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

3D printing in musculoskeletal interface engineering: Current progress and future directions

The musculoskeletal system relies on critical tissue interfaces for its function; however, these interfaces are often compromised by injuries and diseases. Restoration of these interfaces is complex by nature which renders traditional treatments inadequate. An emerging solution is three-dimensional printing, which allows for precise fabrication of biomimetic scaffolds to enhance tissue regeneration. This review summarizes the utility of 3D printing in creating scaffolds for musculoskeletal interfaces, mainly focusing on advanced techniques such as multi-material printing, bioprinting, and 4D printing. We emphasize the significance of mimicking natural tissue gradients and the selection of appropriate biomaterials to ensure scaffold success. The review outlines state-of-the-art 3D printing technologies, varying from extrusion, inkjet and laser-assisted bioprinting, which are crucial for producing scaffolds with tailored mechanical and biological properties. Applications in cartilage-bone, intervertebral disc, tendon/ligament-bone, and muscle-tendon junction engineering are discussed, highlighting the potential for improved integration and functionality. Furthermore, we address challenges in material development, printing resolution, and the in vivo performance of scaffolds, as well as the prospects for clinical translation. The review concludes by underscoring the transformative potential of 3D printing to advance orthopedic medicine, offering a roadmap for future research at the intersection of biomaterials, drug delivery, and tissue engineering.

Regenerative properties of bone marrow mesenchymal stem cell derived exosomes in rotator cuff tears

ABSTRCT

Rotator cuff injury (RCI), characterized by shoulder pain and restricted mobility, represents a subset of tendon-bone insertion injuries (TBI). In the majority of cases, surgical reconstruction of the affected tendons or ligaments is required to address the damage. However, numerous clinical failures have underscored the suboptimal outcomes associated with such procedures. Further investigations have revealed that these failures are largely attributable to delayed healing at the tendon-bone interface, excessive formation of vascularized scar tissue, and inadequate integration of tendon grafts within bone tunnels. As a result, the healing process of rotator cuff injuries faces significant challenges.Bone marrow-derived mesenchymal stem cell exosomes (BMSC-exos) have emerged as a prominent focus of research within the field of bioengineering, owing to their remarkable potential to regulate cellular proliferation and differentiation, modulate immune responses, and facilitate tissue repair and regeneration following cellular damage. In this review, we explore the anti-inflammatory, angiogenic, anti-scarring, and bone metabolism-modulating effects of BMSC-exos in the context of rotator cuff injury. Additionally, we address the limitations and ongoing challenges within current research, offering insights that could guide the clinical application of BMSC-exos in the treatment of rotator cuff injuries in the future.

Reliable Fabrication of Mineral-Graded Scaffolds by Spin-Coating and Laser Machining for Use in Tendon-to-Bone Insertion Repair

A reliable method for fabricating biomimetic scaffolds with a controllable mineral gradient to facilitate the surgical repair of tendon-to-bone injuries and the regeneration of the enthesis is reported. The gradient in mineral content is created by sequentially spin-coating with hydroxyapatite/poly(ε-caprolactone) suspensions containing hydroxyapatite nanoparticles in decreasing concentrations. To produce pores and facilitate cell infiltration, the spin-coated film is released and patterned with an array of funnel-shaped microchannels by laser machining. The unique design provided both mechanical (i.e., substrate stiffness) and biochemical (e.g., hydroxyapatite content) cues to spatially control the graded differentiation of mesenchymal stem cells. Immunocytochemical analysis of human mesenchymal stem cell-seeded scaffolds after 14 days of culture demonstrated the formation of a spatial phenotypic cell gradient from osteoblasts to mineralized chondrocytes based on the level of mineralization in the scaffold. By successfully recreating compositional and cellular features of the native tendon enthesis, the biomimetic scaffolds offer a promising avenue for improved tendon-to-bone repair.

Achieving tendon enthesis regeneration across length scales

Surgical reattachment of tendon to bone is a clinical challenge, with unacceptably high retear rates in the early period after repair. A primary reason for these repeated tears is that the multiscale toughening mechanisms found at the healthy tendon enthesis are not regenerated during tendon-to-bone healing. The need for technologies to improve these outcomes is pressing, and the tissue engineering community has responded with many advances that hold promise for eventually regenerating the multiscale tissue interface that transfers loads between the two dissimilar materials, tendon, and bone. This review provides an assessment of the state of these approaches, with the aim of identifying a critical agenda for future progress.

Advanced topology of triply periodic minimal surface structure for osteogenic improvement within orthopedic metallic screw

Metallic screws are one of the most common implants in orthopedics. However, the solid design of the screw has often resulted in stress shielding and postoperative loosening, substantially impacting its long-term fixation effect after surgery. Four additive manufacturing porous structures (Fischer-Koch S, Octet, Diamond, and Double Gyroid) are now introduced into the screw to fix those issues. Upon applying the four porous structures, elastic modulus in the screw decreased about 2∼15 times to reduce the occurrence of stress shielding, and bone regeneration effect on the screw surface increased about 1∼50 times to improve bone tissue regrowing. With more bone tissue regrowing on the inner surface of porous screw, a stiffer integration between screw and bone tissue will be achieved, which improves the long-term fixation of the screw tremendously. The biofunctions of the four topologies on osteogenesis have been fully explored, which provides an advanced topology optimization scheme for the screw utilized in orthopedic fixation.

