Compression-induced degeneration of the intervertebral disc: an in vivo mouse model and finite-element study

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.

Localization and distribution of cartilage oligomeric matrix protein in the rat intervertebral disc

STUDY DESIGN

Whole rat intervertebral disc (IVD), as well as the anulus fibrosus (AF) and the nucleus pulposus (NP) were studied using immunoblot, immunohistochemistry, and reverse-transcription followed by polymerase chain reaction (RT-PCR) methods to investigate the expression and distribution of cartilage oligomeric matrix protein (COMP).

OBJECTIVES

To investigate the expression and distribution patterns of COMP in normal IVD.

SUMMARY OF BACKGROUND DATA

COMP is an extracellular matrix protein abundantly expressed in articular and growth plate cartilage, as well as bone, ligament, tendon, and synovium. The potential importance of COMP to the spine has been underscored by its mutations that lead to skeletal dysplasia with characteristic platyspondyly. However, the expression and distribution of COMP in spine and IVD has not been illustrated before.

METHODS

The presence of COMP protein was investigated by immunoblotting using a COMP antibody F8 on protein extractions from whole IVD and AF or NP. To compare the expression levels of COMP between lumbar and tail IVDs, and between AF and NP of the IVD, wet weight of the tissues were used for normalization. To show that COMP can be made by IVD cells in situ, RT-PCR was used to investigate the COMP mRNA message. The distribution patterns of COMP in IVD were investigated using immunohistochemistry studies with COMP antibody F8.

RESULTS

COMP is expressed at both the protein and mRNA levels in both the AF and NP of both the lumbar spine and tail IVD. Immunohistochemistry studies show that COMP is found in the extracellular matrix of the IVD, exhibiting lamellar distribution pattern in the AF region. When normalized to wet weight, COMP is found to be expressed at higher levels in the lumbar than the tail IVD, and within the IVD, greater in the AF than the NP region.

CONCLUSIONS

Our results demonstrate the expression of COMP in both the AF and NP of the IVD. COMP is a component of the extracellular matrix of AF and NP, with a lamellar distribution pattern in the AF. Our data suggest that COMP may play a role in the normal structure of IVD.

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.

Rat spinal motion segment in organ culture: a cell viability study

STUDY DESIGN

This study investigated tissue integrity and viability of cells in an organ culture system of intervertebral disc (IVD) with adjoining vertebral bodies.

OBJECTIVE

The goal of this study was to design a methodology to maintain an IVD motion segment in organ culture, thereby preserving viability and tissue architecture.

SUMMARY OF BACKGROUND DATA

Study of IVD mechanobiology in vitro necessitates availability of vertebral bodies for controlled application of complex loads.

METHODS

IVD motion segments were dissected from rat lumbar segments and maintained in organ culture and cell viability was evaluated histochemically using NitroBlue Tetrazolium. Tissue integrity and morphology were evaluated using conventional histologic techniques.

RESULTS

The in vitro organ culture of motion segments maintained the viability and tissue integrity for 14 days. More than 95% viability in all three regions of interest (anulus fibrosus, nucleus pulposus, end plates) was maintained for 14 days in culture.

CONCLUSION

Our initial results suggest that long-term motion segment culture is practical, and the inclusion of vertebral bodies will facilitate anchoring during biomechanical stimulation. Thus, we expect the culture system to provide us with an excellent model for studying the pathomechanics of IVD degeneration and the effects of mechanical stimulation on the biology of IVD cells.

Stepwise reduction of functional spinal structures increase vertebral translation and intradiscal pressure

To date, there are only a few studies that provide data to efficiently calibrate finite element models for the spine due to its complexity. In a recent study, we quantified the range of motion rotation and the lordosis angle. This paper provides complementary results regarding two more parameters, intradiscal pressure and vertebral translation. All parameters were obtained as a function of stepwise anatomical reduction, loading direction and magnitude. Eight lumbar spinal segments (L4-5) with a median age of 52 years (38-59 years) and no signs of disc degeneration were used for the in vitro testing. A miniaturized pressure probe was implanted into the nucleus. An ultrasound-based motion-tracking system was employed to record spatial movements of several landmarks on the specimens. The center of L4, the anterior, posterior, left and right point of the lower endplate of L4 were digitized as landmarks and its translation was determined. Specimens were loaded with pure moments (1-10Nm) in the three principal anatomical planes at a loading rate of 1.0 degrees /s. Anatomy was stepwise reduced by cutting different ligaments, facet capsules and joints and removing nucleus. Translation analysis showed that the L4 center point had its largest displacement in sagittal direction and almost none vertically. Removal of the supra- and interspinous, flaval ligaments showed a slight increase and further removal of structures, a higher increase of translation. Axial rotation also was accompanied with L4 to elevate when torsion was applied. This effect was found to be larger with progressing defects. Nucleotomy exhibited the most unstable situation for specimens. Results of the intradiscal pressure indicated a large increase after removing the facet capsules and joints. Furthermore, it was found that intradiscal pressure correlated well with data of range of motion for rotation. Predicting and simulating clinical defects, surgical intervention or treatment methods requires a well performed calibration based on in vitro data, whereas it is important to adapt all including structures with as many known parameters as possible. Results provided by these studies may be used as a database for researchers aiming to calibrate or validate finite element models of L4-5 segments.

Intradiscal pressure measurements in normal discs, compressed discs and compressed discs treated with axial posterior disc distraction: an experimental study on the rabbit lumbar spine model

Intervertebral disc (IVD) pressure measurement is an appropriate method for characterizing spinal loading conditions. However, there is no human or animal model that provides sufficient IVD pressure data. The aim of our study was to establish physiological pressure values in the rabbit lumbar spine and to determine whether temporary external disc compression and distraction were associated with pressure changes. Measurements were done using a microstructure-based fibreoptic sensor. Data were collected in five control rabbits (N, measurement lying prone at segment L3/4 at day 28), five rabbits with 28 days of axial compression (C, measurement at day 28) and three rabbits with 28 days of axial compression and following 28 days of axial distraction (D, measurement at day 56). Disc compression and distraction was verified by disc height in lateral radiographs. The controls (N) showed a level-related range between 0.25 MPa-0.45 MPa. The IVD pressure was highest at level L3/4 (0.42 MPa; range 0.38-0.45) with a decrease in both cranial and caudal adjacent segments. The result for C was a significant decrease in IVD pressure (0.31 MPa) when compared with controls (P=0.009). D showed slightly higher median IVD pressure (0.32 MPa) compared to C, but significantly lower levels when compared with N (P=0.037). Our results indicate a high range of physiological IVD pressure at different levels of the lumbar rabbit spine. Temporary disc compression reduces pressure when compared with controls. These data support the hypothesis that temporary external compression leads to moderate disc degeneration as a result of degradation of water-binding disc matrix or affected active pumping mechanisms of nutrients into the disc. A stabilization of IVD pressure in discs treated with temporary distraction was observed.

Accelerated intervertebral disc degeneration in scoliosis versus physiological ageing develops against a background of enhanced anabolic gene expression

Molecular consequences of long-term deformation and altered mechanical loading of intervertebral disc (IVD) tissue in scoliosis have yet to be elucidated. We hypothesized that histological disc degeneration is faster in scoliosis than in normal ageing and that this is reflected by an altered gene expression profile. A semiquantitative histodegeneration score (HDS) revealed significantly enhanced degeneration in scoliosis (HDS 5.3) versus age-matched control IVDs (HDS 2.25; p = 0.001). Gene expression analysis by cDNA array and RT-PCR demonstrated higher mRNA levels for extracellular-matrix molecules like aggrecan, biglycan, decorin, lumican, chondromodulin, and COL2A1 in scoliotic discs versus normal discs of identical degeneration score. No differences were evident for catabolic molecules like MMP3, MMP13, MMP17, and TIMP1. In sum, morphologic disc degeneration was accelerated by about 2 decades in scoliosis versus physiological ageing and developed against a background of stronger anabolic matrix metabolism at younger age or in response to the altered mechanical environment of the tissue.

Modeling changes in intervertebral disc mechanics with degeneration

Mechanical response of the spine to various dynamic loading conditions can be analyzed by way of in vitro and in vivo studies. Ethical concerns, interpretation of conclusions reached in animal studies, and lack of detailed stress distributions in the disc components are the major disadvantages of relying solely on in vivo studies. Intraspecimen variability, difficulty in including muscle activity, and inability to mimic fluid exchange into the disc during unloading are some of the disadvantages of in vitro models. The poroelastic finite element models can provide a method of understanding the relationship between biomechanical performance of the disc due to cyclic loading and disc degeneration. A poroelastic finite element model, including regional variation of strain-dependent permeability and osmotic pressure, was used to study the effect of disc degeneration on biomechanical properties as well as propagation of failure in the disc components when cyclic loading was applied to the lumbar disc. The results predicted that healthy discs were much more flexible than degenerated discs, and the disc stiffness decreased with increasing the number of load cycles independent of degenerative condition. Failure was found to progress as the drained elastic properties of the disc components decreased due to the presence of failure. Poroelastic finite element modeling, including strain-dependent permeability and osmotic pressure, is the most advanced analytical tool currently available that can be used to understand how cyclic loading affects the biomechanical characteristics of a degenerated lumbar disc. However, a complete understanding of behavior of the intervertebral disc will ultimately be achieved only with use of a combination of computational models together with in vitro and in vivo experimental methods. Finite element models of discs with varying degrees of disc degeneration will help clinicians understand the initiation and progression of disc failure and degeneration and will assist in the development of approaches to stimulate the regeneration of disc tissues.

