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Research Papers

A Geometric Capacity–Demand Analysis of Maternal Levator Muscle Stretch Required for Vaginal Delivery

[+] Author and Article Information
Paige V. Tracy

Department of Biomedical Engineering,
University of Michigan,
Ann Arbor, MI 48109
e-mail: voigtpai@umich.edu

John O. DeLancey

Department of Obstetrics and Gynecology,
University of Michigan,
Ann Arbor, MI 48109

James A. Ashton-Miller

Department of Mechanical Engineering,
University of Michigan,
Ann Arbor, MI 48109

1Corresponding author.

Manuscript received September 1, 2015; final manuscript received December 11, 2015; published online January 27, 2016. Editor: Beth A. Winkelstein.

J Biomech Eng 138(2), 021001 (Jan 27, 2016) (12 pages) Paper No: BIO-15-1434; doi: 10.1115/1.4032424 History: Received September 01, 2015; Revised December 11, 2015

Because levator ani (LA) muscle injuries occur in approximately 13% of all vaginal births, insights are needed to better prevent them. In Part I of this paper, we conducted an analysis of the bony and soft tissue factors contributing to the geometric “capacity” of the maternal pelvis and pelvic floor to deliver a fetal head without incurring stretch injury of the maternal soft tissue. In Part II, we quantified the range in demand, represented by the variation in fetal head size and shape, placed on the maternal pelvic floor. In Part III, we analyzed the capacity-to-demand geometric ratio, g, in order to determine whether a mother can deliver a head of given size without stretch injury. The results of a Part I sensitivity analysis showed that initial soft tissue loop length (SL) had the greatest effect on maternal capacity, followed by the length of the soft tissue loop above the inferior pubic rami at ultimate crowning, then subpubic arch angle (SPAA) and head size, and finally the levator origin separation distance. We found the more caudal origin of the puborectal portion of the levator muscle helps to protect it from the stretch injuries commonly observed in the pubovisceral portion. Part II fetal head molding index (MI) and fetal head size revealed fetal head circumference values ranging from 253 to 351 mm, which would increase up to 11 mm upon face presentation. The Part III capacity-demand analysis of g revealed that, based on geometry alone, the 10th percentile maternal capacity predicted injury for all head sizes, the 25th percentile maternal capacity could deliver half of all head sizes, while the 50th percentile maternal capacity could deliver a head of any size without injury. If ultrasound imaging could be operationalized to make measurements of ratio g, it might be used to usefully inform women on their level of risk for levator injury during vaginal birth.

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Figures

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Fig. 4

Graphic illustration of the sensitivity analyses in caudal view. Top: Nominal configuration using the convention in Fig.3. Middle left: varying SL (thick black band). Middle right: varying soft tissue origin placement on pelvis (black arrow heads). Bottom left: varying SPAA. Bottom right: varying head size (gray circle). The variation in soft tissue length reduction in downward rotation is not shown. Factors were varied by ± 10%.

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Fig. 3

Caudal view of anterior pelvis with variables used in the maternal capacity calculations. The soft tissue loop originates high on the pelvis (filled arrow heads) and wraps around the fetal head (gray circular structure). The portion of the soft tissue loop in contact with the fetal head is represented by the thick black band, while the portion not in contact with the fetal head is represented by the dashed lines. θ = SPAA. Arch = pelvis/subpubic arch.

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Fig. 2

Upper left: left lateral view of 3D model of the pelvis (green), showing the high origin location (arrow) of the PVM (orange) and the PVM insertion on the PB/AS (light blue). Upper right: 3D model of the pelvis (green), showing the PRM (dark purple, lower right of that image) originating from the PM (white). In the upper two figures, A, P, L, R, and I denote anterior, posterior, left, right, and inferior, respectively. Lower left: The pubic symphysis is projected in the sagittal view seen in a view from the left showing a downward rotation of the PVM loop. Note the wrapping of the PVM around the inferior pubic ramus at point 2 at ultimate crowning. The portion of the PVM between points 1 and 2 lying above the inferior pubic ramus point, 2, is the “noncontact” length because it cannot contact and encircle the fetal head due to the rigidity of the pubic bone. That part of the PVM lying between points 2 and 3 lies below the pubic ramus at 2 so it can contact and encircle the fetal head to allow it to pass inside the loop formed by the PVM. Lower right: This illustrates the downward rotation of the PRM from the prelabor to the ultimate crowning position. Note the absence of PRM wrapping.

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Fig. 1

Schematic illustration of the LA muscles. The subcomponents of the PVM (PPM; PAM, and PVaM) are shown. Left: schematic view of the LA muscles from below after the vulvar structures and PM have been removed showing the arcus tendineus levator ani (ATLA); external anal sphincter (EAS); PAM; PB uniting the two ends of the PPM; ICM; PRM. Right: the LA muscle seen from above looking over the sacral promontory (SAC) showing the PVaM. The urethra, vagina, and rectum have been transected just above the pelvic floor. (The internal obturator muscles have been removed to clarify levator muscle origins.) Recently, it has become clear that the origin of the PRM lies more caudal than is suggested in the illustration at left [8]. Copyright © DeLancey 2003 [9].

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Fig. 5

Cranial and right side views of the fetal head showing the SD, MD, BD, and FD. Figure adapted from Sobre et al. [24].

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Fig. 6

Vertex presentation. Male fetal head circumference (in mm) presenting to the birth canal in a vertex presentation. The shading (green) or the diagonal indicates a region of equal population distribution values for molding and head size. The intensity of the (blue) shading at lower left indicates the degree of maximal molding of small fetal heads, while the intensity of the (red) shading at upper right indicates the degree of lack of molding of large fetal heads.

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Fig. 7

Predicted maternal capacity-to-fetal head demand ratio, g, for the PVM loop with wrapping. The intensity of the (red) shading indicates the degree of cephalolevator disproportion for the PVM.

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Fig. 8

Predicted maternal capacity-to-fetal head demand ratio, g, for the PRM loop. The intensity of (red) shading indicates the degree of cephalolevator disproportion for the PRM.

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Fig. 9

Face presentation. Male fetal head circumference (in mm) presenting to the birth canal during face presentation. The shading (green) of the diagonal indicates a region of equal population distribution values for molding and head size. The intensity of the (blue) shading at lower left indicates the degree of maximal molding of small fetal heads, while the intensity of (red) shading at upper right indicates the degree of lack of molding of large fetal heads.

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Fig. 10

Predicted maternal capacity-to-fetal head demand ratio, g, for the PVM loop without wrapping. The intensity of the (red) shading indicates the degree of cephalolevator disproportion for the PVM in this special case.

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