Intervertebral Disc Model Research Articles

Needle-scalpel therapy inhibits the apoptosis of nucleus pulposus cells via the SDF-1/CXCR4 axis in a rat degenerative cervical intervertebral disc model.

As a common disease, cervical spondylosis (CS) results from the degeneration of the cervical intervertebral disc. However, there are still no effective clinical strategies for the treatment of this disease. Needle-scalpel (Ns), a therapy guided by traditional Chinese medicine theory, alleviates intervertebral disc degradation and is widely used in the clinic to treat CS. Stromal cell-derived factor-1 (SDF-1) and its receptor CXC receptor 4 (CXCR4) in nucleus pulposus cells play an important role in CS onset and development. This study aimed to explore whether Ns can relieve pain and regulate the SDF-1/CXCR4 axis in nucleus pulposus cells to inhibit apoptosis, thereby delaying cervical intervertebral disc degradation in a rat model of CS. It was found that the Ns-treated groups exhibited higher mechanical allodynia scores than the model group, and H&E staining, MRI, and scanning electron microscopy revealed that Ns therapy inhibited intervertebral disc degeneration. Additionally, Ns therapy significantly inhibited increases in the RNA and protein expression levels of SDF-1 and CXCR4. Furthermore, these treatments alleviated the apoptosis of nucleus pulposus cells, which manifested as a decline in the proportion of apoptotic nucleus pulposus cells and inhibition of the decrease in the levels of Bcl-2/Bax. These findings indicated that Ns mitigated CS-induced pain, inhibited the apoptosis of nucleus pulposus cells, and alleviated intervertebral disc degeneration in CS rats. These effects may be mediated by specifically regulating the SDF-1/CXCR4 signaling axis. Based on these findings, we conclude that Ns might serve as a promising therapy for the treatment of CS.

Effect of compressive and tensile forces on glucose concentration and cell viability within the intervertebral disc: A finite element study

Understanding the role of mechanical force on tissue nutrient transport is essential, as sustained force may affect nutrient levels within the disc and initiate disc degeneration. This study aims to evaluate the time-dependent effects of different compressive force amplitudes as well as tensile force on glucose concentration and cell viability within the disc. Based on the mechano-electrochemical mixture theory, a multiphasic finite element model of the lumbar intervertebral disc was developed. The minimum glucose concentration and minimum cell density in both normal and degenerated discs were predicted for different compressive force amplitudes, tensile force, and corresponding creep time. Under high compressive force, the minimum glucose concentration exhibited an increasing and then decreasing trend with creep time in the normal disc, whereas that of the degenerated disc increased, then decreased, and finally increased again. At steady state, a higher compressive force was accompanied by a lower glucose concentration distribution. In the degenerated disc, the minimum cell density was negatively correlated with creep time, with a greater range of affected tissue under a higher compressive force. For tensile force, the minimum glucose concentration of the degenerated disc raised over time. This study highlighted the importance of creep time, force magnitude, and force type in affecting nutrient concentration and cell viability. Sustained weight-bearing activities could deteriorate the nutrient environment of the degenerated disc, while tensile force might have a nonnegligible role in effectively improving nutrient levels within the degenerated disc.

Targeted multifidus muscle activation reduces fibrosis of multifidus muscle following intervertebral disc injury

