Impact of material properties of intervertebral disc on dynamic response of the human lumbar spine to vertical vibration: a finite element sensitivity study

Impact of extracellular matrix and collagen network properties on the cervical intervertebral disc response to physiological loads: A parametric study

Current intervertebral disc finite element models are hard to validate since they describe multi-physical phenomena and contain a huge number of material properties. This work aims to simplify numerical validation/identification studies by prioritizing the sensitivity of intervertebral disc behavior to mechanical properties. A 3D fiber-reinforced hyperelastic model of a C6-C7 intervertebral disc is used to carry out the parametric study. 10 parameters describing the extracellular matrix and the collagen network behaviors are included in the parametric study. The influence of varying these parameters on the disc response is estimated during physiological movements of the head, including compression, lateral bending, flexion, and axial rotation. The obtained results highlight the high sensitivity of the disc behavior to the stiffness of the annulus fibrosus extracellular matrix for all the studied loads with a relative increase in the disc apparent stiffness by 67% for compression and by 57% for axial rotation when the annulus stiffness increases from 0.4 to 2 MPa. It is also shown that varying collagen network orientation, stiffness, and stiffening in the studied configuration range have a noticeable effect on rotational motions with a relative apparent stiffness difference reaching 6.8%, 10%, and 22%, respectively, in lateral bending. However, the collagen orientation does not affect disc response to axial load.

Effect of whole-body vibration at different frequencies on the lumbar spine: A finite element study based on a whole human body model

Many previous studies have found that occupational drivers commonly suffered from low back pain, and low back pain and degeneration of the intervertebral disc might be associated with vibration conditions. However, the biomechanical mechanisms of whole-body vibration that caused pain and injury were not clear. In this study, a validated whole human body finite element model was used, and vibration loads at frequencies of 3, 5, 7 and 9 Hz were loaded to evaluate the frequency effects on the spine. The results showed that the responses of the spine were strong at the 5 Hz vibration load. Vibration loads would produce alternating stresses and bulges in the annulus fibrosus and change the direction of the pressure in the nucleus pulposus. The posterior region of the intervertebral disc showed greater stress fluctuations than the anterior region. The Risk Factors showed that long-term exposure to whole-body vibrations at 5 and 7 Hz might have greater adverse effects on the spine. The findings of this study confirmed that vibrations near the resonance frequency of the human body would cause more injuries to the spine than other frequencies. Alternating stress and bulge might cause fatigue and the degeneration of the intervertebral disc, which might be the mechanisms of spinal injury caused by whole-body vibration, and the posterior regions of the intervertebral disc were more susceptible to degeneration. Some appropriate measures should be taken to reduce the adverse effects of whole-body vibration on spinal health.

Biomechanical Evaluation of Transforaminal Lumbar Interbody Fusion with Coflex-F and Pedicle Screw Fixation: Finite Element Analysis of Static and Vibration Conditions

OBJECTIVE

To investigate the biomechanics of transforaminal lumbar interbody fusion (TLIF) with interspinous process device (IPD) or pedicle screw fixation under both static and vibration conditions by the finite element (FE) method.

METHOD

A validated FE model of the L1-5 lumbar spine was used in this study. This FE model derived from computed tomography images of a healthy female adult volunteer of appropriate age. Then the model was modified to simulate L3-4 TLIF. Four conditions were compared: (i) intact; (ii) TLIF combined with bilateral pedicle screw fixation (BPSF); (iii) TLIF combined with U-shaped IPD Coflex-F (CF); and (iv) TLIF combined with unilateral pedicle screw fixation (UPSF). The intact and surgical FE models were analyzed under static and vibration loading conditions respectively. For static loading conditions, four motion modes (flexion, extension, lateral bending, and axial rotation) were simulated. For vibration loading conditions, the dynamic responses of lumbar spine under sinusoidal vertical load were simulated.

