Mechanical response of a lumbar motion segment in axial torque alone and combined with compression

Biomechanical Analysis of Porous Additive Manufactured Cages for Lateral Lumbar Interbody Fusion: A Finite Element Analysis

BACKGROUND

A porous additive manufactured (AM) cage may provide stability similar to that of traditional solid cages and may be beneficial to bone ingrowth. The biomechanical influence of various porous cages on stability, subsidence, stresses in cage, and facet contact force has not been fully described. The purpose of this study was to verify biomechanical effects of porous AM cages.

METHODS

The surgical finite element models with various cages were constructed. The partially porous titanium (PPT) cages and fully porous titanium (FPT) cages were applied. The mechanical parameters of porous materials were obtained by mechanical test. Then the porous AM cages were compared with solid titanium (TI) cage and solid polyetheretherketone (PEEK) cage. The 4 motion modes were simulated. Range of motion (ROM), cage stress, end plate stress, and facet joint force (FJF) were compared.

RESULTS

For all the surgical models, ROM decreased by >90%. Compared with TI and PPT cages, PEEK and FPT cages substantially reduced the maximum stresses in cage and end plate in all motion modes. Compared with PEEK cages, the stresses in cage and end plate for FPT cages decreased, whereas the ROM increased. Comparing FPT cages, the stresses in cage and end plate decreased with increasing porosity, whereas ROM increased with increasing porosity. After interbody fusion, FJF was substantially reduced in all motion modes except for flexion.

CONCLUSIONS

Fully porous cages may offer an alternative to solid PEEK cages in lateral lumbar interbody fusion. However, it may be prudent to further increase the porosity of the cage.

Normative kinematics of reaching and dexterity tasks: moving towards a quantitative baseline for Functional Capacity Evaluations (FCEs)

Purpose: This work generates a comprehensive description of upper extremity and torso kinematics of a healthy population during reaching and dexterity Functional Capacity Evaluation (FCE) tasks. Methods: Upper limb and torso kinematic data were collected from 30 young, healthy participants as they performed three common FCE tasks: repetitive reaching, fingertip dexterity, and hand and forearm dexterity. Kinematic profiles were created for all clinically relevant angles of the torso, shoulder, elbow, and wrist. Results: These provocative tasks require large ranges of motion and create high demand postures for the upper limb, specifically at the shoulder. Arm elevation was up to 90°, while humeral internal rotation of 25° was observed. Torso angles were typically below 30° from neutral and elbow flexion remained within 90°–120° for nearly all tasks. Wrist ulnar deviation ranged from 0° to 26° for both wrists. Conclusion: The normative data created in this investigation provide a description of healthy motion during reaching and dexterity tasks. These normative curves are the initial step towards understanding movement that would contraindicate return to work during an FCE. This work supports a future clinical goal of being able to identify persons at risk of further injury or disability if returned to work too early.

Biomechanics of anterior plating failure in treating distractive flexion injury in the caudal subaxial cervical spine

BACKGROUND

Operative level is a potential biomechanical risk factor for construct failure during anterior fixation for distractive flexion injury. No biomechanical study of this concept has been reported, although it is important in clinical management.

METHODS

To explore the mechanism of this concept, a previously validated three-dimensional C2-T1 finite element model was modified to simulate surgical procedure via the anterior approach for treating single-level distractive flexion injury, from C2-C3 to C7-T1. Four loading conditions were used including no-compression, follower load, axial load, and combined load. Construct stability at the operative level was assessed.

FINDINGS

Under these loading conditions with the head's weight simulated, segmental stability decreases when the operative level shifts cephalocaudally, especially at C6-C7 and C7-T1, the stress of screw-bone interface increases cephalocaudally, and in the same operative level, the caudal screws always carries more load than the cephalad ones. All these predicted results are consistent with failure patterns observed in clinical reports. In the contrast, under other loading conditions without the weight of head, no obvious segmental divergence was predicted.

INTERPRETATION

This study supports that the biomechanical mechanism of this phenomenon includes eccentric load from head weight during sagittal movements and difference of moment arms. Our study suggests that anterior fixation is not recommended for treating distractive flexion injury at the caudal segments of the subaxial cervical spine, especially at C6-C7 and C7-T1, because of the intrinsic instability in these segments. Combined posterior rigid fixation with anterior fixation should be considered for these segments.

Biomechanical analysis of lumbar decompression surgery in relation to degenerative changes in the lumbar spine - Validated finite element analysis

BACKGROUND

There are no studies about the biomechanical analysis of lumbar decompression surgery in relation to degenerative changes of the lumbar spine. Therefore, the purpose of this study was to compare, by using finite element (FE) analysis, the biomechanical changes of the lumbar spine in terms of annulus stress and nucleus pressure after two different kinds of lumbar decompression surgery in relation to disc degenerative changes.

METHODS

The validated intact and degenerated FE models (L2-5) were used in this study. In these two models, two different decompression surgical scenarios at L3-4, including conventional laminectomy (ConLa) and the spinous process osteotomy (SpinO), were simulated. Therefore, a total of six models were simulated. Under preloading, 7.5 Nm moments of flexion, extension, lateral bending, and torsion were imposed. In each model, the maximal von Mises stress on the annulus fibrosus and nucleus pressure at the index segment (L3-4) and adjacent segments (L2-3 and L4-5) were analyzed.

RESULTS

The ConLa model and disc degeneration model demonstrated a larger annulus stress at the decompression level (L3-4) under all four moments than were seen in the SpinO model and healthy disc model, respectively. Therefore, the ConLa model with moderate disc degeneration showed the highest annulus stress at the decompression level (L3-4). However, the percent change of annulus stress at L3-4 from the intact model to the matched decompression model was less in the moderate disc degeneration model than in the healthy disc model.

CONCLUSIONS

Although the ConLa model with moderate disc degeneration showed the highest annulus stress, the degenerative models would be less influenced by the decompression technique.

