In vivo performance of an acellular disc-like angle ply structure (DAPS) for total disc replacement in a small animal model

Finite element modeling to predict the influence of anatomic variation and implant placement on performance of biological intervertebral disc implants

Background

Tissue-engineered intervertebral disc (TE-IVD) constructs are an attractive therapy for treating degenerative disc disease and have previously been investigated in vivo in both large and small animal models. The mechanical environment of the spine is notably challenging, in part due to its complex anatomy, and implants may require additional mechanical support to avoid failure in the early stages of implantation. As such, the design of suitable support implants requires rigorous validation.

Methods

We created a FE model to simulate the behavior of the IVD cages under compression specific to the anatomy of the porcine cervical spine, validated the FE model using an animal model, and predicted the effects of implant location and vertebral angle of the motion segment on implant behavior. Specifically, we tested anatomical positioning of the superior vertebra and placement of the implant. We analyzed corresponding stress and strain distributions.

Results

Results demonstrated that the anatomical geometry of the porcine cervical spine led to concentrated stress and strain on the posterior side of the cage. This stress concentration was associated with the location of failure of the cages reported in vivo, despite superior mechanical properties of the implant. Furthermore, placement of the cage was found to have profound effects on migration, while the angle of the superior vertebra affected stress concentration of the cage.

Conclusions

This model can be utilized both to inform surgical procedures and provide insight on future cage designs and can be adopted to models without the use of in vivo animal models.

Elastic Fibers in the Intervertebral Disc: From Form to Function and toward Regeneration

Despite extensive efforts over the past 40 years, there is still a significant gap in knowledge of the characteristics of elastic fibers in the intervertebral disc (IVD). More studies are required to clarify the potential contribution of elastic fibers to the IVD (healthy and diseased) function and recommend critical areas for future investigations. On the other hand, current IVD in-vitro models are not true reflections of the complex biological IVD tissue and the role of elastic fibers has often been ignored in developing relevant tissue-engineered scaffolds and realistic computational models. This has affected the progress of IVD studies (tissue engineering solutions, biomechanics, fundamental biology) and translation into clinical practice. Motivated by the current gap, the current review paper presents a comprehensive study (from the early 1980s to 2022) that explores the current understanding of structural (multi-scale hierarchy), biological (development and aging, elastin content, and cell-fiber interaction), and biomechanical properties of the IVD elastic fibers, and provides new insights into future investigations in this domain.

Intervertebral Disc Degeneration: Biomaterials and Tissue Engineering Strategies toward Precision Medicine

Intervertebral disc degeneration is a common cause of discogenic low back pain resulting in significant disability. Current conservative or surgical intervention treatments do not reverse the underlying disc degeneration or regenerate the disc. Biomaterial-based tissue engineering strategies exhibit the potential to regenerate the disc due to their capacity to modulate local tissue responses, maintain the disc phenotype, attain biochemical homeostasis, promote anatomical tissue repair, and provide functional mechanical support. Despite preliminary positive results in preclinical models, these approaches have limited success in clinical trials as they fail to address discogenic pain. This review gives insights into the understanding of intervertebral disc pathology, the emerging concept of precision medicine, and the rationale of personalized biomaterial-based tissue engineering tailored to the severity of the disease targeting early, mild, or severe degeneration, thereby enhancing the efficacy of the treatment for disc regeneration and ultimately to alleviate discogenic pain. Further research is required to assess the relationship between disc degeneration and lower back pain for developing future clinically relevant therapeutic interventions targeted towards the subgroup of degenerative disc disease patients.

Can Manganese Dioxide Microspheres be Used as Intermediaries to Alleviate Intervertebral Disc Degeneration With Strengthening Drugs?

