The ultra-structural organization of the elastic network in the intra- and inter-lamellar matrix of the intervertebral disc

The mechanical properties measurement could be affected by forceps: a technical note on sampling human annulus fibrosus

PURPOSE

To compare the mechanical properties of human annulus fibrosus obtained by forceps versus bistoury and observe whether the measurement could be affected by forceps sampling method.

METHODS

In this study, the mechanical properties of the the extracellular matrix (ECM) of human annulus fibrosus, including elastic modulus and stiffness, were investigated using atomic force microscope (AFM). Tissue was obtained from patients during operation using a bistoury or nucleus pulposus forceps. Tissues obtained with the nucleus pulposus forceps were considered as the forceps group and those obtained with a bistoury were considered as the bistoury group.

RESULTS

There was no significant difference observed between the forceps and bistoury group according to histological staining. The elastic modulus of the forceps group was 0.41 ± 0.08 MPa, and that of bistoury group was 0.53 ± 0.13 MPa, and the difference between the two groups was statistically significant (p < 0.05). The stiffness of the forceps group was 0.024 ± 0.003 N/m, and that of the bistoury group was 0.037 ± 0.003 N/m, and the difference between the two groups was statistically significant (p < 0.05).

CONCLUSION

The results indicate that the forceps sampling method has a substantial negative effect on the micromechanical properties of the ECM of the annulus fibrosus. Bistoury sampling method is recommended as the experimental subject for exploring the micromechanics mechanisms of cervical degenerative disease.

Biomechanics of annulus fibrosus: Elastic fiber simplification and degenerative impact on damage initiation and propagation

This study addresses three primary objectives related to lumbar intervertebral disc (IVD) biomechanics under ramping quasi-static loading conditions. First, we explore the conditions justifying the simplification of axisymmetric elastic fiber families into single fiber bundles through discretized strain energy functions. Simulations reveal that a concentration factor exceeding 10 allows for a consistent deviation below 10% between simplified and non-simplified responses. Second, we investigate the impact of elastic fibers on the physiological stiffness in IVDs, revealing minimal influence on biological motions but significant effects on degeneration. Lastly, we examine the initiation and progression of annulus fibrosus (AF) damage. Our findings confirm the validity of simplifying elastic fiber families and underscore the necessity of considering elastic fiber damage in biomechanical studies of AF tissues. Elastic fibers contribute to increased biaxial stretch stiffness, and their damage significantly affects the loading capacity of the inner AF. Additionally, degeneration significantly alters the susceptibility to damage in the AF, with specific regions exhibiting higher vulnerability. Damage tends to extend circumferentially and radially, emphasizing the regional variations in collagen and elastic fiber properties. This study offers useful insights for refining biomechanical models, paving the way for a more comprehensive understanding of IVD responses and potential clinical implications.

Role of intra-lamellar collagen and hyaluronan nanostructures in annulus fibrosus on lumbar spine biomechanics: insights from molecular mechanics-finite element-based multiscale analyses

Annulus fibrosus' (AF) ability to transmit multi-directional spinal motion is contributed by a combination of chemical interactions among biomolecular constituents-collagen type I (COL-I), collagen type II (COL-II), and proteoglycans (aggrecan and hyaluronan)-and mechanical interactions at multiple length scales. However, the mechanistic role of such interactions on spinal motion is unclear. The present work employs a molecular mechanics-finite element (FE) multiscale approach to investigate the mechanistic role of molecular-scale collagen and hyaluronan nanostructures in AF, on spinal motion. For this, an FE model of the lumbar segment is developed wherein a multiscale model of AF collagen fiber, developed from COL-I, COL-II, and hyaluronan using the molecular dynamics-cohesive finite element multiscale method, is incorporated. Analyses show AF collagen fibers primarily contribute to axial rotation (AR) motion, owing to angle-ply orientation. Maximum fiber strain values of 2.45% in AR, observed at the outer annulus, are 25% lower than the reported values. This indicates native collagen fibers are softer, attributed to the softer non-fibrillar matrix and higher interfibrillar sliding. Additionally, elastic zone stiffness of 8.61 Nm/° is observed to be 20% higher than the reported range, suggesting native AF lamellae exhibit lower stiffness, resulting from inter-collagen fiber bundle sliding. The presented study has further implications towards the hierarchy-driven designing of AF-substitute materials.

Mechanical consequences to the annulus fibrosus following rapid internal pressurization and endplate fracture under restrained-expansion conditions

Intervertebral disc herniation is not a common injury in the adolescent population, but the correlation between trauma and herniation warrants concern. Previous research demonstrated the capacity for rapid internal pressurization to reduce the mechanical integrity of the intervertebral disc's annulus fibrosus, even in the absence of fracture. The purpose of this study was to modify previous internal pressurization procedures towards a more transferable injury model, then investigate the capacity for these procedures to damage the mechanical integrity of the annulus fibrosus. Porcine cervical motion segments with intact facet joints were confined between a vice and force plate under 300 N of static compression, then a single, manual, rapid internal pressurization was delivered. Posterolateral annulus samples were extracted and situated in a 180° peel test configuration, exposing the interlamellar matrix of samples to separations of 0.5 mm/s, until complete separation of the sample occurred. Multilayer tensile testing was performed on superficial and mid-span samples of annulus by applying uniaxial tension of 1 %/s to 50 % strain. Compared to unpressurized controls, rapid pressurization causing fracture resulted in reduced lamellar adhesion and increased toe-region stress and strain properties in the annulus. Morphological assessment reported similar fracture patterns between endplate fractures achieved in the present experiment and endplate fractures documented in human patients. Mechanical plus morphological results suggest that rapid internal pressurization resulting in endplate fracture may represent a potent mechanism for subsequent damage to the intervertebral disc.

