Jean-François Witz

Influence of Geometry and Mechanical Properties on the Accuracy of Patient-Specific Simulation of Women Pelvic Floor

The woman pelvic system involves multiple organs, muscles, ligaments, and fasciae where different pathologies may occur. Here we are most interested in abnormal mobility, often caused by complex and not fully understood mechanisms. Computer simulation and modeling using the finite element (FE) method are the tools helping to better understand the pathological mobility, but of course patient-specific models are required to make contribution to patient care. These models require a good representation of the pelvic system geometry, information on the material properties, boundary conditions and loading. In this contribution we focus on the relative influence of the inaccuracies in geometry description and of uncertainty of patient-specific material properties of soft connective tissues. We conducted a comparative study using several constitutive behavior laws and variations in geometry description resulting from the imprecision of clinical imaging and image analysis. We find that geometry seems to have the dominant effect on the pelvic organ mobility simulation results. Provided that proper finite deformation non-linear FE solution procedures are used, the influence of the functional form of the constitutive law might be for practical purposes negligible. These last findings confirm similar results from the fields of modeling neurosurgery and abdominal aortic aneurysms.

Monitoring of Transient Phenomena in Sliding Contact Application to Friction Brakes

This work focuses on the study of transient phenomena, in particular the non-uniformity and space–time variation of friction forces and surface temperature of a brake disc during stop-braking. Friction tests were conducted on a braking tribometer. The friction forces in the contact were measured using a 3D piezoelectric sensor, while the disc surface temperature was investigated by means of a high frequency fibre-optic two-colour pyrometer. An optical lap-top device was used to keep track of disc revolutions, and an original programme was written to plot the space–time variations of the measured parameters. This new original approach helps better understand the coupling between thermal and tribological phenomena occurring during braking.

An improved lagrangian thermography procedure for the quantification of the temperature fields within polycrystals

Polycrystalline metallic materials are made of an aggregate of grains more or less well oriented with respect to the loading axis. During mechanical loading, the diversity of grain orientations leads to a heterogeneous deformation at the local scale. It is well known that most of the plastic work generated during the deformation process reappears in the form of heat, whereas a certain proportion remains latent in the material and is associated with microstructural changes. To access the local stored energy during deformation processes, experimental energy balances are needed at a suitable scale. Thus, simultaneous measurements of thermal and kinematic fields were made in-house at the microstructural scale of a 316L stainless steel submitted to a macroscopic monotonic tensile test. The aim of the present study is to propose a complete calibration strategy allowing us to estimate the thermal variations of each material point along its local and complex deformation path. This calibration strategy is a key element for achieving experimental granular energy balances and has to overcome two major experimental problems: the dynamics of each infrared focal plane array sensor that leads to undesired spatial and temporal noise and the complexity of the local loading path that must be captured by simultaneous complementary measurement. The improvement of such a multifield strategy is crucial for performing properly the experimental and local energy balances required to build new energetically based damage criteria.

Experimental study of the mechanical behavior of an explanted mesh: The influence of healing

To better understand the in vivo mechanical behavior of synthetic mesh implants, we designed a specific experimental protocol for the mechanical characterization of explanted mesh under uniaxial tension. The implantation of a mesh leads to the development of scar tissue and the formation of a new composite made of native tissue, a mesh implant and scar tissues. This study focused on three points: determining the minimum representative size of mesh implants required for mechanical test samples, highlighting the influence of healing, and defining the healing time required to ensure stabilized mechanical properties. First, we determined the minimum representative size of mesh implants for the mechanical characterization with a study on a synthetic composite made of mesh and an elastomeric matrix mimicking the biological tissues. The size of the samples tested was gradually decreased. The downsizing process was stopped, when the mechanical properties of the composite were not preserved under uniaxial tension. It led to a sample representative size 3cm long and 2cm wide between the grips. Then an animal study was conducted on Wistar rats divided into eight groups. One group was set as control, consisting of the healthy abdominal wall. The other seven groups underwent surgery as follows: one placebo (i.e., without mesh placement), and six with a mesh installation on the abdominal wall and healing time. The rats were sacrificed after different healing times ranging from 1 to 5 months. We observed the influence of healing and healing time on the mechanical response under uniaxial tension of the new composite formed by scar, native tissue, and textile. It seems that 2 months are required to ensure the stabilization of the mechanical properties of the implanted mesh. We were not able to tell the control group (native abdominal wall) from the placebo group (native and scar tissue). This protocol was tested on two different prostheses after 3 months of healing. With this protocol, we were able to differentiate one mesh from another after host integration.

