Effects of slip severity and loading directions on the stability of isthmic spondylolisthesis: a finite element model study

Biomechanical response of decompression alone in lower grade lumbar degenerative spondylolisthesis--A finite element analysis

BACKGROUND

Previous studies have demonstrated the clinical efficacy of decompression alone in lower-grade spondylolisthesis. A higher rate of surgical revision and a lower rate of back pain relief was also observed. However, there is a lack of relevant biomechanical evidence after decompression alone for lower-grade spondylolisthesis.

PURPOSE

Evaluating the biomechanical characteristics of total laminectomy, hemilaminectomy, and facetectomy for lower-grade spondylolisthesis by analyzing the range of motion (ROM), intradiscal pressure (IDP), annulus fibrosus stress (AFS), facet joints contact force (FJCF), and isthmus stress (IS).

METHODS

Firstly, we utilized finite element tools to develop a normal lumbar model and subsequently constructed a spondylolisthesis model based on the normal model. We then performed total laminectomy, hemilaminectomy, and one-third facetectomy in the normal model and spondylolisthesis model, respectively. Finally, we analyzed parameters, such as ROM, IDP, AFS, FJCF, and IS, for all the models under the same concentrate force and moment.

RESULTS

The intact spondylolisthesis model showed a significant increase in the relative parameters, including ROM, AFS, FJCF, and IS, compared to the intact normal lumbar model. Hemilaminectomy and one-third facetectomy in both spondylolisthesis and normal lumbar models did not result in an obvious change in ROM, IDP, AFS, FJCF, and IS compared to the pre-operative state. Moreover, there was no significant difference in the degree of parameter changes between the spondylolisthesis and normal lumbar models after undergoing the same surgical procedures. However, total laminectomy significantly increased ROM, AFS, and IS and decreased the FJCF in both normal lumbar models and spondylolisthesis models.

CONCLUSION

Hemilaminectomy and one-third facetectomy did not have a significant impact on the segment stability of lower-grade spondylolisthesis; however, patients with LDS undergoing hemilaminectomy and one-third facetectomy may experience higher isthmus stress on the surgical side during rotation. In addition, total laminectomy changes the biomechanics in both normal lumbar models and spondylolisthesis models.

A patient specific computational biomechanical model for the entire lumbosacral spinal unit with imposed spondylolysis

BACKGROUND

A biomechanical model of the lumbosacral spinal unit between L1-S1 was developed to investigate the behavior of normal and select pathological states. Our aims were to generate predictive structural models for mechanical deformation including critical stresses in the spine components and to investigate the probability of subsequent lumbar spine fractures in the presence of unilateral spondylolysis.

METHODS

A non-linear three-dimensional finite element pathology-free model of the L1-S1 lumbosacral unit was generated using patient-specific computerized tomography scans and calibrated by comparing it to experimental data of a range of motion modes consisting of flexion, extension, left and right lateral bending, and left and right axial rotation. Unilateral and bilateral pars defects were created on the isthmus of L5 to simulate spondylolysis.

FINDINGS

Results showed that under flexion, left lateral bending and right axial rotation, stresses were higher on the contralateral L5 pars-interarticularis, whereas, no significant changes occurred on the left-right isthmus of the L2-L4 and S1. Significant changes in the range of motion compared to the pathology-free model were observed in bilateral spondylolysis not only adjacent to the pars defect area but also in other lumbar spine levels.

INTERPRETATION

The proposed pathology-free lumbosacral unit model showed good correlation with experimental tests for all loading cases. In unilateral spondylolysis, a subsequent pars defect was observed within the same vertebra. The overall modeling approach can be used to study different pathological states.

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.

Biomechanics of high-grade spondylolisthesis with and without reduction

The clinical advantages of reducing spondylolisthesis over fusion in situ have several intuitive reasons such as restore the spinal column into a more anatomic relationship and alignment. However, there is only little evidence in the literature supporting the theoretical advantages of reduction, and its effect on spinopelvic alignment remains poorly defined. In this study, a comprehensive finite element model was developed to analyze the biomechanics of the spine after spinal fusion at L5-S1 in both types of high-grade spondylolisthesis (balanced and unbalanced pelvis). The relevant clinical indices (i.e. spondylolisthesis grade and Dubousset lumbosacral angle), the displacement of L4-L5, pressure within the annulus and nucleus, and stress at L4-L5 were evaluated and compared. The model can well predict the changes of the important clinical indices during the surgery. For a balanced pelvis, the reduction has a minimal effect on the biomechanical conditions at the adjacent level during postsurgical activities. In the unbalanced case, reduction induced larger deformation in the lumbosacral region and a higher stress concentration at adjacent level. Whether such a stress concentration can lead to long-term disc degeneration is not known. The results provide additional information for the clinician considering reduction of high-grade spondylolisthesis.

