Dynamic magnetic resonance imaging to quantify pelvic organ mobility after treatment for uterine descent: differences between surgical procedures.

INTRODUCTION AND HYPOTHESIS: Pelvic organ mobility is defined as the displacement of pelvic organs between rest and maximal straining. We hypothesized that pelvic organ mobility after vaginal sacrospinous hysteropexy (SSHP) might be increased compared with other surgeries for uterine descent, which may contribute to the high occurrence of postoperative cystocele after this surgery. Pelvic organ mobility and the vaginal axes after SSHP are compared with other surgical procedures for uterine descent: vaginal hysterectomy with uterosacral suspension (VH) and laparoscopic sacrohysteropexy (LSH). METHODS: In this prospective pilot study, 15 women were included (5 for each procedure). Six months postoperatively, POP-Q examination and dynamic MRI were performed and questionnaires were filled out regarding prolapse complaints. Pelvic organ mobility on MRI was defined as vertical displacement of pelvic organs at rest and maximal straining. The displacements and angles were measured using an image registration method. Furthermore, the angle of displacement of cervix/vaginal vault and vaginal axes were assessed. RESULTS: No anatomical recurrences of pelvic organ prolapse were found. No difference in pelvic organ mobility was demonstrated. After VH, a more posterior position of the upper vagina was found compared with SSHP and LSH. CONCLUSIONS: Based on these data, the higher recurrence risk in the anterior compartment after SSHP cannot be explained. Larger sample sizes, studying women with recurrence or de novo cystocele after SSHP or using an upright MRI scanner would be of interest to further assess the relationship between pelvic organ mobility and the occurrence of anterior vaginal wall prolapse.

Pregnancy impact on uterosacral ligament and pelvic muscles using a 3D numerical and finite element model: preliminary results.

INTRODUCTION AND HYPOTHESIS: We studied the geometry of and changes in structures that play an important role in stabilizing the pelvic system during pregnancy using a numerical system at different gestational ages and postpartum. METHODS: We developed a parturient numerical model to assess pelvic structures at different gestational stages (16, 32, and 38 weeks) and postpartum (2 months and 1 year) using magnetic resonance imaging (MRI). Organs, muscles, and ligaments were segmented to generate a 3D model of the pelvis. We studied changes in the length of uterosacral ligaments (USL) and thickness of the puborectal portion of the levator ani muscle (LAM) during and after pregnancy. We used this model to perform finite element (FE) simulation and analyze deformations of these structures under stress from the increase in uterine weight. RESULTS: Analysis reveals an increase in the length of US ligaments at 16, 32, and 38 weeks. Two months after delivery, it decreases without returning to the length at 16 weeks of pregnancy. Similar changes were observed for the puborectal portion of the LAM. Variations observed in these structures are not equivalent to other anatomical structures of pelvic suspension. FE simulation with increased uterus weight does not lead to those findings. CONCLUSION: This analysis brings new elements and a new focus for discussion relating to changes in pelvic balance of parturient women that are not simply linked to the increase in uterine volume.

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

INTRODUCTION AND HYPOTHESIS: We 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. METHODS: We 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. RESULTS: The 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. CONCLUSION: Our 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.

Elastin density: Link between histological and biomechanical properties of vaginal tissue in women with pelvic organ prolapse?

INTRODUCTION AND HYPOTHESIS: The aim of the study was to correlate histological and biomechanical characteristics of the vaginal wall in women with pelvic organ prolapse (POP). METHODS: Tissue samples were collected from the anterior [point Ba; POP Questionnaire (POP-Q)] and/or posterior (point Bp; POP-Q) vaginal wall of 15 women who underwent vaginal surgery for POP. Both histological and biomechanical assessments were performed from the same tissue samples in 14 of 15 patients. For histological assessment, the density of collagen and elastin fibers was determined by combining high-resolution virtual imaging and computer-assisted digital image analysis. For biomechanical testing, uniaxial tension tests were performed to evaluate vaginal tissue stiffness at low (C0) and high (C1) deformation rates. RESULTS: Biomechanical testing highlights the hyperelastic behavior of the vaginal wall. At low strains (C0), vaginal tissue appeared stiffer when elastin density was low. We found a statistically significant inverse relationship between C0 and the elastin/collagen ratio (p = 0.048) in the lamina propria. However, at large strain levels (C1), no clear relationship was observed between elastin density or elastin/collagen ratio and stiffness, likely reflecting the large dispersion of the mechanical behavior of the tissue samples. CONCLUSION: Histological and biomechanical properties of the vaginal wall vary from patient to patient. This study suggests that elastin density deserves consideration as a relevant factor of vaginal stiffness in women with POP.

