A PATIENT-SPECIFIC MULTIBODY MODEL OF SCOLIOTIC SPINE FOR SURGICAL CORRECTION PREDICTION IN THE CORONAL PLANE

Ph.DDOCTOR OF PHILOSOPH

Computational modelling of the scoliotic spine: A literature review

Scoliosis is a deformity of the spine that in severe cases requires surgical treatment. There is still disagreement among clinicians as to what the aim of such treatment is as well as the optimal surgical technique. Numerical models can aid clinical decision-making by estimating the outcome of a given surgical intervention. This paper provided some background information on the modelling of the healthy spine and a review of the literature on scoliotic spine models, their validation, and their application. An overview of the methods and techniques used to construct scoliotic finite element and multibody models was given as well as the boundary conditions used in the simulations. The current limitations of the models were discussed as well as how such limitations are addressed in non-scoliotic spine models. Finally, future directions for the numerical modelling of scoliosis were addressed

Computational modelling of the scoliotic spine: A literature review

open4siScoliosis is a deformity of the spine that in severe cases requires surgical treatment. There is still disagreement among clinicians as to what the aim of such treatment is as well as the optimal surgical technique. Numerical models can aid clinical decision-making by estimating the outcome of a given surgical intervention. This paper provided some background information on the modelling of the healthy spine and a review of the literature on scoliotic spine models, their validation, and their application. An overview of the methods and techniques used to construct scoliotic finite element and multibody models was given as well as the boundary conditions used in the simulations. The current limitations of the models were discussed as well as how such limitations are addressed in non-scoliotic spine models. Finally, future directions for the numerical modelling of scoliosis were addressed.Marco Viceconti and Giorgio Davico were supported by the EU funded project Mobilise-D. The charity Reuse-WithLove is gratefully acknowledged for the financial support to this research.openGould, Samuele L; Cristofolini, Luca; Davico, Giorgio; Viceconti, MarcoGould, Samuele L; Cristofolini, Luca; Davico, Giorgio; Viceconti, Marc

Intervertebral disc characterization by shear wave elastography: An in vitro preliminary study.

Published onlineJOURNAL ARTICLEAuthor's accepted (post-print) manuscriptThe final version of record is available at http://dx.doi.org/10.1177/0954411914540279Patient-specific numerical simulation of the spine is a useful tool both in clinic and research. While geometrical personalization of the spine is no more an issue, thanks to recent technological advances, non-invasive personalization of soft tissue's mechanical properties remains a challenge. Ultrasound elastography is a relatively recent measurement technique allowing the evaluation of soft tissue's elastic modulus through the measurement of shear wave speed. The aim of this study was to determine the feasibility of elastographic measurements in intervertebral disc. An in vitro approach was chosen to test the hypothesis that shear wave speed can be used to evaluate intervertebral disc mechanical properties and to assess measurement repeatability. In total, 11 oxtail intervertebral discs were tested in compression to determine their stiffness and apparent elastic modulus at rest and at 400 N. Elastographic measurements were performed in these two conditions and compared to these mechanical parameters. The protocol was repeated six times to determine elastographic measurement repeatability. Average shear wave speed over all samples was 5.3 ± 1.0 m/s, with a repeatability of 7% at rest and 4.6% at 400 N; stiffness and apparent elastic modulus were 266.3 ± 70.5 N/mm and 5.4 ± 1.1 MPa at rest, respectively, while at 400 N they were 781.0 ± 153.8 N/mm and 13.2 ± 2.4 MPa, respectively. Correlations were found between elastographic measurements and intervertebral disc mechanical properties; these preliminary results are promising for further in vivo application.ParisTech BiomecAM chair programProteorParisTechYves Cotrel Foundation

Planning the Surgical Correction of Spinal Deformities: Toward the Identification of the Biomechanical Principles by Means of Numerical Simulation

The set of surgical devices and techniques to perform spine deformity correction has widened dramatically. Nevertheless, the rate of complications due to mechanical failure remains rather high. Indeed, basic research about the principles of deformity correction and the optimal surgical strategies (i.e. the choice of the fusion length, the most appropriate instrumentation, the degree of tolerable correction) did not progress as much as the techniques. In this work, a software approach for the biomechanical simulation of the correction of patient-specific spinal deformities aimed to the identification of its biomechanical principles is presented. The method is based on three dimensional reconstructions of the spinal anatomy obtained from biplanar radiographic images. A user-friendly graphical interface allows for the planning of the deformity correction and to simulate the instrumentation. Robust meshing of the instrumented spine is provided by using consolidated computational geometry and meshing libraries. Based on finite element simulation, the program predicts the loads acting in the instrumentation as well as in the biological tissues. A simple test case (reduction of a low grade spondylolisthesis at L3-L4) was simulated as a proof-of-concept. Despite the limitations of this approach, the preliminary outcome is promising and encourages a wide effort towards its refinement

