How Static and Cyclic Loading Affect the Mechanical Properties of the Porcine Annulus Fibrosus.

This study sought to evaluate the effects of prolonged cyclic loading on the tissue-level mechanical properties of the spinal annulus fibrosus. Functional spinal units (FSUs) were obtained from porcine cervical spines at the C3-C4 and C5-C6 levels. Following a 15-min preload of 300 N of axial compression, the FSUs were split into three groups: the cyclic loading group cycled between 0.35 MPa and 0.95 MPa for 2 h (n = 8); the static loading group was compressed at 0.65 MPa for 2 h (n = 10); and a control group which only underwent the 300 N preload (n = 11). Following loading, samples of the annulus were excised to perform intralamellar tensile testing and interlamellar 180 deg peel tests. Variables analyzed from the intralamellar test were stress and strain at the end of the toe region, stress and strain at initial failure (yield point), Young's modulus, ultimate stress, and strain at ultimate stress. Variables evaluated from the interlamellar tests were lamellar adhesion strength, adhesion strength variability, and stiffness. The analysis showed no significant differences between conditions on any measured variable; however, there was a trend (p = 0.059) that cyclically loaded tissues had increased adhesion strength variability compared to the static and control conditions. The main finding of this study is that long-duration axial loading did not impact the intra- or interlamellar mechanical properties of the porcine annulus. A trend of increased adhesion strength variability in cyclically loaded samples could indicate a potential predisposition of the annulus to delamination.

Combined flexion and compression negatively impact the mechanical integrity of the annulus fibrosus.

PURPOSE: To observe the effect of static flexion, in combination with compression, on the intralamellar and interlamellar matrix properties of the annulus fibrosus. METHODS: C3/C4 cervical functional spinal units of porcine specimens were selected. Following preloading, all specimens were loaded under 1200 N axial compression in either a neutral or static end range flexion posture (15º) for 2 h. Following loading, six annulus samples were dissected from each disc: four single-layer and two multi-layer samples. The multi-layer samples underwent peel tests to quantify the mechanical properties of the interlamellar matrix while the single-layer samples underwent tensile tests to quantify the mechanical properties of the intralamellar matrix. Statistical comparisons between properties were performed to determine differences between postural condition, extraction location, and extraction depth. RESULTS: Flexion elicited a decrease in lamellar adhesive strength (p = 0.045) and in single-layer failure strain (p = 0.03) when compared to a neutral posture. Flexion also had extraction depth-specific effects namely increased intralamellar matrix stiffness in the inner annulus when compared to neutral (p = 0019). Flexion also resulted in a significant decrease in toe region strain for the inner region of the annulus (p = 0.035). The inner region of the annulus was shown to have a significant increase in stress at 30% strain when compared to the outer region after flexion (p = 0.041). CONCLUSION: The current findings suggest that the mechanical properties of the interlamellar and intralamellar matrices are sensitive to flexion, creating an environment that promotes an increased potential for damage to occur.

A Novel Testing Method to Quantify Mechanical Properties of the Intact Annulus Fibrosus Ring From Rat-Tail Intervertebral Discs.

The annulus fibrosus is the ring-like exterior of the intervertebral disc, which is composed of concentrically organized layers of collagen fiber bundles. The mechanical properties of the annulus have been studied extensively; however, tests are typically performed on extracted fragments or multilayered samples of the annulus and not on the annulus as a whole. The purpose of this study was twofold: (1) to develop a novel testing technique to measure the mechanical properties of the intact, isolated annulus; and (2) to perform a preliminary analysis of the rate-dependency of these mechanical properties. Twenty-nine whole annulus ring samples were dissected from 11 skeletally mature Sprague Dawley rat tails and underwent a tensile failure test at either 2%/s (n = 16) or 20%/s (n = 13). Force and displacement were sampled at 100 Hz and were subsequently normalized to stress and strain. Various mechanical properties were derived from the stress-strain curves and statistically compared between the rates. All mechanical variables, with the exception of initial failure stress, were found to be unaffected by rate. Interestingly, initial failure stress was higher for samples tested at the slower rate compared to the higher rate which is atypical for viscoelastic tissues. Although in general rate did not appear to impact the annulus ring response to tensile loading, this novel, intact annular ring testing technique provides an alternative way to quantify mechanical properties of the annulus.

