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

Influence of graded facetectomy and laminectomy on spinal biomechanics

Facetectomy and laminectomy are techniques for decompressing lumbosacral spinal stenosis. Resections of posterior bony or ligamentous parts normally lead to a decrease in stability. The degree of instability depends on the extent of resection, the loading situation and the condition of the intervertebral discs. The correlation between these parameters is not well understood. In order to investigate how these parameters relate to one another, a three-dimensional, non-linear finite element model of the lumbosacral spine was created. Intersegmental rotations, intradiscal pressures, stresses, strains and forces in the facet joints were calculated while simulating an intact spine as well as different extents of resection (left and bilateral hemifacetectomy, hemilaminectomy and bilateral laminectomy, two-level laminectomy), disc conditions (intact and degenerated) and loading situations (pure moment loads, standing and forward bending). The results of the modelling showed that a unilateral hemifacetectomy increases intersegmental rotation for the loading situation of axial rotation. Expanding the resection to bilateral hemifacetectomy increases intersegmental rotation even more, while further resection up to a bilateral laminectomy has only a minor additional effect. Hemilaminectomy and laminectomy only differ in their effect for ventriflexion and muscle-supported forward bending. Two-level laminectomy increases the intersegmental rotation only for standing. Degenerated discs result in smaller intersegmental rotations and higher disc stresses at the respective levels. Decompression procedures affect the examined biomechanical parameters less markedly in degenerated than in intact discs. Resection of posterior bony or ligamentous elements has a stronger influence on the amount than on the distribution of stresses and deformations in a disc. It has only a minor effect on the biomechanical behaviour of the adjacent region. Spinal stability is decreased after a laminectomy for forward bending, and after a two-level laminectomy for standing. For axial rotation, spinal stability is decreased even after a hemifacetectomy. Patients should therefore avoid excessive axial rotation after such a treatment.

Factors influencing stresses in the lumbar spine after the insertion of intervertebral cages: finite element analysis

Intervertebral cages in the lumbar spine have been an advancement in spinal fusion to relieve low back pain. Even though initial stability is accepted as a requirement for fusion, there are other factors. The load transfer and its effect on the tissues adjacent to the cage may also play an essential role, which is not easily detectable with experimental tests. In this study the effects of an intervertebral cage insertion on a lumbar functional spinal unit were investigated using finite element analyses. The influences of cage material, cancellous bone density and spinal loading for the stresses in a functional spinal unit were evaluated. Three-dimensional (3D) finite element models of L2-L3 were developed for this purpose. An anterior approach for a monobloc, box-shaped cage was modelled. Models with cage were compared to the corresponding intact ones. The results showed that inserting a cage increased the maximum von Mises stress and changed the load transfer in the adjacent structures. Varying the cage material or the loading conditions had a much smaller influence than varying the cancellous bone density. The denser the cancellous bone, the more the stress was concentrated underneath the cage, while the remaining regions were unloaded. This study showed that the density of the underlying cancellous bone is a more important factor for the biomechanical behaviour of a motion segment stabilized with a cage, and its eventual clinical success, than the cage material or the applied load. Inserting an intervertebral cage markedly changed the load transfer. The altered stress distribution may trigger bone remodelling and explain damage of the underlying vertebrae.

The effect of cement augmentation on the load transfer in an osteoporotic functional spinal unit: finite-element analysis

STUDY DESIGN

The effect of cement augmentation on an osteoporotic lumbar functional spinal unit was investigated using finite-element analysis.

OBJECTIVE

To evaluate the influence of cement augmentation on load transfer, stresses, and strains.

SUMMARY OF BACKGROUND DATA

Osteoporosis is the most frequent skeletal disease of the elderly, leading to weakness of the bony structures. Cement injection into vertebral bodies has been used to treat osteoporotic compression fractures of the spine. The clinical results are encouraging. Experimental biomechanical studies showed significant increases in stiffness and strength of treated bodies. However, little is known about the consequences for the adjacent, nontreated levels.

METHODS

Three-dimensional finite-element models of L2-L3 were developed and the material properties adapted to simulate osteoporosis. The influence of augmentation level as well as uni- and bipedicular filling with polymethylmethacrylate were investigated. Compression, flexion, and lateral bending were simulated.

RESULTS

Augmentation increased the pressure in the nucleus pulposus and the deflection of the adjacent endplate. The stresses and strains in the vertebrae next to an augmentation were increased, and their distribution was changed. Larger areas were subjected to higher stresses and strains. The treatment clearly altered the load transfer. Changes to the overall stress and strain distribution were less pronounced for unipedicular augmentation.

CONCLUSIONS

Cement augmentation restores the strength of treated vertebrae, but leads to increased endplate bulge and an altered load transfer in adjacent vertebrae. This supports the hypothesis that rigid cement augmentation may facilitate the subsequent collapse of adjacent vertebrae. Further study is required to determine the optimal reinforcement material and filling volume to minimize this effect.

Biomechanical evaluation of reconstructed lumbosacral spine after total sacrectomy

When a sacral tumor involves the first sacral vertebra, total sacrectomy is necessary. It is mandatory to reconstruct the continuity between the spine and the pelvis after total sacrectomy. In this study, strain and stress on the instruments and the bones were evaluated for two reconstruction methods: a modified Galveston reconstruction (MGR) and a triangular frame reconstruction (TFR). Compressive loading tests were performed using polyurethane vertebral models, and a finite element model of a lumbar spine and pelvis was constructed. Then three-dimensional MGR and TFR models were reconstructed, and finite element analysis was performed to account for the stress on the bones and instruments. With MGR, excessive stress was concentrated at the spinal rod, and there was a strong possibility that the rod between the spine and the pelvis might fail. Although there was no stress concentration on the instruments with TFR, excessive stress on the iliac bone around the sacral rod was more than the yielding stress of the iliac bone. Such stress may cause loosening of the sacral rod from the iliac bone. If the patient were to stand or sit immediately after MGR or TFR, instrumentation failure or loosening might occur.

Comparison of the mechanical behavior of the lumbar spine following mono- and bisegmental stabilization

OBJECTIVE

To determine whether the mechanical behavior of the entire lumbar spine differs following mono- and bisegmental stabilization.

DESIGN

The mechanical behavior of the lumbar spine was studied using the finite element method.

BACKGROUND

Nonunion is somewhat more frequent after bi- than after monosegmental stabilization of the spine. Little is known about differences between the mechanical behavior associated with these procedures.

METHODS

A three-dimensional nonlinear finite element model of the lumbar spine with internal spinal fixators and bone grafts was used to study mechanical behavior after mono- and bisegmental fixation with and without stabilization of the bridged vertebra. Finite element analyses were performed to determine the influence of four different graft positions, five loading conditions, and six different pretensions in the longitudinal fixator rod. The following parameters were considered: the maximum contact pressure at the interface between the bone graft and vertebral body, the force transmitted by the bone graft, and the size of the contact area between the graft and the vertebral body.

RESULTS

Our model shows no clear differences between mono- and bisegmental fixation. Additional stabilization of the bridged vertebra exerts a partly adverse influence on the parameters studied. Pretension in the bridged region has a strong effect on the mechanical behavior.

CONCLUSIONS

The mechanical behavior of the lumbar spine after mono- and bisegmental stabilization is similar. Thus biological factors and the surgical procedure are probably decisive in determining the fusion rate.

RELEVANCE

Knowledge of the mechanical behavior after stabilization of the spine may help to improve the fusion rate. Our results suggest that the mechanical factors studied have only a minor influence on fusion rate and that other factors, such as incomplete resection of cartilage plate and poor local blood supply, are more decisive.

Prenatal rotation of the lumbar spine and its relevance for the development of the zygapophyseal joints

STUDY DESIGN

The intrauterine axial rotational characteristics of the lumbar spine were analyzed and quantified sonographically.

