Tissue Engineering a Biological Repair Strategy for Lumbar Disc Herniation

Review of state-of-the-art micro and macro-bioreactors for the intervertebral disc

Lower back pain continues to be a global epidemic, limiting quality of life and ability to work, due in large part to symptomatic disc degeneration. Development of more effective and less invasive biological strategies are needed to treat disc degeneration. In vitro models such as macro- or micro-bioreactors or mechanically active organ-chips hold great promise in reducing the need for animal studies that may have limited clinical translatability, due to harsher and more complex mechanical loading environments in human discs than in most animal models. This review highlights the complex loading conditions of the disc in situ, evaluates state-of-the-art designs for applying such complex loads across multiple length scales, from macro-bioreactors that load whole discs to organ-chips that aim to replicate cellular or engineered tissue loading. Emphasis was placed on the rapidly evolving more customizable organ-chips, given their greater potential for studying the progression and treatment of symptomatic disc degeneration. Lastly, this review identifies new trends and challenges for using organ-chips to assess therapeutic strategies.

An international analysis of stem cell research in intervertebral disc degeneration

Stem cell therapy has been increasingly investigated as a promising strategy for intervertebral disc degeneration (IDD). However, no international analysis of stem cell research has yet been conducted. This study aimed to analyze the major characteristics of published reports of stem cell use for IDD and to present a global insight into stem cell research. The study period spanned from the inception of the Web of Science database to 2021. A search strategy using specific keywords was implemented to retrieve relevant publications. The numbers of documents, citations, countries, journals, article types, and stem cell types were evaluated. A total of 1170 papers were retrieved. The analysis showed a significant increase in the number of papers over time (p < 0.001). High-income economies accounted for the majority of papers (758, 64.79 %). China produced the most articles (378, 32.31 %), followed by the United States (259, 22.14 %), Switzerland (69, 5.90 %), United Kingdom (54, 4.62 %), and Japan (47, 4.02 %). The United States ranked first in terms of the number of citations (10,346), followed by China (9177) and Japan (3522). Japan ranked first in terms of the number of citations per paper (74.94), followed by United Kingdom (58.54) and Canada (53.74). When standardized by population, Switzerland ranked first, followed by Ireland and Sweden. When gross domestic product was considered, Switzerland ranked first, followed by Portugal and Ireland. The number of papers was positively correlated with gross domestic product (p < 0.001, r = 0.673); however, there was no significant correlation with population (p = 0.062, r = 0.294). Mesenchymal stem cells were the most investigated stem cells, followed by nucleus pulposus-derived stem cells and adipose-derived stem cells. A sharp increase in stem cell research was observed in the field of IDD. China produced the most, although several European countries were more productive relative to their populations and economies.

Graded-Three-Dimensional Cell-Encapsulating Hydrogel as a Potential Biologic Scaffold for Disc Tissue Engineering

BACKGROUND

Intervertebral disk (IVD) degeneration, which can cause lower back pain, is a major predisposing factor for disability and can be managed through multiple approaches. However, there is no satisfactory strategy currently available to reconstruct and recover the natural properties of IVDs after degeneration. As tissue engineering develops, scaffolds with embedded cell cultures have proved critical for the successful regeneration of IVDs.

METHODS

In this study, an integrated scaffold for IVD replacement was developed. Through scanning electron microscopy and other mechanical measurements, we characterized the physical properties of different hydrogels. In addition, we simulated the physiological structure of natural IVDs. Nucleus pulposus (NP) cells and annulus fibrosus-derived stem cells (AFSCs) were seeded in gelatin methacrylate (GelMA) hydrogel at different concentrations to evaluate cell viability and matrix expression.

RESULTS

It was found that different concentrations of GelMA hydrogel can provide a suitable environment for cell survival. However, hydrogels with different mechanical properties influence cell adhesion and extracellular matrix component type I collagen, type II collagen, and aggrecan expression.

CONCLUSION

This tissue-engineered IVD implant had a similar structure and function as the native IVD, with the inner area mimicking the NP tissue and the outer area mimicking the stratified annulus fibrosus tissue. The new integrated scaffold demonstrated a good simulation of disc structure. The preparation of efficient and regeneration-promoting tissue-engineered scaffolds is an important issue that needs to be explored in the future. It is hoped that this work will provide new ideas and methods for the further construction of functional tissue replacement discs.

Hui Medicine Moxibustion Promotes the Absorption of Lumbar Disc Herniation and the Recovery of Motor Function in Rats through Fas/FasL Signaling Pathway

Objectives

To study the resorption of the herniated lumbar disc (RHLD) and its mechanism in the SD rats of lumbar intervertebral disc herniation treated with Hui medicine moxibustion (HMM).

Methods

Forty SD rats were randomly divided into four groups, normal group, lumbar disc herniation (LDH) group, HMM group, and antagonist (HMM+Met12) group, with 10 rats in each group. The rat model of LDH was prepared with the method of lumbar epidural emplacement of the caudal intervertebral disc. In the HMM group and HMM+Met12 groups, 4 weeks after modeling, HMM therapy was performed in the lumbar spine for 3 months with 1 time per day and 20 min each time, the samples were collected 8 weeks after the treatment. The histological degeneration was observed through HE staining, and the neovascularization of intervertebral disc tissues was detected by the expression of CD34 and vascular endothelial growth factor (VEGF). The apoptosis of nucleus pulpous cells was detected by TUNEL assay, and the activity of caspase-3, -8, and -9 and extracellular matrix enzymes was detected by western blotting.