RESULT

Under static loading conditions, compared with intact case, BPSF decreased range of motion (ROM) by 92%, 95%, 89% and 92% in flexion, extension, lateral bending and axial rotation, respectively. While CF decreased ROM by 87%, 90%, 69% and 80%, and UPSF decreased ROM by 84%, 89%, 66% and 82%, respectively. Compared with CF, UPSF increased the endplate stress by 5%-8% in flexion, 7%-10% in extension, 2%-4% in lateral bending, and decreased the endplate stress by 16%-19% in axial rotation. Compared with CF, UPSF increased the cage stress by 9% in flexion, 10% in extension, and decreased the cage stress by 3% in lateral bending, and 13% in axial rotation. BPSF decreased the stress responses of endplates and cage compared with CF and UPSF. Compared BPSF, CF decreased the facet joint force (FJF) by 6%-13%, and UPSF decreased the FJF by 4%-12%. During vibration loading conditions, compared with BPSF, CF reduced maximum values of the FJF by 16%-32%, and vibration amplitudes by 22%-35%, while UPSF reduced maximum values by 20%-40%, and vibration amplitudes by 31%-45%.

CONCLUSION

Compared with other surgical models, BPSF increased the stability of lumbar spine, and also showed advantages in cage stress and endplate stress. CF showed advantages in IDP and FJF especially during vertical vibration, which may lead to lower risk of adjacent segment degeneration. CF may be an effective alternative to pedicle screw fixation in TLIF procedures.

Finite Element Analysis of a Bionate Ring-Shaped Customized Lumbar Disc Nucleus Prosthesis

Study design: Biomechanical study of a nucleus replacement with a finite element model. Objective: To validate a Bionate 80A ring-shaped nucleus replacement. Methods: The ANSYS lumbar spine model made from lumbar spine X-rays and magnetic resonance images obtained from cadaveric spine specimens were used. All materials were assumed homogeneous, isotropic, and linearly elastic. We studied three options: intact spine, nucleotomy, and nucleus implant. Two loading conditions were evaluated at L₃-L₄, L₄-L₅, and L₅-S₁ discs: a 1000 N axial compression load and this load after the addition of 8 Nm flexion moment in the sagittal plane plus 8 Nm axial rotation torque. Results: Maximum nucleus implant axial compression stresses in the range of 16–34 MPa and tensile stress in the range of 5–16 MPa, below Bionate 80A resistance were obtained. Therefore, there is little risk of permanent implant deformation or severe damage under normal loading conditions. Nucleotomy increased segment mobility, zygapophyseal joint and end plate pressures, and annulus stresses and strains. All these parameters were restored satisfactorily by nucleus replacement but never reached the intact status. In addition, annulus stresses and strains were lower with the nucleus implant than in the intact spine under axial compression and higher under complex loading conditions. Conclusions: Under normal loading conditions, there is a negligible risk of nucleus replacement, permanent deformation or severe damage. Nucleotomy increased segmental mobility, zygapophyseal joint pressures, and annulus stresses and strains. Nucleus replacement restored segmental mobility and zygapophyseal joint pressures close to the intact spine. End plate pressures were similar for the intact and nucleus implant conditions under both loading modes. Manufacturing customized nucleus implants is considered feasible, as satisfactory biomechanical performance is confirmed.

Biomechanical investigation of topping-off technique using an interspinous process device following lumbar interbody fusion under vibration loading