Comprehensive modeling of annulus fibrosus: From biphasic refined characterization to damage accumulation under viscous loading

The annulus fibrosus (AF), a permeable, hydrated, and fiber-reinforced soft tissue, exhibits complex responses influenced by fluid pressure, osmotic pressure, and structural mechanics. Existing models struggle to comprehensively represent these intricate interactions and the heterogeneous solid responses within the AF. Additionally, the mechanisms driving differential damage accumulation between non-degenerative and degenerative intervertebral discs remain poorly understood. In this study, we introduce a biphasic-swelling damage model for the AF. We conceptually develop and rigorously validate this model through tissue-level tests employing various loading modes, consistently aligning model predictions with experimental data. Leveraging parametric geometric algorithms and custom Python scripts, we construct models simulating both non-degenerative and degenerative discs. Following calibration, we subject these models to viscous loading protocols. Our findings reveal the posterior AF's susceptibility to damage, contingent upon loading rate and water content. We elucidate the underlying mechanisms by examining the temporal evolution of fluid pressure, osmotic pressure, and the regionally dependent fiber network. This research presents a highly accurate model of the AF, providing valuable insights into disc damage. Future research endeavors should expand this model to incorporate ionic transport and diffusion, enabling a more profound exploration of intervertebral disc mechanobiology. This comprehensive model contributes to a better understanding of AF behavior and may inform therapeutic strategies for disc-related pathologies. STATEMENT OF SIGNIFICANCE: This research presents a comprehensive model of the annulus fibrosus (AF), a crucial component of the intervertebral disc that provides structural support and resists deformation. The study introduces a biphasic-swelling damage model for the AF and validates it through tissue-level tests. The model accounts for fluid pressure, osmotic pressure, and matrix mechanics, providing a more accurate representation of the AF's behavior. The study also investigates the differential damage accumulation between non-degenerative and degenerative discs, shedding light on the mechanisms driving disc degeneration. The findings have significant implications for medical treatments and interventions, as they highlight the posterior AF's susceptibility to damage. This research is of great interest to readers interested in biomechanics, tissue engineering, and medical treatments for disc degeneration.

On the identification of the ultra-structural organization of elastic fibers and their effects on the integrity of annulus fibrosus

Due to the complicated structure of the elastic fiber network in annulus fibrosus, existing in-silico studies either simplified or just overlooked its distribution pattern. Nonetheless, experimental and simulation results have proven that elastic fibers are of great importance to maintaining the structural integrity of annulus fibrosus and therefore to ensuring the load-bearing ability of intervertebral discs. Such needs call for a fine model. This work aims at developing a biphasic annulus fibrosus model by incorporating the accurate distribution pattern of collagen and elastic fibers. Both the structural parameters and intrinsic mechanical parameters were successfully identified using single lamella and inter-lamella microscopy anatomy and micromechanical testing data. The proposed model was then used to implement finite element simulations on various anterior and posterolateral multi-lamellae annulus fibrosus specimens. In general, simulation results agree well with available experimental and simulation data. On this basis, the effects of elastic fibers on the integrity of annulus fibrosus were further investigated. It was found that elastic fibers significantly influence the free swelling, radial stretching and circumferential shear performances of annulus fibrosus. Nonetheless, no significant effects were found for the circumferential stretching capability. The proposed biphasic model considers for the first time the distribution characteristics of elastic fibers at two scales, including both the principal orientations of all fiber families and the detailed distribution pattern within each family. Better understandings on the functions of collagen and elastic fibers can therefore be realized. To further enhance its prediction capability, the current model can be extended in the future by taking the fiber-matrix interaction as well as progressive damages into consideration.

Structure-function characterization of the transition zone in the intervertebral disc

Understanding the structure-function relationship in the intervertebral disk (IVD) is crucial for the development of novel tissue engineering strategies to regenerate IVD and the establishment of accurate computational models for low back pain research. A large number of studies have improved our knowledge of the mechanical and structural properties of the nucleus pulposus (NP) and annulus fibrosus (AF), two of the main regions in the IVD. However, few studies have focused on the AF-NP interface (transition zone; TZ). Therefore, the current study aims to, for the first time, characterize the cyclic and failure mechanical properties of the TZ region under physiological loading (1, 3, and 5%s^-1 strain rates) and investigate the structural integration mechanisms between the NP, TZ, and AF regions. The results of the current study reveal significant effects of region (NP, TZ, and AF) and strain rates (1, 3, and 5%s^-1) on stiffness (p < 0.001). In addition, energy absorption is significantly higher for the AF compared to the TZ and NP (p <0.001) as well as between the TZ and NP (p <0.001). The current research finds adaptation, direct penetration, and entanglement between TZ and AF fibers as three common mechanisms for structural integration between the TZ and AF regions. STATEMENT OF SIGNIFICANCE: Despite a large number of studies that have mechanically, structurally, and biologically characterized the nucleus pulposus (NP) and annulus fibrosus (AF) regions, few studies have focused on the NP-AF interface region (known as Transition Zone; TZ) in the IVD; hence, our understanding of the TZ structure-function relationship is still incomplete. Of particular importance, the cyclic mechanical properties of the TZ, compared to the adjacent regions (NP and AF), are yet to be explored and the precise nature of the structural integration between the NP and AF via the TZ region is not yet known. The current study explores both the mechanical and structural properties of the TZ region to ultimately identify the mechanism of integration between the NP and AF.

