A finite element model validates an external mechanism for opening the urethral tube prior to micturition in the female

Existing theories for micturition in the female mandate total relaxation of the pelvic floor while detrusor pressure pushes the urethra open. However, video X-ray and electromyogram data indicate that micturition is preceded by active outwards opening of the outflow tract by backward-/downward-acting muscle vectors. If the detrusor pressure alone is enough to expand the tube, why does the active opening take place? The aim was to model the urethral tube in detail to assess the relative importance of the active opening mechanism and detrusor pressure. Finite element methods were used to model the urethral tube in detail. Nonlinear-elastic properties similar to urethral component tissues were taken from the literature. The boundary conditions applied to the tube model included internal pressure due to detrusor contraction (60 cm H2O) and various displacements and constraints such as pubourethral and pubovesical ligament attachments. In order to achieve opening dimensions similar to those in the lateral X-ray under the action of detrusor pressure alone, the pressure had to be increased by two orders of magnitude above normal levels. Normal detrusor pressure alone is not sufficient to achieve opening of the urethra against the elasticity of the constituent tissues, suggesting that normal micturition requires an active mechanism provided by backward-/downward-acting pelvic floor muscles, as predicted by the integral theory.