Pathophysiological aspects of cystocele with a 3D finite elements model.

PURPOSES: The objective of this study is to design a 3D biomechanical model of the female pelvic system to assess pelvic organ suspension theories and understand cystocele mechanisms. METHODS: A finite elements (FE) model was constructed to calculate the impact of suspension structure geometry on cystocele. The sample was a geometric model of a control patient's pelvic organs. The method used geometric reconstruction, implemented by the biomechanical properties of each anatomic structure. Various geometric configurations were simulated on the FE method to analyse the role of each structure and compare the two main anatomic theories. RESULTS: The main outcome measure was a 3D biomechanical model of the female pelvic system. The various configurations of bladder displacement simulated mechanisms underlying medial, lateral and apical cystocele. FE simulation revealed that pubocervical fascia is the most influential structure in the onset of median cystocele (essentially after 40 % impairment). Lateral cystocele showed a stronger influence of arcus tendineus fasciae pelvis (ATFP) on vaginal wall displacement under short ATFP lengthening. In apical cystocele, the uterosacral ligament showed greater influence than the cardinal ligament. Suspension system elongation increased displacement by 25 % in each type of cystocele. CONCLUSIONS: A 3D digital model enabled simulations of anatomic structures underlying cystocele to better understand cystocele pathophysiology. The model could be used to predict cystocele surgery results and personalising technique by preoperative simulation.

Comparative analysis of pelvic ligaments: a biomechanics study.

INTRODUCTION AND HYPOTHESIS: Pelvic 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 METHODS: Samples 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. RESULTS: We 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. CONCLUSION: Pelvic 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.

Simulation of the mobility of the pelvic system: influence of fascia between organs.

The mobility of pelvic organs is the result of an equilibrium called Pelvic Static characterizing the balance between the properties and geometries of organs, suspensions and support system. Any imbalance in this complex system can cause of pelvic static disorder. Genital prolapse is a common hypermobility pathology which is complex, multi factorial and its surgical management has high rate of complications. The use of 3 D numerical models and simulation enables the role of the various suspension structures to be objectively studied and quantified. Fascias are connective tissues located between organs. Although their role are described as important in various descriptions of pelvic statics, their influence and role has never been quantitatively objectified. This article presents a refine Finite Element (FE) model for a better understanding of biomechanical contribution of inter-organ fascia. The model is built from MRI images of a young volunteer, the mechanical properties derived from literature data to take into account the age of the patient and new experimental results have enabled an order of magnitude of the mechanical properties of the fascias to be defined. The FE results allows to quantify the biomechanical role of the fascia on pelvic mobility quantified by an analysis of dynamic MRI images and a local mapping of the gap between calculated and measured displacements. This improved numerical model integrating the fascias makes it possible to describe pelvic mobilities with a gap of 1 mm between numerical simulations and measurements, whereas without taking them into account this gap locally reaches 20 mm.

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

INTRODUCTION AND HYPOTHESIS: despite 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. METHODS: samples from 11 fresh women cadavers without prolapse were collected. Uniaxial tension tests were performed and stress-strain curves were obtained. RESULTS: biomechanical 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). CONCLUSIONS: skin 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.

[Biomechanical modeling of pelvic organ mobility: towards personalized medicine]. / Modélisation biomécanique des mobilités des organes pelviens: vers une médecine personnalisée.

