Finite Element simulation of buckling-induced vein tortuosity and influence of the wall constitutive properties

International audienceThe mechanisms giving rise to vein tortuosity, which is often associated with varicosis, are poorly understood. Recent works suggest that significant biological changes in the wall of varicose veins may precede the mechanical aspects of the disease. To test the hypothesis of tortuosity being a consequence of these changes, a Finite Element model was developed based on previous experimental work on vein buckling. The model was then used to evaluate the effect of alterations of the mechanical behavior of the wall on tortuosity onset and severity. The results showed that increasing anisotropy toward the circumferential direction promotes tortuosity. An increase in wall stiffness tends to decrease the level of tortuosity but interestingly, if the vein segment is little or not pre-stretched such increase will not prevent, or it will even promote, the onset of tortuosity. These results provide additional arguments supporting the hypothesis of tortuosity being the consequence of biologically-induced changes in the varicose vein wall. Based on a 3D model of the leg and in vivo identification of the material properties of varicose veins, a clinical validation of these findings is being developed

Biomechanical response of varicose veins to elastic compression: A numerical study.

International audienceA patient-specific finite-element (FE) model of the human leg is developed to model the stress distribution in and around a vein wall in order to determine the biomechanical response of varicose veins to compression treatment. The aim is to investigate the relationship between the local pressure on the soft tissues induced by wearing the compression garment and the development and evolution of varicose veins and various skin-related diseases such as varicose veins and ulcers. Because experimental data on the mechanical properties of healthy superficial veins and varicose veins are scarce in literature, ultrasound images of in vivo varicose veins are acquired and analysed to extract the material constants using Finite Element Model Updating. The decrease in trans-mural pressure, which conditions the effectiveness of compressive treatments, is computed from the simulation results. This constitutes the original added value of the developed model as decrease in trans-mural pressures cannot be assessed experimentally by any other means. Results show that external compression is effective in decreasing the trans-mural pressure, thereby having a positive effect in the control and treatment of vein-related diseases

A mechano-biological model of multi-tissue evolution in bone

International audienceSuccessfully simulating tissue evolution in bone is of significant importance in predicting various biological processes such as bone remodeling, fracture healing and osseointegration of implants. Each of these processes involves in different ways the permanent or transient formation of different tissue types, namely bone, cartilage and fibrous tissues. The tissue evolution in specific circumstances such as bone remodeling and fracturing healing is currently able to be modeled. Nevertheless, it remains challenging to predict which tissue types and organization can develop without any a priori assumptions. In particular, the role of mechano-biological coupling in this selective tissue evolution has not been clearly elucidated. In this work, a multi-tissue model has been created which simultaneously describes the evolution of bone, cartilage and fibrous tissues. The coupling of the biological and mechanical factors involved in tissue formation has been modeled by defining two different tissue states: an immature state corresponding to the early stages of tissue growth and representing cell clusters in a weakly neo-formed Extra Cellular Matrix (ECM), and a mature state corresponding to well-formed connective tissues. This has allowed for the cellular processes of migration, proliferation and apoptosis to be described simultaneously with the changing ECM properties through strain driven diffusion, growth, maturation and resorption terms. A series of finite element simulations were carried out on idealized cantilever bending geometries. Starting from a tissue composition replicating a mid-diaphysis section of a long bone, a steady-state tissue formation was reached over a statically loaded period of 10,000 h (60 weeks). The results demonstrated that bone formation occurred in regions which are optimally physiologically strained. In two additional 1000 h bending simulations both cartilaginous and fibrous tissues were shown to form under specific geometrical and loading cases and cartilage was shown to lead to the formation of bone in a beam replicating a fracture healing initial tissue distribution. This finding is encouraging in that it is corroborated by similar experimental observations of cartilage leading bone formation during the fracture healing process. The results of this work demonstrate that a multi-tissue mechano-biological model of tissue evolution has the potential for predictive analysis in the design and implementations of implants, describing fracture healing and bone remodeling processes