PurposeAerobic exercise produces beneficial outcomes in patients with low back pain and partially attenuates the fibrotic changes to the multifidus in a model of intervertebral disc (IVD) degeneration. More targeted exercise might be required to fully attenuate these fibrotic alterations. This study aimed to investigate whether activation of the multifidus induced by neurostimulation could reduce fibrosis of the multifidus in a model of IVD degeneration in sheep.MethodsIVD degeneration was induced in 18 merino sheep via a partial thickness unilateral annulus fibrosus lesion to the L1/2 and L3/4 IVDs. All sheep received an implantable neurostimulation device that provides stimulation of the L2 medial branch of the dorsal ramus. Three months after surgery, the animals were assigned to Injury or Activated groups. Activated animals received neurostimulation and the Injury group received no stimulation. Six months after surgery, the multifidus was harvested at L2 and L4. Van Gieson’s, Sirius Red and immunofluorescence staining for Collagen-I and -III and quantitative PCR was used to examine fibrosis. Muscle harvested from a previous study without IVD injury was used as a control.ResultsNeurostimulation of the multifidus attenuated IVD degeneration dependent increases in the connective tissue, including Collagen-I but not Collagen-III, compared to the Injury group at L4. No measures of the multifidus muscle at L2, which received no stimulation, differed between the Injury and Activated groups.ConclusionsThese data reveal that targeted activation of the multifidus muscle attenuates IVD degeneration dependent fibrotic alterations to the multifidus.

SIMULATION OF PHYSIOLOGICAL AND DETRIMENTAL LOADING IN WHOLE INTERVERTEBRAL DISC ORGAN MODELS

Mechanical loading is important to maintain the homeostasis of the intervertebral disc (IVD) under physiological conditions but can also accelerate cell death and tissue breakdown in a degenerative state. Bioreactor loaded whole organ cultures are instrumental for investigating the effects of the mechanical environment on the IVD integrity and for preclinical testing of new therapies under simulated physiological conditions. Thereby the loading parameters that determine the beneficial or detrimental reactions largely depend on the IVD model and its preparation. Within this symposium we are discussing the use of bovine caudal IVD culture models to reproduce tissue inflammation or matrix degradation with or without bioreactor controlled mechanical loading. Furthermore, the outcome parameters that define the degenerative state of the whole IVD model will be outlined. Besides the disc height, matrix integrity, cell viability and phenotype expression, the tissue secretome can provide indications about potential interactions of the IVD with other cell types such as neurons. Finally, a novel multiaxial bioreactor setup capable of mimicking the six degrees-of-freedom loading environment of IVDs will be introduced that further advances the relevance of preclinical ex-vivo testing.

ADVANCED BIOREACTOR STUDIES OF REGION-SPECIFIC RESPONSE IN THE INTERVERTEBRAL DISC TO COMPRESSION, FLEXION/EXTENSION, AND TORSION

Intervertebral disc (IVD) degeneration is inadequately understood due to the lack of in vitro systems that fully mimic the mechanical and biological complexity of this organ. We have recently made an advancement by developing a bioreactor able to simulate physiological, multiaxial IVD loading and maintain the biological environment in ex vivo IVD models [1].To validate this new bioreactor system, we simulated natural spine movement by loading 12 bovine IVDs under a combination of static compression (0.1 MPa), cyclic flexion/extension (±3˚, ±6˚ or 0-6˚) and cyclic torsion (±2˚, ±4˚ or 0-4˚) for more than 10’000 (0.2 Hz) or 100’000 (1 Hz) cycles over 14 days. A higher number of cycles increased the release of glycosaminoglycans and nitric oxide, as an inflammation marker, whereas fewer cycles maintained these two factors at physiological levels. All applied protocols upregulated the expression of MMP13 in the outermost annulus fibrosus (AF), indicating a collagen degradation response. This was supported by fissures observed in the AF after a longer loading duration. Increasing loading cycles induced high cell death in the nucleus pulposus and inner AF, while with fewer cycles, high cell viability was maintained in all IVD regions, irrespective of the magnitude of rotation.Less frequent multiaxial loading maintains IVD homeostasis while more frequent loading initiates an IVD degenerative profile. Specifically, the morphological and molecular changes were localized in the AF, which can be associated with combined flexion/extension and torsion. More loading cycles induced region-specific cell death and a higher release of extracellular matrix molecules from the innermost IVD regions, likely associated with longer exposure to static compression. Altogether, we demonstrated the advantages of the multiaxial bioreactor to study region-specific response in the IVD, which will allow a more profound investigation of IVD degeneration under different combinations of motions.

