Evaluation of a patient-specific finite-element model to simulate conservative treatment in adolescent idiopathic scoliosis

PublishedJournal ArticleAuthor's accepted manuscript.Study design: Retrospective validation study. Objectives: To propose a method to evaluate, from a clinical standpoint, the ability of a finite-element model (FEM) of the trunk to simulate orthotic correction of spinal deformity and to apply it to validate a previously described FEM. Summary of background data: Several FEMs of the scoliotic spine have been described in the literature. These models can prove useful in understanding the mechanisms of scoliosis progression and in optimizing its treatment, but their validation has often been lacking or incomplete. Methods: Three-dimensional (3D) geometries of 10 patients before and during conservative treatment were reconstructed from biplanar radiographs. The effect of bracing was simulated by modeling displacements induced by the brace pads. Simulated clinical indices (Cobb angle, T1-T12 and T4-T12 kyphosis, L1-L5 lordosis, apical vertebral rotation, torsion, rib hump) and vertebral orientations and positions were compared to those measured in the patients' 3D geometries. Results: Errors in clinical indices were of the same order of magnitude as the uncertainties due to 3D reconstruction; for instance, Cobb angle was simulated with a root mean square error of 5.7°, and rib hump error was 5.6°. Vertebral orientation was simulated with a root mean square error of 4.8° and vertebral position with an error of 2.5 mm. Conclusions: The methodology proposed here allowed in-depth evaluation of subject-specific simulations, confirming that FEMs of the trunk have the potential to accurately simulate brace action. These promising results provide a basis for ongoing 3D model development, toward the design of more efficient orthoses.ParisTech BiomecAM chair programProteorParisTechYves Cotrel Foundation

Biomechanical analysis and modeling of different vertebral growth patterns in adolescent idiopathic scoliosis and healthy subjects

<p>Abstract</p> <p>Background</p> <p>The etiology of AIS remains unclear, thus various hypotheses concerning its pathomechanism have been proposed. To date, biomechanical modeling has not been used to thoroughly study the influence of the abnormal growth profile (i.e., the growth rate of the vertebral body during the growth period) on the pathomechanism of curve progression in AIS. This study investigated the hypothesis that AIS progression is associated with the abnormal growth profiles of the anterior column of the spine.</p> <p>Methods</p> <p>A finite element model of the spinal column including growth dynamics was utilized. The initial geometric models were constructed from the bi-planar radiographs of a normal subject. Based on this model, five other geometric models were generated to emulate different coronal and sagittal curves. The detailed modeling integrated vertebral body growth plates and growth modulation spinal biomechanics. Ten years of spinal growth was simulated using AIS and normal growth profiles. Sequential measures of spinal alignments were compared.</p> <p>Results</p> <p>(1) Given the initial lateral deformity, the AIS growth profile induced a significant Cobb angle increase, which was roughly between three to five times larger compared to measures utilizing a normal growth profile. (2) Lateral deformities were absent in the models containing no initial coronal curvature. (3) The presence of a smaller kyphosis did not produce an increase lateral deformity on its own. (4) Significant reduction of the kyphosis was found in simulation results of AIS but not when using the growth profile of normal subjects.</p> <p>Conclusion</p> <p>Results from this analysis suggest that accelerated growth profiles may encourage supplementary scoliotic progression and, thus, may pose as a progressive risk factor.</p

Étude biomécanique du traitement de la scoliose idiopathique par orthèse: effets des paramètres de conception des corsets sur les corrections géométriques et sur les contraintes internes du rachis.

