Changes in nuclear composition following cyclic compression of the intervertebral disc in an in vivo rat-tail model

Mechanical Factors Regulate Annulus Fibrosus (AF) Injury Repair and Remodeling: A Review

Low back pain is a common chronic disease that can severely affect the patient's work and daily life. The breakdown of spinal mechanical homeostasis caused by intervertebral disc (IVD) degeneration is a leading cause of low back pain. Annulus fibrosus (AF), as the outer layer structure of the IVD, is often the first affected part. AF injury caused by consistent stress overload will further accelerate IVD degeneration. Therefore, regulating AF injury repair and remodeling should be the primary goal of the IVD repair strategy. Mechanical stimulation has been shown to promote AF regeneration and repair, but most studies only focus on the effect of single stress on AF, and lack realistic models and methods that can mimic the actual mechanical environment of AF. In this article, we review the effects of different types of stress stimulation on AF injury repair and remodeling, suggest possible beneficial load combinations, and explore the underlying molecular mechanisms. It will provide the theoretical basis for designing better tissue engineering therapy using mechanical factors to regulate AF injury repair and remodeling in the future.

TRPV4 differentially controls inflammatory cytokine networks during static and dynamic compression of the intervertebral disc

Background

The ion channel transient receptor potential vanilloid 4 (TRPV4) critically transduces mechanical forces in the IVD, and its inhibition can prevent IVD degeneration due to static overloading. However, it remains unknown whether different modes of loading signals through TRPV4 to regulate the expression of inflammatory cytokines. We hypothesized that TRPV4 signaling is essential during static and dynamic loading to mediate homeostasis and mechanotransduction.

Methods

Mouse functional spine units were isolated and either cyclically compressed for 5 days (1 Hz, 1 h, 10% strain) or statically compressed (24 h, 0.2 MPa). Conditioned media were monitored at 6 h, 24 h, 2 days, and 5 days, with and without TRPV4 inhibition. Effects of TRPV4 activation was also evaluated without loading. The media was analyzed for a panel of 44 cytokines using a microbead array and then a correlative network was constructed to explore the regulatory relationships during loading and TRPV4 inhibition. After the loading regimen, the IVDs were evaluated histologically for degeneration.

Results

Activation of TRPV4 led to an increase interleukin-6 (IL-6) family of cytokines (IL-6, IL-11, IL-16, and leukemia inhibitory factor [LIF]) and decreased the T-cell (CCL3, CCL4, CCL17, CCL20, CCL22, and CXCL10) and monocyte (CCL2 and CCL12) recruiting chemokines by the IVD. Dynamic and static loading each provoked unique chemokine correlation networks. The inhibition of TRPV4 during dynamic loading dysregulated the relationship between LIF and other cytokines, while the inhibition of TRPV4 during static loading disrupted the connectivity of IL-16 and VEGFA.

Conclusions

We demonstrated that TRPV4 critically mediates the cytokine production following dynamic and static loading. The activation of TRPV4 upregulated a diverse set of cytokines that may suppress the chemotaxis of T-cells and monocytes, implicating the role of TRPV4 in maintaining the immune privilege of healthy IVD.

Preclinical in vivo animal models of intervertebral disc degeneration. Part 1: A systematic review

Intervertebral disc degeneration (IVDD), a widely recognized cause of lower back pain, is the leading cause of disability worldwide. A myriad of preclinical in vivo animal models of IVDD have been described in the literature. There is a need for critical evaluation of these models to better inform researchers and clinicians to optimize study design and ultimately, enhance experimental outcomes. The purpose of this study was to conduct an extensive systematic literature review to report the variability of animal species, IVDD induction method, and experimental timepoints and endpoints used in in vivo IVDD preclinical research. A systematic literature review of peer-reviewed manuscripts featured on PubMed and EMBASE databases was conducted in accordance with PRISMA guidelines. Studies were included if they reported an in vivo animal model of IVDD and included details of the species used, how disc degeneration was induced, and the experimental endpoints used for analysis. Two-hundred and fifty-nine (259) studies were reviewed. The most common species, IVDD induction method and experimental endpoint used was rodents(140/259, 54.05%), surgery (168/259, 64.86%) and histology (217/259, 83.78%), respectively. Experimental timepoint varied greatly between studies, ranging from 1 week (dog and rodent models), to >104 weeks in dog, horse, monkey, rabbit, and sheep models. The two most common timepoints used across all species were 4 weeks (49 manuscripts) and 12 weeks (44 manuscripts). A comprehensive discussion of the species, methods of IVDD induction and experimental endpoints is presented. There was great variability across all categories: animal species, method of IVDD induction, timepoints and experimental endpoints. While no animal model can replicate the human scenario, the most appropriate model should be selected in line with the study objectives to optimize experimental design, outcomes and improve comparisons between studies.

Intervertebral Disc-on-a-Chip as Advanced In Vitro Model for Mechanobiology Research and Drug Testing: A Review and Perspective

Discogenic back pain is one of the most diffused musculoskeletal pathologies and a hurdle to a good quality of life for millions of people. Existing therapeutic options are exclusively directed at reducing symptoms, not at targeting the underlying, still poorly understood, degenerative processes. Common intervertebral disc (IVD) disease models still do not fully replicate the course of degenerative IVD disease. Advanced disease models that incorporate mechanical loading are needed to investigate pathological causes and processes, as well as to identify therapeutic targets. Organs-on-chip (OoC) are microfluidic-based devices that aim at recapitulating tissue functions in vitro by introducing key features of the tissue microenvironment (e.g., 3D architecture, soluble signals and mechanical conditioning). In this review we analyze and depict existing OoC platforms used to investigate pathological alterations of IVD cells/tissues and discuss their benefits and limitations. Starting from the consideration that mechanobiology plays a pivotal role in both IVD homeostasis and degeneration, we then focus on OoC settings enabling to recapitulate physiological or aberrant mechanical loading, in conjunction with other relevant features (such as inflammation). Finally, we propose our view on design criteria for IVD-on-a-chip systems, offering a future perspective to model IVD mechanobiology.

TRPV4 mediates cell damage induced by hyperphysiological compression and regulates COX2/PGE2 in intervertebral discs

BACKGROUND

Aberrant mechanical loading of the spine causes intervertebral disc (IVD) degeneration and low back pain. Current therapies do not target the mediators of the underlying mechanosensing and mechanotransduction pathways, as these are poorly understood. This study investigated the role of the mechanosensitive transient receptor potential vanilloid 4 (TRPV4) ion channel in dynamic compression of bovine nucleus pulposus (NP) cells in vitro and mouse IVDs in vivo.

METHODS

Degenerative changes and the expression of the inflammatory mediator cyclooxygenase 2 (COX2) were examined histologically in the IVDs of mouse tails that were dynamically compressed at a short repetitive hyperphysiological regime (vs sham). Bovine NP cells embedded in an agarose-collagen hydrogel were dynamically compressed at a hyperphysiological regime in the presence or absence of the selective TRPV4 antagonist GSK2193874. Lactate dehydrogenase (LDH) and prostaglandin E2 (PGE2) release, as well as phosphorylation of mitogen-activated protein kinases (MAPKs), were analyzed. Degenerative changes and COX2 expression were further evaluated in the IVDs of trpv4-deficient mice (vs wild-type; WT).

RESULTS

Dynamic compression caused IVD degeneration in vivo as previously shown but did not affect COX2 expression. Dynamic compression significantly augmented LDH and PGE2 releases in vitro, which were significantly reduced by TRPV4 inhibition. Moreover, TRPV4 inhibition during dynamic compression increased the activation of the extracellular signal-regulated kinases 1/2 (ERK) MAPK pathway by 3.13-fold compared to non-compressed samples. Trpv4-deficient mice displayed mild IVD degeneration and decreased COX2 expression compared to WT mice.

CONCLUSIONS

TRPV4 therefore regulates COX2/PGE2 and mediates cell damage induced by hyperphysiological dynamic compression, possibly via ERK. Targeted TRPV4 inhibition or knockdown might thus constitute promising therapeutic approaches to treat patients suffering from IVD pathologies caused by aberrant mechanical stress.

