Duration-dependent influence of dynamic torsion on the intervertebral disc: an intact disc organ culture study

Effects of hydrostatic pressure, osmotic pressure, and confinement on extracellular matrix associated responses in the nucleus pulposus cells ex vivo

Spinal movement in both upright and recumbent positions generates changes in physicochemical stresses including hydrostatic pressure (HP), deviatoric stress, and confinement within the intradiscal compartment. The nucleus pulposus (NP) of the intervertebral disc is composed of highly negatively charged extracellular matrix (ECM), which increases osmotic pressure (OP) and generates tissue swelling. In pursuing regenerative therapies for intervertebral disc degeneration, the effects of HP on the cellular responses of NP cells and the ECM environment remain incompletely understood. We hypothesized that anabolic turnover of ECM in NP tissue is maintained under HP and confinement. We first clarified the effects of the relationships among HP, OP, and confinement on swelling NP explants isolated from bovine caudal intervertebral discs over 12 hours. We found that the application of confinement and constant HP significantly inhibits the free swelling of NP (p < 0.01) and helps retain the sulfated glycosaminoglycan. Since confinement and HP inhibited swelling, we incubated confined NPs under HP in high-osmolality medium mimicking ECM-associated OP for 7 days and demonstrated the effects of HP on metabolic turnover of ECM molecules in NP cells. The aggrecan core protein gene was significantly upregulated under confinement and constant HP compared to confinement and no HP (p < 0.01). We also found that confinement and constant HP helped to significantly retain smaller cell area (p < 0.01) and significantly prevent the severing of actin filaments compared to no confinement and HP (p < 0.01). Thus, we suggest that NP's metabolic turnover and cellular responses are regulated by the configuration of intracellular actin and fibrillar ECMs under HP.

Understanding the etiopathogenesis of lumbar intervertebral disc herniation: From clinical evidence to basic scientific research

Lumbar intervertebral disc herniation, as a leading cause of low back pain, productivity loss, and disability, is a common musculoskeletal disorder that results in significant socioeconomic burdens. Despite extensive clinical and basic scientific research efforts, herniation etiopathogenesis, particularly its initiation and progression, is not well understood. Understanding herniation etiopathogenesis is essential for developing effective preventive measures and therapeutic interventions. Thus, this review seeks to provide a thorough overview of the advances in herniation-oriented research, with a discussion on ongoing challenges and potential future directions for clinical, translational, and basic scientific investigations to facilitate innovative interdisciplinary research aimed at understanding herniation etiopathogenesis. Specifically, risk factors for herniation are identified and summarized, including familial predisposition, obesity, diabetes mellitus, smoking tobacco, selected cardiovascular diseases, disc degeneration, and occupational risks. Basic scientific experimental and computational research that aims to understand the link between excessive mechanical load, catabolic tissue remodeling due to inflammation or insufficient nutrient supply, and herniation, are also reviewed. Potential future directions to address the current challenges in herniation-oriented research are explored by combining known progressive development in existing research techniques with ongoing technological advances. More research on the relationship between occupational risk factors and herniation, as well as the relationship between degeneration and herniation, is needed to develop preventive measures for working-age individuals. Notably, researchers should explore using or modifying existing degeneration animal models to study herniation etiopathogenesis, as such models may allow for a better understanding of how to prevent mild-to-moderately degenerated discs from herniating.