A Microstructure-Based Mechanistic Approach to Detect Degeneration Effects on Potential Damage Zones and Morphology of Young and Old Human Intervertebral Discs

There is an increasing demand to develop predictive medicine through the creation of predictive models and digital twins of the different body organs. To obtain accurate predictions, real local microstructure, morphology changes and their accompanying physiological degenerative effects must be taken into account. In this article, we present a numerical model to estimate the long-term aging effect on the human intervertebral disc response by means of a microstructure-based mechanistic approach. It allows to monitor in-silico the variations in disc geometry and local mechanical fields induced by age-dependent long-term microstructure changes. Both lamellar and interlamellar zones of the disc annulus fibrosus are constitutively represented by considering the main underlying microstructure features in terms of proteoglycans network viscoelasticity, collagen network elasticity (along with content and orientation) and chemical-induced fluid transfer. With age, a noticeable increase in shear strain is especially observed in the posterior and lateral posterior regions of the annulus which is in correlation with the high vulnerability of elderly people to back problems and posterior disc hernia. Important insights about the relation between age-dependent microstructure features, disc mechanics and disc damage are revealed using the present approach. These numerical observations are hardly obtainable using current experimental technologies which makes our numerical tool useful for patient-specific long-term predictions.

Comparison of temporomandibular joint disc, meniscus, and intervertebral disc in fundamental characteristics and tissue engineering

The temporomandibular joint (TMJ) disc, meniscus and intervertebral disc (IVD) are three fibrocartilage discs, which play critical roles in our daily life. Their degeneration contributes to diseases such as TMJ disorders, osteoarthritis and degenerative disc disease, affecting patients' quality of life and causing substantial morbidity and mortality. Interestingly, similar in some aspects of fundamental characteristics, they exhibit differences in other aspects such as biomechanical properties. Highlighting these similarities and differences can not only benefit a comprehensive understanding of them and their pathology but also assist in future research of tissue engineering. Likewise, comparing their tissue engineering in cell sources, scaffold and stimuli can guide imitation and improvement of their engineered discs. However, the anatomical structure, function, and biomechanical characteristics of the IVD, TMJ, and Meniscus have not been compared in any meaningful depth needed to advance current tissue engineering research on these joints, resulting in incomplete understanding of them and their pathology and ultimately limiting future research of tissue engineering. This review, for the first time, comprehensively compares three fibrocartilage discs in those aspects to cast light on their similarities and differences.

Biomechanical effect of age-related structural changes on cervical intervertebral disc: A finite element study

Previous literature has investigated the biomechanical response of healthy and degenerative discs, but the biomechanical response of suboptimal healthy intervertebral discs received less attention. The purpose was to compare the biomechanical responses and risk of herniation of young healthy, suboptimal healthy, and degenerative intervertebral discs. A cervical spine model was established and validated using the finite element method. Suboptimal healthy, mildly, moderately, and severely degenerative disc models were developed. Disc height deformation, range of motion, intradiscal pressure, and von Mises stress in annulus fibrosus were analyzed by applying a moment of 4 Nm in flexion, extension, lateral bending, and axial rotation with 100 N compressive loads. Disc height deformation in young healthy, suboptimal healthy, mildly, moderately, and severely degenerative discs was 40%, 37%, 21%, 12%, and 8%, respectively. The decreasing order of the range of motion was young healthy spine > suboptimal healthy spine > mildly degenerative spine > moderately degenerative spine > severely degenerative spine. The mean stress of annulus ground substance in the suboptimal healthy disc was higher than in the young healthy disc. The mean stress of inter-lamellar matrix and annulus ground substance in moderately and severely degenerative discs was higher than in other discs. Age-related structural changes and degenerative changes increased the stiffness and reduced the elastic deformation of intervertebral discs. Decreased range of motion due to the effects of aging or degeneration on the intervertebral disc, may cause compensation of adjacent segments and lead to progressive degeneration of multiple segments. The effect of aging on the intervertebral disc increased the risk of annulus fibrosus damage from the biomechanical point of view. Moderately and severely degenerative discs may have a higher risk of herniation due to the higher risk of damage and layers separation of annulus fibrosus caused by increased stress in the annulus ground substance and inter-lamellar matrix.

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.

Detailed mechanical characterization of the transition zone: New insight into the integration between the annulus and nucleus of the intervertebral disc

The Nucleus Pulposus (NP) and Annulus Fibrous (AF) are two primary regions of the intervertebral disc (IVD). The interface between the AF and NP, where the gradual transition in structure and type of fibers are observed, is known as the Transition Zone (TZ). Recent structural studies have shown that the TZ contains organized fibers that appear to connect the NP to the AF. However, the mechanical characteristics of the TZ are yet to be explored. The current study aimed to investigate the mechanical properties of the TZ at the anterolateral (AL) and posterolateral (PL) regions in both radial and circumferential directions of loading using ovine IVDs (N = 28). Young's and toe moduli, maximum stress, failure strain, strain at maximum stress, and toughness were calculated mechanical parameters. The findings from this study revealed that the mechanical properties of the TZ, including young's modulus (p = 0.001), failure strain (p < 0.001), strain at maximum stress (p = 0.002), toughness (p = 0.027), and toe modulus (p = 0.005), were significantly lower for the PL compared to the AL region. Maximum stress was not significantly different between the PL and AL regions (p = 0.164). We found that maximum stress (p = 0.002), failure strain (p < 0.001), and toughness (p = 0.001) were significantly different in different loading directions. No significant differences for modulus (young's; p = 0.169 and toe; p = 0.352) and strain at maximum stress (p = 0.727) were found between the radial and circumferential loading directions. STATEMENT OF SIGNIFICANCE: To date there has not been a study that has investigated the mechanical characterization of the annulus (AF)-nucleus (NP) interface (transition zone; TZ) in the intervertebral disc (IVD), nor is it known whether the posterolateral (PL) and anterolateral (AL) regions of the TZ exhibit different mechanical properties. Accordingly, the TZ mechanical properties have been rarely used in the development of computational IVD models and relevant tissue-engineered scaffolds. The current research reported the mechanical properties of the TZ region and revealed that its mechanical properties were significantly lower for the PL compared to the AL region. These new findings enhance our knowledge about the nature of AF-NP integration and may help to develop more realistic tissue-engineered or computational IVD models.

