Disc Body Unit Research Articles

On the modeling of human intervertebral disc annulus fibrosus: Elastic, permanent deformation and failure responses

As a primary load-resisting component, annulus fibrosus (AF) maintains structural integrity of the entire intervertebral disc. Experiments have demonstrated that permanent deformation and damage take place in the tissue under mechanical loads. Development of an accurate model to capture the complex behaviour of AF tissue is hence crucial in disc model studies. We, therefore, aimed to develop a non-homogenous model to capture elastic, inelastic and failure responses of the AF tissue and the entire disc model under axial load. Our model estimations satisfactorily agreed with results of existing uniaxial (along fiber, circumferential and axial directions) and biaxial tissue-level tests. The model accurately predicted the failure of the tissue in various directions in uniaxial extension. Collagen fiber content, type and orientation substantially altered AF tissue responses in uni- and bi-axial tests. Although collagen fiber content and type mostly affected failure stress, fiber orientation significantly influenced the tissue failure strain. The entire L2-L3 disc model accurately replicated load–displacement as well as loading-unloading responses of the disc under compression-tension forces. Preconditioning of the disc-body unit substantially stiffened response. Poisson’s ratio of both AF and nucleus considerably affected compression-displacement responses of the disc (173% increase in compression at 1.49 mm displacement when it was changed from 0.499 to 0.49999). Any AF constitutive model should be calibrated under various tissue-level loads and directions as well as the entire disc model responses since using a single tissue-level loading (e.g. uniaxial) for calibration can lead to unrealistic responses in other tests (e.g., biaxial). Special attentions should be given to the choice of Poisson’s ratio and the realistic consideration of preconditioning load.

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 N m 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.

Torsion-Induced Pressure Distribution Changes in Human Intervertebral Discs

Biomechanical testing of human cadaveric lumbar specimens was performed to evaluate the effects of torsional torque on intradiscal pressure and disc height. Evaluate the effects of small torsion torques on intradiscal pressure and disc height in human lumbar specimens. Nuclear depressurization in addition to an instantaneous disc height increase were found in previous porcine research when small (<2 degrees) axial vertebral rotations were applied. If applicable to human spines, this phenomenon may support spinal manipulation for the relief of low back pain. Six human lumbar cadaveric functional spine units (FSU) were loaded in the neutral position with 600 N axial compression. Intranuclear pressure measurements were then obtained at 0, 0.5, 1.0, and 2.0 Nm of torsion. Posterior elements were removed and measurements were repeated for the disc body unit (DBU). There was no statistically significant difference in nuclear pressure or intervertebral disc height with different torsion torques among or between the FSUs and DBUs. However, a disc height increase ranging from 0.13 mm to 0.16 mm occurred with the insertion of a 1.85-mm diameter pressure probe cannula. Small torsion torques showed no significant difference in intradiscal pressures or disc heights. This is an unlikely mechanism for the perceived benefits of spinal manipulation.

The mechanical effects of intervertebral disc lesions

Objective. To determine the mechanical effect of individual concentric tears, radial tears and rim lesions of the intervertebral disc anulus. Design. In vitro dynamic mechanical testing of sheep discs comparing the mechanical behaviour before and after lesion creation. Background. Structural changes to the disc in the form of anular lesions are a feature of disc degeneration and degeneration has been related to changes in the mechanical function of the disc. However, the effect of individual lesions is unknown. Methods. Fifteen ovine, lumbar disc body units were tested in flexion/extension, lateral bending and axial rotation. Concentric tears, radial tears and rim lesions were experimentally introduced and the motions repeated. The mechanical response after lesion creation was compared to the undamaged response. Results. It was found that an anterior rim lesion reduced the peak resistive moment produced by the disc in extension, lateral bending and axial rotation. Concentric tears and radial tears did not affect the peak resistive moment, however, radial tears reduced the hysteresis of response in flexion/extension and lateral bending. The neutral zone was not affected by the presence of disc lesions. Conclusions. These results show that rim lesions reduce the disc's ability to resist motion. Radial tears change the hysteresis of response indicating an altered stress distribution in the disc. Relevance These changes may lead to overloading of the spinal ligaments, muscles and zygapophysial joints, possibly damaging these structures. This suggests a mechanism for a cycle of degeneration that is instigated by small changes in the mechanical integrity of the intervertebral disc.

BIOMECHANICAL EFFECT OF THORACIC POSTERIOR VERTEBRAL ELEMENTS ON PATTERNS OF THE LOCI OF INSTANTANEOUS AXES OF ROTATION IN SAGITTAL PLANE

The objective of this study was to investigate the effect of the thoracic posterior vertebral elements on the kinematics of T10–T11 motion segment in sagittal plane by assessing the locations and loci of the instantaneous axes of rotation (IARs) under flexion and extension pure moments using finite element (FE) method. The IAR has proven to be a useful parameter of vertebral motion and it is an indicator of spinal instability. An anatomically accurate FE model of thoracic T10–T11 functional spinal unit (FSU) was used to characterize the loci of centers of rotation for the intact T10–T11 FSU and disc body unit (without posterior vertebral elements) under flexion and extension pure moments. The centers of rotation predicted by the intact model and disc body unit of thoracic T10–T11 for both flexion and extension were directly below the geometrical center of the moving vertebra. However, the loci of the IARs were significantly affected by the posterior vertebral elements. The loci of instantaneous axes of rotation for the intact model were tracked superoanteriorly and inferoposteriorly for flexion and extension with rotation, respectively. While, for the disc body unit, the loci were detected to diverge lateroinferiorly from the mid-height of the intervertebral disc, they converge medio-inferiorly toward the superior endplate of the inferior vertebra T11 with increased moment. These findings may offer an insight to better understanding the kinematics of the human thoracic spine and provide clinically relevant information for the evaluation of spinal stability and implant devices functionality.