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.

CircRNA-CIDN mitigated compression loading-induced damage in human nucleus pulposus cells via miR-34a-5p/SIRT1 axis

Background

Intervertebral disc degeneration (IDD) is a major contributor to lower back pain, however, the molecular and pathogenetic mechanisms underlying IDD are poorly understood. As a high-risk factor for IDD, compression stress was reported to induce apoptosis of nucleus pulposus (NP) cells and extracellular matrix (ECM) degradation during IDD progression. Circular RNA (circRNA) is a class of endogenous non-coding RNA (ncRNA) and has been reported to function in several diseases. However, whether and how circRNA regulates compression-induced damage of NP cells remains vague. Here, we aimed to investigate the key role of circRNA in compression loading-induced IDD.

Methods

We analysed the circRNA expression of three samples from compression-treated NP cells and three control samples using circRNA microarray assays and further investigated the circRNA involved in compression-induced damage of NP cells (circRNA-CIDN). We investigated the effects of circRNA-CIDN on compression-induced cell apoptosis and NP ECM degradation in vitro and ex vivo. We observed that circRNA-CIDN bound to miRNAs as a miRNA sponge based on luciferase and RNA immunoprecipitation (RIP) assays.

Findings:

CircRNA-CIDN was significantly downregulated in compression-treated human NP cells, as validated by circRNA microarray and qRT-PCR analysis, and overexpressing circRNA-CIDN inhibited compression-induced apoptosis and NP ECM degradation. Further studies demonstrated that circRNA-CIDN served as a sponge for miR-34a-5p, an important miRNA that enhanced compression-induced damage of NP cells via repressing the silent mating type information regulation 2 homolog 1 (SIRT1). CircRNA-CIDN was also verified to contain IDD development in an ex vivo IDD model.

Interpretation

Our results revealed that circRNA-CIDN binding to miR-34a-5p played an important role in mitigating compression loading-induced nucleus pulposus cell damage via targeting SIRT1, providing a potential therapeutic strategy for IDD treatment.

Funding

National Natural Science Foundation of China (81772391, 81974348), Fundamental Research Funds for the Central Universities (2017KFYXJJ248).

Mechanobiology of annulus fibrosus and nucleus pulposus cells in intervertebral discs

Low back pain (LBP) is a chronic condition that can affect up to 80% of the global population. It is the number one cause of disability worldwide and has enormous socioeconomic consequences. One of the main causes of this condition is intervertebral disc (IVD) degeneration. IVD degenerative processes and inflammation associated with it has been the subject of many studies in both tissue and cell level. It is believed that the phenotype of the resident cells within the IVD directly affects homeostasis of the tissue. At the same time, IVDs located between vertebral bodies of spine are under various mechanical loading conditions in vivo. Therefore, investigating how mechanical loading can affect the behaviour of IVD cells has been a subject of many research articles. In this review paper, following a brief explanation of the anatomy of the IVD and its resident cells, we compiled mechanobiological studies of IVD cells (specifically, annulus fibrosus and nucleus pulposus cells) and synthesized and discussed the key findings of the field.

Small Molecule-Based Strategy Promotes Nucleus Pulposus Specific Differentiation of Adipose-Derived Mesenchymal Stem Cells

Adipose tissue-derived mesenchymal stem cells (ADSCs) are promising for regenerating degenerated intervertebral discs (IVDs), but the low efficiency of nucleus pulposus (NP)-specific differentiation limits their clinical applications. The Sonic hedgehog (Shh) signaling pathway is important in NP-specific differentiation of ADSCs, and Smoothened Agonist (SAG) is a highly specific and effective agonist of Shh signaling. In this study, we proposed a new differentiation strategy with the use of the small molecule SAG. The NP-specific differentiation and extracellular matrix (ECM) synthesis of ADSCs were measured in vitro , and the regenerative effects of SAG pretreated ADSCs in degenerated IVDs were verified in vivo . The results showed that the combination of SAG and transforming growth factor-β3 (TGF-β3) is able to increase the ECM synthesis of ADSCs. In addition, the gene and protein expression levels of NP-specific markers were increased by treatment with SAG and TGF-β3. Furthermore, SAG pretreated ADSCs can also improve the disc height, water content, ECM content, and structure of degenerated IVDs in vivo . Our new differentiation scheme has high efficiency in inducing NP-specific differentiation of ADSCs and is promising for stem cell-based treatment of degenerated IVDs.

Intervertebral disc degeneration induced by long-segment in-situ immobilization: a macro, micro, and nanoscale analysis

Background

Cervical spine fixation or immobilization has become a routine treatment for spinal fracture, dislocation, subluxation injuries, or spondylosis. The effects of immobilization of intervertebral discs of the cervical spine is unclear. The goal of this study was to evaluate the effects of long-segment in-situ immobilization of intervertebral discs of the caudal vertebra, thereby simulating human cervical spine immobilization.

Methods

Thirty-five fully grown, male Sprague-Dawley rats were used. Rats were randomly assigned to one of five groups: Group A, which served as controls, and Groups B, C, D, and E, in which the caudal vertebrae were in-situ immobilized using a custom-made external device that fixed four caudal vertebrae (Co7-Co10). After 2 weeks, 4 weeks, 6 weeks, and 8 weeks of in-situ immobilization, the caudal vertebrae were harvested, and the disc height, the T2 signal intensity of the discs, disc morphology, the gene expression of discs, and the structure and the elastic modulus of discs was measured.

Results

The intervertebral disc height progressively decreased, starting at the 6th week. At week 6 and week 8, disc degeneration was classified as grade III, according to the modified Pfirrmann grading system criteria. Long-segment immobilization altered the gene expression of discs. The nucleus pulposus showed a typical cell cluster phenomenon over time. The annulus fibrosus inner layer began to appear disordered with fissure formation. The elastic modulus of collagen fibrils within the nucleus pulposus was significantly decreased in rats in group E compared to rats in group A (p < 0.05). On the contrary, the elastic modulus within the annulus was significantly increased in rats in group E compared to rats in group A (p < 0.05).

Conclusion

Long-segment in-situ immobilization caused target disc degeneration, and positively correlated with fixation time. The degeneration was not only associated with changes at the macroscale and microscale, but also indicated changes in collagen fibrils at the nanoscale. Long-segment immobilization of the spine (cervical spine) does not seem to be an innocuous strategy for the treatment of spine-related diseases and may be a predisposing factor in the development of the symptomatic spine.

Mechanotransduction and cell biomechanics of the intervertebral disc

Mechanical loading of the intervertebral disc (IVD) initiates cell-mediated remodeling events that contribute to disc degeneration. Cells of the IVD, nucleus pulposus (NP) and anulus fibrosus (AF), will exhibit various responses to different mechanical stimuli which appear to be highly dependent on loading type, magnitude, duration, and anatomic zone of cell origin. Cells of the NP, the innermost region of the disc, exhibit an anabolic response to low-moderate magnitudes of static compression, osmotic pressure, or hydrostatic pressure, while higher magnitudes promote a catabolic response marked by increased protease expression and activity. Cells of the outer AF are responsive to physical forces in a manner that depends on frequency and magnitude, as are cells of the NP, though they experience different forces, deformations, pressure, and osmotic pressure in vivo. Much remains to be understood of the mechanotransduction pathways that regulate IVD cell responses to loading, including responses to specific stimuli and also differences among cell types. There is evidence that cytoskeletal remodeling and receptor-mediated signaling are important mechanotransduction events that can regulate downstream effects like gene expression and posttranslational biosynthesis, all of which may influence phenotype and bioactivity. These and other mechanotransduction events will be regulated by known and to-be-discovered cell-matrix and cell-cell interactions, and depend on composition of extracellular matrix ligands for cell interaction, matrix stiffness, and the phenotype of the cells themselves. Here, we present a review of the current knowledge of the role of mechanical stimuli and the impact upon the cellular response to loading and changes that occur with aging and degeneration of the IVD.