Modeling multiaxial damage regional variation in human annulus fibrosus

In the present article, a fully three-dimensional human annulus fibrosus model is developed by considering the regional variation of the complex structural organization of collagen network at different scales to predict the regional anisotropic multiaxial damage of the intervertebral disc. The model parameters are identified using experimental data considering as elementary structural unit, the single annulus lamellae stretched till failure along the micro-sized collagen fibers. The multi-layered lamellar/inter-lamellar annulus model is constructed by considering the effective interactions between adjacent layers and the chemical-induced volumetric strain. The regional dependent model predictions are analyzed under various loading modes and compared to experimental data when available. The stretching along the circumferential and radial directions till failure serves to check the predictive capacities of the annulus model. Model results under simple shear, biaxial stretching and plane-strain compression are further presented and discussed. Finally, a full disc model is constructed using the regional annulus model and simulations are presented to assess the most likely failed areas under disc axial compression. STATEMENT OF SIGNIFICANCE: The damage in annulus soft tissues is a complex multiscale phenomenon due to a complex structural arrangement of collagen network at different scales of hierarchical organization. A fully three-dimensional constitutive representation that considers the regional variation of the structural complexity to estimate annulus multiaxial mechanics till failure has not yet been developed. Here, a model is developed to predict deformation-induced damage and failure of annulus under multiaxial loading histories considering as time-dependent physical process both chemical-induced volumetric effects and damage accumulation. After model identification using single lamellae extracted from different disc regions, the model predictability is verified for various multiaxial elementary loading modes representative of the spine movement. The heterogeneous mechanics of a full human disc model is finally presented.

Micro-mechanical damage of needle puncture on bovine annulus fibrosus fibrils studied using polarization-resolved Second Harmonic Generation(P-SHG) microscopy

Needle injection has been widely used in spinal therapeutic or diagnostic processes, such as discography. The use of needles has been suspected in causing mild disc degeneration which can lead to long-term back pain. However, the localised microscopic damage caused by needles has not been well studied. The local progressive damage on a microscopic level caused by needle punctures on the surface of bovine annulus fibrosus was investigated. Four different sizes of needle were used for the puncture and twenty-nine bovine intervertebral discs were studied. Polarization-resolved second harmonic generation and fluorescent microscopy were used to study the local microscopic structural changes in collagen and cell nuclei due to needle damage. Repeated 70 cyclic loadings at ±5% of axial strain were applied after the needle puncture in order to assess progressive damage caused by the needle. Puncture damage on annulus fibrosus were observed either collagen fibre bundles being pushed aside, being cut through or combination of both with part being lift or pushed in. The progressive damage was found less relevant to the needle size and more progressive damage was only observed using the larger needle. Two distinct populations of collagen, in which one was relatively more organised than the other population, were observed especially after the puncture from skewed distribution of polarization-SHG analysis. Cell shape was found rounder near the puncture site where collagen fibres were damaged.

Modeling of human intervertebral disc annulus fibrosus with complex multi-fiber networks

Collagen fibers within the annulus fibrosus (AF) lamellae are unidirectionally aligned with alternating orientations between adjacent layers. AF constitutive models often combine two adjacent lamellae into a single equivalent layer containing two fiber networks with a crisscross pattern. Additionally, AF models overlook the inter-lamellar matrix (ILM) as well as elastic fiber networks in between lamellae. We developed a nonhomogenous micromechanical model as well as two coarser homogenous hyperelastic and microplane models of the human AF, and compared their performances against measurements (tissue level uniaxial and biaxial tests as well as whole disc experiments) and seven published hyperelastic models. The micromechanical model had a realistic non-homogenous distribution of collagen fiber networks within each lamella and elastic fiber network in the ILM. For small matrix linear moduli (<0.2 MPa), the ILM showed substantial anisotropy (>10%) due to the elastic fiber network. However, at moduli >0.2 MPa, the effects of the elastic fiber network on differences in stress-strain responses at different directions disappeared (<10%). Variations in sample geometry and boundary conditions (due to uncertainty) markedly affected stress-strain responses of the tissue in uniaxial and biaxial tests (up to 16 times). In tissue level tests, therefore, simulations should represent testing conditions (e.g., boundary conditions, specimen geometry, preloads) as closely as possible. Stress/strain fields estimated from the single equivalent layer approach (conventional method) yielded different results from those predicted by the anatomically more accurate apparoach (i.e., layerwise). In addition, in a disc under a compressive force (symmetric loading), asymmetric stress-strain distributions were computed when using a layerwise simulation. Although all developed and selected published AF models predicted gross compression-displacement responses of the whole disc within the range of measured data, some showed excessively stiff or compliant responses under tissue-level uniaxial/biaxial tests. This study emphasizes, when constructing and validating constitutive models of AF, the importance of the proper simulation of individual lamellae as distinct layers, and testing parameters (sample geometric dimensions/loading/boundary conditions).