Examining the protective role of the posterior elements of the spine against endplate fractures in a porcine model.

Previous studies have shown that the posterior elements/facet joints provide strength to the overall functional spine unit (FSU) by taking 3-25% of vertical compressive load off the intervertebral disc (IVD). However, little is known regarding whether this offloading has a protective effect against endplate fracture. Therefore, the purpose of this study was to investigate if the posterior elements provide a protective role to the endplate in porcine cervical spines under fracture-inducing conditions. Twenty-two cervical porcine FSUs (C5/6 level) were randomized into two groups: 1) a control group which had their posterior elements left intact (n = 11); 2) an experimental group which had the posterior elements removed (n = 11). Each FSU underwent a previously reported rapid IVD pressurization protocol in order to create endplate fractures. Briefly, hydraulic fluid was rapidly injected into the IVD via a standard inflation needle inserted through the anterior annulus which was connected to a hydraulic pump and pressure transducer. Post pressurization, each FSU was dissected to determine the presence and size of endplate fracture. Peak pressurization and rate of pressurization were not found to differ between intact and cut specimens (p = 0.313 and 0.101, respectively). In contrast, significantly, more cut FSUs sustained an endplate fracture (11/11) compared to intact FSUs (5/11); p = 0.012. Further, cut FSUs resulted in a fracture area 1.91 times greater in size compared to the fractures seen in the intact FSUs (p = 0.011). Therefore, posterior elements appear to decrease the risk and severity of endplate fracture.

TAK-242 treatment and its effect on mechanical properties and gene expression associated with IVD degeneration in SPARC-null mice.

PURPOSE: Intervertebral disc (IVD) degeneration is accompanied by mechanical and gene expression changes to IVDs. SPARC-null mice display accelerated IVD degeneration, and treatment with (toll-like receptor 4 (TLR4) inhibitor) TAK-242 decreases proinflammatory cytokines and pain. This study examined if chronic TAK-242 treatment impacts mechanical properties and gene expression associated with IVD degeneration in SPARC-null mice. METHODS: Male and female SPARC-null and WT mice aged 7-9 months were given intraperitoneal injections with TAK-242 or an equivalent saline vehicle for 8 weeks (3x/per week, M-W-F). L2-L5 spinal segments were tested in cyclic axial tension and compression. Gene expression analysis (RT-qPCR) was performed on male IVD tissues using Qiagen RT2 PCR arrays. RESULTS: SPARC-null mice had decreased NZ length (p = 0.001) and increased NZ stiffness (p < 0.001) compared to WT mice. NZ length was not impacted by TAK-242 treatment (p = 0.967) despite increased hysteresis energy (p = 0.024). Tensile stiffness was greater in SPARC-null mice (p = 0.018), and compressive (p < 0.001) stiffness was reduced from TAK-242 treatment in WT but not SPARC-null mice (p = 0.391). Gene expression analysis found upregulation of 13 ECM and 5 inflammatory genes in SPARC-null mice, and downregulation of 2 inflammatory genes after TAK-242 treatment. CONCLUSIONS: TAK-242 had limited impacts on SPARC-null mechanical properties and did not attenuate NZ mechanical changes associated with IVD degeneration. Expression analysis revealed an increase in ECM and inflammatory gene expression in SPARCnull mice with a reduction in inflammatory expression due to TAK-242 treatment. This study provides insight into the role of TLR4 in SPARC-null mediated IVD degeneration.

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

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

Mechanical Consequence of Induced Intervertebral Disc Degeneration in the SPARC-Null Mouse.

