Numerical exploration of the combined effect of nutrient supply, tissue condition and deformation in the intervertebral disc

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.

Unveiling interactions between intervertebral disc morphologies and mechanical behavior through personalized finite element modeling

Introduction: Intervertebral Disc (IVD) Degeneration (IDD) is a significant health concern, potentially influenced by mechanotransduction. However, the relationship between the IVD phenotypes and mechanical behavior has not been thoroughly explored in local morphologies where IDD originates. This work unveils the interplays among morphological and mechanical features potentially relevant to IDD through Abaqus UMAT simulations. Methods: A groundbreaking automated method is introduced to transform a calibrated, structured IVD finite element (FE) model into 169 patient-personalized (PP) models through a mesh morphing process. Our approach accurately replicates the real shapes of the patient's Annulus Fibrosus (AF) and Nucleus Pulposus (NP) while maintaining the same topology for all models. Using segmented magnetic resonance images from the former project MySpine, 169 models with structured hexahedral meshes were created employing the Bayesian Coherent Point Drift++ technique, generating a unique cohort of PP FE models under the Disc4All initiative. Machine learning methods, including Linear Regression, Support Vector Regression, and eXtreme Gradient Boosting Regression, were used to explore correlations between IVD morphology and mechanics. Results: We achieved PP models with AF and NP similarity scores of 92.06\% and 92.10\% compared to the segmented images. The models maintained good quality and integrity of the mesh. The cartilage endplate (CEP) shape was represented at the IVD-vertebra interfaces, ensuring personalized meshes. Validation of the constitutive model against literature data showed a minor relative error of 5.20%. Discussion: Analysis revealed the influential impact of local morphologies on indirect mechanotransduction responses, highlighting the roles of heights, sagittal areas, and volumes. While the maximum principal stress was influenced by morphologies such as heights, the disc's ellipticity influenced the minimum principal stress. Results suggest the CEPs are not influenced by their local morphologies but by those of the AF and NP. The generated free-access repository of individual disc characteristics is anticipated to be a valuable resource for the scientific community with a broad application spectrum.

Lactic acid promotes nucleus pulposus cell senescence and corresponding intervertebral disc degeneration via interacting with Akt

The accumulation of metabolites in the intervertebral disc is considered an important cause of intervertebral disc degeneration (IVDD). Lactic acid, which is a metabolite that is produced by cellular anaerobic glycolysis, has been proven to be closely associated with IVDD. However, little is known about the role of lactic acid in nucleus pulposus cells (NPCs) senescence and oxidative stress. The aim of this study was to investigate the effect of lactic acid on NPCs senescence and oxidative stress as well as the underlying mechanism. A puncture-induced disc degeneration (PIDD) model was established in rats. Metabolomics analysis revealed that lactic acid levels were significantly increased in degenerated intervertebral discs. Elimination of excessive lactic acid using a lactate oxidase (LOx)-overexpressing lentivirus alleviated the progression of IVDD. In vitro experiments showed that high concentrations of lactic acid could induce senescence and oxidative stress in NPCs. High-throughput RNA sequencing results and bioinformatic analysis demonstrated that the induction of NPCs senescence and oxidative stress by lactic acid may be related to the PI3K/Akt signaling pathway. Further study verified that high concentrations of lactic acid could induce NPCs senescence and oxidative stress by interacting with Akt and regulating its downstream Akt/p21/p27/cyclin D1 and Akt/Nrf2/HO-1 pathways. Utilizing molecular docking, site-directed mutation and microscale thermophoresis assays, we found that lactic acid could regulate Akt kinase activity by binding to the Lys39 and Leu52 residues in the PH domain of Akt. These results highlight the involvement of lactic acid in NPCs senescence and oxidative stress, and lactic acid may become a novel potential therapeutic target for the treatment of IVDD.