Results

HMM treatment significantly improved the behavioral ability of rats with LDH surgery. The morphological structure was obviously destroyed in the LDH group, but disc structure was significantly repaired in the HMM group, and mild structure alterations were observed in the HMM+Met12 group. Higher levels of CD34 and VEGF were detected in the HMM group indicating that neovascularization is formed. The expression level of FasL was significantly increased in the HMM group. The protein expression levels of cleaved-caspase-3, cleaved-caspase-8, and cleaved-caspase-9 in nucleus pulposus (NP) tissues were also elevated when treated with HMM, and the TUNEL staining showed the same results. The protein expression levels of matrix metalloproteinases- (MMP-) 1, MMP-2, MMP-3, MMP-13, and ADAMTS-4 were markedly promoted in the HMM group. Met12, a small peptide antagonist of FasL, significantly reduced the effects of HMM.

Conclusion

HMM can promote the formation of neovascularization of lumbar intervertebral disc, support the apoptosis of NP cells through Fas/FasL signaling, and regulate the degradation of extracellular matrix enzyme, which then accelerates the absorption of lumbar intervertebral disc herniation and the recovery of motor function in rats.

Non-enzymatic glycation of annulus fibrosus alters tissue-level failure mechanics in tension

Advanced-glycation end products (AGEs) are known to accumulate in biological tissues with age and at an accelerated rate in patients with diabetes and chronic kidney disease. Clinically, diabetes has been linked to increased frequency and severity of back pain, accelerated disc degeneration, and an increased risk of disc herniation. Despite significant clinical evidence suggesting that diabetes-induced AGEs may play a role in intervertebral disc failure and substantial previous work investigating the effects of AGEs on bone, cartilage, and tendon mechanics, the effects of AGEs on annulus fibrosus (AF) failure mechanics have not yet been reported. Thus, the aim of this study was to determine the relationship between physiological levels of AGEs and AF tensile mechanics at two distinct loading rates. In vitro glycation treatments with methylglyoxal were applied to minimize changes in tissue hydration and induce two distinct levels of AGEs based on values measured from human AF tissues. In vitro glycation increased modulus by 48-99% and failure stress by 45-104% versus control and decreased post-failure energy absorption capacity by 15-32% versus control (ANOVA p < 0.0001 on means; range given across two loading rates and glycation levels). AGE content correlated strongly with modulus (R = 0.74, p < 0.0001) and failure stress (R = 0.70, p < 0.0001) and moderately with post-failure energy absorption capacity (R = 0.62, p < 0.0001). Failure strain was reduced by 10-17% at the high-glycation level (ANOVA p = 0.01). Tissue water content remained near or just above fresh-tissue levels for all groups. The alterations in mechanics with glycation reported here are consistent with trends from other connective tissues but do not fully explain the clinical predisposition of diabetics to disc herniation. The results from this study may be used in the development of advanced computational models that aim to study disc disease progression and to provide a deeper understanding of altered structure-function relationships that may lead to tissue dysfunction and failure with aging and disease.

Saline-polyethylene glycol blends preserve in vitro annulus fibrosus hydration and mechanics: An experimental and finite-element analysis

Precise control of tissue water content is essential for ensuring accurate, repeatable, and physiologically relevant measurements of tissue mechanics and biochemical composition. While previous studies have found that saline and polyethylene glycol (PEG) blends were effective at controlling tendon and ligament hydration levels, this work has yet to be extended to the annulus fibrosus (AF). Thus, the first objective of this study was to determine and validate an optimal buffer solution for targeting and maintaining hydration levels of tissue-level AF specimens in vitro. This was accomplished by measuring the transient swelling behavior of bovine AF specimens in phosphate-buffered saline (PBS) and PEG buffers across a wide range of concentrations. Sub-failure, failure, and post-failure mechanics were measured to determine the relationship between changes in tissue hydration and tensile mechanical response. The effect of each buffer solution on tissue composition was also assessed. The second objective of this study was to assess the feasibility and effectiveness of using multi-phasic finite element models to investigate tissue swelling and mechanical responses in different external buffer solutions. A solution containing 6.25%w/v PBS and 6.25%w/v PEG effectively maintained tissue-level AF specimen hydration at fresh-frozen levels after 18 h in solution. Modulus, failure stress, failure strain, and post-failure toughness of specimens soaked in this solution for 18 h closely matched those of fresh-frozen specimens. In contrast, specimens soaked in 0.9%w/v PBS swelled over 100% after 18 h and exhibited significantly diminished sub-failure and failure properties compared to fresh-frozen controls. The increased cross-sectional area with swelling contributed to but was not sufficient to explain the diminished mechanics of PBS-soaked specimens, suggesting additional sub-tissue scale mechanisms. Computational simulations of these specimens generally agreed with experimental results, highlighting the feasibility and importance of including a fluid-phase description when models aim to provide accurate predictions of biological tissue responses. As numerous previous studies suggest that tissue hydration plays a central role in maintaining proper mechanical and biological function, robust methods for controlling hydration levels are essential as the field advances in probing the relationship between tissue hydration, aging, injury, and disease.