Intervertebral disc (IVD) degeneration is associated with low back pain (LBP) and accompanied by mechanical changes to the spine. Secreted protein acidic and rich in cysteine (SPARC) is a protein that contributes to the functioning and maintenance of the extracellular matrix. SPARC-null mice display accelerated IVD degeneration and pain-associated behaviors. This study examined if SPARC-null mice also display altered spine mechanics as compared to wild-type (WT) mice. Lumbar spines from SPARC-null (n = 36) and WT (n = 18) mice aged 14-25 months were subjected to cyclic axial tension and compression to determine neutral zone (NZ) length and stiffness. Three separate mechanical tests were completed for each spine to determine the effect of the number of IVDs tested in series (one versus two versus three IVDs). SPARC-null spine NZs were both stiffer (p < 0.001) and smaller in length (p < 0.001) than WT spines. There was an effect of the number of IVDs tested in series for NZ length but not NZ stiffness when collapsed across condition (SPARC-null and WT). Correlation analysis revealed a weak negative correlation (r = -0.24) between age and NZ length in SPARC-null mice and a weak positive correlation (r = 0.30) between age and NZ stiffness in WT mice. In conclusion, SPARC-null mice had stiffer and smaller NZs than WT mice, regardless of the number of IVDs in series being tested. The increased stiffness of these IVDs likely influences mobility at these spinal joints thereby potentially contributing to low back pain.

Unloaded Organ Culturing Has a Detrimental Effect on the Axial Mechanical Properties of the Intervertebral Disc.

Healthy function of intervertebral discs (IVDs) depends on their tissue mechanical properties. Native cells embedded within IVD tissues are responsible for building, maintaining, and repairing IVD structures in response to genetic, biochemical, and mechanical signals. Organ culturing provides a method for investigating how cells respond to these stimuli in their natural architectural environment. The purpose of this study was to determine how organ culturing affects the mechanical characteristics of functional spine units (FSUs) across the entire range of axial loading, including the neutral zone (NZ), using a rat tail model. Rat tail FSUs were organ cultured at 37 °C in an unloaded state in standard culture media for either 1-day (n = 8) or 6-days (n = 12). Noncultured FSUs (n = 12) were included as fresh control specimens. Axial mechanical properties were tested by applying cyclical compression and tension. A novel mathematical approach was developed to fully characterize the relationship between load, stiffness, and deformation through the entire range of loading. Culturing FSUs for 1-day did not affect any of the axial mechanical outcome measures compared to noncultured IVDs; however, culturing for 6 days increased the size of NZ by 112% and decreased the stiffness in NZ, compressive, and tensile regions by 53%, 19%, and 15%, respectively, compared to noncultured FSUs. These results highlight the importance of considering how the mechanical integrity of IVD tissues may affect the transmission of mechanical signals to cells in unloaded organ culturing experiments.

Immuno-stimulatory capacity of decorin in the rat tail intervertebral disc and the mechanical consequence of resultant inflammation.

PURPOSE: Determine whether decorin is immuno-stimulatory to rat tail IVD cells and to characterize the mechanical consequence of inflammation at the whole rat tail IVD level. METHODS: Cultured rat tail annulus fibrosus (AF) cells were exposed to decorin, a resident IVD small leucine-rich proteoglycan (SLRP), with and without the presence of a toll-like receptor (TLR) 4 inhibitor, TAK-242. Resultant expression of pro-inflammatory cytokine and chemokines (MCP-1; MIP-2; RANTES; IL-6; TNFα) were quantified over 24 h. Whole rat tail IVD cultures (n = 50) were also treated with decorin (two concentrations: 0.5 and 5.0 µg/mL) with and without TAK-242 (via nucleus pulpous injection with a 33-gauge needle), and resultant mechanical properties were measured. RESULTS: AF cells exposed to decorin showed significant increases in pro-inflammatory cytokine and chemokine production; this was significantly blunted with the presence of TAK-242. Whole IVDs injected with decorin showed a dose-dependent decrease in neutral zone and tensile stiffness and an increase in neutral zone size. When TAK-242 was injected into the IVD with the decorin, mechanical stiffness was preserved and not different from sham controls (injected with PBS). CONCLUSION: AF cells are capable of detecting decorin and inducing inflammation. Decorin further resulted in a functional deterioration in IVD mechanical integrity. TAK- 242, a TLR4 inhibitor, blunted chemokine production at the cellular level and preserved mechanical stiffness in the whole IVD.

