Mechanical damage to the intervertebral disc annulus fibrosus subjected to tensile loading

Strain transfer in the annulus fibrosus under applied flexion

A detailed understanding of the anatomical and mechanical environment in the intervertebral disc at the scale of the cell is necessary for the design of tissue engineering repair strategies and to elucidate the role of mechanical factors in pathology. The objective of this study was to measure and compare the macroscale to microscale strains in the outer annulus fibrosus in various cellular regions of intact discs over a range of applied flexion. Macroscale strains were measured on the annulus fibrosus surface, and contrasted to in situ microscale strains using novel confocal microscopy techniques for dual labeling of the cell and the extracellular matrix. Fiber oriented surface strains were significantly higher than in situ fiber strains, which implies a mechanism of load redistribution that minimizes strain along the fibers. Non-uniformity of the strains and matrix distortion occurred immediately and most interestingly varied little with increase in flexion (3-16 degrees), suggesting that inter-fiber shear is important in the initial stages of strain redistribution. Fiber oriented intercellular strains were significantly larger and compressive compared to in situ strains in other regions of the extracellular matrix indicating that the mechanical environment in this region may be unique. Further examination of the structural morphology in this pericellular region is needed to fully understand the pathway of strain transfer from the tissue to the cell. This study provides new knowledge on the complex in situ micro-mechanical environment of the annulus fibrosus that is essential to understanding the mechanobiological behavior of this tissue.

Fabrication and modeling of dynamic multipolymer nanofibrous scaffolds

Aligned nanofibrous scaffolds hold tremendous potential for the engineering of dense connective tissues. These biomimetic micropatterns direct organized cell-mediated matrix deposition and can be tuned to possess nonlinear and anisotropic mechanical properties. For these scaffolds to function in vivo, however, they must either recapitulate the full dynamic mechanical range of the native tissue upon implantation or must foster cell infiltration and matrix deposition so as to enable construct maturation to meet these criteria. In our recent studies, we noted that cell infiltration into dense aligned structures is limited but could be expedited via the inclusion of a distinct rapidly eroding sacrificial component. In the present study, we sought to further the fabrication of dynamic nanofibrous constructs by combining multiple-fiber populations, each with distinct mechanical characteristics, into a single composite nanofibrous scaffold. Toward this goal, we developed a novel method for the generation of aligned electrospun composites containing rapidly eroding (PEO), moderately degradable (PLGA and PCL/PLGA), and slowly degrading (PCL) fiber populations. We evaluated the mechanical properties of these composites upon formation and with degradation in a physiologic environment. Furthermore, we employed a hyperelastic constrained-mixture model to capture the nonlinear and time-dependent properties of these scaffolds when formed as single-fiber populations or in multipolymer composites. After validating this model, we demonstrated that by carefully selecting fiber populations with differing mechanical properties and altering the relative fraction of each, a wide range of mechanical properties (and degradation characteristics) can be achieved. This advance allows for the rational design of nanofibrous scaffolds to match native tissue properties and will significantly enhance our ability to fabricate replacements for load-bearing tissues of the musculoskeletal system.

Localized Intervertebral Disc Injury Leads to Organ Level Changes in Structure, Cellularity, and Biosynthesis

A literature review and new data are presented to evaluate the influence of intervertebral disc (IVD) injury on biomechanics, cellularity, inflammation, and biosynthesis. Literature and new experimental evidence support the hypothesis that localized injury in the disc can lead to immediate and long-term organ level changes in biomechanics and biology of the IVD. Biomechanical properties defining motion segment bending behaviors sensitive to injuries that affect anulus fibrosus (AF) integrity and nucleus pulposus (NP) pressurization. Axial mechanics and IVD height measurements show sensitivity to puncture and other injuries that reduce NP pressurization. Torsional biomechanics are strongly affected by the extent and location of AF lesions but are less sensitive to reduced NP pressurization. IVD injuries such as puncture and stab incisions may also lead to a cascade of biological changes consistent with degeneration, including loss of cellularity, altered biosynthesis and inflammation. New results on effects of 25G needle injection of saline into a bovine IVD organ culture model demonstrated a loss of cellularity and down-regulation of matrix gene expression, providing a specific example of how a minor injury affects the IVD organ response. We conclude that localized injuries in the IVD can induce an organ level degenerative cascade through biomechanical and biological mechanisms, and their interactions. Attempts at IVD repair should target the dual biomechanical roles of the anulus of maintaining nucleus pressurization and transmitting loads across the vertebrae. Biologically, it remains important to maintain IVD cellularity and biosynthesis rates following injury to prevent downstream degenerative changes.

