(B-62) Simulation of Uterus’ Cyclic Active Contraction and Delivery of Deformable Fetus in Ls-dyna

Introduction:: Childbirth, also referred to as delivery or labor, is the final phase of pregnancy when one or more fetuses pass through the birth canal from the uterus and is a biomechanical process. However, approximately twenty percent of labors are difficult or dysfunctional, which leads to a high risk of injuries to both the infant and the mother resulting from the stress, strain, and stretch of the biological tissues or anatomical structures of the fetus and the mother during the delivery process. The ability to phenomenologically model the labor and delivery process will increase our understanding of how variations in anatomy or the characteristics of labor might impact injury risk. In this study, we developed an FEM model in LS-DYNA to simulate uterine cyclic active contraction, which was driven by the contractile fibers inside the uterine wall. The model was then capable of delivering a fetal model through the birth canal.

Materials and Methods:: The geometry of the uterus (Fig 1a) was designed in NX software with a length of 35 cm, a maximum width of 15.6 cm, a thickness of 1 cm, and a diameter of 10 cm at the cervix. Meshing of the uterus was done in Hypermesh software using hexahedral elements (Fig 1b). Each element was bonded with truss beam elements. These beams were assigned to seven regions (Fig 1c). These beams were used to provide the contraction force. The cyclic activation curves (Fig 1d) for the seven beam regions were assumed based on uterine physiologic behavior, with three consecutive contractions modeled. The total length of each contraction was 60 seconds. The solid portion of the uterus was modeled as a hyperelastic material with a Neo Hookean model (c1= 0.03 MPa). The bony pelvis was modeled as a rigid body. The fetus was modeled as three components (head, neck and body), with the head as a rigid body, the fetal neck an elastic structure (E = 50 MPa, v = 0.49), and the fetus’ body as Neo Hookean hyperelastic (c1 = 0.07 MPa). The pelvic muscle floor, rectum, and vagina were modeled as hyperelastic materials with a Mooney Rivlin model (c1 = 0.016 MPa, and c2= 0.004 MPa). The cervix and the bony pelvis were fixed in space. The nodes connecting the pelvic soft tissues and the bony pelvis as well as the nodes on the top edges of the vagina were also fixed in space. No external loads were applied.

Results, Conclusions, and Discussions:: Results: The simulation results of the uterus are provided in Fig 2. The uterus started to contract from the fundus, and the contraction wave propagated to the lower part once the uterus was activated. By t = 30 s, all regions contracted, and the largest stress was 0.13 MPa at the fundus (Fig 2b). By t = 50 s, the stress for different regions among the uterus each reached their maximum value for the first contraction (Fig 2c), and then decreased slightly due to the decrease of the activation level in the first cycle allowing relaxation of the uterus. After that, the stress increased again in the second contraction. The maximum stress for different regions within the uterus in the second cycle (Fig 2d) was higher than that in the first cycle (Fig 2c).
The fetal displacement with time during the delivery process is shown in Fig 3. Contraction of the uterus pushed the fetus forward through the pelvic structures and the vagina. The fetus naturally rotated in a clockwise direction as it moved through the birth canal. The displacement continued to increase until 50 s in the first contraction, then decreased slightly in the first recovery phase, and further increased again in the second contraction cycle. The fetal head was delivered during the second contraction, at which point the simulation stopped. The overall displacement of the fetus during the delivery process was about 320 mm.
Conclusions: An FEM model to simulate the uterine cyclic active contraction and delivery of a deformable fetus through the birth canal was developed in LS-DYNA. In this model, the uterus was able to perform the large deformation, cyclic active contraction, and propagation of the contraction wave needed to achieve delivery. For the fetus, the delivery displacement increased significantly and then decreased slightly with the rise and the decline of the activation level in different contractions, respectively.
Discussions: The nerves inside the neck, which are important if the goal is to investigate the mechanisms of injuries to the fetus during the labor – especially brachial plexus injury – were not included in the current model.

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