Intervertebral Disc Mechanics With Nucleotomy: Differences Between Simple and Dual Loading

Painful herniated discs are treated surgically by removing extruded nucleus pulposus (NP) material (nucleotomy). NP removal through enzymatic digestion is also commonly performed to initiate degenerative changes to study potential biological repair strategies. Experimental and computational studies have shown a decrease in disc stiffness with nucleotomy under single loading modalities, such as compression-only or bending-only loading. However, studies that apply more physiologically relevant loading conditions, such as compression in combination with bending or torsion, have shown contradicting results. We used a previously validated bone-disc-bone finite element model (Control) to create a Nucleotomy model to evaluate the effect of dual loading conditions (compression with torsion or bending) on intradiscal deformations. While disc joint stiffness decreased with nucleotomy under single loading conditions, as commonly reported in the literature, dual loading resulted in an increase in bending stiffness. More specifically, dual loading resulted in a 40% increase in bending stiffness under flexion and extension and a 25% increase in stiffness under lateral bending. The increase in bending stiffness was due to an increase and shift in compressive stress, where peak stresses migrated from the NP-annulus interface to the outer annulus. In contrast, the decrease in torsional stiffness was due to greater fiber reorientation during compression. In general, large radial strains were observed with nucleotomy, suggesting an increased risk for delamination or degenerative remodeling. In conclusion, the effect of nucleotomy on disc mechanics depends on the type and complexity of applied loads.

Inorganic polyphosphates stimulates matrix production in human annulus fibrosus cells

INTRODUCTION

Ubiquitously found in all life forms, inorganic polyphosphates (polyP) are linear polymers of repeated orthophosphate units. Present in intervertebral disc tissue, polyP was previously shown to increase extracellular matrix production in nucleus pulposus (NP) cells. However, the effects of polyP on human annulus fibrosus (hAF) cell metabolism is not known.

METHODS AND RESULTS

Here, hAF cells cultured in the presence of 0.5 to 1 mM polyP, chain length 22 (polyP-22), showed an increase in glycosaminoglycan content, proteoglycan and collagen synthesis, and aggrecan and collagen type 1 gene expression. Gene expression level of matrix metalloproteinases 1 was reduced while matrix metalloproteinases 3 level was increased in hAF cells treated with 1 mM polyP. Adenosine triphosphate (ATP) synthesis was also significantly increased in hAF cell culture 72 hours after the exposure to 1 mM polyP-22.

CONCLUSIONS

PolyP thus has both anabolic and bioenergetic effects in AF cells, similar to that observed in NP cells. Together, these results suggest polyP as a potential energy source and a metabolic regulator of disc cells.

Advances in Tissue Engineering for Disc Repair

Intervertebral disc (IVD) degeneration is a leading cause of chronic low back pain (LBP) that results in serious disability and significant economic burden. IVD degeneration alters the disc structure and spine biomechanics, resulting in subsequent structural changes throughout the spine. Currently, treatments of chronic LBP due to IVD degeneration include conservative treatments, such as pain medication and physiotherapy, and surgical treatments, such as removal of herniated disc without or with spinal fusion. However, none of these treatments can completely restore a degenerated disc and its function. Thus, although the exact pathogenesis of disc degeneration remains unclear, there are studies examining the effectiveness of biological approaches, such as growth factor injection, gene therapy, and cell transplantation, in promoting IVD regeneration. Furthermore, tissue engineering using a combination of cell transplantation and biomaterials has emerged as a promising new approach for repair or restoration of degenerated discs. The main purpose of this review was to provide an overview of the current status of tissue engineering applications for IVD regenerative therapy by performing literature searches using PubMed. Significant advances in tissue engineering have opened the door to a new generation of regenerative therapies for the treatment of chronic discogenic LBP.

Fiber engagement accounts for geometry-dependent annulus fibrosus mechanics: A multiscale, Structure-Based Finite Element Study

A comprehensive understanding of biological tissue mechanics is crucial for designing engineered tissues that aim to recapitulate native tissue behavior. Tensile mechanics of many fiber-reinforced tissues have been shown to depend on specimen geometry, which makes it challenging to compare data between studies. In this study, a validated multiscale, structure-based finite element model was used to evaluate the effect of specimen geometry on multiscale annulus fibrosus tensile mechanics through a fiber engagement analysis. The relationships between specimen geometry and modulus, Poisson's ratio, tissue stress-strain distributions, and fiber reorientation behaviors were investigated at both tissue and sub-tissue levels. It was observed that annulus fibrosus tissue level tensile properties and stress transmission mechanisms were dependent on specimen geometry. The model also demonstrated that the contribution of fiber-matrix interactions to tissue mechanical response was specimen size- and orientation-dependent. The results of this study reinforce the benefits of structure-based finite element modeling in studies investigating multiscale tissue mechanics. This approach also provides guidelines for developing optimal combined computational-experimental study designs for investigating fiber-reinforced biological tissue mechanics. Additionally, findings from this study help explain the geometry dependence of annulus fibrosus tensile mechanics previously reported in the literature, providing a more fundamental and comprehensive understanding of tissue mechanical behavior. In conclusion, the methods presented here can be used in conjunction with experimental tissue level data to simultaneously investigate tissue and sub-tissue scale mechanics, which is important as the field of soft tissue biomechanics advances toward studies that focus on diminishing length scales.