A Novel Fiber-Reinforced Poroviscoelastic Bovine Intervertebral Disc Finite Element Model for Organ Culture Experiment Simulations

Intervertebral disc (IVD) degeneration and methods for repair and regeneration have commonly been studied in organ cultures with animal IVDs under compressive loading. With the recent establishment of a novel multi-axial organ culture system, accurate predictions of the global and local mechanical response of the IVD are needed for control system development and to aid in experiment planning. This study aimed to establish a finite element model of bovine IVD capable of predicting IVD behavior at physiological and detrimental load levels. A finite element model was created based on the dimensions and shape of a typical bovine IVD used in the organ culture. The nucleus pulposus (NP) was modeled as a neo-Hookean poroelastic material and the annulus fibrosus (AF) as a fiber-reinforced poroviscoelastic material. The AF consisted of 10 lamella layers and the material properties were distributed in the radial direction. The model outcome was compared to a bovine IVD in a compressive stress-relaxation experiment. A parametric study was conducted to investigate the effect of different material parameters on the overall IVD response. The model was able to capture the equilibrium response and the relaxation response at physiological and higher strain levels. Permeability and elastic stiffness of the AF fiber network affected the overall response most prominently. The established model can be used to evaluate the response of the bovine IVD at strain levels typical for organ culture experiments, to define relevant boundaries for such studies, and to aid in the development and use of new multi-axial organ culture systems.

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.

Integration of clinical perspective into biomimetic bioreactor design for orthopedics

The challenges to accommodate multiple tissue formation metrics in conventional bioreactors have resulted in an increased interest to explore novel bioreactor designs. Bioreactors allow researchers to isolate variables in controlled environments to quantify cell response. While current bioreactor designs can effectively provide either mechanical, electrical, or chemical stimuli to the controlled environment, these systems lack the ability to combine all these stimuli simultaneously to better recapitulate the physiological environment. Introducing a dynamic and systematic combination of biomimetic stimuli bioreactor systems could tremendously enhance its clinical relevance in research. Thus, cues from different tissue responses should be studied collectively and included in the design of a biomimetic bioreactor platform. This review begins by providing a summary on the progression of bioreactors from simple to complex designs, focusing on the major advances in bioreactor technology and the approaches employed to better simulate in vivo conditions. The current state of bioreactors in terms of their clinical relevance is also analyzed. Finally, this review provides a comprehensive overview of individual biophysical stimuli and their role in establishing a biomimetic microenvironment for tissue engineering. To date, the most advanced bioreactor designs only incorporate one or two stimuli. Thus, the cell response measured is likely unrelated to the actual clinical performance. Integrating clinically relevant stimuli in bioreactor designs to study cell response can further advance the understanding of physical phenomenon naturally occurring in the body. In the future, the clinically informed biomimetic bioreactor could yield more efficiently translatable results for improved patient care.

Investigating the physiological relevance of ex vivo disc organ culture nutrient microenvironments using in silico modeling and experimental validation

BACKGROUND

Ex vivo disc organ culture systems have become a valuable tool for the development and pre-clinical testing of potential intervertebral disc (IVD) regeneration strategies. Bovine caudal discs have been widely selected due to their large availability and comparability to human IVDs in terms of size and biochemical composition. However, despite their extensive use, it remains to be elucidated whether their nutrient microenvironment is comparable to human degeneration.

AIMS

This work aims to create the first experimentally validated in silico model which can be used to predict and characterize the metabolite concentrations within ex vivo culture systems.

MATERIALS & METHODS

Finite element models of cultured discs governed by previously established coupled reaction-diffusion equations were created using COMSOL Multiphysics. Experimental validation was performed by measuring oxygen, glucose and pH levels within discs cultured for 7 days, in a static compression bioreactor.

RESULTS

The in silico model was successfully validated through good agreement between the predicted and experimentally measured concentrations. For an ex vivo organ cultured in high glucose medium (4.5 g/L or 25 mM) and normoxia, a larger bovine caudal disc (Cd1-2 to Cd3-4) had a central concentration of ~2.6 %O2, ~8 mM of glucose and a pH value of 6.7, while the smallest caudal discs investigated (Cd6-7 and Cd7-8), had a central concentration of ~6.5 %O2, ~12 mM of glucose and a pH value of 6.9.

DISCUSSION

This work advances the knowledge of ex vivo disc culture microenvironments and highlights a critical need for optimization and standardization of culturing conditions.

