Mathias Brieu

Biomechanical properties of vaginal tissue: preliminary results

The aim of this study is to characterise the biomechanical properties of vaginal tissue to develop an accurate cure of pelvic organ prolapse (POP). Prolapsed vaginal tissues were extracted during the prolapse cure of five patients (POP) and on five cadavers without noticed pelvic floor dysfunction (non-pelvic organ prolapse) with agreement of the ethics committee. Uni-axial tension was performed, and the results were analysed. Individual reproducibility of experimental results was good, and the results highlight the non-linear relationship between stress (force per unit of surface) and strain (l − l0 / l0) and very large deformation before rupture appearance. This experimental study has proven for the first time that the mechanical behaviour of vaginal tissue has to be defined as hyperelastic with a large deformation. This response has to be taken into account to develop accurate synthetic prostheses for POP cure and in the numerical simulation of the pelvic floor.

Biomechanical properties of prolapsed or non-prolapsed vaginal tissue: impact on genital prolapse surgery

Introduction and hypothesisOur aim is to characterize prolapsed and non-prolapsed vaginal tissue, and thus offer a better understanding of the genital prolapse physiopathology and an improvement of surgical treatments.MethodsVaginal tissue was collected in 30 patients with prolapse (POP) and ten fresh cadavers without prolapse (nPOP) with a favorable advice of Ethics Committee. Uniaxial tension tests were performed. Statistical comparisons of rigidity under moderate deformation and under large deformation have been performedResultsPOP is significantly stiffer than nPOP tissue, both on anterior and posterior walls. A significant difference between POP and nPOP tissues was highlighted when anterior or posterior vaginal walls were respectively compared.ConclusionsThese results might explain the higher rate of relapse when repair is autologous, using already defective and more rigid vaginal tissue. This study suggests that it might be interesting to adapt the characteristics of prosthetic implants to the vaginal face concerned by the prolapsus.

Directional model for isotropic and anisotropic hyperelastic rubber-like materials

A material direction-dependent constitutive model has been formulated for large deformations for isotropic and anisotropic rubber-like materials. Although such materials are usually isotropic, anisotropic behavior has been observed in calendered plates of filled rubbers. Strain energy density function characterizing rubber-like materials is usually dependent on principal stretch ratios and thus is unable to account for anisotropy, whereas the proposed strain energy density depends on material directions and accounts for anisotropy. The material directions have simply been chosen using regular solid geometry. The strain energy density is given as the sum, over all material directions, of elementary directional strain energy densities. Then the elementary strain energy form is phenomenologically determined to account for the state of strain dependence of the material response. The model response is compared to uniaxial tension experimental data for anisotropic hyperelastic rubber-like materials and to uniaxial and biaxial tension for isotropic rubber-like materials.

Comparative analysis of pelvic ligaments: a biomechanics study

Introduction and hypothesisPelvic organ prolapse (POP) affects one third of women of all ages and is a major concern for gynecological surgeons. In pelvic reconstructive surgery, native ligaments are widely used as a corrective support, while their biomechanical properties are unknown. We hypothesized differences in the strength of various pelvic ligaments and therefore, aimed to evaluate and compare their biomechanical properties.Materials and methodsSamples from the left and right broad, round, and uterosacral ligaments from 13 fresh female cadavers without pelvic organ prolapse were collected. Uniaxial tension tests at a constant rate of deformation were performed and stress–strain curves were obtained.ResultsWe observed a non-linear stress–strain relationship and a hyperelastic mechanical behavior of the tissues. The uterosacral ligaments were the most rigid whether at low or high deformation, while the round ligament was more rigid than the broad ligament.ConclusionPelvic ligaments differ in their biomechanical properties and there is fairly good evidence that the uterosacral ligaments play an important role in the maintenance of pelvic support from a biomechanical point of view.

Vagina, abdominal skin, and aponeurosis: do they have similar biomechanical properties?