Intervertebral disc generation tissue engineering treatment platform: gelatin-methacryloyl hydrogel composite system.

Intervertebral disc generation tissue engineering treatment platform: gelatin-methacryloyl hydrogel composite system.

Intervertebral disc-on-a-chip: a precision engineered toolbox for low back pain studies

Current in vitro intervertebral disc (IVD) models do not fully recapitulate the complex mechanobiology of native tissue, and so far there is no strategy to effectively evaluate IVD regeneration. The development of a modular microfluidic on-chip model is expected to enhance the physiological relevance of experimental data leading to successful clinical outcomes.

The role of biomechanical factors in models of intervertebral disc degeneration across multiple length scales.

Low back pain is the leading cause of disability, producing a substantial socio-economic burden on healthcare systems worldwide. Intervertebral disc (IVD) degeneration is a primary cause of lower back pain, and while regenerative therapies aimed at full functional recovery of the disc have been developed in recent years, no commercially available, approved devices or therapies for the regeneration of the IVD currently exist. In the development of these new approaches, numerous models for mechanical stimulation and preclinical assessment, including in vitro cell studies using microfluidics, ex vivo organ studies coupled with bioreactors and mechanical testing rigs, and in vivo testing in a variety of large and small animals, have emerged. These approaches have provided different capabilities, certainly improving the preclinical evaluation of these regenerative therapies, but challenges within the research environment, and compromises relating to non-representative mechanical stimulation and unrealistic test conditions, remain to be resolved. In this review, insights into the ideal characteristics of a disc model for the testing of IVD regenerative approaches are first assessed. Key learnings from in vivo, ex vivo, and in vitro IVD models under mechanical loading stimulation to date are presented alongside the merits and limitations of each model based on the physiological resemblance to the human IVD environment (biological and mechanical) as well as the possible feedback and output measurements for each approach. When moving from simplified in vitro models to ex vivo and in vivo approaches, the complexity increases resulting in less controllable models but providing a better representation of the physiological environment. Although cost, time, and ethical constraints are dependent on each approach, they escalate with the model complexity. These constraints are discussed and weighted as part of the characteristics of each model.

Biomechanical consequences of the intervertebral disc centre of rotation kinematics during lateral bending and axial rotation

The location of the instantaneous centre of rotation (ICR) of a lumbar unit has a considerable clinical importance as a spinal health estimator. Consequently, many studies have been conducted to measure or estimate the ICR during rotations in the three anatomical planes; however the results reported are widely scattered. Even if some inter-subjects variability is to be expected, such inconsistencies are likely explained by the differences in methods and experiments. Therefore, in this paper we seek to model three behaviours of the ICR during lateral bending and axial rotation based on results published in the literature. In order to assess the metabolic and mechanical sensibility to the assumption made on the ICR kinematics, we used a previously validated three dimensional non-linear poroelastic model of a porcine intervertebral disc to simulate physiological lateral and axial rotations. The impact of the geometry was also briefly investigated by considering a 11^circ wedge angle. From our simulations, it appears that the hypothesis made on the ICR location does not significantly affect the critical nutrients concentrations but gives disparate predictions of the intradiscal pressure at the centre of the disc (variation up to 0.7 MPa) and of the displacement fields (variation up to 0.4 mm). On the contrary, the wedge angle does not influence the estimated intradiscal pressure but leads to minimal oxygen concentration decreased up to 33% and increased maximal lactate concentration up to 13%. While we can not settle on which definition of the ICR is more accurate, this work suggests that patient-specific modeling of the ICR is required and brings new insights that can be useful for the development of new tools or the design of surgical material such as total lumbar disc prostheses.

An update on animal models of intervertebral disc degeneration and low back pain: Exploring the potential of artificial intelligence to improve research analysis and development of prospective therapeutics.