Improving the Process of Adjusting the Parameters of Finite Element Models of Healthy Human Intervertebral Discs by the Multi-Response Surface Method

The kinematic behavior of models that are based on the finite element method (FEM) for modeling the human body depends greatly on an accurate estimate of the parameters that define such models. This task is complex, and any small difference between the actual biomaterial model and the simulation model based on FEM can be amplified enormously in the presence of nonlinearities. The current paper attempts to demonstrate how a combination of the FEM and the MRS methods with desirability functions can be used to obtain the material parameters that are most appropriate for use in defining the behavior of Finite Element (FE) models of the healthy human lumbar intervertebral disc (IVD). The FE model parameters were adjusted on the basis of experimental data from selected standard tests (compression, flexion, extension, shear, lateral bending, and torsion) and were developed as follows: First, three-dimensional parameterized FE models were generated on the basis of the mentioned standard tests. Then, 11 parameters were selected to define the proposed parameterized FE models. For each of the standard tests, regression models were generated using MRS to model the six stiffness and nine bulges of the healthy IVD models that were created by changing the parameters of the FE models. The optimal combination of the 11 parameters was based on three different adjustment criteria. The latter, in turn, were based on the combination of stiffness and bulges that were obtained from the standard test FE simulations. The first adjustment criteria considered stiffness and bulges to be equally important in the adjustment of FE model parameters. The second adjustment criteria considered stiffness as most important, whereas the third considered the bulges to be most important. The proposed adjustment methods were applied to a medium-sized human IVD that corresponded to the L3–L4 lumbar level with standard dimensions of width = 50 mm, depth = 35 mm, and height = 10 mm. Agreement between the kinematic behavior that was obtained with the optimized parameters and that obtained from the literature demonstrated that the proposed method is a powerful tool with which to adjust healthy IVD FE models when there are many parameters, stiffnesses, and bulges to which the models must adjust.

Impact of spinal rod stiffness on porcine lumbar biomechanics: Finite element model validation and parametric study

A three-dimensional finite element model of the porcine lumbar spine (L1-L6) was used to assess the effect of spinal rod stiffness on lumbar biomechanics. The model was validated through a comparison with in vitro measurements performed on six porcine spine specimens. The validation metrics employed included intervertebral rotations and the nucleus pressure in the first instrumented intervertebral disc. The numerical results obtained suggest that rod stiffness values as low as 0.1 GPa are required to reduce the mobility gradient between the adjacent and instrumented segments and the nucleus pressures across the porcine lumbar spine significantly. Stiffness variations above this threshold value have no significant effect on spine biomechanics. For such low-stiffness rods, intervertebral rotations in the instrumented zone must be monitored closely in order to guarantee solid fusion. Looking ahead, the proposed model will serve to examine the transverse process hooks and variable stiffness rods in order to further smooth the transition between the adjacent and instrumented segments, while preserving the stability of the instrumented zone, which is needed for fusion.

Effectiveness of pedicle screw inclusion at the fracture level in short-segment fixation constructs for the treatment of thoracolumbar burst fractures: a computational biomechanics analysis

When treating thoracolumbar burst fractures (BF), short-segment posterior fixation (SSPF) represents a less invasive alternative to the traditional long-segment posterior fixation (LSPF) approach. However, hardware failure and loss of sagittal alignment have been reported in patients treated with SSPF. Including pedicle screws at the fracture level in SSPF constructs has been proposed to improve stiffness and reliability of the construct. Accordingly, the biomechanical performance of the proposed construct was compared to LSPF via a computational analysis. Pedicle screws at fracture level improved the performance of the short-segment construct. However, LSPF still represent a biomechanically superior option for treating thoracolumbar BF.

Effect of collagen fibre orientation on intervertebral disc torsion mechanics

The intervertebral disc is a complex fibro-cartilaginous material, consisting of a pressurized nucleus pulposus surrounded by the annulus fibrosus, which has an angle-ply structure. Disc injury and degeneration are noted by significant changes in tissue structure and function, which significantly alters stress distribution and disc joint stiffness. Differences in fibre orientation are thought to contribute to changes in disc torsion mechanics. Therefore, the objective of this study was to evaluate the effect of collagen fibre orientation on internal disc mechanics under compression combined with axial rotation. We developed and validated a finite element model (FEM) to delineate changes in disc mechanics due to fibre orientation from differences in material properties. FEM simulations were performed with fibres oriented at [Formula: see text] throughout the disc (uniform by region and fibre layer). The initial model was validated by published experimental results for two load conditions, including [Formula: see text] axial compression and [Formula: see text] axial rotation. Once validated, fibre orientation was rotated by [Formula: see text] or [Formula: see text] towards the horizontal plane, resulting in a decrease in disc joint torsional stiffness. Furthermore, we observed that axial rotation caused a sinusoidal change in disc height and radial bulge, which may be beneficial for nutrient transport. In conclusion, including anatomically relevant fibre angles in disc joint FEMs is important for understanding stress distribution throughout the disc and will be important for understanding potential causes for disc injury. Future models will include regional differences in fibre orientation to better represent the fibre architecture of the native disc.

Biomechanical advantages of robot-assisted pedicle screw fixation in posterior lumbar interbody fusion compared with freehand technique in a prospective randomized controlled trial-perspective for patient-specific finite element analysis

BACKGROUND CONTEXT

There have been conflicting results on the surgical outcome of lumbar fusion surgery using two different techniques: robot-assisted pedicle screw fixation and conventional freehand technique. In addition, there have been no studies about the biomechanical issues between both techniques.

PURPOSE

This study aimed to investigate the biomechanical properties in terms of stress at adjacent segments using robot-assisted pedicle screw insertion technique (robot-assisted, minimally invasive posterior lumbar interbody fusion, Rom-PLIF) and freehand technique (conventional, freehand, open approach, posterior lumbar interbody fusion, Cop-PLIF) for instrumented lumbar fusion surgery.

STUDY DESIGN

This is an additional post-hoc analysis for patient-specific finite element (FE) model.

PATIENT SAMPLE

The sample is composed of patients with degenerative lumbar disease.

OUTCOME MEASURES

Intradiscal pressure and facet contact force are the outcome measures.

METHODS

Patients were randomly assigned to undergo an instrumented PLIF procedure using a Rom-PLIF (37 patients) or a Cop-PLIF (41), respectively. Five patients in each group were selected using a simple random sampling method after operation, and 10 preoperative and postoperative lumbar spines were modeled from preoperative high-resolution computed tomography of 10 patients using the same method for a validated lumbar spine model. Under four pure moments of 7.5 Nm, the changes in intradiscal pressure and facet joint contact force at the proximal adjacent segment following fusion surgery were analyzed and compared with preoperative states.

RESULTS

The representativeness of random samples was verified. Both groups showed significant increases in postoperative intradiscal pressure at the proximal adjacent segment under four moments, compared with the preoperative state. The Cop-PLIF models demonstrated significantly higher percent increments of intradiscal pressure at proximal adjacent segments under extension, lateral bending, and torsion moments than the Rom-PLIF models (p=.032, p=.008, and p=.016, respectively). Furthermore, the percent increment of facet contact force was significantly higher in the Cop-PLIF models under extension and torsion moments than in the Rom-PLIF models (p=.016 under both extension and torsion moments).

CONCLUSIONS

The present study showed the clinical application of subject-specific FE analysis in the spine. Even though there was biomechanical superiority of the robot-assisted insertions in terms of alleviation of stress increments at adjacent segments after fusion, cautious interpretation is needed because of the small sample size.

