Olivier Mayeur

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.

Mobility and stress analysis of different surgical simulations during a sacral colpopexy, using a finite element model of the pelvic system

Introduction and hypothesisWe aim to analyze the combined influence of the size of the mesh, the number of sutures, the combined use of an anterior and posterior mesh, and the tension applied to the promontory, on the mobility of the pelvic organs and on the sutures, using a Finite Element (FE) model of the female pelvic system during abdominal sacral colpopexy.MethodsWe used a FE model of the female pelvic system, which allowed us to simulate the mobility of the pelvic system and to evaluate problems related to female prolapse. The meshes were added to the geometrical model and then transferred to computing software. This analysis allowed us to compare the stress and mobility during a thrust effort in different situations.ResultsThe bigger the mesh, the less mobility of both anterior and posterior organs there would be. This is accompanied by an increase in stress at the suture level. The combination of a posterior mesh with an anterior one decreases mobility and stress at the suture level. There is a particularly relevant stressing zone on the suture at the cervix. The increase in the number of sutures induces a decrease in the tension applied at each suture zone and has no impact on organ mobility.ConclusionOur model enables us to simulate and analyze an infinite number of surgical hypotheses. Even if these results are not validated at a clinical level, we can observe the importance of the association of both an anterior and a posterior mesh or the number of sutures.

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.

The role of childbirth research simulators in clinical practice.

Birthing simulators have been developed for two different purposes. One is to achieve a delivery model for exclusively educational purposes. The other is to analyze the stresses and constraints on the pelvic system during labor. The construction of researchmodels follows four steps [1]: acquiring the geometric model (the finite element) from an imaging segmentation, defining the biomechanical properties, defining the boundary conditions of the structures, and finally performing the damage analysis (Fig. 1). Most teams who have previously worked on the pathophysiological link between childbirth and urogenital prolapseweremainly interested in the pelvic floor muscles. For example, Ashton-Miller and DeLancey [2] showed that stretch in the pelvic floor muscles can be increased at the end of the second stage of labor, especially in the pubovisceral area of the levator animuscle.Magnetic resonance imaging provides evidence that areas with the strongest stretching are at increased risk of injury, especially when forceps are used [2]. Ashton-Miller and DeLancey hypothesized that these lesions are at the origin of the urogenital prolapse. Parente et al. [3] studied the strength of the pelvic floor muscle against the fetal descent. They suggested that activation of the pelvic floor muscles during childbirth can be an obstacle to the descent of the fetus and can increase the risk of pelvic floor injury. They showed that the more cephalic the flexion, the less resistance against muscle fetal descent and that the risk of muscle stress is diminished [3]. Lepage et al. [1] developed a simulator taking into account the ligamentous system. During the passage of a fetal head in the 50th percentile, the uterosacral ligaments undergo a 30% deformation and are involved in the occurrence of urogenital prolapse [4]. These clinical findings suggest that deflected presentations, posterior varieties, and fetal extractions increase the risk of perineal injury, and demonstrate the pathophysiological link between childbirth and urogenital prolapse. The difficulty is the inability to obtain fresh pregnant tissues to test the biomechanical properties. Work is underway in our team to determine the properties of each patient using dynamic MRI and intravaginal pressure sensors. Subsequently, it could be possible to use simulation in personalized medicine by adapting a model to fetal and maternal anthropomorphic properties and then to predict each mother’s delivery process. A cephalopelvic confrontation could be considered and perineal muscle injuries could be predicted, which could thus help physicians to justify a preventive cesarean delivery or conduct preventive care. In conclusion, research simulators are beneficial in clinical practice. The simulation could have implications for public health, with the goal of decreasing maternal and neonatal morbidity and mortality.

Pedagogical childbirth simulators: utility in obstetrics

Simulation training is an appealing and useful addition to health facilities. Simulation centers are organized to maximize network resources. Simulation training is used for certification or recertification of health professionals and is now an integral part of the methods used in continuing professional development. Simulation has played a unique role in obstetrics. This article is a narrative review describing the different types of childbirth simulators, whether anatomical, virtual, or instrumented. The article identifies the role of each simulator in the training of obstetricians and the role of these instruments in simulation centers.

Anisotropy and strain rate effects on bovine cortical bone: combination of high-resolution imaging and dynamic loading.

Biomechanics of impact requires high accuracy responses at both mesoand macro-scales of the skeleton. The effect of strain rate and the direction of loading are also crucial for a goodpredictionof injuries.Bone isdescribed in the literature as a hard-mineralised tissue composed of fibres, an organic matrix and inorganic salts. Cortical bone has porosity under 30%, whereas the porosity in cancellous bone varies from 30% to 90%. Anisotropies, heterogeneities and nonlinearity aspects of cortical bone have already been studied through different mechanical tests (Lindhom et al. 1964; Novitskaya et al. 2011).Nevertheless, the accurate investigation of strain rate effect is still one of the challenges in bone characterisation. This study combines high-resolution imaging techniquewith themechanical responses of cortical bone under compression loadings at high strain rates.

Analysis of the cortical bone thickness of human thorax based on multi-scale imaging techniques

Recent studies issued by the World Health Organization (2009) reported that road accidents will be the third highest cause of premature deaths in 2020. Besides these general considerations, research aiming to improve road safety is still needed to provide advanced numerical tools for future evaluations of new cars. Numerous models of the human body have already been developed butwere limited in their biofidelity. The thorax is one of the segments frequently involved in road accidents and the complex ribs geometry complicates the characterisation tasks. One such task concerns the description of rib geometry and more precisely the thinness of cortical bone. The cortical bone distribution in the ribs is one of the major factors influencing the thorax response when submitted to an impact (Kemper et al. 2007). Previous studies investigating this parameter using physical measurements (Roberts and Chen 1972) and photographs (Yoganandan andPintar 1998) suffer froma lack of accuracy. Recently, medical CT scans, vet CT scans (Charpail et al. 2005) and mCT scans (Li et al. 2009) have been used for a better characterisation. This study presents a new method to investigate the whole rib geometry using accurate multilevel scale acquisition devices.