Mechanical behavior of annulus fibrosus: a microstructural model of fibers reorientation

Experimental uniaxial tensile tests have been carried out on annulus tissue samples harvested on pig and lamb lumbar intervertebral discs. When subjecting the samples to loading cycles, the stress-strain curves exhibit strong nonlinearities and hysteresis. This particular behavior results from the anisotropic microstructure of annulus tissue composed of woven oriented collagen fibers embedded in the extracellular matrix. During uniaxial tension, the collagen fibers reorient toward the loading direction increasing its global stiffness. To describe this behavior, we propose a heuristic two-dimensional rheological model based on three mechanical and one geometrical characteristics. The latter one is the fibers orientation angle becoming the key parameter that govern the macroscopic mechanical behavior. The experimental results are used to identify the physical properties associated with the rheological model, leading to an accurate representation of the stress-strain curve over a complete loading cycle. In this framework, the fibers reorientation can solely account for the rigidity increase while the hysteresis is associated with liquid viscous flows through the matrix. Based on this representation, unusual coupling effects between strains and fluid flows can be observed, that would significantly affect the cell nutrients transport mechanisms.

Reduced nucleus pulposus glycosaminoglycan content alters intervertebral disc dynamic viscoelastic mechanics

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

Measurement of geometric deformation of lumbar intervertebral discs under in-vivo weightbearing condition

Quantitative data of spinal intervertebral disc deformation is instrumental for investigation of spinal disc pathology. In this study, we employed a combined dual fluoroscopic imaging system and the MR imaging technique to determine the lumbar disc deformation in living human subjects. Discs at L2-3, L3-4 and L4-5 levels were investigated in 8 normal subjects. The geometric deformation of the discs under full body weight loading condition (upright standing) was determined using the supine, non-weightbearing condition as a reference. The average maximum tensile deformation was -21% in compression and 24% in tension, and maximum shear deformation on the disc surface reached 26%. The data indicated that different portions of the disc are under different tensile and shear deformation. Further, discs of L2-3, L3-4 and L4-5 have different deformation behavior under the physiological weightbearing condition. In general, the higher level discs have higher deformation values. The technique used in this study can be used to investigate the deformation behaviors of diseased discs as well as the efficacy of different surgical modalities at restoring normal disc deformation patterns.

The elastic fibre network of the human lumbar anulus fibrosus: architecture, mechanical function and potential role in the progression of intervertebral disc degeneration

Elastic fibres are critical constituents of dynamic biological structures that functionally require elasticity and resilience. The network of elastic fibres in the anulus fibrosus of the intervertebral disc is extensive, however until recently, the majority of histological, biochemical and biomechanical studies have focussed on the roles of other extracellular matrix constituents such as collagens and proteoglycans. The resulting lack of detailed descriptions of elastic fibre network architecture and mechanical function has limited understanding of the potentially important contribution made by elastic fibres to healthy disc function and their possible roles in the progression of disc degeneration. In addition, it has made it difficult to postulate what the consequences of elastic fibre related disorders would be for intervertebral disc behaviour, and to develop treatments accordingly. In this paper, we review recent and historical studies which have examined both the structure and the function of the human lumbar anulus fibrosus elastic fibre network, provide a synergistic discussion in an attempt to clarify its potentially critical contribution both to normal intervertebral disc behaviour and the processes relating to its degeneration, and recommend critical areas for future research.

Micro-structure and mechanical properties of annulus fibrous of the L4-5 and L5-S1 intervertebral discs

BACKGROUND

Nowadays, the study on the structure of the annulus fibrous of intervertebral discs centers on the overall and microscopic structure. There are, however, few investigations about the structures and mechanical properties of the annulus fibrous of intervertebral discs in each point and layer.

METHODS

We observed the tiny anatomical structures and mechanical properties of the adult annulus fibrous of intervertebral discs (L4-5, L5-S1) at the fibrous layer level. Each annulus fibrous of intervertebral disc was delaminated through microsurgical technique. Eight testing points were evenly taken at every layer, and the angles between the fabric direction and the horizontal plane were measured. Meanwhile, five testing specimens were taken from each testing point on every layer along the fabric direction angle, with length (l)=15-20mm, width (b)=1-1.5mm, and thickness (t)=0.1-0.5mm.