Physiological and degenerative loading of bovine intervertebral disc in a bioreactor: A finite element study of complex motions

Intervertebral disc (IVD) degeneration and regenerative therapies are commonly studied in organ-culture experiments with uniaxial compressive loading. Recently, in our laboratory, we established a bioreactor system capable of applying loads in six degrees-of-freedom (DOF) to bovine IVDs, which replicates more closely the complex multi-axial loading of the IVD in vivo. However, the magnitudes of loading that are physiological (able to maintain cell viability) or mechanically degenerative are unknown for load cases combining several DOFs. This study aimed to establish physiological and degenerative levels of maximum principal strains and stresses in the bovine IVD tissue and to investigate how they are achieved under complex load cases related to common daily activities. The physiological and degenerative levels of maximum principal strains and stresses were determined via finite element (FE) analysis of bovine IVD subjected to experimentally established physiological and degenerative compressive loading protocols. Then, complex load cases, such as a combination of compression + flexion + torsion, were applied on the FE-model with increasing magnitudes of loading to discover when physiological and degenerative tissue strains and stresses were reached. When applying 0.1 MPa of compression and ±2-3° of flexion and ±1-2° of torsion the investigated mechanical parameters remained at physiological levels, but with ±6-8° of flexion in combination with ±2-4° of torsion, the stresses in the outer annulus fibrosus (OAF) exceeded degenerative levels. In the case of compression + flexion + torsion, the mechanical degeneration likely initiates at the OAF when loading magnitudes are high enough. The physiological and degenerative magnitudes can be used as guidelines for bioreactor experiments with bovine IVDs.

Computational Modeling Intervertebral Disc Pathophysiology: A Review

Lower back pain is a medical condition of epidemic proportion, and the degeneration of the intervertebral disc has been identified as a major contributor. The etiology of intervertebral disc (IVD) degeneration is multifactorial, depending on age, cell-mediated molecular degradation processes and genetics, which is accelerated by traumatic or gradual mechanical factors. The complexity of such intertwined biochemical and mechanical processes leading to degeneration makes it difficult to quantitatively identify cause–effect relationships through experiments. Computational modeling of the IVD is a powerful investigative tool since it offers the opportunity to vary, observe and isolate the effects of a wide range of phenomena involved in the degenerative process of discs. This review aims at discussing the main findings of finite element models of IVD pathophysiology with a special focus on the different factors contributing to physical changes typical of degenerative phenomena. Models presented are subdivided into those addressing role of nutritional supply, progressive biochemical alterations stemming from an imbalance between anabolic and catabolic processes, aging and those considering mechanical factors as the primary source that induces morphological change within the disc. Limitations of the current models, as well as opportunities for future computational modeling work are also discussed.

The Influence of Axial Compression on the Cellular and Mechanical Function of Spinal Tissues; Emphasis on the Nucleus Pulposus and Annulus Fibrosus: A Review

In light of the correlation between chronic back pain and intervertebral disc (IVD) degeneration, this literature review seeks to illustrate the importance of the hydraulic response across the nucleus pulposus (NP)-annulus fibrosus (AF) interface, by synthesizing current information regarding injurious biomechanics of the spine, stemming from axial compression. Damage to vertebrae, endplates (EPs), the NP, and the AF, can all arise from axial compression, depending on the segment's posture, the manner in which it is loaded, and the physiological state of tissue. Therefore, this movement pattern was selected to illustrate the importance of the bracing effect of a pressurized NP on the AF, and how injuries interrupting support to the AF may contribute to IVD degeneration.