OBJECTIVES

To determine whether axial rotational movements of the lumbar spine occur prenatally, which may influence the morphogenesis of the zygapophysial joints.

SUMMARY OF BACKGROUND DATA

Previously, the development of lumbar zygapophysial sagittal joint orientation was linked to the increase in torque of the torso as upright posture and locomotion are adopted. However, the potential influence of prenatal axial rotational movement of the lumbar spine on the development of the zygapophysial joints has not been examined.

METHODS

For this study, 52 healthy fetuses, gestation age 9 to 36 weeks, were monitored sonographically for rotational movements of the lumbar spine. By multiplanar reconstruction of the sonography images, exemplary segmental rotational values were acquired. This method was validated by directly measuring the manually induced rotation of the formalin-fixated fetus in addition to multiplanar reconstruction.

RESULTS

Axial rotations of the lumbar spine were seen in most of the examined fetuses ages 9 to 36 gestation weeks. Rotations are best assessable with multiplanar reconstruction in the second trimenon, when segmental rotation of the lower lumbar spine reaches a mean of 8 degrees (range, 4-10 degrees ).

CONCLUSIONS

Fetal axial rotation of the lower lumbar spine reaches a degree that may functionally influence the morphologic development of the articular processes of the zygapophysial joints. In contrast to the prevalent hypothesis, lumbar spine development is already functionally influenced prenatally by axial rotation.

Effect of bone graft characteristics on the mechanical behavior of the lumbar spine

There is little information about the influence of bone graft size, position and elasticity on the mechanical behavior of the lumbar spine. Intersegmental motion, intradiscal pressure and stresses in the lumbar spine were calculated using a three-dimensional, nonlinear finite element model which included an internal spinal fixation device and a bone graft. Cross-sectional area, position, and elastic modulus of the graft were varied in this study. Bone grafts, especially very stiff ones, increase stresses on adjacent endplates. Though larger grafts lead to less contact pressure, it is difficult to judge the quality of different bone graft positions. In general, ventral flexion results in lower maximum contact pressure than lateral bending. There is always little intersegmental rotation in the bridged region compared with that of an intact spine.A larger graft with low stiffness should be favored from a mechanical point of view. Patients should avoid lateral bending of the upper body shortly after surgery.

Estimation of muscle forces in the lumbar spine during upper-body inclination

OBJECTIVE

To estimate the muscle forces during upper-body inclination and to determine their influence on stress distribution in the annulus fibrosus of the lumbar spine discs.

DESIGN

The muscle forces and stresses were calculated using a non-linear finite element model of the lumbar spine.

BACKGROUND

Little is known about the influence of muscle forces on the deformation of, and stresses in, the lumbar spine. In most studies, muscle forces are neglected.

METHODS

Three-dimensional non-linear finite element models of the ligamentous lumbar spine, with and without internal spinal fixators, were created. They were validated by use of experimental data from in vitro measurements on cadaver specimens. In a second step, the influence of muscle forces on stresses in the annulus fibrosus of the lumbar spine discs was investigated in a parameter study. This was done for different inclination angles of the upper-body.

RESULTS

Good agreement between analytical and experimental results proved achievable when loading with pure moments in the three main planes of the lumbar spine. For inclination of the upper-body, the flexion angle clearly has a strong influence on the stresses in the lumbar spine while the influence of local muscles was small. The stress distribution in the discs differed considerably when the muscle forces are neglected and only a pure moment is applied.

CONCLUSIONS

This study confirmed earlier ones that have shown that muscle forces should not be neglected when studying the stresses in the lumbar spine. The local dorsal muscles, however, have only a small influence on the stresses in the discs.

RELEVANCE

For investigations of the biomechanical effects of spinal implants and surgical procedures, experimental or analytical methods are used. Due to the complexity involved, as well as to a lack of information, muscle forces are often neglected. Our study showed that muscles do in fact have a major influence on the mechanical behaviour of the spine and should always be taken into account.

Comparative and functional anatomy of the mammalian lumbar spine

As an essential organ of both weight bearing and locomotion, the spine is subject to the conflict of providing maximal stability while maintaining crucial mobility, in addition to maintaining the integrity of the neural structures. Comparative morphological adaptation of the lumbar spine of mammals, especially in respect to locomotion, has however received only limited scientific attention. Specialised features of the human lumbar spine, have therefore not been adequately highlighted through comparative anatomy. Mathematical averages were determined of 14 measurements taken on each lumbar vertebrae of ten mammalian species (human, chimpanzee, orang-utan, kangaroo, dolphin, seal, Przewalski's horse, cheetah, lama, ibex). The revealed traits are analysed with respect to the differing spinal loading patterns. All examined mammalian lumbar spines suggest an exact accommodation to specific biomechanical demands. The lumbar spine has reacted to flexion in a predominant plane with narrowing of the vertebral bodies in quadrupeds. Torsion of the lumbar spine is withstood by an increase in the transverse distance between the inferior articular processes in the upper lumbar spine in primates, but lower lumbar spine in humans, quadrupeds and the seal. Sagittal zygapophyseal joint areas resist torsion in the seal and humans. Ventral shear is resisted by frontal zygapophyseal joint areas in humans and primates, and dorsal shear by encompassing joints in the ibex. The human fifth lumbar vertebra is remarkable in possessing the largest endplate surface area and the widest distance between the inferior articular processes, as an indicator of the high degree of axial load and torsion in bipedalism.

An immunohistochemical study of the dorsal capsule of the lumbar and thoracic facet joints

STUDY DESIGN

The molecular composition of the extracellular matrix in the dorsal capsules of lumbar and thoracic facet joints was analyzed immunohistochemically.

OBJECTIVES

To determine whether the immunohistochemical profile of the lumbar joint capsule suggests a role of the capsule in limiting axial rotation of the lumbar motion segment.

SUMMARY OF BACKGROUND DATA

During axial rotation of the lumbar vertebrae, the axis of rotation shifts toward the facet joints in the direction of rotation. Thus, the capsule of the opposing joint should become tensed and wrap around the inferior articular process. Previous studies suggest that wrap-around ligaments are fibrocartilaginous. However, thoracic joint capsules are largely shielded from such loading and should be purely fibrous.

METHODS

Dorsal capsules were removed from lumbar and thoracic facet joints of six adult cadavers. Specimens were immunolabeled with monoclonal antibodies for collagens, chondroitin, dermatan and keratan sulfates, versican, tenascin, aggrecan and link protein. Antibody binding was detected using the Vectastain ABC 'Elite' peroxidase kit (Vector Laboratories, Inc., Burlingame, CA).

RESULTS

Both lumbar and thoracic joint capsules immunolabelled for most glycosaminoglycans and for Type I, III and VI collagens. However, labeling for Type II collagen, chondroitin-6-sulfate, aggrecan, and link protein was restricted to lumbar capsules. Such labeling was constantly seen at entheses and occasionally in the midsubstance.

CONCLUSIONS

The molecular composition of the lumbar joint capsule suggests that it acts as a fibrocartilaginous, 'wrap-around' ligament that withstands compression in addition to tension during torsional movements of the lumbar spine. It wraps around the inferior articular process as rotation occurs and limits further movement.

Kinematics of rotational mobilisation of the lumbar spine

OBJECTIVE

The purposes of this study were to measure the movements of the lumbar spine produced by rotational mobilisation, and to study the effects of different grades of mobilisation on the movements produced.

DESIGN

Kinematics of rotational mobilisation was assessed with an electromagnetic tracking device.

BACKGROUND

Rotational mobilisation is frequently used in the treatment of back pain, but there was no information on its mechanical effects.

METHODS

Movements of the lumbar spine were measured in 14 healthy volunteers when they were subjected to grades I to IV left rotational mobilisation.