RÉSUMÉ La scoliose est une déformation tridimensionnelle évolutive de la colonne vertébrale et de la cage thoracique. Pour des déformations modérées, le principal traitement utilisé est le traitement par corset. Son objectif est, à court-terme, de réduire les déformations scoliotiques et, à long-terme, d’en empêcher la progression. Toutefois le traitement par corset tel qu’il est effectué actuellement n’est pas optimal. La conception des corsets repose encore principalement sur des principes empiriques et l’expérience variée des orthésistes. Aucune étude, clinique ou numérique, n’a étudié directement l’effet des paramètres de conception d’un corset sur son efficacité. De nombreuses controverses existent encore de ce fait sur les paramètres de conception optimaux. De même, aucune étude, expérimentale ou numérique, n’a tenté de prouver que le traitement par corset permet de modifier favorablement les contraintes agissant sur les plaques de croissance d’un sujet scoliotique, démontrant ainsi de façon théorique l’efficacité du traitement à empêcher la progression des déformations. L’objectif général de ce projet est donc d’étudier l’effet du design des corsets sur la correction immédiate des déformations scoliotiques et sur les contraintes agissant sur les plaques de croissance. L’hypothèse que nous souhaitons vérifier est que le traitement par corset peut annuler l’asymétrie des contraintes de compression s’exerçant sur les plaques de croissance à l’apex des courbures scoliotiques mais que cet effet est dépendant des paramètres de conception du corset, ce qui nécessite un ajustement optimal. Cette étude a été divisée en 5 parties. Une méthode a tout d’abord été développée pour représenter les forces de gravité sur un modèle éléments finis (MEF) du tronc d’un patient scoliotique tout en respectant sa géométrie 3D. Un processus d’optimisation a permis de déterminer les forces à soustraire au MEF, dont la géométrie a été construit à partir d’une reconstruction 3D par radiographies biplanaires du patient, afin d’obtenir suite à l’application de la gravité un modèle correspondant à la géométrie réelle du patient. La différence entre la position 3D des vertèbres issue des radiographies et la position simulée des vertèbres du modèle EF après application de la gravité s’est avéré être inférieure à 3 mm. Les contraintes de compression et les moments d’inflexion latérale agissant sur les plateaux vertébraux ont été calculés. Il a été constaté que dans le plan frontal la concavité des courbures scoliotiques était soumise à des contraintes de compression moyennes supérieures de 0.1 à 0.4 MPa à celles de la convexité.----------ABSTRACT Scoliosis is defined as a three-dimensional deformity of the spine and rib cage. For moderate deformities, bracing is the most common treatment. Its aim is to reduce the scoliotic deformities in a short-term perspective and to prevent their progression in a long-term perspective. The brace treatment is however not optimal as it is practiced today. The braces design is mostly based on empirical principles and on the experience of the orthotists. The effects of the design parameters of a brace on its efficiency have never been studied, experimentally or numerically. As a consequence, the optimal brace design parameters are still controversial. No study demonstrated that the brace treatment modifies favorably the stresses in the vertebral growth plates of a scoliotic patient, proving thus that the brace treatment is theoretically efficient in preventing the scoliotic deformities from progressing. The objective of this project was consequently to study the effect of the brace design on the immediate correction of the scoliotic deformities and on the spinal stresses. The hypothesis we want to verify is that the brace treatment is able to nullify the asymmetry of the compressive stresses exerted on the growth plates at the apex of the scoliotic curves but this effect depends on the design parameters of the brace and an optimal adjustment is thus required. This study was divided into 5 parts. A simulation process was firstly developed to represent the gravity forces in a finite element model (FEM) of the trunk of a scoliotic patient. An optimization process computed the forces to substract to the FEM, based on the 3D reconstruction of biplanar x-rays of the patient, in order to obtain after the inclusion of the gravity forces a model corresponding to the actual geometry of the patient. The difference in the vertebral positions between the geometry acquired form radiographs and the computed geometry of the model including the gravity forces was inferior to 3 mm. The forces and compressive stresses in the scoliotic spine were then computed. An asymmetrical load in the coronal plane, particularly at the apices of the scoliotic curves, was present. Difference of mean compressive stresses between concavity and convexity of the scoliotic curves ranged between 0.1 and 0.4 MPa

Design, Optimization, and Evaluation of a Fusionless Device to Induce Growth Modulation and Correct Spinal Curvatures in Adolescent Idiopathic Scoliosis