Finite element modelling of hybrid stabilization systems for the human lumbar spine

Intersomatic fusion is a very popular treatment for spinal diseases associated with intervertebral disc degeneration. The effects of three different hybrid stabilization systems on both range of motion and intradiscal pressure were investigated, as there is no consensus in the literature about the efficiency of these systems. Finite element simulations were designed to predict the variations of range of motion and intradiscal pressure from intact to implanted situations. After hybrid stabilization system implantation, L4-L5 level did not lose its motion completely, while L5-S1 had no mobility as a consequence of disc removal and fusion process. BalanC hybrid stabilization system represented higher mobility at the index level, reduced intradiscal pressure of adjacent level, but caused to increment in range of motion by 20% under axial rotation. Higher tendency by 93% to the failure was also detected under axial rotation. Dynesys hybrid stabilization system represented more restricted motion than BalanC, and negligible effects to the adjacent level. B-DYN hybrid stabilization system was the most rigid one among all three systems. It reduced intradiscal pressure and range of motion at the adjacent level except from motion under axial rotation being increased by 13%. Fracture risk of B-DYN and Dynesys Transition Optima components was low when compared with BalanC. Mobility of the adjacent level around axial direction should be taken into account in case of implantation with BalanC and B-DYN systems, as well as on the development of new designs. Having these findings in mind, it is clear that hybrid systems need to be further tested, both clinically and numerically, before being considered for common use.

Macrophages Down-Regulate Gene Expression of Intervertebral Disc Degenerative Markers Under a Pro-inflammatory Microenvironment

Low back pain is a highly prevalent clinical problem and intervertebral disc (IVD) degeneration is now accepted as the major pathophysiological mechanism responsible for this condition. Accumulating evidence suggests that inflammation plays a crucial role in the progression of human IVD degeneration, with macrophages being pointed as the key immune cell players in this process since their infiltration in degenerated IVD samples has been extensively demonstrated. Since they are highly plastic, macrophages can play different roles depending on the microenvironmental cues. The study of inflammation associated with IVD degeneration has been somehow neglected and one of the reasons is related with lack of adequate models. To overcome this, we established and characterized a new model of IVD organ culture under pro-inflammatory conditions to further dissect the role of macrophages in IVD associated immune response. For that, human monocyte-derived macrophages were co-cultured either with bovine caudal IVD punches in the presence of the pro-inflammatory cytokine IL-1β, or IVD-conditioned medium (CM), to investigate how IVD-produced factors influence macrophage phenotype. After 72 h, metabolic activity, gene expression and cytokine profile of macrophages and IVD cells were measured. Our results show that macrophages and IVDs remain metabolically active in the presence of IL-1β, significantly upregulate CCR7 gene expression and increase production of IL-6 on macrophages. When treating macrophages with IL-1β-IVD-CM, CCR7 upregulation follows the same trend, while for IL-6 an opposite effect was observed. On the other hand, macrophages interfere with IVD ECM remodeling, decreasing MMP3 expression and downregulating aggrecan and collagen II gene expression in the presence of IL-1β. Overall, the co-culture model established in this study can be considered a suitable approach to address the cellular and molecular pathways that regulate macrophage-IVD crosstalk, suggesting that degenerated IVD tissue tends to polarize human macrophages toward a more pro-inflammatory profile, which seems to aggravate IVD degeneration. This model could be used to improve the knowledge of the mechanisms that link IVD degeneration and the immune response.

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.

Application of Finite Element Analysis for Investigation of Intervertebral Disc Degeneration: from Laboratory to Clinic

Due to the ethical concern and inability to detect inner stress distributions of intervertebral disc (IVD), traditional methods for investigation of intervertebral disc degeneration (IVDD) have significant limitations. Many researchers have demonstrated that finite element analysis (FEA) is an effective tool for the research of IVDD. However, the specific application of FEA for investigation of IVDD has not been systematically elucidated before. In the present review, we summarize the current finite element models (FEM) used for the investigation of IVDD, including the poroelastic nonlinear FEM, diffusive-reactive theory model and cell-activity coupled mechano-electrochemical theory model. We further elaborate the use of FEA for the research of IVDD pathogenesis especially for nutrition and biomechanics associated etiology, and the biological, biomechanical and clinical influences of IVDD. In addition, the application of FEA for evaluation and exploration of various treatments for IVDD is also elucidated. We conclude that FEA is an excellent technique for research of IVDD, which could be used to explore the etiology, biology and biomechanics of IVDD. In the future, FEA may help us to achieve the goal of individualized precision therapy.