Supine spinal magnetic resonance imaging with straightened lower extremities in spondylolisthesis: a comparison with the conventional technique

OBJECTIVES

To compare the degree of slip in spondylolisthesis on supine magnetic resonance (MR) images obtained with flexed and straightened lower extremities.

METHODS

Supine spinal MR studies were performed in 100 cases of symptomatic spondylolisthesis with flexed and then straightened lower extremities. The angle of lumbar lordosis (by Cobb's method) and the degree of slip (by Taillard's method) were compared between the two sets of images.

RESULTS

The mean angle of lumbar lordosis increased from 51.65±8.57° on MR images with flexed lower limbs to 57.39±9.05° on MR images with straightened lower limbs (p<0.001; mean percent increase: 11.51%). Similar change was also observed for the mean degree of slip (from 25.80±7.74% to 28.68±7.93%, p<0.001; mean percent increase: 12.60%). After MR imaging with straightened lower extremities 22 out of 54 initially grade I cases had grade II disease (p<0.001).

CONCLUSIONS

Supine magnetic resonance imaging with straightened lower extremities detects higher degree of slippage in symptomatic patients with spondylolisthesis compared to conventional MRI with flexed lower extremities.

Analysis of Inadvertent Intradiscal Injections during Lumbar Transforaminal Epidural Injection

BACKGROUND

Recently, there have been several case reports and retrospective studies about the incidence of intradiscal (ID) injection during transforaminal epidural steroid injection (TFESI). Inadvertent ID injection is not a rare complication, and it carries the risk of developing diskitis, although there has been no report of diskitis after TFESI. We prospectively evaluated the incidence of inadvertent ID injection during lumbar TFESI and analyzed the contributing factors.

METHODS

Ten patients received 2-level TFESI, and the remaining 229 patients received 1-level TFESI. When successful TFESI was performed, 2 ml of contrast dye was injected under real-time fluoroscopy to check for any inadvertent ID spread. A musculoskeletal radiologist analyzed all magnetic resonance images (MRIs) of patients who demonstrated inadvertent ID injection. When reviewing MRIs, the intervertebral foramen level where ID injection occurred was carefully examined, and any anatomical structure which narrowing the foramen was identified.

RESULTS

Among the 249 TFESI, we identified 6 ID injections; thus, there was an incidence of 2.4%. Four patients had isthmic spondylolisthesis, and the level of spondylolisthesis coincided with the level of ID injection. We further examined the right or left foramen of the spondylolisthesis level and identified the upward migrated disc material that was narrowing the foramen.

CONCLUSIONS

Inadvertent ID injection during TFESI is not infrequent, and pain physicians must pay close attention to the type and location of disc herniation.

Biomechanical loading of the sacrum in adolescent idiopathic scoliosis

BACKGROUND

The pelvis maintains an important role in transferring loads from the upper body to the lower extremities and hence contributes to the standing postural balance. Even though changes in spino-pelvic relative alignment are involved in the pathophysiology of scoliosis, the mechanism through which the transferred load between the spine and pelvis is related to the spinal deformity is not well understood.

METHODS

A personalized finite element model of the spine and pelvis was constructed for 11 right main thoracic and 23 left thoracolumbar/lumbar adolescent idiopathic scoliosis and 12 asymptomatic controls. The compressive stress distribution on the sacrum endplate was computed. The position of the stress distribution barycenter on the sacrum superior endplate in reference to the central hip vertical axis was projected on the transverse plane and compared between scoliotic subgroups and controls.