RÉSUMÉ La scoliose est une déformation musculo-squelettique complexe et tridimensionnelle de la colonne vertébrale. Les mécanismes de progression de la scoliose sont liés au principe de Hueter-Volkmann. Selon cette théorie, les chargements asymétriques des plaques de croissance altèrent la croissance du rachis (cunéiformisation des vertèbres). Une courbure scoliotique présentant un angle de Cobb supérieur à 50° nécessite généralement une intervention chirurgicale avec fusion rachidienne. Cette chirurgie implique des procédures particulièrement invasives et coûteuses, ce qui a incité plusieurs chercheurs à tenter de développer d‘autres alternatives. Des techniques minimalement invasives et sans fusion ont ainsi été élaborées pour contrôler et corriger un mauvais alignement de la colonne vertébrale avant qu'une progression trop importante des déformations scoliotiques ne se produise. Ces techniques tentent d'exploiter la croissance vertébrale résiduelle afin de corriger la cunéiformisation locale et d‘aboutir à un réalignement progressif du rachis. Les traitements sans fusion semblent également mettre en péril la santé du disque intervertébral à long terme et se limitent à une correction 2D (plan frontal) de déformations intrinsèquement 3D. Mieux comprendre biomécaniquement la progression des déformations scoliotiques permettrait de développer des dispositifs sans fusion plus efficaces. Cela serait une contribution importante et innovatrice à l'amélioration du traitement de la scoliose idiopathique adolescente (SIA). L'objectif global de cette thèse était le développement, l‘optimisation, et l‘évaluation expérimentale d'implants sans fusion afin de moduler la croissance et de corriger les déformations scoliotiques. Les objectifs spécifiques étaient de 1) développer un modèle par éléments finis (MEF) de la colonne vertébrale intégrant une modélisation de la croissance; 2) exploiter ce MEF pour étudier les facteurs biomécaniques impliqués dans les mécanismes de progression de la SIA; 3) exploiter le MEF pour analyser la biomécanique des dispositifs sans fusion existant actuellement et repérer les améliorations pouvant être apportées à ces dispositifs; et 4) exploiter la plate-forme de conception conçue (analyses in silico, in situ, et in vivo) pour développer, optimiser, et valider de nouveaux dispositifs sans fusion modulateurs de croissance pour la correction des déformations de la SIA.----------ABSTRACT Scoliosis is a spinal musculoskeletal deformity defined by a 3D deformity of the spine. The pathomechanism of scoliotic progression may be in part explained by the Hueter-Volkmann principle. This theory describes how increased loading of growth plates will reduce regular growth rates while the converse is also accurate. Further, when extended to the pathogenesis of scoliosis, it defines how asymmetric loading of the vertebral bodies leads to the progression of the deformity via vertebral wedging. Currently, a scoliotic curve reaching a magnitude of 50° Cobb deformation requires surgical intervention involving instrumentation and spinal fusion. The process of fusion is among the most invasive and expensive procedures, which has motivated several researchers to develop other alternatives. The development of a less invasive technique, to control and correct a spinal misalignment before undesirable progression occurs, has subsequently been explored. Several fusionless devices have been developed that attempt to manipulate vertebral growth to correct vertebral wedging and, consequently, realign the spine. However, to date, these approaches have yet to be adopted in a clinical context. Moreover, devices actively pursued seemed to imperil the long term health of the intervertebral disc while corrective attempts are restricted to the unilateral manipulation of a 3D deformity. Therefore, enhanced biomechanical understanding of AIS pathomechanism in conjunction with the development of early and less invasive interventions would offer an important contribution to the improved treatment of AIS. The global objective of this thesis was to design, optimize, and evaluated experimentally fusionless device concepts to induce growth modulation and correct spinal curvatures in adolescent idiopathic scoliosis (AIS). The specific objectives were to: 1) develop a FEM of the spine with integrated growth dynamics; 2) exploit the FEM to explore biomechanical factors involved in the pathomechanism of AIS; 3) exploit the FEM to analyze biomechanically current fusionless growth sparring devices to identify available avenues of improvement; and 4) exploit the devised developmental platform (in silico, in situ, and in vivo analyses) to develop, optimize, and validate novel and improved fusionless growth modulating devices for AIS

Utilization of Finite Element Analysis Techniques for Adolescent Idiopathic Scoliosis Surgical Planning