Biomechanical Effects of Human Lumbar Discography: In Vitro Experiments and Their Finite Element Validation

STUDY DESIGN

An experimental and computational finite element analysis of human lumbar spine discography and its resulting effects on disk biomechanics.

OBJECTIVE

To characterize the changes in stress and displacement of the human lumbar spine disks after puncture due to discography.

SUMMARY OF BACKGROUND DATA

Discography of the intervertebral disk (IVD) may be used to diagnose pathology of the disk and determine whether it may be a source for chronic back pain. It has recently been suggested that discography may lead to IVD degeneration, and has been a cause of controversy among spine care physicians.

MATERIALS AND METHODS

Both in vivo experiment using cadaveric specimens and a finite element model of the same L3-L5 lumbar spine was developed using computed tomography scans. Discography was simulated in the model as an area in the disk affected by needle puncture. The material properties in the nucleus pulposus were adjusted to match experimental data both before and after puncture.

RESULTS

Puncture of the IVD leads to increased deformation and increased stresses in the annulus fibrosis region of the disk. Pressure in the nucleus pulposus was found to decrease after puncture. Experimental and computational results correlated well.

CONCLUSIONS

Puncturing the IVD changes disk biomechanics and hence may lead to progressive spine degenerations in particular in the punctured disks.

Finite element method-based study for effect of adult degenerative scoliosis on the spinal vibration characteristics

Finite element analysis was used to investigate the responses of five healthy subjects and five adult degenerative scoliosis (ADS) subjects to cyclic vibration. The dynamic responses of the healthy and scoliotic spines to the sinusoidal cyclic vibrations have been investigated in previous studies by simulation or experimental approaches. However, no simulation or experimental results were available for the ADS subjects. The effect of the ADS on the vibrational characteristics of spines remained unknown. The objective of this study was to compare differences of the dynamic responses to the cyclic vibration input between the healthy subjects and subjects with ADS. Based on the simulations results in this study, the scoliotic spines are more sensitive to the cyclic vibrations than the healthy spines. More resonant frequencies were predicted in the scoliotic spines than the healthy spines. The scoliotic deformity in the spine was to make the vibrational response of the spine significantly more complex at the apical scoliotic region. This study suggested that ADS could severely increase spinal response to the cyclic vibrations, which could potentially lead to further scoliotic deformity in the spine.

Impact of anchor type on porcine lumbar biomechanics: Finite element modelling and in-vitro validation

BACKGROUND

Rigid posterior implants used for spinal stabilization can be anchored to the vertebrae using pedicle screws or screws combined with transverse process hooks. In the present study, a finite element model of a porcine lumbar spine instrumented with screws and hooks is presented and validated.

METHODS

The porcine lumbar spine model was validated using in-vitro measurements on six porcine specimens. Validation metrics included intervertebral rotations (L1 to L6) and nucleus pressure in the topmost cranial instrumented disc. The model was used to compare the biomechanical effect of anchor types.

FINDINGS

Good agreement was observed between the model and validation experiments. For upper transverse hooks construct, intervertebral rotations increased at the upper instrumented vertebra and decreased at the adjacent level. Additionally, nucleus pressures and stress on the annulus decreased in the adjacent disc and increased in the upper instrumented disc. The pull-out forces predicted for both anchor configurations were significantly lower than the pull-out strength found in the literature.

INTERPRETATION

These numerical observations suggest that upper transverse process hooks constructs reduce the mobility gradient and cause less stress in the adjacent disc, which could potentially reduce adjacent segment disease and proximal junction kyphosis incidence without increasing the risk of fixation failure. Future work needs to assess the long-term effect of such constructs on clinical and functional outcomes.

Effect of Graded Facetectomy on Lumbar Biomechanics

Facetectomy is an important intervention for spinal stenosis but may lead to spinal instability. Biomechanical knowledge for facetectomy can be beneficial when deciding whether fusion is necessary. Therefore, the aim of this study was to investigate the biomechanical effect of different grades of facetectomy. A three-dimensional nonlinear finite element model of L3–L5 was constructed. The mobility of the model and the intradiscal pressure (IDP) of L4-L5 for standing were inside the data from the literature. The effect of graded facetectomy on intervertebral rotation, IDP, facet joint forces, and maximum von Mises equivalent stresses in the annuli was analyzed under flexion, extension, left/right lateral bending, and left/right axial rotation. Compared with the intact model, under extension, unilateral facetectomy increased the range of intervertebral rotation (IVR) by 11.7% and IDP by 10.7%, while the bilateral facetectomy increased IVR by 40.7% and IDP by 23.6%. Under axial rotation, the unilateral facetectomy and the bilateral facetectomy increased the IVR by 101.3% and 354.3%, respectively, when turned to the right and by 1.1% and 265.3%, respectively, when turned to the left. The results conclude that, after unilateral and bilateral facetectomy, care must be taken when placing the spine into extension and axial rotation posture from the biomechanical point of view.

Biomechanics of the human intervertebral disc: A review of testing techniques and results

Many experimental testing techniques have been adopted in order to provide an understanding of the biomechanics of the human intervertebral disc (IVD). The aim of this review article is to amalgamate results from these studies to provide readers with an overview of the studies conducted and their contribution to our current understanding of the biomechanics and function of the IVD. The overview is presented in a way that should prove useful to experimentalists and computational modellers. Mechanical properties of whole IVDs can be assessed conveniently by testing 'motion segments' comprising two vertebrae and the intervening IVD and ligaments. Neural arches should be removed if load-sharing between them and the disc is of no interest, and specimens containing more than two vertebrae are required to study 'adjacent level' effects. Mechanisms of injury (including endplate fracture and disc herniation) have been studied by applying complex loading at physiologically-relevant loading rates, whereas mechanical evaluations of surgical prostheses require slower application of standardised loading protocols. Results can be strongly influenced by the testing environment, preconditioning, loading rate, specimen age and degeneration, and spinal level. Component tissues of the disc (anulus fibrosus, nucleus pulposus, and cartilage endplates) have been studied to determine their material properties, but only the anulus has been thoroughly evaluated. Animal discs can be used as a model of human discs where uniform non-degenerate specimens are required, although differences in scale, age, and anatomy can lead to problems in interpretation.

The effects of muscle weakness on degenerative spondylolisthesis: A finite element study

BACKGROUND

Whether muscle weakness is a cause, or result, of degenerative spondylolisthesis is not currently well understood. Little biomechanical evidence is available to offer an explanation for the mechanism behind exercise therapy. Therefore, the aim of this study is to investigate the effects of back muscle weakness on degenerative spondylolisthesis and to tease out the biomechanical mechanism of exercise therapy.