Development and Biodynamic Simulation of a Detailed Musculo-Skeletal Spine Model

Ph.DDOCTOR OF PHILOSOPH

Robotic Exoskeletons for Torso Study, Training, and Assistance

Robotic exoskeletons are important tools in medicine for characterizing certain aspects of diseases, enabling physical therapy treatments, or providing assistance to those with impairments. One area in particular where these devices can make an impact is the study and treatment of scoliosis. First, I adapt a design of a robotic torso exoskeleton to serve the population most susceptible to scoliosis, female adolescents. I used the device to compare the torso stiffness of members of this group with and without scoliosis, and found an interaction effect of degree of freedom (DOF) and torso segment on translational stiffness, and an interaction effect of DOF and group on rotational stiffness. These results can inform the models used to create rigid orthoses for conservative treatment or to simulate the effects of surgical procedures. Second, I explore the effects of different types of augmented sensory feedback commonly used in scoliosis physical therapy. I compare visual and force feedback provided by the exoskeleton on one’s ability to replicate static poses and dynamic movements. I find that while force feedback leads to faster initial improvement, visual feedback may enable the user to learn finer details of the movement. Third, I design a torso exoskeleton for people with neuromotor impairments. People who are not able to sit up independently are at a high risk of developing neuromuscular scoliosis, and must balance the benefits of treatment with rigid orthoses, with the limits that these devices place on functional movements. The device allows users four degrees of freedom, to support functional movements such as reaching and pressure relief maneuvers, but prevents lateral translation and axial rotation, which can contribute to neuromuscular scoliosis. Together, these results demonstrate the potential for robotic exoskeletons in torso study, training, and assistance

Biomechanical Simulator for the Surgical Correction of Sagittal Balance in Adult Spinal Deformity

RÉSUMÉ Pour maintenir une posture érigée minimisant les dépenses énergétiques, l’alignement de la colonne vertébrale dans le plan sagittal est d’une grande importance. Dans le contexte des déformations de la colonne vertébrale chez l'adulte, un mauvais alignement dans le plan sagittal demande une dépense énergétique plus élevée et est associé à la douleur et à une perte de fonction. Le maintien d'une posture érigée dans de telles conditions implique une activation accrue des muscles du tronc et l'utilisation de mécanismes compensatoires pour contrebalancer le débalancement antérieur du haut du corps. L'instrumentation chirurgicale est indiquée chez les patients souffrant de grandes douleurs et de handicaps lorsque les traitements non chirurgicaux ne sont plus suffisants. Cette procédure consiste à insérer des vis dans les pédicules des vertèbres et à redresser la colonne vertébrale à l’aide de tiges métalliques, ce qui conduit à la fusion permanente de la colonne vertébrale. Pour la correction de déformations importantes et manquant de flexibilité dans le plan sagittal, l'ostéotomie de soustraction pédiculaire (OSP) est une procédure souvent utilisée pour rétablir le profil sagittal normal de la colonne lombaire. Cette technique implique la résection des éléments postérieurs de la vertèbre ainsi qu’un coin d’os dans le corps vertébral pour créer une forte angulation de la colonne vertébrale. C'est une procédure très exigeante en raison des risques de complications mécaniques. De nombreux facteurs de risque ayant une incidence sur les taux de complications mécaniques après une instrumentation chirurgicale avec OSP ont été identifiés dans le cadre d’études cliniques. Les patients ayant eu des complications mécaniques avaient reçu une correction significativement plus grande de l’axe vertical sagittal, un cintrage plus grand des tiges dans le plan sagittal et une ostéotomie