Theoretical Explorations Generate New Hypotheses About the Role of the Cartilage Endplate in Early Intervertebral Disk Degeneration

Altered cell nutrition in the intervertebral disk (IVD) is considered a main cause for disk degeneration (DD). The cartilage endplate (CEP) provides a major path for the diffusion of nutrients from the peripheral vasculature to the IVD nucleus pulposus (NP). In DD, sclerosis of the adjacent bony endplate is suggested to be responsible for decreased diffusion and disk cell nutrition. Yet, experimental evidence does not support this hypothesis. Hence, we evaluated how moderate CEP composition changes related to tissue degeneration can affect disk nutrition and cell viability. A novel composition-based permeability formulation was developed for the CEP, calibrated, validated, and used in a mechano-transport finite element IVD model. Fixed solute concentrations were applied at the outer surface of the annulus and the CEP, and three cycles of daily mechanical load were simulated. The CEP model indicated that CEP permeability increases with the degeneration/aging of the tissue, in accordance with recent measurements reported in the literature. Additionally, our results showed that CEP degeneration might be responsible for mechanical load-induced NP dehydration, which locally affects oxygen and lactate levels, and reduced glucose concentration by 16% in the NP-annulus transition zone. Remarkably, CEP degeneration was a condition sine-qua-non to provoke cell starvation and death, while simulating the effect of extracellular matrix depletion in DD. This theoretical study cast doubts about the paradigm that CEP calcification is needed to provoke cell starvation, and suggests an alternative path for DD whereby the early degradation of the CEP plays a key role.

The potential effect of type 2 diabetes mellitus on lumbar disc degeneration: a retrospective single-center study

Background

Diabetes mellitus (DM) and low back pain which is mainly caused by degeneration of the intervertebral discs (IVDs) both are major public health problem worldwide. The present study was designed to investigate the association between type 2 diabetes mellitus (T2D) and severity of lumbar disc degeneration (LDD).

Methods

We retrospectively reviewed patients with low back pain visiting our spine clinic in 2014. Low back pain patients all have the lumbar MRI imaging and no previous treatment. One hundred fifty patients without T2D (group A) and 622 patients with T2D meeting the criteria were included. Sex, age, body mass index (BMI), high blood pressure (HBP), history of smoking, alcohol use, and duration of T2D were recorded. Patients with T2D were assigned to a well-controlled group (group B, n = 380) and a bad-controlled group (group C, n = 242). In group B, T2D duration of 148 patients was ≤ 10 years (group B1) and 232 patients > 10 years (group B2). In group C, T2D duration of 100 patients was ≤ 10 years (group C1) and 142 patients > 10 years (group C2). The severity of LDD was evaluated using the five-level Pfirrmann grading system. Data were analyzed using SPSS 19.0.

Results

Demographic data except age showed no difference among groups (P > 0.5). Compared to patients without T2D, patients with T2D showed more severe disc degeneration after removal of age effects (P < 0.05). From L1/2 to L5/S1, the average Pfirrmann scores between groups A and B1 showed no difference(P > 0.05); groups B2, C1, and C2 showed higher average Pfirrmann scores than group A (P < 0.05). Groups B2 and C2 showed higher average Pfirrmann scores than groups B1 and C1 (P < 0.05). Groups C1 and C2 showed higher average Pfirrmann scores than groups B1 and B2 (P < 0.05). From L1/2 to L5/S1, the severity of LDD was highly positively related to T2D duration both in groups B and C (P < 0.05).

Conclusions

T2D duration > 10 years and a bad control of T2D were risk factors for LDD. The longer T2D duration was, the more severe disc degeneration would be.