METHODS

A nonlinear 3-D finite element model of L3-L5 was constructed. Forces representing global back muscles and global abdominal muscles, follower loads and an upper body weight were applied. The force of the global back muscles was reduced to 75%, 50% and 25% to simulate different degrees of back muscle weakness. An additional boundary condition which represented the loads from other muscles after exercise therapy was set up to keep the spine in a neutral standing position. Shear forces, intradiscal pressure, facet joint forces and von Mises equivalent stresses in the annuli were calculated.

FINDINGS

The intervertebral rotations of L3-L4 and L4-L5 were within the range of in vitro experimental data. The calculated intradiscal pressure of L4-L5 for standing was 0.57MPa, which is similar to previous in vivo data. With the back muscles were reduced to 75%, 50% and 25% force, the shear force moved increasingly in a ventral direction. Due to the additional stabilizing force and moment provided by boundary conditions, the shear force varied less than 15%.

INTERPRETATION

Reducing the force of global back muscles might lead to, or aggravate, degenerative spondylolisthesis with forward slipping from biomechanical point of view. Exercise therapy may improve the spinal biomechanical environment. However, the intrinsic correlation between back muscle weakness and degenerative spondylolisthesis needs more clinical in vivo study and biomechanical analysis.

Finite Element Study of a Lumbar Intervertebral Disc Nucleus Replacement Device

Nucleus replacement technologies are a minimally invasive alternative to spinal fusion and total disc replacement that have the potential to reduce pain and restore motion for patients with degenerative disc disease. Finite element modeling can be used to determine the biomechanics associated with nucleus replacement technologies. The current study focuses on a new nucleus replacement device designed as a conforming silicone implant with an internal void. A validated finite element model of the human lumbar L3-L4 motion segment was developed and used to investigate the influence of the nucleus replacement device on spine biomechanics. In addition, the effect of device design changes on biomechanics was determined. A 3D, L3-L4 finite element model was constructed from medical imaging data. Models were created with the normal intact nucleus, the nucleus replacement device, and a solid silicone implant. Probabilistic analysis was performed on the normal model to provide quantitative validation metrics. Sensitivity analysis was performed on the silicone Shore A durometer of the device. Models were loaded under axial compression followed by flexion/extension, lateral bending, or axial rotation. Compressive displacement, endplate stresses, reaction moment, and annulus stresses were determined and compared between the different models. The novel nucleus replacement device resulted in similar compressive displacement, endplate stress, and annulus stress and slightly higher reaction moment compared with the normal nucleus. The solid implant resulted in decreased displacement, increased endplate stress, decreased annulus stress, and decreased reaction moment compared with the novel device. With increasing silicone durometer, compressive displacement decreased, endplate stress increased, reaction moment increased, and annulus stress decreased. Finite element analysis was used to show that the novel nucleus replacement device results in similar biomechanics compared with the normal intact nucleus.

Preserving Posterior Complex Can Prevent Adjacent Segment Disease following Posterior Lumbar Interbody Fusion Surgeries: A Finite Element Analysis

Objective

To investigate the biomechanical effects of the lumbar posterior complex on the adjacent segments after posterior lumbar interbody fusion (PLIF) surgeries.

Methods

A finite element model of the L1–S1 segment was modified to simulate PLIF with total laminectomy (PLIF-LAM) and PLIF with hemilaminectomy (PLIF-HEMI) procedures. The models were subjected to a 400N follower load with a 7.5-N.m moment of flexion, extension, torsion, and lateral bending. The range of motion (ROM), intradiscal pressure (IDP), and ligament force were compared.

Results

In Flexion, the ROM, IDP and ligament force of posterior longitudinal ligament, intertransverse ligament, and capsular ligament remarkably increased at the proximal adjacent segment in the PLIF-LAM model, and slightly increased in the PLIF-HEMI model. There was almost no difference for the ROM, IDP and ligament force at L5-S1 level between the two PLIF models although the ligament forces of ligamenta flava remarkably increased compared with the intact lumbar spine (INT) model. For the other loading conditions, these two models almost showed no difference in ROM, IDP and ligament force on the adjacent discs.

Conclusions

Preserved posterior complex acts as the posterior tension band during PLIF surgery and results in less ROM, IDP and ligament forces on the proximal adjacent segment in flexion. Preserving the posterior complex during decompression can be effective on preventing adjacent segment degeneration (ASD) following PLIF surgeries.

Sensitivity of lumbar spine response to follower load and flexion moment: finite element study

The follower load (FL) combined with moments is commonly used to approximate flexed/extended posture of the lumbar spine in absence of muscles in biomechanical studies. There is a lack of consensus as to what magnitudes simulate better the physiological conditions. Considering the in-vivo measured values of the intradiscal pressure (IDP), intervertebral rotations (IVRs) and the disc loads, sensitivity of these spinal responses to different FL and flexion moment magnitudes was investigated using a 3D nonlinear finite element (FE) model of ligamentous lumbosacral spine. Optimal magnitudes of FL and moment that minimize deviation of the model predictions from in-vivo data were determined. Results revealed that the spinal parameters i.e. the IVRs, disc moment, and the increase in disc force and moment from neutral to flexed posture were more sensitive to moment magnitude than FL magnitude in case of flexion. The disc force and IDP were more sensitive to the FL magnitude than moment magnitude. The optimal ranges of FL and flexion moment magnitudes were 900-1100 N and 9.9-11.2 Nm, respectively. The FL magnitude had reverse effect on the IDP and disc force. Thus, magnitude for FL or flexion that minimizes the deviation of all the spinal parameters together from the in-vivo data can vary. To obtain reasonable compromise between the IDP and disc force, our findings recommend that FL of low magnitude must be combined with flexion moment of high intensity and vice versa.

Mechanical assessment of the effects of metastatic lytic defect on the structural response of human thoracolumbar spine

To investigate the effects of a clinical lytic defect on the structural response of human thoracolumbar functional spinal unit. A novel CT-compatible mechanical test system was used to image the deformation of a T12-L1 motion segment and measure the change in strain response under compressive loads ranging from 50 to 750 N. A lytic lesion (LM) with cortex involvement (33% by volume) was introduced to the upper vertebral body and the CT experiments were repeated. Finite element models, established from the CT volumes, were used to investigate the defect's effects on the structural response and the state of principal and shear stresses within the affected and adjacent vertebrae. The lytic lesion resulted in severe loss of the vertebral structural competence, resulting in significant, non-linear, and asymmetric increase in the experimentally measured strains and computed stresses within both vertebrae (p < 0.01). At the cortex, the tensile strains were significantly increased, while compressive strains significantly decreased, (p < 0.05). Both the vertebral bone and cortex regions adjacent to the defect showed significant increase in computed compressive, tensile, and shear stresses (p < 0.01). Changes in stress and strain distribution within the affected and adjacent vertebral bone and the experimentally observed bulging and buckling of the vertebral cortices suggested that initiation of catastrophic vertebral failure may occur under load magnitudes encountered in daily living. Although the effect of LM on the global deformation of the spine was well-predicted, our results show that FE predictions of local strain changes must be carefully assessed for clinical relevance. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1808-1819, 2016.

