) via a fibrocartilage interface. Mechanically, the enthesis sustains a dynamic loading environment that includes tensile, compressive, and shear forces. The structural components of the enthesis act to minimize stress concentrations and control stretch at the interface. Current surgical repair of the enthesis, such as in rotator cuff repair and anterior cruciate ligament reconstruction, aim to bridge the gap between the injured ends via reattachment of soft-to-hard tissues or graft replacement. In this review, we discuss the multiscale, morphological, and mechanical characteristics of the fibrocartilage attachment. Additionally, we review historical and recent clinical approaches to treating enthesis injury. Lastly, we explore new technological advancements in tissue-engineered biomaterials that have shown promise in preclinical studies.

Cellular interactions regulate stem cell differentiation in tri-culture

Currently, the mechanism governing the regeneration of the soft tissue-to-bone interface, such as the transition between the anterior cruciate ligament (ACL) and bone, is not known. Focusing on the ACL-to-bone insertion, this study tests the novel hypothesis that interactions between cells from the ligament (fibroblasts) and bone (osteoblasts) initiate interface regeneration. Specifically, these heterotypic cell interactions direct the fibrochondrogenic differentiation of interface-relevant cell populations, defined here as ligament fibroblasts and bone marrow stromal cells (BMSC). The objective of this study is to examine the effects of heterotypic cellular interactions on BMSC or fibroblast growth and biosynthesis, as well as expression of fibrocartilage-relevant markers in tri-culture. The effects of cell-cell physical contact and paracrine interactions between fibroblasts and osteoblasts were also determined. It was found that, in tri-culture with fibroblasts and osteoblasts, BMSC exhibited greater fibrochondrogenic potential than ligament fibroblasts. The growth of BMSC decreased while proteoglycan production and TGF-β3 expression increased. Moreover, tri-culture regulated BMSC response via paracrine factors, and interestingly, fibroblast-osteoblast contact further promoted proteoglycan and TGF-β1 synthesis as well as induced SOX9 expression in BMSC. Collectively, the findings of this study suggest that fibroblast-osteoblast interactions play an important role in regulating the stem cell niche for fibrocartilage regeneration, and the mechanisms of these interactions are directed by paracrine factors and augmented with direct cell-cell contact.

Polymer fiber-based models of connective tissue repair and healing

Physiologically relevant models of wound healing are essential for understanding the biology of connective tissue repair and healing. They can also be used to identify key cellular processes and matrix characteristics critical for the design of soft tissue grafts. Modeling the various stages of repair post tendon injury, polymer meshes of varying fiber diameter (nano-1 (390 nm) < nano-2 (740 nm) < micro (1420 nm)) were produced. Alignment was also introduced in the nano-2 group to model matrix undergoing biological healing rather than scar formation. The response of human tendon fibroblasts on these model substrates were evaluated over time as a function of fiber diameter and alignment. It was observed that the repair models of unaligned nanoscale fibers enhanced cell growth and collagen synthesis, while these outcomes were significantly reduced in the mature repair model consisting of unaligned micron-sized fibers. Organization of paxillin and actin on unaligned meshes was enhanced on micro- compared to nano-sized fibers, while the expression and activity of RhoA and Rac1 were greater on nanofibers. In contrast, aligned nanofibers promoted early cell organization, while reducing excessive cell growth and collagen production in the long term. These results show that the early-stage repair model of unaligned nanoscale fibers elicits a response characteristic of the proliferative phase of wound repair, while the more mature model consisting of unaligned micron-sized fibers is more representative of the remodeling phase by supporting cell organization while suppressing growth and biosynthesis. Interestingly, introduction of fiber alignment in the nanofiber model alters fibroblast response from repair to healing, implicating matrix alignment as a critical design factor for circumventing scar formation and promoting biological healing of soft tissue injuries.

Anterior Cruciate Ligament: Structure, Injuries and Regenerative Treatments

Anterior cruciate ligament (ACL) is one of the most vulnerable ligaments of the knee. ACL impairment results in episodic instability, chondral and meniscal injury and early osteoarthritis. The poor self-healing capacity of ACL makes surgical treatment inevitable. Current ACL reconstructions include a substitution of torn ACL via biological grafts such as autograft, allograft. This review provides an insight of ACL structure, orientation and properties followed by comparing the performance of various constructs that have been used for ACL replacement. New approaches, undertaken to induce ACL regeneration and fabricate biomimetic scaffolds, are also discussed.