Degenerative disc disease (DDD) is a pathological condition associated with intervertebral discs (IVDs) that causes chronic back pain. IVD degeneration has become a significant issue in contemporary society. To date, numerous biological therapies have been applied to alleviate the progression of DDD, among which therapeutic protein injection is the most direct and convenient. However, there are some limitations to applying direct protein injection therapy, the most significant being that the efficacy of this method has a short duration, which is a major factor in its effectiveness and the resulting patient satisfaction. How do we solve this problem? Or how can the effectiveness of the treatment be enhanced? It has been proved that manganese dioxide (MnO₂) microspheres, widely used in environmental science, not only regulate the expression of cell genes and cytokines in the microenvironment, but also have the ability to release drugs slowly. We propose that direct injection of protein encapsulated in hollow MnO₂ (h-MnO₂) microspheres could solve the problem of rapid drug release. In addition, the use of a MnO₂ and protein injection in the treatment of DDD may have a synergistic effect, which would be highly significant for the degradation of pro-inflammatory factors in the DDD microenvironment. Therefore, the combination of MnO₂ and protein may provide a new therapeutic approach to alleviate the progression of DDD.

Recent Progress in Electrospun Nanofiber-Based Degenerated Intervertebral Disc Repair

Annulus fibrosus fissure and fibrosis of nucleus pulposus are severe morphological characteristics of intervertebral disc degeneration. Currently, surgery or drugs are used to relieve pain in such cases. Tissue engineering is a new multidisciplinary strategy with great potential for use in joint replacement and organ regeneration. Based on the natural anatomy of intervertebral discs, intervertebral disc scaffolds are fabricated by exploiting the special arrangement of extracellular matrix fibers. Electrospun nanofibers possess clear advantages in repairing degenerated intervertebral discs. This article reviews and summarizes recently developed methods for improving and fabricating electrospun nanofiber annulus fibrosus scaffolds in terms of nanofiber alignment, material selection, loading additives, and the progress made in combining other advanced technologies with electrospun nanofibers. In addition, the improvement in mechanical properties and biocompatibility of nucleus pulposus scaffolds by electrospun nanofiber-reinforced hydrogels is discussed. Finally, complete intervertebral disc scaffolds can be fabricated using the disc-like angle-ply structure and other emerging fabrication methods. Taken together, electrospun nanofiber intervertebral disc scaffolds are promising for clinical applications.

Three-Dimensional-Printed Flexible Scaffolds Have Tunable Biomimetic Mechanical Properties for Intervertebral Disc Tissue Engineering

The intervertebral disc (IVD) exhibits complex structure and biomechanical function, which supports the weight of the body and permits motion. Surgical treatments for IVD degeneration (e.g., lumbar fusion, disc replacement) often disrupt the mechanical environment of the spine which lead to adjacent segment disease. Alternatively, disc tissue engineering strategies, where cell-seeded hydrogels or fibrous biomaterials are cultured in vitro to promote matrix deposition, do not recapitulate the complex IVD mechanical properties. In this study, we use 3D printing of flexible polylactic acid (FPLA) to fabricate a viscoelastic scaffold with tunable biomimetic mechanics for whole spine motion segment applications. We optimized the mechanical properties of the scaffolds for equilibrium and dynamic moduli in compression and tension by varying fiber spacing or porosity, generating scaffolds with de novo mechanical properties within the physiological range of spine motion segments. The biodegradation analysis of the 3D printed scaffolds showed that FPLA exhibits lower degradation rate and thus has longer mechanical stability than standard PLA. FPLA scaffolds were biocompatible, supporting viability of nucleus pulposus (NP) cells in 2D and in FPLA+hydrogel composites. Composite scaffolds cultured with NP cells maintained baseline physiological mechanical properties and promoted matrix deposition up to 8 weeks in culture. Mesenchymal stromal cells (MSCs) cultured on FPLA adhered to the scaffold and exhibited fibrocartilaginous differentiation. These results demonstrate for the first time that 3D printed FPLA scaffolds have de novo viscoelastic mechanical properties that match the native IVD motion segment in both tension and compression and have the potential to be used as a mechanically stable and biocompatible biomaterial for engineered disc replacement.

