On the Mechanism of Levator Ani Muscle Injury during Vaginal Birth.

Abstract

Vaginal birth is the single largest modifiable risk factor for female pelvic floor disorders, such as pelvic organ prolapse and excretory incontinence. Defects in the levator ani muscle (LA) near its pubic origin immediately after vaginal birth are known to correlate with a three-fold increase in prolapse later in life. A current knowledge gap concerns the normal anatomy, the detailed injury mechanism of this specific injury region, and accurate measures of second stage labor events. An anatomical study of the origin of the pubovisceral portion of the levator ani muscle (PVM) revealed a systematic change in morphology (Ch. 2.1): the medial origin formed a direct oblique attachment, while the lateral origin arose from the catenary-like levator arch. The fiber directions of the different LA subdivisions were quantified using MRIs (Ch. 2.2). The projected angle of the PVM fibers was found to differ by an average of 58° from that of the puborectal muscle (PRM) in the mid-sagittal plane, suggesting different mechanical roles in the pelvic floor structure. Another histological study revealed that the PVM originates medially from the PB via a fibrous entheses tangentially from the periosteum of the PB (Ch. 2.3). A 2-D simplified FE model representing the PVM showed a significant strain energy concentration at the inferior margin of the scarf enthesis (Ch. 3.1). This suggests why injury of the LA can initiate at that location. A 3-D FE model of vaginal birth corroborated these findings and demonstrated why not only the PVM origin (or enthesis) but also the levator arch were at higher risk of injury than the midsection (Ch. 3.2). Finally, a novel computer vision measurement system was developed for measuring perineal surface deformation during late second stage of labor (Ch. 4). The results from two women showed that the deformation during the final push was up to twice that of earlier pushes. We conclude that during vaginal birth, not only excessive stretch particularly at the end of the second stage but also characteristic anatomical pattern of the PVM origin play major roles in causing the injury.