Effect of loading rate and hydration on the mechanical properties of the disc

Current understanding of lumbar intervertebral disc degeneration: a review with emphasis upon etiology, pathophysiology, and lumbar magnetic resonance imaging findings

Degeneration of the lumbar intervertebral discs (IVDs) is highly prevalent in adults and is nearly universal in the elderly population. Degenerative changes within, and adjacent to, the IVDs are likely to contribute to a variety of pain syndromes; however, the exact association between these findings and symptoms remains speculative. Recent research has provided new information regarding the etiology, pathophysiology, and clinical relevance of degeneration of the IVD. This information will assist clinicians and researchers in understanding the development and clinical course of lumbar disc degeneration, as well as its potential impact upon patients seeking physical therapy care for back pain. The purposes of this clinical commentary are to review the structure and metabolic capacity of the normal and degenerative lumbar IVD, and to discuss factors that influence the onset and progression of disc degeneration. Lumbar magnetic resonance images will be used to illustrate the common findings associated with this condition.

pH-Responsive microgel dispersions for repairing damaged load-bearing soft tissue

An important challenge for colloid scientists is to design injectable dispersions that provide structural support for damaged soft tissue and enable regeneration of tissue over the longer term. In this article we highlight a new area of research that aims to produce pH-responsive microgel dispersions that restore the mechanical properties of damaged, load-bearing, soft tissue. Chronic back pain due to degeneration of the intervertebral disc (IVD) is a major health problem and is the primary potential application for the work discussed. pH-Responsive microgel dispersions contain cross-linked polymer particles that swell when the pH approaches the pKa of the incorporated ionic co-monomer. The work considered here involves microgel particles containing MAA (methacrylic acid). The particles show pronounced pH-triggered swelling. The concentrated microgel dispersions change from a fluid to a gel at pH values greater than ca. 6.2, which is within the physiological pH range. The rheological properties are pH-dependent and can be adjusted using particle composition or concentration. Degenerated IVDs containing injected, gelled, microgel dispersions show improved mechanical properties. The disc height under biomechanically meaningful loads can be restored to values observed in non-degenerated IVDs. We also discuss the steps required to provide a minimally invasive injectable microgel system for restoring both the IVD mechanical properties and regenerating tissue in vivo. The approach discussed should also be suitable for other soft tissue types in the body.

Diffusion-weighted magnetic resonance imaging of normal and degenerative lumbar intervertebral discs: a new method to potentially quantify the physiologic effect of physical therapy intervention

STUDY DESIGN

Observational, repeated measures design.

OBJECTIVES

To determine the reliability of the apparent diffusion coefficient (ADC) calculated from diffusion-weighted magnetic resonance images (MRI) of the nuclear region of lumbar intervertebral discs (IVDs), to investigate the differences in the ADC based upon T2-signal intensity, and to examine the test-retest variation in these measures obtained from subjects undergoing serial, diffusion-weighted MRI scans.

BACKGROUND

Impaired diffusion of water within the lumbar IVD is a central characteristic of degenerative disc disease. Diffusion-weighted MRI scans can provide quantitative estimates of water diffusion and may be useful to evaluate the physiologic effects of healing or the change in hydration related to interventions such as traction, manual therapy, or exercise on normal and degenerative lumbar IVDs.

METHODS AND MEASURES

Thirty subjects underwent T2 -weighted and diffusion-weighted lumbar MRI scans. Twenty-one of these subjects underwent a second diffusion-weighted MRI scan 4 to 7 weeks after the initial scan. The ADC was calculated from midsagittal diffusion-weighted images for the IVDs of L1-2 to L5-S1. To assess reliability, repeated measures of the ADC were performed on the first 16 scans. The T2-signal of the nuclear region of each disc was classified as hyperintense, intermediate, or hypointense, and its relationship to the mean ADC of the nuclear region was determined. Test-retest variation in the ADC was described using the coefficient of variation (CV), plus or minus the width of the 95% confidence interval of the standard error of measurement (SEM).

RESULTS

Intraclass correlation coefficients for estimates of intrarater and interrater reliability ranged from 0.95 to 0.99 and the SEM ranged from 0.006 to 0.026 X 10-3 mm2/s. The mean ADC was significantly greater for hyperintense IVDs compared to intermediate and hypointense IVDs. The CV plus or minus the 95% CI of the SEM between scans ranged from 9.0% to 13.6% for all discs, 6.1% to 10.1% for hyperintense discs, and 13.1% to 23.7% for intermediate discs. The prevalence of hypointense discs was too low to make meaningful judgments about their normal degree of variation over time.

CONCLUSION

The ADC of the nuclear region of the lumbar IVDs may be reliably measured from diffusion-weighted images. Degenerative discs had lower mean ADC values than normal discs but demonstrated greater variation between scans. Diffusion-weighted imaging may be a useful procedure to assess change in diffusion of water in lumbar discs that occurs over time.