réalisée à un niveau plus caudal. Il a également été démontré que jusqu'à 40% des patients gardaient un alignement sagittal antérieur après une chirurgie avec OSP et qu'un alignement sagittal non neutre était associé à des taux plus élevés de révision chirurgicale. Même si des objectifs chirurgicaux globaux ont été définis avec la classification SRS-Schwab pour la correction du déséquilibre sagittal, la stratégie chirurgicale optimale spécifique au patient reste mal définie. En outre, malgré les études cliniques et biomécaniques, les relations entre les contraintes mécaniques dans l'instrumentation et les différents paramètres de correction dans le plan sagittal (degré de correction sagittale par variation de l'angle de l’OSP et de l'angle de cintrage des tiges, niveau vertébral de l’OSP et nombre de tiges) sont encore mal comprises. Les connaissances biomécaniques sur les facteurs de risque et leurs effets sur les complications mécaniques liées aux OSP telles que le bris des tiges sont encore limitées et une meilleure compréhension de l'impact biomécanique des OSP pourrait être un excellent outil pour aider les chirurgiens dans leur planification préopératoire de la correction du déséquilibre sagittal.----------ABSTRACT To maintain an erect posture minimizing energy expenditure, the alignment of the spine in the sagittal plane is of great importance. In adult spine deformity (ASD), sagittal misalignment requires higher energy expenditure and is associated with pain and loss of function. Maintaining an erect posture in such conditions involves increased trunk muscles activation and the use of compensatory mechanisms to counter balance the shift of the upper body. Surgical instrumentation is indicated for patients with high pain and disabilities when non-surgical treatments are not sufficient. This procedure consists in inserting screws in the pedicles of the vertebrae and straightening the spine with metal rods connected to the pedicle screws, leading subsequently to the permanent fusion of the spine. For the correction of large and rigid deformities in the sagittal plane, pedicle subtraction osteotomy (PSO) is a procedure used to restore normal sagittal profile of the lumbar spine. This technique involves a wedge-shaped resection of the vertebral body along with all posterior elements of the vertebra to locally increase the lumbar lordosis. It is a highly demanding procedure due to the risks of mechanical complications. Patients with mechanical complications after PSO had a significantly greater correction of the sagittal vertical axis, higher sagittal contour of the rods, and osteotomy performed at a more caudal level. It was also reported that up to 40% of patients kept an anterior sagittal alignment after surgery with PSO and a non-neutral sagittal alignment is associated with higher rates of revision surgery. Even though global surgical objectives have been defined through the SRS-Schwab ASD classification for the correction of sagittal imbalance, patient-specific optimal surgical strategy is still poorly defined. Also, despite clinical and biomechanical investigations, relations between stresses in the instrumentation and different sagittal correction parameters (amount of sagittal correction through varying PSO wedge angle and rod sagittal contouring angle, vertebral level of the PSO and number of rods) is still not well understood. Biomechanical knowledge of the reported risk factors and their effects on mechanical complications related to PSO such as rod breakage are still limited and a better understanding of the PSO’s biomechanical impact could be a great tool to assist surgeons in their preoperative planning of sagittal imbalance correction. Therefore, this project aims to address the following research question: « How do PSO resection angle, rod curvature, vertebral level of the PSO, and number of rods biomechanically impact the correction of sagittal balance and loads in the construct, and how should they be adjusted to reduce the risks of mechanical failure in adult spinal deformity?

