Daniel George

Bulk modulus and volume variation measurement of the liver and the kidneys in vivo using abdominal kinetics during free breathing

This article presents a method of predictive simulation, patient-dependant, in real time of the abdominal organ positions during free breathing. The method, that considers both influence of the abdominal breathing and thoracic breathing, needs a tracking of the patient skin and a model of the patient-specific modification of the diaphragm shape. From a measurement of the abdomen viscera kinematic during free breathing, we evaluate through a finite element analysis, the stress field sustained by the organs for a hyperelastic mechanical behaviour using large strain theory. From this analysis, we deduce an in vivo Poissons ratio and a homogeneous bulk modulus of the liver and kidneys, and compare it to the ones in vitro available in the literature.

Modeling and Simulation of the Cooling Process of Borosilicate Glass

For a better understanding of the thermomechanical behavior of glasses used for nuclear waste vitrification, the cooling process of a bulk borosilicate glass is modeled using the finite element code Abaqus. During this process, the thermal gradients may have an impact on the solidification process. To evaluate this impact, the simulation was based on thermal experimental data from an inactive nuclear waste package. The thermal calculations were made within a parametric window using different boundary conditions to evaluate the variations of temperature distributions for each case. The temperature differences throughout the thickness of solidified glass were found to be significantly non-uniform throughout the package. The temperature evolution in the bulk glass was highly responsive to the external cooling rates applied; thus emphasizing the role of the thermal inertia for this bulky glass cast.

Mechanically-driven bone remodeling simulation: Application to LIPUS treated rat calvarial defects

In this paper we numerically simulate the phenomenon of bone growth in bone defects as driven by external mechanical excitation. Bone growth is accounted for through a continuum model that allows simulation of the filling of a defect. The influence of the model boundary conditions is also discussed. Two and three dimensional simulations are presented, explicitly showing the bone regeneration process inside the cavity on a weekly basis. Numerical results are qualitatively compared with literature experimental data from a rat calvarial defect exposed to low-intensity pulsed ultrasound. The obtained results show trend correlations with the targeted phenomenological observations and allow us to perform a first evaluation of the proposed model parameters to be optimized for clinically relevant situations, even if a systematic experimental campaign is still needed to precisely identify the bio-mechanical parameters involved.

Second-gradient models accounting for some effects of microstructure on remodelling of bones reconstructed with bioresorbable materials

Every year, over two million people worldwide sustain a bone grafting procedure to repair bone defects stemming from a disease or a traumatic event. The standard procedure today is to use autologous bone grafts to repair such defects. However, the need for two surgery procedures (harvesting and implantation) and morbidity at the extraction site are common problems which have been an incentive to look for alternative treatments. One of these alternatives is to use allograft bone, i.e. specially treated bone retrieved from corpses or from patients receiving a hip prosthesis, but these materials may trigger immunological reactions or even lead to disease transmission. A more future-orientated approach is to look for synthetic bioresorbable bone graft substitutes. Indeed, an improvement of the understanding of chemical and biological processes intervening during the resorption of synthetic bone graft substitutes and during the concomitant bone formation should allow a better design of these materials and lead to superior performances. We propose to use a second-gradient, two-solid mixture model to describe growth/resorption phenomena in bone tissues grafted with bioresorbable materials as driven by mechanical loads. The needed theoretical model must be able to account for the interplay between mechanical and biological phenomena which are known to be important for bone tissue synthesis and for resorption of both bone tissue and biomaterial. The possibility of considering the effect of microstructure on reconstructed bone remodelling will be taken into account by using a second-gradient theory.

Multiphysics of bone remodeling: A 2D mesoscale activation simulation

In this work, we present an evolutive trabecular model for bone remodeling based on a boundary detection algorithm accounting for both biology and applied mechanical forces, known to be an important factor in bone evolution. A finite element (FE) numerical model using the Abaqus/Standard® software was used with a UMAT subroutine to solve the governing coupled mechanical-biological non-linear differential equations of the bone evolution model. The simulations present cell activation on a simplified trabeculae configuration organization with trabecular thickness of 200µm. For this activation process, the results confirm that the trabeculae are mainly oriented in the active direction of the principal mechanical stresses and according to the principal applied mechanical load directions. The trabeculae surface activation is clearly identified and can provide understanding of the different bone cell activations in more complex geometries and load conditions.

Mechanical equilibrium of forces and moments applied on orthodontic brackets of a dental arch: Correlation with literature data on two and three adjacent teeth

Although orthodontics have greatly improved over the years, understanding of its associated biomechanics remains incomplete and is mainly based on two dimensional (2D) mechanical equilibrium and long-time clinical experience. Little experimental information exists in three dimensions (3D) about the forces and moments developed on orthodontic brackets over more than two or three adjacent teeth. We define here a simplified methodology to quantify 3D forces and moments applied on orthodontic brackets fixed on a dental arch and validate our methodology using existing results from the literature by means of simplified hypotheses.