FINDINGS

The medio-lateral position of the stress distribution barycenter on the sacrum superior endplate was significantly different between the scoliotic subgroups and controls (p<0.05). The stress distribution barycenter on the sacrum superior endplate was located at the right side of the central hip vertical axis in 82% of the right main thoracic patients and to the left side of the central hip vertical axis in 91% of the left thoracolumbar/lumbar patients.

INTERPRETATION

Analysis of the transferred load to the sacrum provided insight into the biomechanical spino-pelvic interaction in 3D, showing that a thoracolumbar/lumbar scoliotic curve has an increased influence on sacral loads when compared to a main thoracic scoliotic curve.

Effects of degenerated intervertebral discs on intersegmental rotations, intradiscal pressures, and facet joint forces of the whole lumbar spine

The effects of intervertebral disc (IVD) degeneration on biomechanics of the lumbar spine were analyzed. Finite element models of the lumbar spine with various degrees of IVD degeneration at the L4-L5 functional spinal unit (FSU) were developed and validated. With progression of degeneration, intersegmental rotation at the degenerated FSU decreased in flexion-extension and left-right lateral bending, intradiscal pressure at the adjacent FSUs increased in flexion and lateral bending, and facet joint forces at the degenerated FSU increased in lateral bending and axial rotation. These results could provide fundamental information for understanding the mechanism of injuries caused by IVD degeneration.

Biomechanical evaluation of predictive parameters of progression in adolescent isthmic spondylolisthesis: a computer modeling and simulation study

Background

Pelvic incidence, sacral slope and slip percentage have been shown to be important predicting factors for assessing the risk of progression of low- and high-grade spondylolisthesis. Biomechanical factors, which affect the stress distribution and the mechanisms involved in the vertebral slippage, may also influence the risk of progression, but they are still not well known. The objective was to biomechanically evaluate how geometric sacral parameters influence shear and normal stress at the lumbosacral junction in spondylolisthesis.

Methods

A finite element model of a low-grade L5-S1 spondylolisthesis was constructed, including the morphology of the spine, pelvis and rib cage based on measurements from biplanar radiographs of a patient. Variations provided on this model aimed to study the effects on low grade spondylolisthesis as well as reproduce high grade spondylolisthesis. Normal and shear stresses at the lumbosacral junction were analyzed under various pelvic incidences, sacral slopes and slip percentages. Their influence on progression risk was statistically analyzed using a one-way analysis of variance.

Results

Stresses were mainly concentrated on the growth plate of S1, on the intervertebral disc of L5-S1, and ahead the sacral dome for low grade spondylolisthesis. For high grade spondylolisthesis, more important compression and shear stresses were seen in the anterior part of the growth plate and disc as compared to the lateral and posterior areas. Stress magnitudes over this area increased with slip percentage, sacral slope and pelvic incidence. Strong correlations were found between pelvic incidence and the resulting compression and shear stresses in the growth plate and intervertebral disc at the L5-S1 junction.

Conclusions

Progression of the slippage is mostly affected by a movement and an increase of stresses at the lumbosacral junction in accordance with spino-pelvic parameters. The statistical results provide evidence that pelvic incidence is a predictive parameter to determine progression in isthmic spondylolisthesis.

Response analysis of the lumbar spine during regular daily activities--a finite element analysis

A non-linear poroelastic finite element model of the lumbar spine was developed to investigate spinal response during daily dynamic physiological activities. Swelling was simulated by imposing a boundary pore pressure of 0.25 MPa at all external surfaces. Partial saturation of the disc was introduced to circumvent the negative pressures otherwise computed upon unloading. The loading conditions represented a pre-conditioning full day followed by another day of loading: 8h rest under a constant compressive load of 350 N, followed by 16 h loading phase under constant or cyclic compressive load varying in between 1000 and 1600 N. In addition, the effect of one or two short resting periods in the latter loading phase was studied. The model yielded fairly good agreement with in-vivo and in-vitro measurements. Taking the partial saturation of the disc into account, no negative pore pressures were generated during unloading and recovery phase. Recovery phase was faster than the loading period with equilibrium reached in only approximately 3h. With time and during the day, the axial displacement, fluid loss, axial stress and disc radial strain increased whereas the pore pressure and disc collagen fiber strains decreased. The fluid pressurization and collagen fiber stiffening were noticeable early in the morning, which gave way to greater compression stresses and radial strains in the annulus bulk as time went by. The rest periods dampened foregoing differences between the early morning and late in the afternoon periods. The forgoing diurnal variations have profound effects on lumbar spine biomechanics and risk of injury.

