A Hypothesis Generating the Mechanical Systems Underlying Posterior Vaginal Prolapse Based on Observed Displacements by Dynamic Magnetic Resonance Imaging

The aim of this study was to analyze quantified displacements of the posterior vaginal wall (PVW) on dynamic magnetic resonance imaging (MRI), which may generate hypotheses for the detailed mechanisms that underlie the development of posterior vaginal prolapse.Pelvic dynamic MRI scans were obtained for 12 women with normal vaginal structure (stage 0) and 62 women with 4 consecutive stages (1-4) of posterior vaginal prolapse. Structural locations (apex vagina, distal vagina, and mid-perineal body [PB]) and equidistant points along the PVW (points 4-6 were considered as midvagina) were identified, and PVW length, straight distance of PVW, levator ani parameters (levator hiatus length [LHL], levator hiatus width [LHW], levator plate angle, anorectal angle, and M line [ML]), urogenital hiatus, and prolapse diameter were measured at rest and maximal Valsalva, respectively. The displacement of these measurements was obtained.From stage 0 to 2, the variables LHL, LHW, levator plate angle, anorectal angle, and ML increased gradually, but midvagina, distal vagina, and mid-PB were the opposite. From stage 2 to 3, apex vagina, midvagina, distal vaginal, mid-PB, LHL, LHW, and ML raised rapidly and peaked at stage 3, then declined at stage 4. In addition, the correlation coefficients between each measurement from stage 2 to 3 were statistically higher than those from stage 0 to 2.Quantified displacements of the PVW and its supporting structure were shown on dynamic MRI, and the mechanical mechanisms were hypothesized regarding the interaction between pressure and the support force contributing to the deformation of the PVW and the supporting structures.