In vivo correlates between daily physical activity and intervertebral disc health

Physical activity impacts health and disease in multiple body tissues including the intervertebral discs. Fluid flow within the disc is an indicator of disc health that can be observed using diffusion weighted magnetic resonance imaging. We monitored activity levels of 26 participants, age 35-55 yrs, using Actigraph accelerometers for 4 days to evaluate vigorous-intensity activity, moderate to vigorous intensity activity, and sedentary time. Participants underwent structural and diffusion weighted magnetic resonance imaging to evaluate intervertebral disc health and fluid flow. They also underwent bone density scans, carotid artery ultrasounds, a treadmill test, and a physical exam for pain, range of motion, and instability. These measures were used to correlate MRI indicators of intervertebral disc health with participant activity levels. Participants with any vigorous-intensity physical activity compared with no vigorous-intensity activity had significantly greater L5/S1 apparent diffusion coefficient values (p = 0.002), corresponding to higher freedom of diffusive movement for cellular nutrients and metabolic waste. Sagittal T2 values in the L5/S1 were also higher (p = 0.004), corresponding to a higher water content in the discs. Higher apparent diffusion coefficients were also found in participants with more than 30 min compared with less than 30 min of daily moderate to vigorous physical activity (p = 0.03), and in participants with less than 67% awake time as sedentary time compared with more than 67% sedentary time (p = 0.03). Increased dynamic loading through physical activity and decreased static loading from sedentary time benefit intervertebral disc health. Physical activity, particularly vigorous activity, is beneficial in helping maintain intervertebral disc health. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1313-1323, 2018.

Genipin cross-linked type II collagen/chondroitin sulfate composite hydrogel-like cell delivery system induces differentiation of adipose-derived stem cells and regenerates degenerated nucleus pulposus

Nucleus pulposus (NP) degeneration is usually the origin of intervertebral disc degeneration and consequent lower back pain. Although adipose-derived stem cell (ADSC)-based therapy is regarded to be promising for the treatment of degenerated NP, there is a lack of viable cell carriers to transplant ADSCs into the NP while maintaining cell function. In this study, we developed a type II collagen/chondroitin sulfate (CS) composite hydrogel-like ADSC (CCSA) delivery system with genipin as the cross-linking agent. The induction effect of the scaffold on ADSC differentiation was studied in vitro, and a rat coccygeal vertebrae degeneration model was used to investigate the regenerative effect of the CCSA system on the degenerated NP in vivo. The results showed that the CCSA delivery system cross-linked with 0.02% genipin was biocompatible and promoted the expressions of NP-specific genes. After the injection of the CCSA system, the disc height, water content, extracellular matrix synthesis, and structure of the degenerated NP were partly restored. Our CCSA delivery system uses minimally invasive approaches to promote the regeneration of degenerated NP and provides an exciting new avenue for the treatment of degenerative disc disease.

STATEMENT OF SIGNIFICANCE

Nucleus pulposus (NP) degeneration is usually the origin of intervertebral disc degeneration and consequent lower back pain. Stem cell-based tissue engineering is a promising method in NP regeneration, but there is a lack of viable cell carriers to transplant ADSCs into the NP while maintaining cell function. In this study, we developed a type II collagen/chondroitin sulfate (CS) composite hydrogel-like ADSC (CCSA) delivery system with genipin as the cross-linking agent. Although several research groups have studied the fabrication of injectable hydrogel with biological matrix, our study differs from other works. We chose type II collagen and CS, the two primary native components in the NP, as the main materials and combined them according to the natural ratio of collagen and sGAG in the NP. The delivery system is preloaded with ADSCs and can be injected into the NP with a needle, followed by in situ gelation. Genipin is used as a cross-linker to improve the bio-stability of the scaffold, with low cytotoxicity. We investigated the stimulatory effects of our scaffold on the differentiation of ADSCs in vitro and the regenerative effect of the CCSA delivery system on degenerated NP in vivo.

Static Compression Induces ECM Remodeling and Integrin α2β1 Expression and Signaling in a Rat Tail Caudal Intervertebral Disc Degeneration Model

STUDY DESIGN

A three-level rat tail caudal intervertebral disc (IVD) degeneration (IVDD) model was established to study effects of static compression on extracellular matrix (ECM) remodeling and integrin signaling in IVDs during IVDD.

OBJECTIVE

The aim of this study was to investigate the effect of compression force on ECM remodeling and integrin signaling in IVDs during IVDD.

SUMMARY OF BACKGROUND DATA

Integrins sense mechanical environment alteration via binding to ECM ligands and trigger intracellular signaling for pathological ECM remodeling during IVDD. However, the role of compression force in ECM remodeling and integrin signaling during IVDD remains elusive.

METHODS

Compared with the classical one-level rat tail IVDD model that exerts axial stress on the 8th to 9th caudal vertebral bodies, a three-level model was established by using an Ilizarov-type apparatus to exert stress on the 7th to 10th caudal vertebral bodies in rat tails for four weeks. To exclude side effects from surgical stab injury on manipulated discs, intact coccygeal (Co) disc Co8-9 was analyzed.

RESULTS

In three-level IVDD model, significant degeneration of the Co8-9 disc was observed. Quantitative real-time polymerase chain reaction (qRT-PCR) showed elevated mRNA expression of collagen types I, III, and V; matrix metalloproteinases (MMPs) 2, 3, 9, 13, 14; and decreased mRNA expression of collagen type II in Co8-9 disc. Compression loading altered the expression of integrin α2β1 (upregulated) and α10β1 (downregulated) in NP cells, and activated integrin downstream signaling. By contrast, one-level model showed more severe disc degeneration and ECM remodeling. Integrin α1, α2, α11, and β1 were upregulated, whereas α10 was downregulated. Similar activation of integrin signaling was observed.

CONCLUSION

Static compression altered collagen and MMP expression, and promoted β1 integrin expression and signaling in IVD. Compared with one-level rat tail IVDD model, three-level model showed milder effects on disc degeneration, ECM remodeling, and integrin expression, suggesting one-level model might involve other causes that induce IVDD via mechanisms independent of compression force.

LEVEL OF EVIDENCE

N/A.

Optical Coherence Tomographic Elastography Reveals Mesoscale Shear Strain Inhomogeneities in the Annulus Fibrosus

STUDY DESIGN

Basic science study using in vitro tissue testing and imaging to characterize local strains in annulus fibrosus (AF) tissue.

OBJECTIVE

To characterize mesoscale strain inhomogeneities between lamellar and inter-/translamellar (ITL) matrix compartments during tissue shear loading.

SUMMARY OF BACKGROUND DATA

The intervertebral disc is characterized by significant heterogeneities in tissue structure and plays a critical role in load distribution and force transmission in the spine. In particular, the AF possesses a lamellar architecture interdigitated by a complex network of extracellular matrix components that form a distinct ITL compartment. Currently, there is not a firm understanding of how the lamellar and ITL matrix coordinately support tissue loading.

METHODS

AF tissue samples were prepared from frozen porcine lumbar spines and mounted onto custom fixtures of a materials testing system that incorporates optical coherence tomography (OCT) imaging to perform tissue elastography. Tissues were subjected to 20 and 40% nominal shear strain, and OCT images were captured and segmented to identify regions of interest corresponding to lamellar and ITL compartments. Images were analyzed using an optical flow algorithm to quantify local shear strains within each compartment.

RESULTS

Using histology and OCT, we first verified our ability to visualize and discriminate the ITL matrix from the lamellar matrix in porcine AF tissues. Local AF strains in the ITL compartment (22.0 ± 13.8, 31.1 ± 16.9 at 20% and 40% applied shear, respectively) were significantly higher than corresponding strains in the surrounding lamellar compartment (12.1 ± 5.6, 15.3 ± 5.2) for all tissue samples (P < 0.05).

CONCLUSION

Results from this study demonstrate that the lamellar and ITL compartments of the AF distribute strain unevenly during tissue loading. Specifically, shear strain is significantly higher in the ITL matrix, suggesting that these regions may be more susceptible to tissue damage and more mechanobiologically active.

