Development of a multilayer fetal membrane material model calibrated using bulge inflation mechanical tests.

The fetal membranes are an essential mechanical structure for pregnancy, protecting the developing fetus in an amniotic fluid environment and rupturing before birth. In cooperation with the cervix and the uterus, the fetal membranes support the mechanical loads of pregnancy. Structurally, the fetal membranes comprise two main layers: the amnion and the chorion. The mechanical characterization of each layer is crucial to understanding how each layer contributes to the structural performance of the whole membrane. The in-vivo mechanical loading of the fetal membranes and the amount of tissue stress generated in each layer throughout gestation remains poorly understood, as it is difficult to perform direct measurements on pregnant patients. Finite element analysis of pregnancy offers a computational method to explore how anatomical and tissue remodeling factors influence the load-sharing of the uterus, cervix, and fetal membranes. To aid in the formulation of such computational models of pregnancy, this work develops a fiber-based multilayer fetal membrane model that captures its response to previously published bulge inflation loading data. First, material models for the amnion, chorion, and maternal decidua are formulated, informed, and validated by published data. Then, the behavior of the fetal membrane as a layered structure was analyzed, focusing on the respective stress distribution and thickness variation in each layer. The layered computational model captures the overall behavior of the fetal membranes, with the amnion being the mechanically dominant layer. The inclusion of fibers in the amnion material model is an important factor in obtaining reliable fetal membrane behavior according to the experimental dataset. These results highlight the potential of this layered model to be integrated into larger biomechanical models of the gravid uterus and cervix to study the mechanical mechanisms of preterm birth.

Strength of round and uterosacral ligaments: a biomechanical study.

PURPOSE: To investigate the tensile biomechanical properties of round and uterosacral ligaments. METHODS: Tissue samples were obtained from 15 female cadavers without pelvic organ prolapse. Uniaxial tensile tests were performed to obtain stiffness and maximum stress of round and uterosacral ligaments. Correlations were calculated using the Pearson correlation coefficient. Statistical differences between groups were tested using Student's paired and unpaired t test. RESULTS: There was a great variability in the measurements of stiffness and maximum stress in pelvic ligaments. The round ligaments demonstrated stiffness of 9.1 ± 1.6 MPa (mean ± SEM) (ranging from 2 to 25.6 MPa) and maximum stress of 4.3 ± 0.7 MPa (ranging from 1.2 to 11.5 MPa). The stiffness of the uterosacral ligaments was 14.1 ± 1.4 MPa (ranging from 5.7 to 26.1 MPa) with maximum stress of 6.3 ± 0.8 MPa (ranging from 2.2 to 11.9 MPa). There was a strong positive correlation between stiffness and maximum stress in female pelvic ligaments (ρ = 0.851; p < 0.001). The uterosacral ligaments demonstrated higher stiffness and maximum stress compared to the round ligaments (p = 0.006 and p = 0.034; respectively). Age, body mass index and menopausal status were not associated with the biomechanical proprieties of round and uterosacral ligaments. Nulliparous women had lower uterosacral stiffness (15.5 ± 1.3 vs. 10 ± 1.8 MPa; p = 0.033) and maximum stress (8.2 ± 0.9 vs. 4.2 ± 1.1 MPa; p = 0.028) compared to parous women. CONCLUSION: The uterosacral ligaments are significantly more resistant than round ligaments. Parturition seems to enhance the stiffness and maximum stress of the ligaments.

Structural and thermal properties of polypropylene mesh used in treatment of stress urinary incontinence.

Besides material biocompatibility, it is possible to infer that both vaginal and urethral erosion rates associated with sub-urethral synthetic slings may be related to the mechanical properties of the meshes and also to their other properties. With the aim of understanding what distinguishes the different polypropylene meshes, used for the treatment of the stress urinary incontinence (SUI), their structural and thermal properties were investigated. Five different mesh types were tested (Aris, Auto Suture, Avaulta, TVTO and Uretex). Differential scanning calorimetry (DSC) and infrared spectroscopy (FTIR) tests were performed. Furthermore, geometry (electron microscope), linear density and relative density (pyknometer) of the meshes were investigated. The meshes are made of the isotactic polypropylene homopolymer. Aris mesh presented the smallest fibre diameter, linear density and the level of crystallinity among all the meshes used for the treatment of the SUI. This study shows that there is a direct relationship between the fibre diameter, linear density, level of crystallinity and flexural stiffness of the polypropylene meshes used for the treatment of the SUI.