Motion Guides for Assisted Manipulation

This paper explores the use of passive guides to assist a human in manipulating heavy loads. A guide acts as a frictionless rail which confines the load to a one-dimensional curve in its configuration space. In this paper we formulate the problem of designing guides to effectively assist the human, and we apply the formulation to two types of materials handling tasks: pushing a heavy cart and single-arm reaching motions. Guides may be implemented by fixed rails or programmable constraint machines. Initial experiments with humans suggest the potential benefits of guided manipulation.

Biomechanical response of lumbar facet joints under follower preload: a finite element study

Background

Facet joints play a significant role in providing stability to the spine and they have been associated with low back pain symptoms and other spinal disorders. The influence of a follower load on biomechanics of facet joints is unknown. A comprehensive research on the biomechanical role of facets may provide insight into facet joint instability and degeneration.

Method

A nonlinear finite element (FE) model of lumbar spine (L1-S1) was developed and validated to study the biomechanical response of facets, with different values of follower preload (0 N,500 N,800 N,1200 N), under loadings in the three anatomic planes. In this model, special attention was paid to the modeling of facet joints, including cartilage layer. The asymmetry in the biomechanical response of facets was also discussed. A rate of change (ROC) and an average asymmetry factor (AAF) were introduced to explore and evaluate the preload effect on these facet contact parameters and on the asymmetry under different loading conditions.

Results

The biomechanical response of facets changed according to the loading condition. The preload amplified the facet force, contact area and contact pressure in flexion-extension; the same effect was observed on the ipsilateral facet while an opposite effect could be seen on the contralateral facet during lateral bending. For torsion loading, the preload increased contact area, decreased the mean contact pressure, but had almost no effect on facet force. However, all the effects of follower load on facet response became weaker with the increase of preload. The greatest asymmetry of facet response could be found on the ipsilateral side during lateral bending, followed by flexion, bending (contralateral side), extension and torsion. This asymmetry could be amplified by preload in the bending (ipsilateral), torsion loading group, while being reduced in the flexion group.

Conclusions

An analysis combining patterns of contact pressure distribution, facet load, contact area and contact pressure can provide more insight into the biomechanical role of facets under various moment loadings and follower loads. The effect of asymmetry on facet joint response should be fully considered in biomechanical studies of lumbar spine, especially in post structures subjected to physiological loadings.

Electronic supplementary material

The online version of this article (doi:10.1186/s12891-016-0980-4) contains supplementary material, which is available to authorized users.

Biomechanical effects of fusion levels on the risk of proximal junctional failure and kyphosis in lumbar spinal fusion surgery

BACKGROUND

Spinal fusion surgery is a widely used surgical procedure for sagittal realignment. Clinical studies have reported that spinal fusion may cause proximal junctional kyphosis and failure with disc failure, vertebral fracture, and/or failure at the implant-bone interface. However, the biomechanical injury mechanisms of proximal junctional kyphosis and failure remain unclear.

METHODS

A finite element model of the thoracolumbar spine was used. Nine fusion models with pedicle screw systems implanted at the L2-L3, L3-L4, L4-L5, L5-S1, L2-L4, L3-L5, L4-S1, L2-L5, and L3-S1 levels were developed based on the respective surgical protocols. The developed models simulated flexion-extension using hybrid testing protocol.

FINDINGS

When spinal fusion was performed at more distal levels, particularly at the L5-S1 level, the following biomechanical properties increased during flexion-extension: range of motion, stress on the annulus fibrosus fibers and vertebra at the adjacent motion segment, and the magnitude of axial forces on the pedicle screw at the uppermost instrumented vertebra.

INTERPRETATIONS

The results of this study demonstrate that more distal fusion levels, particularly in spinal fusion including the L5-S1 level, lead to greater increases in the risk of proximal junctional kyphosis and failure, as evidenced by larger ranges of motion, higher stresses on fibers of the annulus fibrosus and vertebra at the adjacent segment, and higher axial forces on the screw at the uppermost instrumented vertebra in flexion-extension. Therefore, fusion levels should be carefully selected to avoid proximal junctional kyphosis and failure.

On the load-sharing along the ligamentous lumbosacral spine in flexed and extended postures: Finite element study

A harmonic synergy between the load-bearing and stabilizing components of the spine is necessary to maintain its normal function. This study aimed to investigate the load-sharing along the ligamentous lumbosacral spine under sagittal loading. A 3D nonlinear detailed Finite Element (FE) model of lumbosacral spine with realistic geometry was developed and validated using wide range of numerical and experimental (in-vivo and in-vitro) data. The model was subjected to 500 N compressive Follower Load (FL) combined with 7.5 Nm flexion (FLX) or extension (EXT) moments. Load-sharing was expressed as percentage of total internal force/moment developed along the spine that each spinal component carried. These internal forces and moments were determined at the discs centres and included the applied load and the resisting forces in the ligaments and facet joints. The contribution of the facet joints and ligaments in supporting bending moments produced additional forces and moments in the discs. The intervertebral discs carried up to 81% and 68% of the total internal force in case of FL combined with FLX and EXT, respectively. The ligaments withstood up to 67% and 81% of the total internal moment in cases of FL combined with EXT and FLX, respectively. Contribution of the facet joints in resisting internal force and moment was noticeable at levels L4-S1 only particularly in case of FL combined with EXT and reached up 29% and 52% of the internal moment and force, respectively. This study demonstrated that spinal load-sharing depended on applied load and varied along the spine.

Injury mechanisms of the ligamentous cervical C2-C3 Functional Spinal Unit to complex loading modes: Finite Element study

The cervical spine sustains high rate complex loading modes during Motor Vehicle Crashes (MVCs) which may produce severe injuries accompanied with soft and/or hard tissue failure. Although previous numerical and experimental studies have provided insights on the cervical spine behavior under various loading scenarios, its response to complex impact loads and the resulting injury mechanisms are not fully understood. A validated Finite Element (FE) model of the ligamentous cervical C2-C3 Functional Spinal Unit (FSU) was utilized to assess the spinal response to six combined impact loading modes; flexion-extension combined with compression and distraction, and lateral bending and axial rotation combined with distraction. The FE model used time and rate-dependent material laws which permit assessing bone fracture and ligament failure. Spinal load-sharing, stresses in the spinal components, intradiscal pressure (IDP) change in the nucleus as well as contact pressure in the facet joints were predicted. Bone and ligaments failure occurrence and initiation instants were investigated. Results showed that spinal load-sharing varied with loading modes. Lateral bending combined with distraction was the most critical loading mode as it increased stresses and strains significantly and produced failure in most of the spinal components compared to other modes. The facet joints and surrounding cancellous bone as well as ligaments particularly the capsular (CL) and flavum (FL) ligaments were the most vulnerable structures to rapid flexion-extension, axial rotation and lateral bending combined with distraction or compression. The excessive stress and strain resulted from these loading modes produced rupture of the CL and FL ligaments and failure in the cancellous bone. The detection of failure initiation as well as fracture assessment demonstrated the vulnerability of ligaments to tensile combined loads and the major contribution of the bony structures in resisting compressive combined loads. Findings of this study may potentially assist in the development of injury prevention and treatment strategies.

