A new understanding of the mechanical mechanism of posterior vaginal prolapse based on magnetic resonance imaging.

According to the theory of force translation, the mechanical mechanism underlying posterior vaginal prolapse (PVP) can be speculated upon by measuring the displacement of the pelvic floor supporting structures with magnetic resonance imaging (MRI). Displacement of the posterior vaginal vault (Vp), the square root of the area under the curve between the sacrococcygeal inferior pubic point line (SCIPP) and the middle third of the posterior vaginal wall (PVW) (Sc'), the midperineal body (mid-PB), the H line, the M line, the estimated levator ani subtended volume (eLASV) and the levator hiatus width (LHW) were measured while participants performed during the Valsalva maneuver on MR images. These measurements were evaluated at different stages of PVP (n = 10, 12, 9, and 17 for stages 0, I, II, and ≥ III, respectively) with one-way analysis of variance (ANOVA), and the displacement difference ratio was used to describe the distribution process of force transfer. In Phase 1, the displacement difference ratios of Vp and Sc' far exceeded those of mid-PB and eLASV; in Phase 2, the displacement difference ratio of eLASV increased significantly to more than ten times that of in Phase 1, whereas the displacement difference ratio of the mid-PB was unchanged; in Phase 3, the mid-PB displacement difference ratio increased by nearly 33 times that in Phase 2. Specific interactions between the pelvic floor muscles and connective tissues may occur during the course of PVW prolapse.