Comparative effect of skeletal stem cells versus bone marrow mesenchymal stem cells on rotator cuff tendon-bone healing

Background

Bone marrow mesenchymal stem cells (BMSCs) have immense potential in applications for the enhancement of tendon-bone (T-B) healing. Recently, it has been well-reported that skeletal stem cells (SSCs) could induce bone and cartilage regeneration. Therefore, SSCs represent a promising choice for cell-based therapies to improve T-B healing. In this study, we aimed to compare the therapeutic potential of SSCs and BMSCs for tendon-bone healing.

Methods

SSCs and BMSCs were isolated by flow cytometry, and their proliferation ability was measured by CCK-8 assay. The osteogenic, chondrogenic, and adipogenic gene expression in cells was detected by quantitative real-time polymerase chain reaction (qRT-PCR). C57BL/6 mice underwent unilateral supraspinatus tendon detachment and repair, and the mice were then randomly allocated to 4 groups: control group (tendon-bone interface without any treatment), hydrogel group (administration of blank hydrogel into the tendon-bone interface), hydrogel + BMSCs group (administration of hydrogel with BMSCs into the tendon-bone interface), and hydrogel + SSCs group (administration of hydrogel with SSCs into the tendon-bone interface). Histological staining, Micro-computed tomography (Micro-CT) scanning, biomechanical testing, and qRT-PCR were performed to assay T-B healing at 4 and 8 weeks after surgery.

Results

SSCs showed more cell proportion, exhibited stronger multiplication capacity, and expressed higher osteogenic and chondrogenic markers and lower adipogenic markers than BMSCs. In vivo assay, the SSCs group showed a better-maturated interface which was characterized by richer chondrocytes and more proteoglycan deposition, as well as more newly formed bone at the healing site and increased mechanical properties when compared to other there groups. qRT-PCR analysis revealed that the healing interface in the SSCs group expressed more transcription factors essential for osteogenesis and chondrogenesis than the interfaces in the other groups.

Conclusions

Overall, the results demonstrated the superior therapeutic potential of SSCs over BMSCs in tendon-bone healing.

The translational potential of this article

This current study provides valuable insights that SSCs may be a more effective cell therapy for enhancing T-B healing compared to BMSCs.

Regenerative Anterior Cruciate Ligament Healing in Youth and Adolescent Athletes: The Emerging Age of Recovery Science

Anterior cruciate ligament (ACL) injuries mainly arise from non-contact mechanisms during sport performance, with most injuries occurring among youth or adolescent-age athletes, particularly females. The growing popularity of elite-level sport training has increased the total volume, intensity and frequency of exercise and competition loading to levels that may exceed natural healing capacity. Growing evidence suggests that the prevailing mechanism that leads to non-contact ACL injury from sudden mechanical fatigue failure may be accumulated microtrauma. Given the consequences of primary ACL injury on the future health and quality of life of youth and adolescent athletes, the objective of this review is to identify key "recovery science" factors that can help prevent these injuries. Recovery science is any aspect of sports training (type, volume, intensity, frequency), nutrition, and sleep/rest or other therapeutic modalities that may prevent the accumulated microtrauma that precedes non-contact ACL injury from sudden mechanical fatigue failure. This review discusses ACL injury epidemiology, current surgical efficacy, the native ACL vascular network, regional ACL histological complexities such as the entheses and crimp patterns, extracellular matrix remodeling, the concept of causal histogenesis, exercise dosage and ligament metabolism, central nervous system reorganization post-ACL rupture, homeostasis regulation, nutrition, sleep and the autonomic nervous system. Based on this information, now may be a good time to re-think primary ACL injury prevention strategies with greater use of modified sport training, improved active recovery that includes well-planned nutrition, and healthy sleep patterns. The scientific rationale behind the efficacy of regenerative orthobiologics and concomitant therapies for primary ACL injury prevention in youth and adolescent athletes are also discussed.

A Combined Treatment of BMP2 and Soluble VEGFR1 for the Enhancement of Tendon-Bone Healing by Regulating Injury-Activated Skeletal Stem Cell Lineage

BACKGROUND

Bone morphogenetic protein 2 (BMP2) is an appealing osteogenic and chondrogenic growth factor for promoting tendon-bone healing. Recently, it has been reported that soluble vascular endothelial growth factor (VEGF) receptor 1 (sVEGFR1) (a VEGF receptor antagonist) could enhance BMP2-induced bone repair and cartilage regeneration; thus, their combined application may represent a promising treatment to improve tendon-bone healing. Moreover, BMP2 could stimulate skeletal stem cell (SSC) expansion and formation, which is responsible for wounded tendon-bone interface repair. However, whether the codelivery of BMP2 and sVEGFR1 increases tendon enthesis injury-activated SSCs better than does BMP2 alone needs further research.