Patient Positioning for Surgeries of the Spine : How Does it Impact Spinal Geometry and How Can it be Exploited to Improve Surgical Procedures

RÉSUMÉ Les cas les plus graves de déformation rachidienne, telles que la scoliose, nécessitent une intervention chirurgicale afin de traiter les symptômes et de réaligner la colonne vertébrale. Au cours de l'intervention chirurgicale, les patients sont habituellement maintenus dans une position en décubitus ventral et une instrumentation est utilisée pour corriger et fixer la géométrie de la colonne. Il a été démontré que le positionnement des patients sur des cadres chirurgicaux a un impact sur la géométrie rachidienne, mais ceci n'est pas exploité afin de faciliter et améliorer les procédures chirurgicales. Les cadres disponibles commercialement ont des capacités limitées de positionnement du patient qui puisse être modifiable durant l'intervention. Aussi, afin d‟exploiter éventuellement les diverses possibilités de positionnement, on doit connaître l‟impact de ces positions sur la modulation de la géométrie de la colonne vertébrale du patient opéré. Ce projet a été effectué en parallèle avec la conception et la construction d'un nouveau cadre de positionnement multifonctionnel (MFPF) pour les chirurgies du rachis qui permet le positionnement des membres inférieurs ainsi que le déplacement vertical du thorax. Le MFPF lui-même était une combinaison de deux cadres précédents permettant le positionnement chirurgical: le DPF (permettant le réglage de coussins sur le tronc et l'application de forces correctives) et le "leg positionner" (permettant la flexion et l'extension des membres inférieurs). La modélisation par éléments finis (MEF) a été utilisée pour étudier le positionnement de patient sur le DPF. Les objectifs spécifiques de ce projet étaient: 1) d'adapter et développer une MEF de la colonne vertébrale, cage thoracique, bassin, et des membres inférieurs qui soit capable de simuler les effets géométriques sur la colonne vertébrale résultant du positionnement en décubitus ventral et de l‟ajustement des capacités de positionnement du MFPF; 2) effectuer des essais expérimentaux sur le positionnement en décubitus ventral et les capacités de positionnement du MFPF et utiliser les résultats pour valider le MFF; 3) exploiter le MEF pour développer de nouvelles possibilités de positionnement sur le MFPF permettant de moduler la géométrie de la colonne vertébrale et évaluer ces nouvelles positions expérimentalement avec des accessoires construit pour le MFPF; et 4) exploiter la MEF afin d'étudier l'impact de la combinaison des----------ABSTRACT The most severe cases of spinal deformity, such as scoliosis, require surgical intervention in order to treat symptoms and re-align the spine. During surgical procedures, patients are typically kept in the prone position while surgical instrumentation is utilized to manipulate and fix spinal geometry. Patient positioning on surgical frames has been shown to have an impact on spinal geometry which can be exploited in order to facilitate and improve upon surgical procedures. Current commercial surgical frames have no or limited patient positioning capabilities. In order to best take advantage of a surgical frame‟s positioning capabilities, knowledge must be gained on how they will impact a given patient‟s spinal geometry. This project was done in parallel with the design and construction of a new Multi-Functional Positioning Frame (MFPF) for spinal surgeries which allowed for lower limb positioning and thoracic vertical displacement. The MFPF itself was a combination of two previously developed surgical positioning devices: the Dynamic Positioning Frame (DPF) (allowing thoracic cushion adjustment and corrective force application) and the “leg positioner” (allowing hip flexion and extension). Finite element modeling (FEM) was previously used to study patient positioning on the DPF. The global objective of this thesis was to study how patient positioning on a frame can be used in order to improve scoliosis instrumentation procedures through the intra-operative manipulation of spinal geometry. The specific objectives of this project were: 1) adapt and develop a FEM of the spine, thoracic cage, pelvis, and adjacent structures that is able to simulate the geometric effects on the spine resulting from prone positioning and feature adjustment on the MFPF; 2) experimentally test the impact of prone positioning and feature adjustment on the MFPF and utilize the results to validate the FEM; 3) exploit the FEM in order to study additional surgical positions allowing modification of spinal geometrical parameters not possible on the original MFPF design and experimentally assess these new positions using proof of concept features constructed for the MFPF; and 4) exploit the FEM in order to study the impact of combined MFPF positioning parameters on the geometry of the spine (especially the leg positioning and thoracic components) including developing a method allowing for individual an

Anatomy, Implant Selection and Placement Influence Spine Mechanics Associated with Total Disc Replacement

Through aging and injury, the intervertebral disc of the lumbar spine can undergo degeneration, leading to collapse of the vertebrae and low back pain, a symptom that affects half the adult population in any given year. In an effort to reduce low back pain, total disc replacement treatment removes the degenerated disc, restores natural height and lordosis of the segment, and preserves motion at the joint. Patient anatomy, implant selection, and implant placement play significant roles in a patient\u27s outcomes after total disc replacement surgery. Thus, the objective of the work presented in this thesis was to develop a suite of statistical and computational tools describe population-based anatomy and to support component selection and placement in TDR surgical procedures with the goal of improving implant design and patient outcomes. The statistical modeling approach quantified shape and alignment variation of the lumbar spine by characterizing variability of shape and size of individual vertebra, relative alignment of relevant segments, and overall anatomy of the lumbar spine. Statistical shape models of single vertebrae revealed that the primary mode of variation correlated to vertebral body size variation (average R2 = 0.82 across vertebrae), which can inform sizing lines for total disc replacements. Strong correlations of disc height to the second (R2 = 0.82) and third (R2 = 0.88) principal components of the shape-alignment models of the L4-L5 and L5-S1 segments are useful in assisting clinicians diagnose pathologies, screening patients for treatment options, and pre-operatively planning for surgical treatment. Statistical models of the entire spine reveal how vertebral shape changes influence the spine as a whole. The subject-specific templating approach of total disc replacement surgeries accurately predicted ROM in a cohort of twenty two patients implanted with the ProDisc-L device and suggested changes to total disc replacement size selection and alignment to improve ROM. Predicted ROM was 11.8% different to actual ROM. Improvements in ROM could have been achieved in over 85% of the cases had the proposed templating process been employed, which showed that pre-operative templating can be an important tool to achieve maximum ROM and optimal clinical outcomes. Computational pilot evaluations of subjects implanted with the Activ-L device provided insight into the mechanical behavior of a total disc replacement featuring a center inlay that can translate within the inferior end plate. Results indicated that greater translation of the inlay related to greater overall ROM. Subjects implanted with the Activ-L achieved greater ideal range of motion than subjects with a ProDisc-L, a device featuring an inlay that is fixed within the inferior end plate. Further investigations into this work can reveal design considerations that significantly influence ROM and patient outcomes