Interlamellar matrix governs human annulus fibrosus multiaxial behavior

Establishing accurate structure–property relationships for intervertebral disc annulus fibrosus tissue is a fundamental task for a reliable computer simulation of the human spine but needs excessive theoretical-numerical-experimental works. The difficulty emanates from multiaxiality and anisotropy of the tissue response along with regional dependency of a complex hierarchic structure interacting with the surrounding environment. We present a new and simple hybrid microstructure-based experimental/modeling strategy allowing adaptation of animal disc model to human one. The trans-species strategy requires solely the basic knowledge of the uniaxial circumferential response of two different animal disc regions to predict the multiaxial response of any human disc region. This work demonstrates for the first time the determining role of the interlamellar matrix connecting the fibers-reinforced lamellae in the disc multiaxial response. Our approach shows encouraging multiaxial predictive capabilities making it a promising tool for human spine long-term prediction.

The ultrastructural organization of elastic fibers at the interface of the nucleus and annulus of the intervertebral disk

There has been no study to describe the ultrastructural organization of elastic fibers at the interface of the nucleus pulposus and annulus fibrosus of the intervertebral disk (IVD), a region called the transition zone (TZ). A previously developed digestion technique was optimized to eliminate cells and non-elastin ECM components except for the elastic fibers from the anterolateral (AL) and posterolateral (PL) regions of the TZ in ovine IVDs. Not previously reported, the current study identified a complex elastic fiber network across the TZ for both AL and PL regions. In the AL region, this network consisted of major thick elastic fibers (≈ 1 µm) that were interconnected with delicate (< 200 nm) elastic fibers. While the same ultrastructural organization was observed in the PL region, interestingly the size of the elastic fibers was smaller (< 100 nm) compared to those that were located in the AL region. Quantitative analysis of the elastic fibers revealed significant differences in the size (p < 0.001) and the orientation of elastic fibers (p = 0.001) between the AL and PL regions, with a higher orientation and larger size of elastic fibers observed in the AL region. The gradual elimination of cells and non-elastin extracellular matrix components identified that elastic fibers in the TZ region in combination with the extracellular matrix created a honeycomb structure that was more compact at the AF interface compared to that located close to the NP. Three different symmetrically organized angles of rotation (0⁰ and ±90⁰) were detected for the honeycomb structure at both interfaces, and the structure was significantly orientated at the TZ-AF compared to the TZ-NP interface (p = 0.003).

Elastic fibers: The missing key to improve engineering concepts for reconstruction of the Nucleus Pulposus in the intervertebral disc

The increasing prevalence of low back pain has imposed a heavy economic burden on global healthcare systems. Intense research activities have been performed for the regeneration of the Nucleus Pulposus (NP) of the IVD; however, tissue-engineered scaffolds have failed to capture the multi-scale structural hierarchy of the native tissue. The current study revealed for the first time, that elastic fibers form a network across the NP consisting of straight and thick parallel fibers that were interconnected by wavy fine fibers and strands. Both straight fibers and twisted strands were regularly merged or branched to form a fine elastic network across the NP. As a key structural feature, ultrathin (53 ± 7 nm), thin (215 ± 20 nm), and thick (890 ± 12 nm) elastic fibers were observed in the NP. While our quantitative analysis for measurement of the thickness of elastic fibers revealed no significant differences (p < 0.633), the preferential orientation of fibers was found to be significantly different (p < 0.001) across the NP. The distribution of orientation for the elastic fibers in the NP represented one major organized angle of orientation except for the central NP. We found that the distribution of elastic fibers in the central NP was different from those located in the peripheral regions representing two symmetrically organized major peaks (±45⁰). No significant differences in the maximum fiber count at the major angles of orientation (±45⁰) were observed for both peripheral (p = 0.427) and central NP (p = 0.788). Based on these new findings a structural model for the elastic fibers in the NP was proposed. The geometrical presentation, along with the distribution of elastic fibers orientation, resulting from the present study identifies the ultrastructural organization of elastic fibers in the NP important towards understanding their mechanical role which is still under investigation. Given the results of this new geometrical analysis, more-accurate multiscale finite element models can now be developed, which will provide new insights into the mechanobiology of the IVD. In addition, the results of this study can potentially be used for the fabrication of bio-inspired tissue-engineered scaffolds and IVD models to truly capture the multi-scale structural hierarchy of IVDs. STATEMENT OF SIGNIFICANCE: Visualization of elastic fibers in the nucleus of the intervertebral disk under high magnification was not reported before. The present research utilized extracellular matrix partial digestion to address significant gaps in understanding of nucleus microstructure that can potentially be used for the fabrication of bio-inspired tissue-engineered scaffolds and disk models to truly capture the multi-scale structural hierarchy of discs.

Advanced Strategies for the Regeneration of Lumbar Disc Annulus Fibrosus

Damage to the annulus fibrosus (AF), the outer region of the intervertebral disc (IVD), results in an undesirable condition that may accelerate IVD degeneration causing low back pain. Despite intense research interest, attempts to regenerate the IVD have failed so far and no effective strategy has translated into a successful clinical outcome. Of particular significance, the failure of strategies to repair the AF has been a major drawback in the regeneration of IVD and nucleus replacement. It is unlikely to secure regenerative mediators (cells, genes, and biomolecules) and artificial nucleus materials after injection with an unsealed AF, as IVD is exposed to significant load and large deformation during daily activities. The AF defects strongly change the mechanical properties of the IVD and activate catabolic routes that are responsible for accelerating IVD degeneration. Therefore, there is a strong need to develop effective therapeutic strategies to prevent or reconstruct AF damage to support operational IVD regenerative strategies and nucleus replacement. By the way of this review, repair and regenerative strategies for AF reconstruction, their current status, challenges ahead, and future outlooks were discussed.