Investigation of nano-mechanical properties of annulus fibrosus using atomic force microscopy

We describe the use of atomic force microscopy (AFM) to investigate the nanomechanical properties of annulus fibrosus (AF)-the outer fibrous layer of an intervertebral disc (IVD) encapsulating the inner jelly-like mass known as the nucleus pulposus (NP). Disk disease, degenerated discs, slipped discs, and herniated discs are common terms often linked to back pain and are caused due to degeneration of IVD. Due to the variations in the structure and biochemical composition of the IVD, studies of macromechanical properties in the motion segment or AF may lack all significant nanomechanical responses or behaviors. Existing studies do not report the micro or nano level of mechanics of IVD components and whether the nanomechanics of this tissue mimic its macromechanical behavior is not known. Our studies used AFM to investigate the regional micromechanical properties of the AF that have been otherwise difficult due to small sample size of the tissue. Five different zones including peripheral and central were tested mechanically as well as biochemically. Qualitative biochemical staining and quantitative values of nanomechanical properties of different zones are compared and discussed in detail. The results of nanomechanical investigations described in this study not only reveal its mimic at macroscopic level, they represent an important step towards establishing a framework for testing and comparing tissue engineered IVD replacements with native tissues.

Different effects of static versus cyclic compressive loading on rat intervertebral disc height and water loss in vitro

STUDY DESIGN

In vitro biomechanical study on rat caudal motion segments to evaluate association between compressive loading and water content under static and cyclic conditions.

OBJECTIVE

To test hypotheses: 1) there is no difference in height loss and fluid (volume) loss of discs loaded in compression under cyclic (0.15-1.0 MPa) and static conditions with the same root-mean-square (RMS) magnitudes (0.575 MPa); and 2) after initial disc bulge, tissue water loss is directly proportional to height loss under static loading.

SUMMARY OF BACKGROUND DATA

Disc degeneration affects water content, elastic and viscoelastic behaviors. There is limited understanding of the association between transient water loss and viscoelastic creep in a controlled in vitro environment where inferences may be made regarding mechanisms of viscoelasticity.

METHODS

A total of 126 caudal motion segments from 21 Wistar rats were tested in compression using 1 of 6 protocols: Static loading at 1.0 MPa for 9, 90, and 900 minutes, Cyclic loading at 0.15 to 1.0 MPa/1 Hz for 90 minutes, Mid-Static loading at 0.575 MPa for 90 minutes, and control. Water content was then measured in anulus and nucleus regions.

RESULTS

Percent water loss was significantly greater in nucleus than anulus regions, suggesting some water redistribution, with average values under 1 MPa static loading of 23.0% and 14.9% after 90 minutes and 26.9% and 17.6% after 900 minutes, respectively. Cyclic loading resulted in significantly greater height loss (0.506 +/- 0.108 mm) than static loading with the same RMS value (0.402 +/- 0.096 mm), but not significantly less than static loading at peak value (0.539 +/- 0.122 mm). Significant and strong correlations were found between percent water loss and disc height loss, suggesting water was lost through volume decrease.

CONCLUSION

Peak magnitude of cyclic compression and not RMS value was most important in determining height change and water loss, likely due to differences between disc creep and recovery rates. Water redistribution from nucleus to anulus occurred under loading consistent with an initial elastic compression (and associated disc bulge) followed by a reduction in disc volume over time.

Dehydration rates of meniscus and articular cartilage in vitro using a fast and accurate laser-based coordinate digitizing system

When used in in vitro studies, soft tissues such as the meniscus and articular cartilage are susceptible to dehydration and its effects, such as changes in size and shape as well as changes in structural and material properties. To quantify the effect of dehydration on the meniscus and articular cartilage, the first two objectives of this study were to (1) determine the percent change in meniscal dimensions over time due to dehydration, and (2) determine the percent change in articular cartilage thickness due to dehydration. To satisfy these two objectives, the third objective was to develop a new laser-based three-dimensional coordinate digitizing system (3-DCDS II) that can scan either the meniscus or articular cartilage surface within a time such that there is less than a 5% change in measurements due to dehydration. The new instrument was used to measure changes in meniscal and articular cartilage dimensions of six cadaveric specimens, which were exposed to air for 120 and 130 min, respectively. While there was no change in meniscal width, meniscal height decreased linearly by 4.5% per hour. Articular cartilage thickness decreased nonlinearly at a rate of 6% per hour after 10 min, and at a rate of 16% per hour after 130 min. The system bias and precision of the new instrument at 0 degrees slope of the surface being scanned were 0.0 and 2.6 microm, respectively, while at 45 degrees slope the bias and precision were 31.1 and 22.6 microm, respectively. The resolution ranged between 200 and 500 microm. Scanning an area of 60 x 80 mm (approximately the depth and width of a human tibial plateau) took 8 min and a complete scan of all five sides of a meniscus took 24 min. Thus, the 3-DCDS II can scan an entire meniscus with less than 2% change in dimensions due to dehydration and articular cartilage with less than 0.4% change. This study provides new information on the amount of time that meniscal tissue and articular cartilage can be exposed to air before marked changes in size and shape, and possibly biomechanical, structural and material properties, occur. The new 3-DCDS II designed for this study provides fast and accurate dimensional measurements of both soft and hard tissues.