Introduction and hypothesisDespite minimal fundamental works, there is an increasing use of meshes in urogynecology. The concept is mainly based on experiences with abdominal wall surgery. We aimed to compare the biomechanical properties of vaginal tissue, abdominal aponeurosis, and skin.MethodsSamples from 11 fresh women cadavers without prolapse were collected. Uniaxial tension tests were performed and stress–strain curves were obtained.ResultsBiomechanical properties of the vagina, aponeurosis, and skin differed significantly. The aponeurosis was much more rigid and less extendible than the vagina and skin. Vaginal tissue was less rigid but more extendible than skin. There was no difference between the vagina and skin at low strains (p = 0.341), but a highly significant difference at large strains (p = 0.005).ConclusionsSkin and aponeurosis are not suited to predict vaginal tissue biomechanics. We should be cautious when transferring experiences from abdominal wall surgery to vaginal reconstructive surgery.

Experiments and finite element modelling for the study of prolapse in the pelvic floor system

Pelvic prolapse affects one woman in three of all ages combined and is quite common for more than 60% of patients over 60 years of age. The treatment of this pathological problem is one of the biggest challenges to the gynaecologist today. The rate of surgical intervention failure is quite significant. The recurrence of prolapse could be related to inadequate surgical technique or the pathology or/and biomechanical deficiency of the soft tissues. The modelling and simulation of the behaviour of the pelvic cavity could be a major tool for specific evaluation of pelvic status. A first stage of this model is being developed and reported. The computer-aided design model of the organs of the pelvic floor is created using magnetic resonance image data and the ligament boundary conditions are defined. A multi-organ geometric model is thus created and studied.

A New Biaxial Tension Test Fixture for Uniaxial Testing Machine—A Validation for Hyperelastic Behavior of Rubber-like Materials

A new mechanism for a biaxial tension test is developed for loading an in-plane specimen simultaneously in two principal directions. This mechanism can be adapted to any uniaxial tension test machine and thereby it reduces the cost of conducting tests on expensive machines. It provides a uniform state of equibiaxial tension necessary for procedure characterizing the biaxial loading of any material system and particularly for understanding the hyperelastic behavior law of rubber-like materials for large deformations. The mechanism can also be utilized for evaluating an interaction coefficient of anisotropic or orthotropic materials like reinforced composites that can help in characterizing and predicting failure behavior. As a sample case, the experimental results obtained by this new mechanism are validated with the existing models for two rubber-like materials undergoing hyperelasticity.

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.

Improved viscoelastic model for laminate composite under static and dynamic loadings

Organic matrix composite materials are rate-dependent. Approaches exist for static and for dynamic cases but not for both. In the present study, a viscoelastic spectral model, classically used for low strain rates, was successfully identified on dynamic test data as being suitable for high strain rates. The associated identified model was no longer available for low strain rates. An improved formulation of the model was devised, to be representative for both high and low strain rates. Results exhibited the desired rate dependence and good agreement with experimental data.

3D simulation of pelvic system numerical simulation for a better understanding of the contribution of the uterine ligaments

Introduction and hypothesisGenital prolapse remains a complex pathological condition. Physiopathology remains poorly understood, aetiology is multi-factorial, surgery is not always satisfying, as the rate of relapse cannot be overlooked. More over a good anatomical result will not always guarantee functional satisfaction. The aim of our study is to have a better understanding of the involvement of uterine ligaments in pelvic statics via 3D simulation.MethodsSimulation of pelvic mobility is performed with a validated numerical model in a normal situation (standing up to lying down) or induced pathological ones where parts of the constitutive elements of the model are virtually “cut” independently. Displacements are then discussed.ResultsNumerical results have been compared with dynamic MRI for two volunteers. Dynamic sequences had 90 images, and 180 simulations have been validated. Results are coherent with clinical data and the literature, thus validating our mechanical approach. Uterine ligaments are involved in pelvic statics, but their lesions are not sufficient to generate a genital prolapse. Round ligaments play a part in uterine orientation; the utero-sacral ligaments support the uterus when standing up.ConclusionsPelvic normal and pathological mobility study via modelling and 3D simulation is a new strategy in understanding the complex multifactorial physiopathology of genital prolapse. This approach must be validated in a larger series of patients. Nevertheless, pelvic ligaments seem to play an important role in statics, especially, in agreement with a literature survey, utero-sacral ligaments in a standing position.