Biomimetic strategies for engineering composite tissues

The formation of multiple tissue types and their integration into composite tissue units presents a frontier challenge in regenerative engineering. Tissue-tissue synchrony is crucial in providing structural support for internal organs and enabling daily activities. This review highlights the state-of-the-art in composite tissue scaffold design, and explores how biomimicry can be strategically applied to avoid over-engineering the scaffold. Given the complexity of biological tissues, determining the most relevant parameters for recapitulating native structure-function relationships through strategic biomimicry will reduce the burden for clinical translation. It is anticipated that these exciting efforts in composite tissue engineering will enable integrative and functional repair of common soft tissue injuries and lay the foundation for total joint or limb regeneration.

Construction of finite element model and stress analysis of anterior cruciate ligament tibial insertion

Objective:

The aim of the present study was to develop a more realistic finite element (FE) model of the human anterior cruciate ligament (ACL) tibial insertion and to analyze the stress distribution in the ACL internal fibers under load.

Methods:

The ACL tibial insertions were processed histologically. With Photoshop software, digital images taken from the histological slides were collaged, contour lines were drawn, and different gray values were filled based on the structure. The data were exported to Amira software and saved as “.hmascii” file. This document was imported into HyperMesh software. The solid mesh model generated using HyperMesh software was imported into Abaqus software. The material properties were introduced, boundary conditions were set, and load was added to carry out the FE analysis.

Results:

The stress distribution of the ACL internal fibers was uneven. The lowest stress could be observed in the ACL lateral fibers under tensile and shear load.

Conclusion:

The establishment of ACL tibial insertion FE model and mechanical analysis could reveal the stress distribution in the ACL internal fibers under load. There was greater load carrying capacity in the ACL lateral fibers which could sustain greater tensile and shear forces.

A Comparative Study on the Biomechanical and Histological Properties of Bone-to-Bone, Bone-to-Tendon, and Tendon-to-Tendon Healing: An Achilles Tendon-Calcaneus Model in Goats

BACKGROUND

Surgical repair around the bone-tendon insertion (BTI) may involve bone-to-bone (BB), bone-to-tendon (BT), or tendon-to-tendon (TT) reattachment with varying healing outcome.

HYPOTHESIS

The repair of Achilles tendon-calcaneus (ATC) by reattachment of homogeneous tissue (BB or TT) would heal faster, with respect to tensile properties at the healing complex, than those of reattachment of heterogeneous tissues (BT) over time.

STUDY DESIGN

Controlled laboratory study.

METHODS

Forty-seven adolescent male Chinese goats were divided into BB, BT, and TT groups. Osteotomy of the calcaneus, reattachment of Achilles tendon to the calcaneus after removal of the insertion, and tenotomy of the Achilles tendon were performed to simulate BB, BT, and TT repair, respectively. The ATC healing complexes were harvested at 6, 12, or 24 weeks postoperatively. Mechanical and morphological properties of the healing ATC complexes were assessed by tensile testing and qualitative histology, respectively. The contralateral intact ATC complex was used as the control.

RESULTS

Failure load of BT was 33.4% lower than that of TT (P = .0243) at week 12. Ultimate strength of BT was 50.2% and 45.3% lower than that of TT at weeks 12 (P = .0002) and 24 (P = .0001), respectively. Tissue morphological characteristics of the BB and TT groups showed faster remodeling. The BT group showed limited regeneration of fibrocartilage zone and excessive formation of fibrous tissue at the healing interface.

CONCLUSION

BTI repair between homogeneous tissues (BB and TT healing) showed better healing quality with respect to mechanical and histological assessments than did healing between heterogeneous tissues (BT healing).

CLINICAL RELEVANCE

Anatomic reconstruction of ATC complex injury may be a primary concern when selecting the proper surgical approach. However, it is recommended to select fracture fixation (BB) or tendon repair (TT) instead of bone-tendon reattachment (BT) if possible to ensure better outcome at the healing interface.