Characterization of chondroitinase-induced lumbar intervertebral disc degeneration in a sheep model intended for assessing biomaterials

Intervertebral disc (IVD) degeneration (IVDD) leads to structural and functional changes. Biomaterials for restoring IVD function and promoting regeneration are currently being investigated; however, such approaches require validation using animal models that recapitulate clinical, biochemical, and biomechanical hallmarks of the human pathology. Herein, we comprehensively characterized a sheep model of chondroitinase-ABC (ChABC) induced IVDD. Briefly, ChABC (1 U) was injected into the L_1/2 , L_2/3 , and L_3/4 IVDs. Degeneration was assessed via longitudinal magnetic resonance (MR) and radiographic imaging. Additionally, kinematic, biochemical, and histological analyses were performed on explanted functional spinal units (FSUs). At 17-weeks, ChABC treated IVDs demonstrated significant reductions in MR index (p = 0.030) and disc height (p = 0.009) compared with pre-operative values. Additionally, ChABC treated IVDs exhibited significantly increased creep displacement (p = 0.004) and axial range of motion (p = 0.007) concomitant with significant decreases in tensile (p = 0.034) and torsional (p = 0.021) stiffnesses and long-term viscoelastic properties (p = 0.016). ChABC treated IVDs also exhibited a significant decrease in NP glycosaminoglycan: hydroxyproline ratio (p = 0.002) and changes in microarchitecture, particularly in the NP and endplates, compared with uninjured IVDs. Taken together, this study demonstrated that intradiscal injection of ChABC induces significant degeneration in sheep lumbar IVDs and the potential for using this model in evaluating biomaterials for IVD repair, regeneration, or fusion.

Historical Review of Combined Experimental and Computational Approaches for Investigating Annulus Fibrosus Mechanics

Intervertebral disc research has sought to develop a deeper understanding of spine biomechanics, the complex relationship between disc health and back pain, and the mechanisms of spinal injury and repair. To do so, many researchers have focused on characterizing tissue-level properties of the disc, where the roles of tissue subcomponents can be more systematically investigated. Unfortunately, experimental challenges often limit the ability to measure important disc tissue- and subtissue-level behaviors, including fiber-matrix interactions, transient nutrient and electrolyte transport, and damage propagation. Numerous theoretical and numerical modeling frameworks have been introduced to explain, complement, guide, and optimize experimental research efforts. The synergy of experimental and computational work has significantly advanced the field, and these two aspects have continued to develop independently and jointly. Meanwhile, the relationship between experimental and computational work has become increasingly complex and interdependent. This has made it difficult to interpret and compare results between experimental and computational studies, as well as between solely computational studies. This paper seeks to explore issues of model translatability, robustness, and efficient study design, and to propose and motivate potential future directions for experimental, computational, and combined tissue-level investigations of the intervertebral disc.

Intervertebral disc swelling maintains strain homeostasis throughout the annulus fibrosus: A finite element analysis of healthy and degenerated discs

Tissues in the intervertebral disc have a large capacity to absorb water, partially due to the high glycosaminoglycan (GAG) content, which decreases linearly from the nucleus pulposus (NP) in the center to the outer annulus. Our recent work showed that fiber network and GAG distribution contributes to development of residual stresses and strains that were compressive in the inner annulus to tensile in the outer annulus. GAG loss in the inner annulus, as observed with early to moderate degeneration, reduced swelling capacity and circumferential-direction stress by over 50%. However, our previous model was not capable of evaluating interactions between the NP and annulus fibrosus (AF) during swelling. In this study, we evaluated the effect of degeneration (GAG content or swelling capacity) on residual stress development throughout the disc. Simulations of moderate to severe degeneration showed a 40% decrease in NP swelling capacity, with a 25% decrease in AF and cartilaginous endplate swelling. Together, these changes in tissue swelling resulted in a decrease in NP pressure (healthy = 0.21 MPa; severe degeneration = 0.03 MPa) that was comparable to observations in human discs. There was a 60% decrease in circumferential-direction residual deformations with early degeneration. Radial-direction stretch switched from compressive to tensile with degeneration, which may increase the risk for tears or delamination. Degeneration had a significant impact on residual stress/stretch and fiber stretch in the posterior AF, which is important for understanding herniation risk. In conclusion, degenerative changes in disc geometry and intradiscal deformations was recreated by only altering NP and AF GAG composition. Since most computational models simulate degeneration by altering material stiffness, this work highlights the importance of directly simulating biochemical composition and distribution to study disc biomechanics with degeneration. STATEMENT OF SIGNIFICANCE: Tissues in the intervertebral disc have a large swelling capacity, due to its high glycosaminoglycan content. Our recent work demonstrated the importance of fiber network and glycosaminoglycan distribution residual stresses and strains development. In this study, we evaluated the effect of swelling on intradiscal deformations between the nucleus pulposus and annulus fibrosus. We also investigated the effect of degenerative glycosaminoglycan loss on swelling-based intradiscal deformations of the intact disc and its subcomponents. Decreases in nucleus glycosaminoglycan content resulted in morphological changes observed with degenerated discs and may help to explain mechanisms behind the increases in annular tears and mechanical dysfunction with degeneration.