CONCLUSION

Ultimately, for assessment of cell-based therapies and successful clinical translation based on nutritional demands, it is imperative that the critical metabolite values within organ cultures (minimum glucose, oxygen and pH values) are physiologically relevant and comparable to the stages of human degeneration.

In vitro nucleus pulposus tissue model with physicochemical stresses

Intervertebral discs (IVDs) are exposed to changes in physicochemical stresses including hydrostatic and osmotic pressure via diurnal spinal motion. Homeostasis, degeneration, and regeneration in IVDs have been studied using in vitro, ex vivo, and animal models. However, incubation of nucleus pulposus (NP) cells in medium has limited capability to reproduce anabolic turnover and regeneration under physicochemical stresses. We developed a novel pressure/perfusion cell culture system and a semipermeable membrane pouch device for enclosing isolated NP cells for in vitro incubation under physicochemical stresses. We assessed the performance of this system to identify an appropriate stress loading regimen to promote gene expression and consistent accumulation of extracellular matrices by bovine caudal NP cells. Cyclic hydrostatic pressure (HP) for 4 days followed by constant HP for 3 days in high osmolality (HO; 450 mOsm/kg H2O) showed a trend towards upregulated aggrecan expression and dense accumulation of keratan sulfate without gaps by the NP cells. Furthermore, a repetitive regimen of cyclic HP for 2 days followed by constant HP for 1 day in HO (repeated twice) significantly upregulated gene expression of aggrecan (P < .05) compared to no pressure and suppressed matrix metalloproteinase-13 expression (P < .05) at 6 days. Our culture system and pouches will be useful to reproduce physicochemical stresses in NP cells for simulating anabolic, catabolic, and homeostatic turnover under diurnal spinal motion.

Slow depressurization following intradiscal injection leads to injectate leakage in a large animal model

Needle injection has been indicated as the most practical method of delivering therapeutic agents to the intervertebral disc due to the disc's largely avascular nature. As the disc is characterized by both high stiffness and low permeability, injection requires substantial pressure, which may not relax on practical time scales. Additionally, needle puncture results in a localized disruption to the annulus fibrosus that can provide a leakage pathway for pressurized injectate. We hypothesized that intradiscal injection would result in slow relaxation of injectate pressure, followed by leakage upon needle retraction. This hypothesis was tested via controlled injection of fluorescently labeled saline into bovine caudal discs via a 21 gauge needle. Injections were performed with 10% of total disc volume injected at 3%/s followed by a 4-minute dwell. An analytical poroelastic model was calibrated to the experimental data and used to estimate injectate delivery with time. Experimental results confirmed both pressurization (with a peak of 199 ± 45 kPa) and slow recovery (final pressure of 81 ± 23 kPa). Injectate leakage through the needle puncture was verified following needle retraction in all samples. Histological sections of the discs displayed a clear defect at each disc's injection site with strong fluorescent labeling indicating a leakage pathway. The modeling results suggest that less than one-fourth of the injected volume was absorbed by the tissue in 4 minutes. Taken together these results suggest that needle injection is a feasible, albeit inefficient method for delivery of therapeutic agents into the intervertebral disc. Particular care should be taken to aspirate un-absorbed injectate prior to needle retraction to prevent leakage and exposure of surrounding tissues.

Spinal injury in alpine winter sports: a review

INTRODUCTION

Alpine winter sports have become increasingly popular over recent decades, with a similar increase in accident incidence. This review provides an overview of the most recent literature concerning spinal injury epidemiology, mechanisms, patterns and prevention strategies in the context of alpine winter sports.

MATERIAL AND METHODS

The PubMed, Cochrane Library, and EMBASE databases were searched using the keywords spine injury, alpine injury, spine fracture, skiing injuries, snowboard injuries. 64 published studies in English and German met a priori inclusion criteria and were reviewed in detail by the authors.