Mechanical tensile properties of the anterolateral ligament

Background

In a noticeable percentage of patients anterolateral rotational instabilities (ALRI) remain after an isolated ACL reconstruction. Those instabilities may occur due to an insufficiently directed damage of anterolateral structures that is often associated with ACL ruptures. Recent publications describe an anatomical structure, termed the anterolateral ligament (ALL), and suggest that this ligament plays a significant role in the pathogenesis of ALRI of the knee joint. However, only limited knowledge about the biomechanical characteristics and tensile properties of the anterolateral ligament exists.

Methods

The anterolateral ligament was dissected in four fresh-frozen human cadaveric specimens and all surrounding tissue removed. The initial length of the anterolateral ligament was measured using a digital caliper. Tensile tests with load to failure were performed using a materials testing machine. The explanted anterolateral ligaments were histologically examined to measure the cross-sectional area.

Results

The mean ultimate load to failure of the anterolateral ligament was 49.90 N (± 14.62 N) and the mean ultimate strain was 35.96% (± 4.47%). The mean length of the ligament was 33.08 mm (± 2.24) and the mean cross-sectional area was 1.54 mm² (± 0.48 mm²). Including the areal measurements the maximum tension was calculated to be 32.78 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $\frac {N}{{mm}^{2}}$ \end{document}Nmm2 (± 4.04 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $\frac {N}{{mm}^{2}}$ \end{document}Nmm2).

Conclusions

The anterolateral ligament is an anatomical structure with tensile properties that are considerably weaker compared to other peripheral structures of the knee. Knowledge of the anterolateral ligament’s tensile strengths may help to better understand its function and with graft choices for reconstruction procedures.

Enthesis fibrocartilage cells originate from a population of Hedgehog-responsive cells modulated by the loading environment

Tendon attaches to bone across a specialized tissue called the enthesis. This tissue modulates the transfer of muscle forces between two materials, i.e. tendon and bone, with vastly different mechanical properties. The enthesis for many tendons consists of a mineralized graded fibrocartilage that develops postnatally, concurrent with epiphyseal mineralization. Although it is well described that the mineralization and development of functional maturity requires muscle loading, the biological factors that modulate enthesis development are poorly understood. By genetically demarcating cells expressing Gli1 in response to Hedgehog (Hh) signaling, we discovered a unique population of Hh-responsive cells in the developing murine enthesis that were distinct from tendon fibroblasts and epiphyseal chondrocytes. Lineage-tracing experiments revealed that the Gli1 lineage cells that originate in utero eventually populate the entire mature enthesis. Muscle paralysis increased the number of Hh-responsive cells in the enthesis, demonstrating that responsiveness to Hh is modulated in part by muscle loading. Ablation of the Hh-responsive cells during the first week of postnatal development resulted in a loss of mineralized fibrocartilage, with very little tissue remodeling 5 weeks after cell ablation. Conditional deletion of smoothened, a molecule necessary for responsiveness to Ihh, from the developing tendon and enthesis altered the differentiation of enthesis progenitor cells, resulting in significantly reduced fibrocartilage mineralization and decreased biomechanical function. Taken together, these results demonstrate that Hh signaling within developing enthesis fibrocartilage cells is required for enthesis formation.

Hard-Soft Tissue Interface Engineering

The musculoskeletal system is comprised of three distinct tissue categories: structural mineralized tissues, actuating muscular soft tissues, and connective tissues. Where connective tissues - ligament, tendon and cartilage - meet with bones, a graded interface in mechanical properties occurs that allows the transmission of load without creating stress concentrations that would cause tissue damage. This interface typically occurs over less than 1 mm and contains a three order of magnitude difference in elastic stiffness, in addition to changes in cell type and growth factor concentrations among others. Like all engineered tissues, the replication of these interfaces requires the production of scaffolds that will provide chemical and mechanical cues, resulting in biologically accurate cellular differentiation. For interface tissues however, the scaffold must provide spatially graded chemical and mechanical cues over sub millimetre length scales. Naturally, this complicates the manufacture of the scaffolds and every stage of their subsequent cell seeding and growth, as each region has different optimal conditions. Given the higher degree of difficulty associated with replicating interface tissues compared to surrounding homogeneous tissues, it is likely that the development of complex musculoskeletal tissue systems will continue to be limited by the engineering of connective tissues interfaces with bone.