Multiscale composite model of fiber-reinforced tissues with direct representation of sub-tissue properties

In many fiber-reinforced tissues, collagen fibers are embedded within a glycosaminoglycan-rich extrafibrillar matrix. Knowledge of the structure-function relationship between the sub-tissue properties and bulk tissue mechanics is important for understanding tissue failure mechanics and developing biological repair strategies. Difficulties in directly measuring sub-tissue properties led to a growing interest in employing finite element modeling approaches. However, most models are homogeneous and are therefore not sufficient for investigating multiscale tissue mechanics, such as stress distributions between sub-tissue structures. To address this limitation, we developed a structure-based model informed by the native annulus fibrosus structure, where fibers and the matrix were described as distinct materials occupying separate volumes. A multiscale framework was applied such that the model was calibrated at the sub-tissue scale using single-lamellar uniaxial mechanical test data, while validated at the bulk scale by predicting tissue multiaxial mechanics for uniaxial tension, biaxial tension, and simple shear (13 cases). Structure-based model validation results were compared to experimental observations and homogeneous models. While homogeneous models only accurately predicted bulk tissue mechanics for one case, structure-based models accurately predicted bulk tissue mechanics for 12 of 13 cases, demonstrating accuracy and robustness. Additionally, six of eight structure-based model parameters were directly linked to tissue physical properties, further broadening its future applicability. In conclusion, the structure-based model provides a powerful multiscale modeling approach for simultaneously investigating the structure-function relationship at the sub-tissue and bulk tissue scale, which is important for studying multiscale tissue mechanics with degeneration, disease, or injury.

Multi-laminate annulus fibrosus repair scaffold with an interlamellar matrix enhances impact resistance, prevents herniation and assists in restoring spinal kinematics

Focal defects in the annulus fibrosus (AF) of the intervertebral disc (IVD) arising from herniation have detrimental impacts on the IVD's mechanical function. Thus, biomimetic-based repair strategies must restore the mechanical integrity of the AF to help support and restore native spinal loading and motion. Accordingly, an annulus fibrosus repair patch (AFRP); a collagen-based multi-laminate scaffold with an angle-ply architecture has been previously developed, which demonstrates similar mechanical properties to native outer AF (oAF). To further enhance the mimetic nature of the AFRP, interlamellar (ILM) glycosaminoglycan (GAG) was incorporated into the scaffolds. The ability of the scaffolds to withstand simulated impact loading and resist herniation of native IVD tissue while contributing to the restoration of spinal kinematics were assessed separately. The results demonstrate that incorporation of a GAG-based ILM significantly increased (p < 0.001) the impact strength of the AFRP (2.57 ± 0.04 MPa) compared to scaffolds without (1.51 ± 0.13 MPa). Additionally, repair of injured functional spinal units (FSUs) with an AFRP in combination with sequestering native NP tissue and a full-thickness AF tissue plug enabled the restoration of creep displacement (p = 0.134), short-term viscous damping coefficient (p = 0.538), the long-term viscous (p = 0.058) and elastic (p = 0.751) damping coefficients, axial neutral zone (p = 0.908), and axial range of motion (p = 0.476) to an intact state. Lastly, the AFRP scaffolds were able to prevent native IVD tissue herniation upon application of supraphysiologic loads (5.28 ± 1.24 MPa). Together, these results suggest that the AFRP has the strength to sequester native NP and AF tissue and/or implants, and thus, can be used in a composite repair strategy for IVDs with focal annular defects thereby assisting in the restoration of spinal kinematics.

Bone morphogenetic protein-7 retards cell subculture-induced senescence of human nucleus pulposus cells through activating the PI3K/Akt pathway

BACKGROUND

Allogeneic disc cell is the main cellular resource in tissue engineering (TE)-based strategy to retard disc degeneration. However, the accessible disc cells often exhibit senescent phenotype when they are subcultured in vitro Hence, alleviating senescence of human disc cells during cell subculture is important for TE-based strategy to regenerate degenerative disc tissue.

OBJECTIVE

The present study was aimed to investigate whether bone morphogenetic protein-7 (BMP-7) can alleviate subculture-induced senescence of human nucleus pulposus (NP) cells in vitro Methods: NP cells from human disc tissue were subcultured in vitro for six passages. Exogenous BMP-7 was added along with the culture medium to investigate its effects on senescence of NP cells. The inhibitor LY294002 was used to investigate the role of the PI3K/Akt pathway.