RESULTS

In the starting positions, the spines were found to be flexed, axially rotated to the left and laterally bent to the right. As the mobilisation grade increased, the spine was axially rotated further into the range. Rotational mobilisation was found to produce oscillatory movements of the lumbar spine in all three anatomical planes. It produced axial rotation which was accompanied by lateral bending in the opposite direction and sagittal rotation. The mean frequency of the oscillatory movements was 1.4 Hz. The amplitude of the oscillations was small, and was found to be increased in grades II and III mobilisation.

CONCLUSION

Rotational mobilisation may be able to restore lost movements of the lumbar spine in any of the three anatomical planes.

The dynamic response of L(2)/L(3) 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.

Load sharing within a human lumbar vertebral body using the finite element method

STUDY DESIGN

A finite element parametric study was performed to investigate the structural roles of the vertebral cortical shell and the trabecular centrum.

OBJECTIVES

To address the debated issue of the relative load-carrying role of the vertebral cortical shell.

SUMMARY OF BACKGROUND DATA

Several experimental and computational studies have been aimed at quantifying the load-carrying roles of the human vertebral cortical shell and trabecular centrum. These studies, however, have supported no consensus.

METHODS

A finite element model of three lumbar vertebral bodies was developed to predict the fraction of the total compressive load acting on the lumbar vertebral body, under two different loading conditions, that was supported by the cortical shell. Parametric variations in vertebral material and geometric properties were examined to determine how this fraction was influenced by such changes.

RESULTS

The fraction of the compressive load supported by the cortical shell was found to be strongly dependent on the distance from the endplate, increasing from about 34% at either endplate to approximately 63% at the midtransverse plane. This fraction was independent of the loading characteristics, proportional to the properties of the cortex, and inversely proportional to the modulus of the centrum. Additionally, the cortical shell force fraction was affected significantly by changes in the overall vertebral geometry.

CONCLUSIONS

Our findings indicate that the structural dominance of the cortical shell and centrum alternate depending on the location within the vertebral body. However, as age-related bone loss progresses, the load-carrying role of the cortical shell could increase significantly.

The pathomechanism of spondylolytic spondylolisthesis in immature primate lumbar spines in vitro and finite element assessments

STUDY DESIGN

Immature Chacma baboon (Papio ursinus) spine specimens were used to determine load-displacement behavior as related to disc injury. This was accomplished through the application of A-P shear force until failure of FSUs with pars defects. Several finite element models (FEMs) of the FSU were developed to study the mechanism of slippage in immature baboon lumbar spines.

OBJECTIVES

The purpose was to show that spondylolisthesis (olisthesis) always occurs through the growth plate using a model similar to immature human lumbar spines. Using FEMs, the roles of facet orientation, pars interarticularis thickness, and a weak growth-plate in producing slippage were examined.

SUMMARY OF BACKGROUND DATA

Progression from spondylolysis (lysis) to olisthesis occurs, most often, during the adolescent growth spurt. The biomechanical literature dealing with the slippage mechanism in the immature lumbar spine does not provide a clear understanding and is sparse.

METHODS

Several groups of FSUs were subjected to A-P shear force until failure. The results provided displacement at failure as a function of disc injury and flexion-extension fatigue. A bilateral pars defect was created in each specimen prior to application of A-P shear force using an MTS machine. Failure sites were assessed radiographically and histologically. A nonlinear 3-D FEM of the intact L4-L5 was created from CT scans. The model was modified to predict the effects of a pars fracture, a thin pars, a weak growth plate, and facet orientation on the shear load through the growth plate and stresses in the pars.

RESULTS

Experimentally, failures always occurred through the growth-plate in the disc intact and disc-incised groups. In the intact FEM, the growth plate carried21% of the applied A-P shear force. The load increased when the facets were more sagittally oriented. The effect of thin pars and/or weaker growth plate was an increase in stresses in the pars. Changes in the load through the growth plate were minimal.

CONCLUSIONS

The weakest link in immature baboon lumbar functional spinal units (FSUs) with lysis during an A-P shear load was the growth plate, between the cartilaginous and osseous end plates. Surgeons may assess this lesion on MRI views, thereby predicting the possible development and preventing progression of olisthesis. Finite element model results predict that more sagittally orientated facets and/or a pars fracture are prerequisites for olisthesis to occur.

The effects of motion on trunk biomechanics

OBJECTIVE

To review the literature that evaluates the influence of trunk motion on trunk strength and structural loading.

BACKGROUND

In recent years, trunk dynamics have been identified as potential risk factors for developing low-back disorders. Consequently, a better understanding of the underlying mechanisms involved in trunk motion is needed.

METHODS

This review summarizes the results of 53 studies that have evaluated trunk motion and its impact on several biomechanical outcome measures. The biomechanical measures consisted of trunk strength, intra-abdominal pressure, muscle activity, imposed trunk moments, and spinal loads. Each of these biomechanical measures was discussed in relation to the existing knowledge within each plane of motion (extension, flexion, lateral flexion, twisting, and asymmetric extension).

RESULTS

Trunk strength was drastically reduced as dynamic motion increased, and males were impacted more than females. Intra-abdominal pressure seemed to only be affected by trunk dynamics at high levels of force. Trunk moments were found to increase monotonically with increased trunk motion. Both agonistic and antagonistic muscle activities were greater as dynamic characteristics increased. As a result, the three-dimensional spinal loads increase significantly for dynamic exertions as compared to isometric conditions.

CONCLUSIONS

Trunk motion has a dramatic affect on the muscle coactivity, which seems to be the underlying source for the decrease strength capability as well as the increased muscle force, IAP, and spinal loads. This review suggests that the ability of the individual to perform a task "safely" might be significantly compromised by the muscle coactivity that accompanies dynamic exertions. It is also important to consider various workplace and individual factors when attempting to reduce the impact of trunk motions during dynamic exertions. Relevance This review provides insight as to why trunk motions are important risk factors to consider when attempting to control low-back disorders in the workplace. It is apparent that trunk motion increases the risk of low-back disorders. To better control low-back disorders in industry, more comprehensive knowledge about the impact of trunk motion is needed. A better understanding of muscle coactivity may ultimately lead to reducing the risk associated with dynamic exertions.

The influence of psychosocial stress, gender, and personality on mechanical loading of the lumbar spine

STUDY DESIGN

The effects of psychosocial stress on muscle activity and spinal loading were evaluated in a laboratory setting.

OBJECTIVE

To evaluate the influence of psychosocial stress, gender, and personality traits on the functioning of the biomechanical system and subsequent spine loading.

SUMMARY OF BACKGROUND DATA

Physical, psychosocial, and individual factors all have been identified as potential causal factors of low back disorders. How these factors interact to alter the loading of the spine has not been investigated.

METHODS

Twenty-five subjects performed sagittally symmetric lifts under stressful and nonstressful conditions. Trunk muscle activity, kinematics, and kinetics were used to evaluate three-dimensional spine loading using an electromyographic-assisted biomechanical model. A personality inventory characterized the subject's personality traits. Anxiety inventories and blood pressure confirmed reactions to stress.

RESULTS

Psychosocial stress increased spine compression and lateral shear, but not in all subjects. Differences in muscle coactivation accounted for these stress reactions. Gender also influenced spine loading; Women's anterior-posterior shear forces increased in response to stress, whereas men's decreased. Certain personality traits were associated with increased spine loading compared with those with an opposing personality trait and explained loading differences between subjects.

CONCLUSIONS

A potential pathway between psychosocial stress and spine loading has been identified that may explain how psychosocial stress increases risk of low back disorders. Psychosocially stressful environments solicited more of a coactivity response in people with certain personality traits, making them more susceptible to spine loading increases and suspected low back disorder risk.