FINDINGS AND INTERPRETATION

Through tension tests, we first measured the stress/strain curves to obtain the fitting curves and equations. Thus, mechanical property parameters such as the elastic moduli, damage strain, and damage stress of the testing points were obtained along with relevant equations. The results are as follows: first, there was no obvious difference between the micro-structures and mechanical properties of the annulus fibrous of intervertebral discs L4-5 and L5-S1. Second, the fiber orientation angle at each measurement point gradually increases with the increase of the fibrous layer from the outside to the inside along the radial direction in the horizontal plane. The minimum fiber orientation angle was 25-30 degrees . The fiber orientation angle at the same layer gradually increases from front to back. Furthermore, the fiber orientation angle was 70-90 degrees right in the middle of the back of the annulus fibrous of the lumbar intervertebral disc. The fiber orientation is basically consistent with the posterior longitudinal ligament going. Through the normalized equation and normalized line, the fiber orientation angle alpha at any point in any layer can be easily obtained. Lastly, the elastic moduli of each testing point decreased with the layers increasing along the radius from the outside to the inside. The damage stress decreased linearly from the outside to the inside.

Altered collagen fiber kinematics define the onset of localized ligament damage during loading

Detecting the initiation of mechanical injury to biological tissue, and not just its ultimate failure, is critical to a sensitive and specific characterization of tissue tolerance, development of quantitative relationships between macro- and microstructural tissue responses, and appropriate interpretation of physiological responses to loading. We have developed a novel methodological approach to detect the onset and spatial location of structural damage in collagenous soft tissue, before its visible rupture, via identification of atypical regional collagen fiber kinematics during loading. Our methods utilize high-speed quantitative polarized light imaging to identify the onset of tissue damage in ligament regions where mean collagen fiber rotation significantly deviates from its behavior during noninjurious loading. This technique was validated by its ability to predict the location of visible rupture ( P = 0.0009). This fiber rotation-based metric of damage identifies potential facet capsular ligament injury beginning well before rupture, at 51 ± 12% of the displacement required to produce tissue failure. Although traditional macroscale strain metrics fail to identify the location of microstructural damage, initial injury detection determined by altered fiber rotation was significantly correlated ( R = 0.757, P = 0.049) with tissue yield (defined by a decrease in stiffness), supporting the capabilities of this method. Damaged regions exhibited higher variance in fiber direction than undamaged regions ( P = 0.0412).

A new laser scanning technique for imaging intervertebral disc displacement and its application to modeling nucleotomy

BACKGROUND

Nucleotomy is a standard procedure for treating disc prolapse. It can reduce intervertebral disc height, flattening and displacing the disc, which could lead to a painful narrowing of the foramina due to nerve root compression. The purpose of this study was to investigate the disc displacement of a complete spinal segment with and without nucleotomy. We hypothesized that a nucleotomy under a certain load combination might amplify disc displacement.

METHODS

A laser scanner was developed for recording three-dimensional disc displacement of six loaded L4-5 specimens for three conditions: intact, disc with vertebral bodies and subsequent nucleotomy. Specimens were exposed to pure moments of 7.5 N m in the three principal anatomical directions. Disc displacement was obtained at maximal deflection. A finite element model was validated and subsequently utilized to determine disc displacement. The task of the finite element model was to provide supplemental data for the posterolateral region, which could not be measured from intact specimens.

FINDINGS

Disc displacement measurements of intact specimens were limited to the anterior part of discs, whereas the finite element model was able to provide the missing data of the dorsal disc region. The simulation of load combinations showed that the highest disc displacement was 1.9 mm at the lateral or posterolateral region. The nucleotomy increased the disc displacement up to 2.1mm, whereas the displacement zenith migrated posterolaterally.

INTERPRETATION

These results could be a possible explanation for disadvantages of nucleotomy as a treatment. With the methodology presented here, we would be able to assess the performance of nucleus implants by determining the disc displacement map. This could also give us appropriate information of the annular deformation, which is needed for the development of motion preserving implants.