Human bone morphogenetic protein 7 transfected nucleus pulposus cells delay the degeneration of intervertebral disc in dogs

The main reason for intervertebral disc (IVD) degeneration is the decrease in the quantity and activity of IVD cells with subsequent reduction of the extracellular matrix (ECM). In this study, we investigated a cell-based repair strategy by injecting nucleus pulposus cells (NPCs) transduced with human bone morphogenetic protein (hBMP7) by adeno-associated virus-2 into the canine degenerative IVD to determine whether NPCs expressing hBMP7 could delay the degeneration of the IVD. Fourteen canines received annular punctures to induce disc degeneration. Eight weeks later, saline (group A), allogeneic NPCs (group B), or allogeneic NPCs transduced with hBMP7 (group C) were injected into the degenerative discs. Twelve weeks after the injection, MRI scan showed that the degeneration process of groups C was slower and less severe compared with that of groups B and C. The IVD stability in group C was superior to that in groups A and B in left-right bending and rotation. HE, safranin-O staining, and ELISA indicated that the degenerative degree of the IVD in group C was significantly milder than that in groups A and B. The study demonstrated that the implantation of NPCs-hBMP7 could effectively maintained the structural integrity, ECM, and biomechanical properties of the canine degenerated discs. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1311-1322, 2017.

Human cartilage endplate permeability varies with degeneration and intervertebral disc site

Despite the critical functions the human cartilage endplate (CEP) plays in the intervertebral disc, little is known about its structural and mechanical properties and their changes with degeneration. Quantifying these changes with degeneration is important for understanding how the CEP contributes to the function and pathology of the disc. Therefore the objectives of this study were to quantify the effect of disc degeneration on human CEP mechanical properties, determine the influence of superior and inferior disc site on mechanics and composition, and simulate the role of collagen fibers in CEP and disc mechanics using a validated finite element model. Confined compression data and biochemical composition data were used in a biphasic-swelling model to calculate compressive extrafibrillar elastic and permeability properties. Tensile properties were obtained by applying published tensile test data to an ellipsoidal fiber distribution. Results showed that with degeneration CEP permeability decreased 50-60% suggesting that transport is inhibited in the degenerate disc. CEP fibers are organized parallel to the vertebrae and nucleus pulposus and may contribute to large shear strains (0.1-0.2) and delamination failure of the CEP commonly seen in herniated disc tissue. Fiber-reinforcement also reduces CEP axial strains thereby enhancing fluid flux by a factor of 1.8. Collectively, these results suggest that the structure and mechanics of the CEP may play critical roles in the solute transport and disc mechanics.

A Degenerative/Proinflammatory Intervertebral Disc Organ Culture: An Ex Vivo Model for Anti-inflammatory Drug and Cell Therapy

Resolution of intervertebral disc (IVD) degeneration-associated inflammation is a prerequisite for tissue regeneration and could possibly be achieved by strategies ranging from pharmacological to cell-based therapies. In this study, a proinflammatory disc organ culture model was established. Bovine caudal disc punches were needle punctured and additionally stimulated with lipopolysaccharide (10 μg/mL) or interleukin-1β (IL-1β, 10-100 ng/mL) for 48 h. Two intradiscal therapeutic approaches were tested: (i) a nonsteroidal anti-inflammatory drug, diclofenac (Df) and (ii) human mesenchymal stem/stromal cells (MSCs) embedded in an albumin/hyaluronan hydrogel. IL-1β-treated disc organ cultures showed a statistically significant upregulation of proinflammatory markers (IL-6, IL-8, prostaglandin E2 [PGE2]) and metalloproteases (MMP1, MMP3) expression, while extracellular matrix (ECM) proteins (collagen II, aggrecan) were significantly downregulated. The injection of the anti-inflammatory drug, Df, was able to reduce the levels of proinflammatory cytokines and MMPs and surprisingly increase ECM protein levels. These results point the intradiscal application of anti-inflammatory drugs as promising therapeutics for disc degeneration. In parallel, the immunomodulatory role of MSCs on this model was also evaluated. Although a slight downregulation of IL-6 and IL-8 expression could be found, the variability among the five donors tested was high, suggesting that the beneficial effect of these cells on disc degeneration needs to be further evaluated. The proinflammatory/degenerative IVD organ culture model established can be considered a suitable approach for testing novel therapeutic drugs, thus reducing the number of animals in in vivo experimentation. Moreover, this model can be used to address the cellular and molecular mechanisms that regulate inflammation in the IVD and their implications in tissue degeneration.