Cell deformation and micromechanical environment in the intervertebral disc

The purpose of this research was to explore the in situ anatomic and mechanical environment of disc cells. Laser scanning confocal microscopy was used to characterize three-dimensional morphology of intervertebral disc cells, micromechanical deformation and interaction with extracellular matrix, and functional intercellular communication. Bovine coccygeal discs were used for both the anatomic and micromechanical investigations. Anulus fibrosus cells had a complex morphology with sinuous processes woven into the extracellular matrix, particularly in the outer aspect of the anulus where they were also interconnected via functional gap junctions. They were also found in an extensive pericellular matrix of type-VI collagen, joining as many as ten cells into linear cell arrays that could be extracted from the matrix as functional units. Mechanically, collagen fibril sliding was demonstrated to govern cell mechanics and strain transfer in the anulus fibrosus during loading activities. Lamellar cells were largely protected from direct tensile strains in the matrix, with minimal intercellular strains. However, intercellular strains between lamellar cells in adjacent arrays were large, illustrating shearing between linear cell arrays. Appreciable shear was observed across the lamellar cell bodies as well as to the cellular processes woven into the matrix. These findings demonstrated the morphologic and micromechanical complexity of anulus fibrosus cells. The knowledge of the in situ environment of disc cells will provide a base to investigate the mechanical implications of disc degeneration on the cellular environment and to better understand how mechanical and genetic risk factors can impact the cells that are essential to maintaining the intervertebral disc.

Mechanobiology of the intervertebral disc and relevance to disc degeneration

Mechanical loading of the intervertebral disc may contribute to disc degeneration by initiating degeneration or by regulating cell-mediated remodeling events that occur in response to the mechanical stimuli of daily activity. This article is a review of the current knowledge of the role of mechanical stimuli in regulating intervertebral disc cellular responses to loading and the cellular changes that occur with degeneration. Intervertebral disc cells exhibit diverse biologic responses to mechanical stimuli, depending on the loading type, magnitude, duration, and anatomic zone of cell origin. The innermost cells respond to low-to-moderate magnitudes of static compression, osmotic pressure, or hydrostatic pressure with increases in anabolic cell responses. Higher magnitudes of loading may give rise to catabolic responses marked by elevated protease gene or protein expression or activity. The key regulators of these mechanobiologic responses for intervertebral disc cells will be the micromechanical stimuli experienced at the cellular level, which are predicted to differ from that measured for the extracellular matrix. Large hydrostatic pressures, but little volume change, are predicted to occur for cells of the nucleus pulposus during compression, while the highly oriented cells of the anulus fibrosus may experience deformations in tension or compression during matrix deformations. In general, the pattern of biologic response to applied loads suggests that the cells of the nucleus pulposus and inner portion of the anulus fibrosus experience comparable micromechanical stimuli in situ and may respond more similarly than cells of the outer portion of the anulus fibrosus. Changes in these features with degeneration are critically understudied, particularly degeneration-associated changes in cell-level mechanical stimuli and the associated mechanobiology. Little is known of the mechanisms that regulate cellular responses to intervertebral mechanobiology, nor is much known with regard to the precise mechanical stimuli experienced by cells during loading. Mechanical factors appear to regulate responses of the intervertebral disc cells through mechanisms involving intracellular Ca(2+) transients and cytoskeletal remodeling that may regulate downstream effects such as gene expression and posttranslational biosynthesis. Future studies should address the broader biologic responses to mechanical stimuli in intervertebral disc mechanobiology, the involved signaling mechanisms, and the apparently important interactions among mechanical factors, genetic factors, cytokines, and inflammatory mediators that may be critical in the regulation of intervertebral disc degeneration.

Changes in gene expression and protein distribution at different stages of mechanically induced disc degeneration--an in vivo study on the New Zealand white rabbit

The objective of the study was to improve the biological understanding of degenerative disc disease using a rabbit model in which different stages of disc degeneration are induced by variation of the duration of loading with an external compression-device applying 2.4 MPa. Gene expression and protein distribution were analyzed in controls and after 1, 28, and 56 days of hyperphysiologic loading. To evaluate extracellular matrix genes, quantitative real-time reverse-transcriptase polymerase chain reaction was applied for collagen I, collagen II, biglycan, decorin, fibromodulin, fibronectin, aggrecan, and osteonectin. As representatives of catabolic, anticatabolic, and anabolic factors, matrix metalloproteinase-13 (MMP-13), tissue inhibitor of matrix metalloproteinase-1 (TIMP-1), and bone morphogenetic protein-2 (BMP-2) were chosen. To evaluate protein distribution, immunohistochemistry was performed for collagen I, collagen II, and BMP-2/4. Matrix gene expression was characterized by two major developments: collagen I and II, biglycan, and decorin showed early elevation followed by later downregulation to control levels, whereas fibromodulin, fibronectin, aggrecan, and osteonectin showed continuous upregulation or remained at similar levels. Induction of MMP-13 gene expression was found in degenerated discs. TIMP-1 and BMP-2 were elevated immediately after hyperphysiologic loading and presented highest levels in the 56-day group. Immunohistochemistry showed less collagen II and BMP-2/4 positive cells after compression. In conclusion, elevated matrix gene expression represents an early cellular response to hyperphysiologic loading. As degeneration progresses, some matrix genes increase upregulation, whereas others start downregulation. Continuous upregulation of catabolic, anticatabolic, and anabolic factors indicates their important role in the degeneration process.

In vitro organ culture of the bovine intervertebral disc: effects of vertebral endplate and potential for mechanobiology studies

STUDY DESIGN

Whole bovine coccygeal discs were cultured under static load, with or without vertebral endplates (VEPs), and assessed for cell viability, biochemical stability, biosynthetic activity, and biosynthetic responsiveness to changes in mechanical load.

OBJECTIVES

To assess the effects of VEPs on biochemical and cellular stability of disc cells during in vitro culture of large disc explants. To determine whether cultured discs could respond to mechanical perturbation.

SUMMARY OF BACKGROUND DATA

Previous methods for culturing the intervertebral disc have focused on rabbit and rat discs, but the small size of these discs limits the relevance of these culture systems to the human condition. Bovine coccygeal discs have similar dimensions to the human lumbar disc (i.e., similar size and nominal stresses), but long-term culture of these discs has not been reported.

METHODS

Bovine coccygeal discs were harvested with or without VEPs, cultured under static load (5 kg, approximately 0.25 MPa, in situ swelling pressure) for up to 1 week, and evaluated for changes in hydration, glycosaminoglycan content, cell viability, and biosynthetic activity. Additionally, the biochemical and biosynthetic response of discs cultured without VEP to increasing the load to a 20-kg (approximately 1 MPa, the estimated stress in human lumbar disc during heavy lifting) static load for 6 hours was assessed.

RESULTS

During the first 24 hours, culturing discs with endplates was moderately better with regards to maintaining in situ anulus hydration and nucleus glycosaminoglycan levels. The endplates, however, obstructed media flow to the disc, resulting in a marked decrease in cell viability after 1 week of culture. Nucleus pulposus cell viability was maintained in discs cultured without endplates, but there was a significant drop in biosynthetic activity within 2 days of culture. Despite this drop, the disc cells in the discs without VEP remained biosynthetically responsive to changes in mechanical loading.

CONCLUSIONS

It is possible to maintain cell viability and the biosynthetic responsiveness of large discs for up to 1 week in vitro when the discs are cultured under static load and without VEP.

Role of endplates in contributing to compression behaviors of motion segments and intervertebral discs

The purpose of this study was to gain an improved understanding of the mechanical behavior of the intervertebral disc in the presence and absence of the vertebral endplates. Mechanical behaviors of rat caudal motion segments, vertebrae and isolated disc explants under two different permeability conditions were investigated and viscoelastic behaviors were evaluated using a stretched-exponential function to describe creep and recovery behaviors. The results demonstrated that both vertebrae and discs underwent significant deformations in the motion segment even under relatively low-loading conditions. Secondly, disruption of the collagenous network had minimal impact on equilibrium deformations of disc explants as compared to disc deformations occurring in the motion segments provided that vertebral deformations were accounted for; however, differences in endplate permeability conditions had a significant effect on viscoelastic behaviors. Creep occurred more quickly than recovery for motion segment and explant specimens. In addition, disc explants and motion segments both exhibited non-recoverable deformations under axial compression under low- and high-loading conditions. Results have important implications for interpreting the role of vertebral endplates in contributing to disc mechanical behaviors and direct application to mechanobiology studies involving external loading to rodent tail intervertebral discs.

