L4-L5 Motion Research Articles

A multiphasic FE analysis of an L4-L5 motion segment with focus on the inhomogeneous intervertebral disc

A multiphasic FE analysis of an L4-L5 motion segment with focus on the inhomogeneous intervertebral disc

Material Sensitivity Study on Lumbar Motion Segment (L2-L3) Under Sagittal Plane Loadings Using Probabilistic Method

In this study, the probabilistic responses of a three-dimensional finite element L2-L3 motion segment, with and without posterior elements, tested under sagittal plane loadings, are presented. Understanding the effect of biologic uncertainties and variations on the biomechanical response provides an insight into spinal behavior under normal and degenerated conditions. The biologic variability of 19 spinal components (nucleus, annulus, ligament, cortical/cancellous bone, endplate, and ligaments) in the motion segment was incorporated using statistical distributions into the model. A total of 2000 runs were performed using Monte Carlo probabilistic algorithms to compute the probabilistic response. This study establishes the relative importance of the spinal components in resisting the loading modes. The results show that for an intact motion segment, posterior ligaments are more dominant than intervertebral disc in resisting flexion moment. In extension, the capsular ligaments were found to be the most influential parameter. The intervertebral disc (ie, nucleus and annulus) affects the angular response of the disc body segment more than the hard tissues (ie, cortical and cancellous bone). The application of the probabilistic analysis provides a new approach whereby the influences of inherent uncertainties and variations in biologic structures can be studied and the biomechanical response assessed.

Segmental Motion Adjacent to an Instrumented Lumbar Fusion

We present an in vitro biomechanical comparison of adjacent segment motion at the cranial segment (L3-L4) for an L4-L5 versus an L4-S1 fusion model using cadaveric lumbosacral spines. The purpose is to determine the biomechanical effect on the unfused cranial segment of extending a short lumbar fusion to the sacrum versus stopping at L5. Radiographic evidence of adjacent segment degeneration can occur as a late sequela in patients following lumbar and lumbosacral spinal fusions. It is believed that altered biomechanics adjacent to the fusion construct contribute to these degenerative changes. Little is known regarding changes in cranial adjacent segment mechanics resulting from inclusion of the sacrum compared to ending a fusion at L5. Seven human cadaveric lumbosacral spines were instrumented with pedicle screws at L4, L5, and S1. Rods were placed from L4-L5 and from L4-S1 to simulate the corresponding fusion models. A material testing system was used to apply load-controlled moments to the spines in flexion-extension, lateral bending, and axial rotation. Electromagnetic sensors were used to record 6 df motion across the L3-L4, L4-L5, and L5-S1 motion segments. Angular displacements were recorded and system stiffness was calculated for each spine and construct. A paired sample t test was used to determine significance of recorded differences. Under flexion-extension loading, the angular displacement in the sagittal plane at L3-L4 for the L4-S1 model was 9.0 degrees compared to 7.8 degrees for the L4-L5 model (+15%; P = 0.002). Under lateral bending loading, L3-L4 motion in the coronal plane for the L4-S1 model was 12.8 degrees and was 14.5 degrees for the L4-L5 model (-12%; P = 0.002). In axial rotation testing, L3-L4 torsional motion for the L4-S1 model was equivalent to the L4-L5 model. Overall system stiffness increased for the L4-S1 model compared with the L4-L5 model. In this load-controlled model, extending fusion across L5-S1 did not consistently increase motion at L3-L4. While it may be difficult to translate this finding to a clinical setting, avoiding fusion to the sacrum in a lower lumbar fusion may not provide significant benefit from the standpoint of avoiding adjacent segment disease.

Effect of facetectomy on lumbar spinal stability under sagittal plane loadings.