Adolescent Idiopathic Scoliosis, a three-dimensional deformity of the thoracolumbar spine, affects approximately 1-3% of patients ages 10-18. Surgical correction and treatment of the spinal column is a costly and high-risk task that is consistently complicated by factors such as patient-specific spinal deformities, curve flexibility, and surgeon experience. The following dissertation utilizes finite element analysis to develop a cost-effective, building-block approach by which surgical procedures and kinematic evaluations may be investigated. All studies conducted are based off a volumetric, thoracolumbar finite element (FE) model developed from computer-aided design (CAD) anatomy whose components are kinematically validated with in-vitro data. Spinal ligament stiffness properties derived from the literature are compared for kinematic assessment of a thoracic functional spinal unit (FSU) and benchmarked with available in-vitro kinematic data. Once ligament stiffness properties were selected, load sharing among soft tissues (e.g., ligaments and intervertebral disc) within the same FSU is then assessed during individual steps of a posterior correction procedure commonly used on scoliosis patients. Finally, the entire thoracolumbar spine is utilized to mechanically induce a mild scoliosis profile through an iterative preload and growth procedure described by the Hueter-Volkmann law. The mild scoliosis model is then kinematically compared with an asymptomatic counterpart. The thoracic deformation exhibited in the mild scoliosis model compared well with available CT datasets. Key findings of the studies confirm the importance of appropriately assigning spinal ligament properties with traditional toe and linear stiffness regimes to properly characterize thoracic spine FE models. Stiffness properties assigned within spinal FE models may also alter how intact ligaments and intervertebral discs respond to external loads during posterior correction procedures involving serial ligament removal, and thus can affect any desired post-surgical outcomes. Lastly, the thoracolumbar spine containing mild scoliosis experiences up to a 37% reduction in global range of motion compared to an asymptomatic spine, while also exhibiting larger decreases in segmental axial rotations at apical deformity levels. Future studies will address kinematic behavior of a severe scoliosis deformity and set the stage for column-based osseoligamentous load sharing assessments during surgical procedures

Biomechanical analysis and modeling of lumbar belt: Preliminary study.

International audienceLow back pain is a major public health problem in European Countries. In France, about 50% of population is suffering of this pathology every year (Fassier 2011). Because of health care cost and sick leave (Fassier 2011; Leclerc et al. 2009), low back pain has both societal and economic adverse consequences. Many treatments are proposed. However no guideline is provided to physician. Treatment depends on patient, on low back pain type and evolution and also on physician knowledge and believes. Medical devices, as lumbar belt might be proposed to treat low back pain. Several clinical trials have shown their efficacy (Calmels et al. 2009). Nevertheless, both mechanical and physiological effects of lumbar belts remain unclear. In this study, the application of a lumbar belt on the trunk is simulated by a finite element model. It is often assumed that the pain comes from the toe of the intervertebral discs and is related only to the intradiscal pressure and the thoracolumbar posture. Beside, abdominal pressure is used by belt manufacturers as a marker of the lumbar belt efficiency, because a change in the abdominal pressure could bring a change in the thoracolumbar posture and consequently on the intradiscal pressure. That's why the goal of this study is to determine the mechanical effect of wearing lumbar belt: i) on abdominal pressure; ii) on thoracolumbar posture; iii) on intervertebral disc pressure

Evaluation of a patient-specific finite element model to simulate conservative treatment in adolescent idiopathic scoliosis

Study design: Retrospective validation study Objectives: To propose a method to evaluate, from a clinical standpoint, the ability of a finite element model (FEM) of the trunk to simulate orthotic correction of spinal deformity, and to apply it to validate a previously described FEM Summary of background data: Several FEMs of the scoliotic spine have been described in the literature. These models can prove useful in understanding the mechanisms of scoliosis progression and in optimizing its treatment, but their validation has often been lacking or incomplete. Methods: Three-dimensional geometries of ten patients before and during conservative treatment were reconstructed from bi-planar radiographs. The effect of bracing was simulated by modeling displacements induced by the brace pads. Simulated clinical indices (Cobb angle, T1-T12 and T4-T12 kyphosis, L1-L5 lordosis, apical vertebral rotation, torsion, rib hump) and vertebral orientations and positions were compared to those measured in the patients’ three-dimensional geometries. Results: Errors in clinical indices were of the same order of magnitude as the uncertainties due to 3D reconstruction; for instance, Cobb angle was simulated with a root mean square error of 5.7° and rib hump error was 6.4°. Vertebral orientation was simulated with a root mean square error of 4.8° and vertebral position with an error of 2.5 mm. Conclusions: The methodology proposed here allowed in-depth evaluation of subject-specific simulations, confirming that FEMs of the trunk have the potential to accurately simulate brace action. These promising results provide a basis for ongoing 3D model development, toward the design of more efficient orthoses.The authors are grateful to the ParisTech BiomecAM chair program on subject-specific musculoskeletal modelling for funding (with the support of Proteor, ParisTech and Yves Cotrel Foundations) and to EOS imaging for logistic support in data collection