Female pelvic mobility is crucial for urinary, bowel and sexual function and for vaginal delivery. This mobility is ensured by a complex organ suspension system composed of ligaments, fascia and muscles. Impaired pelvic mobility affects one in three women of all ages and can be incapacitating. Surgical management has a high failure rate, largely owing to poor knowledge of the organ support system, including the barely discernible ligamentous system. We propose a 3D digital model of the pelvic cavity based on MRI images and quantitative tools, designed to locate the pelvic ligaments. We thus obtain a coherent anatomical and functional model which can be used to analyze pelvic pathophysiology. This work represents a first step towards creating a tool for localizing and characterizing the source of pelvic imbalance. We examine possible future applications of this model, in terms of personalized therapy and prevention.

Pelvic organ prolapse meshes: Can they preserve the physiological behavior?

Implants for the cure of female genital prolapse still show numerous complications cases that sometimes have dramatic consequences. These implants must be improved to provide physiological support and restore the normal functionalities of the pelvic area. Besides the trend towards lighter meshes, a better understanding of the in vivo role and impact of the mesh implantation is required. This work investigates the mechanical impact of meshes after implantation with regards to the behavior of the native tissues. Three meshes were studied to assess their mechanical and biological impact on the native tissues. An animal study was conducted on rats. Four groups (n = 17/group) underwent surgery. Rats were implanted on the abdominal wall with one of the three polypropylene knitted mesh (one mesh/group). The last group served as control and underwent the same surgery without any mesh implantation. Post-operative complications, contraction, mechanical rigidities, and residual deformation after cyclic loading were collected. Non-parametric statistical comparisons were performed (Kruskal-Wallis) to observe potential differences between implanted and control groups. Mechanical characterization showed that one of the three meshes did not alter the mechanical behavior of the native tissues. On the contrary, the two others drastically increased the rigidities and were also associated with clinical complications. All of the meshes seem to reduce the geometrical lengthening of the biological tissues that comes with repetitive loads. Mechanical aspects might play a key role in the compatibility of the mesh in vivo. One of the three materials that were implanted during an animal study seems to provide better support and adapt more properly to the physiological behavior of the native tissues.

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

INTRODUCTION AND HYPOTHESIS: Our 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. METHODS: Vaginal 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 performed RESULTS: POP 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. CONCLUSIONS: These 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.

Complete 3 dimensional reconstruction of parturient pelvic floor.

INTRODUCTION: The women pelvic floor is a complex system, which seems to endure several modifications during pregnancy and childbirth. Our primary purpose was to build an extensive 3 dimensional (3D) numerical anatomical model of the women pelvic floor. METHODS: First, the role and the location of each organ, muscle, or ligament, were identified through an extensive literature review. Then, different entities were selected because of their visibility and importance in the pelvic floor. Each entity was identified using anatomical knowledge, and outlined on 2 dimensional (2D) MRI images, that were carried out on 4 pregnant women, using sequences T1, T2 and proton density weighted, through AVIZO program. The overlay of these 2D outlines produced a 3D geometrical reconstruction, which was then reworked with the program CATIA to obtain a usable geometric model. RESULTS: We identified and integrated 15 anatomical structures to the geometrical model, including organs, ligament and muscles from the pelvis and perineum. This geometrical model allowed us to obtain a visual interactive representation with 3D images. These different steps resulted in the creation of a complete numerical model of the female pelvic floor, which might be used in Finite Element simulation. CONCLUSION: A new complete and accurate 3D numerical anatomical model of the women pelvic floor was elaborated. It presents simultaneously analytical prospects, through the observation of the strains and deformations that are imposed on the different structures, and educational prospects, through the detailed visual representation of several situations.

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.

Successful immunosuppressant-free heterotopic transplantation of tracheal allografts in the pig.