Mechanical effects of continuous passive motion on the lumbar spine in seating

The aim of this study was to develop a model which describes the mechanical spinal response to small alternating pelvic stimulation induced by an active rotational movement of a normal chair. The rotary continuous passive motion (RCPM) of the seat about a vertical axis of only 0.6 degrees resulted in an increased in spinal length as opposed to the normal daily shrinkage, and back patients experienced pain relief. Passive and active exercies have been broadly applied for treating and healing spinal disorders. A rigid body package (ADAMS Android) was used to translate the stimulation of the ischial tubersoity in caudo-cranial handing-over visualisation. The parameters of the model were set so that the values of the global stiffness and geometry of the intervertebral discs could be changed. In vivo validation of the model was based on force and moment measurements using an internal AO fixator. The predicitons of the model concerning natural frequency (4.5Hz) in vertical direction and the axial torsion response on small pelvic torsion are comparable with experimental data.

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

STUDY DESIGN

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.

OBJECTIVE

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.

SUMMARY OF BACKGROUND DATA

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Spine loading and trunk kinematics during team lifting

Two-person or team lifting is a popular method for handling materials under awkward or heavy lifting conditions. While many guidelines and standards address safe lifting limits for individual lifting, there are no such limits for team lifting, and these lifts are poorly understood. The literature associated with team lifting offers some interesting paradoxes. Many studies have indicated that people lift less per individual under team conditions compared with one-person lifting. Yet, at least one study has reported an increase in team-lifting capacity when subjects were height-matched. The current study explored the spine loading characteristics of one- and two-person lifting teams when subjects lifted under several sagittally symmetric and asymmetric conditions. Spine compression was lower for two person lifts for a given weight, while lifting in sagittally symmetric conditions whereas lateral shear became much greater for two-person lifts under asymmetric lifting conditions. This study has linked these changes to differences in trunk kinematic patterns adopted during one- versus two-person lifting.

Relation between spinal load factors and the high-risk probability of occupational low-back disorder

Spinal compression is traditionally assumed the principal biomechanical mechanism associated with occupationally related low-back disorders (LBD). However, there is little conclusive evidence demonstrating that compression is related to occupational LBD. The objective of this research was to examine whether axial compression in the lumbar spine can predict the probability that a lifting task should be classified as high risk for LBD. Furthermore, the improvement in predictive ability was examined when analyses include 3-D, dynamic biomechanical factors. Ten experienced warehouse workers transferred 12 pallet loads of boxes in a simulation of warehouse working conditions. Biomechanical estimates of 2-D static and 3-D dynamic spinal compression, shear loads and tissue strains were achieved from the subjects during each lifting exertion. Each lift was also assessed for probability of high LBD risk classification. Regression analyses were performed to examine the relationship between biomechanical and epidemiological factors. Results indicate 2-D static estimates of spinal compression describe approximately 13% of the probability of high LBD risk variability. Dynamic estimates of spinal compression describe > 44% of the variability. A multifactor regression model including 3-D spinal loads and tissue strains further improved the predictive ability, but the improvement was not statistically significant. This research demonstrates the biomechanical source of low-back pain is dynamic, multifaceted and multidimensional. Significant improvements in ergonomics assessments can be achieved by examining interactions of dynamic biomechanical factors. Unfortunately, this improved predictive ability is generated at the high cost of computational complexity. However, less realistic biomechanical representations may ignore the injury mechanisms associated with the greater number of workplace injuries. Thus, improved understanding of the dynamic biomechanical interactions influencing the tolerance and injury mechanisms of the spine may permit more accurate assessments of workplace injury factors associated with LBD and reduced incidence of occupationally related low-back pain.

A comprehensive analysis of low-back disorder risk and spinal loading during the transferring and repositioning of patients using different techniques

Although patient handlers suffer from low-back injuries at an alarming rate worldwide, there has been limited research quantifying the risk for the specific tasks performed by the patient handlers. The current study used both a comprehensive evaluation system (low-back disorder risk model) and theoretical model (biomechanical spinal loading model) to evaluate risk of LBD of 17 participants (12 experienced and five inexperienced) performing several patient handling tasks. Eight of the participants were female and nine were male. Several patient transfers were evaluated as well as repositioning of the patient in bed; these were performed with one and two people. The patient transfers were between bed and wheelchair (fixed and removable arms) and between commode chair and hospital chair. A 'standard' patient (a 50 kg co-operative female; non-weight bearing but had use of upper body) was used in all patient handling tasks. Overall, patient handling was found to be an extremely hazardous job that had substantial risk of causing a low-back injury whether with one or two patient handlers. The greatest risk was associated with the one-person transferring techniques with the actual task being performed having a limited effect. The repositioning techniques were found to have significant risk of LBD associated with them with the single hook method having the highest LBD risk and spinal loads that exceeded the tolerance limits (worst patient handling job). The two-person draw sheet repositioning technique had the lowest LBD risk and spinal loads but still had relatively high spinal loads and LBD risk. Thus, even the safest of tasks (of the tasks evaluated in this study) had significant risk. Additionally, the current study represented a 'best' case scenario since the patient was relatively light and co-operative. Thus, patient handling in real situations such as in a nursing home, would be expected to be worse. Therefore, to have an impact on LBD, it is necessary to provide mechanical lift assist devices.

Total trunk muscle force and spinal compression are lower in asymmetric moments as compared to pure extension moments

The aim of the present study was to test the assumption that asymmetric trunk loading requires a higher total muscle force and consequently entails a higher compression forces on the spine as compared to symmetric loading. When the trunk musculature is modelled in sufficient detail, optimisation shows that there is no mechanical necessity for an increase in total muscle force (or compression force) with task asymmetry. A physiologically based optimisation does also not predict an increase in total muscle force or spinal loading with asymmetry. EMG data on 14 trunk muscles collected in eight subjects showed antagonistic coactivity to be present in both conditions. However, estimates of total muscle force based on the EMG were lower when producing an asymmetric moment. In conclusion, producing an asymmetric moment appears to cause slightly lower forces on the lumbosacral joint as compared to a symmetric moment. Only lateral shear forces increase with asymmetry but these remain well below failure levels.

Surface strain distribution on thoracic and lumbar vertebrae under axial compression. The role in burst fractures

STUDY DESIGN

The surface strain distribution on the thoracic and lumbar vertebrae during axial compressive loading was examined.

OBJECTIVES

To examine the general mechanical behavior of the thoracic and lumbar vertebrae to evaluate their role in burst fractures.

SUMMARY OF BACKGROUND DATA

Burst fractures are generally characterized by injury to the middle column and fracturing of the superior endplate. However, results in previous biomechanical investigations have not shown how these fractures are initiated during compression.

METHODS

Twenty-one thoracic and lumbar vertebrae (5 T10, 10 L1, and 6 L4) with upper and lower vertebrae were studied. Three-axis rosette strain gauges were cemented to 11 sites on the vertebral surface. An axial compressive load was applied, and the strain was measured in each specimen. The strain recorded by each rosette gauge was converted into a tensile, compressive, and shear component.

RESULTS

The highest tensile and compressive strain was recorded at the base of the pedicle. Shear strain in the vertebral body was significantly higher than that in the lamina at all three spinal levels. At L1 and L4, the tensile strain at the superior vertebral rim was higher than that at the inferior rim.

CONCLUSIONS

The high tensile and compressive strains found at the base of the pedicle of T10, L1, and L4 indicate that the base of the pedicle is the site of fracture initiation. The higher tensile strain at the superior vertebral rim of L1 and L4 supports the clinical observation of the thoracolumbar burst fractures.

Contact analysis of a posterior cervical spine plate using a three-dimensional canine finite element model

The development of a three-dimensional finite element model of a posteriorly plated canine cervical spine (C3-C6) including contact nonlinearities is described. The model was created from axial CT scans and the material properties were derived from the literature. The model demonstrated sufficient accuracy from the results of a mesh convergence test. Significant steps were taken toward establishing model validation by comparison of plate surface strains with a posteriorly plated canine cervical spine under three-point bending. This model was developed to better characterize the contact pressures at the various interfaces under average physiologic canine loading. The analysis showed that the screw-plate interfaces had the highest values of all the mechanical parameters evaluated.