Construction of biomimetic artificial intervertebral disc scaffold via 3D printing and electrospinning

Intervertebral disc (IVD) degeneration is a clinically disease that seriously endangers people's health. Tissue engineering provides a promising method to repair and regenerate the damaged IVD physiological function. Successfully tissue-engineered IVD scaffold should mimic the native IVD histological and macro structures. Here, 3D printing and electrospinning were combined to construct an artificial IVD composite scaffold. Poly lactide (PLA) was used to print the IVD frame structure, the oriented porous poly(l-lactide)/octa-armed polyhedral oligomeric silsesquioxanes (PLLA/POSS-(PLLA)₈) fiber bundles simulated the annulus fibrosus (AF), and the gellan gum/poly (ethylene glycol) diacrylate (GG/PEGDA) double network hydrogel loaded with bone marrow mesenchymal stem cells (BMSCs) simulated the nucleus pulposus (NP) structure. Morphological and mechanical tests showed that the structure and mechanical properties of the IVD scaffold were similar to that of the natural IVD. The compression modulus of the scaffold is about 10 MPa, which is comparable to natural IVD and provides good mechanical support for tissue repair and regeneration. At the same time, the porosity and mechanical properties of the scaffold can be regulated through the 3D model design. In the AF structure, the fiber bundles are oriented concentrically with each subsequent layer oriented 60° to the spinal column, and can withstand the tension generated during the deformation of the NP. In the NP structure, BMSCs were evenly distributed in the hydrogel and could maintain high cell viability. Animal experiment results demonstrated that the biomimetic artificial IVD scaffold could maintain the disc space and produce the new extracellular matrix. This engineered biomimetic IVD scaffold is a promising biomaterial for individualized IVD repair and regeneration.

Tissue-Engineering of Human Intervertebral Disc: A Concise Review

Intervertebral disc (IVD) represents a structure of crucial structural and functional importance for human spine. Pathology of IVD institutes a frequently encountered condition in current clinical practice. Degenerative Disc Disease (DDD), the principal clinical representative of IVD pathology, constitutes an increasingly diagnosed spinal disorder associated with substantial morbidity and mortality in recent years. Despite the considerable incidence and socioeconomic burden of DDD, existing treatment modalities including conservative and surgical methods have been demonstrated to provide a limited therapeutic effect, being not capable of interrupting or reversing natural progress of underlying disease. These limitations underline the requirement for development of novel, innovative and more effective therapeutic strategies for DDD management. Within this literature framework, compromised IVD replacement with a viable IVD construct manufactured with Tissue-Engineering (TE) methods has been recommended as a promising therapeutic strategy for DDD. Existing preliminary preclinical data demonstrate that proper combination of cells from various sources, different scaffold materials and appropriate signaling molecules renders manufacturing of whole-IVD tissue-engineered constructs a technically feasible process. Aim of this narrative review is to critically summarize current published evidence regarding particular aspects of IVD-TE, primarily emphasizing in providing researchers in this field with practicable knowledge in order to enhance clinical translatability of their research and informing clinical practitioners about the features and capabilities of innovative TE science in the field of IVD-TE.

Proper animal experimental designs for preclinical research of biomaterials for intervertebral disc regeneration

Low back pain is a vital musculoskeletal disease that impairs life quality, leads to disability and imposes heavy economic burden on the society, while it is greatly attributed to intervertebral disc degeneration (IDD). However, the existing treatments, such as medicines, chiropractic adjustments and surgery, cannot achieve ideal disc regeneration. Therefore, advanced bioactive therapies are implemented, including stem cells delivery, bioreagents administration, and implantation of biomaterials etc. Among these researches, few reported unsatisfying regenerative outcomes. However, these advanced therapies have barely achieved successful clinical translation. The main reason for the inconsistency between satisfying preclinical results and poor clinical translation may largely rely on the animal models that cannot actually simulate the human disc degeneration. The inappropriate animal model also leads to difficulties in comparing the efficacies among biomaterials in different reaches. Therefore, animal models that better simulate the clinical charateristics of human IDD should be acknowledged. In addition, in vivo regenerative outcomes should be carefully evaluated to obtain robust results. Nevertheless, many researches neglect certain critical characteristics, such as adhesive properties for biomaterials blocking annulus fibrosus defects and hyperalgesia that is closely related to the clinical manifestations, e.g., low back pain. Herein, in this review, we summarized the animal models established for IDD, and highlighted the proper models and parameters that may result in acknowledged IDD models. Then, we discussed the existing biomaterials for disc regeneration and the characteristics that should be considered for regenerating different parts of discs. Finally, well-established assays and parameters for in vivo disc regeneration are explored.