Lumbar spinal loading during bowling in cricket: a kinetic analysis using a musculoskeletal modelling approach

The objective of the study was to evaluate two types of cricket bowling techniques by comparing the lumbar spinal loading using a musculoskeletal modelling approach. Three-dimensional kinematic data were recorded by a Vicon motion capture system under two cricket bowling conditions: (1) participants bowled at their absolute maximal speeds (max condition), and (2) participants bowled at their absolute maximal speeds while simultaneously forcing their navel down towards their thighs starting just prior to ball release (max-trunk condition). A three-dimensional musculoskeletal model comprised of the pelvis, sacrum, lumbar vertebrae and torso segments, which enabled the motion of the individual lumbar vertebrae in the sagittal, frontal and coronal planes to be actuated by 210 muscle-tendon units, was used to simulate spinal loading based on the recorded kinematic data. The maximal lumbar spine compressive force is 4.89 ± 0.88BW for the max condition and 4.58 ± 0.54BW for the max-trunk condition. Results showed that there was no significant difference between the two techniques in trunk moments and lumbar spine forces. This indicates that the max-trunk technique may not increase lower back injury risks. The method proposed in this study could be served as a tool to evaluate lower back injury risks for cricket bowling as well as other throwing activities.

The influence of facet joint orientation and tropism on the stress at the adjacent segment after lumbar fusion surgery: a biomechanical analysis

BACKGROUND CONTEXT

Facet joint orientation and tropism influence the biomechanics of the corresponding segment. Therefore, the sagittal orientation or tropism of the facet joint adjacent to the fusion segment seems a potential risk factor for adjacent segment degeneration. However, there have been no biomechanical studies regarding this issue.

PURPOSE

To investigate the association between adjacent facet orientation and facet tropism and stress in adjacent disc/facet joints using finite element (FE) analysis.

STUDY DESIGN

An FE analysis.

METHODS

Four intact (F50, F55, F60, and FT [facet tropism]) and matched L3-L4 fusion (F50, F55, F60, and FT fusion) models with different facet joint orientation (50°, 55°, 60° relative to the coronal plane, and facet tropism, respectively) at both L2-L3 facet joints were simulated. In each model, intradiscal pressures and facet contact force at the L2-L3 segment were investigated under pure moments and anterior shear force.

RESULTS

Compared with the matched-intact model, the F60 fusion model yielded the highest and largest percentage increase of intradiscal pressure at the L2-L3 segment under flexion, torsion moment, and anterior shear force among the F50, F55, and F60 fusion models. F60 fusion model also demonstrated the largest facet contact force under torsion moment among the F50, F55, and F60 fusion models. In all conditions tested, the FT fusion model demonstrated the highest intradiscal pressure and facet contact force of all the models.

CONCLUSIONS

Facet joint orientation and tropism at the adjacent segment influences the overstress of the adjacent segment, especially under the clinical circumstance of increased anterior shear force.

THE BIOMECHANICAL EFFECTS OF CEMENT AUGMENTATION AND PARTIAL VERTEBRAL HEIGHT RESTORATION ON THE LOAD TRANSFER CHANGE OF ADJACENT VERTEBRAE IN VERTEBROPLASTY

Vertebroplasty is commonly used to treat vertebral wedge fractures (VWFs). However, differing degrees of vertebral height restoration (VHR) have been reported after vertebroplasty, and little is known about how grades (steepness) of VWF deformities affect loadings on the fractured and adjacent unfractured vertebrae. Therefore, the goal of this study was to create a non-linear finite element (FE) model of the T10–L2 thoracolumbar segments. With this model, we aimed to evaluate the biomechanical outcomes of three different collapse models (25%, 50%, and 75%) at the T12 vertebra before and after cement augmentation (CA) and with and without VHR. In these VWF simulations, the forces of the erector spinae, the intradiscal pressure, and the maximum von Mises stresses in the endplates and vertebral bodies increased as vertebral deformation increased. Performing CA alone, without restoring vertebral height for the fractured vertebra, did not change the stiffness of multiple spinal segments or the pressures on the adjacent disc, but it did decrease stresses on the endplates and the vertebral bone. A 10% restoration of vertebral height after CA reduced the maximum von Mises stress in the endplates and bone structures more than when CA did not restore vertebral height (no VHR). These results suggest that achieving partial VHR during vertebroplasty may help prevent postvertebroplasty fractures in the fractured and adjacent vertebrae.

Finite element analysis for comparison of spinous process osteotomies technique with conventional laminectomy as lumbar decompression procedure

PURPOSE

The purpose of this study was to evaluate and compare the biomechanical behavior of the lumbar spine after posterior decompression with the spinous process osteotomy (SPiO) technique or the conventional laminectomy (CL) technique using a finite element (FE) model.

MATERIALS AND METHODS

Three validated lumbar FE models (L2-5) which represented intact spine and two decompression models using SPiO and CL techniques at the L3-4 segment were developed. In each model, the ranges of motion, the maximal von Mises stress of the annulus fibrosus, and the intradiscal pressures at the index segment (L3-4) and adjacent segments (L2-3 and L4-5) under 7.5 Nm moments were analyzed. Facet contact forces were also compared among three models under the extension and torsion moments.

RESULTS

Compared to the intact model, the CL and SPiO models had increased range of motion and annulus stress at both the index segment (L3-4) and the adjacent segments under flexion and torsion. However, the SPiO model demonstrated a reduced range of motion and annulus stress than the CL model. Both CL and SPiO models had an increase of facet contact force at the L3-4 segment under the torsion moment compared to that of the intact model. Under the extension moment, however, three models demonstrated a similar facet contact force even at the L3-4 model.

CONCLUSION

Both decompression methods lead to postoperative segmental instability compared to the intact model. However, SPiO technique leads to better segmental stability compared to the CL technique.