Animal models have been invaluable in the identification of molecular events occurring in and contributing to intervertebral disc (IVD) degeneration and important therapeutic targets have been identified. Some outstanding animal models (murine, ovine, chondrodystrophoid canine) have been identified with their own strengths and weaknesses. The llama/alpaca, horse and kangaroo have emerged as new large species for IVD studies, and only time will tell if they will surpass the utility of existing models. The complexity of IVD degeneration poses difficulties in the selection of the most appropriate molecular target of many potential candidates, to focus on in the formulation of strategies to effect disc repair and regeneration. It may well be that many therapeutic objectives should be targeted simultaneously to effect a favorable outcome in human IVD degeneration. Use of animal models in isolation will not allow resolution of this complex issue and a paradigm shift and adoption of new methodologies is required to provide the next step forward in the determination of an effective repairative strategy for the IVD. AI has improved the accuracy and assessment of spinal imaging supporting clinical diagnostics and research efforts to better understand IVD degeneration and its treatment. Implementation of AI in the evaluation of histology data has improved the usefulness of a popular murine IVD model and could also be used in an ovine histopathological grading scheme that has been used to quantify degenerative IVD changes and stem cell mediated regeneration. These models are also attractive candidates for the evaluation of novel anti-oxidant compounds that counter inflammatory conditions in degenerate IVDs and promote IVD regeneration. Some of these compounds also have pain-relieving properties. AI has facilitated development of facial recognition pain assessment in animal IVD models offering the possibility of correlating the potential pain alleviating properties of some of these compounds with IVD regeneration.

Biomimetic Design of 3D Fibrous Mesh Reinforced Hydrogel Replicating the Form and Function of the Intervertebral Disc

While cervical total artificial disc replacement potentially preserves motion of the natural intervertebral disc (IVD), problems also arise involving alterations of the spinal biomechanics. A major challenge lies in restoring mechanics of the natural IVD with appropriate kinematics and biomimetic configuration. A biomimetic artificial IVD model is designed and fabricated using a 3D braided fibrous scaffold and a self‐healable hydrogel matrix. The artificial IVD is characterized by 3D four‐directional fibrous structure resembling natural annulus fibrosus and self‐healable hydrogel‐mimicking natural nucleus pulposus. In the compression tests, the artificial IVD exhibits reasonable mechanical behaviors and desired viscoelastic behaviors similar to the natural IVD. After fatigue loading of 5 million cycles, the artificial IVDs become stiffer, whereas the mechanical values remain within the reasonable range. Finite‐element analysis of the artificial IVD from mesoscale and macroscale analysis indicates the coherent load transfer through both the interconnections within the fiber mesh and the fiber–matrix interface, and the entire IVD shows a stress profilometry similar to natural IVD. In conclusion, a biomimetic prototype of artificial IVD with nature‐mimicking mechanics and structure is fabricated. The presence of interwoven fibrous mesh, hydrogel confinement, and proper interfacial adhesion is all essential for scalable production of the IVD.

Effects of geometric individualisation of a human spine model on load sharing: neuro-musculoskeletal simulation reveals significant differences in ligament and muscle contribution

In spine research, two possibilities to generate models exist: generic (population-based) models representing the average human and subject-specific representations of individuals. Despite the increasing interest in subject specificity, individualisation of spine models remains challenging. Neuro-musculoskeletal (NMS) models enable the analysis and prediction of dynamic motions by incorporating active muscles attaching to bones that are connected using articulating joints under the assumption of rigid body dynamics. In this study, we used forward-dynamic simulations to compare a generic NMS multibody model of the thoracolumbar spine including fully articulated vertebrae, detailed musculature, passive ligaments and linear intervertebral disc (IVD) models with an individualised model to assess the contribution of individual biological structures. Individualisation was achieved by integrating skeletal geometry from computed tomography and custom-selected muscle and ligament paths. Both models underwent a gravitational settling process and a forward flexion-to-extension movement. The model-specific load distribution in an equilibrated upright position and local stiffness in the L4/5 functional spinal unit (FSU) is compared. Load sharing between occurring internal forces generated by individual biological structures and their contribution to the FSU stiffness was computed. The main finding of our simulations is an apparent shift in load sharing with individualisation from an equally distributed element contribution of IVD, ligaments and muscles in the generic spine model to a predominant muscle contribution in the individualised model depending on the analysed spine level.