LEVEL OF EVIDENCE

N/A.

Static and dynamic compression application and removal on the intervertebral discs of growing rats

Fusionless implants are used to correct pediatric progressive spinal deformities, most of them spanning the intervertebral disc. This study aimed at investigating the effects of in vivo static versus dynamic compression application and removal on discs of growing rats. A microloading device applied compression. 48 immature rats (28 d.o.) were divided into two groups (43d, 53d). Each group included four subgroups: control (no surgery), sham (device installed without loading), static (0.2 MPa) and dynamic compressions (0.2 MPa ± 30% with 0.1 Hz). In 43d subgroups, compression was applied for 15 days. In 53d subgroups, compression was followed by 10 days without loading. Disc heights, nucleus/annulus volumetric proportions and nucleus proteoglycan contents were analyzed using one-way ANOVA and post-hoc Tukey comparisons (p < 0.05). Disc heights of 43d and 53d static and dynamic loading rats were lower than shams (p < 0.05). Volumetric proportions remained similar. At 43d, nucleus proteoglycan contents increased in both static and dynamic loading rats. However, at 53d, static loading rats had lower proteoglycan content than dynamic loading rats (p < 0.05). Disc structure is altered following static compression removal, but nucleus proteoglycan content remaining elevated in dynamic group. Dynamic fusionless implants would better preserve disc integrity.

Dynamic Compression Effects on Immature Nucleus Pulposus: a Study Using a Novel Intelligent and Mechanically Active Bioreactor

BACKGROUND

Previous cell culture and animal in vivo studies indicate the obvious effects of mechanical compression on disc cell biology. However, the effects of dynamic compression magnitude, frequency and duration on the immature nucleus pulposus (NP) from an organ-cultured disc are not well understood.

OBJECTIVE

To investigate the effects of a relatively wide range of compressive magnitudes, frequencies and durations on cell apoptosis and matrix composition within the immature NP using an intelligent and mechanically active bioreactor.

METHODS

Discs from the immature porcine were cultured in a mechanically active bioreactor for 7 days. The discs in various compressive magnitude groups (0.1, 0.2, 0.4, 0.8 and 1.3 MPa at a frequency of 1.0 Hz for 2 hours), frequency groups (0.1, 0.5, 1.0, 3.0 and 5.0 Hz at a magnitude of 0.4 MPa for 2 hours) and duration groups (1, 2, 4 and 8 hours at a magnitude of 0.4 MPa and frequency of 1.0 Hz) experienced dynamic compression once per day. Discs cultured without compression were used as controls. Immature NP samples were analyzed using the TUNEL assay, histological staining, glycosaminoglycan (GAG) content measurement, real-time PCR and collagen II immunohistochemical staining.

RESULTS

In the 1.3 MPa, 5.0 Hz and 8 hour groups, the immature NP showed a significantly increase in apoptotic cells, a catabolic gene expression profile with down-regulated matrix molecules and up-regulated matrix degradation enzymes, and decreased GAG content and collagen II deposition. In the other compressive magnitude, frequency and duration groups, the immature NP showed a healthier status regarding NP cell apoptosis, gene expression profile and matrix production.

CONCLUSION

Cell apoptosis and matrix composition within the immature NP were compressive magnitude-, frequency- and duration-dependent. The relatively high compressive magnitude or frequency and long compressive duration are not helpful for maintaining the healthy status of an immature NP.

Low rate loading-induced convection enhances net transport into the intervertebral disc in vivo

BACKGROUND CONTEXT

The intervertebral disc primarily relies on trans-endplate diffusion for the uptake of nutrients and the clearance of byproducts. In degenerative discs, diffusion is often diminished by endplate sclerosis and reduced proteoglycan content. Mechanical loading-induced convection has the potential to augment diffusion and enhance net transport into the disc. The ability of convection to augment disc transport is controversial and has not been demonstrated in vivo.

PURPOSE

To determine if loading-induced convection can enhance small molecule transport into the intervertebral disc in vivo.

STUDY DESIGN

Net transport was quantified via postcontrast enhanced magnetic resonance imaging (MRI) into the discs of the New Zealand white rabbit lumbar spine subjected to in vivo cyclic low rate loading.

METHODS

Animals were administered the MRI contrast agent gadodiamide intravenously and subjected to in vivo low rate loading (0.5 Hz, 200 N) via a custom external loading apparatus for either 2.5, 5, 10, 15, or 20 minutes. Animals were then euthanized and the lumbar spines imaged using postcontrast enhanced MRI. The T1 constants in the nucleus, annulus, and cartilage endplates were quantified as a measure of gadodiamide transport into the loaded discs compared with the adjacent unloaded discs. Microcomputed tomography was used to quantify subchondral bone density.

RESULTS

Low rate loading caused the rapid uptake and clearance of gadodiamide in the nucleus compared with unloaded discs, which exhibited a slower rate of uptake. Relative to unloaded discs, low rate loading caused a maximum increase in transport into the nucleus of 16.8% after 5 minutes of loading. Low rate loading increased the concentration of gadodiamide in the cartilage endplates at each time point compared with unloaded levels.

CONCLUSIONS

Results from this study indicate that forced convection accelerated small molecule uptake and clearance in the disc induced by low rate mechanical loading. Low rate loading may, therefore, be therapeutic to the disc as it may enhance the nutrient uptake and waste product clearance.

Photochemically crosslinked collagen annulus plug: a potential solution solving the leakage problem of cell-based therapies for disc degeneration

Intra-disc injection of mesenchymal stem cells (MSCs) to treat disc degeneration may lead to unfavorable complications, particularly osteophyte formation. Development of an effective method to block the injection portal, prevent the leakage of injected cells and materials and, hence, prevent osteophyte formation is of the utmost importance before MSC-based therapies can be applied in a clinical setting. Here we seek to alleviate the cell leakage problem and the associated complication osteophyte formation by developing an injectable annulus plug to block the injection portal during intra-disc delivery. Specifically, we fabricated a needle-shaped collagen plug by photochemical crosslinking and successfully delivered it intra-discally, in association with MSCs in collagen microsphere carriers, using a custom-made delivery device. The mechanical performance of the plug and its effectiveness in reducing cell leakage were evaluated ex vivo under compression and in torsion push-out tests. The results demonstrate that the plug survived physiologically relevant loadings and significantly reduced leakage and enhanced retention of the injected materials. Finally, a pilot in vivo study in rabbits was conducted to evaluate the performance of the plug. Microcomputed tomography imaging and histology revealed that the plug significantly reduced osteophyte formation. This work suggests the potential of the annulus plug as an adjunct or annulus closure device for intra-disc delivery of cells and materials.

Dynamic and static overloading induce early degenerative processes in caprine lumbar intervertebral discs

Mechanical overloading of the spine is associated with low back pain and intervertebral disc (IVD) degeneration. How excessive loading elicits degenerative changes in the IVD is poorly understood. Comprehensive knowledge of the interaction between mechanical loading, cell responses and changes in the extracellular matrix of the disc is needed in order to successfully intervene in this process. The purpose of the current study was to investigate whether dynamic and static overloading affect caprine lumbar discs differently and what mechanisms lead to mechanically induced IVD degeneration. Lumbar caprine IVDs (n = 175) were cultured 7, 14 and 21 days under simulated-physiological loading (control), high dynamic or high static loading. Axial deformation and stiffness were continuously measured. Cell viability, cell density, and gene expression were assessed in the nucleus, inner- and outer annulus. The extracellular matrix (ECM) was analyzed for water, glycosaminoglycan and collagen content. IVD height loss and changes in axial deformation were gradual with dynamic and acute with static overloading. Dynamic overloading caused cell death in all IVD regions, whereas static overloading mostly affected the outer annulus. IVDs expression of catabolic and inflammation-related genes was up-regulated directly, whereas loss of water and glycosaminoglycan were significant only after 21 days. Static and dynamic overloading both induced pathological changes to caprine lumbar IVDs within 21 days. The mechanism by which they inflict biomechanical, cellular, and extracellular changes to the nucleus and annulus differed. The described cascades provide leads for the development of new pharmacological and rehabilitative therapies to halt the progression of DDD.