RESULTS

Compared with the human disc NP cells cultured in the baseline culture medium, addition of BMP-7 increased cell proliferation potency and telomerase activity, decreased senescence-associated β-galactosidase (SA-β-Gal) activity and G0/G1 phase fraction, and down-regulated the expression of p16 and p53. Moreover, these positive effects of BMP-7 against senescence of human disc NP cells coincided with activation of the PI3K/Akt pathway. Further analysis showed that inhibitor LY294002 partly inhibited these protective effects of BMP-7 against senescence of human disc NP cells.

CONCLUSION

BMP-7 alleviates subculture-induced senescence of human disc NP cells through activating the PI3K/Akt pathway. The present study provides new knowledge on allogeneic disc NP cell-based TE strategy to regenerate degenerative human disc tissue.

Relative Nucleus Pulposus Area and Position Alters Disc Joint Mechanics

Aging and degeneration of the intervertebral disc are noted by changes in tissue composition and geometry, including a decrease in nucleus pulposus (NP) area. The NP centroid is positioned slightly posterior of the disc's centroid, but the effect of NP size and location on disc joint mechanics is not well understood. We evaluated the effect of NP size and centroid location on disc joint mechanics under dual-loading modalities (i.e., compression in combination with axial rotation or bending). A finite element model was developed to vary the relative NP area (NP:Disc area ratio range = 0.21 - 0.60). We also evaluated the effect of NP position by shifting the NP centroid anteriorly and posteriorly. Our results showed that compressive stiffness and average first principal strains increased with NP size. Under axial compression, stresses are distributed from the NP to the annulus, and stresses were redistributed towards the NP with axial rotation. Moreover, peak stresses were greater for discs with a smaller NP area. NP centroid location had a greater impact on intradiscal pressure during flexion and extension, where peak pressures in the posterior annulus under extension was greater for discs with a more posteriorly situated NP. In conclusion, the findings from this study highlight the importance of closely mimicking NP size and location in computational models that aim to understand stress/strain distribution during complex loading and for developing repair strategies that aim to recapitulate the mechanical behavior of healthy discs.

Agarose-Based Hydrogels as Suitable Bioprinting Materials for Tissue Engineering

Hydrogels are useful materials as scaffolds for tissue engineering applications. Using hydrogels with additive manufacturing techniques has typically required the addition of techniques such as cross-linking or printing in sacrificial materials that negatively impact tissue growth to remedy inconsistencies in print fidelity. Thus, there is a need for bioinks that can directly print cell-laden constructs. In this study, agarose-based hydrogels commonly used for cartilage tissue engineering were compared to Pluronic, a hydrogel with established printing capabilities. Moreover, new material mixtures were developed for bioprinting by combining alginate and agarose. We compared mechanical and rheological properties, including yield stress, storage modulus, and shear thinning, as well as construct shape fidelity to assess their potential as a bioink for cell-based tissue engineering. The rheological properties and printability of agarose-alginate gels were statistically similar to those of Pluronic for all tests (p > 0.05). Alginate-agarose composites prepared with 5% w/v (3:2 agarose to alginate ratio) demonstrated excellent cell viability over a 28-day culture period (>∼70% cell survival at day 28) as well matrix production over the same period. Therefore, agarose-alginate mixtures showed the greatest potential as an effective bioink for additive manufacturing of biological materials for cartilage tissue engineering.

Critical aspects and challenges for intervertebral disc repair and regeneration-Harnessing advances in tissue engineering

Low back pain represents the highest burden of musculoskeletal diseases worldwide and intervertebral disc degeneration is frequently associated with this painful condition. Even though it remains challenging to clearly recognize generators of discogenic pain, tissue regeneration has been accepted as an effective treatment option with significant potential. Tissue engineering and regenerative medicine offer a plethora of exploratory pathways for functional repair or prevention of tissue breakdown. However, the intervertebral disc has extraordinary biological and mechanical demands that must be met to assure sustained success. This concise perspective review highlights the role of the disc microenvironment, mechanical and clinical design considerations, function vs mimicry in biomaterial‐based and cell engineering strategies, and potential constraints for clinical translation of regenerative therapies for the intervertebral disc.

Recent advances on 3D printing of patient-specific implants for fibrocartilage tissue regeneration

Fibrocartilage is a very peculiar type of tissue found in intervertebral disc and meniscus. It is characterized by its avascular nature and for the shear and compressive forces that it can be subjected to. The number of individuals affected by the degeneration of fibrocartilaginous tissues has been growing and the poor outcomes of current treatments have led to an increased interest in new alternative approaches. Therefore, the combination of reverse engineering with 3D printing has been extensively explored in order to produce patient-specific implants capable of improving the current clinical outcomes. This review outlines the recent advances achieved in the tissue engineering field, especially focusing on fibrocartilaginous tissue.

Osmotic Pressure Alters Time-dependent Recovery Behavior of the Intervertebral Disc

STUDY DESIGN

Disc recovery behavior under hypo- and hyperosmotic pressure.

OBJECTIVE

To evaluate the effect of osmotic pressure on the unloaded recovery response of healthy discs.

