SAM

Subject Specific Finite Element Mesh Generation of the Pelvis from Biplanar X-ray Images: Application to 120 clinical cases

FOUGERON, Nolwenn — Mon, 01 Jan 2018 00:00:00 GMT

Subject Specific Finite Element Mesh Generation of the Pelvis from Biplanar X-ray Images: Application to 120 clinical cases FOUGERON, Nolwenn; MACRON, Aurélien; TRAVERT, Christophe; SKALLI, Wafa; ROHAN, Pierre-Yves; PILLET, Helene Several Finite Element (FE) models of the pelvis have been developed to comprehensively assess the onset of pathologies and for clinical and industrial applications. However, because of the difficulties associated with the creation of subject-specific FE mesh from CT scan and MR images, most of the existing models rely on the data of one given individual. Moreover, although several fast and robust methods have been developed for automatically generating tetrahedral meshes of arbitrary geometries, hexahedral meshes are still preferred today because of their distinct advantages but their generation remains an open challenge. Recently, approaches have been proposed for fast 3D reconstruction of bones based on X-ray imaging. In this study, we adapted such an approach for the fast and automatic generation of all-hexahedral subject-specific FE models of the pelvis based on the elastic registration of a generic mesh to the subject-specific target in conjunction with element regularity and quality correction. The technique was successfully tested on a database of 120 3D reconstructions of pelvises from biplanar X-ray images. For each patient, a full hexahedral subject-specific FE mesh was generated with an accurate surface representation.

Subject specific hexahedral Finite Element mesh generation of the pelvis from bi-Planar X-ray images

FOUGERON, Nolwenn — Sun, 01 Jan 2017 00:00:00 GMT

Subject specific hexahedral Finite Element mesh generation of the pelvis from bi-Planar X-ray images FOUGERON, Nolwenn; SKALLI, Wafa; ROHAN, Pierre-Yves; PILLET, Helene Several Finite Element (FE) models of the pelvis have been developed to comprehensively assess the onset of pathologies and for clinical and industrial applications. However, because of the difficulties associated with the creation of subject-specific FE mesh from CT scan and MR images, most of the existing models rely on the data of one given individual. Moreover, although several fast and robust methods have been developed for automatically generating tetrahedral meshes of arbitrary geometries, hexahedral meshes are still preferred today because of their distinct advantages but their generation remains an open challenge. Recently, approaches have been proposed for fast 3D reconstruction of bones based on X-ray imaging. In this study, we adapted such an approach for the fast and automatic generation of all-hexahedral subject-specific FE models of the pelvis based on the elastic registration of a generic mesh to the subject-specific target in conjunction with element regularity and quality correction. A full hexahedral subject-specific FE mesh was generated with an accurate surface representation.

Role of Ligaments in the Knee Joint Kinematic Behavior: Development and Validation of a Finite Element Model

GERMAIN, François — Fri, 01 Jan 2016 00:00:00 GMT

Role of Ligaments in the Knee Joint Kinematic Behavior: Development and Validation of a Finite Element Model GERMAIN, François; ROCHCONGAR, Goulven; THOREUX, Patricia; SKALLI, Wafa; ROUCH, Philippe; ROHAN, Pierre-Yves; PILLET, Helene The management of knee instability is a complex problem in orthopedic surgery. To comprehensively assess the biomechanical role of the knee joint and to investigate various aspects of knee mechanics, several Finite Element (FE) knee models have been developed. However, (i) the full validation of these models against tibio-femoral and tibio-patellar kinematic data and (ii) the high numerical costs associated with the computation of the biomechanical response of the knee joint are still main issues. Moreover, the contribution on knee mobility of the different ligaments is still unclear. The aim of this study was therefore to develop an FE model with both extensive validation and low computational for the investigation of the role of ligaments in the joint kinematic behavior. To this end, a 3D FE model, consisting of the distal and proximal part of the femur and tibia, respectively, the patella, the quadriceps tendon, the cartilage, and knee ligaments was developed in ANSYS. For the model evaluation, 23 fresh frozen knee joints were tested in flexion/extension using a validated device. The model-predicted response was within or at the limits of the experimental corridors for all translations and rotations of tibia and patella with regard to the femur. A sensitivity analysis was conducted to evaluate the impact of both the stiffness and initial strain of ligaments on the knee kinematic response. Our results showed the high sensitivity of the model to the mechanical parameters of the ligaments.