RESULTS

There are various mechanisms of injury in alpine winter sports (high speed falls in skiing, jumping failure in snowboarding) whilst regionality and injury severity are broadly similar. The thoracolumbar spine is the most common region for spinal injury. Spinal cord injury is relatively rare, usually accompanying distraction and rotation type fractures and is most commonly localised to the cervical spine. Disc injuries seem to occur more commonly in alpine winter sport athletes than in the general population.

DISCUSSION

Despite awareness of increasing rates and risks of spinal injuries in alpine winter sports, there has been little success in injury prevention.

Molecular and histological characteristics of bovine caudal nucleus pulposus by combined changes in hydrostatic and osmotic pressures in vitro

Intervertebral disc degeneration is ubiquitous among aging patients, and altered matrix homeostasis is one of the key features of this condition. Physicochemical stresses have a significant impact on matrix homeostasis as they lead to progressive degeneration and may be associated with spinal pain and dysfunction. Thus, it is important to understand the cellular and matrix characteristics of nucleus pulposus in response to these stresses, which include hydrostatic and osmotic pressures during alternate loading conditions. We hypothesized that a combination of changes in hydrostatic pressure and in osmotic pressure that mimic normal, daily spinal stress would stimulate anabolic function, whereas a non-realistic combination of those stresses would stimulate catabolic function in nucleus pulposus cells. We examined the effects of these combined stresses, represented by 12 systematic conditions, on the metabolic activities of enzymatically isolated bovine caudal nucleus pulposus in vitro. We measured the gene expression of extracellular matrix (ECM) molecules and proliferating cell nuclear antigen (PCNA) and evaluated the quality of the matrix and the capability of cell proliferation immunohistologically. Combined cyclic hydrostatic pressure at 0.5 MPa, 0.5 Hz, and high osmotic pressure at 450 mOsm upregulated the aggrecan core protein and collagen type-II gene expression significantly (p < 0.05), and showed trends of upregulation of chondroitin sulfate N-acetylgalactosaminyltransferase 1, matrix metalloproteinase-13, and PCNA. ECM, however, contained empty spaces at a high osmotic pressure with and without hydrostatic pressure. Since ECM has highly specialized physicochemical properties, homeostasis should involve not only phenotypic cellular behavior but also turnover of ECM. © 2018 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res 37:466-476, 2019.

Intervertebral disc damage models in organ culture: a comparison of annulus fibrosus cross-incision versus punch model under complex loading

PURPOSE

Comparison of two annulus fibrosus injury models that mimic intervertebral disc (IVD) herniation, enabling the study of IVD behaviour under three loading regimes in a bovine organ culture model.

METHODS

An injury was induced by custom-designed cross-incision tool or a 2-mm biopsy punch in IVDs. Discs were cultured for 14 days under (1) complex (compression and torsion), (2) static, and (3) no load. Disc height, mitochondrial activity, DNA and glycosaminoglycan (GAG) contents, and disc stiffness under complex load were determined. Further, gene expression and histology analysis were performed.

RESULTS

While both injury models did not change the compressional stiffness of IVDs, cross-incision decreased disc height under complex load. Moreover, under complex load, the biopsy punch injury induced down-regulation of several anabolic, catabol ic, and inflammatory genes, whereas cross-incision did not significantly differ from control discs. However, DNA and GAG contents were in the range of the healthy control discs for both injury models but did show lower contents under no load and static load. Injury side and contralateral side of the IVD showed a similar behaviour on the biochemical assays tested.

CONCLUSION

Compressional stiffness, GAG and DNA contents, did not differ between injury models under complex load. This behaviour was partially attributed to the positive influence of complex loading on matrix regeneration and cell viability. However, disc height was reduced for the cross-incision. Relative gene expression changes of the inflammatory and anabolic genes for the biopsy punch approach might indicate that induced damage was too intense to trigger any inflammatory or repair response. These slides can be retrieved under Electronic Supplementary Material.