Topping-off technique has been proposed to prevent adjacent-segment degeneration/disease following spine fusion surgery. Nevertheless, few studies have investigated biomechanics of the fusion surgery with topping-off device under whole-body vibration (WBV). This biomechanical study aimed to investigate the vibration characteristics of human lumbar spine after topping-off surgery, and also to evaluate the effect of bony fusion on spine biomechanics. Based on a healthy finite-element model of lumbosacral spine (L1-sacrum), the models of topping-off surgery before and after bony fusion were developed. The simulated surgical procedures consisted of interbody fusion with rigid stabilizer at L4-L5 segment (rigid fusion) and dynamic stabilizer at degenerated L3-L4 segment. An interspinous implant, Device for Intervertebral Assisted Motion (DIAM, Medtronic Inc., Minnesota, USA), was used as the dynamic stabilizer. The stress responses of spine segments and implants under a vertical cyclic load were calculated and analyzed. The results showed that compared with rigid fusion alone, the topping-off technique significantly decreased disc stress at transition segment (L3-L4) as expected, and resulted in a slight increase in disc stress at its supra-adjacent segment (L2-L3). It indicated that the topping-off stabilization using DIAM might provide a good tradeoff between protection of transition segment and deterioration of its supra-adjacent segment during WBV. Also, it was found that bony fusion decreased stress in L4 inferior endplate and rigid stabilizer but had nearly no effect on stress in DIAM and L3-L4 disc, which was helpful to determine the biomechanical differences before and after bony fusion.

Three-Dimensional Finite Element Analysis of L4-5 Degenerative Lumbar Disc Traction under Different Pushing Heights

Objective

To study and analyze the changes of intervertebral foramen height and area of the degenerative L4-5 intervertebral disc under different pushing heights by the finite element method.

Methods

CT and MRI images of T12-S1 segments were obtained from a healthy volunteer who met the inclusion criteria. A DR machine was used to capture images of the lumbar lateral section before and after simultaneous pushing of the L4 and L5 spinous processes by manipulation called Daogaijinbei, and the measurement showed that the displacement changes of L4 and L5 were both approximately 10 cm, so the pushing height was set at 0-10 cm. A three-dimensional finite element model of the entire normal lumbar spine was established using Mimics 16.0, Geomagic Studio 2014, Hypermesh 13.0, MSC.Patran 2012, and so on. The disc height and nucleus area of the lumbar disc of the normal entire lumbar disc model were adjusted to establish models of the L4-5 disc with mild, moderate, and severe degeneration. Changes of disc height and area of the L4-5 degenerative intervertebral disc under different pushing heights were calculated.

Results

The size of the L4-5 intervertebral foramen was analyzed from the height and area of the intervertebral foramen, and the results showed the following: (1) as for the normal lumbar disc and a lumbar of the L4-5 disc with mild and moderate degeneration, the height of the L4-5 intervertebral foramen and its area both increased during pushing between 0 and 8 cm. After the pushing height reached 8 cm, the height and area of the L4-5 intervertebral foramen gradually became stable; (2) as for the L4-5 disc with severe degeneration, during the process of pushing, the height and area of the L4-5 intervertebral foramen increased slightly, but this change was not obvious.

Conclusions

After the spinal manipulation, the sizes of the L4-5 intervertebral foramen of the L4-5 disc with mild and moderate degeneration were significantly larger than those before pushing; in contrast, the size of L4-5 intervertebral foramen of the L4-5 disc with severe lumbar degeneration was not significantly changed.

Prediction of complications and fusion outcomes of fused lumbar spine with or without fixation system under whole-body vibration

Lumbar fixator has been widely used, which can stabilize the lumbar spine and improve the fusion outcomes, but also lead to many complications. The effects of the internal fixator on biomechanical properties of the fused lumbar spine have been widely concerned for many years. However, most studies only considered the static loads and did not consider the effect of the fixator on the properties of the human lumbar spine under whole-body vibration (WBV). The purpose of this study is to investigate how the fixation system affects the biomechanical characteristics of the lumbar spine, fusion outcomes, and complications under WBV based on the finite element analysis. A three-dimensional nonlinear osteoligamentous finite element model of the intact L1-sacrum spine with muscles was established. A 5-Hz, 40-N sinusoidal vertical load supplemented with a 400-N preload was applied at L1 to simulate the vibration of the human body. For the adjacent segments, the fixation system may increase the risk of the adjacent segment disease under WBV. For the fused segments, the fixation system may decrease the risk of subsidence and cage failure including fatigue failure under WBV. The fixation system may provide a more stable and suitable environment for vertebral cell growth under WBV and lead to better fusion outcomes. This study reveals insights into the effect of the fixation system on the vibration characteristics of the lumbar and provides new information on the fixation system, fusion outcomes, complications, clinical evaluation, and selection of fixation system.