Simulating the Remodelling of Bone around Implants

FRAME, Jamie C. — Sun, 01 Jan 2017 00:00:00 GMT

Simulating the Remodelling of Bone around Implants FRAME, Jamie C.; CORTÉ, Laurent; ALLENA, Rachele; ROHAN, Pierre-Yves Introduction Improper osseointegration of implants leading to poor mechanical anchoring or embrittlement of neighboring bone is a major concern in orthopedic surgery [1?]. This integration is known to depend on the complex interplay between the mechanical environment and the cell activity in the tissues surrounding the implant. In order to accurately predict the success of an implant a robust description of the remodeling behavior of bone is required. Building upon previous research modeling osteogenesis around implants [2] a mechano-biological Finite Element (FE) model is proposed to describe the remodeling processes involved when bone, cartilage and fibrous tissue are submitted to mechanical loads. Method In this work, we describe the mechanostat (the interrelationship between loading conditions and remodeling) of bone [1] by modelling the net effect of cellular activities at the tissue level. For that, we distinguish the immature tissues resulting from the early proliferation steps (growth and diffusion) from the mature tissues obtained after a consolidation of the extra-cellular matrix (mineralization for bone). In each elementary volume element, the creation of new (immature) tissue is dependent upon the level of applied strain and is described by a reaction-diffusion equation. Results Using these equations a simple cantilever cyclic bending simulation was created and loaded to recreate a range of physiological strains experienced during bone remodeling. Preliminary results for bone tissue only are presented in Figure 1. This shows the cantilever boundary conditions and maximum normalized shear strain distribution which produce the evolution of immature and mature bone tissues over time. As the Young’s modulus increases proportionately with the increase in mature tissue density the strain under constant loading conditions is observed to reduce, therefore altering the generation of new tissue types. The model proposed here may offer numerous perspectives as a predictive tool for implant design or for the new therapies against bone resorption.

Combining Freehand Ultrasound-Based Indentation and Inverse Finite Element Modelling for the Identification of Hyperelastic Material Properties of Thigh Soft Tissues

FOUGERON, Nolwenn — Wed, 01 Jan 2020 00:00:00 GMT

Combining Freehand Ultrasound-Based Indentation and Inverse Finite Element Modelling for the Identification of Hyperelastic Material Properties of Thigh Soft Tissues FOUGERON, Nolwenn; HEARING, Diane; ROSE, Jean-Loïc; BONNET, Xavier; ROHAN, Pierre-Yves; PILLET, Helene Finite Element Analysis (FEA) is a numerical modelling tool vastly employed in research facilities to analyse and predict load transmission between the human body and a medical device, such as a prosthesis or an exoskeleton. Yet, the use of Finite Element Modelling (FEM) in a framework compatible with clinical constraints is hindered by, amongst others, heavy and time-consuming assessments of material properties. Ultrasound imaging opens new and unique opportunities for the assessment of in vivo material properties of soft tissues. Confident of these advances, a method combining a freehand ultrasound probe and a force sensor was developed in order to compute the hyperelastic constitutive parameters of the soft tissues of the thigh in both relaxed (R) and contracted (C) muscles configurations. Seven asymptomatic subjects were included for the experiment. Two operators in each configuration performed the acquisitions. Inverse FEM allowed for the optimisation of an Ogden’s hyperelastic constitutive model of soft tissues of the thigh in large displacement. The mean shear modulus identified for configurations R and C were respectively 3.2 ± 1.3 kPa and 13.7 ± 6.5 kPa. The mean alpha parameter identified for configurations R and C were respectively 10 ± 1 and 9 ± 4. An analysis of variance showed that the configuration had an effect on constitutive parameters but not the operator.