Cell viability, proliferation and extracellular matrix production of human annulus fibrosus cells cultured within PDLLA/Bioglass composite foam scaffolds in vitro

The objective of this study was to assess cell viability, attachment, morphology, proliferation, and collagen and sulphated glycosaminoglycan (s-GAG) production by human annulus fibrosus (HAF) cells cultured in vitro in poly(d,l-lactide) (PDLLA)/Bioglass composite foams. PDLLA foams with different percentages (0, 5 and 30wt.%) of Bioglass particles were prepared by thermally induced phase separation (TIPS) and characterized by scanning electron microscopy (SEM). HAF cell viability in the PDLLA/Bioglass foam was analysed using Live/Dead staining. HAF cell attachment was observed using SEM. An assessment of cell proliferation was conducted using the WST-1 assay. The level of s-GAG and collagen produced by HAF cells was quantified using the 1,9-dimethylmethylene blue (DMMB) assay and Sircoltrade mark assay after 4 weeks of culture. The presence of collagen types I and II within the PDLLA/Bioglass composite foams was analysed using immunohistochemistry. Live/dead staining showed that many viable HAF cells were present on the top surface of the foams as well as penetrating into the internal pore structure, suggesting that the PDLLA/Bioglass composite materials are non-toxic and that the presence of Bioglass particles within PDLLA scaffolds does not inhibit HAF cell growth. The SEM observations revealed that more clusters of HAF cells were attached to the pore walls of both the PDLLA/5BG foam and the PDLLA/30BG foam when compared with the PDLLA/0BG foam. WST-1 assay performed over a period of 4 weeks showed an increased tendency of HAF cells to proliferate within both the PDLLA/5BG foam and the PDLLA/30BG foam when compared with both the tissue culture plastic control and the PDLLA/0BG foam, indicating the presence of Bioglass in the foam has a positive effect on HAF cell proliferation. Sircoltrade mark and DMMB assays showed that HAF cells cultured within the PDLLA/30BG foam had a greater ability to deposit collagen and proteoglycan when compared with the control and the PDLLA/0BG foam after 4 weeks in culture, suggesting that the increase of Bioglass content may induce microenvironmental changes which promote the production of extracellular matrix containing abundant collagen and s-GAG. The immunohistochemical analysis of collagen production demonstrated that collagen produced in all cultures was predominantly of type I. These findings provide preliminary evidence for the use of PDLLA/Bioglass composite as cell-carrier materials for future treatments of the intervertebral disc with damaged AF region.

The relation between intervertebral disc bulging and annular fiber associated strains for simple and complex loading

Mechanical failure of the annulus fibrosus is mostly indicated by tears, fissures, protrusions or disc prolapses. Some of these annulus failures can be caused by a high intradiscal pressure. This has an effect on disc bulging. However, it is not fully understood how disc bulging is related to disc loading. Therefore, the aim of this study was to investigate the annular fiber strains and disc bulging under simple and complex spinal loads. A novel laser scanner was used to image surfaces of six L2-3 segments. Specimens were loaded with 500 N or 7.5 Nm in a spine tester while acquiring surface maps. Loading was applied in the three principal main directions and four combined directions. Disc bulging and tissue surface strains in annulus collagen fiber directions were computed. Two conditions were measured; intact and defect (vertebral body-disc-body units). Axial compression resulted in 2.7% fiber associated strains in intact segments and the defect increased strains up to 6.7%. Disc bulging increased from 0.7 mm to 0.87 mm. Flexion produced 7.2% fiber associated strains and 1.63 mm bulge going up to 17.5% and 2.21 mm after the defect. Highest fiber associated strains were found for the combination of axial rotation plus lateral bending with 24.6% and with a maximal bulging of 1.14 mm. It was found that there is no tight relationship between fiber associated strains and disc bulging. This was especially seen for the load combinations. Highest fiber associated strains were found to be located in small posterolateral regions. Fiber associated strains had a much higher magnitude than previously reported fiber associated strains. The results showed that combined loading is most likely to produce higher associated fiber strains compared to single axis loading.