Wavelet Analysis of Experimental Blade Vibrations During Interaction With an Abradable Coating

Minimizing the clearance between turbofan blades and the surrounding casing is a key factor to achieving compressor efficiency. The deposition of an abradable coating on casings is one of the technologies used to reduce this blade-casing clearance and ensure blade integrity in the event of blade-casing contact. Aircraft in-service condi-tions may lead to interactions between the blade tip and the coated casing, during which wear of the abradable coating, blade dynamics and interacting force are critical yet little-understood issues. In order to study blade/abradable-coating interactions of a few tens of milliseconds, experiments were conducted on a dedicated test rig. The experi-mental data were analyzed with the aim of determining the friction-induced vibrational modes of the blade. This involved a time-frequency analysis of the experimental blade strain using the Continuous Wavelet Transform, combined with a modal analysis of the blade. The latter was carried out with two kinds of kinematic boundary conditions at the blade tip: free and modified, by imposing contact with the abradable coating. The interaction data show that the blade vibration modes identified during interactions cor-respond to the free boundary condition, due to the transitional nature of the phenomena and the very short duration of contacts. The properties of the Continous Wavelet Trans-form were then used to identify the occurence of blade-coating contact. Two kinds of blade/abradable-coating interactions were identified: bouncing of the blade over short time periods associated with loss of abradable material, and isolated contacts capable of amplifying the blade vibrations without causing significant wear of the abradable coat-ing. The results obtained were corroborated by high-speed imaging of the interactions.

Experimental Characterization and Simulation of Layer Interaction in Facial Soft Tissues

Anatomically detailed modeling of soft tissue structures such as the forehead plays an important role in physics based simulations of facial expressions, for surgery planning, and implant design. We present ultrasound measurements of through-layer tissue deformation in different regions of the forehead. These data were used to determine the local dependence of tissue interaction properties in terms of variations in the relative deformation between individual layers. A physically based finite element model of the forehead is developed and simulations are compared with measurements in order to validate local tissue interaction properties. The model is used for simulation of forehead wrinkling during frontalis muscle contraction.

Evaluation of Strains on Levator Ani Muscle: Damage Induced During Delivery for a Prediction of Patient Risks

Since childbirth presents a significant risk factor for pathology occurrence of the pelvic floor, analysis of the phenomena involved during a vaginal delivery is a major issue in obstetrics and gynecology researches. Computational biomechanics tool dedicated to the delivery could help to understand the causes of injuries and predict the perineal lesion. From MRI images of four women at different terms of pregnancy, a parametric FE model is generated and allows to analyze the potential damage areas during childbirth, related to strain rate of anatomical structures. The influence of the geometry of levator ani muscle, head size, terms, and cephalic presentations are investigated. The geometrical refinement of anatomical structures influences the strain levels and helps to localized more precisely the most injured areas. Posterior cephalic presentation presents higher injury risk than the anterior one. Maternal geometry at different terms brings equivalent results contrary to the fetal head sizes that have an influence on the strain level and the potential damage induced. This multi-parametric investigation allows us to have a customizable and predictive tool evaluating the potential damages on the pelvis during delivery.

Characterization of the anisotropic mechanical behavior of human abdominal wall connective tissues

Abdominal wall sheathing tissues are commonly involved in hernia formation. However, there is very limited work studying mechanics of all tissues from the same donor which prevents a complete understanding of the abdominal wall behavior and the differences in these tissues. The aim of this study was to investigate the differences between the mechanical properties of the linea alba and the anterior and posterior rectus sheaths from a macroscopic point of view. Eight full-thickness human anterior abdominal walls of both genders were collected and longitudinal and transverse samples were harvested from the three sheathing connective tissues. The total of 398 uniaxial tensile tests was conducted and the mechanical characteristics of the behavior (tangent rigidities for small and large deformations) were determined. Statistical comparisons highlighted heterogeneity and non-linearity in behavior of the three tissues under both small and large deformations. High anisotropy was observed under small and large deformations with higher stress in the transverse direction. Variabilities in the mechanical properties of the linea alba according to the gender and location were also identified. Finally, data dispersion correlated with microstructure revealed that macroscopic characterization is not sufficient to fully describe behavior. Microstructure consideration is needed. These results provide a better understanding of the mechanical behavior of the abdominal wall sheathing tissues as well as the directions for microstructure-based constitutive model.

Patient-Specific Simulation: Non-Destructive Identification Method for Soft Tissue Under Large Strain: Application to Pelvic System

This work presents a non-destructive method to assess mechanical properties of the patient-specific soft tissues of a multi-organ system under large strain. The presented application is focusing on the female pelvic cavity. Based on an experimental data bank of mechanical properties, dynamic MRI’s displacement field analysis, MRI’s geometrical reconstruction, and FE model of the pelvic cavity, a protocol has been developed to identify the material properties of a specific patient’s organs. The purpose of this paper is to tackle that issue by using an inverse finite element analysis. Mechanical properties of the soft tissues are optimized to obtain the MRI’s observed displacement of the cervix on the FE model.

Digital Image Correlation for Large Strain

Due to their high deformability, the elastomers are widely used in various fields of applications. Their hyper-elastic behavior requires means of characterization adapted to contactless, high strain as well as heterogeneous strain field. Digital Image Correlation (DIC) have been used to characterize such material. In case of large strain it might be necessary to re-actualize the reference image to compute the displacement field on every images of the test. Different strategy to apply DIC in large strain are presented. The direct addition of the successive calculated displacement leads to inconsistent results, it is required to make a composition of the successive calculated displacements. Some artefacts due to the multiple interpolation are highlighted and may leads to filter the results. In case of high strain gradients, a method with very small element size and median filter applied directly in correlation process has been used highlighting the ability of large strain DIC in heterogeneous media.