Impact of extracellular matrix and collagen network properties on the cervical intervertebral disc response to physiological loads: A parametric study

Current intervertebral disc finite element models are hard to validate since they describe multi-physical phenomena and contain a huge number of material properties. This work aims to simplify numerical validation/identification studies by prioritizing the sensitivity of intervertebral disc behavior to mechanical properties. A 3D fiber-reinforced hyperelastic model of a C6-C7 intervertebral disc is used to carry out the parametric study. 10 parameters describing the extracellular matrix and the collagen network behaviors are included in the parametric study. The influence of varying these parameters on the disc response is estimated during physiological movements of the head, including compression, lateral bending, flexion, and axial rotation. The obtained results highlight the high sensitivity of the disc behavior to the stiffness of the annulus fibrosus extracellular matrix for all the studied loads with a relative increase in the disc apparent stiffness by 67% for compression and by 57% for axial rotation when the annulus stiffness increases from 0.4 to 2MPa. It is also shown that varying collagen network orientation, stiffness, and stiffening in the studied configuration range have a noticeable effect on rotational motions with a relative apparent stiffness difference reaching 6.8%, 10%, and 22%, respectively, in lateral bending. However, the collagen orientation does not affect disc response to axial load.

Diagnostic Limitations and Aspects of the Lumbosacral Transitional Vertebrae (LSTV)

The regeneration of an intervertebral disc can only be successful if the cause of the degeneration is known and eliminated. The lumbosacral transitional vertebrae (LSTV) offer itself as a model for IVD (intervertebral disc) regeneration. The aim of this work is to support this statement. In our scoliosis outpatient clinic, 1482 patients were radiologically examined, and ambiguous lumbosacral junction underwent MRI examination. Patients with Castellvi classification type II–IV were included and the results are compared with the current literature in PubMed (12 October 2022). The LSTV are discussed as a possible IVD model. A total of 115 patients were diagnosed with LSTV Castellvi type II–IV. A Castellvi distribution type IIA (n-55), IIB (n-24), IIIA (n-20), IIIB (n-10) and IV (n-6) can be found. In all, 64 patients (55.7%) reported recurrent low-back pain (LBP). Scoliosis (Cobb angle >10°) was also confirmed in 72 patients (58 female and 14 male) and 56 (75.7%) had unilateral pathology. The wide variation in the literature regarding the prevalence of the LSTV (4.6–35.6%) is reasoned by the doubtful diagnosis of Castellvi type I. The LSTV present segments with reduced to absent mobility and at the same time leads to overload of the adjacent segments. This possibility of differentiation is seen as the potential for a spinal model.

Engineered Human Intervertebral Disc Model Inducing Degenerative Microglial Proinflammation.

Degeneration of the intervertebral disc (IVD) is a major contributor to low back pain (LBP). IVD degeneration is characterized by abnormal production of inflammatory cytokines secreted by IVD cells. Although the underlying molecular mechanisms of LBP have not been elucidated, increasing evidence suggests that LBP is associated particularly with microglia in IVD tissues and the peridiscal space, aggravating the cascade of degenerative events. In this study, we implemented our microfluidic chemotaxis platform to investigate microglial inflammation in response to our reconstituted degenerative IVD models. The IVD models were constructed by stimulating human nucleus pulposus (NP) cells with interleukin-1β and producing interleukin-6 (129.93 folds), interleukin-8 (18.31 folds), C-C motif chemokine ligand-2 (CCL-2) (6.12 folds), and CCL-5 (5.68 folds). We measured microglial chemotaxis (p < 0.05) toward the conditioned media of the IVD models. In addition, we observed considerable activation of neurodegenerative and deactivation of protective microglia via upregulated expression of CD11b (p < 0.001) and down-regulation of CD206 protein (p < 0.001) by soluble factors from IVD models. This, in turn, enhances the inflammatory milieu in IVD tissues, causing matrix degradation and cellular damage. Our findings indicate that degenerative IVD may induce degenerative microglial proinflammation, leading to LBP development.