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

BIJAR, Ahmad — Thu, 01 Jan 2015 00:00:00 GMT

Atlas-Based Automatic Generation of Subject-Specific Finite Element Tongue Meshes BIJAR, Ahmad; PERRIER, Pascal; PAYAN, Yohan; ROHAN, Pierre-Yves 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.

Three-Dimensional Reconstruction of Foot in the Weightbearing Position From Biplanar Radiographs: Evaluation of Accuracy and Reliability

PERRIER, Antoine — Mon, 01 Jan 2018 00:00:00 GMT

Three-Dimensional Reconstruction of Foot in the Weightbearing Position From Biplanar Radiographs: Evaluation of Accuracy and Reliability PERRIER, Antoine; RAMANOUDJAME, Mira; HAUSSELLE, Jérôme; LELIÈVRE, Henri; SERINGE, Raphael; SKALLI, Wafa; WICART, Philippe; ROHAN, Pierre-Yves The initial assessment and postoperative monitoring of patients with various abnormalities of the foot in clinical routine practice is primarily based on the analysis of radiographs taken in the weightbearing position. Conventional x-ray imaging, however, only provides a 2-dimensional projection of 3-dimensional (3D) bony structures, and the clinical parameters assessed from these images can be affected by projection biases. In the present work, we addressed this issue by proposing an accurate 3D reconstruction method of the foot in the weightbearing position from low-dose biplanar radiographs with clinical index measurement assessment for clinical routine practice. The accuracy of the proposed reconstruction method was evaluated for both shape and clinical indexes by comparing 3D reconstructions of 6 cadaveric adult feet from computed tomographic images and from biplanar radiographs. For the reproducibility study, 3D reconstructions from the biplanar radiographs of the foot of 6 able-bodied subjects were considered, with 2 observers repeating each measurement of anatomic landmarks 3 times. Baseline assessment of important 3D clinical parameters was performed on 17 subjects (34 feet; mean age 27.7, range 20 to 52 years). The average point to surface distance between the 3D stereoradiographic reconstruction and the computed tomographic scan-based reconstruction was 1 mm (range 0mm to 6mm). The selected radiographic landmarks were highly reproducible (95% confidence interval <2.0 mm). The greatest interindividual variability for the clinical parameters was observed for the twisting angle (mean 87°, range 73° to 100°). Such an approach opens the way for routine 3D quantitative analysis of the foot in the weightbearing position.

Feasibility of sub-dermal soft tissue deformation assessment using B-mode ultrasound for pressure ulcer prevention

DORIDAM, Jennifer — Mon, 01 Jan 2018 00:00:00 GMT

Feasibility of sub-dermal soft tissue deformation assessment using B-mode ultrasound for pressure ulcer prevention DORIDAM, Jennifer; MACRON, Aurélien; VERNEY, A.; ROHAN, Pierre-Yves; PILLET, Helene; VERGARI, Claudio Pressure Ulcer (PU) prevention remains a main public health issue. The physio-pathology of this injury is not fully understood, and a satisfactory therapy is currently not available. Recently, several works suggested that mechanical strains are responsible of deformation-induced damage involved in the initiation of Deep Tissue Injury (DTI). A better assessment of the internal behavior could allow to enhance the modeling of the transmission of loads into the different structures composing the buttock. A few studies focused on the experimental in vivo buttock deformation quantification using Magnetic Resonance Imaging (MRI), but its use has important drawbacks. In clinical practice, ultrasound imaging is an accessible, low cost, and real-time technic to study the soft tissue. The objective of the present work was to show the feasibility of using B-mode ultrasound imaging for the quantification of localised soft-tissue strains of buttock tissues during sitting. An original protocol was designed, and the intra-operator reliability of the method was assessed. Digital Image Correlation was used to compute the displacement field of the soft tissue of the buttock during a full realistic loading while sitting. Reference data of the strains in the frontal and sagittal planes under the ischium were reported for a population of 7 healthy subjects. The average of shear strains over the region of interest in the fat layer reached levels up to 117% higher than the damage thresholds previously quantified for the muscular tissue in rats. In addition, the observation of the muscles displacements seems to confirm previous results which already reported the absence of muscular tissue under the ischium in the seated position, questioning the assumption commonly made in Finite Element modeling that deep tissue injury initiates in the muscle underlying the bone.

