Contractions, a risk for premature rupture of fetal membranes: A new protocol with cyclic biaxial tension

Does preconditioning lower the rupture resistance of chorioamniotic membrane?

Premature rupture of fetal membrane occurs in about 3% of all pregnancies. The physical integrity of chorioamnion (CA) membrane should be retained until delivery for a healthy pregnancy. To explore the effect of pre-conditioning and probe size on the mechanical properties of human chorioamniotic sac, the mechanical properties of 17 human chorioamniotic membranes, collected from cesarean delivery, were examined using biaxial puncture tests with and without preconditioning by different probe sizes. For preconditioned samples, the mean ± std. of ultimate rupture stress was calculated to be 1.73 ± 0.13, 1.61 ± 0.29 and 1.78 ± 0.26 MPa for the probe sizes of 3, 5 and 7 mm, respectively. For samples with no preconditioning, these values were calculated to be 2.38 ± 0.29, 2.36 ± 0.37, and 2.59 ± 0.43 MPa for the above-mentioned probe sizes. The force to probe diameter for samples with no preconditioning was in the range of 1087-1301 N/m for the three probe diameters, well in the range of 850-1580 N/m reported by previous studies. Our results show that the preconditioned samples had significantly lower ultimate puncture force and ultimate stress compared to non-preconditioned samples. In addition, a correlation between the probe size and the magnitude of puncture force was observed, while the stress values were not significantly affected by changing probe size.

Ex-vivo mechanical sealing properties and toxicity of a bioadhesive patch as sealing system for fetal membrane iatrogenic defects

Preterm prelabor rupture of membranes (PPROM) is the most frequent complication of fetal surgery. Strategies to seal the membrane defect created by fetoscopy aiming to reduce the occurrence of PPROM have been attempted with little success. The objective of this study was to evaluate the ex-vivo mechanical sealing properties and toxicity of four different bioadhesives integrated in semi-rigid patches for fetal membranes. We performed and ex-vivo study using term human fetal membranes to compare the four integrated patches composed of silicone or silicone-polyurethane combined with dopaminated-hyaluronic acid or hydroxypropyl methylcellulose (HPMC). For mechanical sealing properties, membranes were mounted in a multiaxial inflation device with saline, perforated and sealed with the 4 combinations. We measured bursting pressure and maximum pressure free of leakage (n = 8). For toxicity, an organ culture of membranes sealed with the patches was used to measure pyknotic index (PI) and lactate dehydrogenase (LDH) concentration (n = 5). All bioadhesives achieved appropriate bursting pressures, but only HPMC forms achieved high maximum pressures free of leakage. Concerning toxicity, bioadhesives showed low PI and LDH levels, suggesting no cell toxicity. We conclude that a semi-rigid patch coated with HPMC achieved ex-vivo sealing of iatrogenic defects in fetal membranes with no signs of cell toxicity. These results warrant further research addressing long-term adhesiveness and feasibility as a sealing system for fetoscopy.

Collagen bundling and alignment in equibiaxially stretched human amnion

We study irreversible collagen arrangement processes in ex-vivo human amnions subjected to inflation tests, which simulate the mechanical conditions prior to and during the initiation of labor uterine contractions. The investigation is focused on the center of the membrane where the stresses are maximal and equibiaxial. Second harmonic generation reveals an unexpected collagen rearrangement in the compact layer that is responsible for the structural integrity of the fetal membrane. The observed bundling and alignment of the collagen fibers indicate a deviation from the expected equibiaxial stress state. The statistical analysis of the fiber orientations provides information on two driving forces for collagen alignment: microscale flaws and macroscale deviation from the equibiaxial strain. As the pressure increases, the macroscale effect becomes dominant, and a high density of fibers that are aligned along a specific direction is observed. A model that explains these observations and relates them to the material properties is presented. The results of this study indicate that a temporal increase in intrauterine pressure or uterine cervix dilatation causes irreversible changes in collagen molecular connections that may lead to biological changes, such as the initiation of term and preterm labor.