Toward the Next Generation of Spine Bioreactors: Validation of an Ex Vivo Intervertebral Disc Organ Model and Customized Specimen Holder for Multiaxial Loading

A new generation of bioreactors with integrated six degrees of freedom (6 DOF) aims to mimic more accurately the natural intervertebral disc (IVD) load. We developed and validated in a biological and mechanical study a specimen holder and corresponding ex vivo IVD organ model according to the bioreactor requirements for multiaxial loading and a long-term IVD culture. IVD height changes and cell viability were compared between the 6 DOF model and the standard 1 DOF model throughout the 3 weeks of cyclic compressive loading in the uniaxial bioreactor. Furthermore, the 6 DOF model and holder were loaded for 9 days in the multiaxial bioreactor under development using the same conditions, and the IVDs were evaluated for cell viability. The interface of the IVD model and specimen holder, enhanced with fixation screws onto the bone, was tested in compression, torsion, lateral bending, and tension. Additionally, critical motions such as tension and bending were assessed for a combination of side screws and top screws or side screws and adhesive. The 6 DOF model loaded in the uniaxial bioreactor maintained similar cell viability in the IVD regions as the 1 DOF model. The viability was high after 2 weeks throughout the whole IVD and reduced by more than 30% in the inner annulus fibrous after 3 weeks. Similarly, the IVDs remained highly viabile when cultured in the multiaxial bioreactor. In both models, IVD height changes after loading were in the range of typical physiological conditions. When differently directed motions were applied, the holder-IVD interface remained stable under hyper-physiological loading levels using a side screw approach in compression and torsion and the combination of side and top screws in tension and bending. We thus conclude that the developed holding system is mechanically reliable and biologically compatible for application in a new generation of multiaxial bioreactors.

A MODIFIED FINITE ELEMENT MODEL OF A POROVISCOELASTIC INTERVERTEBRAL DISC

The intervertebral disc (IVD) is the soft tissue between the vertebral bodies, which is responsible for transmitting multi-directional loads through the spine and to allow relative motion between the vertebral bodies. The IVD is composed of three distinct tissues, including the nucleus pulposus, annulus fibrosus, and the cartilaginous endplates. Each of these tissues has a characteristic composition and structure which provide them with unique mechanical properties. Among these, nucleus pulposus and annulus fibrosus due to their intricate time-dependent mechanical response has always been the topic of interest for the researchers. Here, we aimed at establishing a patient-specific 3D finite element (FE) model of human IVD based on the poroviscoelastic constitutive law. The main objective was to use the data of tensile stress-relaxation tests on the annulus and nucleus regions to find the poroviscoelastic material constitutive law. The model assumed that the disc is a two-phase body consisting of a water-saturated solid matrix. To do that, the available data in the literature was used as the primary material properties of our model. Thereafter, a set of compressive and tensile loadings was applied on the established patient-specific model of the IVD and the FE results of the poroviscoelastic model were compared to the experimental data. This allowed us to determine a new set of revised parameter values for the poroviscoelastic model which will have practical implications for any future FE studies.