Molecular pathogenic factors in symptomatic disc degeneration

BACKGROUND CONTEXT

Although symptomatic disc degeneration is thought to be the leading cause of chronic low back pain, no available biologic therapy is yet available to treat this highly prevalent condition.

PURPOSE

In this article, the cellular, biomechanical and molecular alterations that occur during disc degeneration are reviewed to provide a better understanding of this pathologic process.

STUDY DESIGN

The cellular and molecular aspects of disc degeneration are reviewed.

METHODS

The available studies detailing the molecular and cellular changes during disc degeneration are reviewed in an effort to provide a basis for understanding the biologic strategies for disc repair.

RESULTS

Disc degeneration begins early in life and involves a cascade of changes at the cellular and molecular level that results in degradation of the extracellular matrix of the disc, leading to biomechanical failure of this complex structure.

CONCLUSION

With a thorough understanding of the cellular and molecular events causing degeneration of the intervertebral disc, rational strategies for disc repair can be understood and evaluated. It appears that biologic disc repair will be feasible in the future although challenges remain in this blossoming field.

Animal models for human disc degeneration

Despite the significant impairment associated with degenerative disc disease, a clear understanding of its pathogenesis is still lacking. Currently, no particular model parallels the complex nature of human disc degeneration. Naturally occurring animal models have the drawback that the basis for the high rate of disc degeneration is not known. Although the interventions in artificial animal models that create disc degeneration are known, the relationship of those to the events leading to disc degeneration in humans is not. With the recent progress in biomechanics, cell biology and molecular biology, an easily reproducible and valid animal model may help unlock the complex cascade of events surrounding human disc degeneration.

Disc Regeneration: Why, When, and How

There is increasing acknowledgment that patients with back pain who are candidates for surgery, will benefit over the long term from less invasive procedures that facilitate dynamic stabilization, rather than fusion. Dynamic stabilization can be addressed by providing assistance using mechanical devices, or relying on biologic processes such as tissue regeneration and repair. The concept of biologic disc repair has grown in recent years because of improved understanding of the cellular and molecular events of disc aging and degeneration. This article describes approaches to cell therapy, reviews relevant studies, and discusses ways to maximize clinical efficacy. Tissue engineering approaches for disc regeneration and healing have significant clinical potential.

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.

Significance of the mechanical environment during regeneration of the intervertebral disc

The prevalence of low back pain is high, and the intervertebral (IV) disc is regarded as one of the major causes. Various approaches have been reported to either slow down disc degeneration or to repair/regenerate the disc. So far, the effect of the mechanical environment has not been addressed in these approaches, although several investigations have shown its influence on other mesenchymal tissues. In this paper, we propose that the biophysical stimuli from the mechanical environment can directly influence cell type, as well as their metabolic activity during repair/regeneration of the IV disc. To demonstrate the potential of this idea, data from the literature, as well as explorative experimental results, are presented.

A fibronectin fragment alters the metabolism by rabbit intervertebral disc cells in vitro

STUDY DESIGN

A biochemical and gene expression study was conducted to determine the effects of the 30-kDa N-terminal fibronectin fragment (Fn-f) on the glycosaminoglycan content of nucleus pulposus (NP) explant cultures, and on the gene expression profile of NP cells in alginate culture.

OBJECTIVE

To determine the effects of Fn-f on NP cells in alginate culture and disc explant cultures.

SUMMARY OF BACKGROUND DATA

The macroscopic and histologic features of disc degeneration have been well described, but the molecular biology of disc degeneration remains poorly understood. Although fibronectin and fibronectin fragments are known to accumulate in degenerative discs, the role of fibronectin fragments on the degenerative process has not been elucidated. This study sought to define the effects of Fn-f on the expression of key matrix and degradative genes and on disc matrix proteins.

METHODS

New Zealand white rabbits discs were harvested. NP cells were either isolated and grown in alginate culture or cultured as explanted tissue. The cultured cells were exposed to 10 nmol/L, 100 nmol/L, and 1 micromol/L concentrations of 30-kDa N-terminal Fn-f or a control substance and then analyzed histologically, biochemically, and with gene expression studies.

RESULTS

Alginate-cultured NP cells maintained a histologic appearance and phenotypic expression pattern similar to disc cells in vivo. Exposure of these cells to Fn-f led to the up-regulation of the MMP-9, MMP-13, and Fas genes and the down-regulation of the Type II collagen and aggrecan genes. In explant culture, Fn-f exposure led to a 60% reduction in glycosaminoglycan content compared with controls.

CONCLUSION

Treatment of NP cells in vitro with Fn-f led to changes in matrix proteins and gene expression similar to those seen during disc degeneration in vivo. This supports a possible detrimental role of the N-terminal fibronectin fragment in degenerative disc disease.

A three-dimensional collagen matrix as a suitable culture system for the comparison of cyclic strain and hydrostatic pressure effects on intervertebral disc cells

OBJECT

To study intervertebral disc cell mechanobiology, the authors developed experimental systems that allow the application of cyclic strain and intermittent hydrostatic pressure (IHP) on isolated disc cells under equal three-dimensional (3D) culture conditions. The purpose of the study was to characterize disc cell proliferation, viability, morphology, and gene expression in 3D collagen matrices.

METHODS

The effects of cyclic strain (1, 2, 4, and 8% strain; 1 Hz) and IHP (0.25 MPa, 0.1 Hz) on gene expression (real-time polymerase chain reaction) of anabolic and catabolic matrix proteins were investigated and compared with those derived from mechanically unstimulated controls. Intervertebral disc cells proliferated in the collagen gels (mean viability 91.6%) and expressed messenger RNA for collagen I, collagen II, aggrecan, matrix metalloproteinase (MMP)-2, and MMP-3. Morphologically, both spindle-shaped cells with longer processes and rounded cells were detected in the collagen scaffolds. Cyclic strain increased collagen II and aggrecan expression and decreased MMP-3 expression of anulus fibrosus cells. No significant difference between the four strain magnitudes was found. Intermittent hydrostatic pressure tended to increase collagen I and aggrecan expression of nucleus cells and significantly decreased MMP-2 and -3 expression of nucleus cells, whereas aggrecan expression of anulus cells tended to decrease.

CONCLUSIONS

Based on these results, the collagen matrix appeared to be a suitable substrate to apply both cyclic strain and IHP to intervertebral disc cells under 3D culture conditions. Individual variations may be influenced by the extent of degeneration of the disc specimens from which the cells were isolated. This experimental setup may be suitable for studying the influence of degeneration on the disc cell response to mechanical stimuli.

Matrix remodeling expression in anulus cells subjected to increased compressive load

STUDY DESIGN

Mechanobiology study of gene expression changes as a result of compressive overload of anular fibrochondrocytes.

OBJECTIVE

To test hypotheses regarding phenotype shift in genes coding for representative extracellular matrix (ECM) proteins and matrix modulators.

SUMMARY OF THE BACKGROUND DATA

In degenerative disc disease, the transfer of compressive load through the disc shifts largely from the nucleus onto the anulus. In vivo models simulating this condition have shown derangement of the collagenous ultrastructure in the anulus. In vitro models of cultured anulus cells subjected to static compressive stress generally suggest a down-regulation of synthesis. This study evaluated the expression of specific isomers of genes responsible for mechanical viability and metabolism of the disc under cyclic compressive loads.

METHODS

Fibrochondrocytes were digested from the anuli of 3, 2-week-old pigs, embedded in 1.5% alginate gel, and hydrostatically compressed at 0.5 Hz for 3 hours to amplitudes of 10 and 30 atm. These levels represented nominal load transfer through the healthy disc and high load transfer through the degenerative disc. Ribonucleic acid was isolated, reverse transcribed, and evaluated by real-time polymerase chain reaction for expression of type I (C-I) and type II (C-II) collagen, aggrecan, the matrix metalloproteinase (MMP-1), and the transforming growth factor beta (TGFbeta-1). Results were expressed at percentages of uncompressed controls.

RESULTS

The lower pressure of 10 atm resulted in up-regulation of all ECM protein genes. C-I and C-II both averaged 141%, and aggrecan 121% of controls (P < 0.05). MMP-1 and TGFbeta-1 were essentially unchanged. With the pressure increased to 30 atm, C-II remained approximately at the level expressed under lower pressure, but C-I was reduced to 42% of controls (P < 0.05), indicating a phenotype shift. MMP-1 and TGFbeta-1 also were down-regulated to 71% and 54% of controls, respectively (P < 0.05).

CONCLUSIONS

The up-regulation of the ECM genes with nominal pressure highlights the mechanobiological importance of common activity in fibrocartilage homeostasis. Differential regulation of the 2 primary collagen types with high pressure indicates a capacity of the anulus to remodel according to pathomechanical conditions.