OBJECTIVES: It has been demonstrated that both heterotopic and orthotopic transplants of epithelium-denuded cryopreserved tracheal allografts are feasible in immunosuppressant-free rabbits. Validation of these results in large animals is required before considering clinical applications. We evaluated the viability, immune tolerance and strain properties of such tracheal allografts heterotopically transplanted in a pig model. METHODS: Ten tracheal segments, 5 short (5 rings) and 5 long (10 rings), were obtained from male Landrace pigs. The tracheal segments were surgically denuded of their epithelium, then cryopreserved and stored in a tissue bank for 33 to 232 days. After thawing, tracheal segments stented with a silicone tube were wrapped in the omentum in 2 groups of 5 female recipients. The animals did not receive any immunosuppressive drugs. The animals were euthanized from Day 6 to Day 90 in both groups. RESULTS: An effective revascularization of allografts regardless of length was observed. Lymphocyte infiltrate was shown in the early postoperative period and became non-significant after 30 days. Allografts displayed high levels of neoangiogenesis and viable cartilage rings with islets of calcification. Biomechanical measurements demonstrated strain properties similar to those of a fresh tracheal segment from Day 58. CONCLUSIONS: Our results demonstrate the acceptability and satisfactory stiffness of epithelium-denuded cryopreserved tracheal allografts implanted in the omentum, despite the absence of immunosuppressive drugs. Since the omentum has the capability to reach the tracheal region, this approach should be investigated in the setting of orthotopic transplants in a pig model before considering clinical applications.

A nonlinear-elastic constitutive model for soft connective tissue based on a histologic description: Application to female pelvic soft tissue.

To understand the mechanical behavior of soft tissues, two fields of science are essential: biomechanics and histology. Nonetheless, those two fields have not yet been studied together often enough to be unified by a comprehensive model. This study attempts to produce such model. Biomechanical uniaxial tension tests were performed on vaginal tissues from 7 patients undergoing surgery. In parallel, vaginal tissue from the same patients was histologically assessed to determine the elastic fiber ratio. These observations demonstrated a relationship between the stiffness of tissue and its elastin content. To extend this study, a mechanical model, based on an histologic description, was developed to quantitatively correlate the mechanical behavior of vaginal tissue to its elastic fiber content. A satisfactory single-parameter model was developed assuming that the mechanical behavior of collagen and elastin was the same for all patients and that tissues are only composed of collagen and elastin. This single-parameter model showed good correlation with experimental results. The single-parameter mechanical model described here, based on histological description, could be very useful in helping to understand and better describe soft tissues with a view to their characterization. The mechanical behavior of a tissue can thus be determined thanks to its elastin content without introducing too many unidentified parameters.

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.

Biomechanical properties of human pelvic organs.

OBJECTIVE: To comparatively define the biomechanical characteristics of the pelvic organs (the vagina, bladder, and rectum), which are crucial for the maintenance of pelvic support. Despite minimal fundamental studies, meshes are increasingly implanted into the vesicovaginal and rectovaginal spaces to replace disrupted native tissues and to treat pelvic organ prolapse. However, the mechanical characteristics of these materials have not yet been compared with those of the "functional unit," the vagina, bladder, and rectum. METHODS: Samples from 5 fresh female cadavers without prolapse were collected. Uniaxial tension tests under monotonic and cyclic loading were performed and the stress-strain curves obtained. RESULTS: The biomechanical properties of the vaginal, bladder, and rectal tissues differed significantly. We were able to demonstrate a nonlinear relationship between the stress and strain and a visco-hyperelastic behavior with a Mullins effect of damage of the tissues examined. Comparable rigidity was found between the investigated tissues at low strains; however, at large strain levels, marked differences could be observed. The vagina was much more rigid and less extendible than the rectal tissue, which, in turn, was more rigid than the bladder tissue. The anterior and posterior vagina revealed a different stiffness, and the bladder tissue was anisotropic at large strain levels. CONCLUSION: Our results underline the pivotal role of the vaginal tissue for the maintenance of pelvic support. The observed differences with respect to tissue rigidity should have pronounced effects on the physiologic organ function, pointing to the necessity of a differentiated view on using the same prosthetic material for different anatomic locations.

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.

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-l(0)/l(0)) 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.