Architecture-Promoted Biomechanical Performance-Tuning of Tissue-Engineered Constructs for Biological Intervertebral Disc Replacement

Background: Biological approaches to intervertebral disc (IVD) restoration and/or regeneration have become of increasing interest. However, the IVD comprises a viscoelastic system whose biological replacement remains challenging. The present study sought to design load-sharing two-component model systems of circular, nested, concentric elements reflecting the nucleus pulposus and annulus fibrosus. Specifically, we wanted to investigate the effect of architectural design variations on (1) model system failure loads when testing the individual materials either separately or homogeneously mixed, and (2) also evaluate the potential of modulating other mechanical properties of the model systems. Methods: Two sets of softer and harder biomaterials, 0.5% and 5% agarose vs. 0.5% agarose and gelatin, were used for fabrication. Architectural design variations were realized by varying ring geometries and amounts while keeping the material composition across designs comparable. Results: Variations in the architectural design, such as lamellar width, number, and order, combined with choosing specific biomaterial properties, strongly influenced the biomechanical performance of IVD constructs. Biomechanical characterization revealed that the single most important parameter, in which the model systems vastly exceeded those of the individual materials, was failure load. The model system failure loads were 32.21- and 84.11-fold higher than those of the agarose materials and 55.03- and 2.14-fold higher than those of the agarose and gelatin materials used for system fabrication. The compressive strength, dynamic stiffness, and viscoelasticity of the model systems were always in the range of the individual materials. Conclusions: Relevant architecture-promoted biomechanical performance-tuning of tissue-engineered constructs for biological IVD replacement can be realized by slight modifications in the design of constructs while preserving the materials’ compositions. Minimal variations in the architectural design can be used to precisely control structure–function relations for IVD constructs rather than choosing different materials. These fundamental findings have important implications for efficient tissue-engineering of IVDs and other load-bearing tissues, as potential implants need to withstand high in situ loads.

Intervertebral disc degeneration and regeneration: a motion segment perspective

Back and neck pain have become primary reasons for disability and healthcare spending globally. While the causes of back pain are multifactorial, intervertebral disc degeneration is frequently cited as a primary source of pain. The annulus fibrosus (AF) and nucleus pulposus (NP) subcomponents of the disc are common targets for regenerative therapeutics. However, disc degeneration is also associated with degenerative changes to adjacent spinal tissues, and successful regenerative therapies will likely need to consider and address the pathology of adjacent spinal structures beyond solely the disc subcomponents. This review summarises the current state of knowledge in the field regarding associations between back pain, disc degeneration, and degeneration of the cartilaginous and bony endplates, the AF-vertebral body interface, the facet joints and spinal muscles, in addition to a discussion of regenerative strategies for treating pain and degeneration from a whole motion segment perspective.

SIRT1 Inhibits Apoptosis by Promoting Autophagic Flux in Human Nucleus Pulposus Cells in the Key Stage of Degeneration via ERK Signal Pathway

Background

The application of biomolecular interventions in the early stage of intervertebral disc degeneration (IVDD) is considered an ideal method for the treatment of IVDD. However, the precise definition of the "early stage" of IVDD is unclear. Silent information regulation 2 homologue-1 (SIRT1) can protect human degenerative nucleus pulposus (NP) cells from apoptosis by activating autophagy. However, the mechanism of this effect is still unclear. This study tried to confirm the "early stage" of IVDD and the role of NP cell autophagy during IVDD as well as to determine the mechanism by which SIRT1 protects NP cells.

Methods

The characteristics of the NP in various stages of degeneration were assessed to confirm the "early stage" of IVDD. Then, autophagy and apoptosis were detected in NP cells after SIRT1 upregulation/downregulation. Finally, LY294002 and PD98059 were used to inhibit the AKT/ERK pathway to determine the mechanism by which SIRT1 regulates autophagy in NP cells.

Results

Our data showed that mildly degenerative (Pfirrmann grade III with normal height of intervertebral disc) NP cells may be the key target for biomolecular interventions in IVDD and that SIRT1 protects human mildly degenerative NP cells from apoptosis by activating autophagy via the ERK signalling pathway.

