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Research Papers

Effects of Nonlinear Muscle Elasticity on Pelvic Floor Mechanics During Vaginal Childbirth

[+] Author and Article Information
Xinshan Li

Auckland Bioengineering Institute, University of Auckland, Auckland 1010, New Zealandshannon.li@auckland.ac.nz

Jennifer A. Kruger, Martyn P. Nash, Poul M. F. Nielsen

Auckland Bioengineering Institute, University of Auckland, Auckland 1010, New Zealand

http://www.cmiss.org/cmgui/zinc.

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J Biomech Eng 132(11), 111010 (Oct 27, 2010) (6 pages) doi:10.1115/1.4002558 History: Received June 20, 2010; Revised August 10, 2010; Posted September 16, 2010; Published October 27, 2010; Online October 27, 2010

The role of the pelvic floor soft tissues during the second stage of labor, particularly the levator ani muscle, has attracted much interest recently. It has been postulated that the passage of the fetal head through the pelvis may cause excessive stretching of the levator ani muscle, which may lead to pelvic floor dysfunction and pelvic organ prolapse later in life. In order to study the complex biomechanical interactions between the levator ani muscle and the fetal head during the second stage of labor, finite element models have been developed for quantitative analysis of this process. In this study we have simulated vaginal delivery using individual-specific anatomical computer models of the pelvic floor interacting with a fetal head model with minimal restrictions placed upon its motion. Two constitutive relations were considered for the levator ani muscle (of exponential and neo-Hookean forms). For comparison purposes, the exponential relation was chosen to exhibit much greater stiffening at higher strains beyond the range of the experimental data. We demonstrated that increased nonlinearity in the elastic response of the tissues leads to considerably higher (56%) estimated force required for delivery, accompanied by a more homogeneous spatial distribution of maximum principal stretch ratio across the muscle. These results indicate that the form of constitutive relation beyond the presently available experimental data markedly affects the estimated function of the levator ani muscle during vaginal delivery, due to the large strains that occur. Further experimental data at higher strains are necessary in order to more reliably characterize the constitutive behavior required for modeling vaginal childbirth.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

The pelvic floor model shown in right lateral (left) and anterior (right) views, respectively. Yellow: LA; brown: support; silver: pubis.

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Figure 2

A fetal vault model in occiput anterior position shown in right lateral (left) and anterior (right) views, respectively. SOFD: suboccipitofrontal diameter; BPD: biparietal diameter.

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Figure 3

First Piola–Kirchhoff stress versus engineering strain in the fiber direction. The fitted exponential (solid line) and neo-Hookean (dashed line) constitutive relations are overlaid with the experimental data (circles).

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Figure 4

The path of the fetal head during descent for the exponential and neo-Hookean constitutive relations in anterior (left column) and right lateral (right column) views. The vertical dashed lines indicate the anteroposterior location of the ischial spines. The displacements of the center of mass of the fetal head relative to its original position are plotted below. The location of the overall maximum stretch is circled for both constitutive relations at 85 mm descent.

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Figure 5

The estimated force exerted on the fetal head versus its descent based on the two constitutive relations

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Figure 6

Mean and standard deviation of the maximum principal stretch ratio (λmax) in the LA muscle. The dash-dotted line represents descent (80 mm) at which λmax distributions are illustrated in Fig. 7.

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Figure 7

The maximum principal stretch ratio (λmax) distributions across the LA muscle at 80 mm descent for the exponential and the neo-Hookean materials. The meshes are shown in the left lateral, right lateral, and anterior views.

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Figure 8

Histogram of the maximum principal stretch ratio across the LA muscle at 80 mm descent for the exponential and the neo-Hookean material models.

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