Pauline Lecomte-Grosbras

Characterization of free edge effects: influence of mechanical properties, microstructure and structure effects

The elastic property mismatch between plies with different orientations induces stress concentrations near free edges. This free edge effect can cause early delamination of composite structures. Laminates with 15° and –15° plies are studied experimentally to highlight the free-edge effect and the induced micromechanism damage. Full field measurements under tensile loading are performed at macroscopic and mesoscopic scales on edges of the sample. Results show displacement gradients and strain concentrations near interlaminar interfaces. Residual displacement gradients are measured after unloading, which highlights local damage at interlaminar interfaces. Observations at the microscopic scale show that cracks appear at fibre/matrix interfaces and propagate between adjacent fibres along interlaminar interfaces. A comparison of the results obtained on different composites highlights the influence of mechanical properties and material microstructure on edge effects. The study of samples with dropped plies highlights the influence of the combination of both geometric and material singularities on edge effects.

Towards a Better Understanding of Pelvic System Disorders Using Numerical Simulation

Genital prolapse is a pathologic hyper-mobility of the organs that forms the pelvic system. Although this is common condition, the pathophysiology of this disorder is not well known. In order to improve the understanding of its origins, we recreate--virtually--this biomechanical pathology using numerical simulation. The approach builds on a finite element model with parameters measured on several fresh cadavers. The meshes are created from a MRI of a healthy woman and the simulation includes the mechanical interactions between organs (contacts, ligaments, adhesion...). The model is validated through comparison of functional mobilities of the pelvic system observed on a dynamic MRI. We then propose to modify, step by step, the model and its parameters to produce a pathologic situation and have a better understanding of the process. It is not a formal proof but the numerical experiments reinforce the clinical hypothesis on the multifactorial origins of the pathology.

FE Simulation for the Understanding of the Median Cystocele Prolapse Occurrence

Female pelvic organ prolapse is a complex mechanism combining the mechanical behavior of the tissues involved and their geometry defects. The developed approach consists in generating a parametric FE model of the whole pelvic system to analyze the influence of this material and geometric combination on median cystocele prolapse occurrence. In accordance with epidemiological and anatomical literature, the results of the numerical approach proposed show that the geometrical aspects have a stronger influence than material properties. The fascia between the bladder and vagina and paravaginal ligaments are the most important anatomical structures inducing the amplitude of cystocele prolapse. This FE model has also allowed studying the coupled effect, showing a significant influence of the fascia size. The study allows highlighting the origins of the median cystocele prolapse and responds to this major issue of mobility occurrence.

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.

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.

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.

Biomechanics of pelvic system: Towards a definition of the required mechanical properties of implants

Genital prolapse is a prevalent pelvic disorder inducing hypermobility of organs. Its physiopathology is not well understood as highlighted by the high rate of failure of the surgical treatments. A better definition of the pelvic tissues properties is needed to design more functional prostheses. Image registration is first used to describe the structure of the pelvic system. Experimental characterization is done to have a map of the mechanical properties of pelvic soft tissues and compare healthy and pathologic tissues behaviour. Then a model based on macromolecular approach and histologic composition is proposed.