A study of pH-responsive microgel dispersions: from fluid-to-gel transitions to mechanical property restoration for load-bearing tissue

An interesting, and potentially important, challenge for colloid scientists is to design injectable dispersions that enable repair of damaged and degenerated tissue. This work presents a study of the ability of pH-responsive microgel particles to restore the mechanical properties of load-bearing soft tissue. Microgel particles are cross-linked polymer colloid particles that are swollen with solvent. The first part of the study consists of an investigation of the pH-triggered swelling of poly(EA/MAA/BDDA) (ethylacrylate, methacrylic acid and 1,4-butanediol diacrylate) microgel particles using photon correlation spectroscopy (PCS) measurements. The concentrated dispersions exhibit a strong fluid-to-gel transition when the pH is increased to above 6.0, i.e., above this pH they form gelled microgel dispersions. The swelling data are used to aid interpretation of the pH-triggered changes in the gel modulus, as probed using dynamic rheology. The second part of the study involves an investigation of the mechanical properties of artificially degenerated, model intervertebral discs (IVDs) containing gelled microgel dispersions. High concentration microgel dispersions were injected as fluids into the interior of degenerated IVDs and the pH increased by subsequent alkaline solution injection to cause particle swelling and dispersion gelation. Uniaxial compression data measured for the IVDs containing injected microgel dispersions indicate that the pH-induced particle swelling of the microgel restores the mechanical properties of degenerated IVDs to values similar to those measured for normal, non-degenerated, IVDs.

Can periods of static loading be used to enhance the resistance of the spine to cumulative compression?

Results of in vitro studies conducted on isolated bone specimens have indicated a higher tolerance to static load than exists when exposed to cyclic loading, when controlled for creep rate. If this difference in load tolerance exists, it may be exploited to extend the life of vertebral bone exposed to repetitive compression, and potentially alter the development of spinal injury. However, little work has been conducted on functional spinal units to determine if bone displays this characteristic within an intact joint. Additionally, static loading may result in load redistribution within the intervertebral disc forcing more of the compressive load towards the periphery of the endplate away from the nucleus. In order to examine these potential mechanisms, 218 osteoligamentous porcine functional spinal units were assigned to one of 15 loading scenarios. This involved one of three normalized peak load magnitudes (50%, 70% and 90% of estimated compressive tolerance) and one of five normalized static load applications (0%, 50%, 100%, 200% and 1000% of the total dynamic work duration). Load magnitude significantly altered the resistance to cumulative compression with decreased peak magnitudes corresponding to both increased cumulative load tolerance and increased height loss. Static load periods did not alter the resistance of the spinal unit to cumulative compression or impact the number of cycles tolerated to failure. The insertion of static load periods impacted the total survival time to failure, but only for the 1000% static load group, an exposure unlikely to occur for most in vivo exposures. The insertion of static load periods decreased the amount of height loss during testing which may play a protective role by allowing load redistribution within the vertebral bone and intervertebral disc.

Pathophysiology of the intervertebral disc and the challenges for MRI

Through its ability to make relatively noninvasive and repeatable measurements, MRI has a great deal to offer, not only to clinical diagnosis of intervertebral disc disorders but also as a tool for basic research into disc physiology and the etiology of disc degeneration. In this brief review we outline the structure of the disc, the composition and organization of its macromolecules, and the changes that occur during disc degeneration, attempting to summarize features that have been or could become targets of MRI characterization. It is important to recognize, however, the fundamental limitation that most of the changes so far observed in MRI are consequences of alterations in cellular metabolism that occurred months to years previously and provide little insight into the current functional status of the tissue. There is therefore a need to develop MR techniques that directly characterize cellular activity and factors such as nutrient delivery on which it is critically dependent. We therefore briefly review cellular energy metabolism and nutrient transport into the avascular disc and consider the ability of MRI to reveal information about such processes. As a corollary of this discussion we also consider the constraints that the unusual transport properties of the disc impose on the delivery of contrast agents to the disc, since an understanding of these limitations is central to interpretation of the resulting images.