MicroRNAs in Intervertebral Disc Degeneration, Apoptosis, Inflammation, and Mechanobiology

Intervertebral disc (IVD) degeneration is a multifactorial pathological process associated with low back pain, the leading cause of years lived in disability worldwide. Key characteristics of the pathological changes connected with degenerative disc disease (DDD) are the degradation of the extracellular matrix (ECM), apoptosis and senescence, as well as inflammation. The impact of nonphysiological mechanical stresses on IVD degeneration and inflammation, the mechanisms of mechanotransduction, and the role of mechanosensitive miRNAs are of increasing interest. As post-transcriptional regulators, miRNAs are known to affect the expression of 30% of proteincoding genes and numerous intracellular processes. The dysregulation of miRNAs is therefore associated with various pathologies, including degenerative diseases such as DDD. This review aims to give an overview of the current status of miRNA research in degenerative disc pathology, with a special focus on the involvement of miRNAs in ECM degradation, apoptosis, and inflammation, as well as mechanobiology.

Effect of aggrecan degradation on the nanomechanics of hyaluronan in extra-fibrillar matrix of annulus fibrosus: A molecular dynamics investigation

Intervertebral Disc (IVD) Degeneration is one of the primary causes of low back pain among the adult population - the most significant cause being the degradation of aggrecan present in the extra-fibrillar matrix (EFM). Aggrecan degradation is closely associated with loss of water content leading to an alteration in the mechanical behaviour of the IVD. The loss in water content has a significant impact on the chemo-mechanical interplay of IVD biochemical constituents at the fundamental level. This work presents a mechanistic understanding of the effect of hydration, closely associated with aggrecan degradation, on the nanoscale mechanical behaviour of the hyaluronan present in the EFM of the Annulus Fibrosus. For this purpose, explicit three-dimensional molecular dynamics analyses of tensile and compressive tests are performed on a representative atomistic model of the hyaluronan present in the EFM. To account for the degradation of aggrecan, hydration levels are varied from 0 to 75% by weight of water. Analyses show that an increase in the hydration levels decreases the elastic modulus of hyaluronan in tension from ~4.6 GPa to ~2.1 GPa. On the other hand, the increase in hydration level increases the elastic moduli in axial compression from ~1.6 GPa in un-hydrated condition to ~6 GPa in 50% hydrated condition. But as the hydration levels increase to 75%, the elastic modulus reduces to ~3.5 GPa signifying a shift in load-bearing characteristic, from the solid hyaluronan component to the fluid component. Furthermore, analyses show a reduction in the intermolecular energy between hyaluronan and water, under axial tensile loading, indicating a nanoscale intermolecular debonding between hyaluronan and water molecules. This is attributed to the ability of hyaluronan to form stabilizing intra-molecular hydrogen bonds between adjacent residues. Compressive loading, on the other hand, causes intensive coiling of hyaluronan molecule, which traps more water through hydrogen bonding and aids in bearing compressive loads. Overall, study shows that hydration level has a strong influence on the atomistic level interactions between hyaluronan molecules and hyaluronan and water molecules in the EFM which influences the nanoscale mechanics of the Annulus Fibrosus.

Effect of the CCL5-Releasing Fibrin Gel for Intervertebral Disc Regeneration

OBJECTIVE

To explore if chemokine (C-C motif) ligand 5 (CCL5) delivery could recruit annulus fibrosus (AF) cells to the injury sites and facilitate the repair of ruptured AF.

DESIGN

The effects of CCL5 on bovine AF cells in vitro were tested by transwell assay and quantitative real-time polymerase chain reaction. Fibrin gel containing CCL5 was used to treat annulotomized bovine caudal discs cultured under dynamic loading conditions. After 14 days of loading, the samples were collected for histological examination. A pilot animal study was performed using sheep cervical discs to investigate the effect of fibrin gel encapsulated with CCL5 for the treatment of ruptured AF. After 14 weeks, the animals were sacrificed, and the discs were scanned with magnetic resonance imaging before histopathological examination.

RESULTS

CCL5 showed a chemotactic effect on AF cells in a dose-dependent manner. AF cells cultured with CCL5 in vitro did not show any change of the gene expression of CCL5 receptors, catabolic and proinflammatory markers. In vitro release study showed that CCL5 exhibited sustained release from the fibrin gel into the culture media; however, in the organ culture study CCL5 did not stimulate homing of AF cells toward the defect sites. The pilot animal study did not show any repair effect of CCL5.

CONCLUSIONS

CCL5 has a chemotactic effect on AF cells in vitro, but no ex vivo or in vivo regenerative effect when delivered within fibrin gel. Further study with a stronger chemotactic agent and/or an alternate biomaterial that is more conductive of cell migration is warranted.