Mechanobiological stimuli for bone remodeling: mechanical energy, cell nutriments and mobility

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Mechanobiological stimuli for bone remodeling: mechanical energy, cell nutriments and mobility Daniel George, Rachele Allena, Yves Remond

Towards Building a Multiscale Mechanical Model for the Prediction of Acute Subdural Hematomas

Acute subdural hematoma (ASDH) is a potentially devastating, yet curable, extra axial fluid collection within the subdural space situated between the skull and the cortex. It is often due to rupture of bridging veins crossing this subdural space, caused by the brain-skull relative motion. To be able to predict ASDH, a numerical model reflecting the mechanical properties of vascular walls is attractive. With this in mind, a suitable approach consists in modeling the material microstructure at different scales. In a former work [1, 2], R. Abdel Rahman studied the mechanical properties of the bridging veins – superior sagittal sinus junction when a human head is submitted to shock. This work showed the apparition of ASDH over a given value of head rotational acceleration. But lacks in the knowledge of microstructure and of the constituents mechanical properties were put forward in understanding the relations between material mechanical behavior and the apparition of ASDH. Therefore we chose to adopt a multiscale approach to model ASDH apparition. In the current work, several experimental observations have been set up to obtain a sufficient knowledge of the vein wall microstructure which was imprecisely documented to date. Stained thin slices of human brain were observed by optical microscopy. In addition, microtomography was used to assess the collagen fibers orientations. These observations allowed the identification of the different scales needed for modeling the microstructure. Many authors [3–6] deal with the mechanical behavior of vascular walls and of their various constituents but none of them consider multiple scales for modeling [7]. The next step of this work consists in improving the predictive capabilities of the existing model by going down the scales and taking microstructure into account. This methodology enabled the introduction of only physical parameters into the model, which is essential for future predictive capabilities. Finally, a failure criterion for the bridging veins taking into account the different scales has been created and is still being improved. It allows the evaluation of specific disease influence like collagen damage due to physiology. Besides it provides a prediction tool for ASDH useable for optimization of various shock absorbers.Copyright

Experimental quantification of the mechanical forces and moments applied on three adjacent orthodontic brackets

Orthodontic appliances deliver forces and moments that will determine movement of teeth. To analyze this latter, we developed an experimental setup to measure the mechanical forces applied on the teeth and to calculate, through a simplified theoretical analysis, the reactive forces and corresponding moments onto the brackets of three adjacent teeth. To validate the theoretical and experimental results, we use a simplified clinical situation of a maxillary canine in infraclusion and surrounded by its corresponding upper lateral incisor and first premolar. Forces are then measured experimentally and compared with the calculated results. From this, we show the specific dissymmetry of the mechanical forces on each side of the maxillary canine due to the applied mechanical forces and the undesirable induced generated moments occurring on each tooth that will directly impact the bone remodeling process and the final tooth repositioning.

A New Software Suite in Orthognathic Surgery : Patient Specific Modeling, Simulation and Navigation

Orthognathic surgery belongs to the scope of maxillofacial surgery. It treats dentofacial deformities consisting in discrepancy between the facial bones (upper and lower jaws). Such impairment affects chewing, talking, and breathing and can ultimately result in the loss of teeth. Orthognathic surgery restores facial harmony and dental occlusion through bone cutting, repositioning, and fixation. However, in routine practice, we face the limitations of conventional tools and the lack of intraoperative assistance. These limitations occur at every step of the surgical workflow: preoperative planning, simulation, and intraoperative navigation. The aim of this research was to provide novel tools to improve simulation and navigation. We first developed a semiautomated segmentation pipeline allowing accurate and time-efficient patient-specific 3D modeling from computed tomography scans mandatory to achieve surgical planning. This step allowed an improvement of processing time by a factor of 6 compared with interactive segmentation, with a 1.5-mm distance error. Next, we developed a software to simulate the postoperative outcome on facial soft tissues. Volume meshes were processed from segmented DICOM images, and the Bullet open source mechanical engine was used together with a mass-spring model to reach a postoperative simulation accuracy <1 mm. Our toolset was completed by the development of a real-time navigation system using minimally invasive electromagnetic sensors. This navigation system featured a novel user-friendly interface based on augmented virtuality that improved surgical accuracy and operative time especially for trainee surgeons, therefore demonstrating its educational benefits. The resulting software suite could enhance operative accuracy and surgeon education for improved patient care.