Pressure-induced end-plate fracture in the porcine spine: Is the annulus fibrosus susceptible to damage?

PURPOSE: To determine if the mechanical properties of the annulus fibrosus (AF) are altered following end-plate fracture. Vertebral fractures, particularly those in the growth plate, are relatively common among adolescents. What is unclear is whether or not these fractures are also associated with concomitant damage to the intervertebral disc (IVD), in particular the AF. METHODS: The current study employed a high-rate IVD pressurization model to create growth plate fractures in the porcine cervical spine. Posterior AF mechanical properties and laminate adhesion strength were quantified in fractured spines and compared to samples obtained from non-pressurized, un-fractured spines. RESULTS: AF laminate adhesion strength was 31% lower in the fractured spines compared to the un-fractured spines. CONCLUSION: This decrease in laminate adhesion strength suggests that growth plate fracture damage is not isolated to the vertebra and results in microdamage to the interlamellar matrix of the AF. This may increase in the risk of progressive delamination of the AF, which is associated with IVD herniation. These slides can be retrieved under Electronic Supplementary Material.

The aging disc: using an ovine model to examine age-related differences in the biomechanical properties of the intralamellar matrix of single lamellae.

PURPOSE: To determine the effect of age on the biomechanical properties of the intralamellar matrix of single annulus fibrosus (AF) lamellae. METHODS: One intervertebral disc (IVD) was excised from five young (<12 months), five middle-aged (2-4 years) and five older (5-7 years) ovine lumbar spines. From each IVD, a maximum of four single AF lamellae samples were harvested: two from the anterior region and two from the posterior region. Tissues were mounted in a tensile testing apparatus such that tension was applied perpendicular to the orientation of the collagen fibers to isolate the intralamellar matrix. Variables of interest from the stress-strain relationship were: end of toe-region strain and corresponding stress, initial failure stress and strain, and elastic stiffness. RESULTS: When compared to the middle-aged and old samples, the intralamellar matrix of young AF samples displayed significantly higher stress values at the end of the end of toe-region (p = 0.008) and at initial failure (p = 0.002). Further, the young samples were stiffer than both middle-aged and old samples (p = 0.04). CONCLUSIONS: This study was the first to show that the intralamellar matrix of single AF lamellae is weaker and more compliant in middle-aged and old ovine IVDs compared to young IVDs. These findings are likely a result of the remarkable age-related changes that occur that ultimately weaken the IVD as a whole.

Perceived Risk of Low-Back Injury Among Four Occupations.

OBJECTIVE: This study aimed to assess the perception of risk of low-back injury of individuals from four groups: office/administrative employees, dental workers (dentists/dental hygienists), firefighters, and undergraduate students. BACKGROUND: The concept of worker's perception of injury risk has been used to set safe material-handling limits and to determine compliance with health and safety regulations but has not been used to identify perceptual differences among occupations or potential deficiencies in risk awareness. METHOD: Participants (N = 232) were presented with eight images of different low-back postures/tasks and were required to rate their perceived magnitude of low-back risk on a scale from 0 (no risk) to 10 (extreme risk). RESULTS: Office/administrative and dental workers rated postures higher than firefighters and students. Individuals from all groups perceived kyphotic postures as having a higher low-back risk than lordotic postures. Further, office and dental workers, compared to firefighters and students, perceived sitting postures to have a relatively higher level of risk, likely due to these postures being typically adopted by these individuals at work. No relationship between previous low-back pain and risk rating was observed in this study. CONCLUSION: Low-back injury risk perception varies between occupations/groups and may be a result of different exposures. APPLICATION: The results of this study can potentially be used to implement occupation-specific training programs to ensure that the scientific research regarding low-back injuries is being properly conveyed to employees across all sectors.