Finite element modelling of nearly incompressible materials and volumetric locking: a case study

International audienceThe purpose of this paper is to illustrate the influence of the choice of the finite element technology on the occurrence of locking and hourglass instabilities. We chose to focus on the case study of the activation of the posterior genio-glossus (GGp) that is a lingual muscle located at the root of the tongue and inserts in the front to the mandible. The activation of this muscle compresses the tongue in the lower part and generates a forward and upward movement of the tongue body, because of the incompressibility of tongue tissues (for example during the production of the phonemes /i/ or /s/)

The effect of breathing on the in vivo mechanical characterization of linea alba by ultrasound shearwave elastography

The most common surgical repair of abdominal wall hernia consists in implanting a mesh to reinforce hernia defects during the healing phase. Ultrasound shearwave elastography (SWE) is a promising non-invasive method to estimate soft tissue mechanical properties at bedside through shear wave speed (SWS) measurement. Combined with conventional ultrasonography, it could help the clinician plan surgery. In this work, a novel protocol is proposed to reliably assess the stiffness of the linea alba, and to evaluate the effect of breathing and of inflating the abdomen on SWS. Fifteen healthy adults were included. SWS was measured in the linea alba, in the longitudinal and transverse direction, during several breathing cycle and during active abdominal inflation. SWS during normal breathing was 2.3 [2.0; 2.5] m/s in longitudinal direction and 2.2 [1.9; 2.7] m/s in the transversal. Inflating the abdomen increased SWS both in longitudinal and transversal direction (3.5 [2.8; 5.8] m/s and 5.2 [3.0; 6.0] m/s, respectively). The novel protocol significantly improved the reproducibility relative to the literature (8% in the longitudinal direction and 14% in the transverse one). Breathing had a mild effect on SWS, and accounting for it only marginally improved the reproducibility. This study proved the feasibility of the method, and its potential clinical interest. Further studies on larger cohort should focus on improving our understanding of the relationship between abdominal wall properties and clinical outcomes, but also provide a cartography of the abdominal wall, beyond the linea alba

Cortex tissue relaxation and slow to medium load rates dependency can be captured by a two-phase flow poroelastic model

This paper investigates the complex time-dependent behavior of cortex tissue, under adiabatic condition, using a two-phase flow poroelastic model. Motivated by experiments and Biot's consolidation theory, we tackle time-dependent uniaxial loading, confined and unconfined, with various geometries and loading rates from 1 micrometer/sec to 100 micrometer/sec. The cortex tissue is modeled as the porous solid saturated by two immiscible fluids, with dynamic viscosities separated by four orders, resulting in two different characteristic times. These are respectively associated to interstitial fluid and glial cells. The partial differential equations system is discretised in space by the finite element method and in time by Euler-implicit scheme. The solution is computed using a monolithic scheme within the open-source computational framework FEniCS. The parameters calibration is based on Sobol sensitivity analysis, which divides them into two groups: the tissue specific group, whose parameters represent general properties, and sample specific group, whose parameters have greater variations. Our results show that the experimental curves can be reproduced without the need to resort to viscous solid effects, by adding an additional fluid phase. Through this process, we aim to present multiphase poromechanics as a promising way to a unified brain tissue modeling framework in a variety of settings

Atlas-Based Automatic Generation of Subject-Specific Finite Element Tongue Meshes

Generation of subject-specific 3D finite element (FE) models requires the processing of numerous medical images in order to precisely extract geometrical information about subject-specific anatomy. This processing remains extremely challenging. To overcome this difficulty, we present an automatic atlas-based method that generates subject-specific FE meshes via a 3D registration guided by Magnetic Resonance images. The method extracts a 3D transformation by registering the atlas’ volume image to the subject’s one, and establishes a one-to-one correspondence between the two volumes. The 3D transformation field deforms the atlas’ mesh to generate the subject-specific FE mesh. To preserve the quality of the subject-specific mesh, a diffeomorphic non-rigid registration based on B-spline free-form deformations is used, which guarantees a non-folding and one-to-one transformation. Two evaluations of the method are provided. First, a publicly available CT-database is used to assess the capability to accurately capture the complexity of each subject-specific Lung’s geometry. Second, FE tongue meshes are generated for two healthy volunteers and two patients suffering from tongue cancer using MR images. It is shown that the method generates an appropriate representation of the subject-specific geometry while preserving the quality of the FE meshes for subsequent FE analysis. To demonstrate the importance of our method in a clinical context, a subject-specific mesh is used to simulate tongue’s biomechanical response to the activation of an important tongue muscle, before and after cancer surgery

