Finite element modeling and simulation of degeneration and hydrotraction therapy of human lumbar spine segments

A large percent of population is affected by low back pain problems all over the world, starting from the degeneration of the lumbar spinal structure, caused generally by ageing and mechanical overloading. If the degeneration is not too advanced, surgical treatments can be avoid, by applying conservative treatments, like traction therapies. Dry traction is a well-known method, however, often happens that instead of the traction effect and stress relaxation, the compression increases in the discs due to muscle activities. This verifies the importance of the suspension hydro-traction therapy, where the muscles are completely relaxed. The aim of this study was doubled: to model and simulate numerically the age-related and accidental degenerations of lumbar functional spinal units (FSU) and to simulate the mechanical answer of the more or less degenerated lumbar segments for the hydro-traction treatment, by using FE method. The basic question was: how to unload the disc to regain or improve its functional and metabolic ability. FE simulations of the mechanical behaviour of human lumbar FSUs with life-long agerelated and sudden accidental degenerations are presented for tension and compression. Compressive material constants were obtained from the literature, tensional material moduli were determined by parameter identification, using in vivo measured global elongations of segments as control parameters. 3D FE models of a typical FSU of lumbar part L3-S1 were developed extended to several nonlinear and nonsmooth unilateral features of intervertebral discs, ligaments, articular facet joints and attachments. The FE model was validated both for compression and tension, by comparing the numerical calculations with experimental results. The weightbath hydrotraction therapy decreases pain, increases joint flexibility, and improves the quality of life of patients with cervical or lumbar discopathy. Numerical simulations were investigated to clear the biomechanical effects of hydrotraction treatment of more or less degenerated segments to improve the efficiency of the non-invasive conservative treatment

Development of computerized human static strength simulation model for job design

This article describes the development of models to predict population static strengths and low back forces resulting from common manual exertions in industry. The resulting biomechanical models are shown to be valid for their intended purposes, but limitations still exist. In particular, they are meant to aid in evaluating very slow or static exertions, such as when carefully lifting, pushing, or pulling on heavy objects, but do not allow dynamic exertions to be simulated. It is shown that use of these models in the early design of workplaces and equipment is dependent on the use of computerized homonoids and behavioral-based inverse kinematic algorithms in conjunction with CAD systems. © 1997 John Wiley & Sons, Inc.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/35211/1/3_ftp.pd

Biomechanical factors in the progression of idiopathic scoliosis

Idiopathic scoliosis is present when, in upright positions of the trunk, the spine curves to the side for unknown reasons. This paper reviews evidence concerning some biomechanical factors that might underlie the progression of such curves. The review concentrates on studies conducted in our laboratories. Arguments are made, based on biomechanical analyses and experiments, that progression occurs because of defects in the postural control system of the spine.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43994/1/10439_2006_Article_BF02371453.pd