Biomechanical investigation of thoracolumbar spine in different postures during ejection using a combined finite element and multi-body approach

The aim of this study is to investigate the dynamic response of a multi-segment model of the thoracolumbar spine and determine how the sitting posture affects the response under the impact of ejection. A nonlinear finite element model of the thoracolumbar-pelvis complex (T9-S1) was developed and validated. A multi-body dynamic model of a pilot was also constructed so an ejection seat restraint system could be incorporated into the finite element model. The distribution of trunk mass on each vertebra was also considered in the model. Dynamics analysis showed that ejection impact induced obvious axial compression and anterior flexion of the spine, which may contribute to spinal injuries. Compared with a normal posture, the relaxed posture led to an increase in stress on the cortical wall, endplate, and intradiscal pressure of 43%, 10%, 13%, respectively, and accordingly increased the risk of inducing spinal injuries.

Biomechanical comparison of mono-segment transpedicular fixation with short-segment fixation for treatment of thoracolumbar fractures: A finite element analysis

Mono-segment transpedicular fixation is a method for the treatment of certain types of thoracolumbar spinal fracture. Finite element models were constructed to evaluate the biomechanics of mono-segment transpedicular fixation of thoracolumbar fracture. Spinal motion (T10–L2) was scanned and used to establish the models. The superior half of the cortical bone of T12 was removed and the superior half of the cancellous bone of the T12 body was assigned the material properties of injured bone to mimic vertebral fracture. Transpedicular fixation of T11 and T12 was performed to produce a mono-segment fixation model; T11 and L1 were fixed to produce a short-segment fixation model. Motion differences between functional units and von Mises stress on the spine and implants were measured under axial compression, anterior bending, extensional bending, lateral bending and axial rotation. We found no significant difference between mono- and short-segment fixations in the motion of any functional unit. Stress on the T10/T11 nucleus pulposus and T10/T11 and L1/L2 annulus fibrosus increased significantly by about 75% on anterior bending, extensional bending and lateral bending. In the fracture model, stress was increased by 24% at the inferior endplate of T10 and by 43% at the superior endplate of L2. All increased stresses were reduced after fixation and lower stress was observed with mono-segment fixation. In summary, the biomechanics of mono-segment pedicle screw instrumentation was similar to that of conventional short-segment fixation. As a minimally invasive treatment, mono-segment fixation would be appropriate for the treatment of selected thoracolumbar spinal fractures.

Biomechanical analysis of fusion segment rigidity upon stress at both the fusion and adjacent segments: a comparison between unilateral and bilateral pedicle screw fixation

PURPOSE

The purpose of this study was to investigate the effects of unilateral pedicle screw fixation on the fusion segment and the superior adjacent segment after one segment lumbar fusion using validated finite element models.

MATERIALS AND METHODS

Four L3-4 fusion models were simulated according to the extent of decompression and the method of pedicle screws fixation in L3-4 lumbar fusion. These models included hemi-laminectomy with bilateral pedicle screw fixation in the L3-4 segment (BF-HL model), total laminectomy with bilateral pedicle screw fixation (BF-TL model), hemi-laminectomy with unilateral pedicle screw fixation (UF-HL model), and total laminectomy with unilateral pedicle screw fixation (UF-TL model). In each scenario, intradiscal pressures, annulus stress, and range of motion at the L2-3 and L3-4 segments were analyzed under flexion, extension, lateral bending, and torsional moments.

RESULTS

Under four pure moments, the unilateral fixation leads to a reduction in increment of range of motion at the adjacent segment, but larger motions were noted at the fusion segment (L3-4) in the unilateral fixation (UF-HL and UF-TL) models when compared to bilateral fixation. The maximal von Mises stress showed similar patterns to range of motion at both superior adjacent L2-3 segments and fusion segment.

CONCLUSION

The current study suggests that unilateral pedicle screw fixation seems to be unable to afford sufficient biomechanical stability in case of bilateral total laminectomy. Conversely, in the case of hemi-laminectomy, unilateral fixation could be an alternative option, which also has potential benefit to reduce the stress of the adjacent segment.

Biomechanical comparison of transforaminal lumbar interbody fusion with 1 or 2 cages by finite-element analysis

BACKGROUND

Anterior lumbar interbody fusion and posterior lumbar interbody fusion with 1 cage have been shown to have similar biomechanics compared with the use of 2 cages. However, there have been no reports on the biomechanical differences between using 1 or 2 cages in transforaminal lumbar interbody fusion (TLIF) surgery.

OBJECTIVE

To determine the biomechanical differences between the use of 1 or 2 cages in TLIF by finite-element analysis.

METHODS

Three validated finite-element models of the L3-L5 lumbar segment were created (intact model and single- and paired-cage TLIF models). To study the biomechanics, a compressive preload of 400 N over 7.5 N-m was applied to the superior surfaces of the L3 vertebral body to simulate flexion, extension, rotation, and lateral bending.

RESULTS

There was no significant difference in the range of motion between single-cage and paired-cage TLIF models, < 1° for all loading cases. Cage stress was high in the single-cage TLIF model under all loading conditions. Bone graft stress was high in the single-cage TLIF model. Pedicle screw stress was higher in the single-cage compared with the paired-cage TLIF.

CONCLUSION

Single-cage TLIF approximates biomechanical stability and increases the stress of the bone graft. The use of a single cage may simplify the standard TLIF procedure, shorten operative times, decrease cost, and provide satisfactory clinical outcomes. Thus, single-cage TLIF is a useful alternative to traditional 2-cage TLIF.

Biomechanical effects of vertebroplasty on thoracolumbar burst fracture with transpedicular fixation: a finite element model analysis

OBJECTIVE

To investigate the biomechanical effects of augmentation of the fractured vertebrae after posterior instrumentation.

METHODS

By simulating internal fixation plus augmentation with cement, eight tridimensional, anatomically detailed finite element models of the T11-L1 functional spinal junction were developed. Two kinds of models for mimicking different severity of the fracture were established according to the Denis' classification. Augmentation with cement was conducted after reduction with posterior fixation using a universal spine system. These models assumed a three-column loading configuration as follows: compression, anteflexion, extension, lateroflexion and axial rotation. Stress of the implants and spine was evaluated.

RESULTS

Data showed that for severely fractured models, augmentation apparently decreased the von Mises stresses by 50% for the rods and 40% for the screws, about 40% for the inferior endplate of T11, and 50% for the superior endplate of L1 in vertical compression and other load situations.

CONCLUSION

We should only apply vertebroplasty to prevent correction loss and implants failure based on the fact that it could significantly decrease stress of the instrumentations and spine when the vertebrae are severely fractured.

LEVEL OF EVIDENCE

Level IV, biomechanical study.