Occurrence and regional distribution of apoptosis in scoliotic discs

STUDY DESIGN

Seventy surgically obtained intervertebral discs from 9 patients with idiopathic and 7 patients with neuromuscular scoliosis were analyzed for the regional distribution of apoptosis.

OBJECTIVES

To investigate the role of apoptotic mechanisms in scoliotic discs.

SUMMARY OF BACKGROUND DATA

The reasons for the development of scoliosis are complex and yet not completely understood. In herniated lumbar disc tissue, increased apoptosis and a high expression of Fas and Fas ligand and caspase-3 activity have already been reported, suggesting a pivotal role of apoptotic mechanisms in intervertebral disc degeneration. In scoliotic discs, cell death was correlated with disc deformity and changes in nutrient supply and metabolic levels. The role of apoptosis in scoliotic discs, however, is still unclear.

METHODS

Apoptosis was determined by terminal deoxyribonucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) assay and poly(ADP-ribose) polymerase (PARP) p85 immunohistochemistry. Expression of Fas and Fas-ligand was analyzed by immunohistochemistry and reverse transcription polymerase chain reaction analysis.

RESULTS

In all samples analyzed, apoptotic cells could be detected in the nucleus, anulus, and endplate. The number of apoptotic cells was significantly higher in the nucleus compared to the other areas and in the apex versus the nonapex discs. There was no difference between the discs of idiopathic and neuromuscular scoliosis and between the 2 age groups studied (10-17 years and 17-48 years, respectively). A strong expression of Fas and Fas-ligand could be detected in all samples.

CONCLUSION

Increased numbers of apoptotic cells in the nucleus of scoliotic discs and the apex disc suppose a pivotal role of programmed cell death for the progression of this common disorder. The simultaneous increase of Fas and Fas-ligand expression in areas with increased cell death point to an activation of the apoptotic process via the Fas/Fas-L system.

The surgical treatment of the lumbar disc prolapse: nucleotomy with additional transpedicular dynamic stabilization versus nucleotomy alone

STUDY DESIGN

Clinical and radiologic study evaluating the outcome after nucleotomy with dynamic stabilization compared with nucleotomy alone.

OBJECTIVES

To investigate the effect of dynamic stabilization on the progression of segmental degeneration after nucleotomy.

SUMMARY OF BACKGROUND DATA

Nucleotomy as treatment for lumbar disc prolapse in combination with initial segment degeneration may lead to segmental instability. Dynamic stabilization systems restrict segmental motion and thus prevent further degeneration of the lumbar spine. They are designed to avoid the disadvantages of rigid fixation, such as pseudarthrosis and adjacent segment degeneration.

METHODS

Eighty-four patients underwent nucleotomy of the lumbar spine for the treatment of symptomatic disc prolapse. Additional dynamic stabilization (DYNESYS) was performed in 35 of those cases. All patients showed signs of initial disc degeneration (MODIC I). They underwent evaluation before surgery, 3 months after surgery, and at follow-up. The mean duration of follow-up was 34 months. Examinations included radiographs, magnetic resonance imaging (MRI), physical examination, and subjective patient evaluation using Oswestry score and visual analog scale (VAS).

RESULTS

Clinical symptoms, Oswestry score, and VAS improved significantly in both groups after 3 months. At follow-up, a significant increase in the Oswestry score and in the VAS was seen only in the nonstabilized group. In the dynamically stabilized group, no progression of disc degeneration was noted at follow-up, whereas radiologic signs of accelerated segmental degeneration existed in the solely nucleotomized group. There were no implant-associated complications.

CONCLUSIONS

The applied dynamic stabilization system is useful to prevent progression of initial degenerative disc disease of lumbar spinal segments after nucleotomy.

Effects of unisegmental disc compression on adjacent segments: an in vivo animal model

It is controversial whether fusion of discs in the spine leads to increased degeneration on the remaining discs or whether the degenerative changes are merely a part of the inevitable natural history process. To determine the effects of unisegmental compression and subsequent recovery on adjacent segments, we studied histology, radiology and intradiscal pressure using an in vivo rabbit model. Fifteen New Zealand rabbits were divided in to three groups of five. In the first group, the intervertebral disc L4-L5 of the lumbar spine was axially loaded for 28 days with an external loading device. In the second group, the intervertebral disc was compressed for 28 days and allowed to recover for an equal amount of time, with the loading device removed. Five animals underwent a sham operation, in which the external loading device was situated, but their discs remained unloaded for 28 days. The intradiscal pressure was determined in the loaded discs as well as in the cranial and caudal adjacent discs. Lateral radiographs were taken from each subjected intervertebral disc with adjacent vertebral bodies and the cranial and caudal adjacent segments. The compressed discs showed lower intradiscal pressure in comparison with the control group, which remained unloaded. In the cranial and caudal discs adjacent to the loaded discs the average intradiscal pressure was similar to the unloaded controls. The loaded discs demonstrated a significant decrease in disc space. No discs adjacent to the loaded discs changed in height. The lamellar architecture of the inner, middle, and outer annulus became more disorganized in the loaded discs. The nucleus pulposus showed increase of mucoid degeneration and increased cell death. Intervertebral discs from the control group and the adjacent discs to the compressed discs maintained their normal morphology. This study shows that mechanical loading of discs in the spine can cause rapid degeneration. Adjacent discs, however, did not change in terms of radiology, intradiscal pressure, or histology.

Cytomorphology of notochordal and chondrocytic cells from the nucleus pulposus: a species comparison

The nuclei pulposi of the intervertebral discs (IVDs) contain a mixed population of cell types at various stages of maturation. This tissue is formed either by or with the help of cells from the embryonic notochord, which appear to be replaced during development by a population of chondrocyte-like cells of uncertain origin. However, this transition occurs at widely varying times, depending upon the species--or even breed--of the animal being examined. There is considerable debate among spine researchers as to whether the presence of these residual notochordal cells has a significant impact upon IVD degeneration models, and thus which models may best represent the human condition. The present study examines several different species commonly used in lumbar spine investigations to explore the variability of notochordal cells in the IVD.

Effect of bone morphogenetic protein-2 (BMP-2) on matrix production, other BMPs, and BMP receptors in rat intervertebral disc cells

OBJECTIVE

An in vitro experiment study using rat disc cells was carried out to determine the effect of bone morphogenetic protein-2 (BMP-2) on extracellular matrix production, other BMPs, and BMP receptors (BMPRs).

METHODS

Cells from the anulus fibrosus and transition zone were harvested and cultured. When the cells reached 80% confluence, BMP-2 was added to reach a final concentration of 200 ng/mL. Three days later, the culture media were collected for the assay of sulfated glycosaminoglycans (sGAG) and collagen types I and II. The cells were harvested for RNA extraction to determine the genes expressed. All experiments were performed at least three times to ensure repeatability.

RESULTS

BMP-2 significantly increased aggrecan and collagen type II mRNA expression 8.30 and 4.61 times, respectively, and decreased versican mRNA expression 0.54 times as compared with control. Collagen type I production and mRNA level were not changed. BMP-2 significantly increased transforming growth factor-beta1 (TGFbeta1) and BMP-7 mRNA expression 2.32 and 2.45 times, respectively, compared with control. There was no significant change in BMP-6 mRNA expression. BMPR type IB and II mRNA expressions were increased and BMPR type 1A mRNA expression was decreased, but none of these differences was significant.

CONCLUSIONS

The results of this study show that in rat intervertebral disc cells, BMP-2 increases aggrecan and collagen type II mRNA expression and decreases versican gene expression. BMP-2 also up-regulates mRNA expression for BMP-7 and TGFbeta but has no significant effect on the BMPRs.

Degenerative changes of porcine intervertebral disc induced by vertebral endplate injuries

STUDY DESIGN

Graded endplate injuries were performed in porcine lumbar discs. The effects of such injuries were compared to control animals in which a sham operation was performed.

OBJECTIVES

To investigate the effects of endplate injuries on disc tissue.

SUMMARY OF BACKGROUND DATA

Studies have shown that injuries of vertebral endplates are frequently found at autopsy. However, little is known on the effects of acute injuries of vertebral endplates in vivo.

METHODS

Ten domestic pigs were included in the study group. Under general anesthesia, the lower three discs of the lumbar spine were exposed and randomly submitted to multiple endplate injuries, isolated endplate injury, and no treatment. A sham operation was performed in 5 pigs used as control group. Animals were killed 7 months after surgery and the harvested lumbar spine submitted to MRI investigations, histologic, and biochemical analysis.

RESULTS

MRI showed that all but one discs treated with multiple endplate injuries were markedly degenerated while, of the discs treated with an isolated injury, one was markedly degenerated, five slightly degenerated and two were normal (P = 0.01). Histologic analysis showed severe changes in discs treated with multiple injuries. In those who had an isolated injury, changes were less severe and essentially limited to the posterior anulus or the inner anterior anulus. Biochemical analysis showed an inverse correlation between uronate content in the nucleus pulposus and severity of endplate injuries.