Biomechanical study of intervertebral disc degeneration

Degeneration and age affect the biomechanics of the intervertebral disc, by reducing its stiffness, flexibility and shock absorption capacities against daily movement and spinal load. The biomechanical characterization of intervertebral discs is achieved by conducting mechanical testing to vertebra-disc-vertebra segments and applying axial, shear, bend and torsion loads, statically or dynamically, with load magnitudes corresponding to the physiological range. However, traditional testing does not give a view of the load and deformation states of the disc components: nucleus pulposus, annulus fibrosus and endplate. Thus, the internal state of stress and strains of the disc can only be predicted by numerical methods, one of which is the finite element method. The objective of this thesis was, to study the biomechanics of degenerated intervertebral discs to load conditions in compression, bending and torsion, by using mechanical testing and a finite element model of disc degeneration, based on magnetic resonance imaging (MRI). Therefore, lumbar discs obtained from cadavers corresponding to spinal levels L2-L3 and L4-L5 with mild to severe degeneration were used. Intervertebral osteochondrosis and spondylosis deformans were identified, being the disc space collapse, the most striking feature. Next, all discs were tested to static and dynamic load conditions, the results gained corresponded to the disc stiffness (in compression, bending and torsion), stress relaxation and dynamic response. Of these, the stiffness response was used to validate the disc model. The testing results suggest that discs with advanced degeneration over discs with mild degeneration are, less rigid in compression, less stiffer under bending and torsion, showed less radial bulge, and reduce their viscoelastic and damping properties. This study shows that degeneration has an impact on the disc biomechanical properties which can jeopardize normal functionality. Development of one finite element model of disc degeneration started by choosing a MRI of a L2-L3 disc. Segmentation of vertebra bone and disc materials followed, and were based on pixel brightness and radiology fundamentals, then a finite element mesh was created to account for the disc irregular shape. The disc materials were modeled as hyperelastic and the bone materials were modeled as orthotropic and isotropic. Adjustment of material properties was based on integrity of the annulus fibrosus, giving a stiffness value matching that of a mild degeneration disc. Then, validation of the model was performed, and included a study of the distributions of stress and strain under loads of compression, bending and torsion. The results from all load simulations show that the disc undergoes large deformations. In contrast, the vertebrae are subjected to higher stress but with negligible deformations. In compression, the model predicted formation of symmetrical disc bulge which agree with the testing behavior. The nucleus pulposus showed to be the principal load carrier with negative principal stresses and strains. In bending and torsion, the annulus fibrosus showed to be the principal load carrier with large symmetrical principal strains and stresses for the former loading and large shearing for the latter. The study showed the importance of soft tissue deformation, mostly noticed in advanced degeneration. In contrast, the higher stresses in the vertebra over those of the intervertebral disc showed the relevance of bone predisposition to fracture. Such kind of studies, should contribute to the understanding of the biomechanics of the intervertebral disc.La degeneración y edad afectan la biomecánica del disco intervertebral, reduciendo la capacidad de rigidez, flexibilidad y atenuación de impactos, contra el movimiento y carga del raquis. La caracterización biomecánica del disco se realiza con ensayos mecánicos a segmentos de vértebra-disco-vértebra y aplicando cargas axiales, cortantes, flexión y torsión, estáticas ó dinámicas, con magnitudes de carga según el intervalo fisiológico. Sin embargo, las pruebas tradicionales no dan una visión de los estados de carga y deformación de los componentes del disco: núcleo pulposo, anillo fibroso y placa terminal. Por lo tanto, el estado interno de esfuerzos y deformaciones del disco, solo puede ser predicho con métodos numéricos, uno de los cuales es el método de elemento finito. El objetivo de esta tesis fue, estudiar la biomecánica de discos intervertebrales degenerados a las condiciones de carga en compresión, flexión y torsión, mediante el uso de ensayos mecánicos y de un modelo de elementos finitos de la degeneración de disco, basado en imágenes con resonancia magnética (MRI). Por lo tanto, se usaron discos lumbares L2-L3 y L4-L5 obtenidos de cadáveres, con degeneración leve a severa. Se identificó osteocondrosis intervertebral y espondilosis deformante, siendo el colapso del espacio intervertebral el aspecto más relevante. Luego, todos los discos fueron ensayados a condiciones de carga estática y dinámica, y los resultados correspondieron a la rigidez del disco (a compresión, flexión y torsión), a la relajación de tensiones y a la respuesta dinámica. De éstos, la rigidez fue usada para validar el modelo de disco. Los resultados de los ensayos sugieren que los discos con degeneración avanzada sobre aquellos con degeneración leve son, menos rigidos a compresión, menos rigidos a flexión y torsión, presentan menor protuberancia radial, y reducen sus propiedades viscoelásticas y de amortiguamiento. El estudio muestra que la degeneración impacta las propiedades biomecánicas del disco, poniendo en riesgo la funcionalidad normal. El desarollo de un modelo de elementos finitos de la degeneración de disco inició eligiendo una secuencia de resonancia magnética de un disco L2-L3. La segmentación de los materiales del disco y de las vértebras se realizó basado en intensidad de brillo del pixel y en fundamentos de radiología, y se creó una malla de elementos finitos correspondiente a la forma irregular del disco. Los materiales del disco se modelaron como hiperelásticos y los tejidos óseos se modelaron como materiales ortotrópicos e isotrópicos. El ajuste de propiedades de los materiales fue basado en la integridad del anillo fibroso, y dio una rigidez correspondiente a la de un disco con degeneración leve. Luego, se realizó la validación del modelo, e incluyó un estudio de las distribuciones de esfuerzo y deformación a las condiciones de carga en compresión, flexión y torsión. Los resultados de todas las simulaciones de carga mostraron que el disco es sometido a grandes deformaciones. En contraste, las vértebras fueron sometidas a mayores esfuerzos pero con deformaciones insignificantes. En compresión, el modelo predijo la formación de una protuberancia radial simétrica, en concordancia con la experimentación. El núcleo pulposo mostró ser el portador principal de carga, con tensiones y deformaciones principales negativas. En flexión y torsión, el anillo fibroso mostró ser el portador principal de carga, con grandes deformaciones y tensiones principales simétricas para la primera carga, y con grandes tensiones cortantes para la segunda carga. El estudio mostró la importancia de las deformaciones de los tejidos blandos, principalmente notados en la degeneración avanzada. Por el contrario, las tensiones mayores en los cuerpos vertebrales sobre aquellas del disco intervertebral mostraron la relevancia de la predisposición a las fracturas óseas. Este tipo de estudio debe contribuir a la comprensión de la biomecánica del disco intervertebral