Effects of Tobacco Smoking on the Degeneration of the Intervertebral Disc: A Finite Element Study

Tobacco smoking is associated with numerous pathological conditions. Compelling experimental evidence associates smoking to the degeneration of the intervertebral disc (IVD). In particular, it has been shown that nicotine down-regulates both the proliferation rate and glycosaminoglycan (GAG) biosynthesis of disc cells. Moreover, tobacco smoking causes the constriction of the vascular network surrounding the IVD, thus reducing the exchange of nutrients and anabolic agents from the blood vessels to the disc. It has been hypothesized that both nicotine presence in the IVD and the reduced solute exchange are responsible for the degeneration of the disc due to tobacco smoking, but their effects on tissue homeostasis have never been quantified. In this study, a previously presented computational model describing the homeostasis of the IVD was deployed to investigate the effects of impaired solute supply and nicotine-mediated down-regulation of cell proliferation and biosynthetic activity on the health of the disc. We found that the nicotine-mediated down-regulation of cell anabolism mostly affected the GAG concentration at the cartilage endplate, reducing it up to 65% of the value attained in normal physiological conditions. In contrast, the reduction of solutes exchange between blood vessels and disc tissue mostly affected the nucleus pulposus, whose cell density and GAG levels were reduced up to 50% of their normal physiological levels. The effectiveness of quitting smoking on the regeneration of a degenerated IVD was also investigated, and showed to have limited benefit on the health of the disc. A cell-based therapy in conjunction with smoke cessation provided significant improvements in disc health, suggesting that, besides quitting smoking, additional treatments should be implemented in the attempt to recover the health of an IVD degenerated by tobacco smoking.

Intervention of rAAV-hTERT-Transducted Nucleus Pulposus Cells in Early Stage of Intervertebral Disc Degeneration: A Study in Canine Model

OBJECTIVES

To investigate the efficacy of recombinant adeno-associated virus (rAAV)-human telomerase reverse transcriptase (hTERT)-transducted nucleus pulposus cells (NPCs) in disc degeneration process in a canine disc degeneration model.

METHODS

The intervertebral disc degeneration of lumbar (L) 1-2, L3-4, and L5-6, from 12 female mongrels was prepared with the 20-gauge biopsy gun. Four weeks after animal model preparation, intervention experiment with rAAV-hTERT-transducted NPCs was conducted: group A, L1-2, serum-free medium with rAAV-hTERT modified NPCs; group B, L3-4, serum-free medium with NPCs; group C, L5-6, serum-free medium alone. Canines underwent digital radiography and magnetic resonance imaging 1 day before intervention, and 4, 8, and 12 weeks after intervention to evaluate the change of disc height and hydration status of interventional intervertebral discs. Twelve weeks after intervention, histological, biomechanical, and biochemical studies were carried out.