CONCLUSIONS

Injuries of vertebral endplates in porcine discs were found to cause degenerative changes in the disc tissue on MRI, histologic, and biochemical investigations. The severity of such degenerative changes was related to the severity of endplate injuries. Injuries of vertebral endplate may be one of the pathomechanisms leading to early changes in the disc matrix and eventually to abnormal biomechanical behavior of the whole disc. The present animal model seems to be a suitable experimental model for disc degeneration.

Mechanical differences between lumbar and tail discs in the mouse

The mouse lumbar and tail discs are both used as models to study disc degeneration; however, the mechanical behavior of these two levels has not been compared. The objective of this study was to compare the elastic and viscoelastic mechanical properties of lumbar and tail discs of the mouse under axial compression-tension loading. We hypothesized that tail discs would have a larger transition zone (e.g., neutral zone) and would be less stiff in compression. To test these hypotheses, lumbar and tail bone-disc-bone motion segments were loaded in axial compression and tension. The nonlinear elastic mechanical behavior was examined using a tri-linear curvefit. Elastic behavior of lumbar and tail discs was most different in the low-stiffness transition region (neutral zone), where lumbar discs were nearly twice as stiff over half the axial displacement. In addition, viscoelastic behavior, which was examined using a stretch-exponential curvefit, also showed large lumbar and tail differences, where lumbar discs compressed by 60% of their original height and tail discs by 98% after static creep compression. These results demonstrate that tail discs undergo far more axial displacement than lumbar discs under the same load. These findings are relevant to rodent tail models where chronic loads are applied in vivo to study mechanical pathways of degeneration. Furthermore, the tri-linear model, used here to curvefit the nonlinear compression-tension data, quantified stiffness in the transition zone for the first time, which may prove useful in future disc mechanical studies.

Quantitative analysis of gene expression in a rabbit model of intervertebral disc degeneration by real-time polymerase chain reaction

BACKGROUND CONTEXT

Serial analysis of gene expression during the course of intervertebral disc degeneration (IDD) could elucidate valuable insight into pathophysiology and provide a basis for identification of potential targets for the development of novel cellular- and gene-based therapies. However, very few previous studies described the changes in gene expression through the process of IDD using a suitable animal model.

PURPOSE

To use a recently developed rabbit annular stab model and the technique of real-time reverse transcriptase-polymerase chain reaction (RT-PCR) to quantify the change in expression of key rabbit-specific mRNA sequences encoding for selected extracellular matrix (ECM) products, catabolic, anabolic, and anti-catabolic factors in normal and stabbed discs.

STUDY DESIGN

Gene expression analyses were performed to characterize a slowly progressive and reproducible animal model of IDD using real-time RT-PCR.

METHODS

Twelve rabbits underwent an annular stab with a 16-gauge needle to the L2-L3, L3-L4, and L4-L5 discs, and three rabbits served as sham controls. Nucleus pulposus tissues were harvested from the stabbed discs at 3, 6, 12 and 24 weeks after confirmation of degenerative changes by magnetic resonance imaging (MRI) scan. Real-time RT-PCR was performed with the use of rabbit-specific primers for 1) extracellular matrix (ECM) component genes: collagen type Ia and IIa, and aggrecan; 2) catabolic genes: matrix metalloprotease-3 (MMP-3), inducible nitric oxide synthase (iNOS), and interleukin-1beta (IL-1beta); 3) anabolic growth genes: bone morphogenic protein-2, and -7 (BMP-2, -7), transforming growth factor-beta1 (TGF-beta1), and insulin-like growth factor-1 (IGF-1); and 4) anti-catabolic gene: tissue inhibitor of metalloprotease-1 (TIMP-1). These data were normalized to mRNA levels of glyceraldehyde phosphate dehydrogenase (GAPDH), a constitutively expressed gene.

RESULTS

The MRI images confirmed progressive decline in the nucleus pulposus area of high T2 signal and in the signal intensity of the stabbed discs over the 24-week study period consistent with IDD. The ECM components, aggrecan and collagen type IIa mRNA levels had decreased markedly by week 3 and never recovered, whereas type Ia collagen mRNA gradually increased throughout course of degeneration. BMP-2, BMP-7 and IGF-1 mRNA were relatively decreased from weeks 3 to 6 but then increased at weeks 12 and 24 to end at a level near the preoperative level. The TIMP-1 expression fell dramatically to approximately one tenth of the preoperative level by week 3 and remained low throughout the degenerative process. The remaining results, including those from TGF-beta1 and the catabolic genes (MMP-3, IL-1beta, iNOS) demonstrated a double peak characteristic. The gene expression increased by week 3, decreased to a low level at weeks 6 and 12 and then had a second, late peak at 24 weeks.

CONCLUSIONS

The gene expression profiles of ECM components and anabolic, catabolic, and anti-catabolic factors demonstrate many characteristics similar to the findings in human disc degeneration and suggest an inability of the intervertebral disc (IVD) to mount an early anabolic response to injury, thereby offering a possible explanation for the disc's lack of reparative capabilities. Catabolic genes are strongly up-regulated both early and late in degeneration, lending strong support to the hypothesis that an anabolic or catabolic imbalance plays a primary role in IDD. According to the resultant patterns, augmenting early production of BMP-2, BMP-7, IGF-1 or TIMP-1 by gene transfer techniques might possibly alter the progressive course of degeneration as seen in the stab model. The next step will be to transfer these therapeutic genes to regulate the biologic processes and ideally alter the progressive course of disc degeneration.

Cell mechanics and mechanobiology in the intervertebral disc

STUDY DESIGN

A review is presented on current knowledge of the micromechanical factors in the intervertebral disc, their role in modifying cell biology, and changes with degeneration.

OBJECTIVES

To identify current knowledge, knowledge gaps, and areas for future research in micromechanics of the intervertebral disc.

SUMMARY OF BACKGROUND DATA

Mechanical factors play important roles in the initiation and progression of intervertebral disc degeneration. Evidence suggests that substantial biologic remodeling occurs in the intervertebral disc in response to mechanical stimuli that may play a critical role in determining the fate of a degenerating intervertebral disc. Information is needed on the precise mechanical stimuli that these cells experience and the mechanisms that govern their responses.

METHODS

A review is presented of cell morphology, cell mechanics, and the internal strains and other mechanical factors predicted to occur at the cell level. A review of intervertebral disc cell responses to well-controlled physical stimuli is also presented with a focus on in vitro studies of explants and isolated cells.

RESULTS

Important differences in cell morphology, mechanics, micromechanical factors, and mechanobiology are noted to occur between cells of the nucleus pulposus and anulus fibrosus. Changes in these features with degeneration are critically understudied, particularly degeneration-associated changes in cell morphology, cell mechanics, and altered physiology with mechanical loading.

CONCLUSIONS

Information on the mechanisms that govern cell responses to mechanical stimuli in the intervertebral disc are just emerging. Studies must address determination of the factors that control micromechanical stimuli, but also mechanisms by which mechanics may interact with genetic factors to regulate expression and remodeling of extracellular matrix molecules, cytokines and mediators of pain and inflammation in degenerating tissue.

Mechanical conditions that accelerate intervertebral disc degeneration: overload versus immobilization

STUDY DESIGN

A review of the literature on macromechanical factors that accelerate disc degeneration with particular focus on distinguishing the roles of immobilization and overloading.

OBJECTIVE

This review examines evidence from the literature in the areas of biomechanics, epidemiology, animal models, and intervertebral disc physiology. The purpose is to examine: 1) what are the degeneration-related alterations in structural, material, and failure properties in the disc; and 2) evidence in the literature for causal relationships between mechanical loading and alterations in those structural and material properties that constitute disc degeneration.

SUMMARY OF BACKGROUND DATA

It is widely assumed that the mechanical environment of the intervertebral disc at least in part determines its rate of degeneration. However, there are two plausible and contrasting theories as to the mechanical conditions that promote degeneration: 1) mechanical overload; and 2) reduced motion and loading.

RESULTS

There are a greater number of studies addressing the "wear and tear" theory than the immobilization theory. Evidence is accumulating to support the notion that there is a "safe window" of tissue mechanical conditions in which the discs remain healthy.

CONCLUSIONS

It is concluded that probably any abnormal loading conditions (including overload and immobilization) can produce tissue trauma and/or adaptive changes that may result in disc degeneration. Adverse mechanical conditions can be due to external forces, or may result from impaired neuromuscular control of the paraspinal and abdominal muscles. Future studies will need to evaluate additional unquantified interactions between biomechanics and factors such as genetics and behavioral responses to pain and disability.

ISSLS prize winner: Collagen fibril sliding governs cell mechanics in the anulus fibrosus: an in situ confocal microscopy study of bovine discs

STUDY DESIGN

In situ investigation of collagen and cell mechanics in bovine caudal discs using novel techniques of confocal microscopy.

OBJECTIVE

To measure simultaneously the in situ intercellular and collagen matrix mechanics in the inner and outer anulus fibrosus of the intervertebral disc subjected to flexion.