Effects of Panax ginseng-containing herbal plasters on compressed intervertebral discs in an in vivo rat tail model

BACKGROUND

Tienchi (Panax notoginseng) has been used in conservative treatments for back pain as a major ingredient of many herbal medicines. This study aims to investigate the effects of a herbal medicine containing tienchi on compressed intervertebral discs in rats.

METHODS

Using an in vivo rat tail model, intervertebral disc compression was simulated in the caudal 8-9 discs of 25 rats by continuous static compression (11 N) for 2 weeks. An herbal medicine plaster (in which the major ingredient was tienchi) was externally applied to the compressed disc (n=9) for three weeks, and held in place by an adhesive bandage, in animals in the Chinese Medicine (CM) group. The effect of the bandage was evaluated in a separate placebo group (n=9), while no intervention with unrestricted motion was provided to rats in an additional control group (n=7). Disc structural properties were quantified by in vivo disc height measurement and in vitro morphological analysis.

RESULTS

Disc height decreased after the application of compression (P < 0.001). The disc height decreased continuously in the control (P = 0.006) and placebo (P = 0.003) groups, but was maintained in the CM group (P = 0.494). No obvious differences in disc morphology were observed among the three groups (P = 0.896).

CONCLUSION

The tienchi-containing herbal plaster had no significant effect on the morphology of compressed discs, but maintained disc height in rats.

Stemming the Degeneration: IVD Stem Cells and Stem Cell Regenerative Therapy for Degenerative Disc Disease

The intervertebral disc (IVD) is immensely important for the integrity of vertebral column function. The highly specialized IVD functions to confer flexibility and tensile strength to the spine and endures various types of biomechanical force. Degenerative disc disease (DDD) is a prevalent musculoskeletal disorder and is the major cause of low back pain and includes the more severe degenerative lumbar scoliosis, disc herniation and spinal stenosis. DDD is a multifactorial disorder whereby an imbalance of anabolic and catabolic factors, or alterations to cellular composition, or biophysical stimuli and genetic background can all play a role in its genesis. However, our comprehension of IVD formation and theetiology of disc degeneration (DD) are far from being complete, hampering efforts to formulate appropriate therapies to tackle DD. Knowledge of the stem cells and various techniques to manipulate and direct them to particular fates have been promising in adopting a stem-cell based regenerative approach to DD. Moreover, new evidence on the residence of stem/progenitor cells within particular IVD niches has emerged holding promise for future therapeutic applications. Existing issues pertaining to current therapeutic approaches are also covered in this review.

Rat tail static compression model mimics extracellular matrix metabolic imbalances of matrix metalloproteinases, aggrecanases, and tissue inhibitors of metalloproteinases in intervertebral disc degeneration

Introduction

The longitudinal degradation mechanism of extracellular matrix (ECM) in the interbertebral disc remains unclear. Our objective was to elucidate catabolic and anabolic gene expression profiles and their balances in intervertebral disc degeneration using a static compression model.

Methods

Forty-eight 12-week-old male Sprague-Dawley rat tails were instrumented with an Ilizarov-type device with springs and loaded statically at 1.3 MPa for up to 56 days. Experimental loaded and distal-unloaded control discs were harvested and analyzed by real-time reverse transcription-polymerase chain reaction (PCR) messenger RNA quantification for catabolic genes [matrix metalloproteinase (MMP)-1a, MMP-2, MMP-3, MMP-7, MMP-9, MMP-13, a disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS)-4, and ADAMTS-5], anti-catabolic genes [tissue inhibitor of metalloproteinases (TIMP)-1, TIMP-2, and TIMP-3], ECM genes [aggrecan-1, collagen type 1-α1, and collagen type 2-α1], and pro-inflammatory cytokine genes [tumor necrosis factor (TNF)-α, interleukin (IL)-1α, IL-1β, and IL-6]. Immunohistochemistry for MMP-3, ADAMTS-4, ADAMTS-5, TIMP-1, TIMP-2, and TIMP-3 was performed to assess their protein expression level and distribution. The presence of MMP- and aggrecanase-cleaved aggrecan neoepitopes was similarly investigated to evaluate aggrecanolytic activity.

Results

Quantitative PCR demonstrated up-regulation of all MMPs and ADAMTS-4 but not ADAMTS-5. TIMP-1 and TIMP-2 were almost unchanged while TIMP-3 was down-regulated. Down-regulation of aggrecan-1 and collagen type 2-α1 and up-regulation of collagen type 1-α1 were observed. Despite TNF-α elevation, ILs developed little to no up-regulation. Immunohistochemistry showed, in the nucleus pulposus, the percentage of immunopositive cells of MMP-cleaved aggrecan neoepitope increased from 7 through 56 days with increased MMP-3 and decreased TIMP-1 and TIMP-2 immunopositivity. The percentage of immunopositive cells of aggrecanase-cleaved aggrecan neoepitope increased at 7 and 28 days only with decreased TIMP-3 immunopositivity. In the annulus fibrosus, MMP-cleaved aggrecan neoepitope presented much the same expression pattern. Aggrecanase-cleaved aggrecan neoepitope increased at 7 and 28 days only with increased ADAMTS-4 and ADAMTS-5 immunopositivity.

Conclusions

This rat tail sustained static compression model mimics ECM metabolic imbalances of MMPs, aggrecanases, and TIMPs in human degenerative discs. A dominant imbalance of MMP-3/TIMP-1 and TIMP-2 relative to ADAMTS-4 and ADAMTS-5/TIMP-3 signifies an advanced stage of intervertebral disc degeneration.

The effects of dynamic loading on the intervertebral disc

Loading is important to maintain the balance of matrix turnover in the intervertebral disc (IVD). Daily cyclic diurnal assists in the transport of large soluble factors across the IVD and its surrounding circulation and applies direct and indirect stimulus to disc cells. Acute mechanical injury and accumulated overloading, however, could induce disc degeneration. Recently, there is more information available on how cyclic loading, especially axial compression and hydrostatic pressure, affects IVD cell biology. This review summarises recent studies on the response of the IVD and stem cells to applied cyclic compression and hydrostatic pressure. These studies investigate the possible role of loading in the initiation and progression of disc degeneration as well as quantifying a physiological loading condition for the study of disc degeneration biological therapy. Subsequently, a possible physiological/beneficial loading range is proposed. This physiological/beneficial loading could provide insight into how to design loading regimes in specific system for the testing of various biological therapies such as cell therapy, chemical therapy or tissue engineering constructs to achieve a better final outcome. In addition, the parameter space of 'physiological' loading may also be an important factor for the differentiation of stem cells towards most ideally 'discogenic' cells for tissue engineering purpose.

Role of load history in intervertebral disc mechanics and intradiscal pressure generation

Solid-fluid interactions play an important role in mediating viscoelastic behaviour of biological tissues. In the intervertebral disc, water content is governed by a number of factors, including age, disease and mechanical loads, leading to changes in stiffness characteristics. We hypothesized that zonal stress distributions depend on load history, or the prior stresses experienced by the disc. To investigate these effects, rat caudal motion segments were subjected to compressive creep biomechanical testing in vitro using a protocol that consisted of two phases: a Prestress Phase (varied to represent different histories of load) followed immediately by an Exertion Phase, identical across all Prestress groups. Three analytical models were used to fit the experimental data in order to evaluate load history effects on gross and zonal disc mechanics. Model results indicated that while gross transient response was insensitive to load history, there may be changes in the internal mechanics of the disc. In particular, a fluid transport model suggested that the role of the nucleus pulposus in resisting creep during Exertion depended on Prestress conditions. Separate experiments using similarly defined load history regimens were performed to verify these predictions by measuring intradiscal pressure with a fibre optic sensor. We found that the ability for intradiscal pressure generation was load history-dependent and exhibited even greater sensitivity than predicted by analytical models. A 0.5 MPa Exertion load resulted in 537.2 kPa IDP for low magnitude Prestress compared with 373.7 kPa for high magnitude Prestress. Based on these measurements, we developed a simple model that may describe the pressure-shear environment in the nucleus pulposus. These findings may have important implications on our understanding of how mechanical stress contributes to disc health and disease etiology.