Mechanics of arterial subfailure with increasing loading rate

Arterial subfailure leads to delayed symptomatology and high morbidity and mortality rates, particularly for the thoracic aorta and carotid arteries. Although arterial injuries occur during high-velocity automotive collisions, previous studies of arterial subfailure focused on quasi-static loading. This investigation subjected aortic segments to increasing loading rates to quantify effects on elastic, subfailure, and ultimate vessel mechanics. Sixty-two specimens were axially distracted, and 92% demonstrated subfailure before ultimate failure. With increasing loading rate, stress at initial subfailure and ultimate failure significantly increased, and strain at initial subfailure and ultimate failure significantly decreased. Present results indicate increased susceptibility for arterial subfailure and/or dissection under higher-rate extension. According to the present results, automotive occupants are at greater risk of arterial injury under higher velocity impacts due to greater body segment motions in addition to decreased strain tolerance to subfailure and catastrophic failure.

Investigation of thoracolumbar T12–L1 burst fracture mechanism using finite element method

A finite element model of the T12-L1 motion segment was subjected to dynamic vertical impact to investigate vertebral burst fracture mechanism at the thoracolumbar junction. A rigid ball was directed vertically towards a rigid plate fixed on top of the T12 vertebral body to simulate the axial impact. The results show that upon impact, the T12 vertebra exhibited a vibratory motion. At its maximum compression, the endplates bulged towards their vertebral bodies. The central parts of the endplates adjacent to the nucleus experienced the highest effective stress, and localized stress concentration developed correspondingly within the central parts of the cancellous bone adjacent to the endplates. This appears to confirm the hypothesis that nucleus material is forced to enter the vertebral body, pressurizing it further and squeezing the fat and marrow contents out of the cancellous bone. When the nucleus material enters the vertebral body faster than fat and marrow being expulsed, the vertebral body could burst through the anterior and posterior cortical shell. Upon sudden posterior cortex fracture, the transient fragment encroachment could be further into the spinal canal than the final observed locations, as the fragments are retropulsed to the vertebral body during the bursting process.

Comparison of Kinematics Between Thoracolumbar T11-T12 and T12-L1 Functional Spinal Units

The purpose of this study was to compare the kinematics in terms of the locations and loci of instantaneous axes of rotation (IARs) at levels T11-T12 and T12-L1 of thoracolumbar junction (TLJ). The IAR is one of the kinematics characteristics of a functional spinal unit (FSU) in a plane under load. There is little information about loci of IARs in the TLJ. Validated finite element (FE) models of T11-T12 and T12-L1 FSUs were used to determine the locations and loci of IARs in three anatomical planes. In the sagittal plane, the locations and loci of the IARs were located below the intervertebral disc for T11-T12, and situated in the intervertebral disc for T12-L1. In the frontal plane, they were all located around the mid-sagittal plane for T11-T12 and T12-L1. In the transverse plane, they fell in the medio-anterior region of the movable vertebra T11 for T11-T12, and located near the cortical shell of the upper vertebra T12 for T12-L1. These findings may offer an insight to better understanding the kinematics of the human thoracolumbar spine and provide clinically relevant information for the evaluation of spinal stability and functionality of implant devices.

Disc Mechanics With Trans-Endplate Partial Nucleotomy are not Fully Restored Following Cyclic Compressive Loading and Unloaded Recovery

Mechanical function of the intervertebral disc is maintained through the interaction between the hydrated nucleus pulposus, the surrounding annulus fibrosus, and the superior and inferior endplates. In disc degeneration the normal transfer of load between disc substructures is compromised. The objective of this study was to explore the mechanical role of the nucleus pulposus in support of axial compressive loads over time. This was achieved by measuring the elastic slow ramp and viscoelastic stress-relaxation mechanical behaviors of cadaveric sheep motion segments before and after partial nucleotomy through the endplate (keeping the annulus fibrosus intact). Mechanics were evaluated at five conditions: Intact, intact after 10,000cycles of compression, acutely after nucleotomy, following nucleotomy and 10,000cycles of compression, and following unloaded recovery. Radiographs and magnetic resonance images were obtained to examine structure. Only the short time constant of the stress relaxation was altered due to nucleotomy. In contrast, cyclic loading resulted in significant and large changes to both the stiffness and stress relaxation behaviors. Moreover, the nucleotomy had little to no effect on the disc mechanics after cyclic loading, as there were no significant differences comparing mechanics after cyclic loading with or without the nucleotomy. Following unloaded recovery the mechanical changes that had occurred as a consequence of cyclic loading were restored, leaving only a sustained change in the short time constant due to the trans-endplate nucleotomy. Thus the swelling and redistribution of the remaining nucleus pulposus was not able to fully restore mechanical behaviors. This study reveals insights into the role of the nucleus pulposus in disc function, and provides new information toward the potential role of altered nucleus pulpous function in the degenerative cascade.