Conclusion

Our data showed that SIRT1 inhibits apoptosis by promoting the autophagic flux in NP cells via the ERK signalling pathway during the key stage of degeneration. These findings will assist in the development of novel therapeutic approaches for IVDD treatment.

Fabrication, maturation, and implantation of composite tissue-engineered total discs formed from native and mesenchymal stem cell combinations

Low back pain arising from disc degeneration is one of the most common causes of limited function in adults. A number of tissue engineering strategies have been used to develop composite tissue engineered total disc replacements to restore native tissue structure and function. In this study we fabricated a composite engineered disc based on the combination of a porous polycaprolactone (PCL) foam annulus fibrosus (AF) and a hyaluronic acid (HA) hydrogel nucleus pulposus (NP). To evaluate whether native tissue cells or mesenchymal stem cells (MSCs) would perform better, constructs were seeded with native AF/NP cells or with MSCs in the foam and/or gel region. Maturation of these composite engineered discs was evaluated for 9 weeks in vitro culture by biochemical content, histological analysis and mechanical properties. To evaluate the performance of these constructs in the in vivo space, engineered discs were implanted into the caudal spines of athymic rats for 5 weeks. Our findings show that engineered discs comprised of AF/NP cells and MSCs performed similarly and maintained their structure after 5 weeks in vivo. However, for both cell types, loss of proteoglycan was evident in the NP region. These data support the continued development of the more clinically relevant MSCs population for disc replacement applications. STATEMENT OF SIGNIFICANCE: A number of tissue engineering strategies have emerged that are focused on the creation of a composite disc replacement. We fabricated a composite engineered disc based on the combination of a porous foam AF and a HA gel NP. We used these constructs to determine whether the combination of AF/NP cells or MSCs would mature to a greater extent in vitro and which cell type would best retain their phenotype after implantation. Engineered discs comprised of AF/NP cells and MSCs performed similarly, maintaining their structure after 5 weeks in vivo. These data support the successful fabrication and in vivo function of an engineered disc composed of a PCL foam AF and a hydrogel NP using either disc cells or MSCs.

Sacrificial Fibers Improve Matrix Distribution and Micromechanical Properties in a Tissue-Engineered Intervertebral Disc

Tissue-engineered replacement discs are an area of intense investigation for the treatment of end-stage intervertebral disc (IVD) degeneration. These living implants can integrate into the IVD space and recapitulate native motion segment function. We recently developed a multiphasic tissue-engineered disc-like angle-ply structure (DAPS) that models the micro-architectural and functional features of native tissue. While these implants resulted in functional restoration of the motion segment in rat and caprine models, we also noted deficiencies in cell infiltration and homogeneity of matrix deposition in the electrospun poly(ε-caprolactone) outer region (annulus fibrosus, AF) of the DAPS. To address this limitation, here, we incorporated a sacrificial water-soluble polymer, polyethylene oxide (PEO), as a second fiber fraction within the AF region to increase porosity of the implant. Maturation of these PEO-modified DAPS were evaluated after 5 and 10 weeks of in vitro culture in terms of AF biochemical content, MRI T2 values, overall construct mechanical properties, AF micromechanical properties and cell and matrix distribution. To assess the performance of the PEO-modified DAPS in vivo, precultured constructs were implanted into the rat caudal IVD space for 10 weeks. Results showed that matrix distribution was more homogenous in PCL/PEO DAPS, as evidenced by more robust histological staining, organized collagen deposition and micromechanical properties, compared to standard PCL-only DAPS in vitro. Cell and matrix infiltration were also improved in vivo, but no differences in macromechanical properties and a trend towards improved micromechanical properties were observed. These findings demonstrate that the inclusion of a sacrificial PEO fiber fraction in the DAPS AF region improves cellular colonization, matrix elaboration, and in vitro and in vivo function of an engineered IVD implant. STATEMENT OF SIGNIFICANCE: This work establishes a method for improving cell infiltration and matrix distribution within tissue-engineered dense fibrous scaffolds for intervertebral disc replacement. Tissue-engineered whole disc replacements are an attractive alternative to the current gold standard (mechanical disc arthroplasty or vertebral fusion) for the clinical treatment of patients with advanced disc degeneration.