Expansion of FGFR3-positive nucleus pulposus cells plays important roles in postnatal nucleus pulposus growth and regeneration

BackgroundIntervertebral disc degeneration (IVDD) can cause low back pain, a major public health concern. IVDD is characterized with loss of cells especially those in nucleus pulposus (NP), due to the limited proliferative potential and regenerative ability. Few studies, however, have been carried out to investigate the in vivo proliferation events of NP cells and the cellular contribution of a specific subpopulation of NP during postnatal growth or regeneration.MethodsWe generated FGFR3-3*Flag-IRES-GFP mice and crossed FGFR3-CreERT2 mice with Rosa26-mTmG, Rosa26-DTA and Rosa26-Confetti mice, respectively, to perform inducible genetic tracing studies.ResultsExpression of FGFR3 was found in the outer region of NP with co-localized expressions of proliferating markers. By fate mapping studies, FGFR3-positive (FGFR3+) NP cells were found proliferate from outer region to inner region of NP during postnatal growth. Clonal lineage tracing by Confetti mice and ablation of FGFR3·+ NP cells by DTA mice further revealed that the expansion of the FGFR3+ cells was required for the morphogenesis and homeostasis of postnatal NP. Moreover, in degeneration and regeneration model of mouse intervertebral disc, FGFR3+ NP cells underwent extensive expansion during the recovery stage.ConclusionOur present work demonstrates that FGFR3+ NP cells are novel subpopulation of postnatal NP with long-existing proliferative capacity shaping the adult NP structure and participating in the homeostasis maintenance and intrinsic repair of NP. These findings may facilitate the development of new therapeutic approaches for IVD regeneration.

Encapsulation of Human-Bone-Marrow-Derived Mesenchymal Stem Cells in Small Alginate Beads Using One-Step Emulsification by Internal Gelation: In Vitro, and In Vivo Evaluation in Degenerate Intervertebral Disc Model.

Cell microencapsulation in gel beads contributes to many biomedical processes and pharmaceutical applications. Small beads (<300 µm) offer distinct advantages, mainly due to improved mass transfer and mechanical strength. Here, we describe, for the first time, the encapsulation of human-bone-marrow-derived mesenchymal stem cells (hBM-MSCs) in small-sized microspheres, using one-step emulsification by internal gelation. Small (127–257 µm) high-mannuronic-alginate microspheres were prepared at high agitation rates (800–1000 rpm), enabling control over the bead size and shape. The average viability of encapsulated hBM-MSCs after 2 weeks was 81 ± 4.3% for the higher agitation rates. hBM-MSC-loaded microspheres seeded within a glycosaminoglycan (GAG) analogue, which was previously proposed as a mechanically equivalent implant for degenerate discs, kept their viability, sphericity, and integrity for at least 6 weeks. A preliminary in vivo study of hBM-MSC-loaded microspheres implanted (via a GAG-analogue hydrogel) in a rat injured intervertebral disc model demonstrated long-lasting viability and biocompatibility for at least 8 weeks post-implantation. The proposed method offers an effective and reproducible way to maintain long-lasting viability in vitro and in vivo. This approach not only utilizes the benefits of a simple, mild, and scalable method, but also allows for the easy control of the bead size and shape by the agitation rate, which, overall, makes it a very attractive platform for regenerative-medicine applications.

A fully three-dimensional model of interpenetrating collagen fibrillar networks for intervertebral disc mechanics

In this work, a fully three-dimensional model of the human intervertebral disc is proposed within a purely analytical framework in the final goal to create time and cost-efficient patient-specific models. The constitutive representation considers the deformation-induced damage along with the hydration effects on the regional disc mechanics. Special functions are introduced to generate versatile and anatomically accurate disc geometries taking into account morphology regional variation. The complex interpenetrating collagen fibrillar networks are explicitly introduced in the three-dimensional model considering the regional variation throughout the disc. A quantitative evaluation of the predictive capabilities of a human lumbar disc model shows a good agreement with compression and torsion experiments from the literature both for the overall disc stiffness and for the direct MRI kinematic fields. Model predictions are then performed and critically discussed on axially/torsionally loaded human lumbar discs.