RESULTS

The rAAV-hTERT-transducted NPCs were constructed successfully. The mRNA level of hTERT from rAAV-hTERT-transfected NPCs increased obviously. There was no significant change of disc height index observed between groups and within groups. The relative grayscale index (RGI) was maintained 8 weeks after the intervention in group A, whereas in group B and group C, the RGI decreased significantly (p<0.05). No significant differences of the angle of lateral bending and extension-flexion bending were observed in group A compared with other groups (p>0.05). The morphology of disc structure was preserved in group A. In group B, the structure of inner annulus was broken down and the jelly-like nucleus pulposus (NP) tissue transmitted into the fibrocartilaginous tissue. In group C, the jelly-like NP tissue was completely replaced by fibrocartilaginous tissue. In the NP, the content of proteoglycan (PG) and collagen II was higher in group A than in group C (p<0.05). The content of PG was 13, 8.9, and 15.6 times higher than the content of collagen II in group A, group B, and group C, respectively.

CONCLUSIONS

In 12 weeks of observation, rAAV-hTERT-transducted NPCs could delay the degeneration process in the canine model which was superior than the capacity of NPCs in preserving structure integrity, content of extracellular matrix, and mechanical stability.

On the relative relevance of subject-specific geometries and degeneration-specific mechanical properties for the study of cell death in human intervertebral disk models

Capturing patient- or condition-specific intervertebral disk (IVD) properties in finite element models is outmost important in order to explore how biomechanical and biophysical processes may interact in spine diseases. However, disk degenerative changes are often modeled through equations similar to those employed for healthy organs, which might not be valid. As for the simulated effects of degenerative changes, they likely depend on specific disk geometries. Accordingly, we explored the ability of continuum tissue models to simulate disk degenerative changes. We further used the results in order to assess the interplay between these simulated changes and particular IVD morphologies, in relation to disk cell nutrition, a potentially important factor in disk tissue regulation. A protocol to derive patient-specific computational models from clinical images was applied to different spine specimens. In vitro, IVD creep tests were used to optimize poro-hyperelastic input material parameters in these models, in function of the IVD degeneration grade. The use of condition-specific tissue model parameters in the specimen-specific geometrical models was validated against independent kinematic measurements in vitro. Then, models were coupled to a transport-cell viability model in order to assess the respective effects of tissue degeneration and disk geometry on cell viability. While classic disk poro-mechanical models failed in representing known degenerative changes, additional simulation of tissue damage allowed model validation and gave degeneration-dependent material properties related to osmotic pressure and water loss, and to increased fibrosis. Surprisingly, nutrition-induced cell death was independent of the grade-dependent material properties, but was favored by increased diffusion distances in large IVDs. Our results suggest that in situ geometrical screening of IVD morphology might help to anticipate particular mechanisms of disk degeneration.

The role of endplate poromechanical properties on the nutrient availability in the intervertebral disc

OBJECTIVE

To investigate the relevance of the human vertebral endplate poromechanics on the fluid and metabolic transport from and to the intervertebral disc (IVD) based on educated estimations of the poromechanical parameter values of the bony endplate (BEP).

METHODS

50 micro-models of different BEP samples were generated from μCTs of lumbar vertebrae and allowed direct determination of porosity values. Permeability values were calculated by using the micro-models, through the simulation of permeation via computational fluid dynamics. These educated ranges of porosity and permeability values were used as inputs for mechano-transport simulations to assess their effect on both the distributions of metabolites within an IVD model and the poromechanical calculations within the cartilaginous part of the endplate i.e., the cartilage endplate (CEP).

RESULTS

BEP effective permeability was highly correlated to local variations of porosity (R(2) ≈ 0.88). Universal patterns between bone volume fraction and permeability arose from these results and from other experimental data in the literature. These variations in BEP permeability and porosity had negligible effects on the distributions of metabolites within the disc. In the CEP, the variability of the poromechanical properties of the BEP did not affect the predicted consolidation but induced higher fluid velocities.

CONCLUSIONS

The present paper provides the first sets of thoroughly identified BEP parameter values that can be further used in patient-specific poromechanical studies. Representing BEP structural changes through variations in poromechanical properties did not affect the diffusion of metabolites. However, attention might be paid to alterations in fluid velocities and cell mechano-sensing within the CEP.