Prevalence of occupation-related pain among baristas and an examination of low back and shoulder demand during the preparation of espresso-based beverages.

Many baristas complain of low back pain (LBP) and upper extremity discomfort while at work. This study documented the prevalence of LBP and shoulder pain, via questionnaire, among a population of baristas to determine whether cumulative low back loads and shoulder moments are associated with pain reporting. Fifty-nine baristas completed the questionnaire; ten were also video-recorded for biomechanical analysis while making espresso beverages and cumulative and peak low back loads and shoulder moments were calculated. Seventy-three percent of those who completed the questionnaire reported having experienced LBP, and half attributed this pain to their job as a barista. Furthermore, 68% reported having experienced shoulder pain and half also attributed this pain to their job. Those who suffered from LBP had higher peak low back compression and those with shoulder pain had, in general, higher moments about their dominant shoulder.

Mechanical consequences to the annulus fibrosus following rapid internal pressurization and endplate fracture under restrained-expansion conditions.

Intervertebral disc herniation is not a common injury in the adolescent population, but the correlation between trauma and herniation warrants concern. Previous research demonstrated the capacity for rapid internal pressurization to reduce the mechanical integrity of the intervertebral disc's annulus fibrosus, even in the absence of fracture. The purpose of this study was to modify previous internal pressurization procedures towards a more transferable injury model, then investigate the capacity for these procedures to damage the mechanical integrity of the annulus fibrosus. Porcine cervical motion segments with intact facet joints were confined between a vice and force plate under 300 N of static compression, then a single, manual, rapid internal pressurization was delivered. Posterolateral annulus samples were extracted and situated in a 180° peel test configuration, exposing the interlamellar matrix of samples to separations of 0.5 mm/s, until complete separation of the sample occurred. Multilayer tensile testing was performed on superficial and mid-span samples of annulus by applying uniaxial tension of 1 %/s to 50 % strain. Compared to unpressurized controls, rapid pressurization causing fracture resulted in reduced lamellar adhesion and increased toe-region stress and strain properties in the annulus. Morphological assessment reported similar fracture patterns between endplate fractures achieved in the present experiment and endplate fractures documented in human patients. Mechanical plus morphological results suggest that rapid internal pressurization resulting in endplate fracture may represent a potent mechanism for subsequent damage to the intervertebral disc.

The effect of intervertebral disc damage on the mechanical strength of the annulus fibrosus in the adjacent segment.

BACKGROUND CONTEXT: A herniated intervertebral disc (IVD) is a common injury in the human population. Despite the injury being isolated to a singular IVD in the spine, it is important to look at the biomechanical effects that a damaged IVD has on the entire spine, specifically the IVD adjacent to the injury. PURPOSE: This study examined the effects of a damaged IVD on the mechanical properties of the annulus fibrosus (AF) in the adjacent cranial IVD. STUDY DESIGN: Basic science study using an in-vitro porcine model. METHODS: Sixteen porcine cervical spines were used; specifically spinal levels C3/4/5 were assigned to one of two experimental groups: 1) a control group that was not subjected any injuries (n=8); 2) an experimental group that experienced an injury to the anterolateral part of the disc, reaching the nucleus pulposus but without affecting the posterior portion of the AF in the C4/5 functional spine unit (FSU) (n=8). Each specimen underwent a previously published precondition compression protocol of 300 N of compression for 15 minutes followed by a cyclical compression protocol of compression protocol of 0.5 Hz sinusoidal waveform at 300 to 1200 N for 2 hours (3600 cycles). Post compression, the C3/4 AF was dissected to obtain two multilayer samples (one anterior and one posterior) as well as a peel sample (from the posterolateral region). A tensile strength test was conducted to examine the strength of the interlamellar matrix (peel sample) and the overall strength of the AF (multilayer samples). RESULTS: Significant results were found in the peel test samples. Specifically, experimental specimens were less stiff compared than control specimens (p<.01). In addition, experimental specimens also had a lower average strength then control specimens (p<.01). This reduction in both interlamellar strength and stiffness increases the risk of delamination in the experimental samples. In contrast, there were no differences found between the two groups when examining the AF as a whole through the multilayer tests (p>.05). CONCLUSIONS: It appears that a damaged IVD impacts the biomechanics of the spine and specifically the mechanical properties of the adjacent IVD. Specifically, the observed weakening of the interlamellar matrix in these adjacent IVDs may predispose it to delamination and subsequently degeneration or herniation. CLINICAL SIGNIFICANCE: These findings may help clinicians when treating patients who have experienced a disc herniation or severe degeneration, as they may potentially experience accelerated adjacent disc degeneration.