Generation of an in vitro model of the outer annulus fibrosus-cartilage interface

Current treatments for degenerative disc disease do not restore full biological functionality of the intervertebral disc (IVD). As a result, regenerative medicine approaches are being developed to generate a biological replacement that when implanted will restore form and function of the degenerated IVD. Tissue-engineered models to date have focused on the generation of nucleus pulposus and annulus fibrosus IVD components. However, these tissues need to be integrated with a cartilage endplate in order for successful implantation to occur. The purpose of this study was to generate an in vitro annulus fibrosus-cartilage interface model which would enable us to better understand the biological and biomechanical implications of such interfaces. It was hypothesized that in vitro-formed outer annulus fibrosus (OAF) and cartilage tissues would integrate in direct-contact coculture to yield an interface containing extracellular matrix with aspects resembling the native OAF-CEP interface. In vitro-formed tissues were generated using bovine OAF cell-seeded angle-ply, multi-lamellated polycarbonate urethane scaffolds and articular chondrocytes, which were then placed in direct-contact coculture. 2-week old OAF tissues integrated with 3-day old cartilage by 1 week of coculture. Immunohistochemical staining of 2-week interfaces showed that distributions of collagen type I, collagen type II, and aggrecan were similar to the native bovine interface. The apparent tensile strength of the in vitro interface increased significantly between 2 and 4 weeks of coculture. In summary, an annulus fibrosus-cartilage interface model can be formed in vitro which will facilitate the identification of conditions required to generate an entire tissue-engineered disc replacement suitable for clinical use.

[Research progress of hydrogel used for regeneration of nucleus pulposus in intervertebral disc degeneration]

Objective

To summarize the research progress of hydrogels for the regeneration and repair of degenerative intervertebral disc and to investigate the potential of hydrogels in clinical application.

Methods

The related literature about the role of hydrogels in intervertebral disc degeneration especially for nucleus pulposus was reviewed and analyzed.

Results

Hydrogels share similar properties with nucleus pulposus, and it plays an important role in the regeneration and repair of degenerative intervertebral disc, which can be mainly applied in nucleus pulposus prosthesis, hydrogel-based cell therapy, non-cellular therapy, and tissue engineering repair.

Conclusion

Hydrogels are widely used in the regeneration and repair of intervertebral disc, which provides a potential treatment for intervertebral disc degeneration.

Molecular Mechanisms of Intervertebral Disc Degeneration

Intervertebral disc degeneration is a well-known cause of disability, the result of which includes neck and back pain with associated mobility limitations. The purpose of this article is to provide an overview of the known molecular mechanisms through which intervertebral disc degeneration occurs as a result of complex interactions of exogenous and endogenous stressors. This review will focus on some of the identified molecular changes leading to the deterioration of the extracellular matrix of both the annulus fibrosus and nucleus pulposus. In addition, we will provide a summation of our current knowledge supporting the role of associated DNA and intracellular damage, cellular senescence's catabolic effects, oxidative stress, and the cell's inappropriate response to damage in contributing to intervertebral disc degeneration. Our current understanding of the molecular mechanisms through which intervertebral disc degeneration occurs provides us with abundant insight into how physical and chemical changes exacerbate the degenerative process of the entire spine. Furthermore, we will describe some of the related molecular targets and therapies that may contribute to intervertebral repair and regeneration.

Lessons to be learned and future directions for intervertebral disc biomaterials

Biomaterials science has achieved significant advancements for the replacement, repair and regeneration of intervertebral disc tissues. However, the translation of this research to the clinic presents hurdles. The goal of this paper is to identify strategies to recapitulate the intrinsic complexities of the intervertebral disc, to highlight the unresolved issues in basic knowledge hindering the clinical translation, and finally to report on the emerging technologies in the biomaterials field. On this basis, we identify promising research directions, with the hope of stimulating further debate and advances for resolving clinical problems such as cervical and low back pain using biomaterial-based approaches.