Biomechanical Modeling and Characterization of the Postural Parameters in Adolescent Idiopathic Scoliosis

RÉSUMÉ La scoliose est une déformation 3D de la colonne vertébrale qui influence la morphologie et l'alignement de la colonne vertébrale, du bassin et de la cage thoracique. Bien que plusieurs paramètres soient introduits pour identifier et évaluer les courbes chez les sujets scoliotiques, la relation biomécanique entre la colonne vertébrale et le bassin ainsi que ses impacts sur la posture et l'équilibre général des sujets scoliotiques n’est pas encore élucidée. Le but de ce projet doctoral était d'examiner l'interaction spino-pelvienne en mesurant les paramètres biomécaniques chez les sujets atteints de scolioses idiopathiques adolescentes (SIA). La cinématique pelvienne, l'orientation spino-pelvienne relative et le chargement biomécanique lombo-sacré ont été examinés chez des sujets avec des courbures différentes. L’hypothèse que nous souhaitons vérifier est que l'interaction spino-pelvienne (au niveau des paramètres statiques, cinématiques et des chargements biomécaniques à l’interface entre le rachis et le bassin) est non seulement différente entre les SIA et les contrôles, mais varie aussi entre les sujets présentant différents types de scolioses. De plus, l'effet d’une instrumentation chirurgicale du rachis sur l’équilibre ainsi que sur l'interaction biomécanique spino-pelvienne a été étudié post opérativement. Donc, après avoir examiné la littérature pertinente, trois chapitres ont été consacrés pour examiner l'hypothèse générale de ce projet. Chaque chapitre aborde un aspect de l'interaction spino-pelvienne chez les sous-groupes scoliotiques et compare les résultats avec un groupe de contrôles de la même catégorie d'âge-sexe. Bien que l'orientation pelvienne entre les sujets SIA et le groupe contrôle était différente, il n'est pas vérifié dans quelle mesure l'orientation pelvienne et l'alignement spino-pelvien affectent la cinématique du bassin chez les sujets présentant différents types de courbures. Par la suite, l’interférence entre l'orientation du bassin et le mouvement spino-pelvien a été étudiée.----------ABSTRACT Scoliosis is a 3D spinal deformity which impacts the morphology and alignment of the spine, the pelvis, and the ribcage. Although several spinal parameters are introduced to identify and evaluate scoliotic curves, there is not much known about the biomechanical relationship between the spine and the pelvis and its impact on the overall posture and equilibrium of the scoliotic patients. The focus of this Ph.D. project was to investigate the spino-pelvic biomechanical interaction in adolescent idiopathic scoliosis (AIS) more closely. Spine and pelvic kinematic, relative spino-pelvic orientation in static, and lumbosacral biomechanical loading were investigated in subjects with different curve patterns. We hypothesized that spino-pelvic interaction is not only different between AIS and controls, but also varies between subjects with different scoliotic types in static, kinematic, and biomechanical loading. Furthermore the hypothetical effect of the spinal operation on equilibrating the spino-pelvic biomechanical interaction was tested postoperatively. Hence, after reviewing the pertinent literatures, 3 chapters were devoted to investigate the general hypothesis of this project. Each chapter tries to investigate one aspect of the spine and pelvis interaction in scoliotic subgroups and compares the results with an age-gender match group of controls. Although the pelvic alignment in the AIS group was different from the age-gender matched control group, it is not closely verified to what extent the pelvic orientation and the spino-pelvic alignment affect the pelvis kinematic in subjects with different curve types and subsequently its impact on the spino-pelvic movement is not determined. An experimental setup was designed to investigate the pelvic 3D motion during simple trunk movement in vivo

Biomechanical Response of the Epiphyseal Vertebral Growth Plate under Static and Cyclic Compression: A Finite Element Study

