Relationship Between High Intra-Abdominal Pressure and Compliance of the Pelvic Floor Support System in Women Without Pelvic Organ Prolapse: A Finite Element Analysis

Background: The objective of this study was to study the relationship between high intra-abdominal pressure and the compliance of the pelvic floor support system in a normal woman without pelvic organ prolapse (POP), using a finite element model of the whole pelvic support system.Methods: A healthy female volunteer (55 years old) was scanned using magnetic resonance imaging (MRI) during the Valsalva maneuver. According to the pelvic structure contours traced by a gynecologist and anatomic details measured from dynamic MRI, a finite element model of the whole pelvic support system was established, including the uterus, vagina with cavity, cardinal and uterosacral ligaments, levator ani muscle, rectum, bladder, perineal body, pelvis, and obturator internus and coccygeal muscles. This model was imported into ANSYS software, and an implicit iterative method was employed to simulate the biomechanical response with increasing intra-abdominal pressure.Results: Stress and strain distributions of the vaginal wall showed that the posterior wall was more stable than the anterior wall under high intra-abdominal pressure. Displacement at the top of the vagina was larger than that at the bottom, especially in the anterior–posterior direction.Conclusion: These results imply potential injury areas with high intra-abdominal pressure in non-prolapsed women, and provide insight into clinical managements for the prevention and surgical repair plans of POP.

Female squirrel monkeys as models for research on women’s pelvic floor disorders

Animal models enable research on biological phenomena with controlled interventions not possible or ethical in patients. Among species used as experimental models, squirrel monkeys ( Saimiri genus) are phylogenetically related to humans and are relatively easily managed in captivity. Quadrupedal locomotion of squirrel monkeys resembles most other quadrupedal primates in that they utilize a diagonal sequence/diagonal couplets gait when walking on small branches. However, to assume a bipedal locomotion, the human pelvis has undergone evolutionary changes. Therefore, the pelvic bone morphology is not that similar between the female squirrel monkey and woman, but pelvic floor support structures and impacts of fetal size and malpresentation are similar. Thus, this review explores the pelvic floor support structural characteristics of female squirrel monkeys, especially in relation to childbirth to demonstrate similarities to humans.

The evolution of pelvic canal shape and rotational birth in humans

The human foetus needs to rotate when passing through the tight birth canal because of the complex shape of the pelvis. In most women the upper part, or inlet, of the birth canal has a round or mediolaterally oval shape, which is considered ideal for parturition, but it is unknown why the lower part, or outlet, of the birth canal has a pronounced anteroposteriorly oval shape. Here we show that the shape of the lower birth canal affects the ability of the pelvic floor to resist pressure exerted by the abdominal organs and the foetus. Based on a series of finite element analyses, we found that the highest deformation, stress and strain occur in pelvic floors with a circular or mediolaterally oval shape, whereas an anteroposterior elongation increases pelvic floor stability. This suggests that the anteroposterior oval outlet shape is an evolutionary adaptation for pelvic floor support. For the pelvic inlet, by contrast, it has long been assumed that the mediolateral dimension is constrained by the efficiency of upright locomotion. But we argue that upright stance limits the anteroposterior dimension of the inlet. A deeper inlet requires greater pelvic tilt and lumbar lordosis, which compromises spine health and the stability of upright posture. These different requirements on the pelvic inlet and outlet have led to the complex shape of the human pelvic canal and to the evolution of rotational birth.

Development of anatomically based customizable three-dimensional finite-element model of pelvic floor support system: POP-SIM1.0

To develop an anatomically based customizable finite-element (FE) model of the pelvic floor support system to simulate pelvic organ prolapse (POP): POP-SIM1.0. This new simulation platform allows for the construction of an array of models that objectively represent the key anatomical and functional variation in women with and without prolapse to test pathomechanism hypotheses of the prolapse formation. POP-SIM1.0 consists of anatomically based FE models and a suite of Python-based tools developed to rapidly construct FE models by customizing the base model with desired structural parameters. Each model consists of anatomical structures from three support subsystems which can be customized based on magnetic resonance image measurements in women with and without prolapse. The customizable structural parameters include presence of levator ani (LA) avulsion, hiatus size, anterior vaginal wall dimension, attachment fascia length and apical location in addition to the tissue material properties and intra-abdominal pressure loading. After customization, the FE model was loaded with increasing intra-abdominal pressure (0–100 cmH 2 O) and solved using ABAQUS explicit solver. We were able to rapidly construct anatomically based FE models with specific structural geometry which reflects the morphology changes often observed in women with prolapse. At maximum loading, simulated structural deformations have similar anatomical characteristics to those observed during clinical exams and stress magnetic resonance images. Simulation results showed the presence of LA muscle avulsion negatively impacts the pelvic floor support. The normal model with intact muscle had the smallest exposed vaginal length of 11 mm, while the bilateral avulsion produced the largest exposed vaginal length at 24 mm. The unilateral avulsion model had an exposed vaginal length of 18 mm and also demonstrated a tipped perineal body similar to that seen in clinical observation. Increasing the hiatus size, vaginal wall length and fascia length also resulted in worse pelvic floor support, increasing the exposed vaginal length from 18 mm in the base model to 33 mm, 54 mm and 23.5 mm, respectively. The developed POP-SIM1.0 can simulate the anatomical structure changes often observed in women with prolapse. Preliminary results showed that the presence of LA avulsion, enlarged hiatus, longer vaginal wall and fascia length can result in larger prolapse at simulated maximum Valsalva.