Engineering complex orthopaedic tissues via strategic biomimicry

The primary current challenge in regenerative engineering resides in the simultaneous formation of more than one type of tissue, as well as their functional assembly into complex tissues or organ systems. Tissue-tissue synchrony is especially important in the musculoskeletal system, wherein overall organ function is enabled by the seamless integration of bone with soft tissues such as ligament, tendon, or cartilage, as well as the integration of muscle with tendon. Therefore, in lieu of a traditional single-tissue system (e.g., bone, ligament), composite tissue scaffold designs for the regeneration of functional connective tissue units (e.g., bone-ligament-bone) are being actively investigated. Closely related is the effort to re-establish tissue-tissue interfaces, which is essential for joining these tissue building blocks and facilitating host integration. Much of the research at the forefront of the field has centered on bioinspired stratified or gradient scaffold designs which aim to recapitulate the structural and compositional inhomogeneity inherent across distinct tissue regions. As such, given the complexity of these musculoskeletal tissue units, the key question is how to identify the most relevant parameters for recapitulating the native structure-function relationships in the scaffold design. Therefore, the focus of this review, in addition to presenting the state-of-the-art in complex scaffold design, is to explore how strategic biomimicry can be applied in engineering tissue connectivity. The objective of strategic biomimicry is to avoid over-engineering by establishing what needs to be learned from nature and defining the essential matrix characteristics that must be reproduced in scaffold design. Application of this engineering strategy for the regeneration of the most common musculoskeletal tissue units (e.g., bone-ligament-bone, muscle-tendon-bone, cartilage-bone) will be discussed in this review. It is anticipated that these exciting efforts will enable integrative and functional repair of soft tissue injuries, and moreover, lay the foundation for the development of composite tissue systems and ultimately, total limb or joint regeneration.

FTIR-I compositional mapping of the cartilage-to-bone interface as a function of tissue region and age

Soft tissue-to-bone transitions, such as the osteochondral interface, are complex junctions that connect multiple tissue types and are critical for musculoskeletal function. The osteochondral interface enables pressurization of articular cartilage, facilitates load transfer between cartilage and bone, and serves as a barrier between these two distinct tissues. Presently, there is a lack of quantitative understanding of the matrix and mineral distribution across this multitissue transition. Moreover, age-related changes at the interface with the onset of skeletal maturity are also not well understood. Therefore, the objective of this study is to characterize the cartilage-to-bone transition as a function of age, using Fourier transform infrared spectroscopic imaging (FTIR-I) analysis to map region-dependent changes in collagen, proteoglycan, and mineral distribution, as well as collagen organization. Both tissue-dependent and age-related changes were observed, underscoring the role of postnatal physiological loading in matrix remodeling. It was observed that the relative collagen content increased continuously from cartilage to bone, whereas proteoglycan peaked within the deep zone of cartilage. With age, collagen content across the interface increased, accompanied by a higher degree of collagen alignment in both the surface and deep zone cartilage. Interestingly, regardless of age, mineral content increased exponentially across the calcified cartilage interface. These observations reveal new insights into both region- and age-dependent changes across the cartilage-to-bone junction and will serve as critical benchmark parameters for current efforts in integrative cartilage repair.

A multi-scale structural study of the porcine anterior cruciate ligament tibial enthesis

Like the human anterior cruciate ligament (ACL), the porcine ACL also has a double bundle structure and several biomechanical studies using this model have been carried out to show the differential effect of these two bundles on macro-level knee joint function. It is hypothesised that if the different bundles of the porcine ACL are mechanically distinct in function, then a multi-scale anatomical characterisation of their individual enthesis will also reveal significant differences in structure between the bundles. Twenty-two porcine knee joints were cleared of their musculature to expose the intact ACL following which ligament-bone samples were obtained. The samples were fixed in formalin followed by decalcification with formic acid. Thin sections containing the ligament insertion into the tibia were then obtained by cryosectioning and analysed using differential interference contrast (DIC) optical microscopy and scanning electron microscopy (SEM). At the micro-level, the anteromedial (AM) bundle insertion at the tibia displayed a significant deep-rooted interdigitation into bone, while for the posterolateral (PL) bundle the fibre insertions were less distributed and more focal. Three sub-types of enthesis were identified in the ACL and related to (i) bundle type, (ii) positional aspect within the insertion, and (iii) specific bundle function. At the nano-level the fibrils of the AM bundle were significantly larger than those in the PL bundle. The modes by which the AM and PL fibrils merged with the bone matrix fibrils were significantly different. A biomechanical interpretation of the data suggests that the porcine ACL enthesis is a specialized, functionally graded structural continuum, adapted at the micro-to-nano scales to serve joint function at the macro level.