Development and validation of a CO-C7 FE complex for biomechanical study

In this study, the digitized geometrical data of the embalmed skull and vertebrae (C0-C7) of a 68-year old male cadaver were processed to develop a comprehensive, geometrically accurate, nonlinear C0-C7 FE model. The biomechanical response of human neck under physiological static loadings, near vertex drop impact and rear-end impact (whiplash) conditions were investigated and compared with published experimental results. Under static loading conditions, the predicted moment-rotation relationships of each motion segment under moments in midsagittal plane and horizontal plane agreed well with experimental data. In addition, the respective predicted head impact force history and the S-shaped kinematics responses of head-neck complex under near-vertex drop impact and rear-end conditions were close to those observed in reported experiments. Although the predicted responses of the head-neck complex under any specific condition cannot perfectly match the experimental observations, the model reasonably reflected the rotation distributions among the motion segments under static moments and basic responses of head and neck under dynamic loadings. The current model may offer potentials to effectively reflect the behavior of human cervical spine suitable for further biomechanics and traumatic studies.

Human lumbar spine creep during cyclic and static flexion: creep rate, biomechanics, and facet joint capsule strain

There is a high incidence of low back pain (LBP) associated with occupations requiring sustained and/or repetitive lumbar flexion (SLF and RLF, respectively), which cause creep of the viscoelastic tissues. The purpose of this study was to determine the effect of creep on lumbar biomechanics and facet joint capsule (FJC) strain. Specimens were flexed for 10 cycles, to a maximum 10 Nm moment at L5-S1, before, immediately after, and 20 min after a 20-min sustained flexion at the same moment magnitude. The creep rates of SLF and RLF were also measured during each phase and compared to the creep rate predicted by the moment relaxation rate function of the lumbar spine. Both SLF and RLF resulted in significantly increased intervertebral motion, as well as significantly increased FJC strains at the L3-4 to L5-S1 joint levels. These parameters remained increased after the 20-min recovery. Creep during SLF occurred significantly faster than creep during RLF. The moment relaxation rate function was able to accurately predict the creep rate of the lumbar spine at the single moment tested. The data suggest that SLF and RLF result in immediate and residual laxity of the joint and stretch of the FJC, which could increase the potential for LBP.

Segmental variation in the in vitro cell metabolism of nucleus pulposus cells isolated from a series of bovine caudal intervertebral discs

STUDY DESIGN

This study focuses on the association between cell metabolism and molecular matrix composition of nucleus pulposus (NP) tissue with spine level in sequential bovine caudal intervertebral discs.

OBJECTIVE

To explore the hypothesis that the molecular composition of NP tissue and corresponding cell metabolism varies with caudal spine. A secondary hypothesis is tested that potential cellular differences are maintained after monolayer culture.

SUMMARY OF BACKGROUND DATA

In articular cartilage, cell metabolism and molecular matrix composition are influenced by loading history. This may also be a feature of intervertebral discs in series.

METHODS

NP cells (nonpooled or level pooled) were isolated from four sequential bovine caudal intervertebral discs (levels 3-4, 4-5, 5-6, and 6-7) and cultured in alginate beads immediately or following monolayer culture. Levels of 3H-TdR (proliferation) and 35SO4 (GAG synthesis) incorporation were determined from 14 animals. In a separate set of 6 animals, total content of water, DNA, collagen (and type), GAG (and type) were also determined.

RESULTS

The rate of 3H-TdR and 35SO4 incorporation in freshly isolated NP cells increased nonlinearly from level 3-4 to 6-7 (P < 0.05). Monolayer cultured cells retained level-specific differences for 35SO4 and 3H-TdR incorporation similar to that of freshly isolated cells. GAG content and chondroitin sulfate proportion decreased distally (P < 0.05); however, total collagen and Type I proportion increased distally (P < 0.05). No significant differences in water or DNA content could be determined.

CONCLUSIONS

The results support the hypothesis that level-specific differences in NP cell metabolism and molecular composition are dependent on spine level potentially reflecting subtle mechanical differences between levels. Retention of level-specific differences in monolayer may suggest a certain level of cell "programming." This may be important for cellular strategies to repairspecific sites of degeneration.

Value and limitations of using the bovine tail as a model for the human lumbar spine

STUDY DESIGN

The contents of DNA, proteoglycan, type II collagen, and denatured type II collagen in the bovine coccygeal intervertebral discs were examined in situ in relation to disc level, age, and tissue region.

OBJECTIVE

To determine whether bovine coccygeal discs are a suitable model to study human lumbar discs.

SUMMARY OF BACKGROUND DATA

Bovine coccygeal discs have been suggested as a suitable alternative model because they are readily available, in contrast to human discs, and represent a common source of tissue in the disc field. However, it is not known whether the changes in matrix contents in bovine coccygeal discs are similar to those found in the human lumbar spine.