What is the influence of using generic material properties on the estimation of the pelvis sagging when sitting from a Finite Element model of the buttock region?

MACRON, Aurélien — Mon, 01 Jan 2018 00:00:00 GMT

What is the influence of using generic material properties on the estimation of the pelvis sagging when sitting from a Finite Element model of the buttock region? MACRON, Aurélien; DORIDAM, Jennifer; VERNEY, Alexandre; ROHAN, Pierre-Yves; PILLET, Helene Ischial pressure sores are painful, slow healing wounds that develop during prolonged sitting. Its formation is associated with the high internal strains induced by the compression of the soft tissues under the ischium [1]. 3D Finite Element (FE) models have been developed to estimate internal strains in the subdermal soft tissues. Some authors have also investigated the influence of the material properties of the soft tissues [2]. However, the interval of variation of the parameters in these sensitivity studies are not necessarily representative of the variability of subgroups of population. In this contribution, we investigate the influence of using the material properties of one given individual (generic material properties) as representative of a population. The generic material properties were obtained by Finite Element Updating to fit the experimental sagging of the pelvis of one subject when sitting. The 3D subject-specific FE model was generated from the combination of bi-planar Radiography, ultrasound imaging and optical scanner and is composed of the pelvis (rigid body) and 3 homogeneous layers representing the muscle tissue, fat and skin. The adipose tissue and the muscle layer were modelled as an Ogden quasi-incompressible hyperelastic material. The same material parameters were used to estimate the pelvis sagging of 7 healthy subjects. The estimated sagging was compared to the experimental one measured by computing the vertical displacements of both ischial tuberosities visible on the radiographs before and after sitting (Figure 1). For 5 subjects, the differences between both were below 1mm. For the two other subjects, the differences were 4 and 6 mm. These findings suggest that using generic material properties allow to reproduce the biomechanical response of the buttock when sitting for healthy subjects. The same approach could be applied to spinal cord injury population, which will allow to clarify the necessity of personalizing the material properties in models developed for this population.

Development and validation of a new methodology for the fast generation of patient-specific FE models of the buttock for pressure ulcer prevention.

MACRON, Aurélien — Mon, 01 Jan 2018 00:00:00 GMT

Development and validation of a new methodology for the fast generation of patient-specific FE models of the buttock for pressure ulcer prevention. MACRON, Aurélien; DORIDAM, Jennifer; VERNEY, Alexandre; ROHAN, Pierre-Yves; PILLET, Helene Ischial pressure sores are painful, slow healing wounds that develop during prolonged sitting. Its formation is associated with the high internal strains induced by the compression of the soft tissues under the ischium [1]. Although, many 3D Finite Element (FE) models have been developed to predict the mechanical response of the subdermal soft tissues, they are always constructed from segmentation of MRI or CT-Scan acquisitions limiting the studies to only one individual and overlooking the inter-individual variability. In this contribution, we present a new methodology for a fast 3D FE model generation of the buttock for PU prevention. The 3D subject-specific FE model was generated from the combination of bi-planar Radiography, ultrasound imaging and optical scanner and is composed of the pelvis (rigid body) and 3 homogeneous layers representing the muscle tissue, fat and skin. The adipose tissue and the muscle layer were modelled as an Ogden quasi-incompressible hyperelastic material and the material properties were calibrated to fit the experimental data. The validation of the model was performed from external pressure measurement on a population of 6 healthy subjects. The mean difference of the median pressure was 0.32kPa (std 0.8kPa), showing good agreement between the experiments and FE models and representing 2% of the mean value. The low generation time of this model compared to existing methodologies will allow to investigate the influence of pelvis and buttock geometry on the biomechanical response of the subdermal soft tissues under the ischium during sitting.