Pierre-Yves Rohan

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

The 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/).

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

Several Finite Element (FE) models of the pelvis have been developed to comprehensively assess nthe onset of pathologies and for clinical and industrial applications. However, because of the difficulties nassociated with the creation of subject-specific FE mesh from CT scan and MR images, nmost of the existing models rely on the data of one given individual. Moreover, although several nfast and robust methods have been developed for automatically generating tetrahedral meshes nof arbitrary geometries, hexahedral meshes are still preferred today because of their distinct nadvantages but their generation remains an open challenge. Recently, approaches have been nproposed for fast 3D reconstruction of bones based on X-ray imaging. In this study, we adapted nsuch an approach for the fast and automatic generation of all-hexahedral subject-specific FE nmodels of the pelvis based on the elastic registration of a generic mesh to the subject-specific ntarget in conjunction with element regularity and quality correction. A full hexahedral subject-specific FE mesh was generated with an accurate surface nrepresentation.

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

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.

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

Abstract 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. &NA; Level of Clinical Evidence: 5

Development and evaluation of a new methodology for the fast generation of patient-specific Finite Element models of the buttock for sitting-acquired deep tissue injury prevention

The occurrence and management of Pressure Ulcers remain a major issue for patients with reduced mobility and neurosensory loss despite significant improvement in the prevention methods. These injuries are caused by biological cascades leading from a given mechanical loading state in tissues to irreversible tissue damage. Estimating the internal mechanical conditions within loaded soft tissues has the potential of improving the management and prevention of PU. Several Finite Element models of the buttock have therefore been proposed based on either MRI or CT-Scan data. However, because of the limited availability of MRI or CT-Scan systems and of the long segmentation time, all studies in the literature include the data of only one individual. Yet the inter-individual variability cant be overlooked when dealing with patient specific estimation of internal tissue loading. As an alternative, this contribution focuses on the combined use of low-dose biplanar X-ray images, B-mode ultrasound images and optical scanner acquisitions in a non-weight-bearing sitting posture for the fast generation of patient-specific FE models of the buttock. Model calibration was performed based on Ischial Tuberosity sagging. Model evaluation was performed by comparing the simulated contact pressure with experimental observations on a population of 6 healthy subjects. Analysis of the models confirmed the high inter-individual variability of soft tissue response (maximum Green Lagrange shear strains of 213u202f±u202f101% in the muscle). This methodology opens the way for investigating inter-individual factors influencing the soft tissue response during sitting and for providing tools to assess PU risk.

Chapter 22 – Spine

Clinical problems of the human spine have a high prevalence, affecting more than 25.5 million people in 2012. Older adults, in particular, are susceptible to degenerative spine disorders such as deformities or osteoporosis. A basic requirement for proper management of various spinal disorders, effective injury prevention, and rehabilitation is a detailed knowledge of the fundamental biomechanics of the spine. Despite growing interest in biomechanical research on the spine during the last decades, however, many clinical problems remain largely unsolved, because of poor understanding of the underlying degeneration phenomena and the complexity of the spinal construct. In particular, diagnosis is challenging, because of the lack of tools to quantitatively assess soft tissue alteration, and because the most relevant clinical indices for diagnosis are not clearly established. Driven by ever-growing computer power and imaging devices, the development of FE models has become widespread, allowing scientists to overcome some of the existing shortcomings (invasiveness, complexity of the organization of the biological tissues, and complexity of establishing the loads present in the human spine, for example). These have emerged as powerful and reliable tools with considerable applications in surgery planning, in studying the etiology, progression, and effects of spinal deformities and intervertebral discs. These models have enhanced our understanding of the spine and will continue to do so. In our group, numerical work performed using FE modeling has highlighted the paramount influence of both geometric patient-specific modeling and in vivo personalization of tissue mechanical properties. Among the many exciting avenues for future research is the question of the validation of computational modeling and simulation with the perspective of supporting the development of medical devices.

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

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.