Contribution of facet joints, axial compression, and composition to human lumbar disc torsion mechanics

Stresses applied to the spinal column are distributed between the intervertebral disc and facet joints. Structural and compositional changes alter stress distributions within the disc and between the disc and facet joints. These changes influence the mechanical properties of the disc joint, including its stiffness, range of motion, and energy absorption under quasi-static and dynamic loads. There have been few studies evaluating the role of facet joints in torsion. Furthermore, the relationship between biochemical composition and torsion mechanics is not well understood. Therefore, the first objective of this study was to investigate the role of facet joints in torsion mechanics of healthy and degenerated human lumbar discs under a wide range of compressive preloads. To achieve this, each disc was tested under four different compressive preloads (300-1200 N) with and without facet joints. The second objective was to develop a quantitative structure-function relationship between tissue composition and torsion mechanics. Facet joints have a significant contribution to disc torsional stiffness (∼60%) and viscoelasticity, regardless of the magnitude of axial compression. The findings from this study demonstrate that annulus fibrosus GAG content plays an important role in disc torsion mechanics. A decrease in GAG content with degeneration reduced torsion mechanics by more than an order of magnitude, while collagen content did not significantly influence disc torsion mechanics. The biochemical-mechanical and compression-torsion relationships reported in this study allow for better comparison between studies that use discs of varying levels of degeneration or testing protocols and provide important design criteria for biological repair strategies. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Bioreactors with hydrostatic pressures imitating physiological environments in intervertebral discs

Intervertebral discs are normally exposed to a variety of loads and stresses but hydrostatic pressure (HP) could be the main biosignal for chondrogenic cell differentiation and maintenance of this tissue. Although there are simple approaches to intermittently expose cell cultures to HP in separate material testing devices, utilization of biomimetic bioreactors aiming to provide in vitro conditions mimicking those found in vivo, attracts special attention. However, design of such bioreactors is complex due to the requirement of high HP magnitudes (up to 3 MPa) applied in different regimes mimicking pressures arising in intervertebral disc during normal daily activities. Furthermore, efficient mass transfer has to be facilitated to cells within 3D scaffolds, and the engineering challenges include avoidance or removal of gas bubbles in the culture medium before pressurization as well as selection of appropriate, biocompatible construction materials and maintenance of sterility during cultivation. Here, we review approaches to induce HP in 2D and 3D cell cultures categorized into 5 groups: (I) discontinuous systems with direct pressurization of the cultivation medium by a piston, (II) discontinuous systems with indirect pressurization by a compression fluid, (III) continuous systems with direct pressurization of the cultivation medium, static culture, (IV) continuous systems with culture perfusion, and (V) systems applying HP in conjunction with other physical signals. Although the complexity is increasing as additional features are added to the systems, the need to understand HP effects on cells and tissues in a physiologically relevant, yet precisely controlled, environment together with current technological advancements are leading towards innovative bioreactor solutions.

High-magnitude compression accelerates the premature senescence of nucleus pulposus cells via the p38 MAPK-ROS pathway

BACKGROUND

Mechanical overloading can lead to disc degeneration. Nucleus pulposus (NP) cell senescence is aggravated within the degenerated disc. This study was designed to investigate the effects of high compression on NP cell senescence and the underlying molecular mechanism of this process.

METHODS

Rat NP cells seeded in decalcified bone matrix were subjected to non-compression (control) or compression (2% or 20% deformation, 1.0 Hz, 6 hours/day). The reactive oxygen species (ROS) scavenger N-acetylcysteine (NAC) and the p38 MAPK inhibitor SB203580 were used to investigate the roles of the ROS and p38 MAPK pathway under high-magnitude compression. Additionally, we studied the effects of compression (0.1 or 1.3 MPa, 1.0 Hz, 6 hours/day) in a rat disc organ culture.