Three-dimensional measurement of lumbar spine kinematics for fast bowlers in cricket

OBJECTIVE: To determine whether the three-dimensional (3-D) lumbar spine kinematics for the mixed fast bowling technique differed to those of the side-on and front-on fast bowling techniques. BACKGROUND: It has been previously shown that bowlers who utilise a mixed bowling technique are more likely to show lumbar spine pathology than those who bowl with either the side-on or front-on techniques. METHODS: An electromagnetic device (3-Space(R)Fastrak(TM)) operating at 120 Hz captured range of motion and 3-D lumbar spine kinematics during the delivery stride of 20 young high performance subjects. The trajectory of shoulder and pelvic girdle markers were simultaneously captured and these data were used to classify bowlers into either a side-on, front-on or mixed technique group. RESULTS: No significant differences (P<0.004) existed between the side-on/front-on and mixed groups for 12 selected variables derived from the lumbar spine kinematic data. However, an examination of effect sizes revealed evidence that the mixed group showed: a greater amount of left lateral bend and an extended lumbar spine at front foot impact; a body position further from a neutral orientation at lease; and a greater range of motion and angular velocity of the trunk in the lateral bending and flexion/extension axes. CONCLUSIONS: Selected lumbar range of motion and velocity measures tended to be higher for mixed bowlers than side-on/front-on bowlers. RELEVANCE: Overuse injuries to fast bowlers in bricket are common. To better understand the mechanics of injury it is necessary to understand the 3-D rotations of the lumbar spine during this activity.

Lumbar intradiscal pressure measured in the anterior and posterolateral annular regions during asymmetrical loading

OBJECTIVE: To analyze the effect of asymmetrical loading on intradiscal pressure. DESIGN: Human cadaveric lumbar spines were instrumented with multiple pressure sensors and subjected to external mechanical loads. BACKGROUND: Tears and radial fissures in the posterolateral annulus with no evidence of annular protrusion or nuclear extrusion are frequently observed in non-degenerated intervertebral discs. Cadaveric studies have shown that asymmetrical loads lead to posterolateral herniation. Regional overload may be responsible for a progressive structural weakness of the posterolateral annular fibres. METHODS: Three needles each equipped with three independent pressure sensors were inserted in the midplane of the L(3)/L(4) intervertebral disc (n = 16) in the anterior, right posterolateral and left posterolateral regions. Axial rotation was applied in the upright, flexed and extended positions and the pressures recorded. RESULTS: The largest intradiscal pressure increase was observed in the posterolateral inner annular regions, more so in flexion than extension, when combined with axial rotation. Significant centripetal pressure gradients were found only in the posterolateral needles during the upright and flexed positions. CONCLUSIONS: When applying compression and axial rotation, the posterolateral inner annular zones of the intervertebral disc show high stress peaks and centripetal pressure gradients. Asymmetrical loads (rotation) combined with postural changes in the sagittal plane increase these effects, and may be responsible for a chronic mechanical overload of these regions. RELEVANCE: Our findings suggest a predilection for the posterolateral inner annular regions to mechanical failure, especially under asymmetrical loading.

Three dimensional moments in the lumbar spine during asymmetric lifting

OBJECTIVE: To better understand the loads placed on the spine during asymmetric lifting. DESIGN: Analysis of variance was used to test the effects of asymmetry (0, 45, 90 degrees ), event (up, down, min and max), and trial (first and second) on applied spine moments and resulting EMG signals in the lumbar spine. BACKGROUND: Loading conditions resulting from symmetric lifting are well documented in the literature, yet free-style asymmetric tasks have not been intensively studied. METHODS: Infrared markers and EMG electrodes were fixed to ten subjects. Subjects were asked to lift a 45 N weight from three different positions in relation to the sagittal plane. External loads on the L3-L4 and L4-L5 disc planes were calculated. Maximum moments and EMG were compared with those at lift off and set down of the burden. All lifts were repeated twice. RESULTS: Introducing asymmetry to the lift increased lateral bending and twisting moments. Flexion moments increased by approximately 5% after lift off of the load. During the second repetition of the lift, flexion and twisting moments decreased and increased, respectively. EMG signals were greater on the contralateral side during asymmetric lifts. CONCLUSIONS: Asymmetric lifting places complex loads across the trunk, which may become increasingly asymmetric with repeated trials. RELEVANCE: Individuals should take precautions to perform lifts in a symmetric manner and to avoid repetitive asymmetric lifting if possible. Past study indicated that twisting and lateral bending loads, when coupled with flexion loads, resulted in much greater disc compression and should be avoided. This work described the degree to which lateral bending and twisting loads were introduced as lifts deviated from the sagittal plane.

Trunk muscle activities during asymmetric twisting motions

Axial twisting of the torso has been identified via epidemiologic studies as a significant risk factor for occupationally-related low back disorders. However, only recently have biomechanical studies been able to describe how twisting is accomplished through the use of the trunk musculature. These studies have been performed on subjects whose torso twists were performed in an upright posture. In this study, the electromyographic activity of ten trunk muscles was observed while 12 subjects performed twisting exertions in three different trunk postures. These postures included upright twisting, twisting while the trunk was flexed in the sagittal plane, and twisting while the trunk was flexed and rotated asymmetrically. In addition, twisting velocity and direction of motion were changed under the experimental conditions. Under upright twisting conditions, the twisting torque was generated easily and relatively efficiently through the employment of the oblique (internal and external) and latissimus dorsi muscles. When the trunk was flexed the activity of erector spinae muscles increased (about 10-15%) while the external oblique activity decreased (about 3-5%). Twisting while in asymmetric bent postures was accomplished with a reduction in oblique and latissimus dorsi muscle activities (approximately 5%) while the erector spinae muscle activity remained elevated. The change in muscle activity needed to balance the torso during twisting while bending also increased the amount of lateral torque that was produced by the trunk. These findings suggest that studies observing trunk muscle activities and trunk loading while subjects were in upright postures should be interpreted with caution when evaluating the activity of the trunk during occupational activities. Since many occupational twisting tasks are performed in awkward, asymmetric postures, application of results from upright twisting studies might underestimate the risk of these activities.

Spine loading during asymmetric lifting using one versus two hands

This study documented three-dimensional spinal loading associated with asymmetric lifting while using either one or two hands to perform the task. Lift asymmetry was defined as a function of the load origin relative to the sagittal plane of the body. Lifts occurred at 0, 30, or 60 degrees off the sagittal plane on both sides of the body (lifting from the right and from the left relative to the sagittal plane). Ten subjects lifted a 13.7 kg box from one of these origins to a sagittally symmetric destination. Spinal loads were estimated through the use of a validated EMG-assisted model. Spine compression and lateral shear forces increased as the lift origin became more asymmetric. However, spinal compression and lateral shear increased by about twice the rate when lifting from origins to the left of the sagittal plane compared to lifting from origins to the right of the sagittal plane. Anterior-posterior spinal shear decreased as asymmetry increased with larger decreases occurring when lift origins occurred to the right of the sagittal plane. One-hand lifting changed the compression and shear profiles significantly. One-hand lifts using the hand on the same side of the body as the load resulted in compression forces that were approximately equal to those observed when lifting with two hands in a sagittally symmetric position. Anterior-posterior shear decreased and lateral shear increased under these conditions. These results reflect the trade-offs that must be considered among spinal forces during asymmetric lifting while using one or two hands. These findings have significant implications for task assessment interpretation and workplace design.