Biomechanical comparison of instrumentation techniques in treatment of thoracolumbar burst fractures: a finite element analysis

BACKGROUND

There are several surgical techniques currently employed to treat thoracolumbar burst fractures, including anterior fixation, posterior fixation, or combined anterior-posterior fixation. Biomechanical analysis of the various types of surgical techniques is therefore critical to enable selection of the appropriate surgical method for successful spinal fusion. However, the effects of the various spinal fusion techniques on spinal stiffness have not been clearly defined, and the strengths and weaknesses of each fusion technique are still controversial.

METHODS

The biomechanical effects of increasing the number of anterior rods and removing the mid-column in anterior fixation, posterior fixation, and combined anterior-posterior fixation on spinal stiffness in thoracolumbar burst fractures was investigated. Finite element analysis was used to investigate the effects of the three fusion methods on spine biomechanics because of its ability to control for variables related to the material and experimental environment.

RESULTS

The stiffness of the fused spinal junction highly correlates with the selection of an additional posterior fixation. The mid-column decompression showed a significant change in stiffness, although the effect of decompression was much less than that with the application of posterior fixation and the anterior rod number. In addition, two-rod anterior fixation without additional posterior fixation is able to provide enough spinal stability; and one-rod anterior fixation with posterior fixation yields better results in regard to preventing excessive motion and ensuring spinal stability.

CONCLUSIONS

The present study shows that careful consideration is necessary when choosing the anterior rod number and applying posterior fixation and mid-column decompression during surgical treatment of thoracolumbar burst fractures.

Inclusion of regional poroelastic material properties better predicts biomechanical behavior of lumbar discs subjected to dynamic loading

Understanding the relationship between repetitive lifting and the breakdown of disc tissue over several years of exposure is difficult to study in vivo and in vitro. The aim of this investigation was to develop a three-dimensional poroelastic finite element model of a lumbar motion segment that reflects the biological properties and behaviors of in vivo disc tissues including swelling pressure due to the proteoglycans and strain-dependent permeability and porosity. It was hypothesized that when modeling the annulus, prescribing tissue specific material properties will not be adequate for studying the in vivo loading and unloading behavior of the disc. Rather, regional variations of these properties, which are known to exist within the annulus, must also be included. Finite element predictions were compared to in vivo measurements published by Tyrrell et al. (1985) of percent change in total stature for two loading protocols, short-term creep loading and standing recovery and short-term cyclic loading with standing recovery. The model in which the regional variations of material properties in the annulus had been included provided an overall better prediction of the in vivo behavior as compared to the model in which the annulus properties were assumed to be homogenous. This model will now be used to study the relationship between repetitive lifting and disc degeneration.

Moment-rotation responses of the human lumbosacral spinal column

The objective of this study was to test the hypothesis that the human lumbosacral joint behaves differently from L1-L5 joints and provides primary moment-rotation responses under pure moment flexion and extension and left and right lateral bending on a level-by-level basis. In addition, range of motion (ROM) and stiffness data were extracted from the moment-rotation responses. Ten T12-S1 column specimens with ages ranging from 27 to 68 years (mean: 50.6+/-13.2) were tested at a load level of 4.0 N m. Nonlinear flexion and extension and left and right lateral bending moment-rotation responses at each spinal level are reported in the form of a logarithmic function. The mean ROM was the greatest at the L5-S1 level under flexion (7.37+/-3.69 degrees) and extension (4.62+/-2.56 degrees) and at the L3-L4 level under lateral bending (4.04+/-1.11 degrees). The mean ROM was the least at the L1-L2 level under flexion (2.42+/-0.90 degrees), L2-L3 level under extension (1.58+/-0.63 degrees), and L1-L2 level under lateral bending (2.50+/-0.75 degrees). The present study proved the hypothesis that L5-S1 motions are significantly greater than L1-L5 motions under flexion and extension loadings, but the hypothesis was found to be untrue under the lateral bending mode. These experimental data are useful in the improved validation of FE models, which will increase the confidence of stress analysis and other modeling applications.