STATEMENT OF SIGNIFICANCE

Although not life-threatening, intervertebral disc disorders have enormous impact on life quality and disability. Disc function within the human body is mainly mechanical, and therefore the use of biomaterials to rescue disc function and alleviate pain is logical. Despite intensive research, the clinical translation of biomaterial-based therapies is hampered by the intrinsic complexity of this organ. After decades of development, artificial discs or tissue replacements are still niche applications given their issues of integration and displacement with detrimental consequences. The struggles of biological therapies and tissue engineering are therefore understandable. However, recent advances in biomaterial science give new hope. In this paper we identify the most promising new directions for intervertebral disc biomaterials.

Biomaterials for intervertebral disc regeneration: Current status and looming challenges

A biomaterial-based strategy is employed to regenerate the degenerated intervertebral disc, which is considered a major generator of neck and back pain. Although encouraging enhancements in the anatomy and kinematics of the degenerative disc have been gained by biomaterials with various formulations in animals, the number of biomaterials tested in humans is rare. At present, most studies that involve the use of newly developed biomaterials focus on regeneration of the degenerative disc, but not pain relief. In this review, we summarise the current state of the art in the field of biomaterial-based regeneration or repair for the nucleus pulposus, annulus fibrosus, and total disc transplantation in animals and humans, and we then provide essential suggestions for the development and clinical translation of biomaterials for disc regeneration. It is important for researchers to consider the commonly neglected issues instead of concentrating solely on biomaterial development and fabrication.

Physiology and Engineering of the Graded Interfaces of Musculoskeletal Junctions

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

In Vitro Maturation and In Vivo Integration and Function of an Engineered Cell-Seeded Disc-like Angle Ply Structure (DAPS) for Total Disc Arthroplasty

Total disc replacement with an engineered substitute is a promising avenue for treating advanced intervertebral disc disease. Toward this goal, we developed cell-seeded disc-like angle ply structures (DAPS) and showed through in vitro studies that these constructs mature to match native disc composition, structure, and function with long-term culture. We then evaluated DAPS performance in an in vivo rat model of total disc replacement; over 5 weeks in vivo, DAPS maintained their structure, prevented intervertebral bony fusion, and matched native disc mechanical function at physiologic loads in situ. However, DAPS rapidly lost proteoglycan post-implantation and did not integrate into adjacent vertebrae. To address this, we modified the design to include polymer endplates to interface the DAPS with adjacent vertebrae, and showed that this modification mitigated in vivo proteoglycan loss while maintaining mechanical function and promoting integration. Together, these data demonstrate that cell-seeded engineered discs can replicate many characteristics of the native disc and are a viable option for total disc arthroplasty.

Mechanical function near defects in an aligned nanofiber composite is preserved by inclusion of disorganized layers: Insight into meniscus structure and function

The meniscus is comprised of circumferentially aligned fibers that resist the tensile forces within the meniscus (i.e., hoop stress) that develop during loading of the knee. Although these circumferential fibers are severed by radial meniscal tears, tibial contact stresses do not increase until the tear reaches ∼90% of the meniscus width, suggesting that the severed circumferential fibers still bear load and maintain the mechanical functionality of the meniscus. Recent data demonstrates that the interfibrillar matrix can transfer strain energy to disconnected fibrils in tendon fascicles. In the meniscus, interdigitating radial tie fibers, which function to stabilize and bind the circumferential fibers together, are hypothesized to function in a similar manner by transmitting load to severed circumferential fibers near a radial tear. To test this hypothesis, we developed an engineered fibrous analog of the knee meniscus using poly(ε-caprolactone) to create aligned scaffolds with variable amounts of non-aligned elements embedded within the scaffold. We show that the tensile properties of these scaffolds are a function of the ratio of aligned to non-aligned elements, and change in a predictable fashion following a simple mixture model. When measuring the loss of mechanical function in scaffolds with a radial tear, compared to intact scaffolds, the decrease in apparent linear modulus was reduced in scaffolds containing non-aligned layers compared to purely aligned scaffolds. Increased strains in areas adjacent to the defect were also noted in composite scaffolds. These findings indicate that non-aligned (disorganized) elements interspersed within an aligned network can improve overall mechanical function by promoting strain transfer to nearby disconnected fibers. This finding supports the notion that radial tie fibers may similarly promote tear tolerance in the knee meniscus, and will direct changes in clinical practice and provide guidance for tissue engineering strategies.