Fatigue Effects on Manual Lifting Acceleration.

The objective of this research was to study the effects of fatigue on lifting acceleration and posture caused by repetitive manual lifting. Electromyogram, oxygen consumption and heart rate were used as indices of a subject\u27s physiological fatigue. Data were collected for a period of two hours of continuous lifting, at a rate of 4 lifts per minute using the free-style posture. A box of fixed dimensions (30cm x 30cm x 20cm) and fixed weight (15.9 Kg) was used as load. It was found that the average load acceleration increased with work duration. Peak acceleration was found to be maximum in the initial lifting period of zero to 0.3 seconds. Back compressive force was highest at the beginning of a lifting cycle. It was found that the shorter subjects\u27 lifting technique was more stable than taller subjects\u27. It was also found that lifting posture changed from stoop to squat and then back to stoop during two hours of lifting. EMG analysis showed that arms were stressed most during the initial lifting phase (that is in the stoop position). EMG level of a leg was higher when the squat posture was used

Biomechanical Response of Trunk Under Perturbations and in Challenged Seated Balance

RÉSUMÉ Les maux de dos sont reconnus comme un problème de santé répandu et ayant un grand impact socio-économique. Des programmes de prévention, de réadaptation et de traitement devraient être fondés sur une bonne compréhension des fonctions neuro-biomécaniques de la colonne vertébrale dans des conditions normales et de blessure. En raison de difficultés techniques, de coûts excessifs et des enjeux éthiques aux mesures in vivo et in vitro, la modélisation biomécanique a été reconnue comme un outil complémentaire et puissant à cet égard. Une perturbation du tronc qui peut se produire lors du chargement/déchargement soudain ou le déplacement rapide du tronc (ex. chutes et glissades) a été identifié comme un facteur de risque de mal de dos. Les effets d'inertie ainsi que les grandes réponses réflexives des muscles ont le potentiel de générer des charges vertébrales excessives qui peuvent causer des blessures à la colonne vertébrale. En effet, le risque de blessure augmente dans les cas ayant une faible marge de stabilité ou une réponse réflexive neuromusculaire altérée. La rigidité intrinsèque des tissus passifs et des muscles préactivés ainsi que les réponses réflexives de ces muscles permettraient d’améliorer la stabilité et l'équilibre du tronc. Les charges produisant la perturbation, les conditions préexistantes ainsi qu'un dysfonctionnement dans le contrôle des rigidités intrinsèques et réflexives influencent les forces musculaires et les charges imposées sur la colonne vertébrale. L’équilibre postural du tronc en position assise instable a été suggérée autant en réadaptation que pour l'étude des mécanismes de contrôle neuromusculaire de la colonne vertébrale. L'avantage de cette tâche ou exercice pour l'étude de la stabilité de la colonne vertébrale est qu’elle permet d’éliminer l’apport des membres sur le contrôle de la colonne vertébrale en positon assise. En d’autres mots, seul le mouvement de la colonne lombaire permet de rétablir l’équilibre, ce qui en fait une tâche très spécifique. Puisque les réponses réflexes des muscles sont essentielles au contrôle de la colonne vertébrale, un contrôle altéré en raison de la douleur ou d’un dysfonctionnement neuromusculaire peut causer de plus grandes forces réflexives et charges sur la colonne vertébrale qui peuvent augmenter le risque de blessure. Il est important d’estimer les charges sur la colonne vertébrale afin d'évaluer la sécurité relative de cette tâche autant chez des sujets sains que chez des sujets lombalgiques.