SUMMARY OF BACKGROUND DATA

Mechanobiology studies, both in vivo and in vitro, clearly demonstrate that mechanical factors can influence the metabolic activity of disc cells, altering the expression of key extracellular matrix molecules. Essential to elucidating the mechanotransduction mechanisms is a detailed understanding of the in situ mechanical environment of disc cells in response to whole-body mechanical loads.

METHODS

Confocal microscopy was used to simultaneously track and capture in situ images of fluorescently labeled cells and matrix during an applied flexion. The position of the nuclear centroids was calculated before and after applied flexion to quantify the in situ intercellular mechanics of both lamellar and interlamellar cells. The deflection patterns of lines photobleached into the extracellular matrix were used to quantify collagen fibril sliding and collagen fibril strains in situ.

RESULTS

The extracellular matrix was observed to deflect nonuniformly due to the relative sliding of the collagen fibrils. Intercellular displacements within the lamellar layers were also nonuniform, both along a cell row and between adjacent rows. Within a cell row, the intercellular displacements were small (<1%).

CONCLUSIONS

The in situ cell mechanics of anular cells was found to be strongly influenced by collagen fibril sliding in the extracellular matrix and could not be inferred directly from applied tissue loads.

Animal models of intervertebral disc degeneration: lessons learned

STUDY DESIGN

A literature review of intervertebral disc degeneration animal models.

OBJECTIVES

Focus is placed on those models that suggest degeneration mechanisms relevant to human.

SUMMARY OF BACKGROUND DATA

Medical knowledge from observational epidemiology and intervention studies suggest many etiologic causal factors in humans. Animal models can provide basic science data that support biologic plausibility as well as temporality, specificity, and dose-response relationships.

METHODS

Studies are classified as either experimentally induced or spontaneous, where experimentally induced models are subdivided as mechanical (alteration of the magnitude or distribution of forces on the normal joint) or structural (injury or chemical alteration). Spontaneous models include those animals that naturally develop degenerative disc disease.

RESULTS

Mechanobiologic relationships are apparent as stress redistribution secondary to nuclear depressurization (by injury or chemical means) can cause cellular metaplasia, tissue remodeling, and pro-inflammatory factor production. Moderate perturbations can be compensated for by cell proliferation and matrix synthesis, whereas severe perturbations cause architectural changes consistent with human disc degeneration.

CONCLUSIONS

These models suggest that two stages of architectural remodeling exist in humans: early adaptation to gravity loading, followed by healing meant to reestablish biomechanical stability that is slowed by tissue avascularity. Current animal models are limited by an incomplete set of initiators and outcomes that are only indirectly related to important clinical factors (pain and disability).

Recent advances in analytical modeling of lumbar disc degeneration

STUDY DESIGN

Review of the most recent advances in the development of poro-elastic analytical models that include physiologic parameters used for understanding lumbar disc degeneration due to repetitive loading.

OBJECTIVES

To discuss how poro-elastic finite element models that include physiologic parameters such as strain-dependent permeability and porosity and regional variation of poro-elastic material properties of a motion segment can be used to understand the effect of disc degeneration due to cyclic loading on the disc biomechanical properties.

SUMMARY OF BACKGROUND DATA

Mechanical response of the spine to various dynamic loading conditions can be analyzed using in vitro and in vivo studies. Ethical concerns, interpretation of conclusions reached using animal studies, and lack of detailed stress distributions in the disc components are the major disadvantages in using in vivo studies for understanding disc degeneration process. Intraspecimen variability, noninclusion of muscle activity, and difficulty of influx of fluid into the disc during unloading are some of the disadvantages while using in vitro models to understand disc degeneration. The poro-elastic finite element models can provide a method that can circumvent the disadvantages mentioned above and allow a way to understand the relationship between biomechanical performance of the disc due to cyclic loading and disc degeneration.

METHODS

Several types of finite element model were developed in understanding relationship between disc degeneration and associated changes in biomechanical properties. Simplest among them include material and geometric nonlinearity of the disc and was used to predict failure in the disc anulus and endplates under static loading conditions. Response of the lumbar disc under creep loading was studied using poro-elastic models. These models were further refined by including swelling pressure and variable permeability due to change in porosity under load to understand time-dependent deformation of a lumbar disc in a multiple creep compression expansion loading. Regional variation of strain dependent permeability and osmotic pressure was included to further refine the poro-elastic finite element model. This refined model was used to study the effect of disc degeneration on biomechanical properties when cyclic loading was applied to the lumbar disc.

RESULTS

The refined model that included regional variation of strain-dependent permeability and osmotic pressure was validated by comparing diurnal change in total stature measured in vivo. The study showed that disc height loss was larger after considerable number of cyclic loadings both in normal and degenerated discs. Cyclic loading also showed that Grade I discs were much more flexible than Grade IV discs. The disc stiffness also decreased as the load cycle increased.

CONCLUSIONS

A number of different approaches have been used to address the issue of disc degeneration. Poro-elastic finite element model including strain-dependent permeability and osmotic pressure is the most popular analytical tool currently available that can be used to understand how cyclic loading affects the biomechanical characteristics of a degenerated lumbar disc. However, it is important to note that a complete understanding of the behavior of the intervertebral disc will ultimately be arrived using a combination of analytical models, such as the models presented here, in addition to in vitro and in vivo experimental methods.

Anabolic and catabolic mRNA levels of the intervertebral disc vary with the magnitude and frequency of in vivo dynamic compression

The goal of this study was to characterize the anabolic and catabolic mRNA response of the disc to dynamic loading to determine if variations in the magnitude and/or frequency of loading could elicit different cellular responses. Sixty-eight Wistar rats were instrumented with an Ilizarov-type device spanning caudal disc 8-9. Seventy-two hours after surgery, animals were anesthetized and loaded at either 1 or 0.2 MPa at a frequency of 1, 0.2 or 0.01 Hz for 2 h (6 groups). The surgical control (Sham) animals underwent anesthesia with no loading. Loaded (c8-9) and internal-control discs (c6-7 and c10-11) were dissected and annulus and nucleus tissue were separately analyzed by real-time RT-PCR for levels of anabolic (collagen-1A1, collagen-2A1, aggrecan) and catabolic (MMP-3, MMP-13, ADAMTs-4) mRNA. In the nucleus, a frequency-dependent response was seen at 1 MPa with anabolic genes stimulated at 0.01 Hz and catabolic genes at 1 Hz. In the annulus all frequencies resulted in significant up-regulation of catabolic mRNA at 1 MPa loading. In general loading at 0.2 MPa or 0.2 Hz had little effect on gene expression. The results suggest that gene expression of the annulus appears to be more dependent on the magnitude of applied stress, while the nucleus is both magnitude- and frequency-dependent.

Conditioned medium differentially regulates matrix protein gene expression in cells of the intervertebral disc

STUDY DESIGN

Matrix protein gene expression was determined for cells of the anulus fibrosus (AF) and nucleus pulposus (NP) regions of the intervertebral disc when cultured in AF or NP cell-conditioned medium.

OBJECTIVES

To investigate changes in mRNA levels for type I collagen, type II collagen and aggrecan in cells of the AF and NP in response to cell-conditioned medium.

SUMMARY OF BACKGROUND DATA

Cells of the intervertebral disc have been shown to respond to exogenous soluble mediators such as the growth factors TGF-beta and IGF-1. Little is known of their biologic response to endogenous factors that may be secreted locally or by cells of neighboring regions.

METHODS

Porcine cells were cultured for 48 hours in alginate gel in the presence or absence of conditioned medium. Gene expression for aggrecan and collagens was quantified using real-time reverse transcriptase-polymerase chain reaction. RESULTS.: AF cell gene expression was generally stimulated by the conditioned medium of either AF or NP cells. In contrast, the notochordal cell-containing NP cells showed little change in gene expression with either source of conditioned medium.

CONCLUSIONS

Cells of the NP and AF secrete soluble factors in culture at similarly effective doses to stimulate matrix protein gene expression in AF cells of the intervertebral disc. Unlike AF cells, however, NP cell gene expression was not stimulated by any conditioned medium, suggesting that differences exist in the responsiveness of cells of notochordal (NP) and fibrocartilaginous (AF) phenotypes. Understanding these differences between cells of the intervertebral disc may reveal unique stimulatory factors important to repair and regeneration of the degenerated intervertebral disc.

Adjacent segment disease after lumbar or lumbosacral fusion: review of the literature

STUDY DESIGN

Review of the literature.

OBJECTIVES

Review the definition, etiology, incidence, and risk factors associated with as well as potential treatment options.

SUMMARY OF BACKGROUND DATA

The development of pathology at the mobile segment next to a lumbar or lumbosacral spinal fusion has been termed adjacent segment disease. Initially reported to occur rarely, it is now considered a potential late complication of spinal fusion that can necessitate further surgical intervention and adversely affect outcomes.

METHODS

MEDLINE literature search.