Evidence-informed management of chronic low back pain with traction therapy

The management of chronic low back pain (CLBP) has proven to be very challenging in North America, as evidenced by its mounting socioeconomic burden. Choosing among available nonsurgical therapies can be overwhelming for many stakeholders, including patients, health providers, policy makers, and third-party payers. Although all parties share a common goal and wish to use limited health-care resources to support interventions most likely to result in clinically meaningful improvements, there is often uncertainty about the most appropriate intervention for a particular patient. To help understand and evaluate the various commonly used nonsurgical approaches to CLBP, the North American Spine Society has sponsored this special focus issue of The Spine Journal, titled Evidence-Informed Management of Chronic Low Back Pain Without Surgery. Articles in this special focus issue were contributed by leading spine practitioners and researchers, who were invited to summarize the best available evidence for a particular intervention and encouraged to make this information accessible to nonexperts. Each of the articles contains five sections (description, theory, evidence of efficacy, harms, and summary) with common subheadings to facilitate comparison across the 24 different interventions profiled in this special focus issue, blending narrative and systematic review methodology as deemed appropriate by the authors. It is hoped that articles in this special focus issue will be informative and aid in decision making for the many stakeholders evaluating nonsurgical interventions for CLBP.

Elastic fibers enhance the mechanical integrity of the human lumbar anulus fibrosus in the radial direction

The anulus fibrosus of the human lumbar intervertebral disc has a complex, hierarchical structure comprised of collagens, proteoglycans, and elastic fibers. Recent histological studies have suggested that the elastic fiber network may play an important functional role. In this study, it was hypothesized that elastic fibers enhance the mechanical integrity of the extracellular matrix in the radial orientation, perpendicular to the plane containing the collagen fibers. Using a combination of biochemically verified enzymatic treatments and biomechanical tests, it was demonstrated that degradation of elastic fibers resulted in a significant reduction in both the initial modulus and the ultimate modulus, and a significant increase in the extensibility, of radially oriented anulus fibrosus specimens. Separate treatments and mechanical tests were used to account for any changes attributable to non-specific degradation of glycosaminoglycans. Additionally, histological assessments provided a unique perspective on structural changes in the elastic fiber network in radially oriented specimens subjected to tensile deformations. The results of this study demonstrate that elastic fibers play an important and unique role in the mechanical properties of the anulus fibrosus, and provide the basis for the development of improved material models to describe intervertebral disc mechanical behavior.

Porous silk scaffolds can be used for tissue engineering annulus fibrosus

There is no optimal treatment for symptomatic degenerative disc disease which affects millions of people worldwide. One novel approach would be to form a patch or tissue replacement to repair the annulus fibrosus (AF) through which the NP herniates. As the optimal scaffold for this has not been defined the purpose of this study was to determine if porous silk scaffolds would support AF cell attachment and extracellular matrix accumulation and whether chemically decorating the scaffold with RGD peptide, which has been shown to enhance attachment for other cell types, would further improve AF cell attachment and tissue formation. Annulus fibrosus cells were isolated from bovine caudal discs and seeded into porous silk scaffolds. The percent cell attachment was quantified and the cell morphology and distribution within the scaffold was evaluated using scanning electron microscopy. The cell-seeded scaffolds were grown for up to 8 weeks and evaluated for gene expression, histological appearance and matrix accumulation. AF cells attach to porous silk scaffolds, proliferate and synthesize and accumulate extracellular matrix as demonstrated biochemically and histologically. Coupling the silk scaffold with RGD-peptides did not enhance cell attachment nor tissue formation but did affect cell morphology. As well, the cells had higher levels of type II collagen and aggrecan gene expression when compared to cells grown on the non-modified scaffold, a feature more in keeping with cells of the inner annulus. Porous silk is an appropriate scaffold on which to grow AF cells. Coupling RGD peptide to the scaffold appears to influence AF cell phenotype suggesting that it may be possible to select an appropriate scaffold that favours inner annulus versus outer annulus differentiation which will be important for tissue engineering an intervertebral disc.

Quantifying the contributions of structure to annulus fibrosus mechanical function using a nonlinear, anisotropic, hyperelastic model