Biomechanical analysis of two-step traction therapy in the lumbar spine

Traction therapy is one of the most common conservative treatments for low back pain. However, the effects of traction therapy on lumbar spine biomechanics are not well known. We investigated biomechanical effects of two-step traction therapy, which consists of global axial traction and local decompression, on the lumbar spine using a validated three-dimensional finite element model of the lumbar spine. One-third of body weight was applied on the center of the L1 vertebra toward the superior direction for the first axial traction. Anterior translation of the L4 vertebra was considered as the second local decompression. The lordosis angle between the superior planes of the L1 vertebra and sacrum was 44.6° at baseline, 35.2° with global axial traction, and 46.4° with local decompression. The fibers of annulus fibrosus in the posterior region, and intertransverse and posterior longitudinal ligaments experienced stress primarily during global axial traction, these stresses decreased during local decompression. A combination of global axial traction and local decompression would be helpful for reducing tensile stress on the fibers of the annulus fibrosus and ligaments, and intradiscal pressure in traction therapy. This study could be used to develop a safer and more effective type of traction therapy.

Finite element modeling of soft tissues: material models, tissue interaction and challenges

BACKGROUND

Musculoskeletal soft tissues, such as articular cartilage, ligaments, knee meniscus and intervertebral disk, have a complex structure, which provides elasticity and capability to support and distribute the body loads. Soft tissues describe an inhomogeneous and multiphasic structure, and exhibit a nonlinear, time-dependent behavior. Their mechanical response is governed by a substance composed of protein fiber-rich and proteoglycan-rich extracellular matrix and interstitial fluid. Protein fibers (e.g. collagen) give the tissue direction dependent stiffness and strength. To investigate these complex biological systems, the use of mathematical tools is well established, alone or in combination with experimental in vitro and in vivo tests. However, the development of these models poses many challenges due to the complex structure and mechanical response of soft tissues.

METHODS

Non-systematic literature review.

FINDINGS

This paper provides a summary of different modeling strategies with associated material properties, contact interactions between articulating tissues, validation and sensitivity of soft tissues with special focus on knee joint soft tissues and intervertebral disk. Furthermore, it reviews and discusses some salient clinical findings of reported finite element simulations.

INTERPRETATION

Model studies extensively contributed to the understanding of functional biomechanics of soft tissues. Models can be effectively used to elucidate clinically relevant questions. However, users should be aware of the complexity of such tissues and of the capabilities and limitations of these approaches to adequately simulate a specific in vivo or in vitro phenomenon.

The biomechanical influence of the facet joint orientation and the facet tropism in the lumbar spine

BACKGROUND CONTEXT

Facet joint orientation and facet tropism (FT) are presented as the potential anatomical predisposing factors for lumbar degenerative changes that may lead in turn to early degeneration and herniation of the corresponding disc or degenerative spondylolisthesis. However, no biomechanical study of this concept has been reported.

PURPOSE

To investigate the biomechanical influence of the facet orientation and FT on stress on the corresponding segment.

STUDY DESIGN

Finite element analysis.

METHODS

Three models, F50, F55, and F60 were simulated with different facet joint orientations (50°, 55°, and 60° relative to coronal plane) at both L2-L3 facet joints. A FT model was also simulated to represent a 50° facet joint angle at the right side and a 60° facet joint angle at the left side in the L2-L3 segment. In each model, the intradiscal pressures were investigated under four pure moments and anterior shear force. Facet contact forces at the L2-L3 segment were also analyzed under extension and torsion moments and anterior shear force. This study was supported by 5000 CHF grant of 2011 AO Spine Research Korea fund. The authors of this study have no topic-specific potential conflicts of interest related to this study.

RESULTS

The F50, F55, and F60 models did not differ in the intradiscal pressures generated under four pure moments: but under anterior shear force, the F60 and FT models showed increases of intradiscal pressure. The F50 model under extension and the F60 model under torsion each generated an increase in facet contact force. In all conditions tested, the FT model yielded the greatest increase of intradiscal pressure and facet contact force of all the models.

CONCLUSIONS

The facet orientation per se did not increase disc stress or facet joint stress prominently at the corresponding level under four pure moments, but FT could make the corresponding segment more vulnerable to external moments or anterior shear force.

Biomechanical comparison of posterior lumbar interbody fusion and transforaminal lumbar interbody fusion by finite element analysis

BACKGROUND

The transforaminal lumbar interbody fusion (TLIF) procedure may reduce many of the risks and limitations associated with posterior lumbar interbody fusion (PLIF). However, little is known about the biomechanical difference between PLIF and TLIF.

OBJECTIVE

To determine the biomechanical difference between PLIF and TLIF by finite-element analysis.

METHODS

Three validated finite-element models of L3-5 lumbar segment were created (intact model, PLIF model, and TLIF model). To analyze the biomechanics of these models, flexion, extension, rotation, and lateral bending moments of 7.5 N-m with a compressive preload of 400 N were imposed on the superior surfaces of the L3 vertebral body.

RESULTS

The range of motion at the L4-5 level of the PLIF and TLIF models decreased for all loading cases, compared with the intact model. Differences in the range of motion between PLIF and TLIF were not significant at less than 1 degree for all loading cases. The stress of the cage was found to be high in the PLIF model at the cage-endplate interface under all loading conditions. The stress exerted on the pedicle screw was greater in TLIF than PLIF. Particularly in flexion loading, the stress experienced by the pedicle screw in the TLIF model was 70.7% greater than that in the PLIF model.

CONCLUSION

The TLIF procedure increases the approximate biomechanical stability and reduces stress at the cage-endplate interface, except for a slight increase in screw stress. Clinically, the TLIF procedure may reduce many of the risks and limitations associated with PLIF and offer a useful alternative to the PLIF procedure.

What have we learned from finite element model studies of lumbar intervertebral discs in the past four decades?

Finite element analysis is a powerful tool routinely used to study complex biological systems. For the last four decades, the lumbar intervertebral disc has been the focus of many such investigations. To understand the disc functional biomechanics, a precise knowledge of the disc mechanical, structural and biochemical environments at the microscopic and macroscopic levels is essential. In response to this need, finite element model studies have proven themselves as reliable and robust tools when combined with in vitro and in vivo measurements. This paper aims to review and discuss some salient findings of reported finite element simulations of lumbar intervertebral discs with special focus on their relevance and implications in disc functional biomechanics. Towards this goal, the earlier investigations are presented, discussed and summarized separately in three distinct groups of elastic, multi-phasic transient and transport model studies. The disc overall response as well as the relative role of its constituents are markedly influenced by loading rate, magnitude, combinations/preloads and posture. The nucleus fluid content and pressurizing capacity affect the disc compliance, annulus strains and failure sites/modes. Biodynamics of the disc is affected by not only the excitation characteristics but also preloads, existing mass and nucleus condition. The role of fluid pressurization and collagen fiber stiffening diminish with time during diurnal loading. The endplates permeability influences the time-dependent response of the disc in both loaded and unloaded recovery phases. The transport of solutes is substantially influenced by the disc size, tissue diffusivity and endplates permeability.