Trunk strength in combined motions of rotation and flexion/extension in normal young adults

Thirty-eight normal healthy young subjects (14 males, 24 females) with mean ages of 23 years (males) and 21 years (females), performed 36 functional rotational tasks of the trunk. The subject's lower extremities were stabilized in a stabilizing platform, allowing the entire motion of flexion-rotation and extension-rotation to take place in the trunk. Of these tasks, 18 were isometric and the other 18 were isokinetic. The isometric tasks consisted of flexion-rotation and extension-rotation from a 20 degrees, 40 degrees and 60 degrees flexed trunk in 20 degrees, 40 degrees and 60 degrees of axial rotation. The isokinetic activity consisted of flexion-rotation and extension-rotation from upright and flexed postures respectively in 20 degrees, 40 degrees and 60 degrees rotation planes at 15 degrees, 30 degrees and 60 degrees/s angular velocities. The results revealed that the males were significantly stronger than females (p < 0.01) and isometric activities produced significantly greater torque compared to isokinetic efforts (p < 0.01). The degree of trunk flexion was not significant; the angle of rotation, although significant, had only a small effect. The 60 degrees trunk rotation was significantly different from 20 degrees and 40 degrees of trunk rotation. The multiple regressions were all significant (p < 0.01); however, they predicted only 40 to 60% of the variations. Based on the results and analysis, it is suggested that the motion involved rather than the torque may have a consequential effect in the precipitation of back injuries.

Constrained testing conditions affect the axial rotation response of lumbar functional spinal units

STUDY DESIGN

Human cadaveric spine specimens were tested in axial rotation using constrained and unconstrained methods.

OBJECTIVES

To determine the degree to which constrained methods affect the response of the functional spinal unit in axial rotation at lumbar and lumbosacral levels.

SUMMARY OF BACKGROUND DATA

A substantial controversy exists in the literature regarding the appropriateness of different testing methods. No study has been found in which the effect of constraint on axial rotation behavior was objectively examined.

METHODS

Ten human cadaveric spine specimens (five L3-L4, five L5-S1) were tested in axial rotation, using both constrained and unconstrained methods. In the unconstrained test, pure moments were applied to the upper vertebra, and its complete three-dimensional motion was measured using an optoelectronic camera system. In the constrained test, the specimens were loaded in a fixed-axis servohydraulic test machine individually around five rotational axis positions within the vertebral body, and the rotational motion was measured.

RESULTS

The rotational angles in the constrained tests were not different among the five rotational axis positions. However, the maximum rotation from the five axis positions was approximately 40% greater than the minimum rotation, a significant difference. The axial rotational motion of the unconstrained tests was always less than the maximum rotation measured in the constrained test. However, the total rotational angle using the helical axis of motion was not significantly different from the constrained angles.

CONCLUSIONS

The large differences between maximum and minimum rotation angles demonstrate that the behavior of the functional spinal unit in axial rotation is sensitive to the axis's position but the location of the axis is not repeatable. This supports the use of unconstrained methods in spinal testing.

The role of complex, simultaneous trunk motions in the risk of occupation-related low back disorders

STUDY DESIGN

Simultaneous trunk kinematic variables of industrial workers performing jobs with varying degrees of low back disorder risk were quantified, by using a three-dimensional electrogoniometer.

OBJECTIVES

To assess the distinguishing patterns of simultaneous multidimensional (complex) motion parameters of workers performing manual material handling jobs with varying degrees of low back disorder risk.

SUMMARY OF BACKGROUND DATA

There is significant epidemiologic and biomechanical evidence that implicates simultaneously occurring or combined motions and loading as important risk factors follow back disorder. However, the specific levels or magnitudes and patterns of these complex motions at which risk of low back disorder is increased are still unknown.

METHODS

An industrial database of 126 workers and jobs was used to quantify the complex trunk motions of groups with varying degrees of low back disorder risk. Three groups, low-, medium-, and high-risk, were defined on the basis of retrospective injury records of the corresponding jobs. The jobs were further classified into five cells of weight-lift rate combinations. Within each weight-lift rate cell, the three-dimensional trunk motion patterns of workers were analyzed. Bivariate distributions and cumulative distribution functions were used to compare the simultaneous occurrence of complex dynamic motions among risk groups.

RESULTS

High- and medium-risk groups exhibited complex trunk motion patterns involving high magnitudes of combined velocities, especially at extreme sagittal flexion; whereas the low-risk group did not. Postural trunk information alone did not provide a consistent pattern of distinguishing among risk groups.

CONCLUSIONS

Elevated levels of complex simultaneous velocity patterns were unique to groups with increased low back disorder risk. Knowledge of these complex trunk velocity patterns in combination with key workplace factors provides a more sensitive means for identifying low back disorder occupational risk factors than does mere postural information.

Torque resistance of the passive tissues of the trunk at axial rotation

Sitting in a twisted working posture is common when driving an agricultural tractor in the field. To be able to twist backwards, the driver has to overcome the torque from the passive tissues (passive resistance). The purpose of this investigation was to determine the relationship between passive resistance at axial rotation and twisting angle of the trunk when a person is seated. Ten healthy tractor drivers and 10 healthy office workers were passively twisted about the cranio-sacral axis with simultaneous measurements of the torque needed to twist them and the twisting angle. Surface electromyogram was used to control the relaxation of the muscles of the subjects. An exponential function was fitted to the data, which shows that passive resistance increases progressively with twisting angle. No significant differences in trunk passive resistance due to the subject's occupation, direction of twist or their interactions could be found. The fitted function is useful when assessing the load in the trunk in tractor driving, but also for other occupational activities where the trunk is twisted.

Prediction of load sharing among spinal components of a C5-C6 motion segment using the finite element approach

STUDY DESIGN

A finite element model of the ligamentous cervical spinal segment was used to compute loads in various structures in response to clinically relevant loading modes.

OBJECTIVE

To predict biomechanical parameters, including intradisc pressure, tension in ligaments, and forces across facets that are not practical to quantify with an experimental approach.

SUMMARY OF BACKGROUND DATA

Finite element models of the cervical spine in their present form, because of inherent assumptions and simplifications, are not entirely satisfactory for studying the biomechanics of the intact, injured, and stabilized cervical spinal segment.

METHODS

A three-dimensional finite element model of a C5-C6 motion segment was developed from serial computed tomographic scans of a ligamentous cervical spinal segment. This model included nonlinear ligament definition, fully composite intervertebral disc, fluid nucleus, and Luschka's joints. The model-based displacement predictions were in agreement with the experimental data. This model was used to predict load sharing and other related parameters in spinal elements in response to various loading modalities.

RESULTS

In axial compression, 88% of the applied load passed through the disc. The interspinal ligament experienced the most strain (29.5%) in flexion, and the capsular ligaments were strained the most (15.5%) in axial rotation. The maximum intradisc pressure was 0.24 MPa in the flexion with axial compression mode (1.8 Nm + 73.6 N). The anterior and posterior disc bulges increased with the increase in axial compression (up to 800 N).

CONCLUSIONS

The results provide new insight into the role of various elements in transmitting loads. The model represents significant and essential advancement in comparison with previous finite element models, making it possible for such models to be used in investigating a broad spectrum of clinically relevant issues.

An assessment of complex spinal loads during dynamic lifting tasks

STUDY DESIGN

An electromyogram-assisted free-dynamic lifting model was used to quantify the patterns of complex spinal loads in subjects performing various lifting tasks.

OBJECTIVES

To assess in vivo the three-dimensional complex spinal loading patterns associated with high and low risk lifting conditions that matched those observed in industrial settings.

SUMMARY OF BACKGROUND DATA

Combined loading on the spine has been implicated as a major risk factor in occupational low back disorders. However, there is a void in the literature regarding the role of these simultaneously occurring complex spinal loads during manual lifting.

METHODS

Eleven male subjects performed symmetric and asymmetric lifting tasks with varying speed and weight. Reactive forces and moments at L5-S1 were determined through the use of electrogoniometers and a force plate. An electromyogram-assisted model provided the continuous patterns of three-dimensional spinal loads under these complex lifting tasks.