On the defect tolerance of fetal membranes

A series of mechanical experiments were performed to quantify the strength and fracture toughness of human amnion and chorion. The experiments were complemented with computational investigations using a 'hybrid' model that includes an explicit representation of the collagen fibre network of amnion. Despite its much smaller thickness, amnion is shown to be stiffer, stronger and tougher than chorion, and thus to determine the mechanical response of fetal membranes, with respect to both, deformation and fracture behaviour. Data from uniaxial tension and fracture tests were used to inform and validate the computational model, which was then applied to rationalize measurements of the tear resistance of tissue samples containing crack-like defects. Experiments and computations show that the strength of amnion is not significantly reduced by defects smaller than 1 mm, but the crack size induced by perforations for amniocentesis and fetal membrane suturing during fetal surgery might be larger than this value. In line with previous experimental observations, the computational model predicts a very narrow near field at the crack tip of amnion, due to localized fibre alignment and collagen compaction. This mechanism shields the tissue from the defect and strongly reduces the interaction of multiple adjacent cracks. These findings were confirmed through corresponding experiments, showing that no interaction is expected for multiple sutures for an inter-suture distance larger than 1 mm and 3 mm for amnion and chorion, respectively. The experimental procedures and numerical models applied in the present study might be used to optimize needle and/or staple dimensions and inter-suture distance, and thus to reduce the risk of iatrogenic preterm premature rupture of the membranes from amniocentesis, fetoscopic and open prenatal surgery.

Risk Factors for Preterm Birth following Open Fetal Myelomeningocele Repair: Results from a Prospective Cohort

BACKGROUND

Fetal myelomeningocele (fMMC) repair is a therapeutic option in selected cases. This study aimed to identify risk factors for preterm birth (PTB) following open fMMC repair.

METHODS

Sixty-seven women underwent fMMC repair and delivered a baby between 2010 and 2018 at our center. Demographic, surgical, and pregnancy complications, including potential risk factors for PTB such as preterm premature rupture of membranes (PPROM), chorioamniotic membrane separation (CMS), and placental abruption were evaluated.

RESULTS

Maternal body mass index, maternal age, parity, previous uterine surgery, gestational age at fetal surgery, total surgery duration, surgical subcutaneous hematoma, oligohydramnios, and amniotic fluid leakage were not identified as risk factors for PTB. CMS (p = 0.028, 92 vs. 52%) and PPROM (p = 0.001, 95 vs. 52%) were highly associated with PTB. Placental abruption was found more often in women after fMMC repair than in a general obstetrical population (12 vs. 1%) and ended in premature birth in all cases (p = 0.024, 100 vs. 60%). However, the majority of women delivered at a gestational age >35 weeks.

CONCLUSIONS

In our study cohort, risk factors for PTB were PPROM, CMS, and placental abruption, whereas surgery duration did not influence outcome. We conclude that the surgery technique should aim to minimize CMS and amniotic fluid leakage.

Geometry of an inflated membrane in elliptic bulge tests: Evaluation of an ellipsoidal shape approximation by stereoscopic digital image correlation measurements

Elliptic bulge tests are conducted on liver capsule, a fibrous connective membrane, associated with a field measurement method to assess the global geometry of the samples during the tests. The experimental set up is derived from a previous experimental campaign of bulge tests under microscope. Here, a stereoscopic Digital Image Correlation (DIC) system is used to measure global parameters on the test and investigate some assumptions made on the testing conditions which could not been assessed with microscopic measurements. In particular, the assumption of an ellipsoidal shape of the inflated membrane is tested by comparing the actual sample shape measured by stereoscopic DIC with an idealized ellipsoidal shape. Results indicate that a rather constant gap exists between the idealized and actual position. The approximation in the calculation of a macroscopic strain through analytical modeling of the test is estimated here. The study of the liver capsule case shows that important differences can be observed in strain calculation depending on the method and assumptions taken. Therefore, analytical modeling of mechanical tests through ellipsoidal approximation needs to be carefully evaluated in every application. Here the field measurement allows assessing the validity of these modeling assumptions. Moreover, it gives precious details about the boundary conditions of the bulge test and revealed the heterogeneous clamping, highlighted by strain concentrations.

Characterization of irreversible physio-mechanical processes in stretched fetal membranes

We perform bulge tests on live fetal membrane (FM) tissues that simulate the mechanical conditions prior to contractions. Experimental results reveal an irreversible mechanical behavior that appears during loading and is significantly different than the mechanical behavior that appears during unloading or in subsequent loading cycles. The irreversible behavior results in a residual strain that does not recover upon unloading and remains the same for at least 1h after the FM is unloaded. Surprisingly, the irreversible behavior demonstrates a linear stress-strain relation. We introduce a new model for the mechanical response of collagen tissues, which accounts for the irreversible deformation and provides predictions in agreement with our experimental results. The basic assumption of the model is that the constitutive stress-strain relationship of individual elements that compose the collagen fibers has a plateau segment during which an irreversible transformation/deformation occurs. Fittings of calculated and measured stress-strain curves reveal a well-defined single-value property of collagenous tissues, which is related to the threshold strain εth for irreversible transformation. Further discussion of several physio-mechanical processes that can induce irreversible behavior indicate that the most probable process, which is in agreement with our results for εth, is a phase transformation of collagen molecules from an α-helix to a β-sheet structure. A phase transformation is a manifestation of a significant change in the molecular structure of the collagen tissues that can alter connections with surrounding molecules and may lead to critical biological changes, e.g., an initiation of labor.