Interlamellar-induced time-dependent response of intervertebral disc annulus: A microstructure-based chemo-viscoelastic model

The annulus fibrosus of the intervertebral disc exhibits an unusual transversal behavior for which a constitutive representation that considers as well regional effect, chemical sensitivity and time-dependency has not yet been developed, and it is hence the aim of the present contribution. A physically-based model is proposed by introducing a free energy function that takes into account the actual disc annulus structure in relation with the surrounding biochemical environment. The response is assumed to be dominated by the viscoelastic contribution of the extracellular matrix, the elastic contribution of the oriented collagen fibers and the osmo-induced volumetric contribution of the internal fluid content variation. The regional dependence of the disc annulus response due to variation in fibers content/orientation allows a micromechanical treatment of the soft tissue. A finite element model of the annulus specimen is designed while taking into consideration the 'interlamellar' ground substance zone between lamellae of the layered soft tissue. The kinetics is designed using full-field strain measurements performed on specimens extracted from two disc annulus regions and tested under different osmotic conditions. The time-dependency of the tissue response is reported on stress-free volumetric changes, on hysteretic stress and transversal strains during quasi-static stretching at different strain-rates and on their temporal changes during an interrupted stretching. Considering the effective contributions of the internal fluid transfer and the extracellular matrix viscosity, the microstructure-based chemo-mechanical model is found able to successfully reproduce the significant features of the macro-response and the unusual transversal behavior including the strong regional dependency from inner to outer parts of the disc: Poisson's ratio lesser than 0 (auxetic) in lamellae plane, higher than 0.5 in fibers plane, and their temporal changes towards usual values (between 0 and 0.5) at chemo-mechanical equilibrium. The underlying time-dependent mechanisms occurring in the tissue are analyzed via the local numerical fields and important insights about the effective role of the interlamellar zone are revealed for the different disc localizations. STATEMENT OF SIGNIFICANCE: The structural complexity of the annulus fibrosus has only been appreciated through recent experimental contributions and a constitutive representation that considers as well regional effect, chemical sensitivity and time-dependency of the unusual transversal behavior has not yet been developed. Here, a microstructure-based chemo-viscoelastic model is developed to highlight the interlamellar-induced time-dependent response by means of a two-scale strategy. The model provides important insights about the origin of the time-dependent phenomena in disc annulus along with regional dependency, essential for understanding disc functionality.

Synchrotron tomography of intervertebral disc deformation quantified by digital volume correlation reveals microstructural influence on strain patterns

The intervertebral disc (IVD) has a complex and multiscale extracellular matrix structure which provides unique mechanical properties to withstand physiological loading. Low back pain has been linked to degeneration of the disc but reparative treatments are not currently available. Characterising the disc's 3D microstructure and its response in a physiologically relevant loading environment is required to improve understanding of degeneration and to develop new reparative treatments. In this study, techniques for imaging the native IVD, measuring internal deformation and mapping volumetric strain were applied to an in situ compressed ex vivo rat lumbar spine segment. Synchrotron X-ray micro-tomography (synchrotron CT) was used to resolve IVD structures at microscale resolution. These image data enabled 3D quantification of collagen bundle orientation and measurement of local displacement in the annulus fibrosus between sequential scans using digital volume correlation (DVC). The volumetric strain mapped from synchrotron CT provided a detailed insight into the micromechanics of native IVD tissue. The DVC findings showed that there was no slipping at lamella boundaries, and local strain patterns were of a similar distribution to the previously reported elastic network with some heterogeneous areas and maximum strain direction aligned with bundle orientation, suggesting bundle stretching and sliding. This method has the potential to bridge the gap between measures of macro-mechanical properties and the local 3D micro-mechanical environment experienced by cells. This is the first evaluation of strain at the micro scale level in the intact IVD and provides a quantitative framework for future IVD degeneration mechanics studies and testing of tissue engineered IVD replacements. STATEMENT OF SIGNIFICANCE: Synchrotron in-line phase contrast X-ray tomography provided the first visualisation of native intact intervertebral disc microstructural deformation in 3D. For two annulus fibrosus volumes of interest, collagen bundle orientation was quantified and local displacement mapped as strain. Direct evidence of microstructural influence on strain patterns could be seen such as no slipping at lamellae boundaries and maximum strain direction aligned with collagen bundle orientation. Although disc elastic structures were not directly observed, the strain patterns had a similar distribution to the previously reported elastic network. This study presents technical advances and is a basis for future X-ray microscopy, structural quantification and digital volume correlation strain analysis of soft tissue.

The Mechanical Role of the Radial Fiber Network Within the Annulus Fibrosus of the Lumbar Intervertebral Disc: A Finite Elements Study

The annulus fibrosus (AF) of the intervertebral disc (IVD) consists of a set of concentric layers composed of a primary circumferential collagen fibers arranged in an alternating oblique orientation. Moreover, there exists an additional secondary set of radial translamellar collagen fibers which connects the concentric layers, creating an interconnected fiber network. The aim of this study was to investigate the mechanical role of the radial fiber network. Toward that goal, a three-dimensional (3D) finite element model of the L3–L4 spinal segment was generated and calibrated to axial compression and pure moment loading. The AF model explicitly recognizes the two heterogeneous networks of fibers. The presence of radial fibers demonstrated a pronounced effect on the local disc responses under lateral bending, flexion, and extension modes. In these modes, the radial fibers were in a tensile state in the disc region that subjected to compression. In addition, the circumferential fibers, on the opposite side of the IVD, were also under tension. The local stress in the matrix was decreased in up to 9% in the radial fibers presence. This implies an active fiber network acting collectively to reduce the stresses and strains in the AF lamellae. Moreover, a reduction of 26.6% in the matrix sideways expansion was seen in the presence of the radial fibers near the neutral bending axis of the disc. The proposed biomechanical model provided a new insight into the mechanical role of the radial collagen fibers in the AF structure. This model can assist in the design of future IVD substitutes.