A study using an anatomically accurate finite-element model of a L2-L3 motion segment to investigate the biomechanical effects of graded bilateral and unilateral facetectomies of L3 under flexion and extension loadings. To predict the amount of facetectomy on lumbar motion segment that would cause segmental instability, therefore enhancing the understanding concerning the role of the facet under sagittal loadings. This study provides a quantitative study on the role of facets in preserving segmental lumbar stability. Previous analytical models lack of three-dimensional structural characterization and insufficient element representation for facet joints. A validated finite-element L2-L3 model was subjected to sagittal loadings at 7.5 Nm. Effects of ligaments and facets were examined to establish their relative importance on segment response. The effect of iatrogenic changes (graded unilateral and bilateral facetectomy) was then investigated under these loadings to predict the alterations in terms of gross external (angular and coupled) responses, flexibilities, and facet load. This study shows the importance of preserving ligaments to prevent rotational instabilities for motion segment under flexion. The effect of the facetectomy on the motion segment is insignificant under flexion. In extension, unilateral facetectomy and resection on contralateral facet markedly alters the rotational motion and flexibilities as well as coupled motions. Also, unilateral complete facetectomy with resection of less than 100% on contralateral facet generates high facet load. Clinically, this study suggests that it may be appropriate to incorporate additional stabilization procedure in restoring the spinal strength and stability for surgical intervention of unilateral complete facetectomy and resection on contralateral facet. The exploitation of the finite-element method to simulate clinically related situations permits an improved understanding of lumbar spinal stability to assist in defining clinical expectation for various forms of surgical intervention of the operative procedures.

Biomechanical role of lumbar spine ligaments in flexion and extension: determination using a parallel linkage robot and a porcine model.

Porcine lumbar spine specimens were cyclically loaded in flexion and extension with a pure moment. The resistance to flexion and extension of the individual spinal elements was quantified using a sequential resection procedure. To determine the biomechanical contribution of the individual posterior spinal ligaments, including the interconnections between the supraspinous and interspinous ligaments, to resisting flexion-extension moments applied to the lumbar spine. Numerous experimental studies have performed repeated tests with sequential resection to determine the role of spinal ligaments. Most studies have not maintained the kinematics across trials, and therefore these studies have assessed the kinematics and kinetics of injured spines rather than delineating the mechanics of individual ligaments. The L4-L5 motion pathway for pure moment loading (between in 16 Nm of flexion and 12 Nm of extension) of porcine spinal specimens was learned and replayed using a parallel-linkage robot. Each specimen was tested 6 times, with sequential resections performed as follows: intact, cut interconnections between the supraspinous and interspinous ligaments, supraspinous ligament removed, interspinous ligament removed, ligamentum flavum cut, and facet joints removed. The kinematic pathway was repeated for each of these tests, and, accordingly, the changes in loads between trials reflected the mechanics of the cut/removed structures. The interaction between the supraspinous and interspinous ligaments is a significant contributor to the flexion resistance of the porcine lumbar spine, resisting 10.5% of the peak flexion moment. This contribution is similar to the interspinous ligament (11.3%) and almost as large as the supraspinous ligament (14%) and facet joints (14.2%). The supraspinous/interspinous ligament complex was the largest contributor to the resistance of flexion motion (35.9%), followed by the intervertebral disc (25.2%) and the ligamentum flavum (24.7%). The only structures involved in resisting extension were the facet joint complexes, intervertebral disc, and possibly the spinous processes. The supraspinous/interspinous ligament complex is the largest contributor to resisting applied flexion moments in the porcine lumbar spine.

A biomechanical study of lumbar fusion based on a three-dimensional nonlinear finite element method.

Biomechanical analyses under compression, flexion, and extension loading were performed to evaluate the stability of interbody, posterolateral, posterior, and facet fusions using a nonlinear three-dimensional finite element method. The effects of facet fusion on other lumbar fusions were also examined. A three-dimensional L4-L5 motion segment model was developed that took into consideration the material nonlinearities of ligaments and annular fibers and the contact nonlinearities of facet joints. Of all models of fusion, maximum rigidity was obtained in the interbody fusion model. In the posterolateral, posterior, and facet fusion models under compression, axial displacement and flexion rotation were induced. In combination with facet fusion, the interbody, posterolateral, and posterior fusion models demonstrated a decrease in axial displacement of about 6%, 1%, and 5%, respectively, under compression and a decrease in rotation angle of about 22%, 12%, and 48%, respectively, under flexion-extension loading. Stress concentration moved principally toward the fusion site, indicating increased load transfer across the fusion mass. Our findings suggest that a more solid fixation can be expected from lumbar fusion--especially in posterior fusion--if facet fusion is performed.