STATEMENT OF SIGNIFICANCE

This study is driven by the hypothesis that pre-contraction mechanical stretch of the fetal membrane (FM) can lead to a change in the microstructure of the FM, which in turn induces a critical biological (hormonal) change that leads to the initiation of labor. We present mechanical characterizations of live FM tissues that reveal a significant irreversible process and a new model for the mechanical response of collagen tissues, which accounts for this process. Fittings of calculated and measured results reveal a well-defined single-value property of collagenous tissues, which is related to the threshold strain for irreversible transformation. Further discussion indicates that the irreversible deformation is induced by a phase transformation of collagen molecules that can lead to critical biological changes.

Mechanical and microstructural investigation of the cyclic behavior of human amnion

The structural and mechanical integrity of amnion is essential to prevent preterm premature rupture (PPROM) of the fetal membrane. In this study, the mechanical response of human amnion to repeated loading and the microstructural mechanisms determining its behavior were investigated. Inflation and uniaxial cyclic tests were combined with corresponding in situ experiments in a multiphoton microscope (MPM). Fresh unfixed amnion was imaged during loading and changes in thickness and collagen orientation were quantified. Mechanical and in situ experiments revealed differences between the investigated configurations in the deformation and microstructural mechanisms. Repeated inflation induces a significant but reversible volume change and is characterized by high energy dissipation. Under uniaxial tension, volume reduction is associated with low energy, unrecoverable in-plane fiber reorientation.

Deformation mechanisms of human amnion: Quantitative studies based on second harmonic generation microscopy

Multiphoton microscopy has proven to be a versatile tool to analyze the three-dimensional microstructure of the fetal membrane and the mechanisms of deformation on the length scale of cells and the collagen network. In the present contribution, dedicated microscopic tools for in situ mechanical characterization of tissue under applied mechanical loads and the related methods for data interpretation are presented with emphasis on new stepwise monotonic uniaxial experiments. The resulting microscopic parameters are consistent with previous ones quantified for cyclic and relaxation tests, underlining the reliability of these techniques. The thickness reduction and the substantial alignment of collagen fiber bundles in the compact and fibroblast layer starting at very small loads are highlighted, which challenges the definition of a reference configuration in terms of a force threshold. The findings presented in this paper intend to inform the development of models towards a better understanding of fetal membrane deformation and failure, and thus of related problems in obstetrics and other clinical conditions.

Time-dependent mechanical behavior of human amnion: macroscopic and microscopic characterization

Characterizing the mechanical response of the human amnion is essential to understand and to eventually prevent premature rupture of fetal membranes. In this study, a large set of macroscopic and microscopic mechanical tests have been carried out on fresh unfixed amnion to gain insight into the time-dependent material response and the underlying mechanisms. Creep and relaxation responses of amnion were characterized in macroscopic uniaxial tension, biaxial tension and inflation configurations. For the first time, these experiments were complemented by microstructural information from nonlinear laser scanning microscopy performed during in situ uniaxial relaxation tests. The amnion showed large tension reduction during relaxation and small inelastic strain accumulation in creep. The short-term relaxation response was related to a concomitant in-plane and out-of-plane contraction, and was dependent on the testing configuration. The microscopic investigation revealed a large volume reduction at the beginning, but no change of volume was measured long-term during relaxation. Tension-strain curves normalized with respect to the maximum strain were highly repeatable in all configurations and allowed the quantification of corresponding characteristic parameters. The present data indicate that dissipative behavior of human amnion is related to two mechanisms: (i) volume reduction due to water outflow (up to ∼20 s) and (ii) long-term dissipative behavior without macroscopic deformation and no systematic global reorientation of collagen fibers.

