Computational modeling of vacuum-assisted delivery: biomechanics of maternal soft tissues

Abstract

Childbirth is a complex process influenced by physiological, mechanical, and hormonal factors. While natural vaginal delivery is the safest, it is not always feasible due to diverse circumstances. In such cases, assisted delivery techniques, such as vacuum-assisted delivery (VAD), may facilitate vaginal birth. However, this technique can be associated with a higher risk of maternal injuries, potentially resulting in long-term conditions such as pelvic organ prolapse or incontinence. This study investigates the biomechanical impact of VAD on maternal tissues, aiming to reduce these risks. A finite element model was developed to simulate VAD, incorporating maternal musculature, a deformable fetal head, and a vacuum cup. Twelve simulations were conducted, varying contraction durations, resting intervals, and the number of pulls required for fetal extraction. Results revealed that prolonged contraction durations, coupled with extended resting intervals, lead to a reduction in pelvic floor stress. Elevated stress levels were observed when fetal extraction involved two pulls, with an 8.43% decrease in maximum stress from two pulls to four. The peak stress recorded was 0.81 MPa during a 60-second contraction, followed by a 60-second rest period. These findings indicate that longer maneuvers may reduce trauma, as extended pulls allow muscles more time to relax and recover during both contraction and rest phases. Furthermore, an increased number of pulls extends the duration of the maneuver, facilitating fetal rotation and improved adjustment to the birth canal. This study offers crucial insights into the biomechanics of childbirth, providing clinicians with valuable information to enhance maternal outcomes and refine assisted delivery techniques.

Keywords: Assisted delivery simulations; Finite element method; Pelvic floor dysfunctions; Perineal injuries; Vacuum cup.

Fig. 1

Geometric model of the PFM, in red, and perineum. The perineal body is presented in gray, the external anal sphincter in brown, the ischiocavernosus and bulbospongiosus muscles in dark red, the superficial transverse perineal muscle in blue, and deep transverse perineal muscles in pink. The blue lines correspond to two paths defined on the levator hiatus, path 1, and the urogenital hiatus, path 2 (the space through which the urethra and the vagina pass). The dark brown line corresponds to the fixed nodes of the PFM, while the yellow line corresponds to the constrained nodes of the ischiocavernosus and superficial transverse perineal muscles. The left figure corresponds to a superior view of the female pelvis and the right figure to an inferior view

Fig. 2

Representation of the finite element model of the fetal head and vacuum cup. The vacuum cup is positioned at the flexion point of the fetal head, showing the distances to the anterior and posterior fontanelles. The black arrow at the tip of the vacuum cup indicates the direction of the imposed displacement

Fig. 3

Dynamics of the VAD simulation. Lateral and anterior views of the fetal head descent corresponding to the vertical displacement of the vacuum cup from 0 to 100 mm

Fig. 4

Force–deflection curve obtained from the antero-posterior compression test for the fetal head finite element model, compared to the experimental curves by Loyd (2011)

Fig. 5

Stretch of the levator hiatus (PFM) and urogenital hiatus (perineum) during the vertical displacement of the vacuum cup

Fig. 6

Maximum principal stress (in MPa) along the vertical displacement on the vacuum cup measured on Path 2, corresponding to the urogenital hiatus. The results include the finite element model of the perineum for vertical displacements of 37.5 mm and 82.5 mm, highlighting the stress distribution and identifying the most critical regions. Before reaching 37.5 mm, the stretch is primarily due to the extraction of the vacuum cup

Fig. 7

Time evolution of the maximum principal stress in the fetal head (left axis) and the vertical displacement of the vacuum cup (right axis) during the simulated VAD. The shaded blue areas indicate resting phases, while the white areas represent pulling phases

Fig. 8

Maximum principal stress (in MPa) along the vertical displacement of the vacuum cup measured on the PFM path for the different cases under study. The vertical dashed line indicates the displacement at which the resting stage is defined

Fig. 9

Maximum principal stress (in MPa) along the vertical displacement on the vacuum cup considering the fetal extraction with two, three and four pulls

Fig. 10

Force (in N) applied on the vacuum cup to extract the fetal head along its vertical displacement through the birth canal with two, three and four pulls