Prediction of the influence of vertical whole-body vibration on biomechanics of spinal segments after lumbar interbody fusion surgery

BACKGROUND

Previous studies have shown that for healthy spine, cyclic loading encountered due to whole-body vibration exposure generated higher responses in spinal tissues than static loading. However, how whole-body vibration affects spine biomechanics after interbody fusion surgery is poorly understood. This study aimed at comparing the effects of vibration loading on spinal segments between postsurgical and healthy lumbar spines.

METHODS

A validated finite element model of healthy human lumbosacral spine was modified to simulate interbody fusion at L4-L5 level considering the statuses immediately after surgery (before bony fusion) and after bony fusion. Biomechanical responses at its adjacent levels for the healthy and fusion models to a sinusoidal axial vibration load of ±40 N and the corresponding static axal loads (-40 N and 40 N) were computed using transient dynamic and static analyses, respectively.

FINDINGS

For both healthy and fusion models, vibration amplitudes of the predicted responses were significantly higher than the corresponding changing amplitudes under static loads. Specifically, the increasing effect of vibration load in disc bulge, disc stress and intradiscal pressure at L3-L4 level reached 255.9%, 215.0% and 224.4% for the healthy model, 157.4%, 177.8% and 171.8% for the fusion model (before bony fusion), 141.9%, 152.6% and 160.1% for the fusion model (after bony fusion).

INTERPRETATION

Although whole-body vibration is still more dangerous for the lumbar spine after interbody fusion surgery than static loading, the sensitivity of adjacent segment in postsurgical spine to vibration loading is decreased compared with healthy spine, especially when reaching to bony fusion.

Distribution of Young's modulus at various sampling points in a human lumbar spine vertebral body

BACKGROUND CONTEXT

Mathematical modeling for creating computer spine models is one of the basic methods underlying many scientific publications. The accuracy of strength parameters of tissues introduced into such models translates directly into the reliability of obtained results. Experimental determination of Young's modulus (E) in various areas of spongy bone tissue seems to be crucial for creating a reliable spine model without excessive simplifications in the form of a single E value for the whole vertebral body.

PURPOSE

The aim of the study was to determine Young's modulus in different parts of the lumbar vertebral column for samples subjected to compression and bending.

STUDY DESIGN

Cylindrical spongy bone tissue samples were subjected to bending and compression strength tests.

METHODS

The study included 975 pathologically unchanged samples of spongy bone tissue harvested from the lumbar vertebrae of 15 male donors. The samples were subjected to compression or bending strength tests and then Young's modulus was determined for each sample depending on its location in the vertebral body. The samples were tested differently between given locations within one vertebra as well as between vertebrae.

RESULTS

Compressed specimens are characterized by highly significantly different Young's modulus values depending on the location in the vertebral body. Samples No. 7 and No. 9 in the anterior part of the vertebral body have highly significantly higher Young's modulus values compared to those in the posterior part of the vertebral body for all lumbar vertebrae. Samples subjected to bending showed significant differences (p<.05) between samples located closer to the vertebral canal (No.16, No.17) and samples located further away (No.14, No.15) with higher values for the samples located in the posterior part of the vertebral body.

CONCLUSIONS

Accommodating the anisotropic structure of spongy bone in computer models and the application of different Young's module values for areas within one vertebral body will allow one to obtain realistic results of computer simulations used.

CLINICAL SIGNIFICANCE

Determining the exact strength parameters of spongy bone tissue within one vertebra and changes in these properties in subsequent vertebrae will allow to create more accurate computer models of the lumbar spine and the whole spine. This, in turn, will translate into more reliable computer simulations used, among others, to determine the risk of fractures or osteoporotic changes, or simulation of the procedure of spinal fusion.