Human lumbar facet joint capsule strains: II. Alteration of strains subsequent to anterior interbody fixation

Background context In cases of low back pain associated with biomechanical lumbar instability, anterior interbody fixation can be used as a surgical treatment, but its affect on facet joint capsule strains is unknown. Purpose To determine the effect of a single-level anterolateral interbody fixation, the changes in lumbar facet joint capsule strains at the level of and adjacent to the fixation were evaluated. Study design/setting Human cadaveric lumbar spine specimens were tested under displacement control before and after the addition of a single anterior thoracolumbar plate (ATLP) on the L4–L5 motion body. Methods Ligamentous lumbar spine specimens (n = 7) were potted and actuated before and after fixation of the L4–L5 motion segment with an ATLP in motions of extension, flexion, left and right bending. Joint moments were calculated from the applied load and respective moment arms. Intervertebral angulation was measured using biaxial inclinometers mounted onto adjacent vertebrae. Plane strains of the capsules were measured by optically tracking the displacements of small, infrared reflective markers glued to capsule surfaces. Statistical differences (p<.05) in moment, intervertebral angle and capsular strain were assessed using analysis of variance and comparison of linear regression lines. Results Fixation resulted in an increase in moment at the three vertebral levels for all motions. There was also an increase in intervertebral angle at L3–L4 and L5–S1, and a decrease in intervertebral angle at L4–L5 for all motions. Plane strains in the L3–L4 and L5–S1 facet capsules increased as a result of the fixation. L4–L5 facet capsules experienced decreased and increased strains ipsilateral and contralateral, respectively, to the instrumentation. Conclusion Restriction of a vertebral motion segment using a single ATLP increased adjacent capsular strains, which if suprathreshold for capsule nociceptors, could play a role in low back pain.

A biomechanical study of lumbar spondylolysis based on a three-dimensional finite element method

Biomechanical analyses under compression only, and for a combination of flexion, extension, rotation, and lateral bending were performed to evaluate the stress of the interarticular portion of the lumbar vertebra using a nonlinear three-dimensional finite element method. A detailed three-dimensional L4–L5 motion segment model was developed that took into consideration the material nonlinearities of ligaments and annular fibers and the contact nonlinearities of facet joints. For a more accurate examination, the separation of cortical bone and cancellous bone for both posterior and anterior elements were also considered. The stress in the pars interarticularis was weakest under compression alone, but stronger under compression with lateral bending loading, with flexion, with rotation, and with extension. Under each loading condition, the region of the stress concentration was consistent with the separated region of the spondylolysis observed in clinical situations. Since the stress in the pars interarticularis was high under extension and rotation in particular, those loadings were suggested to be relatively high risk factors leading to spondylolysis.

DETERMINATION OF LOAD TRANSMISSION AND CONTACT FORCE AT FACET JOINTS OF L2–L3 MOTION SEGMENT USING FE METHOD

In the human spine, it is well known that facet joints play a significant role in load transmission and providing stability. It has also been hypothesized to be one of most probable sources of low back pain. Experimental determination of the load-bearing role of lumbar intervertebral joints, such as the facets joints, under axial compression has not been a straightforward task. In this study, the role of the facets in load transmission through a L2–L3 motion segment under axial compression is investigated using a L2–L3 finite element (FE) model, incorporated with an accurate three-dimensional geometry of facet joints with the inclusion of surface-to-surface continuum contact representation. The effects of osteoarthritis on facet force and biomechanical behaviors are also investigated by assuming friction at the facet joints. The study shows that the facet joints resisted 8% more in load for joints with osteoarthritics as compared with the normal joints. High percentage increase in contact facet force was also predicted for joint with osteoarthritics deformity. The use of the analytical FE model provided yet another efficient alternative for predicting the load transmission and contact force for degenerative joints, so as to provide a better understanding of the biomechanics of the spine as well as the pathophysiology of the various spinal disorders and degenerative conditions.