STATEMENT OF SIGNIFICANCE

The meniscus is a complex fibrous tissue, whose architecture includes radial tie fibers that run perpendicular to and interdigitate with the predominant circumferential fibers. We hypothesized that these radial elements function to preserve mechanical function in the context of interruption of circumferential bundles, as would be the case in a meniscal tear. To test this hypothesis, we developed a biomaterial analog containing disorganized layers enmeshed regularly throughout an otherwise aligned network. Using this material formulation, we showed that strain transmission is improved in the vicinity of defects when disorganized fiber layers were present. This supports the idea that radial elements within the meniscus improve function near a tear, and will guide future clinical interventions and the development of engineered replacements.

Intervertebral Disk Degeneration and Repair

Intervertebral disk (IVD) degeneration is a natural progression of the aging process. Degenerative disk disease (DDD) is a pathologic condition associated with IVD that has been associated with chronic back pain. There are a variety of different mechanisms of DDD (genetic, mechanical, exposure). Each of these pathways leads to a final common result of unbalancing the anabolic and catabolic environment of the extracellular matrix in favor of catabolism. Attempts have been made to gain an understanding of the process of IVD degeneration with in Vitro studies. These models help our understanding of the disease process, but are limited as they do not come close to replicating the complexities that exist with an in Vivo model. Animal models have been developed to help us gain further understanding of the degenerative cascade of IVD degeneration In Vivo and test experimental treatment modalities to either prevent or reverse the process of DDD. Many modalities for treatment of DDD have been developed including therapeutic protein injections, stem cell injections, gene therapy, and tissue engineering. These interventions have had promising outcomes in animal models. Several of these modalities have been attempted in human trials, with early outcomes having promising results. Further, increasing our understanding of the degenerative process is essential to the development of new therapeutic interventions and the optimization of existing treatment protocols. Despite limited data, biological therapies are a promising treatment modality for DDD that could impact our future management of low back pain.

Cell therapy for the degenerating intervertebral disc

Spinal conditions related to intervertebral disc (IVD) degeneration cost billions of dollars in the US annually. Despite the prevalence and soaring cost, there is no specific treatment that restores the physiological function of the diseased IVD. Thus, it is vital to develop new treatment strategies to repair the degenerating IVD. Persons with IVD degeneration without back pain or radicular leg pain often do not require any intervention. Only patients with severe back pain related to the IVD degeneration or biomechanical instability are likely candidates for cell therapy. The IVD progressively degenerates with age in humans, and strategies to repair the IVD depend on the stage of degeneration. Cell therapy and cell-based gene therapy aim to address moderate disc degeneration; advanced stage disease may require surgery. Studies involving autologous, allogeneic, and xenogeneic cells have all shown good survival of these cells in the IVD, confirming that the disc niche is an immunologically privileged site, permitting long-term survival of transplanted cells. All of the animal studies reviewed here reported some improvement in disc structure, and 2 studies showed attenuation of local inflammation. Among the 50 studies reviewed, 25 used some type of scaffold, and cell leakage is a consistently noted problem, though some studies showed reduced cell leakage. Hydrogel scaffolds may prevent cell leakage and provide biomechanical support until cells can become established matrix producers. However, these gels need to be optimized to prevent this leakage. Many animal models have been leveraged in this research space. Rabbit is the most frequently used model (28 of 50), followed by rat, pig, and dog. Sheep and goat IVDs resemble those of humans in size and in the absence of notochordal cells. Despite this advantage, there were only 2 sheep and 1 goat studies of 50 studies in this cohort. It is also unclear if a study in large animals is needed before clinical trials since some of the clinical trials proceeded without a study in large animals. No animal studies or clinical trials completely restored IVD structure. However, results suggest cause for optimism. In light of the fact that patients primarily seek medical care for back pain, attenuating local inflammation should be a priority in benchmarks for success. Clinicians generally agree that short-term back pain should be treated conservatively. When interventions are considered, the ideal therapy should also be minimally invasive and concurrent with other procedures such as discography or discectomy. Restoration of tissue structure and preservation of spinal motion are desirable.