METHODS

Intervertebral discs from bovine tails were dissected into the nucleus pulposus (NP) and anulus fibrosus (AF). Tissues were weighed and analyzed for matrix contents using specific assays.

RESULTS

Similar to water content, the proteoglycan content was higher in the NP than in the AF. Water content of the bovine NP did not change with age, unlike the proteoglycan content, which decreased. type II collagen content was higher in the NP than in the AF, and both did not change overall significantly with age. The percent of denatured type II collagen decreased with age only in the NP. The DNA content did not vary with age in the AF and in the NP.

CONCLUSION

Differences in matrix contents exist between the bovine coccygeal discs and the human lumbar spine. Thus, caution must be exercised when using the bovine tail as a model for the human lumbar spine in biochemical studies.

Effect of displacement rate on the tensile mechanics of pediatric cervical functional spinal units

This study examined the effect of loading (displacement) rate on the tensile mechanics of cervical spine functional spinal units. A total of 40 isolated functional spinal units (two vertebrae and the adjoining soft tissues) from juvenile male baboons (10+/-0.6-human equivalent years old) were subjected to tensile loading spanning four orders of magnitude from 0.5 to 5000 mm/s. The stiffness, ultimate failure load, and corresponding displacement at failure were measured for each specimen and normalized by spinal geometry to examine the material properties as well as the structural properties. The tensile stiffness, failure load, normalized stiffness, and normalized failure load significantly increased (ANOVA, p<0.001) with increasing displacement rate. From the slowest to fastest loading rate, a two-fold increase in stiffness and four-fold increase in failure load were observed. The tensile failure strains (1.07+/-0.31 mm/mm strain) were not significantly correlated with loading rate (ANOVA, p=0.146). Both the functional (non-destructive stiffness and normalized stiffness) and failure mechanics of isolated functional spinal units exhibited a power-law relationship with displacement rate. Modeling efforts utilizing these rate-dependent characteristics will enhance our understanding of the tensile viscoelastic response of the spine and enable improved dynamic injury prevention schemes.

Single lamellar mechanics of the human lumbar anulus fibrosus

The mechanical behavior of the entire anulus fibrosus is determined essentially by the tensile properties of its lamellae, their fiber orientations, and the regional variation of these quantities. Corresponding data are rare in the literature. The paper deals with an in vitro study of single lamellar anulus lamellae and aims to determine (i) their tensile response and regional variation, and (ii) the orientation of lamellar collagen fibers and their regional variation. Fresh human body-disc-body units (L1-L2, n=11) from cadavers were cut midsagittally producing two hemidisc units. One hemidisc was used for the preparation of single lamellar anulus specimens for tensile testing, while the other one was used for the investigation of the lamellar fiber orientation. Single lamellar anulus specimens with adjacent bone fragments were isolated from four anatomical regions: superficial and deep lamellae (3.9+/-0.21 mm, mean +/- SD, apart from the outer boundary surface of the anulus fibrosus) at ventro-lateral and dorsal positions. The specimens underwent cyclic uniaxial tensile tests at three different strain rates in 0.15 mol/l NaCl solution at 37 degrees C, whereby the lamellar fiber direction was aligned with the load axis. For the characterization of the tensile behavior three moduli were calculated: E(low) (0-0.1 MPa), E(medium) (0.1-0.5 MPa) and E(high) (0.5-1 MPa). Additionally, specimens were tested with the load axis transverse to the fiber direction. From the second hemidisc fiber angles with respect to the horizontal plane were determined photogrammetrically from images taken at six circumferential positions from ventral to dorsal and at three depth levels. Tensile moduli along the fiber direction were in the range of 28-78 MPa (regional mean values). Superficial lamellae have larger E(medium) (p=0.017) and E(high) (p=0.012) than internal lamellae, and the mean value of superficial lamellae is about three times higher than that of deep lamellae. Tensile moduli of ventro-lateral lamellae do not differ significantly from the tensile moduli of dorsal lamellae, and E(low) is generally indifferent with respect to the anatomical region. Tensile moduli transverse to the fiber direction were about two orders of magnitude smaller (0.22+/-0.2 MPa, mean +/- SD, n=5). Tensile properties are not correlated significantly with donor age. Only small viscoelastic effects were observed. The regional variation of lamellar fiber angle phi is described appropriately by a regression line |phi|=23.2 + 0.130 x alpha (r(2)=0.55, p<0.001), where alpha is the polar angle associated with the circumferential position. The single anulus lamella may be seen as the elementary structural unit of the anulus fibrosus, and exhibits marked anisotropy and distinct regional variation of tensile properties and fiber angles. These features must be considered for appropriate physical and numerical modeling of the anulus fibrosus.