몸통 운동시 지향각(Orientation angles)을 이용한 허리 근육의 3차원 위치 좌표 추정 기법

The purpose of this study was to develop a method for estimating 3-D coordinates of lower trunk muscles using orientation angles during a motion. Traditional 3-D motion analysis system with DLT technique was used to track down the locations of eight reference markers which were attached on the back of the subject. In order to estimate the orientations of individual lumbar vertebrae and musculoskeletal parameters of the lower trunk muscle, the rotation matrix of the middle trunk reference frame relative to the lower trunk reference frame was determined and the angular locations of individual lumbar vertebrae were estimated by partitioning the orientation angles (Cardan angles) that represent the relative angles between the rotations of the middle and lower trunks. When the orientation angles of individual intervertebral joints were known at a given instant, the instantaneous coordinates of the origin and insertion for all selected muscles relative to the L5 local reference frame were obtained by applying the transformation matrix to the original coordinates which were relative to a local reference frame (S1, L4, L3, L2, or L1) in a rotation sequence about the Z-, X- and Y-axes. The multiplication of transformation matrices was performed to estimate the geometry and kinematics of all selected muscles. The time histories of the 3-D coordinates of the origin and insertion of all selected muscles relative to the center of the L4-L5 motion segment were determined for each trial.

Hinged-Dynamic Posterior Device Permits Greater Loads on the Graft and Similar Stability as Compared with Its Equivalent Rigid Device: A Three-Dimensional Finite Element Assessment

A three-dimensional finite element model was developed to compare the load sharing capabilities of the hinged (or dynamic) versus rigid states of a posterior lumbar spinal fixation device. The objective was to show that a dynamic state of a fixation system enables more load to be transferred through the graft (anterior column) as compared with an alternative rigid state, without compromising the stability imparted to the stabilized segment. The stability aspects of the rigid and dynamic pedicle screw-rod/plate fixation devices have been reported. Load sharing data of dynamic devices, as compared with rigid devices, are sparse. A three-dimensional nonlinear, experimentally validated, midsagittal symmetric finite element model of a L3–L4 motion segment was used for analysis. The intact model was modified to simulate a facetectomy and stabilization using both the hinged and rigid devices. The nucleus of the intact model was replaced to simulate a graft of cancellous bone, cortical bone, or titanium. The hinged device allowed for greater axial displacement while it maintained rotational stability similar to that of the rigid device. As the stiffness of the nucleus was increased from cancellous bone to titanium, the hinged device allowed a greater load through the nucleus compared with the rigid device predictions. However, the difference in load through the nucleus was minimal for the cortical and titanium grafts. The load transferred through the nucleus was greater for the hinged device, while it imparted rotational stability similar to the rigid device. Thus, a hinged screw-rod device offers a more favorable environment for enhanced fusion of the graft. Finite element models of a destabilized lumbar motion segment supplemented by a hinged or rigid posterior fixation device predicted that the hinged design permits greater load through the nucleus without any decrease in the rotational stability. Such systems are expected to enhance the fusion process.

Load-sharing between anterior and posterior elements in a lumbar motion segment implanted with an artificial disc.

A nonlinear three-dimensional finite element model of the osteoligamentous L3-L4 motion segment was used to predict changes in posterior element loads as a function of disc implantation and associated surgical procedures. To evaluate the effects of disc implantation on the biomechanics of the posterior spinal elements (including the facet joints, pedicles, and lamina) and on the vertebral bodies. Although several artificial disc designs have been used clinically, biomechanical data-particularly the change in loads in the posterior elements after disc implantation-are sparse. A previously validated intact finite element model was implanted with a ball-and-cup-type artificial disc model via an anterior approach. The implanted model predictions were compared with in vitro data. To study surgical variables, small and large windows were cut into the anulus, and the implant was placed anteriorly and posteriorly within the disc space. The anterior longitudinal ligament was also restored. Models were subjected to either 800 N axial compression force alone or to a combination of 10 N-m flexion-extension moment and 400 N axial preload. Implanted model predictions were compared with those of the intact model. Facet loads were more sensitive to the anteroposterior location of the artificial disc than to the amount of anulus removed. Under 800 N axial compression, implanted models with an anteriorly placed artificial disc exhibited facet loads 2.5 times greater than loads observed with the intact model, whereas posteriorly implanted models predicted no facet loads in compression. Implanted models with a posteriorly placed disc exhibited greater flexibility than the intact and implanted models with anteriorly placed discs. Restoration of the anterior longitudinal ligament reduced pedicle stresses, facet loads, and extension rotation to nearly intact levels. The models suggest that, by altering placement of the artificial disc in the anteroposterior direction, a surgeon can modulate motion-segment flexuralstiffness and posterior load-sharing, even though the specific disc replacement design has no inherent rotational stiffness.

