Biomechanical analyses of the maternal pelvic floor during the second stage of labor.

Pelvic floor dysfunction including pelvic organ prolapse, stress urinary incontinence and fecal incontinence, is an important women's health issue. Both pelvic floor dysfunction and localized levator ani muscle atrophy are associated with vaginal birth. To quantify maternal levator muscle tissue stretch during vaginal birth we developed 3-D geometric computer model to simulate delivery of a molded, spherical, fetal head (Chapter 2). The pubovisceral portion of the levator muscle reached a stretch ratio of 3.26 at the end of the second stage of labor, and corresponded with location of the muscle defect on MR images. Pudendal nerve damage is found in women with pelvic floor dysfunction and has been documented as occurring with vaginal birth. So, the above model was modified to simulate the pudendal nerve stretch during vaginal birth (Chapter 3). The largest strain, 33%, was found in the nerve branches innervating the anal sphincter, placing them at highest risk for stretch injury during vaginal birth. In order to investigate the stress distribution within levator ani muscle during vaginal birth, a subject-specific 3-D viscoelastic finite element model was constructed using ABAQUS to simulate the second stage of labor using a realistically-shaped fetal head (Chapter 4). The region of maximum stress was found to occur at the end of the second stage of labor near the origin of the pubovisceral muscle. A 3-D viscoelastic model of the pelvic floor was developed to examine the effect of six different directed pushing techniques on the length of the second stage of labor (Chapter 5). Three maximum voluntary pushes during the uterine contraction resulted in the shortest labor. One push at peak uterine contraction lengthened labor by 16%, while pushing before or after the peak lengthened it by at least 32%. This may have implications for avoiding maternal exhaustion. A simplified mathematical viscoelastic model was used to examine the effect on tissue stretch of varying the relative stiffness of the pubovisceral muscle and perineal body during the second stage of labor (Chapter 6). The results show that a two-fold decrease in perineal body stiffness reduced maximum pubovisceral muscle stretch by 8%, while a 1-cm episiotomy reduced it by the same amount.PhDApplied SciencesBiomedical engineeringHealth and Environmental SciencesMechanical engineeringObstetricsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/126487/2/3253335.pd