Levator ani muscle stretch induced by simulated vaginal birth

Abstract

Objective: To develop a three-dimensional computer model to predict levator ani muscle stretch during vaginal birth.

Methods: Serial magnetic resonance images from a healthy nulliparous 34-year-old woman, published anatomic data, and engineering graphics software were used to construct a structural model of the levator ani muscles along with related passive tissues. The model was used to quantify pelvic floor muscle stretch induced during the second stage of labor as a model fetal head progressively engaged and then stretched the iliococcygeus, pubococcygeus, and puborectalis muscles.

Results: The largest tissue strain reached a stretch ratio (tissue length under stretch/original tissue length) of 3.26 in medial pubococcygeus muscle, the shortest, most medial and ventral levator ani muscle. Regions of the ileococcygeus, pubococcygeus, and puborectalis muscles reached maximal stretch ratios of 2.73, 2.50, and 2.28, respectively. Tissue stretch ratios were proportional to fetal head size: For example, increasing fetal head diameter by 9% increased medial pubococcygeus stretch by the same amount.

Conclusion: The medial pubococcygeus muscles undergo the largest stretch of any levator ani muscles during vaginal birth. They are therefore at the greatest risk for stretch-related injury.

Figures

Figure 1

(Top) Inferior three-quarter view, seen from the left, of the pelvic floor structures appearing behind the ischiopubic rami (gray). That portion of the perineal membrane (blue) connecting the most medial portion (2) of the pubococcygeus muscles to the perineal body is shown. The lateral portions of the perineal membrane have been removed. (Bottom) The pelvic bones (outlined from image above in white) and perineal body have been removed to show a close-up of the arrangement of the iliococcygeal, pubococcygeal, puborectalis muscles, as well as the urethra (umber), vagina (pink), and rectum (brown). Individual muscle bands are identified by a number inscribed near their origin on the arcus tendineus (white). The anal sphincters were segmented directly from in vivo magnetic resonance images, but neither the coapting effect of the venous plexus nor its covering anoderm are shown. © 2003 Biomechanics Research Lab, University of Michigan, Ann Arbor. Lien. Muscle Stretch During Birth. Obstet Gynecol 2004.

Figure 2

Superior and inferior (A and B) and left lateral views of the muscular elements of the female pelvic floor (C and D). (A and B) The pubic symphysis (PS) appears in the midline at the top of each image. The sacrum (not shown) would appear at bottom. (B) Half of the perineal membrane (blue) has been removed to reveal the medialmost pubococcygeus (orange) and puborectalis (magenta) muscles. The arcus tendineus appears bilaterally as the cablelike structures (white) to which the lateral ends of each iliococcygeus muscle (red ) band insert; the midline anococcygeal raphe (white) is also evident. (C) The left half of the bony pelvis has been removed to reveal a lateral exterior view of the muscles. (D) The left-side muscles have also been removed, along with the pelvic organs, to reveal an inner view of the right-side muscles. The internal and external anal sphincters are shown in light and dark brown, respectively. Colors for urethra and vagina are the same as in Figure 1. The anal sphincters were segmented directly from in vivo magnetic resonance images, but neither the coapting effect of the venous plexus nor its covering anoderm are shown. © 2003 Biomechanics Research Lab, University of Michigan, Ann Arbor. Lien. Muscle Stretch During Birth. Obstet Gynecol 2004.

Figure 3

Simulated effect of fetal head descent on the levator ani muscles in the second stage of labor. At top left, a left lateral view shows the fetal head (blue) located posteriorly and inferiorly to the pubic symphysis (PS) in front of the sacrum (S). The sequence of five images at left show the fetal head as it descends 1.1, 2.9, 4.7, 7.9, and 9.9 cm below the ischial spines as the head passes along the curve of Carus (indicated by the transparent, light blue, curved tube). The sequence of five images at right are front-left, three-quarter views corresponding to those shown at left. © 2003 Biomechanics Research Lab, University of Michigan, Ann Arbor. Lien. Muscle Stretch During Birth. Obstet Gynecol 2004.

Figure 4

The relationship between fetal head descent (abscissa, in centimeters; icons at top) and the resulting muscle stretch ratios (ordinate) in selected levator ani muscles. The labels at right identify the pubococcygeus (PC), iliococcygeus (IC), and puborectalis (PR) muscle bands defined and numbered in Figure 1. The largest stretch is induced in the medialmost pubococcygeus (PC2) muscle, the last muscle to be engaged by the fetal head. The shaded region denotes the values of stretch tolerated by nongravid appendicular striated muscle without injury. © 2003 Biomechanics Research Lab, University of Michigan, Ann Arbor. Lien. Muscle Stretch During Birth. Obstet Gynecol 2004.

Figure 5

The upper bar graph compares, by muscle, initial and final muscle lengths corresponding to 1.1- and 9.9-cm model fetal head descent, respectively, as shown anatomically in Figure 3. The lower bar graph shows the maximum corresponding stretch ratio found in each levator ani muscle band. Note that the value of the stretch ratio is not simply proportional to initial or final length. For both graphs, muscles are arranged left to right, in ventral to dorsal order of origin location, following the numbering used in Figure 1. Lien. Muscle Stretch During Birth. Obstet Gynecol 2004.

Figure 6

(A) Normal anatomy in an axial midurethra proton density magnetic resonance image showing the pubovisceral muscle (*) (see Figure 1 for orientation). (B) Woman who has lost a part of the left pubovisceral muscle (displayed on the right side of the image, according to standard medical imaging convention) with lateral displacement of the vagina into the area normally occupied by the muscle. The arrow points to the expected location of the missing muscle. The puborectalis is left intact bilaterally. OI = obturator internus; PB = pubic bone; U = urethra; V = vagina; R = rectum. Lien. Muscle Stretch During Birth. Obstet Gynecol 2004.

Figure 7

Axial, midurethral section through the arch of the pubic bone (see PS, top) and the model levator ani muscles (shown earlier in Figures 1 and 2) corresponding to those from the patients shown in the Figure 6 cross sections. Intact muscles are shown in dark gray. Simulated PC2 muscle atrophy is illustrated by the light gray shading of the left-side PC2 muscle. This location is shown to correspond with the location of muscle atrophy demonstrated in Figure 6. PS = pubic symphysis; U = urethra; V= vagina; R = rectum. Lien. Muscle Stretch During Birth. Obstet Gynecol 2004.