Disc lesions and the mechanics of the intervertebral joint complex.

Correlations between tears in the disc and the mechanics of both the intervertebral joint and vertebral body bone were analyzed. To examine the effect of disc degeneration on the mechanics of spinal motion segments. Degeneration of the intervertebral disc results in changes to the mechanics of the spine. The actual effect of tear type and size on the mechanics of the intervertebral joint is unknown. Thirty spinal specimens (median age, 68 years) were divided into T12-L1, L2-L3, and L4-L5 motion segments. Mechanical tests recorded stiffness in flexion, extension, and torsion. Disc morphology was ascertained by taking three transverse sections of the disc and mapping and measuring the concentric tears, radial tears, and rim lesions. The severity of each tear type within each disc then was quantified. Bone cubes from the adjacent vertebral bodies were tested in compression to determine the elastic moduli and tested to failure in the longitudinal direction. Groups with tears were older and had reduced bone elastic moduli than groups without tears. Extension stiffness for the intact joint tended to increase with increasing tear severity. A decrease in torsional stiffness was present with increased severity of rim lesions at both L2-L3 and L4-L5. Tears in the intervertebral disc are reflected in a reduction in vertebral bone elastic modulus and in changes in the mechanics of the intervertebral joints in flexion, extension, and torsion.

Viscoelastic finite-element analysis of a lumbar motion segment in combined compression and sagittal flexion. Effect of loading rate.

A study using a validated viscoelastic finite-element model of a L2-L3 motion segment to identify the load sharing among the passive elements at different loading rates. To enhance understanding concerning the role of the loading rate (i.e., speed of lifting and lowering during manual material handling tasks) on the load sharing and safety margin of spinal structures. Industrial epidemiologic studies have shown that jobs requiring a higher speed of trunk motion contribute to a higher risk of industrial low back disorders. Consideration of the dynamic loading characteristics, such as lifting at different speeds, requires modeling of the viscoelastic behavior of passive tissues. Detailed systematic analysis of loading rate effects has been lacking in the literature. Complex flexion movement was simulated by applying compression and shear loads at the top of the upper vertebra while its sagittal flexion angle was prescribed without constraining any translations. The lower vertebra was fixed at the bottom. The load reached its maximum values of 2000 N compression and 200 N anterior shear while L2 was flexed to 10 degrees of flexion in the three different durations of 0.3, 1, and 3 seconds to represent fast, medium, and slow movements, respectively. The resisted bending moment, gross load-displacement response of the motion segment, forces in facet joints and ligaments, stresses and strains in anulus fibrosus, and intradiscal pressure were compared across different rates. The distribution of stress and strain was markedly affected by the loading rate. The higher loading rate increased the peak intradiscal pressure (12.4%), bending moment (20.7%), total ligament forces (11.4%), posterior longitudinal ligament stress (15.7%), and anulus fiber stress at the posterolateral innermost region (17.9%), despite the 15.4% reduction in their strain. Consideration of the time-dependent material properties of passive elements is essential to improving understanding of motion segment responses to dynamic loading conditions. Higher loading rate markedly reduces the safety margin of passive spinal elements. When the dynamic tolerance limits of tissues are available, the results provide bases for the guidelines of safe dynamic activities in clinics or industry.

Rate effect on sharing of passive lumbar motion segment under load-controlled sagittal flexion: viscoelastic finite element analysis

Industrial epidemiological studies have shown that jobs requiring a higher speed of trunk motion contribute to a higher risk of industrial low back disorders. Consideration of the loading dynamic characteristics, such as lifting at different speeds, requires modeling of the viscoelastic behavior of passive tissues. Detailed systematic analysis of the effects of loading rate has been lacking in the literature. A validated viscoelastic finite element model of a L2–L3 motion segment was used to identify the load sharing among the passive elements at different loading rates. Force controlled complex flexion movement was simulated by applying load at the top of the upper vertebra without constraining any coupled sagittal rotation, whereas the lower vertebra was fixed at the bottom. The load reached its maximum values of 2000 N compression, 400 N anterior shear, and 20 Nm flexion in three different durations of 0.3, 3 and 30 s to represent fast, medium and slow movement. The global force–displacement response of the motion segment, forces in facet joints and ligaments, stresses and strains in anulus fibrosus, and intradiscal pressure were compared across different rates. The higher rate of loading while reaching a prescribed complex forward flexion loading increased the intradiscal pressure and the stress in the anulus fibers at the posterolateral innermost layers, but reduced the global displacements, ligament forces and facet joint forces. The distribution of stress and strain was markedly affected by the loading rate. Consideration of the time-dependent material properties of passive elements is essential to improve our understanding of the responses of the motion segment to dynamic loading conditions. Speed of the manual materials handling (MMH) tasks should be included as a risk factor in the biomechanical and epidemiological studies and guidelines for safe lifting.

