On the management of maternal pushing during the second stage of labor: a biomechanical study considering passive tissue fatigue damage accumulation

Abstract

Background: During the second stage of labor, the maternal pelvic floor muscles undergo repetitive stretch loading as uterine contractions and strenuous maternal pushes combined to expel the fetus, and it is not uncommon that these muscles sustain a partial or complete rupture. It has recently been demonstrated that soft tissues, including the anterior cruciate ligament and connective tissue in sheep pelvic floor muscle, can accumulate damage under repetitive physiological (submaximal) loads. It is well known to material scientists that this damage accumulation can not only decrease tissue resistance to stretch but also result in a partial or complete structural failure. Thus, we wondered whether certain maternal pushing patterns (in terms of frequency and duration of each push) could increase the risk of excessive damage accumulation in the pelvic floor tissue, thereby inadvertently contributing to the development of pelvic floor muscle injury.

Objective: This study aimed to determine which labor management practices (spontaneous vs directed pushing) are less prone to accumulate damage in the pelvic floor muscles during the second stage of labor and find the optimum approach in terms of minimizing the risk of pelvic floor muscle injury.

Study design: We developed a biomechanical model for the expulsive phase of the second stage of labor that includes the ability to measure the damage accumulation because of repetitive physiological submaximal loads. We performed 4 simulations of the second stage of labor, reflecting a directed pushing technique and 3 alternatives for spontaneous pushing.

Results: The finite element model predicted that the origin of the pubovisceral muscle accumulates the most damage and so it is the most likely place for a tear to develop. This result was independent of the pushing pattern. Performing 3 maternal pushes per contraction, with each push lasting 5 seconds, caused less damage and seemed the best approach. The directed pushing technique (3 pushes per contraction, with each push lasting 10 seconds) did not reduce the duration of the second stage of labor and caused higher damage accumulation.

Conclusion: The frequency and duration of the maternal pushes influenced the damage accumulation in the passive tissues of the pelvic floor muscles, indicating that it can influence the prevalence of pelvic floor muscle injuries. Our results suggested that the maternal pushes should not last longer than 5 seconds and that the duration of active pushing is a better measurement than the total duration of the second stage of labor. Hopefully, this research will help to shed new light on the best practices needed to improve the experience of labor for women.

Keywords: finite element method; low-cycle fatigue failure; management of labor; maternal pushes; pelvic floor muscle; second stage of labor; vaginal delivery.

Figure A.1 –

Finite element mesh of the levator ani muscles.

Figure A.2 –

Non-local damage at a material point x results from an averaging operation of the local quantities of the material points inside the non-local averaging volume. This volume is defined by radius L_c, which is the characteristic length..

Figure 1 –

Histological appearance of ovine pelvic floor muscle-tendon sections stained with Masson’s Trichrome stain. Comparison between an untested control, a), and fatigue-tested specimens (b-c) (60 cycles at between 45% and 60% failure displacement). Fatigued specimens show elongated and deformed muscle fibers (arrows), individual fiber rupture (square) and increased interstitial spaces.

Figure 2–

Superior view of the structure representing the levator ani muscles. a) 3-D geometry of the levator ani muscles, in red, with the pubic bone and symphysis pubis (SP) shown in ivory. The levator muscles attach laterally to the ischial spines and posteriorly to the sacrum, neither of which are shown for simplicity. The outline of the pubovisceral muscle (PVM), the most anterior and caudal portion of the levator ani muscles, is shown with dashed white lines: it takes origin from the pubic bone on either side of the symphysis pubis (SP) and inserts into the perineal body (not shown) near the midline. The vagina, anal sphincter and remaining portions of the levator ani, including the iliococcygeus, coccygeus, and puborectal muscles (PBR) are not delineated for simplicity. The nodes along the green edges were fixed. b) View of the model levator ani muscle showing fiber directions within the muscle and main dimensions (in mm). Black dotted line shows the hiatal length used to calculate the stretch.

Figure 3–

Finite element model of the fetal head showing the a) occipitofrontal diameter, and b) biparietal diameter with head circumference. All dimensions shown are in mm.

Figure 4 -

Expulsive force placed on the model fetal head during simulated directed pushing. a) Profile of a single 10 seconds push and b) Valsalva-push pattern: three 10 second pushes superposed upon the 90 s uterine contraction.

Figure 5 -

Details of the expulsive force placed on the model fetal head during simulated spontaneous pushing. a) 1-push pattern: one 10 second push superposed upon the 90 s uterine contraction b) Profile of a single 5 second push, c) 3-push pattern: three 5 second pushes superposed upon the 90 s uterine contraction. d) 5-push pattern: five 5 second pushes superposed upon the 90 s uterine contraction.

Figure 6 -

Vertical displacement of the fetal head (mm) with time (s) for the four simulated pushing patterns: 1-push, 3-push, 5-push and Valsalva-push. The pictures below show the right lateral view of the fetal head descent for the 1-push simulation at the starting point, at the end of each contraction, and in the final position. The vertical dashed grey lines represent the end of a contraction (C), labelled numerically along the top of the plot.

Figure 7 -

Right lateral view showing the damage distribution in the pelvic floor muscles at the end of the simulation for the four pushing patterns. The peak damage and critical node location are identified.

Figure 8 –

Boxplot grouping the pelvic floor nodal muscle fiber damage values at the end of the simulation for each category (Critical, High, Noteworthy, Moderate and Low) and for the four pushing patterns. Numbers inside the graph area are the number of nodes in each category. Critical: $1\le \widehat{{\text{D}}_{\text{f}1}}<0.8$, High: $0.8<\widehat{{\text{D}}_{\text{f}1}}\le 0.45$, Noteworthy: $0.45<\widehat{{\text{D}}_{\text{f}1}}\le 0.35$, Moderate: $0.35<\widehat{{\text{D}}_{\text{f}1}}\le 0.1$ and Low $0\le \widehat{{\text{D}}_{\text{f}1}}\le 0.1$

Figure 9 -

a) Fiber damage evolution at the critical node for the four simulated pushing patterns, b) Comparison of the fiber maximum damage at the critical node for the simulated pushing patterns, c) Fiber maximum damage at the critical node as a function of the simulations total time, and d) Fiber maximum damage at the critical node as a function of the time spent in active pushing, which is the sum of the duration of the maternal pushes.