RESULTS

The results showed that complex dynamic motions similar to those observed in risky industrial tasks generated substantial levels of combined compressive and shear loads. In addition, higher loading rates were observed under these conditions. Unlike loading magnitudes, loading rate was a better indicator of dynamic loading because it incorporated both the duration and magnitude of net muscle forces contributing to total spinal loading during the lifting conditions.

CONCLUSIONS

Quantification of spinal combined motions and loading in vivo has not been undertaken. This study provided a unified assessment of the effects of combined or coupled motions and moments in the internal loading of the spine. Dynamic lifting conditions similar to those observed in risky industrial situations generated unique complex patterns of spinal loading, which have been implicated to pose a higher risk to the spinal structure. The higher predicted loading and loading rate during asymmetric lifting conditions can be avoided by appropriate ergonomic workplace modifications.

Frozen storage affects the compressive creep behavior of the porcine intervertebral disc

STUDY DESIGN

A biomechanical study of the compressive creep behavior of the porcine intervertebral disc before and after frozen storage.

OBJECTIVE

To determine whether frozen storage alters the creep response, hydration, and nuclear swelling pressure of the intact intervertebral disc.

SUMMARY OF BACKGROUND DATA

The mechanical response of the disc is dominated by swelling and fluid flow, whose effects are time-dependent. Because fluid content, which may change during storage, plays a significant role in the disc's time-dependent behavior, changes in mechanical response due to freezing may have been missed in previous studies that focused on time-independent behavior only.

METHODS

Porcine intervertebral discs were tested in repeated cycles of compressive creep either immediately postmortem or after 3 weeks of frozen storage. Swelling pressure and nuclear hydration were also measured in fresh and frozen discs. A fluid transport model was used to analyze the creep data.

RESULTS

The creep behavior of the intact porcine intervertebral disc is dramatically affected by frozen storage. The apparent permeability of the frozen discs was 82% higher than that of the fresh discs, and the swelling pressure of frozen discs was 25% lower in frozen discs (P < 0.01). The behavior of fresh and frozen discs became more dissimilar with repeated cycles of creep.

CONCLUSIONS

In vitro tests of frozen porcine intervertebral discs do not represent fresh behavior. Frozen storage appears to permanently alter disc behavior. The precise nature of any freezing-induced damage, and whether frozen storage similarly affects human discs, remains to be seen.

Development of a vertebral endplate 3-D reconstruction technique

The increase of low back problems has stimulated the development of different analysis and evaluation techniques. Among these methods, the direct linear transformation (DLT) technique is commonly used to reconstruct the spine in three dimensions by means of its known image coordinates on radiographs. Despite its efficiency and precision, general reconstruction of some standard anatomical landmarks (7-11) does not give all the necessary data for a detailed analysis of the intrinsic geometrical characteristics of lumbar vertebrae. Thus, in order to obtain such geometrical information a three-dimensional (3-D) reconstruction vertebral endplate contour technique has been developed. This technique involves 1) iterative optimization and reconstruction processes of the vertebral endplate centroid and 2) 3-D reconstruction of vertebral endplate contour. Validation based on mathematical simulations demonstrated that two or three iterations are necessary to correct (within 2 mm) the endplate centroid position for simulated error higher than 10 mm. Other validations based on 3-D reconstructions of a chamfered tube and a dry vertebra contours of known dimensions have given mean errors of 2 mm. Application on a healthy subject demonstrated the potential of this 3-D reconstruction technique. Finally, 3-D data obtained on vertebral endplates would allow the development of new clinical measurements that could be used to evaluate the lumbar spine geometrical behavior and orthoses biomechanical effects.

Differences in low back load between kneeling and seated working at ground level

The effects of the load on the low back when working in a kneeling posture were compared to those when working on a chair designed to alleviate work load in harvesting radish. In 10 male subjects data regarding back muscle fatigue and length changes of the spine, and estimates of experienced discomfort were collected. The results show that for all three effects of back load working on the chair leads to lower levels as compared to working kneeling. However, back load and discomfort were lower when working on the chair, back discomfort still increased substantially during the work on the chair. It is thus a sub-optimal solution from an ergonomic point of view, but at present it could be recommended to allow for variation with the conventional working method.

A three-dimensional parameterized finite element model of the lower cervical spine. Study of the influence of the posterior articular facets

In this study, we present a three-dimensional geometrical and mechanical finite element model of the complete lower cervical spine. The geometry of the vertebrae is parameterized which allows the model to fit different morphologies of vertebrae. The results obtained with a reduced functional unit model (without posterior arch) and with a complete functional unit model were compared with those obtained from experimental studies, when moments of flexion, extension lateral flexion and axial torque were applied. General agreement was observed. Since the model was parameterized, it was possible to study the influence of some geometrical parameters on the mechanical behavior of the cervical spine. Particularly, we focused on the influence of the posterior articular facets as their geometry is very different from those of the other spinal levels and as large inter-individual variability can be observed. The orientation of the facets with regard to the horizontal plane appeared to have a large influence on the 'coupled rotation to principal rotation' ratio, notably in lateral flexion.

Spine loading during trunk lateral bending motions

Increases in lateral trunk velocities have been identified as a mechanism for increasing the risk of low-back disorder. Previous studies have identified an increase in coactivation of the trunk musculature during lateral bends, but no studies have evaluated how spine loading changes as lateral trunk velocity increases. Twelve subjects were asked to lift loads laterally at one static and three dynamic velocities. Ten trunk muscle activities and trunk kinematics were documented and used as input parameters to an EMG-assisted model to evaluate spine loading. Muscle coactivation was observed in all lateral bends. Coactivation significantly increased during dynamic trials compared to the static trials. Coactivity increased spinal loads by as much as 25% compared to values predicted by models that did not consider coactivity. Movements to the right significantly increased spine loadings (252 N increase in compression) compared to movements to the left. Spine compression, A-P shear, and lateral shear all increased in the dynamic trials compared to the static conditions. Peak compression increased by an average of 525 N at 45 degrees s-1 compared to static loading. Compression and lateral shear increased monotonically as trunk velocity increased. It is expected that this combined (compression and lateral shear) loading is the mechanism for increased risk observed in industry.

Asymmetry of erector spinae muscle activity in twisted postures and consistency of muscle activation patterns across subjects

STUDY DESIGN

An experimental study involving a repeated measures design was performed.

OBJECTIVES

To describe the muscle activation pattern in extending from a twisted posture; to evaluate interindividual variation of this pattern; and to study the effects of fatigue development.

SUMMARY OF BACKGROUND DATA

Extending the trunk from a twisted posture is a risk factor for low back pain.

METHODS

Twelve male subjects performed contractions at 20%, 40%, 60%, and 100% of maximum in a neutral position and 15 degrees, 30 degrees, and 45 degrees twisted posture. Surface electromyography of six bilateral back muscle tracts was recorded during these 5-second contractions and during sustained contractions at 0 degree and 30 degrees and 40% of maximum.

RESULTS

The data show an asymmetry in muscle activation, increasing with angle of twist. Activity was higher contralateral to the direction of twisting. Linear regression analysis with angle and torque level against electromyographic amplitude yielded a significant fit in 92% of the data sets. In 75% of the subjects, the regression lines were similar. The divergence of the activation pattern in the other subjects appears to be explained by a higher co-contraction level of abdominal muscles. Fatigue caused a shift in activity to the more laterally situated muscle tracts.

CONCLUSIONS

Twisting causes an asymmetry in back muscle activation. Resulting stress concentrations in spinal motion segments, and a reduced spinal stability could underlie the observed relationship between the task investigated and low back pain. Fatigue may reduce further spinal stability. Some subjects display a divergent activation pattern, which probably involves higher spinal compression forces.