The annulus fibrosus of the intervertebral disc is comprised of concentric lamella of oriented collagen fibers embedded in a hydrated proteoglycan matrix with smaller amounts of minor collagens, elastin, and small proteoglycans. Its structure and composition enable the disc to withstand complex loads and result in inhomogeneous, anisotropic, and nonlinear mechanical behaviors. The specific contributions of the annulus fibrosus constituent structures to mechanical function remain unclear. Therefore, the objective of this study was to use a structurally motivated, anisotropic, nonlinear strain energy model of annulus fibrosus to determine the relative contributions of its structural components to tissue mechanical behavior. A nonlinear, orthotropic hyperelastic model was developed for the annulus fibrosus. Terms to describe fibers, matrix, and interactions between annulus fibrosus structures (shear and normal to the fiber directions) were explicitly included. The contributions of these structures were analyzed by including or removing terms and determining the effect on the fit to multidimensional experimental data. Correlation between experimental and model-predicted stress, a Bland-Altman analysis of bias and standard deviation of residuals, and the contribution of structural terms to overall tissue stress were calculated. Both shear and normal interaction terms were necessary to accurately model multidimensional behavior. Inclusion of shear interactions more accurately described annulus fibrosus nonlinearity. Fiber stretch and shear interactions dominated contributions to circumferential direction stress, while normal and shear interactions dominated axial stress. The results suggest that interactions between fibers and matrix, perhaps facilitated by crosslinks, elastin, or minor collagens, augment traditional (i.e., fiber-uncrimping) models of nonlinearity.

Direct measurement of intervertebral disc maximum shear strain in six degrees of freedom: motions that place disc tissue at risk of injury

Human intervertebral disc specimens were tested to determine the regions of largest maximum shear strain (MSS) experienced by disc tissues in each of three principal displacements and three rotations, and to identify the physiological rotations and displacements that may place the disc at greatest risk for large tissue strains and injury. Tearing of disc annulus may be initiated by large interlamellar shear strains. Nine human lumbar discs were tagged with radiographic markers on the endplates, disc periphery and with a grid of wires in the mid-transverse plane and subjected to each of the six principal displacements and rotations. Stereo-radiographs were taken in each position and digitized for reconstruction of the three-dimensional position of each marker. Maximum tissue shear strains were calculated from relative marker displacements and normalized by the input displacement or rotation. Lateral shear, compression, and lateral bending were the motions that produced the mean (95% confidence interval) largest mean MSS of 9.6 (0.7)%/mm, 9.0 (0.5)%/mm, and 5.8 (1.6)%/ degrees , respectively, and which occurred in the posterior, posterolateral and lateral peripheral regions of the disc. After taking into account the reported maximum physiological range of motion for each degree of freedom, motions producing the highest physiological MSS were lateral bending (57.8 (16.2)%) and flexion (38.3 (3.3)%), followed by lateral shear (14.4 (1.1)%) and compression (12.6 (0.7)%).

Effects of exogenous crosslinking on in vitro tensile and compressive moduli of lumbar intervertebral discs

BACKGROUND

Collagen crosslinks may play a vital role in preventing ongoing disc degeneration. Age-accumulating crosslinks have been thought to increase brittleness and reduce fatigue resistance. However recent studies have demonstrated increases in fatigue resistance, joint stability and nutritional flow properties resulting from crosslink augmentation. In this study, multi-directional moduli of bovine lumbar intervertebral discs were measured in vitro, including circumferential tension, radial compression, axial tension, and axial compression in control and crosslinked specimens.

METHODS

Four types of annulus fibrosus specimens were dissected from control and crosslinked discs. Cross-sectional areas were measured using a non-contact laser measurement system and then four separate mechanical tests were conducted using a materials testing machine with custom-made loading fixtures.

FINDINGS

The circumferential specimens demonstrated the highest moduli in both low stiffness and linear elastic regions. After a crosslink treatment, the modulus increased more in circumferential tension compared to axial tension and more in axial compression compared to radial compression. Other tensile properties had higher increases in circumferential tension compared to axial tension after crosslinking.

INTERPRETATION

Assuming form follows function, circumferential tension is the predominant type of stress experienced by non-degenerated annulus fibrosus. The anisotropic mechanical properties of the annulus fibrosus is non-uniformly affected by crosslink augmentation. Dominant effects were in the directions with greater inherent stiffnesses. These results suggest some beneficial effects of crosslink augmentation on the mechanical properties of the annulus fibrosus: increase in ultimate strength, yield strength, toughness, and modulus in the principal stress directions.