Calibration of the fat and muscle hyperelastic material parameters for the assessment of the internal tissue deformation in relation to pressure ulcer prevention

Our group aims at proposing a protocol for evaluating the subject-specific risk for sacral pressure ulcer (PU). The idea is to model the way soft tissue will be deformed under the external pressure and to compute internal strains. It has been indeed showed that high strains can create deep tissue injuries (Ceelen et al., 2008). Because of the clinical constraints, we propose to use Ultrasound (US) to build the subject-specific biomechanical model of the sacral soft tissue as an alternative to 3D MR imaging (because of cost, time and accessibility). This paper focuses on the way such a subject-specific model will be designed and more specifically on the methodology we propose for estimating the constitutive parameters of the internal sacral soft tissue, namely the adipose tissue and the muscle. The values of these parameters have indeed a strong influence on the computed internal strainsThis project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 811965

Multi-compartment poroelastic models of perfused biological soft tissues: implementation in FEniCSx

Soft biological tissues demonstrate strong time-dependent and strain-rate mechanical behavior, arising from their intrinsic visco-elasticity and fluid-solid interactions (especially at sufficiently large time scales). The time-dependent mechanical properties of soft tissues influence their physiological functions and are linked to several pathological processes. Poro-elastic modeling represents a promising approach because it allows the integration of multiscale/multiphysics data to probe biologically relevant phenomena at a smaller scale and embeds the relevant mechanisms at the larger scale. The implementation of multi-phasic flow poro-elastic models however is a complex undertaking, requiring extensive knowledge. The open-source software FEniCSx Project provides a novel tool for the automated solution of partial differential equations by the finite element method. This paper aims to provide the required tools to model the mixed formulation of poro-elasticity, from the theory to the implementation, within FEniCSx. Several benchmark cases are studied. A column under confined compression conditions is compared to the Terzaghi analytical solution, using the L2-norm. An implementation of poro-hyper-elasticity is proposed. A bi-compartment column is compared to previously published results (Cast3m implementation). For all cases, accurate results are obtained in terms of a normalized Root Mean Square Error (RMSE). Furthermore, the FEniCSx computation is found three times faster than the legacy FEniCS one. The benefits of parallel computation are also highlighted.Comment: https://github.com/Th0masLavigne/Dolfinx_Porous_Media.gi

Optimal bone structure is dependent on the interplay between mechanics and cellular activities

Bone is a tissue with the remarkable capacity to adapt its structure to an optimized microstructural form depending on variations in the loading conditions. The remodeling process in bone produces distinct tissue distributions such as cortical and trabecular bone but also fibrous and cartilage tissues. Although it has been demonstrated that mechanical factors play a decisive role in the architectural optimization, it may also follow that biological factors have an influence. This interplay between loading and physiology has not been previously reported but is paramount for a proper assessment of bone remodeling outcomes. In this work we present a mechanostat model for bone remodeling which is shown to predict the mechanically driven homeostasis. It is further demonstrated that the steady-state reached is innately dependent upon the loading magnitudes and directions. The model was then adjusted to demonstrate the influence of specific biological factors such as cell proliferation, migration and resorption. Furthermore, two scenarios were created to replicate the physiological conditions of two bone disorders – osteoporosis and osteopetrosis – where the results show that there is a significant distinction between the homeostatic structures reached in each case and that the tissue adaptations follow similar trends to those observed in clinical studies