Modeling of facet articulation as a nonlinear moving contact problem: sensitivity study on lumbar facet response.

A finite element (FE) based scheme for modeling facet articulation in a spinal motion segment is proposed. The algorithm presented models the facet articulation as a nonlinear progressive contact problem. This algorithm is used to perform a nonlinear FE analysis of a complete L3-L4 motion segment. The role of facets in load transmission through a motion segment and its sensitivity to facet geometric parameters (i.e., spatial orientation of the facets and the gap between the facet articular surfaces) on this load transmission are studied. Compression, flexion, extension, and torsion loads are used in this study. The effect of facetectomy on gross segment response and disk fiber strains is studied by comparing the response of FE models of motion segment with and without facets. Large facet loads are obtained when the motion segment is subjected to torsional and large extension rotations, whereas minimal facet loads are observed under compression and flexion loading. Removal of facets reduces the segment stiffness considerably in torsion and results in higher strain levels in disk fibers. The facet load transmission is sensitive to facet geometric parameters, i.e., spatial orientation and initial facet joint gap. The facet loads increase uniformly with decrease in initial gap between the facet articular surfaces under compression, extension, and torsional loads. The sensitivity to spatial orientation angles of the facet is, however, found to vary with the type of loading. This sensitivity may account for the wide variation in the facet response reported in literature.

The dynamic response of [formula omitted] motion segment in cyclic axial compressive loading

Objective. The dynamic response and load sharing amongst passive elements of an L2–L3 motion segment during axial compressive cyclic loading was investigated. Design. A validated viscoelastic nonlinear finite element model of L2–L3 was used for a detailed stress/strain analysis during axial cyclic loading. Background. The repetitive loading of the spine has been implicated as a risk factor in developing low back disorders. However, the quantitative description of injury mechanisms and the internal load sharing have been lacking. Methods. The applied cyclic axial compressive loading was controlled, peak to peak, from 600 to 1000 N at 0.5 Hz for 15 cycles. The stress/strain and strain energy density of various elements were quantified and the effects of cyclic loading on these parameters were investigated. Results. The axial stiffness of the motion segment decreased, while intradiscal pressure (IDP) and the strain in anulus fibers of the outermost lamella increased. The axial stresses of outer lamellae in the anulus matrix reduced, in contrast to the increased strain at the endplate. Conclusions. The load sharing amongst the passive elements of the motion segment changed. The response of the motion segment to the same external axial load depends on the history of loading. The anulus fibers in the innermost layer were slack due to compression, hence not at risk of failure. The loss of disc height and increased disc bulge led to higher strain in anulus fibers of outermost layer. In future, more complex loading conditions with-a longer duration should be considered.

1997 Volvo Award winner in biomechanical studies. Kinematic behavior of the porcine lumbar spine: a chronic lesion model.

Experimental models of intervertebral disc and facet joint degeneration were created in vivo in the porcine lumbar spine for studying spinal kinematics, using a dynamic technique. To quantify the changes in spinal kinematics and the stabilizing capacity of the lumbar musculature caused by chronic lesions in the intervertebral disc and facet joints. Segmental kinematics are detrimentally altered by acute injury to passive structures of the motion segment. However, stimulation of the surrounding musculature adds stability to the motion segment. The in vivo kinematics of a degenerated lumbar motion segment and the stabilizing function of the surrounding musculature have not been quantified dynamically. Forty-four pigs were used in six chronic lesions models: sham, disc anulus, disc nucleus, facet capsule, facet joint slit, and facet joint wedge. Three months after injury, an instrumented linkage was used to measure continuously the sagittal kinematics of the L3-L4 motion segment during flexion-extension, with and without stimulation of the lumbar paraspinal musculature. Flexion-extension end point and maximum ranges of motion, and hysteresis were analyzed. Significant alterations in the kinematics caused by chronic lesions were observed, particularly when using the maximum range of motion and when comparing changes in axial translation. Muscular stimulation reduced the hysteresis in the sham, facet capsule, and disc nucleus groups; however, increased hysteresis was observed in the remaining lesion groups. The kinematic behavior of motion segments with chronic lesions was established. The maximum range of motion, which must be measured using a dynamic technique, was a more sensitive parameter for identifying changes in segmental kinematics caused by chronic lesions than was the end range of motion. The lumbar musculature was less efficient overall in stabilizing the motion segment, possibly because of altered mechanisms in the neuromuscular feedback system.