SUMMARY OF BACKGROUND DATA

The intervertebral disc is a poroviscoelastic material that experiences large fluctuations in water composition throughout a diurnal loading cycle. Fluid flow out of the disc occurs through mechanical loading, whereas fluid flow into the disc occurs through passive diffusion because of an imbalance of ions between the disc and its surrounding environment. Osmotic pressure has been used to alter water uptake and tissue hydration.

METHODS

Motion segments were prepared from the caudal spine sections of the skeletally mature bovines. A 300-N compressive load was applied for 2 hours before unloaded recovery for 12 hours. Hypo- and hyperosmotic pressure was used to alter the rate of water uptake and disc height recovery during unloaded recovery. A 5-parameter rheological model was used to describe the disc's time-dependent recovery behavior.

RESULTS

The elastic response was not altered by changes in osmotic pressure; however, viscoelastic recovery was highly dependent on saline osmolarity and recovery time. The fast response of viscoelastic recovery was not dependent on osmotic pressure. The time constant for the slow response decreased whereas the slow response stiffness increased as osmotic pressure increased.

CONCLUSION

The fast response of viscoelastic recovery is governed by flow-independent recovery, whereas the slow response is related to flow-dependent recovery. The rate and magnitude of flow-dependent recovery are highly sensitive to changes in osmotic pressure of the saline bath. There is an osmotic pressure that reduces disc recovery behavior to an elastic response or flow-independent recovery.

LEVEL OF EVIDENCE

N/A.

Hypoxia-inducible factor (HIF)-1alpha knockout accelerates intervertebral disc degeneration in mice

INTRODUCTION

The abnormality of nucleus pulposus (NP) plays a critical role in intervertebral disc (IVD) degeneration, in which NP cells show apoptosis and fibrosis, leading to the ability of the disc to transfer and distribute loads between the vertebrae is decreased. Considering that hypoxia inducible factor-1α (HIF-1α) is abundantly expressed in NP and that it mediates cell proliferation, migration and apoptosis in various cell types, we hypothesized that NP-HIF-1α plays an important role in NP and evaluate whether NP-HIF-1α is involved in IVD degeneration.

MATERIAL AND METHODS

Sonic Hedgehog-Cre^+/- mice were crossed with HIF-1α^flox/flox mice to generate NP specific HIF-1α-deficient (HIF-1α^-/-) mice. Magnetic resonance imaging (MRI) study was used to evaluate NP dehydration and X-ray study was used to acquire the changes of disc height. Histological changese, content of glycoproteins and the in situ expression of aggrecan were evaluated by hematoxylin & eosin (H&E) staining, safranin-O/fast green staining and immunohistochemistry assay, respectively. Western bloting was used to detect the change of extracellular matrix in IVD.

RESULTS

Firstly, the results of in situ hybridization confirmed that HIF-1α in NP was successfully knocked out in HIF-1α^-/- mice. Next, for HIF-1α deficiency mice, imaging study shows IVD was narrowed in X-ray and signal intensity of NP was decreased in MR T2-weight imaging. Accordingly, the size and cell number of NP and proteoglycan content was decreased in NP-HIF-1α^-/- mice. Finally, Western bloting shows that protein level of collagen II and aggrecan, two main matrix in disc, were both decreased in NP-HIF-1α^-/- mice.

CONCLUSIONS

The present study demonstrates that HIF-1α is essential for NP development and homeostasis and the deficiency of NP-HIF-1α leads to IVD degeneration in mice.

A modular approach to creating large engineered cartilage surfaces

Native articular cartilage has limited capacity to repair itself from focal defects or osteoarthritis. Tissue engineering has provided a promising biological treatment strategy that is currently being evaluated in clinical trials. However, current approaches in translating these techniques to developing large engineered tissues remains a significant challenge. In this study, we present a method for developing large-scale engineered cartilage surfaces through modular fabrication. Modular Engineered Tissue Surfaces (METS) uses the well-known, but largely under-utilized self-adhesion properties of de novo tissue to create large scaffolds with nutrient channels. Compressive mechanical properties were evaluated throughout METS specimens, and the tensile mechanical strength of the bonds between attached constructs was evaluated over time. Raman spectroscopy, biochemical assays, and histology were performed to investigate matrix distribution. Results showed that by Day 14, stable connections had formed between the constructs in the METS samples. By Day 21, bonds were robust enough to form a rigid sheet and continued to increase in size and strength over time. Compressive mechanical properties and glycosaminoglycan (GAG) content of METS and individual constructs increased significantly over time. The METS technique builds on established tissue engineering accomplishments of developing constructs with GAG composition and compressive properties approaching native cartilage. This study demonstrated that modular fabrication is a viable technique for creating large-scale engineered cartilage, which can be broadly applied to many tissue engineering applications and construct geometries.