Modelo de degeneração do disco intervertebral por punção da cauda de ratos Wistar: avaliação histológica e radiográfica

OBJETIVO: descrever a caracterização histológica e radiográfica do método de indução da degeneração do disco intervertebral da cauda de ratos Wistar induzida por meio de punção. MÉTODOS: ratos Wistar machos adultos foram anestesiados, radiografados e submetidos à punção dos discos intervertebrais localizados entre a sexta e a sétima e a oitava e nona vértebras coccígeas. Para a punção foi utilizada agulha de 20G, que foi introduzida até o ânulo fibroso, e foi realizada dupla rotação de 360º, mantendo-se a mesma posição durante 30 segundos antes da retirada. O disco intermediário aos segmentos lesados (7-8) não foi puncionado e foi utilizado como controle. Foi selecionado o período pós-lesão de 30 dias (n=9) para sacrifício e análise dos discos intervertebrais. Os animais foram radiografados 30 dias após a lesão para análise da altura do disco intervertebral. Os segmentos da cauda foram removidos, fixados e desmineralizados, processados e corados com Hematoxilina-Eosina para avaliação histológica. RESULTADOS: a análise radiográfica revelou a redução significativa da altura dos discos lesados em relação ao controle. A avaliação histológica revelou alterações no núcleo pulposo e ânulo fibroso dos discos lesados em relação ao controle. Não foram observadas diferenças na intensidade de lesão entre os discos proximal e distal. CONCLUSÃO: a degeneração do disco intervertebral da cauda de ratos Wistar induzida por meio de punção mostrou ser método reprodutível para estudo da degeneração do disco intervertebral. Esse modelo mostrou validade para avaliação experimental de novas intervenções terapêuticas nos processos de degeneração do disco intervertebral.

Effects of traction on structural properties of degenerated disc using an in vivo rat-tail model

STUDY DESIGN

An in vivo rat-tail model was adopted to study the structural changes of degenerated intervertebral disc after different traction protocols.

OBJECTIVE

To investigate the effects of traction with different modes and magnitudes on disc with simulated degeneration.

SUMMARY OF BACKGROUND DATA

Traction has been commonly used in clinical practice for treating low back pain. Its effects on disc with degeneration have not been fully investigated.

METHODS

Forty-seven mature rats were used. Continuous static compression of 11 N was applied to the rat caudal 8-9 disc for 2 weeks to simulate disc degeneration. Tractions with different modes (static or intermittent) and magnitudes (1.4 N or 4.2 N) were applied to the degenerated disc for 3 weeks. The disc height was quantified in vivo on days 4, 18, and 39. The treated discs were then harvested for morphologic analysis.

RESULTS

Significant decrease in disc height with degenerative morphologic changes was observed after the application of the static compression. The changes in disc height after the application of traction were found to be magnitude dependent. Continuous decrease in disc height was observed after 4.2-N traction, whereas the disc height maintained after traction of 1.4 N. However, no obvious morphologic change was found in comparison with the degenerated discs without traction.

CONCLUSION

Although traction was not demonstrated to have restored disc with degeneration, traction with relatively low magnitude was found to have significant beneficial effect in maintaining disc height of degenerated disc, and it might be a potential intervention to slow down the process of degeneration. Future studies of the effects of low-magnitude traction on degenerated disc are recommended.

The immediate effect of repeated loading on the compressive strength of young porcine lumbar spine

The human spine is exposed to repeated loading during daily activities and more extremely during sports. Despite this, there remains a lack of knowledge regarding the immediate effects on the spine due to this mode of loading. Age-specific spinal injury patterns has been demonstrated and this implies differences in reaction to load mode and load history The purpose of the present study was to investigate the impact of cyclic pre-loading on the biomechanical properties and fracture patterns of the adolescent spine in an experimental model. Eight functional spinal units from four young porcine spines were harvested. The functional spinal units were cyclic loaded with 20,000 cycles and then axially compressed to failure. The compression load at failure, ultimate stress and viscoelastic parameters were calculated. The functional spinal units were examined with plain radiography, computer tomography and MRI before and after the loading, and finally macroscopically and histologically. The median compression load at failure in this study was 8.3 kN (range 5.6-8.7 kN). The median deformation for all cases was 2.24 mm (range 2.30-2.7 mm) and stiffness was 3.45 N/mm (range 3.5-4.5 N/mm). A fracture was seen on radiograph in one case, on CT and macroscopically in seven, and on MRI and histologically in all eight cases. The cyclic loaded functional spinal units in the present study were not more sensitive to axial compression than non-cyclic loaded functional spinal units from young porcine. The endplate and the growth zone were the weakest part in the cyclic loaded functional spinal units. Disc signal reduction and disc height reduction was found on MRI. The E-modulus value found in this study was of the same order of magnitude as found by others using a porcine animal model.

Cellular mechanobiology of the intervertebral disc: new directions and approaches

The more we learn about the intervertebral disc (IVD), the more we come to appreciate the intricacies involved in transmission of forces through the ECM to the cell, and in the biological determinants of its response to mechanical stress. This review highlights recent developments in our knowledge of IVD physiology and examines their impact on cellular mechanobiology. Discussion centers around the continually evolving cellular and microstructural anatomy of the nucleus pulposus (NP) and the annulus fibrosus (AF) in response to complex stresses generated in support of axial load and spinal motion. Particular attention has been given to cells from the immature NP and the interlamellar AF, and assessment of their potential mechanobiologic contributions to the health and function of the IVD. In addition, several innovative approaches that have been brought to bear on studying the interplay between disc cells and their micromechanical environment are discussed. Techniques for "engineering" cellular function and technologies for fabricating more structurally defined biomaterial scaffolds have recently been employed in disc research. Such tools can be used to elucidate the biological and physical mechanisms by which different IVD cell populations are regulated by mechanical stress, and contribute to advancement of preventative and therapeutic measures.

Developing consistently reproducible intervertebral disc degeneration at rat caudal spine by using needle puncture

OBJECT

The goal in this study was to develop a convenient, less-invasive animal model to monitor progression of intervertebral disc (IVD) degeneration for future testing of new treatments for disc degeneration.

METHODS

Level 5/6 and 7/8 IVDs of rat caudal spine were stabbed laterally with 18- or 21-gauge hypodermic needles to a depth of 5 mm from the subcutaneous surface with the aid of fluoroscopy. In vivo MR imaging studies were performed at 4, 8, and 12 weeks postsurgery to monitor progression of IVD degeneration. Histological analysis including H & E and safranin O staining, and immunohistochemical studies of collagen type II and bone morphogenetic protein receptor type II (BMPRII) were assessed at 12 weeks postsurgery.

RESULTS

The 18- and 21-gauge needle-stabbed discs illustrated decreases in both the T2 density and MR imaging index starting at 4 weeks, with no evidence of spontaneous recovery by 12 weeks. Histological staining demonstrated a decreased nucleus pulposus (NP) area, and the NP-anulus fibrosus border became unclear during the progression of disc degeneration. Similar patterns of degenerative signs were also shown in both safranin O- and collagen type II-stained sections. The BMPRII immunohistochemical analysis of stabbed discs demonstrated an increase in BMPRII expression in the remaining NP cells and became stronger in anulus fibrosus with the severity of disc degeneration.

CONCLUSIONS

After introducing an 18- or 21-gauge needle into the NP area of discs in the rat tail, the stabbed disc showed signs of degeneration in terms of MR imaging and histological outcome measurements. Changes in BMPRII expression in this animal model provide an insight for the effectiveness of delivering BMPs into the region responsible for chondrogenesis for disc repair. This convenient, less-invasive, reproducible, and cost-effective model may be a useful choice for testing novel treatments for disc degeneration.