An experimental study of lumbar destabilization. Restabilization and bone density.

An investigation of the effects of bone density on lumbar spine stability using destabilizing and restabilizing procedures. To measure cadaveric vertebral bone densities computed tomographic scans and to correlate the measured densities with lumbar spine stability in the intact and during sequential destabilization and restabilization. The stabilizing effects of lumbar pedicle screw fixation have been widely described. Numerous construct failure mechanisms have been observed, including screw loosening in osteoporosis. Although previous studies have analyzed the effect of bone density on the compression strength of bone similar to that used in interbody fusion and the relationship of pedicle screw pull-out strength to vertebral bone density, a combined study of bone density and construct stability using an interbody bone spacer with pedicle fixation has not been performed. Bone densities were measured in 20 human cadaveric lumbar spines using computed tomography scans and a hydroxyapatite phantom. After the specimens were mounted in a testing frame, the L4-L5 motion segments were subjected to cyclic axial compression-torsional loads, and axial and rotational intervertebral displacements were monitored. Laminectomy, facetectomy, and pedicle screw-plate fixation were performed sequentially in three specimens. Ten others underwent these procedures with an additional destabilization procedure, discectomy, after facetectomy. Seven others underwent the same sequence as the previous group, followed by the insertion of interbody bone. Cyclic testing was resumed after each procedure. Average bone densities varied widely among the specimens. Average bone densities of the pedicle and of the vertebral body for individual specimens were well-correlated (r = 0.897). Displacements were recorded as a percentage of the intact state before destabilization; average percentages are reported as follows: axial displacements increased after facetectomy (145%) and subsequent discectomy (251%), and rotational displacements increased after facetectomy (295%) and discectomy (390%). Instrumentation without interbody bone resulted in specimens with decreased axial (126%) and rotational (156%) displacements. The addition of interbody bone further decreased axial (111%) and rotational (117%) displacements. The rotational stabilization provided by instrumentation was well-correlated with vertebral bone density (r = 0.804). This correlation was enhanced by the use of interbody bone (r = 0.939). The unstable lumbar spine can be partially stabilized using fixation. Interbody bone provides additional stability. The immediate stability provided by pedicle screws is greater in lumbar vertebrae with higher bone density.

Mechanical response of the lumbar spine to seated postural loads.

In vitro force and deformation measurements formed the basis for determinate, quasistatic analysis of principal forces in the seated lumbar spine. To explore the relationship between seated postures and the mechanical response in component tissues of lumbar intervertebral joints. Despite the high prevalence of low back pain syndrome, the precise mechanisms relating specific mechanical loads to spinal degeneration are not well understood. Simultaneous, time-dependent measurement of anterior column forces and articular facet forces has not been presented previously. consequently, a determinate analysis of principal component forces has not been possible. Twelve lumbar spines (L1-S1) were subjected to constant loading conditions while in flexed and extended seated postures. Time-dependent forces were measured in the anterior column at the L4 and L5 superior endplates and in the four facets of the L3-L4 and L4-L5 motion segments. A quasi-static analysis of sagittal plane forces was used to compute the remaining principal joint forces, including ligament, disc shear, and facet impingement forces. Component forces changed under static loading in both postures. There were significant differences between the mechanical responses of the two postures. Although the vertical creep displacement was greater in the extended seated posture (3.22 mm versus 2.11 mm), the escalation of forces was more severe in the flexed posture. The results suggest a mechanism of force balancing in lordotic postures under static loads, whereas flexed postures produce large increases to the tensile forces in the region of the posterior anulus.