Finite element investigation on the dynamic mechanical properties of low-frequency vibrations on human L2-L3 spinal motion segments with different degrees of degeneration

Exposure to low-frequency vibration is harmful to human lumbar health. However, the dynamic mechanical properties of lumbar spines with varying degrees of degeneration during time-dependent vibration remain incompletely understood. In this study, four poroelastic finite element models of human L2-L3 spinal motion segments, including the non-degeneration and the mild, moderate, and serious degeneration, were established. One-hour low-frequency vibrations with different frequencies were applied. Then, the dynamic mechanical properties of different degenerated lumbar models under the same vibration and the same lumbar model under vibrations at different frequencies were investigated. The results indicated and implied that the negative influences of 1-h vibration on the dynamic mechanical properties of the non-degenerated and mildly degenerated models were similar, but became obvious for the moderately and seriously degenerated models with time. Therefore, the damage caused by low-frequency vibration on the degenerated spinal motion segments was more serious compared with that on the healthy one. Meanwhile, the dynamic mechanical properties of the same lumbar model under vibrations at different frequencies expressed the negligible differences when the vibration frequency was not close to the lumbar natural frequency. Thus, the effects of the 1-h vibrations at different frequencies on one spinal motion segment were similar. Vibration frequency sensitivity analysis on the dynamic characteristics of human L2-L3 spinal motion segments with different degrees of degeneration.

Influence of different postures under vertical impact load on thoracolumbar burst fracture

Clinical studies have extensively shown that burst fractures can cause severe and long-term neurological deficits. However, the mechanism of burst fracture is not clear, and the influence of different spinal postures on burst fracture is still unidentified. The study aimed at investigating the influence of different postures under vertical impact load on thoracolumbar burst fracture. A detailed nonlinear finite element model of T12-L2 segment was developed to investigate these problems. In this work, a rigid ball was used to vertically impact the finite element spinal segment, which emulated the process of burst fracture as in experimental condition. During the process, amounting to 9 different postures (normal, flexion, extension, right/left lateral bending of 8°, right/left axial rotation of 4° and 8°) were studied. Totally five failure modes were observed. Six different parameters, including vertebral height, vertebral bulge, interpedicular widening, vertebral kyphotic angle, posterior vertebral body angle, and joint facet contact force, were observed to evaluate the corresponding severity of burst fracture. Burst fracture in extension was the severest, and the loss of vertebral height in flexion was the most. The different postures in these simulations changed the morphology of intervertebral disc and facet joints force, resulting in different types of fracture.

The effect of non-fusion dynamic stabilization on biomechanical responses of the implanted lumbar spine during whole-body vibration

BACKGROUND AND OBJECTIVE

Non-fusion dynamic stabilization surgery is increasingly popular for treating degenerative lumbar disc disease. However, changes in spine biomechanics after application of posterior dynamic fixation devices during whole-body vibration (WBV) remain unclear. The study aimed to examine the effects of non-fusion dynamic stabilization on biomechanical responses of the implanted lumbar spine to vertical WBV.

METHODS

By modifying L4-L5 segment of the healthy human L1-sacrum finite element model, single-level disc degeneration, dynamic fixation using the BioFlex system and anterior lumbar interbody fusion (ALIF) with rigid fixation were simulated, respectively. Dynamic responses of stress and strain in the spinal levels for the healthy, degenerated, BioFlex and ALIF models under an axial cyclic loading were investigated and compared.

RESULTS

The results showed that endplate stress at implant level was lower in the BioFlex model than in the degenerated and ALIF models, but stress of the connecting rod in the BioFlex system was greater than that in the rigid fixation system used in the ALIF. Compared with the healthy model, stress and strain responses in terms of disc bulge, annulus stress and nucleus pressure at adjacent levels were decreased in the degenerated, BioFlex and ALIF models, but no obvious difference was observed in these responses among the three models.

CONCLUSIONS

This study may be helpful to understand variations in vibration characteristics of the lumbar spine after application of non-fusion dynamic stabilization system.