Anular delamination strength of human lumbar intervertebral disc.

INTRODUCTION: Progression of intervertebral disc (IVD) herniation does not occur exclusively in a linear manner through the anulus fibrosus (AF), but can migrate circumferentially due to localized AF delamination. Consequently, resistance to delamination is an important factor in determining risk of herniation progression. The inter-lamellar matrix located between the AF layers is responsible for resisting this delamination; however, its mechanical properties are largely unknown. This study aimed to determine the mechanical properties of the inter-lamellar matrix in human AF samples via a peel test. MATERIALS AND METHODS: Seventeen human IVDs (degeneration grades of 2-3) were obtained from six lumbar spines. From these 17 discs, 53 tissue samples were obtained from the superficial and deep regions of the anterior and posterior AF. Samples were dissected into a 'T' configuration to facilitate a T-peel test (or 180-degree peel test) by initiating delamination between the two middle AF layers. RESULTS: Peel strength was found to be 33 % higher in tissues obtained from the superficial AF region as compared with the deep region (p = 0.047). CONCLUSION: This finding may indicate a higher resistance to delamination in the superficial AF, and as a result, delamination and herniation progression may occur more readily in the deeper layers of the AF.

Delamination of the Annulus Fibrosus of the Intervertebral Disc: Using a Bovine Tail Model to Examine Effect of Separation Rate.

The intervertebral disc (IVD) is a complex structure, and recent evidence suggests that separations or delamination between layers of the annulus may contribute to degeneration development, a common cause of low back pain The purpose of the present experiment was to quantify the mechanical response of the layer-adjoining interlamellar matrix at different rates of separation. Understanding the rate-dependency of the interlamellar matrix, or the adhesion between adjacent layers of the disc, is important as the spine experiences various loading velocities during activities of daily living. Twelve discs were dissected from four bovine tails (three extracts per tail). Two multi-layered annulus samples were collected from each IVD (total = 24, mean bond width = 3.82 ± 0.96 mm) and randomly assigned to a 180° peel test at one of three delamination rates; 0.05 mm/s, 0.5 mm/s, or 5 mm/s. Annulus extracts were found to have similar maximal adhesion strengths (p = 0.39) and stiffness (p = 0.97) across all rate conditions. However, a significant difference in lamellar adhesion strength variability was observed between the 5 mm/s condition (0.96 N/mm ± 0.31) when compared to the 0.5 mm/s (0.50 N/mm ± 0.19) and 0.05 mm/s (0.37 N/mm ± 0.13) conditions (p < 0.05). Increased variability may be indicative of non-uniform strength due to inconsistent adhesion throughout the interlamellar matrix, which is exacerbated by increased rates of loading. The observed non-uniform strength could possibly lead to a scenario more favourable to the development of microtrauma, and eventual delamination.

An in vitro 3D annulus fibrosus cell culture model with type I collagen: An examination of cell-matrix interactions.