Engineering anisotropic biomimetic fibrocartilage microenvironment by bioprinting mesenchymal stem cells in nanoliter gel droplets

Over the past decade, bioprinting has emerged as a promising patterning strategy to organize cells and extracellular components both in two and three dimensions (2D and 3D) to engineer functional tissue mimicking constructs. So far, tissue printing has neither been used for 3D patterning of mesenchymal stem cells (MSCs) in multiphase growth factor embedded 3D hydrogels nor been investigated phenotypically in terms of simultaneous differentiation into different cell types within the same micropatterned 3D tissue constructs. Accordingly, we demonstrated a biochemical gradient by bioprinting nanoliter droplets encapsulating human MSCs, bone morphogenetic protein 2 (BMP-2), and transforming growth factor β1 (TGF- β1), engineering an anisotropic biomimetic fibrocartilage microenvironment. Assessment of the model tissue construct displayed multiphasic anisotropy of the incorporated biochemical factors after patterning. Quantitative real time polymerase chain reaction (qRT-PCR) results suggested genomic expression patterns leading to simultaneous differentiation of MSC populations into osteogenic and chondrogenic phenotype within the multiphasic construct, evidenced by upregulation of osteogenesis and condrogenesis related genes during in vitro culture. Comprehensive phenotypic network and pathway analysis results, which were based on genomic expression data, indicated activation of differentiation related mechanisms, via signaling pathways, including TGF, BMP, and vascular endothelial growth factor.

The mature athlete: aging tendon and ligament

CONTEXT

Aging changes the biology, healing capacity, and biomechanical function of tendons and ligaments and results in common clinical pathologies that present to orthopedic surgeons, primary care physicians, physical therapists, and athletic trainers. A better understanding of the age-related changes in these connective tissues will allow better patient care.

EVIDENCE ACQUISITION

The PubMed database was searched in December 2012 for English-language articles pertaining to age-related changes in tendons and ligaments.

LEVEL OF EVIDENCE

Level 5.

RESULTS

The mature athlete faces challenges associated with age-dependent changes in the rotator cuff, Achilles tendon, lateral humeral epicondylar tendons, quadriceps tendon, and patellar tendon. The anterior cruciate ligament and the medial collateral ligament are the most studied intra-articular and extra-articular ligaments, and both are associated with age-dependent changes.

CONCLUSION

Tendons and ligaments are highly arranged connective tissue structures that maintain joint motion and joint stability. These structures are subject to vascular and compositional changes with increasing age that alter their mechanotransduction, biology, healing capacity, and biomechanical function. Emerging research into the etiology of age-dependent changes will provide further information to help combat the age-related clinical complications associated with the injuries that occur to tendons and ligaments.

Decellularized dermis-polymer complex provides a platform for soft-to-hard tissue interfaces

To develop a soft-to-hard tissue interface, we made a decellularized dermis/poly(methyl methacrylate) (PMMA) complex by soaking the decellularized dermis in methyl methacrylate (MMA) and an initiator, and then polymerizing the MMA. The decellularized tissue was chosen because of its good biocompatibility and the easiness of suturing it, and MMA because of its hard tissue compatibility and wide use in the biomedical field. The MMA filled the cavities in the dermis and polymerized within 10 min. No leaking or polymer aggregation was observed, implying that a homogenous tissue-polymer complex had formed. The cell infiltration and the integration between the tissue and the dermis occurred in vivo, whereas the cells could not infiltrate the tissue-polymer complex. This implies that the interface tissue should possess both complex and noncomplex parts, where the cells infiltrate the noncomplex part and stop when they encounter the complex part, integrating the soft and hard tissue or hard polymer.