A model of spine, ribcage and pelvic responses to a specific lumbar manipulative force in relaxed subjects

One class of manipulative techniques commonly used during assessment and treatment of spinal disorders involves the patient lying face down while the therapist slowly applies a posteroanterior force to a selected vertebra. The aim of this investigation was to develop a model which was capable of predicting the vertebral displacements resulting from such a manipulative force, applied to the lumbar spine. A linear three-dimensional finite element model was generated using both previously published and original data to define the geometry and material properties. The complete model included the ribcage, thoraco-lumbar spine and pelvis with their associated soft tissues. The model simulated the relaxed state in a normal subject so the muscle forces were assumed to be negligible. Sensitivity analysis suggested that if the model was to be used to simulate the behaviour of individual subjects, then the model dimensions and pelvic constraints should be matched to the particular subject. The model validity was studied by comparing the predicted responses with those that have been observed in living human subjects. The model predictions were found to be in good agreement with the mean observed human responses, with predicted displacements being within one standard deviation of the mean observed values. This agreement suggests that the model is useful for predicting the linear region responses to slowly applied lumbar posteroanterior forces. The simulations predicted that appreciable global vertebral displacements (up to 1.5 mm) and rotations (up to 1 degree) occurred as far away as the middle and lower thoracic spine during low lumbar loading. Intervertebral translations were predicted to be 1 mm or more at up to four intervertebral joints away from the point of load application.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanical risk factors for occupationally related low back disorders

A continuing challenge for ergonomists has been to determine quantitatively the types of trunk motion and how much trunk motion contributes to the risk of occupationally-related low back disorder (LBD). It has been difficult to include this motion information in workplace assessments since the speed at which trunk motion becomes dangerous has not been determined. An in vivo study was performed to assess the contribution of three-dimensional dynamic trunk motions to the risk of LBD during occupational lifting in industry. Over 400 industrial lifting jobs were studied in 48 varied industries. The medical records in these industries were examined so that specific jobs historically categorized as either low, medium, or high risk for occupationally-related LBD could be identified. A tri-axial electrogoniometer was worn by workers and documented the three-dimensional angular position, velocity, and acceleration characteristics of the lumbar spine while workers worked at these low, medium, or high risk jobs. Workplace and individual characteristics were also documented for each of the repetitive lifting tasks. A multiple logistic regression model indicated that a combination of five trunk motion and workplace factors predicted well both medium risk and high risk occupational-related LBD. These factors included lifting frequency, load moment, trunk lateral velocity, trunk twisting velocity, and trunk sagittal angle. Increases in the magnitude of these factors significantly increased the risk of LBD. The analyses have enabled us to determine the LBD risk associated with combined changes in the magnitudes of the five factors. The results indicate that by suitably varying these five factors observed during the lift collectively, the odds of high risk group membership may decrease by over ten times. These results were related to the biomechanical, ergonomic, and epidemiologic literature. The five trunk motion and workplace factors could be used as quantitative, objective measures to redesign the workplace so that the risk of occupationally-related LBD is minimized.

Distraction and compression loads enhance spine torsional stiffness

We have assessed the degree to which compression and distraction forces applied to the spine of the magnitude achievable through instrumentation systems increase torsional stiffness, thereby possibly enhancing the environment for arthrodesis. A functional relationship between spine torsional stiffness and axial compression and distraction forces is described. To establish this relationship whole human thoracolumbar spines as well as individual motion segments were tested in torsion under a variety of axial loading conditions consistent with forces applied clinically. These studies indicate that applying axial loads to the whole thoracolumbar spine through the action of compression or distraction increases the stiffness of the spine in torsion. Compression and distraction forces increased the torsional stiffness of thoracic segments, but only compression forces were found to significantly increase the stiffness of lumbar segments. The soft tissue structures of both the anterior and the posterior columns have the ability to increase torsional stiffness with axial loading.

Influence of geometrical factors on the behavior of lumbar spine segments: a finite element analysis

The main objective of this study was the assessment of the influence of geometrical factors on the behavior of lumbar segments. To this end, a three-dimensional, parameterized, finite element model of the lumbar spine was used, and the results were compared with inhouse experimental results and with the few published experimental results available concerning either the geometry of the tested samples or the differences observed at different vertebral levels. Furthermore, in order to appreciate the relative importance of the geometry, the influence of the variation of some other parameters was studied, such as the orientation of the facet joints, the gap between the articular processes, and the Young's modulus of the disk fibers. As a first approach, a series of computations was carried out in order to evaluate the role of geometry in the mechanical behavior differences observed at different levels. It has been found that geometrical factors do exert a noticeable influence on the behavior of the spine, especially those which interfere with the dimensions of the intervertebral disk.

Mechanical behavior of the human lumbar and lumbosacral spine as shown by three-dimensional load-displacement curves

The lumbar region is a frequent site of spinal disorders, including low-back pain, and of spinal trauma. Clinical studies have established that abnormal intervertebral motions occur in some patients who have low-back pain. A knowledge of normal spinal movements, with all of the inherent complexities, is needed as a baseline. The present study documents the complete three-dimensional elastic physical properties of each lumbar intervertebral level from the level between the first and second lumbar vertebrae through the level between the fifth lumbar and first sacral vertebrae. Nine whole fresh-frozen human cadaveric lumbar-spine specimens were used. Pure moments of flexion-extension, bilateral axial torque, and bilateral lateral bending were applied, and three-dimensional intervertebral motions were determined with use of stereophotogrammetry. The motions were presented in the form of a set of six load-displacement curves, quantitating intervertebral rotations and translations. The curves were found to be non-linear, and the motions were coupled. The ranges of motion were found to compare favorably with reported values from in vivo studies.

Nonlinear stress analysis of the whole lumbar spine in torsion--mechanics of facet articulation

The response of the whole ligamentous lumbar spine in axial torsion is studied. Attention is focused on the inter-segmental variations, role of articular facets, presence of coupled movements, intervertebral stresses and the effects of a structural alteration at a level on the response. A detailed three-dimensional finite element model (L1-S1) was used for nonlinear stress analyses under torques of up to 15 N m. In right axial torque, extra cases of larger gap limit for more effective articulation, of loss of L4-L5 disc fluid content, and of removal of L4-L5 compression facet were also studied. The intersegmental results varied from one level to the next. The facet contact occurred at the lateral posterior regions of articular surfaces. The interference gap distance between facet articular surfaces, and not the orientation of articular surfaces, was the primary factor in the relative effectiveness of facets in resisting torsion. Coupled motions of upward translation and flexion rotation were noted at all levels under torques in both left and right directions. Loss of disc fluid content at the L4-L5 level resulted in larger facet contact force and axial rotation at the same level. Removal of the L4-L5 compression facet substantially increased the axial rotation, intradiscal pressure, maximum fibre strain, and strains in capsular ligaments at the same L4-L5 level. In axial torque, the disc failure appeared to be unlikely. Relatively large facet forces were generated that further increased with a loss in disc fluid content.

An EMG-assisted model of loads on the lumbar spine during asymmetric trunk extensions

An EMG-assisted, low-back, lifting model is presented which simulates spinal loading as a function of dynamic, asymmetric, lifting exertions. The purpose of this study has been to develop a model which overcomes the limitations of previous models including static or isokinetic mechanics, inaccurate predictions of muscle coactivity, static interpretation of myoelectric activity, and physiologically unrealistic or variable muscle force per unit area. The present model predicts individual muscle forces from processed EMG data, normalized as a function of trunk angle and asymmetry, and modified to account for muscle length and velocity artifacts. The normalized EMGs are combined with muscle cross-sectional area and intrinsic strength capacity as determined on a per subject basis, to represent tensile force amplitudes. Dynamic internal and external force vectors are employed to predict trunk moments, spinal compression, lateral and anterior shear forces. Data from 20 subjects performing a total of 2160 exertions showed good agreement between predicted and measured values under all trunk angle, asymmetry, velocity, and acceleration conditions. The design represents a significant step toward accurate, fully dynamic modeling of the low-back in multiple dimensions. The benefits of such a model are the insights provided into the effects of motion induced, muscle co-activity on spinal loading in multiple dimensions.