Background: Disorders of the intervertebral disc (IVD) are widely known to result in low back pain; one of the most common debilitating conditions worldwide. As a multifaceted condition, both inflammatory environment and mechanical factors can play a crucial role in IVD damage, and in particular, in the annulus fibrosus (AF), the highly collagenous outer ring of the IVD. As a result, a better understanding of how cells from the IVD, and specifically the AF, interact and respond to their environment is imperative. Goal: The goal of this study is to use collagen type I as an in vitro three-dimensional extracellular matrix for AF cells of IVD and briefly examine both the cellular and mechanical effect of exposure to an inflammatory stimulant. Methods: We utilized type I collagen as a 3D in vitro model material for culturing AF cells of Sprague Dawley rat tail IVDs. Results: We showed that the cultured cells are viable and metabolically active; these cells also induced a distinct and significant contraction on their collagen matrix. Furthermore, to demonstrate potential versatility of our model our model and its versatility, we used lipopolysaccharide (LPS), as a known inflammatory stimulant in IVDs, to manipulate the cells and their interaction. LPS treatment resulted in detectable changes to the contraction cells induced on the collagen matrix and affected the mechanical properties of these constructs.

A comparison of uniaxial and biaxial mechanical properties of the annulus fibrosus: a porcine model.

The annulus fibrosus of the intervertebral disk experiences multidirectional tension in vivo, yet the majority of mechanical property testing has been uniaxial. Therefore, our understanding of how this complex multilayered tissue responds to loading may be deficient. This study aimed to determine the mechanical properties of porcine annular samples under uniaxial and biaxial tensile loading. Two-layer annulus samples were isolated from porcine disks from four locations: anterior superficial, anterior deep, posterior superficial, and posterior deep. These tissues were then subjected to three deformation conditions each to a maximal stretch ratio of 1.23: uniaxial, constrained uniaxial, and biaxial. Uniaxial deformation was applied in the circumferential direction, while biaxial deformation was applied simultaneously in the circumferential and compressive directions. Constrained uniaxial consisted of a stretch ratio of 1.23 in the circumferential direction while holding the tissue stationary in the axial direction. The maximal stress and stress-stretch ratio (S-S) moduli determined from the biaxial tests were significantly higher than those observed during both the uniaxial tests (maximal stress, 97.1% higher during biaxial; p=0.002; S-S moduli, 117.9% higher during biaxial; p=0.0004) and the constrained uniaxial tests (maximal stress, 46.8% higher during biaxial; S-S moduli, 82.9% higher during biaxial). These findings suggest that the annulus is subjected to higher stresses in vivo when under multidirectional tension.

The effect of compressive loading rate on annulus fibrosus strength following endplate fracture.

Intervertebral disc degeneration poses a considerable healthcare challenge, although the process is not well understood. Endplate fracture marks severe biomechanical compromise in a segment and may be correlated with degeneration of the disc. The purpose of this experiment was to investigate the relationship between endplate fracture velocity and damage to the annulus fibrosus. Following overnight-thawing, 27 frozen porcine cervical spines were dissected into motion segments (vertebra-disc-vertebra) and compressed until fracture at one of three loading rates (fast=15 mm/s, medium=1.5 mm/s, and slow=0.15 mm/s), or remained unfractured (control). Two annular samples were extracted and mechanically tested from each segment: 1) Bilayer samples underwent uniaxial tension to a stretch-ratio of 1.5; 2) Multilayer samples were delaminated with a 180° peel test configuration. All three rates of compression resulted in specimen fracture observed in the endplate and/or vertebra with varying degree of severity. Significant differences were detected in compressive strength and stiffness of motion segments when loaded at different rates of compression; interestingly these differences were not observed in the mechanical properties of the annulus fibrosus suggesting that at slow rates of loading, fracture of the endplate precedes destruction of the annulus fibrosus. In corroboration of these findings, gross and histological analysis reported no signs of annular disruption, strengthening assertions that annular damage did not occur.