RESULTS

The most common abnormal finding at the adjacent segment is disc degeneration. Biomechanical changes consisting of increased intradiscal pressure, increased facet loading, and increased mobility occur after fusion and have been implicated in causing adjacent segment disease. Progressive spinal degeneration with age is also thought to be a major contributor. From a radiographic standpoint, reported incidence during average postoperative follow-up observation ranging from 36 to 369 months varies substantially from 5.2 to 100%. Incidence of symptomatic adjacent segment disease is lower, however, ranging from 5.2 to 18.5% during 44.8 to 164 months of follow-up observation. The rate of symptomatic adjacent segment disease is higher in patients with transpedicular instrumentation (12.2-18.5%) compared with patients fused with other forms of instrumentation or with no instrumentation (5.2-5.6%). Potential risk factors include instrumentation, fusion length, sagittal malalignment, facet injury, age, and pre-existing degenerative changes.

CONCLUSION

Biomechanical alterations likely play a primary role in causing adjacent segment disease. Radiographically apparent, asymptomatic adjacent segment disease is common but does not correlate with functional outcomes. Potentially modifiable risk factors for the development of adjacent segment disease include fusion without instrumentation, protecting the facet joint of the adjacent segment during placement of pedicle screws,fusion length, and sagittal balance. Surgical management, when indicated, consists of decompression of neural elements and extension of fusion. Outcomes after surgery, however, are modest.

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

While in vitro studies have shown that mechanical loading can result in changes in the composition of intervertebral disc matrix, the effects of cyclic loading in vivo have not been considered. The objective of this study was to assess the effect of static and cyclic compression of different frequencies on the nuclear composition of the intervertebral disc. Thirty-six Sprague-Dawley rats were randomly divided into a control group (no pin insertion, no loading), a sham group (pins inserted in sixth and seventh caudal vertebrae, no loading), a static loading group (compression applied via pins) and cyclic loading groups (loading at 0.5, 1.5 or 2.5 Hz). Loading was applied for 1 h each day from the third to 17th day following pin insertion, and the caudal 5-6, 6-7 and 7-8 discs harvested to quantify proteoglycan content, collagen content and chondrocyte density in the nucleus pulposus. Static compression resulted in a significant reduction in total proteoglycan content as compared with the adjacent control disc, but this effect was not seen in any of the cyclic loading groups. However, comparison with the sham group appears to indicate an overall decrease in total proteoglycan content at the targeted and adjacent levels following cyclic loading. The 0.5 Hz loading group showed a significantly greater total proteoglycan content than all other compression groups, and also showed a lower total collagen content than the sham group. Results suggest that frequency dependent changes in composition occur in response to cyclic loading, but are not limited to the directly loaded disc alone. Further studies are required to verify this, but the choice of control appears to need careful consideration in all studies of this nature.

Studies of human intervertebral disc cell function in a constrained in vitro tissue culture system

STUDY DESIGN

This is a laboratory-based study examining a novel in vitro culture system for intervertebral disc tissue.

OBJECTIVES

Address the hypothesis that "the novel culture system will preserve intervertebral disc tissue matrix and cell function and prevent cellular apoptosis for periods up to 21 days."

SUMMARY OF BACKGROUND DATA

Studies of cell function in human intervertebral disc tissue are scarce. In vivo study of human intervertebral disc cells remains impracticable; in situ molecular biology in histologic sections lacks a dynamic dimension; and as for in vitro studies, cell culture often lacks physiologic relevance and explant cultures are subject to loss of tissue integrity and altered cell behavior. There is a biologic and therapeutic need for a satisfactory explant culture system for studying human intervertebral disc tissue in a controlled environment.

METHODS

Samples of human intervertebral disc tissue, obtained at surgery, were examined for a number of tissue and cell parameters immediately after excision (controls) and following culture of tissue samples either in a plastic ring or unconstrained in tissue culture medium for up to 3 weeks. Data were compared between cultured tissue and controls.

RESULTS

By comparison with control tissue, unconstrained explants swelled, tissue structure was disturbed, and there were profound changes in cell function. By contrast, tissue cultured in plastic rings maintained tissue structure, and after 3 weeks, the cellular parameters were the same as in controls.

CONCLUSIONS

This is the first reported system to preserve cell function of human discal explants for long periods in tissue culture. It will be a useful tool for a wide range of investigations of intervertebral disc biology that have not hitherto been possible.

Experimental disc degeneration due to endplate injury

The aim of this study was to create an experimental model of disc degeneration that closely mimicked human disc degeneration. In six domestic pigs, an L4 cranial endplate perforation into the nucleus pulposus was made. Three months postoperatively, compressive testing was performed on the L2-L4 motion segments, and intradiscal pressure was measured in the intervening discs. Histochemical and morphologic examinations were made on the excised degenerated and adjacent discs. A significant reduction in water content was observed in the outer anterior annulus of the degenerated disc. In the nucleus, the proteoglycan content was significantly reduced, as well as the cellularity, although not significantly. The nucleus lost its gel-like structure and was discolored, and there was delamination of annular layers. Intradiscal pressure in the nucleus was significantly lower in the degenerated disc. In conclusion, experimental degeneration of the intervertebral disc induced by endplate penetration resembled human disc degeneration, as exemplified by biochemical and structural changes.

The effect of blocking a nutritional pathway to the intervertebral disc in the dog model

BACKGROUND

The hypothesis that injecting bone cement adjacent to one or both endplates would bring about degeneration in the intervening disc was tested.

METHODS

In 11 dogs, bone cement was injected just below the superior endplates of L1, L2, and L3 to block the nutritional supply through these endplates to the three intervertebral discs T13-L1, L1-L2, and L2-L3. In one other dog, both the superior and the inferior endplates of the same discs (T13-L1, L1-L2, and L2-L3) were blocked with bone cement. All 12 dogs were euthanized between 31 and 70 weeks after the surgery. The three experimental discs (T13-L1, L1-L2, and L2-L3) and two control discs (T12-T13 and L4-L5) were excised and assessed using enzyme-linked immunosorbent assay (ELISA) and histology.

RESULTS

Radiographs of the lumbar spine at the time of death did not show any signs of disc bulging, disc space narrowing, or peripheral osteophyte formation in any of the 12 dogs. The experimental discs as well as the control discs appeared normal in every dog. After the discs were bisected, they were carefully inspected for any visible signs of degeneration. The experimental discs showed no clear signs of disc degeneration and were not distinguishable from the control discs on a gross level. The numerical results from the ELISA showed that in the experimental discs as opposed to the control discs, there were significant increases in proteoglycan content in both the nucleus (P = 0.033) and annulus (P = 0.01) and clear histologic changes in some of the discs.

CONCLUSION

The results show that injecting bone cement adjacent to one or both endplates for up to 70 weeks does not produce degeneration in any visible form in the intervening disc. There were no disc bulging, no apparent annular fissures, and no disc spacing narrowing. There were, however, increases in protoglycan content in both the nucleus and the annulus and clear histologic changes in some of the discs.

Young investigator award winner: validation of the mouse and rat disc as mechanical models of the human lumbar disc

STUDY DESIGN

Measure the mechanical properties of the mouse and rat disc in compression and torsion.

OBJECTIVES

Validate mouse and rat disc as a biomechanical model of the human disc by comparing the normalized properties in compression and torsion loading.

SUMMARY OF BACKGROUND DATA

Rodents have been widely used as models to study disc degeneration; however, mechanical assessments of the rodent disc have been limited. Mouse and rat disc mechanical properties have not been determined.

METHODS

Mechanically test mouse and rat motion segments from both the lumbar and the caudal levels in axial compression and torsion. Normalize the stiffness using disc geometry and compare with human motion segment stiffness taken from the literature. Compare lumbar and caudal levels with each other within each species, and test for correlation between mechanics and body weight.

RESULTS

The average compression stiffness, normalized by geometry, was 2-4 MPa and compared well with human motion segment stiffness in compression (3-9 MPa). The average torsion stiffness, normalized by disc geometry, was 5-11 MPa and compared well with human motion segment stiffness in torsion (2-9 MPa). Differences between the lumbar and caudal levels were observed. For the caudal tail, no correlation between body weight and any compression property was observed, but for the lumbar spine, some correlations were observed. CONCLUSIONS.: This study provides validation for the mouse and rat disc as a mechanical model of the human disc. Correlations between lumbar spine properties and animal body weight provide support for the use of quadruped animal lumbar spines as mechanical models of the bipedal human spine. The differences between lumbar and tail mechanics need further exploration. These findings are important in light of the extensive use of the rodent in disc studies and the expected future utility of genetically engineered mice.