PURPOSE

To study the effect of BMP2 combined with sVEGFR1 on tendon-bone healing and injury-activated SSC lineage.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 128 C57BL/6 mice that underwent unilateral supraspinatus tendon detachment and repair were randomly assigned to 4 groups: (1) untreated control group; (2) hydrogel group, which received a local injection of the blank hydrogel at the injured site; (3) BMP2 group, which received an injection of hydrogel with BMP2; and (4) BMP2 with sVEGFR1 group, which received an injection of hydrogel with BMP2 and sVEGFR1. Histology, micro-computed tomography, and biomechanical tests were conducted to evaluate tendon-bone healing at 4 and 8 weeks after surgery. In addition, flow cytometry was performed to detect the proportion of SSCs and their downstream differentiated subtypes, including bone, cartilage, and stromal progenitors; osteoprogenitors; and pro-chondrogenic progenitors within supraspinatus tendon enthesis at 1 week postoperatively.

RESULTS

The repaired interface in BMP2 with sVEGFR1 group showed a significantly improved collagen fiber continuity, increased fibrocartilage, greater newly formed bone, and elevated mechanical properties compared with the other 3 groups. There were more SSCs; bone, cartilage, and stromal progenitors; osteoprogenitors; and pro-chondrogenic progenitors in the BMP2 with sVEGFR1 group than that in the other groups.

CONCLUSION

Our study suggests that the combined delivery of BMP2 and sVEGFR1 could promote tendon-bone healing and stimulate the expansion of SSCs and their downstream progeny within the injured tendon-bone interface.

CLINICAL RELEVANCE

Combining BMP2 with sVEGFR1 may be a good clinical treatment for wounded tendon enthesis healing.

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.

Nanofibrous scaffolds for the healing of the fibrocartilaginous enthesis: advances and prospects

With the current developmental advancements in nanotechnology, nanofibrous scaffolds are being widely used. The healing of fibrocartilaginous enthesis is a slow and complex process, and while existing treatments have a certain effect on promoting their healing, these are associated with some limitations. The nanofibrous scaffold has the advantages of easy preparation, wide source of raw materials, easy adjustment, easy modification, can mimic the natural structure and morphology of the fibrocartilaginous enthesis, and has good biocompatibility, which can compensate for existing treatments and be combined with them to promote the repair of fibrocartilaginous enthesis. The nanofibrous scaffold can promote the healing of fibrocartilaginous enthesis by controlling the morphology and ensuring controlled drug release. Hence, the use of nanofibrous scaffold with stimulative response features in the musculoskeletal system has led us to imagine its potential application in fibrocartilaginous enthesis. Therefore, the healing of fibrocartilaginous enthesis based on a nanofibrous scaffold may be a novel therapeutic approach.

A Review of the Role of Tendon Stem Cells in Tendon-Bone Regeneration

Tendon-bone injuries are a prevalent health concern associated with sports and other physically demanding activities. These injuries have a limited innate healing ability, often leading to the formation of scar tissue rather than the regeneration of healthy tendon tissue. This scar tissue results from excessive fibrosis during the early healing process and often leads to reduced tendon function and an increased risk of reinjury. Traditionally, surgical reconstruction has been the primary treatment for tendon-bone injuries. However, restoring the natural structure and mechanical properties of tendons after surgical reconstruction presents a considerable challenge. Recently, the potential of stem cell therapy has been explored as an alternative treatment approach. In particular, a new type of pluripotent stem cell known as tendon stem cells (TDSCs) has been identified within tendon tissue. These cells exhibit the potential for self-renewal and multidirectional differentiation, meaning they can differentiate into fibroblasts and chondrocytes. These differentiated cells can aid in the repair and regeneration of new tissues by producing collagen and other matrix molecules that provide structural support. TDSCs have become a focal point in research for treating tendon-bone injuries and related conditions. The potential use of these cells provides a basis for both basic research and clinical applications, particularly in understanding the tendon-bone healing process and identifying factors that affect the ability of TDSCs to promote this healing. This review article aims to analyze the role of TDSCs in tendon-bone healing, understanding their therapeutic potential and contributing to the development of effective treatment strategies for tendon-bone injuries.

Osteoarthritis models: From animals to tissue engineering

Osteoarthritis (OA) is a chronic degenerative osteoarthropathy. Although it has been revealed that a variety of factors can cause or aggravate the symptoms of OA, the pathogenic mechanisms of OA remain unknown. Reliable OA models that accurately reflect human OA disease are crucial for studies on the pathogenic mechanism of OA and therapeutic drug evaluation. This review first demonstrated the importance of OA models by briefly introducing the OA pathological features and the current limitations in the pathogenesis and treatment of OA. Then, it mainly discusses the development of different OA models, including animal and engineered models, highlighting their advantages and disadvantages from the perspective of pathogenesis and pathology analysis. In particular, the state-of-the-art engineered models and their potential were emphasized, as they may represent the future direction in the development of OA models. Finally, the challenges in obtaining reliable OA models are also discussed, and possible future directions are outlined to shed some light on this area.