An Anisotropic Model for Annulus Tissue and Enhanced Finite Element Analyses of Intact Lumbar Disc Bodies

The current investigation addresses advanced FE analyses of intact lumbar intervertebral discs. Human disc body units basically consist of a fluid filled cavity (nucleus pulposus) approximately at the disc center and surrounding annulus tissue reinforced by collagen fibers (annulus fibrosus). For the latter component a constitutive model is presented which accounts for nonlinear anisotropic stress response. In contrast to state-of-the-art material descriptions mostly used in the literature (linear elasticity), the current approach provides numerical accuracy in the finite strain regime. Thus, local strain and stress distributions in the annulus fibrosus can properly be determined. Because of the highly nonlinear stress response of the constitutive equations, nonlinear material parameters are required. Although tensile properties of the human annulus fibrosus can already be found in the literature, these material data are not suitable for the current concept as will be shown. The entire constitutive theory is therefore based on a new experimental procedure for the determination of local, nonlinear and anisotropic stress response in annulus tissue. The constitutive model proves its applicability in representative numerical examples. In contrast to state-of-the-art FE analyses, the results do not exhibit parasitic strain (and stress) distributions in annulus tissue under physiological loading conditions. Hence, the current approach seems to be very attractive for refined spinal implant design.

Can variations in intervertebral disc height affect the mechanical function of the disc?

The finite element method was used to investigate the effect of variations in disc height on the mechanical behavior of the intervertebral disc. The effect of disc height on the mechanical behavior of a human lumbar spine segment in terms of axial displacement, intradiscal pressure, posterolateral disc bulge, tensile stress in the peripheral anulus fibers, and longitudinal stress distribution at the end plate-vertebra interface was evaluated. Disc height varies with individuals, disc level, abnormal conditions, and clinical management. A three-dimensional finite element model of L2-L3 disc body unit was developed. Parametric studies were undertaken by studying discs of three different heights: 8 mm, 10 mm, and 12 mm, whereas disc cross sectional area, finite element mesh density, and all other parameters were kept constant. The model accounted for geometric nonlinearity but assumed that the material properties were linear. Variations in disc height had a significant influence on the axial displacement, the posterolateral disc bulge, and the tensile stress in the peripheral anulus fibers, but the effect on the intradiscal pressure and the longitudinal stress distribution at the endplate vertebra interface was minimal. Variations in disc height may compromise the general conclusions reached from experimental work and analytic studies in which geometric parameters (especially disc height and disc cross-sectional area) are not taken into consideration.

The 1H-NMR evaluation of biomechanical function in human lumbar disc

The purpose of this study was to determine the usefulness of relaxation time of nuclear magnetic resonance in evaluating biomechanical functions of human lumbar discs. Using L3-L4 disc-body units obtained from fresh human cadavers en bloc, proton density imaging was performed with a NMR-CT. Furthermore, intradiscal pressure, tan delta, dynamic stiffness, and relaxation time were measured. The results were as follows: Proton density imaging was useful in detecting the degeneration of the human lumbar disc. Intradiscal pressure, tan delta, and relaxation time decreased with advancing age whereas dynamic stiffness increased. The correlation coefficient of relaxation time to tan delta and dynamic stiffness was 0.80 (p less than 0.01) and -0.67 (p less than 0.05), respectively, which reflected dynamic viscoelasticity of the human lumbar disc. In conclusion, relaxation time seems to be useful in evaluating biomechanical functions of human lumbar discs.

Stress analysis of the lumbar disc-body unit in compression. A three-dimensional nonlinear finite element study.

It has been argued that a clarification of the mechanical causes of low-back pain requires a knowledge of the states of stress and strain throughout the lumbo-sacral spine. Since a purely experimental approach cannot provide this information, analytical model studies, to supplement measurements, are called for. In the present study, a general three-dimensional finite element program has been developed and applied for the analysis of the lumbar L2-3 disc-body unit. The analysis accounts for both the material and the geometric nonlinearities and is based on a representation of the annulus as a composite of collagenous fibers embedded in a matrix of ground substance. The geometry of the model analyzed is based on in vitro measurements. The validity of the model and the analysis procedure has been established by a comparison of those predictions that are also amenable to direct measurements, eg, the response of the disc-body unit to compressive load in terms of axial displacement, disc bulge, end-plate bulge, and intradiscal pressure. The states of stress and strain have then been computed in the cancellous bone, cortical shell, and the subchondral endplate of the intervertebral body and in the annulus fibers and ground substance of the disc when the unit is subjected to a compressive load. The results indicate that for a normal disc with an incompressible nucleus, the most vulnerable elements under compressive load are the cancellous bone and the end-plate adjacent to the nucleus space. On the other hand, for a degenerated disc, simulated in an extreme fashion by assuming it to be void of the nucleus, the analysis predicts the annulus bulk material to be also susceptible to failure. The annulus fibers do not appear to be vulnerable to rupture when the disc-body unit is subjected to pure compressive force.