Perineal descent predicted by a pelvic bone factor: the pelvic incidence angle.

Purpose: By measuring the pelvic incidence angle, we assessed the relationship between pelvic floor disorders and pelvic morphology, which allowed us to document for the first time the hypothesis that pelvic incidence may be a predictive factor of perineal descent.

Methods: In a retrospective study of 197 women, the perineal descent at rest and during straining was assessed by defecography. The pelvic incidence angle (53 degrees +/- 9 degrees , independent of the subject position) was defined as the angle between the line perpendicular to the sacral plate at its midpoint and the line connecting this point to the middle of the femoral heads axis. The pelvic incidence angle was correlated with the incidence of pelvic floor descent.

Results: In those with pelvic floor descent at rest compared with those without, pelvic incidence angle was significantly larger (64 degrees vs. 53 degrees , P < 10E-06). As a predictive factor of perineal descent at rest, a great pelvic incidence (>62 degrees ) had a sensitivity (73 percent), specificity (82 percent), positive predictive value (81 percent), and negative predictive value (75 percent).

Conclusions: A large pelvic incidence (>62 degrees ) may be a predictive factor of perineal descent at rest before the apparition of other acquired factors. With pelvic incidence >62 degrees , a large overhang between the insertions increases the strains on the perineum, which is rather horizontal.