Calculation of membrane tension in selected sections of the pelvic floor

A mathematical model to estimate membrane tensions (Mt) at the urogenital hiatus and midpelvis in patients with and without prolapse is proposed. For that purpose the complex structures of the pelvic floor were simplified and, based on assumptions concerning geometry and loading conditions, Laplace's law was used to calculate Mt. The pelvic cavity is represented by an ellipsoid in which the midpelvic and hiatal sections are described by an ellipse. The downwards forces within the pelvis (F(in)) are in equilibrium with the support forces within its walls (F(w)). F(in) is the abdominal pressure (PABD) multiplied by the area A of the ellipse. The force inside the tissues (F(w)) is distributed along the circumference of the ellipse C. The Mt can be approximated as Mt = (PABD.A)/C (N/m). Mt-α accounts for the angle α which describes tissue orientation with respect to the anatomical section and is calculated as Mt-α = Mt/sin(α).

We conducted a retrospective study on archived magnetic resonance imaging scans (n = 20) and ultrasound images in patients with (n = 50) or without prolapse (n = 50) and measured actual geometrical variables. PABD was measured in patients with and without prolapse (n = 20).

Mt at the urogenital hiatus at rest is 0.35 N/cm. They significantly increase with the Valsalva manoeuvre, by a factor of 2.3 (without prolapse) to 3.6 (with prolapse).

Calculated Mt are much lower than what is reported for the abdominal cavity. Prolapse patients have significantly larger Mt, which during the Valsalva manoeuvre increase more than in healthy subjects.