----------ABSTRACT Back pain is known as a prevalent health crisis with large socioeconomic impact on societies. Effective prevention, rehabilitation and treatment programs should be founded on solid understanding of the spine functional neuro-biomechanics in normal and injured conditions. Due to technical difficulties, excessive cost and ethical concerns with in vivo and in vitro measurements, in silico biomechanical modeling has been recognized as a complementary and powerful tool in this respect. Trunk perturbation that may happen in sudden loading/unloading or rapid displacement of trunk (during falls and slips for example) has been found as a risk factor for back pain. Inertial effects as well as large reflexive response of muscles could generate excessive spinal loads that may cause back pain or spinal injuries. The risk of injury further increases in cases with low margin of stability or impaired neuromuscular reflex response. Intrinsic stiffness of passive tissues and active muscles along with muscles reflexive responses have been suggested as the mechanisms that enhance stability and balance of the trunk. Perturbation load, pre-perturbation conditions as well as dysfunction in any of active-passive mechanisms alter their contributions in balance control and as a result influence muscle forces and spinal loads. The risk of pain and injury likely increases as well. Challenged sitting has been suggested as an approach for therapeutic applications as well as investigation of the neuromuscular control mechanisms of spine. The advantage of this method in studying spine stability is in eliminating the effect of lower extremities that are fixed to the seat on the control of the spine in seated subjects. Since muscle reflex responses are essential in control of the trunk, impaired control due to pain or neural dysfunction cause larger reflexive forces and spinal loads that may increase the risk of injury and pain. It is important to estimate the spinal loads in order to assess the relative safety of a task for both healthy and chronic low back pain (CLBP) groups. Three objectives are set in this study. The first one is to determine the effect of pre-perturbation conditions and perturbation load magnitude on the spine biodynamics response subject to sudden loads

AN EMG OPTIMIZATION MODEL OF THE KINETIC DEMANDS ON THE LOWER BACK DURING ASYMMETRICAL GAIT AND LOAD CARRIAGE

Gait asymmetries are associated with a high incidence of lower back pain (LBP). Although there are several causes of gait asymmetry (i.e. amputation, injury, or deformities), lower back kinetic demands have not been quantified and suitably compared due to experimental limitations in these clinical populations. Further, the impact of gait asymmetry on lower back demands during carrying tasks has not been established. This dissertation addressed these issues by artificially and safely inducing gait asymmetry in healthy able-bodied participants during walking and carrying tasks. LBP risk was assessed by L5/S1 vertebral joint force levels estimated with an OpenSim musculoskeletal model of the lower back adapted to incorporate participant-specific responses using an EMG optimization approach. The model was evaluated systematically for force estimate efficacy and sensitivity to input parameters prior to gait asymmetry assessments. Twelve participants performed walking and carrying tasks on a treadmill at individually scaled speeds while kinematics, external kinetics, and muscle activities (EMG) were recorded. Walking conditions consisted of unperturbed symmetrical gait, and asymmetrical gait induced by perturbing the right leg with a 2.54 cm shoe leveler, ~1 kg ankle weight, combined weight and shoe leveler, or a clinical walking boot that restricted ankle joint motion and added mass. Load carrying was performed while holding 7.5% and 15% bodyweight dumbbells in one or two hands during symmetric gait and asymmetric gait induced by the walking boot. The perturbations were successful in producing different degrees of gait asymmetry. However, L5/S1 joint forces were not significantly different between conditions despite unique spatiotemporal asymmetries. This indicates that LBP in those with gait asymmetry may not be due solely to level planar walking. During carrying tasks, gait asymmetry induced by the walking boot increased some metrics of lower back loading. Further, carrying a load in the hand contralateral to the walking boot produced larger forces than when carried on the same side. These results emphasize the importance of evaluating specific sources of gait asymmetry during daily activities other than walking when assessing LBP risk and would encourage more inclusive ergonomic carrying guidelines