Reduced nucleus pulposus glycosaminoglycan content alters intervertebral disc dynamic viscoelastic mechanics

The intervertebral disc functions over a range of dynamic loading regimes including axial loads applied across a spectrum of frequencies at varying compressive loads. Biochemical changes occurring in early degeneration, including reduced nucleus pulposus glycosaminoglycan content, may alter disc mechanical behavior and thus may contribute to the progression of degeneration. The objective of this study was to determine disc dynamic viscoelastic properties under several equilibrium loads and loading frequencies, and further, to determine how reduced nucleus glycosaminoglycan content alters dynamic mechanics. We hypothesized that (1) dynamic stiffness would be elevated with increasing equilibrium load and increasing frequency, (2) the disc would behave more elastically at higher frequencies, and finally, (3) dynamic stiffness would be reduced at low equilibrium loads under all frequencies due to nucleus glycosaminoglycan loss. We mechanically tested control and chondroitinase ABC injected rat lumbar motion segments at several equilibrium loads using oscillatory loading at frequencies ranging from 0.05 to 5Hz. The rat lumbar disc behaved non-linearly with higher dynamic stiffness at elevated compressive loads irrespective of frequency. Phase angle was not affected by equilibrium load, although it decreased as frequency was increased. Reduced glycosaminoglycan decreased dynamic stiffness at low loads but not at high equilibrium loads and led to increased phase angle at all loads and frequencies. The findings of this study demonstrate the effect of equilibrium load and loading frequencies on dynamic disc mechanics and indicate possible mechanical mechanisms through which disc degeneration can progress.

Effects of static compression with different loading magnitudes and durations on the intervertebral disc: an in vivo rat-tail study

STUDY DESIGN

An in vivo rat-tail model was used to study the effects of static compression with different loading magnitudes and durations on the intervertebral disc.

OBJECTIVE

To investigate the effects of static compression with different loading magnitudes and durations on the intervertebral disc over a period of time.

SUMMARY OF BACKGROUND DATA

A disc degeneration model is essential for studying therapeutic effects on degenerated disc. Static compression can induce degenerative-like changes in the intervertebral disc. However, the consequences of the simulation model over a period of resting have not been clearly documented, which may have confounding effects on the experimental outcome.

METHODS

Thirty-five rats were used. Static compressions with different loads (11 or 17 N) and durations (1 hour daily or continuous) were applied to the rat-tail caudal 8-9 disc for 2 weeks, and followed with 3 weeks of rest. The disc height was quantified in vivo on days 4, 18, and 39. The rats were killed and the discs were harvested for morphologic examination on day 39 after the disc height measurement.

RESULTS

Significant decrease in disc height was observed after continuous static compression for both 11 and 17 N, and continued during the resting period. The morphologic evaluation of the continuous compressed disc showed a decreased nuclear size, reduced number of nuclear cells, and irregular nuclear shape with inward bulging of disorganized annular collagen lamellas. Daily compression of 1 hour was found to induce a transient increase in disc height, but restored after the 3-week resting period. Favorable morphologic changes, including vacuolated nuclear cells and oval nuclear shape with well-organized annular collagen lamellas, were seen in the rat disc specimens with daily compression of 1 hour.

CONCLUSION

Disc degenerative-like changes without recovery were demonstrated in the rat caudal disc after continuous compression. The changes in disc height and disc morphology were found to be dependent on the duration of load application and may have clinical implication.

Matrix remodeling during intervertebral disc growth and degeneration detected by multichromatic FAST staining

Various imaging techniques have been used to assess degeneration of the intervertebral disc, including many histological methods, but cartilage-oriented histological stains do not clearly show the comparatively complex structures of the disc. In addition, there is no integrated method to assess efficiently both the compartmental organization and matrix composition in disc samples. In this study, a novel histological method, termed FAST staining, has been developed to investigate disc growth and degeneration by sequential staining with fast green, Alcian blue, Safranin-O, and tartrazine to generate multichromatic histological profiles (FAST profiles). This identifies the major compartments of the vertebra-disc region, including the cartilaginous endplate and multiple zones of the annulus fibrosus, by specific FAST profile patterns. A disc degeneration model in rabbit established using a previously described puncture method showed gradual but profound alteration of the FAST profile during disc degeneration, supporting continual alteration of glycosaminoglycan. Changes of the FAST profile pattern in the nucleus pulposus and annulus fibrosus of the postnatal mouse spine suggested matrix remodeling activity during the growth of intervertebral discs. In summary, we developed an effective staining method capable of defining intervertebral disc compartments in detail and showing matrix remodeling events within the disc. The FAST staining method may be used to develop a histopathological grading system to evaluate disc degeneration or malformation.

A simple disc degeneration model induced by percutaneous needle puncture in the rat tail

STUDY DESIGN

: We evaluated the degenerative changes to rat tail vertebral discs induced by percutaneous needle puncture, and we compared 2 puncture styles for the depth of needle puncture and the rate of disc degeneration.

OBJECTIVE

: To develop a simple animal model of disc degeneration.

SUMMARY OF BACKGROUND DATA

: The study of biologically based treatments for degenerative disc disease depends largely on animal models. Annulus needle puncture in the lumbar spine inducing disc degeneration in rabbits has proven successful, but a similar method has not been evaluated in the tail discs of rats, even though it might produce a desirable model for disc repair studies.

METHODS

: Two consecutive rat tail vertebral discs, proximal and distal to the eighth coccygeal vertebra, were randomized for injury and control. The disc selected for injury was punctured percutaneously using a 20-gauge needle with either full penetration or half penetration. The discs were harvested 1, 2, and 4 weeks later. Measurements included disc height on molybdenum target digital radiographs, biochemistry (water content, glycosaminoglycans, and hydroxyproline), and histology.

RESULTS

: Needle punctures with full or half penetration caused significant disc space narrowing and progressive histologic changes of degeneration as early as 1 and 2 weeks after injury, respectively. Significant decrease in glycosaminoglycan content was observed at 4 weeks in the half-penetration puncture discs and at 2 and 4 weeks in discs punctured penetratively. Penetrative puncture resulted in a faster decrease in disc height, lower glycosaminoglycan content, and higher grades of histologic degeneration. The water and hydroxyproline content of the discs did not change appreciably.

CONCLUSION

: Tail disc percutaneous needle puncture is a simple method for inducing disc degeneration and the rate of degeneration is positively related to the depth of needle puncture. This model still has some limitations that should be taken into consideration when results of disc regeneration research in this model are interpreted and extrapolated to human.

Effects of electroacupuncture on a degenerated intervertebral disc using an in-vivo rat-tail model

Electroacupuncture (EA) has long been used as conservative treatment for low back pain (LBP). Its effect on relief of back pain has been demonstrated in many clinical studies. However, whether it has any effect on the biological properties of an intervertebral disc, which is one of the major causes of LBP, is still unclear. The aim of this study was, therefore, to investigate the effects of EA with different simulation frequencies on an intervertebral disc with simulated degeneration using an in-vivo rat-tail model. In this study, 33 rats were used. Disc degeneration was simulated in the rat caudal 8-9 disc via continuous static compressive loading of 11 N for 2 weeks. EA with a frequency of 2 or 100 Hz was then applied to the degenerated disc for 3 weeks with 3 sessions/week and 20 min/session. The intervertebral disc height was measured before and after compression as well as after EA intervention for 3 weeks. The static compression was found to result in a reduction in the disc height of about 22 per cent. There was no evidence that this change could be reversed after resting or the EA intervention. However, EA at 100 Hz was found to induce a further decrease in disc height, which was not shown for the rats after resting or EA at 2 Hz. The results of this study showed that effects of EA on disc degeneration are frequency dependent and adverse effects could result if EA at a certain frequency was used.

Compression asymmetric static experimental model of degenerative disk diseases

We propose an experimental model of degenerative-dystrophic changes in intervertebral disks of rat tail. The results of X-ray examination and histological studies showed that degenerative changes in the disk tissues caused by experimental compression of intervertebral disks in rat tails are identical to those in humans.