New insights into the viscoelastic and failure mechanical properties of the elastic fiber network of the inter-lamellar matrix in the annulus fibrosus of the disc

The mechanical role of elastic fibers in the inter-lamellar matrix (ILM) is unknown; however, it has been suggested that they play a role in providing structural integrity to the annulus fibrosus (AF). Therefore, the aim of this study was to measure the viscoelastic and failure properties of the elastic fiber network in the ILM of ovine discs under both tension and shear directions of loading. Utilizing a technique, isolated elastic fibers within the ILM from ovine discs were stretched to 40% of their initial length at three strain rates of 0.1% s^-1 (slow), 1% s^-1 (medium) and 10% s^-1 (fast), followed by a ramp test to failure at 10% s^-1. A significant strain-rate dependent response was found, particularly at the fastest rate for phase angle and normalized stiffness (p < 0.001). The elastic fibers in the ILM demonstrated a significantly higher capability for energy absorption at slow compared to medium and fast strain rates (p < 0.001). These finding suggests that the elastic fiber network of the ILM exhibits nonlinear elastic behavior. When tested to failure, a significantly higher normalized failure force was found in tension compared to shear loading (p = 0.011), which is consistent with the orthotropic structure of elastic fibers in the ILM. The results of this study confirmed the mechanical contribution of the elastic fiber network to the ILM and the structural integrity of the AF. This research serves as a foundation for future studies to investigate the relationship between degeneration and ILM mechanical properties.

STATEMENT OF SIGNIFICANCE

The mechanical role of elastic fibres in the inter-lamellar matrix (ILM) of the disc is unknown. The viscoelastic and failure properties of the elastic fibre network in the ILM in both tension and shear directions of loading was measured for the first time. We found a strain-rate dependent response for the elastic fibres in the ILM. The elastic fibres in the ILM demonstrated a significantly higher capability for energy absorption at slow compared to medium and fast strain rates. When tested to failure, a significantly higher normalized failure force was found in tension compared to shear loading, which is consistent with the orthotropic structure of elastic fibres in the ILM.

New findings confirm the viscoelastic behaviour of the inter-lamellar matrix of the disc annulus fibrosus in radial and circumferential directions of loading

While few studies have improved our understanding of composition and organization of elastic fibres in the inter-lamellar matrix (ILM), its clinical relevance is not fully understood. Moreover, no studies have measured the direct tensile and shear failure and viscoelastic properties of the ILM. Therefore, the aim of this study was, for the first time, to measure the viscoelastic and failure properties of the ILM in both the tension and shear directions of loading. Using an ovine model, isolated ILM samples were stretched to 40% of their initial length at three strain rates of 0.1%s^-1 (slow), 1%s^-1 (medium) and 10%s^-1 (fast) and a ramp test to failure was performed at a strain rate of 10%s^-1. The findings from this study identified that the stiffness of the ILM was significantly larger at faster strain rates, and energy absorption significantly smaller, compared to slower strain rates, and the viscoelastic and failure properties were not significantly different under tension and shear loading. We found a strain rate dependent response of the ILM during dynamic loading, particularly at the fastest rate. The ILM demonstrated a significantly higher capability for energy absorption at slow strain rates compared to medium and fast strain rates. A significant increase in modulus was found in both loading directions and all strain rates, having a trend of larger modulus in tension and at faster strain rates. The finding of no significant difference in failure properties in both loading directions, was consistent with our previous ultra-structural studies that revealed a well-organized (±45°) elastic fibre orientation in the ILM. The results from this study can be used to develop and validate finite element models of the AF at the tissue scale, as well as providing new strategies for fabricating tissue engineered scaffolds.

STATEMENT OF SIGNIFICANCE

While few studies have improved our understanding of composition and organization of elastic fibres in the inter-lamellar matrix (ILM) of the annulus in the disc no studies have measured the direct mechanical failure and viscoelastic properties of the ILM. The findings from this study identified that the stiffness of the ILM was significantly larger at faster strain rates, and energy absorption significantly smaller, compared to slower strain rates. The failure properties of the ILM were not significantly different under tension and shear.

Ultrastructural organization of elastic fibres in the partition boundaries of the annulus fibrosus within the intervertebral disc

The relationship between elastic fibre disorders and disc degeneration, aging and progression of spine deformity have been discussed in a small number of studies. However, the clinical relevance of elastic fibres in the annulus fibrosus (AF) of the disc is poorly understood. Ultrastructural visualization of elastic fibres is an important step towards understanding their structure-function relationship. In our previous studies, a novel technique for visualization of elastic fibres across the AF was presented and their ultrastructural organization in intra- and inter-lamellar regions was compared. Using the same novel technique in the present study, the ultrastructural organization of elastic fibres in the partition boundaries (PBs), which are located between adjacent collagen bundles, is presented for the first time. Visualization of elastic fibres in the PBs in control and partially digested (digested) samples was compared, and their orientation in two different cutting planes (transverse and oblique) were discussed. The ultrastructural analysis revealed that elastic fibres in PBs were a well-organized dense and complex network having different size and shape. Adjacent collagen bundles in a cross section (CS) lamella appear to be connected to each other, where elastic fibres in the PBs were merged in parallel or penetrated into the collagen bundles. There was no significant difference in directional coherency coefficient of elastic fibres between the two different cutting planes (p = .35). The present study revealed that a continuous network of elastic fibres may provide disc integrity by connecting adjacent bundles of CS lamellae together. Compared to our previous studies, the density of the elastic fibre network in PBs was lower, and fibre orientation was similar to the intra-lamellar space and inter-lamellar matrix.

STATEMENT OF SIGNIFICANCE

A detailed ultrastructural study in the partition boundaries of the annulus fibrosus within the disc revealed a well-organized elastic fibre network with a complex ultrastructure. The continuous network of elastic fibres may provide disc integrity by connecting adjacent bundles of cross section lamellae together. The density of the elastic fibre network in PBs was lower, and fibre orientation was similar to the intra-lamellar space and the inter-lamellar matrix.