A dynamic investigation of the burst fracture process using a combined experimental and finite element approach

Spinal burst fractures account for about 15% of spinal injuries and, because of their predominance in the younger population, there are large associated social and healthcare costs. Although several experimental studies have investigated the burst fracture process, little work has been undertaken using computational methods. The aim of this study was to develop a finite element model of the fracture process and, in combination with experimental data, gain a better understanding of the fracture event and mechanism of injury. Experimental tests were undertaken to simulate the burst fracture process in a bovine spine model. After impact, each specimen was dissected and the severity of fracture assessed. Two of the specimens tested at the highest impact rate were also dynamically filmed during the impact. A finite element model, based on CT data of an experimental specimen, was constructed and appropriate high strain rate material properties assigned to each component. Dynamic validation was undertaken by comparison with high-speed video data of an experimental impact. The model was used to determine the mechanism of fracture and the postfracture impact of the bony fragment onto the spinal cord. The dissection of the experimental specimens showed burst fractures of increasing severity with increasing impact energy. The finite element model demonstrated that a high tensile strain region was generated in the posterior of the vertebral body due to the interaction of the articular processes. The region of highest strain corresponded well with the experimental specimens. A second simulation was used to analyse the fragment projection into the spinal canal following fracture. The results showed that the posterior longitudinal ligament became stretched and at higher energies the spinal cord and the dura mater were compressed by the fragment. These structures deformed to a maximum level before forcing the fragment back towards the vertebral body. The final position of the fragment did not therefore represent the maximum dynamic canal occlusion.

Structural behavior of human lumbar spinal motion segments

The objectives of this study were to obtain linearized stiffness matrices, and assess the linearity and hysteresis of the motion segments of the human lumbar spine under physiological conditions of axial preload and fluid environment. Also, the stiffness matrices were expressed in the form of an 'equivalent' structure that would give insights into the structural behavior of the spine. Mechanical properties of human cadaveric lumbar L2-3 and L4-5 spinal motion segments were measured in six degrees of freedom by recording forces when each of six principal displacements was applied. Each specimen was tested with axial compressive preloads of 0, 250 and 500 N. The displacements were four slow cycles of +/-0.5mm in anterior-posterior and lateral displacements, +/-0.35 mm axial displacement, +/-1.5 degrees lateral rotation and +/-1 degrees flexion-extension and torsional rotations. There were significant increases with magnitude of preload in the stiffness, hysteresis area (but not loss coefficient) and the linearity of the load-displacement relationship. The mean values of the diagonal and primary off-diagonal stiffness terms for intact motion segments increased significantly relative to values with no preload by an average factor of 1.71 and 2.11 with 250 and 500 N preload, respectively (all eight tests p<0.01). Half of the stiffness terms were greater at L4-5 than L2-3 at higher preloads. The linearized stiffness matrices at each preload magnitude were expressed as an equivalent structure consisting of a truss and a beam with a rigid posterior offset, whose geometrical properties varied with preload. These stiffness properties can be used in structural analyses of the lumbar spine.

Mechanically induced disruption of the healthy bovine intervertebral disc

STUDY DESIGN

An intact bovine caudal disc model was used to investigate how combinations of biomechanical parameters influence the severity of disruption during compressive loading.

OBJECTIVES

To quantify the combined influence of flex-ion, hydration level, and compressive loading rate on nuclear disruption.

SUMMARY OF BACKGROUND DATA

The risk of disc pro-lapse is known to increase when the disc is loaded flexed. However, there are few experimental data available quantifying the extent to which loading parameters might interact to produce disruption in the healthy disc.

METHODS

Reproducible states of full and partial hydration were established for 96 isolated caudal discs. These discs were then subjected to compression under combined conditions of high or low hydration, zero or full flexion, and moderate or low loading rate. The extent of disc disruption was assessed macroscopically using a damage weighting procedure.

RESULTS

Maximum disruption of the intact, healthy disc occurred under combined conditions of full hydration and flexion. Loading rate, whether at 0.004 MPa/sec or 4 MPa/sec, had little influence when applied without flexion, but there was an increased risk of disruption when the moderate rate was combined with full flexion.

CONCLUSIONS

The investigation demonstrates that significant levels of disruption can be induced by mechanical loading in intact discs free of degenerative change. Our findings support the hypothesis that mechanical injury to a healthy disc might initiate a process of degeneration.

Dynamic stiffness and damping of human intervertebral disc using axial oscillatory displacement under a free mass system

The aim of this study was to analyse the dynamic response of the human intervertebral disc to vibration in a physiologically relevant frequency spectrum. Eight lumbar intervertebral discs were harvested. After preparation, each sample was subjected to a pre-loading and then dynamic compression (from 5 to 30 Hz). The dynamic compression was applied using an experimental set-up comprising a free weight loading from above and a driving oscillatory displacement from below (closest to the in vivo loading). A viscoelastic model enabled the calculation of stiffness and damping from the transfer function. From 5 Hz to 30 Hz the stiffness values are between 0.19 and 3.66 (MN/m) and the damping values between 32 and 2094 (Ns/m). The mean resonant frequency was found at 8.7 Hz. These dynamic characteristics of the intervertebral disc could be used in a three-dimensional finite elements model of the human body to study its response to vibration in the driving position.