Intervertebral disc degeneration: the role of the mitochondrial pathway in annulus fibrosus cell apoptosis induced by overload

Degeneration of the intervertebral disk (IVD) is a major pathological process implicated in low back pain and is a prerequisite to disk herniation. Although mechanical stress is an important modulator of the degeneration, the underlying molecular mechanism remains unclear. The association of human IVD degeneration, assessed by magnetic resonance imaging, with annulus fibrosus cell apoptosis and anti-cytochrome c staining revealed that the activation of the mitochondria-dependent apoptosome was a major event in the degeneration process. Mouse models of IVD degeneration were used to investigate the role of the mechanical stress in this process. The application of mechanical overload (1.3 MPa) for 24 hours induced annulus fibrosus cell apoptosis and led to severe degeneration of the mouse disks. Immunostaining revealed cytochrome c release but not Fas-L generation. The role of the caspase-9-dependent mitochondrial pathway in annulus fibrosus cell apoptosis induced by overload was investigated further with the use of cultured rabbit IVD cells in a stretch device. Mechanical overload (15% area change) induced apoptosis with increased caspase-9 activity and decreased mitochondrial membrane potential. Furthermore, Z-LEHD-FMK, a caspase-9 inhibitor, but not Z-IETD-FMK, a caspase-8 inhibitor, attenuated the overload-induced apoptosis. Our results from human samples, mouse models, and annulus fibrosus culture experiments demonstrate that the mechanical overload-induced IVD degeneration is mediated through the mitochondrial apoptotic pathway in IVD cells.

Biological response of the intervertebral disc to dynamic loading

Disc degeneration is a chronic remodeling process that results in alterations of matrix composition and decreased cellularity. This study tested the hypothesis that dynamic mechanical forces are important regulators in vivo of disc cellularity and matrix synthesis. A murine model of dynamic loading was developed that used an external loading device to cyclically compress a single disc in the tail. Loads alternated at a 50% duty cycle between 0MPa and one of two peak stresses (0.9 or 1.3MPa) at one of two frequencies (0.1 or 0.01Hz) for 6h per day for 7 days. An additional group received static compression at 1.3MPa for 3h/day for 7 days. A control group wore the device with no loading. Sections of treated discs were analyzed for morphology, proteoglycan content, apoptosis, cell areal density, and aggrecan and collagen II gene expression. Dynamic loading induced differential effects that depended on frequency and stress. No significant changes to morphology, proteoglycan content or cell death were found after loading at 0.9MPa, 0.1Hz. Loading at lower frequency and/or higher stress increased proteoglycan content, matrix gene expression and cell death. The results have implications in the prevention of intervertebral disc degeneration, suggesting that loading conditions may be optimized to promote maintenance of normal structure and function.

In situ intercellular mechanics of the bovine outer annulus fibrosus subjected to biaxial strains

In situ intercellular strains in the outer annulus fibrosus of bovine caudal discs were determined under two states of biaxial strain. Confocal microscopy was used to track and capture images of fluorescently labelled nuclei at applied Lagrangian strains in the axial direction (E(A)(S)) of 0%, 7.5% and 15% while the circumferential direction (E(C)(S)) was constrained to either 0% or -2.5%. The position of the nuclear centroids were calculated in each image and used to investigate the in situ intercellular mechanics of both lamellar and interlamellar cells. The intercellular Lagrangian strains measured in situ were non-uniform and did not correspond with the biaxial Lagrangian strains applied to the tissue. A row-oriented analysis of intercellular unit displacements within the lamellar layers found that the magnitudes of unit displacements between cells along a row (delta;(II)) were small (|delta;(IIavg)|=1.6% at E(C)(S)=0%, E(A)(S)=15%; |delta;(IIavg)|=3.0% at E(C)(S)=-2.5%, E(A)(S)=15%) with negative unit displacements occurring greater than one-third of the time. Evidence of interlamellar shear and increased intercellular Lagrangian strains among the cells within the interlamellar septa suggested that their in situ mechanical environment may be more complex. The in situ intercellular strains of annular cells were strongly dependent upon the local structure and behaviour of the extracellular matrix and did not correspond with applied tissue strains. This knowledge has immediate relevance for in vitro investigations of disc mechanobiology, and will also provide a base to investigate the mechanical implications of disc degeneration at the cellular level.

In vivo growth factor treatment of degenerated intervertebral discs

STUDY DESIGN

An in vivo model was used to investigate the response of degenerated discs to various exogenous growth factors.

OBJECTIVES

To study growth factor-induced alterations of the spatial and temporal patterns of disc cellularity and matrix gene expression.

SUMMARY OF BACKGROUND DATA

Cell proliferation and proteoglycan synthesis have been stimulated by growth factors in normal disc cells, suggesting that growth factors may play a therapeutic role for degeneration. However, the response in situ in degenerated discs has not been characterized.

METHODS

Degeneration was induced in murine caudal discs by static compression. Degenerated discs were given single or multiple injections of growth and differentiation factor-5, transforming growth factor-beta, insulin-like growth factor-1, basic fibroblast growth factor, or saline as control. Comparisons of disc morphology, anular cell density, proliferating cells, disc height, and aggrecan and type II collagen gene expression were made either 1 week or 4 weeks after treatment.

RESULTS

In some growth and differentiation factor-5 and transforming growth factor-beta treated discs, expansion of inner anular fibrochondrocyte populations into the nucleus was observed. The cells actively expressed aggrecan and type II collagen mRNA. A lesser effect was observed for insulin-like growth factor-1 and little or no effect for basic fibroblast growth factor. Differences in cell density and proliferating cells were not significant between treatments but suggested a trend of increased cellularity and proliferation following growth factor treatment. A statistically significant increase in disc height 4 weeks after growth and differentiation factor-5 treatment was measured.

CONCLUSIONS

Anular fibrochondrocytes in degenerated discs are responsive to some growth factors in vivo. The results have implications in the early intervention of disc degeneration to arrest or slow the degenerative process.

The potential of gene therapy for the treatment of disc degeneration

Research in biologic methods of treating disc degeneration is still in its infancy. Many different strategies are being evaluated, but the gene therapy strategy stands out because of its potential for long-term efficacy. Choosing the correct gene for use in gene therapy is critically important. Of the many different classes of potentially therapeutic genes, the regulatory genes hold the most promise. Of the different gene therapy delivery methods, the most work has been performed with viral vectors, either ex vivo or in vivo. Current research now is turning toward in vivo experiments in rabbits. Efficacy and safety will be demonstrated first with smaller animal models. Beyond that, nonhuman primate experiments demonstrating efficacy and safety will be the penultimate step before initiation of human studies.

The compressive creep properties of normal and degenerated murine intervertebral discs

Identifying mechanisms by which degeneration alters intervertebral disc material properties and biomechanical behavior is important for clarifying back pain risk factors as well as for evaluating the efficacy of novel interventions. Our goal was to quantify and characterize degeneration-dependent changes in the disc's response to compression using a previously established murine model of disc degeneration. We performed compressive creep tests on normal and degenerated murine intervertebral discs and parameterized the biomechanical response using a previously established fluid-transport model. Using a series of biochemical and histological assays, we sought to determine how biomechanical alterations were attributable to degeneration-related changes in tissue morphology. We observed that with moderate degeneration, discs lost height (mean+/-std. dev. of 0.44+/-0.01 vs. 0.36+/-0.01 mm, p<0.0001), increased in proteoglycan content (31+/-4 vs. 43+/-2 microg/ml of extract, p<0.0002), became less stiff (2.17+/-0.66 vs. 1.56+/-0.44 MPa, p<0.053), and crept more. Model results suggested that the increased creep response was mainly due to a diminished strain-dependent nuclear swelling pressure. We also noted that the model-derived tissue properties varied with the applied load magnitude for both normal and degenerated discs. Overall, our data demonstrate that architectural remodeling stimulated by excessive loading diminishes the disc's ability to resist compression. These results are similar to degeneration-dependent changes reported for human discs.

Differential effects of static and dynamic compression on meniscal cell gene expression

Cells of the meniscus are exposed to a wide range of time-varying mechanical stimuli that may regulate their metabolic activity in vivo. In this study, the biological response of the meniscus to compressive stimuli was evaluated in vitro, using a well-controlled explant culture system. Gene expression for relevant extracellular matrix proteins was quantified using real-time RT-PCR following a 24 h period of applied static (0.1 MPa compressive stress) or dynamic compression (0.08-0.16 MPa). Static and dynamic compression were found to differentially regulate mRNA levels for specific proteins of the extracellular matrix. Decreased mRNA levels were observed for decorin ( approximately 2.1 fold-difference) and type II collagen ( approximately 4.0 fold-difference) following 24 h of dynamic compression. Decorin mRNA levels also decreased following static compression ( approximately 4.5 fold-difference), as did mRNA levels for both types I ( approximately 3.3 fold-difference) and II collagen ( approximately 4.0 fold-difference). Following either static or dynamic compression, mRNA levels for aggrecan, biglycan and cytoskeletal proteins were unchanged. It is noteworthy that static compression was associated with a 2.6 fold-increase in mRNA levels for collagenase, or MMP-1, suggesting that the homeostatic balance between collagen biosynthesis and catabolism was altered by the mechanical stimuli. These findings demonstrate that the biosynthetic response of the meniscus to compression is regulated, in part, at the transcriptional level and that transcription of types I and II collagen as well as decorin may be regulated by common mechanical stimuli.