Reliability of radiographic intervertebral disc height measurement for in vivo rat-tail model

Quantification of changes in intervertebral disc height is critical for studying intervertebral disc degeneration. Reliability of disc height measurement is therefore especially important for in vivo studies using animal models of disc degeneration. In this study, the effects of image intensity and percentage of disc width used for disc height measurement from radiographic images were evaluated in a rat-tail model. Radiographs were taken for 10 Sprague-Dawley rats using a standardized protocol. Average disc heights of the caudal 8-9 discs were determined using original and intensity adjusted images with different percentages of disc width. The average disc height was found to be significantly affected by both the image intensity and the percentage of disc width measured. A higher reliability was found in the measurement for image with adjusted intensity and using smaller disc width. Image intensity is suggested to be controlled and the disc width should be taken into account in quantifying the disc height.

Are animal models useful for studying human disc disorders/degeneration?

Intervertebral disc (IVD) degeneration is an often investigated pathophysiological condition because of its implication in causing low back pain. As human material for such studies is difficult to obtain because of ethical and government regulatory restriction, animal tissue, organs and in vivo models have often been used for this purpose. However, there are many differences in cell population, tissue composition, disc and spine anatomy, development, physiology and mechanical properties, between animal species and human. Both naturally occurring and induced degenerative changes may differ significantly from those seen in humans. This paper reviews the many animal models developed for the study of IVD degeneration aetiopathogenesis and treatments thereof. In particular, the limitations and relevance of these models to the human condition are examined, and some general consensus guidelines are presented. Although animal models are invaluable to increase our understanding of disc biology, because of the differences between species, care must be taken when used to study human disc degeneration and much more effort is needed to facilitate research on human disc material.

A Removable Precision Device for In-Vivo Mechanical Compression of Rat Tail Intervertebral Discs

The rat tail intervertebral disc has emerged as an important model to examine the mechanisms for mechanically induced degeneration and remodeling. Previous methods used to apply high precision axial compressive loading to a rat tail intervertebral disc in vivo either required anesthesia, or the permanent mounting of a loading device to the animal, and were not well described in the literature. Therefore, a new device to apply compressive loading to the rat tail intervertebral disc was developed and validated. The rat tail compressive loading system utilized a pneumatically driven device weighing 18 g, and was capable of delivering a 12.6 N sinusoidal or square waveform at frequencies up to 1.0 Hz. The system improved on previous methods in its modular construction, relative ease of fabrication, compatibility with existing tail model technology and overall cost effectiveness. The removable system eliminated the need for anesthesia and through a modular, cost effective, design allowed for the simultaneous loading of multiple animals. This system expanded the ability to accurately, ethically, and efficiently apply dynamic compressive loads to the rat tail intervertebral disc for extended periods of time in order to address questions related to disc mechanobiology.

Spatially varying material properties of the rat caudal intervertebral disc

STUDY DESIGN

The use of a microscopy based material testing technique to assess the local material properties of rat caudal intervertebral discs under uniaxial compression.

OBJECTIVES

To better understand the cell environment of rat caudal intervertebral discs during mechanical loading and elucidate better the role of the nucleus pulposus to the overall disc material properties.

SUMMARY OF BACKGROUND DATA

Rat tail models of disc degeneration have been widely used for their similarity with the degeneration phenomena in human beings. Degenerative patterns in the disc are often inhomogeneous, however, only average material properties of rodent discs have been studied. Knowledge of the spatially varying properties within the disc is necessary to understand the disc cell milieu during tissue loading.

METHODS

Rat caudal motion segments were tested intact, sectioned, and with alterations of nucleus pulposus using microscopy based techniques. Local displacements and strains were obtained using digital image correlation. Strains and load measurements were used to get the average apparent Young's modulus, peak stress, local Young's modulus, and local Poisson's ratio.

RESULTS

There was no difference observed in the average apparent Young's modulus among experimental groups. Peak stresses decreased significantly when the nucleus pulposus was replaced with extremely fluid-like materials. The axial displacement field showed 3 distinct linear distributions in samples which were sectioned. The center region in all groups had significantly smaller axial strain and showed a higher local Young's modulus.

CONCLUSIONS

The average equilibrium Young's modulus may be dependent on short-range ultrastructural organization. Spatially varying material properties within the intervertebral disc may be caused by orientation of fiber bundles in the different regions of the anulus fibrosus. The fiber bundles are better able to resist compressive loads when oriented parallel rather than perpendicular to the loading direction. At equilibrium, the anulus fibrosus also appears to have a shielding effect independent of the material filling up the nucleus pulposus space.

Dependence of mechanical behavior of the murine tail disc on regional material properties: a parametric finite element study

In vivo rodent tail models are becoming more widely used for exploring the role of mechanical loading on the initiation and progression of intervertebral disc degeneration. Historically, finite element models (FEMs) have been useful for predicting disc mechanics in humans. However, differences in geometry and tissue properties may limit the predictive utility of these models for rodent discs. Clearly, models that are specific for rodent tail discs and accurately simulate the disc's transient mechanical behavior would serve as important tools for clarifying disc mechanics in these animal models. An FEM was developed based on the structure, geometry, and scale of the mouse tail disc. Importantly, two sources of time-dependent mechanical behavior were incorporated: viscoelasticity of the matrix, and fluid permeation. In addition, a novel strain-dependent swelling pressure was implemented through the introduction of a dilatational stress in nuclear elements. The model was then validated against data from quasi-static tension-compression and compressive creep experiments performed previously using mouse tail discs. Finally, sensitivity analyses were performed in which material parameters of each disc subregion were individually varied. During disc compression, matrix consolidation was observed to occur preferentially at the periphery of the nucleus pulposus. Sensitivity analyses revealed that disc mechanics was greatly influenced by changes in nucleus pulposus material properties, but rather insensitive to variations in any of the endplate properties. Moreover, three key features of the model-nuclear swelling pressure, lamellar collagen viscoelasticity, and interstitial fluid permeation-were found to be critical for accurate simulation of disc mechanics. In particular, collagen viscoelasticity dominated the transient behavior of the disc during the initial 2200 s of creep loading, while fluid permeation governed disc deformation thereafter. The FEM developed in this study exhibited excellent agreement with transient creep behavior of intact mouse tail motion segments. Notably, the model was able to produce spatial variations in nucleus pulposus matrix consolidation that are consistent with previous observations in nuclear cell morphology made in mouse discs using confocal microscopy. Results of this study emphasize the need for including nucleus swelling pressure, collagen viscoelasticity, and fluid permeation when simulating transient changes in matrix and fluid stress/strain. Sensitivity analyses suggest that further characterization of nucleus pulposus material properties should be pursued, due to its significance in steady-state and transient disc mechanical response.

The effects of short-term load duration on anabolic and catabolic gene expression in the rat tail intervertebral disc

The goal of this study was to determine the time-dependent response of the intervertebral disc cells to in vivo dynamic compression. Forty-seven skeletally mature Wistar rats (>12 months old) were instrumented with an Ilizarov-type device spanning caudal disc 8-9. Using a load magnitude (1 MPa) and frequency (1.0 Hz) that were previously shown to significantly alter mRNA levels in the disc, the effects of 0.5 and 4 h of loading were investigated and compared to a sham group and our previous 2 h results. Annulus and nucleus tissue of loaded (c8-9) and internal control discs (c6-7 and c10-11) were separately analyzed by real-time RT-PCR for levels of mRNA coding for various anabolic (collagen-1A1, collagen-2A1, aggrecan) and catabolic (MMP-3, MMP-13, ADAMTs-4) proteins. In the annulus, mRNA levels increased for Collagen types I & II, and MMP 3 & 13 with increasing load duration. In contrast, the nucleus had the largest increases in aggrecan, ADAMTs-4, MMP-3 and MMP-13 after 2 h of loading, with aggrecan and MMP-13 mRNA levels returning to control values after 4 h of loading. Taken in context with our previous studies, we conclude that intervertebral disc cells from the nucleus and annulus have distinct responses to dynamic mechanical compression in vivo with sensitivity to compression magnitude, frequency and duration.