Repairing Fetal Membranes with a Self-adhesive Ultrathin Polymeric Film: Evaluation in Mid-gestational Rabbit Model

Preterm premature rupture of membranes causes 40% of all preterm births, affecting 150000 women each year in the United States. Prenatal diagnostic procedures and surgical interventions increase incidence of adverse events, leading to iatrogenic membrane rupture after a fetoscopic procedure in 45% of cases. We propose an ultrathin, self-adherent, poly-L-lactic acid patch ("nanofilm") as a reparative wound closure after endoscopic/fetoscopic procedures. These nanofilms are compatible with application in wet conditions and with minimally invasive instrumentation. Ex vivo studies to evaluate the nanofilm were conducted using human chorion-amnion (CA) membranes. A custom-built inflation device was used for mechanical characterization of CA membranes and for assessment of nanofilm adhesion and sealing of membrane defects up to 3 mm in size. These ex vivo tests demonstrated the ability of the nanofilm to seal human CA defects ranging in size from 1 to 3 mm in diameter. In vivo survival studies were conducted in 25 mid-gestational rabbits, defects were created by perforating the uterus and the CA membranes and subsequently using the nanofilm to seal these wounds. These in vivo studies confirmed the successful sealing of defects smaller than 3 mm observed ex vivo. Histological analysis of whole harvested uteri 7 days after surgery showed intact uterine walls in 59% of the nanofilm repaired fetuses, along with increased uterine size and intrauterine development in 63% of the cases. In summary, we have developed an ultrathin, self-adhesive nanofilm for repair of uterine membrane defects.

About Puncture Testing Applied for Mechanical Characterization of Fetal Membranes

Puncture testing has been applied in several studies for the mechanical characterization of human fetal membrane (FM) tissue, and significant knowledge has been gained from these investigations. When comparing results of mechanical testing (puncture, inflation, and uniaxial tension), we have observed discrepancies in the rupture sequence of FM tissue and significant differences in the deformation behavior. This study was undertaken to clarify these discrepancies. Puncture experiments on FM samples were performed to reproduce previous findings, and numerical simulations were carried out to rationalize particular aspects of membrane failure. The results demonstrate that both rupture sequence and resistance to deformation depend on the samples' fixation. Soft fixation leads to slippage in the clamping, which reduces mechanical loading of the amnion layer and results in chorion rupturing first. Conversely, the stiffer, stronger, and less extensible amnion layer fails first if tight fixation is used. The results provide a novel insight into the interpretation of ex vivo testing as well as in vivo membrane rupture.

Photobleaching as a tool to measure the local strain field in fibrous membranes of connective tissues

Connective tissues are complex structures which contain collagen and elastin fibers. These fiber-based structures have a great influence on material mechanical properties and need to be studied at the microscopic scale. Several microscopy techniques have been developed in order to image such microstructures; among them are two-photon excited fluorescence microscopy and second harmonic generation. These observations have been coupled with mechanical characterization to link microstructural kinematics to macroscopic material parameter evolution. In this study, we present a new approach to measure local strain in soft biological tissues using a side-effect of fluorescence microscopy: photobleaching. Controlling the loss of fluorescence induced by photobleaching, we create a pattern on our sample that we can monitor during mechanical loading. The image analysis allows three-dimensional displacements of the patterns at various loading levels to be computed. Then, local strain distribution is derived using the finite element discretization on a four-node element mesh created from our photobleached pattern. Photobleaching tests on a human liver capsule have revealed that this technique is non-destructive and does not have any impact on mechanical properties. This method is likely to have other applications in biological material studies, considering that all collagen-elastin fiber-based biological tissues possess autofluorescence properties and thus can be photobleached.

Second harmonic generation microscopy of fetal membranes under deformation: normal and altered morphology

INTRODUCTION

Insight into the microstructure of fetal membrane and its response to deformation is important for understanding causes of preterm premature rupture of the membrane. However, the microstructure of fetal membranes under deformation has not been visualized yet. Second harmonic generation microscopy, combined with an in-situ stretching device, can provide this valuable information.

METHODS

Eight fetal membranes were marked over the cervix with methylene blue during elective caesarean section. One sample per membrane of reflected tissue, between the placenta and the cervical region, was cyclically stretched with a custom built inflation device. Samples were mounted on an in-situ stretching device and imaged with a multiphoton microscope at different deformation levels. Microstructural parameters such as thickness and collagen orientation were determined. Image entropy was evaluated for the spongy layer.

RESULTS

The spongy layer consistently shows an altered collagen structure in the cervical and cycled tissue compared with the reflected membrane, corresponding to a significantly higher image entropy. An increased thickness of collagenous layers was found in cervical and stretched samples in comparison to the reflected tissue. Significant collagen fibre alignment was found to occur already at moderate deformation in all samples.

CONCLUSIONS

For the first time, second harmonic generation microscopy has been used to visualize the microstructure of fetal membranes. Repeated mechanical loading was shown to affect the integrity of the amnion-chorion interface which might indicate an increased risk of premature rupture of fetal membrane. Moreover, mechanical loading might contribute to morphological alterations of the fetal membrane over the cervical region.