A Novel Method for Repeatable Failure Testing of Annulus Fibrosus

Tears in the annulus fibrosus (AF) of the intervertebral disk can result in disk herniation and progressive degeneration. Understanding AF failure mechanics is important as research moves toward developing biological repair strategies for herniated disks. Unfortunately, failure mechanics of fiber-reinforced tissues, particularly tissues with fibers oriented off-axis from the applied load, is not well understood, partly due to the high variability in reported mechanical properties and a lack of standard techniques ensuring repeatable failure behavior. Therefore, the objective of this study was to investigate the effectiveness of midlength (ML) notch geometries in producing repeatable and consistent tissue failure within the gauge region of AF mechanical test specimens. Finite element models (FEMs) representing several notch geometries were created to predict the location of bulk tissue failure using a local strain-based criterion. FEM results were validated by experimentally testing a subset of the modeled specimen geometries. Mechanical testing data agreed with model predictions (∼90% agreement), validating the model's predictive power. Two of the modified dog-bone geometries (“half” and “quarter”) effectively ensured tissue failure at the ML for specimens oriented along the circumferential-radial and circumferential-axial directions. The variance of measured mechanical properties was significantly lower for notched samples that failed at the ML, suggesting that ML notch geometries result in more consistent and reliable data. In addition, the approach developed in this study provides a framework for evaluating failure properties of other fiber-reinforced tissues, such as tendons and meniscus.

Hydrogel based cartilaginous tissue regeneration: recent insights and technologies

Hydrogel based technologies has been extensively employed in both exploratory research and clinical applications to address numerous existing challenges in the regeneration of articular cartilage and intervertebral disc.

Experimental Application of Bone Marrow Mesenchymal Stem Cells for the Repair of Intervertebral Disc Annulus Fibrosus

Background

This study provides experimental results on the applicability of bone marrow mesenchymal stem cells (BMSCs) for the repair of intervertebral disc annulus fibrosus in rabbits.

Material/Method

Thirty healthy rabbits were randomized into an observation group (n=15) and a control group (n=15). Both groups underwent degeneration of intervertebral disc annulus fibrosus. The observation group was treated with a solution of BMSCs and dexamethasone sodium phosphate, while the control group was treated with dexamethasone sodium phosphate only.

Results

The two groups were compared for efficacy and pathological conditions after treatment. Both disc height index and level of type II collagen in nucleus pulposus were significantly higher in the observation group than in the control group at 2, 4, 8, and 12 weeks after degeneration (p<0.05 for all comparisons). The percentages of grade 0 and grade 1 were significantly higher in the observation group than in the control group (p<0.05 for both grade 0 and 1 comparisons), while the percentage of grade 4 and grade 5 were significantly lower in the observation group than in the control group (p<0.05 for both grade 4 and 5 comparisons).

Conclusions

BMSCs cultured in vitro can effectively repair intervertebral disc annulus fibrosus, which is of positive significance, and thus is clinically recommended.

The effect of six degree of freedom loading sequence on the in-vitro compressive properties of human lumbar spine segments

The complex, direction-dependent, poro-viscoelastic properties of the intervertebral disc (disc) suggest that investigations of the six degree of freedom (6DOF) behaviour may be susceptible to inter-test variation in mechanical response if the disc does not return to initial conditions between loading directions. No studies have quantified the effects of sequential multi-directional loading on the consistency of the compressive response of the disc throughout a 6DOF testing protocol. Therefore, the objective of this study was to determine the effect of 6DOF loading on the compressive properties (stiffness and phase angle) of human discs, as evaluated by a reference compression test performed after each single DOF test. Fourteen intact human functional spinal units (FSU) were tested in each of ±6DOFs (shear directions followed by bending and compression) across four orders of magnitude loading frequencies (0.001-1Hz), followed by reference compression tests while subjected to physiological preload, hydration, and body temperature conditions in a hexapod robot. Repeated measures ANOVA revealed significant within-subjects effects between the reference compression tests for modulus (p<0.001), stiffness (p<0.001), and phase angle (p=0.008). Significant post-hoc pairwise comparisons were initially seen between the control and other reference compression tests for stiffness and modulus after the shear DOFs, however, no significant differences were present after the final reference compression test compared to control. More pronounced effects were seen for stiffness in comparison to modulus and phase angle. These effects may be due to three potentials factors, which include the sequence of testing, the cohort of degenerative specimens, and/or cumulative creep due to the constant application of a follower load. While the sequence of test directions was chosen to minimise the biphasic effect, there may be other sequences, which could result in minimal changes in compressive properties.

Autophagy: A double-edged sword in intervertebral disk degeneration

Autophagy is a homeostatic mechanism through which intracellular damaged organelles and proteins are degraded and recycled in response to increased metabolic demands or stresses. Although primarily cytoprotective, dysfunction of autophagy is often associated with many degenerative diseases, including intervertebral disc (IVD) degeneration (IDD). As a main contributing factor to low back pain, IDD is the pathological basis for various debilitating spinal diseases. Either higher or lower levels of autophagy are observed in degenerative IVD cells. Despite the precise role of autophagy in disc degeneration that is still controversial, with difference from protection to aggravation, targeting autophagy has shown promise for mitigating disc degeneration. In the current review, we summarize the changes of autophagy in degenerative IVD cells and mainly discuss the relationship between autophagy and IDD. With continued efforts, modulation of the autophagic process could be a potential and attractive therapeutic strategy for degenerative disc disease.