RESULTS

Both in scaffold and organ cultures, high-magnitude compression (20% deformation or 1.3 MPa) increased senescence-associated β-galactosidase (SA-β-Gal) activity, senescence marker (p16 and p53) expression, G1 cell cycle arrest, and ROS generation, and decreased cell proliferation, telomerase activity and matrix (aggrecan and collagen II) synthesis. Further analysis of the 20% deformation group showed that NAC inhibited NP cell senescence but had no obvious effect on phospho-p38 MAPK expression and that SB203580 significantly attenuated ROS generation and NP cell senescence.

CONCLUSIONS

High-magnitude compression can accelerate NP cell senescence through the p38 MAPK-ROS pathway.

Long-term load duration induces N-cadherin down-regulation and loss of cell phenotype of nucleus pulposus cells in a disc bioreactor culture

Long-term exposure to a mechanical load causes degenerative changes in the disc nucleus pulposus (NP) tissue. A previous study demonstrated that N-cadherin (N-CDH)-mediated signalling can preserve the NP cell phenotype. However, N-CDH expression and the resulting phenotype alteration in NP cells under mechanical compression remain unclear. The present study investigated the effects of the compressive duration on N-CDH expression and on the phenotype of NP cells in an ex vivo disc organ culture. Porcine discs were organ cultured in a self-developed mechanically active bioreactor for 7 days. The discs were subjected to different dynamic compression durations (1 and 8 h at a magnitude of 0.4 MPa and frequency of 1.0 Hz) once per day. Discs that were not compressed were used as controls. The results showed that long-term compression duration (8 h) significantly down-regulated the expression of N-CDH and NP-specific molecule markers (Brachyury, Laminin, Glypican-3 and Keratin 19), attenuated Alcian Blue staining intensity, decreased glycosaminoglycan (GAG) and hydroxyproline (HYP) contents and decreased matrix macromolecule (aggrecan and collagen II) expression compared with the short-term compression duration (1 h). Taken together, these findings demonstrate that long-term load duration can induce N-CDH down-regulation, loss of normal cell phenotype and result in attenuation of NP-related matrix synthesis in NP cells.

Influence of Complex Loading Conditions on Intervertebral Disc Failure

STUDY DESIGN

High resolution imaging investigation of the failure of ovine lumbar intervertebral discs under complex loading.

OBJECTIVE

To investigate how different loading combinations influence the mechanism and extent of intervertebral disc failure.

SUMMARY OF BACKGROUND DATA

Even though there has been extensive research on how an intervertebral disc fails under various conditions, failure mechanisms remain unclear. In addition, the influence of different loading directions on the mode and extent of failure under complex loading was never systematically investigated.

METHODS

Thirty ovine lumbar spinal segments were loaded in a newly developed, dynamic, 6-degree-of-freedom (6-DOF) disc loading simulator under five combinations of the following loading parameters: 0°-13° flexion, 0°-10° lateral bending, 0°-4° axial rotation, 0-800 N axial compression. A total of 1000 cycles at 2 Hz were done. After testing, imaging of the discs was performed in an ultra-high field magnetic resonance imaging (11.7 T) scanner and with a micro-computed tomography scanner.

RESULTS

A total of 13 large endplate junction failures (EPJFs) occurred, of which all but one maintained an intact cartilaginous endplate. Ten out of 13 EPJFs occurred caudally. Four solely annulus failures occurred affecting only the outer posterior annulus. A herniation was not observed. The maximum moments measured in any group (median) were 52.5 N · m flexion, 16.5 N · m lateral bending, and 14.0 N · m axial rotation.

CONCLUSION

Complex loading protocols could lead to EPJFs (76%) and annulus failures (24%) in vitro. The combination of flexion, lateral bending, axial rotation, and axial compression bears the highest risk for caudal EPJF. Flexion without lateral bending and vice versa has the lowest risk for failure. Both axial compression and axial rotation seem to have a smaller influence than flexion and lateral bending. It seems that a herniation requires an additional failure of the cartilaginous endplate, likely initiated by further axial compressive load.

LEVEL OF EVIDENCE

4.