Cervical facet capsular ligament yield defines the threshold for injury and persistent joint-mediated neck pain

The cervical facet joint has been identified as a source of neck pain, and its capsular ligament is a likely candidate for injury during whiplash. Many studies have shown that the mechanical properties of ligaments can be altered by subfailure injury. However, the subfailure mechanical response of the facet capsular ligament has not been well defined, particularly in the context of physiology and pain. Therefore, the goal of this study was to quantify the structural mechanics of the cervical facet capsule and define the threshold for altered structural responses in this ligament during distraction. Tensile failure tests were preformed using isolated C6/C7 rat facet capsular ligaments (n=8); gross ligament failure, the occurrence of minor ruptures and ligament yield were measured. Gross failure occurred at 2.45+/-0.60 N and 0.92+/-0.17 mm. However, the yield point occurred at 1.68+/-0.56 N and 0.57+/-0.08 mm, which was significantly less than gross failure (p<0.001 for both measurements). Maximum principal strain in the capsule at yield was 80+/-24%. Energy to yield was 14.3+/-3.4% of the total energy for a complete tear of the ligament. Ligament yield point occurred at a distraction magnitude in which pain symptoms begin to appear in vivo in the rat. These mechanical findings provide insight into the relationship between gross structural failure and painful loading for the facet capsular ligament, which has not been previously defined for such neck injuries. Findings also present a framework for more in-depth methods to define the threshold for persistent pain and could enable extrapolation to the human response.

Intervertebral disc biomechanical analysis using the finite element modeling based on medical images

In this paper, a 3D geometric model of the intervertebral and lumbar disks has been presented, which integrated the spine CT and MRI data-based anatomical structure. Based on the geometric model, a 3D finite element model of an L1-L2 segment was created. Loads, which simulate the pressure from above were applied to the FEM, while a boundary condition describing the relative L1-L2 displacement is imposed on the FEM to account for 3D physiological states. The simulation calculation illustrates the stress and strain distribution and deformation of the spine. The method has two characteristics compared to previous studies: first, the finite element model of the lumbar are based on the data directly derived from medical images such as CTs and MRIs. Second, the result of analysis will be more accurate than using the data of geometric parameters. The FEM provides a promising tool in clinical diagnosis and for optimizing individual therapy in the intervertebral disc herniation.

In vitro studies of annulus fibrosus disc cell attachment, differentiation and matrix production on PDLLA/45S5 Bioglass composite films

The aim of this study was to investigate the potential of using PDLLA/45S5 (PDLLA--poly(D,L-lactide)) Bioglass composite films for the culture of annulus fibrosus (AF) cells in vitro with a view to a tissue engineering application. PDLLA films incorporated with different percentages (0, 5 and 30 (wt%)) of Bioglass particles were prepared by solvent casting and characterized by scanning electron microscopy (SEM), water contact angle and white-light interferometry. Bovine AF cell morphology and attachment were analysed using SEM. Cytoskeletal organization was determined by actin labelling with FITC-phalloidin using fluorescence microscopy. The amount of sulphated glycosaminoglycan (sGAG) and collagen produced by AF cells were quantified using the 1,9-dimethylmethylene blue (DMMB) and Sircol assays after 4 weeks in culture. Composite films of PDLLA filled with Bioglass are an appropriate substrate for annulus cells and these films promote the production of an extracellular matrix (ECM) containing abundant sGAGs and collagen. These findings provide a basis for the understanding of the production of ECM molecules by cells cultured on 2D PDLLA/45S5 Bioglass composite films. The results will provide new insights into the design and development of composites containing Bioglass and resorbable polymers as scaffolds for intervertebral disc tissue repair.

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.

Mechanisms for mechanical damage in the intervertebral disc annulus fibrosus

Intervertebral disc degeneration results in disorganization of the laminate structure of the annulus that may arise from mechanical microfailure. Failure mechanisms in the annulus were investigated using composite lamination theory and other analyses to calculate stresses in annulus layers, interlaminar shear stress, and the region of stress concentration around a fiber break. Scanning electron microscopy (SEM) was used to evaluate failure patterns in the annulus and evaluate novel structural features of the disc tissue. Stress concentrations in the annulus due to an isolated fiber break were localized to approximately 5 microm away from the break, and only considered a likely cause of annulus fibrosus failure (i.e., radial tears in the annulus) under extreme loading conditions or when collagen damage occurs over a relatively large region. Interlaminar shear stresses were calculated to be relatively large, to increase with layer thickness (as reported with degeneration), and were considered to be associated with propagation of circumferential tears in the annulus. SEM analysis of intervertebral disc annulus fibrosus tissue demonstrated a clear laminate structure, delamination, matrix cracking, and fiber failure. Novel structural features noted with SEM also included the presence of small tubules that appear to run along the length of collagen fibers in the annulus and a distinct collagenous structure representative of a pericellular matrix in the nucleus region.

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.