Objective
The purpose of this study was to investigate the influence of fetal head flexion during vaginal delivery with a 3-dimensional computational finite element model.
Study Design
A finite element model of the pelvic skeletal structure, pelvic floor, and fetus was developed. The movements of the fetus during birth were simulated in engagement, descent, flexion, internal rotation, and extension of the fetal head. The opposite forces against the fetal descendent and the stress of the pelvic floor muscles were obtained on simulations with different degrees of head flexion.
Results
The simulated increase in fetal head flexion is associated with lower values of opposite forces against the fetal descent. The descending fetus with abnormal head flexion also meets resistance in later stations. Lower stress on the pelvic floor was demonstrated with simulated increase in fetal head flexion during vaginal delivery.
Conclusion
This analytic evidence suggests that the fetal head flexion during vaginal delivery may facilitate birth and protect the pelvic floor.
The positional changes in the presenting part that is required to navigate the pelvic canal are known as the mechanisms of labor. During its descent, the fetus acquires the smallest possible cross section to pass through the birth canal. The descending fetus meets resistance from the cervix, walls of the pelvis, or pelvic floor muscles, which normally results in a head flexion. The chin is brought into close contact with the thorax, and the longer occipitofrontal diameter is substituted by the shorter suboccipitobregmatic diameter. The anteroposterior diameter reduces from nearly 11.5-9.5 cm. These interactions between fetus presentation and birth canal can be considered a biomechanical system.
The process of childbirth and the mechanisms of labor have been studied for decades. Computational models have emerged as a quantitative analysis tool that is based on biomechanical principles that are applied to enhance our understanding of childbirth. These methods may help clinicians to understand possible factors that lead to birth-induced pelvic floor muscle injuries and dysfunctions.
The finite element (FE) method is a tool of mathematic simulation that consists in the discretization of a continuous model in small pieces, maintaining the same properties of the original model. The FE method was developed in the 1950s to analyze complex structures that were subjected to mechanical loads. The FE method offers the advantage of being able to model structures and indirectly quantify their mechanical behavior under any theoretic conditions. This technique has been proposed as an effective tool to investigate the effect of vaginal delivery on pelvic floor.
Our understanding of the mechanisms underlying labor and parturition remains inadequate. The evaluation of the biomechanical effects of the descending fetus on the pelvic floor during delivery cannot be measured in vivo. Therefore, the aim of this study was to investigate the influence of fetal head flexion on the pelvic floor stress field and forces during vaginal delivery, using a 3-dimensional computational FE model.
Materials and Methods
To estimate the biomechanical repercussion of fetal head flexion during vaginal delivery, an FE model was developed that was based in the previous works by the authors.
Pelvic floor structures
Geometric information that was obtained from measurements conducted by Janda et al was used to construct the FE model of the pelvic floor muscles. Institutional review board approval was not necessary because only geometric information, in the form of point coordinates, was used on this work.
With the use of the available points, a set of lines were created that allowed the definition of surfaces that were then used for the creation of the FE mesh. The FE model for the pelvic floor was then connected to a model of the pelvic skeletal structure, as shown in Figure 1 . No displacement boundary conditions were applied to all the nodes that are connected to the pelvic bone, ligament, and coccyx in the FE simulation (represented on Figure 1 by the “attached curve”).
Fetus
An FE mesh of the fetus was also used for the simulations ( Figure 2 ), based on the previous work of the authors; the dimensions of the fetus head were adjusted and the position of the arms and legs were repositioned to be in accordance with the literature for a full-term fetus. The principal dimensions of the fetal head were the following: suboccipitobregmatic diameter, 9.5 cm; suboccipitofrontal diameter, 10.5 cm; occipitofrontal diameter, 11.5 cm; mentovertical diameter, 13.0 cm; and submentobregmatic diameter, 9.5 cm. The principal geometric and material parameters are shown in the Table .
| Variable | Measure |
|---|---|
| Fetus | |
| Suboccipitobregmatic diameter | 9.5 cm |
| Suboccipitofrontal diameter | 10.5 cm |
| Occipitofrontal diameter | 11.5 cm |
| Mentovertical diameter | 13.0 cm |
| Submentobregmatic diameter | 9.5 cm |
| Mother | |
| Obstetric conjugate | 11.4 cm |
| Sagittal outlet | 11.1 cm |
| Interspinous diameter | 12.0 cm |
| Intertuberous diameter | 11.1 cm |
| Transverse diameter | 12.6 cm |
| Constitutive model parameters | |
| c | 1.85 × 10 –2 N/mm 2 |
| b | 1.173 |
| A | 2.80 × 10 –2 N/mm 2 |
| a | 0.6215 |
| D | 1 × 10 –4 mm 2 /N |
| T 0 M | 0.682 Pa |
The movements of the fetus during birth were simulated in engagement, descent, flexion, internal rotation, and extension of the fetal head ( Figure 3 , A). The movements of the fetus, in occipitoanterior presentation, were imposed by controlling the movements of several reference points that belonged to the fetus model. Inside the fetus model, 4 groups of elements (head, shoulders, back, and pelvis) were considered to be rigid, and a reference point was assigned to each group. By controlling the displacements and rotations of these 4 points, the movements of the fetus were totally defined.