The effect of hydration on the stiffness of intervertebral discs in an ovine model

OBJECTIVE

To determine the hydration-over-time behaviour of ovine intervertebral discs and intact joints in a saline bath at body temperature and the effect this has on their stiffness compared to air at ambient temperature.

DESIGN

The hydration-over-time behaviour and stiffness of the ovine functional spinal unit and disc were quantified.

BACKGROUND

The fluid content of an intervertebral disc is not constant but varies with external load and load history. The stiffness of ovine functional spinal units in a hydrated environment and how this compares to testing in air have not been quantified.

METHODS

Intervertebral discs and functional spinal units were weighed and soaked in a saline water bath at 37 degrees C and reweighed each hour for 6 h. They were then allowed to stand in air at room temperature while the time to return to initial weight was recorded. Functional spinal units were randomly assigned to two groups. Axial compression, flexion, extension, lateral bending and axial torsion tests were performed on both the intact functional spinal unit and isolated disc. Group 1 was tested in air then in a saline water bath at 37 degrees C with the testing order reversed for Group 2.

RESULTS

Hydration of the disc reached a plateau after an average 3-4 h of soaking with the largest increase seen in the first hour. Four hours, standing in air at room temperature, was required to return specimens to their initial weight. The functional spinal unit stiffness was significantly lower for those specimens tested in the bath compared to air.

CONCLUSIONS

Ovine intervertebral discs show similar hydration-over-time behaviour when compared to human discs. Stiffnesses in different modes of loading were significantly different when tested in a hydrated environment compared with the standard method of testing in air.

RELEVANCE

It has been shown that there are biomechanical and biochemical similarities between sheep and human intervertebral discs. Despite these similarities, no studies have looked at how ovine intervertebral discs behave over time in a hydrated environment. In humans, hydration levels are an important aspect of intervertebral disc degeneration. There is also a relationship between decreased hydration levels and increased stiffness. This study demonstrates the similarities between human and ovine hydration-over-time behaviour. The importance of intervertebral disc hydration and its effects on stiffness under different modes of loading were also demonstrated and have not been previously shown using the ovine model. In this context, the results from this study provide further support for the use of the ovine model.

Biomechanical factors influencing nuclear disruption of the intervertebral disc

STUDY DESIGN

A disc model with full anular division was used to investigate how different biomechanical parameters influence the severity of nuclear disruption during compressive loading.

OBJECTIVE

To quantify the manner in which flexion, hydration, and loading rate contribute to the breakdown in the intrinsic cohesive structure of the nucleus pulposus.

SUMMARY OF BACKGROUND DATA

The risk of disc herniation is known to increase when the disc is loaded in flexed positions. However, there is a lack of experimental data showing how a combination of flexion with different loading rates and hydration levels affects the extent of nuclear disruption.

METHODS

A reproducible state of full hydration was established for isolated bovine caudal discs. A period of static preloading at an applied stress of 1 MPa was used to obtain a consistent state of partial hydration. Then 96 discs were subjected to a full-thickness division of the anulus fibrosus and compressed while hydration level, degree of flexion, and rate of loading were varied systematically.

RESULTS

A full spectrum of nuclear damage was observed in the tests, ranging from no detectable disruption to sudden sequestration of the entire nucleus. These results were quantified, and a general correlation was established between the severity of disruption and the different loading parameters.

CONCLUSIONS

The degree of flexion and the level of hydration were shown to play an important role in influencing the tendency of the nucleus to break loose and extrude through a preexisting anular division. Interestingly, the rate of loading appeared to have only a minor effect on the severity of damage induced in discs that incorporated a full depth anular division.

Structure and function of the lumbar intervertebral disk in health, aging, and pathologic conditions

This report is a comprehensive review of the basic and clinical science relating to the morphology and function of the intervertebral disc of the lumbar spine. The purpose is to review the anatomy, physiology, and biomechanics of the intervertebral disc of the lumbar spine in health, with aging, and in pathologic conditions. The complex morphology and ultrastructure of the intervertebral disc of the lumbar spine in the human provide the critical elements that permit normal mobility and transmission of force through the vertebral column. Alterations in this structure are manifest in a variety of clinical conditions routinely encountered in orthopaedic physical therapy practice. These structural and biomechanical changes are related to degenerative changes that occur in association with aging and trauma. Knowledge of the gross morphology and ultrastructure of the intervertebral disc and pathobiologic processes underlying associated conditions is essential to orthopaedic practice.