RÉSUMÉ La scoliose idiopathique de l’adolescent (SIA), qui implique une déformation tridimensionnelle de la colonne vertébrale, affecte 1 à 3% des adolescents, principalement des filles. Les influences mécaniques sur la croissance du rachis jouent un rôle important dans la progression de la courbure chez les patients SIA, notamment pendant les périodes de croissance rapide comme l’adolescence. Les traitements sans fusion du rachis, via la modulation mécanique locale de la croissance osseuse, ont montré des avancées prometteuses pour le traitement précoce des déformations modérées de la colonne vertébrale. Des études in vivo récentes sur les plaques de croissance de rats, utilisant des chargements statiques et dynamiques équivalents au niveau de la contrainte moyenne appliquée, ont montré que les chargements dynamiques étaient aussi efficaces que les chargements statiques en termes de modulation de croissance, mais moins dommageables pour l’intégrité de la plaque de croissance en comparaison aux chargements statiques [1, 2]. Cependant, il a été montré que la combinaison de hautes fréquences et d’amplitudes d’oscillations conduit à des inflammations tissulaires. Comme approche complémentaire à l’approche expérimentale pour investiguer la biomécanique du rachis, la modélisation numérique fournit une plateforme pour approfondir nos connaissances sur les structures du rachis et leurs comportements mécaniques [3]. En utilisant les simulations numériques, un paramètre peut aisément être modifié afin d’investiguer son effet spécifique en maintenant les autres paramètres constants. La modélisation par éléments finis constitue l’une des méthodes numériques couramment utilisées ; elle se base sur une méthode numérique rapide pour des analyses de contraintes et de déformations de problèmes complexes, en évitant les limites et difficultés associées aux études expérimentales. Bien que les modèles poroélastiques aient été développés pour investiguer la réponse dépendante du temps des tissus rachidiens, le comportement biomécanique comparatif de tissus cartilagineux, tels que les plaques de croissance sous chargements statiques vs. dynamiques, n’a pas encore été clairement déterminé [3, 4] et pourrait apporter des connaissances sur la compréhension de l’interaction entre les chargements mécaniques et le métabolisme tissulaire. L’objectif principal de ce projet était d’étudier la réponse biomécanique de la plaque de croissance soumise à des compressions statique et dynamique, en utilisant des modèles par éléments finis. Afin d’atteindre cet objectif, un modèle axisymétrique poroélastique de la plaque----------ABSTRACT Adolescent idiopathic scoliosis (AIS), a 3D deformity of the spine, affects 1-3% of adolescents, mainly females. Mechanical influences on spinal growth play an important role in AIS curve progression, mainly during the rapid growth periods such as adolescence. Fusionless corrective techniques of the spine, by means of local mechanical modulation of bone growth, have shown promising advances in the early treatment of moderate spinal deformities. Recent in vivo studies on rat growth plates using matched static and cyclic loadings in terms of average stress showed that cyclic loads were as efficient as static loads in terms of growth modulation but less detrimental to the growth plate integrity compared to static loads [1, 2]. However, it was shown that the combination of high frequency and oscillation amplitude resulted in infection in the rats. As a complementary approach to investigate spinal biomechanics from experiments, computational modeling provides a platform to extend our knowledge about spinal structures and their mechanical behavior [3]. Using computational modeling, one parameter can be changed easily to investigate its effect while the other parameters are kept constant. Finite element modeling is one of the widely used computational methods; it provides a fast numerical method for stress and strain analysis in complex problems, while avoiding limitations and difficulties associated with experimental studies. Although poroelastic models have been developed to investigate the time-dependent response of the spinal tissues, the comparative biomechanical behavior of cartilaginous tissues such as growth plates under static vs. cyclic loads has yet to be fully understood [3, 4] and could provide insights on understanding of the interaction of mechanical loading and tissue metabolism. The objective of this project was to study the biomechanical response of the growth plate to static and cyclic compressions using finite element models. To achieve this objective, an axisymmetric biphasic model of growth plate was first developed to investigate stress components and deformation within the model for different transversal permeabilities and peripheral pore pressures (part one). Then, a finite element model of a spinal functional unit was used to investigate the same parameters as well as fluid content using a more realistic model (part two). This thesis aimed at verifying the hypothesis that Cyclic and static compressive loads show the same total stress but different pore pressure (stress in fluid phase) and effective stress (stress in solid matrix) within growth plates