Urethral support in female urinary continence part 2: a computational, biomechanical analysis of Valsalva

In Part 1, we observed urethral mechanics during Valsalva that oppose current continence theories. In this study, we utilize a finite element model to elucidate the role of supportive tissues on the urethra during Valsalva. By determining the sensitivity of urethral motion and deformations to variations in tissue stiffnesses, we formulate new hypotheses regarding mechanisms of urethral passive closure.

Anatomy was segmented from a nulliparous, continent woman at rest. The model was tuned such that urethral motion during Valsalva matched that observed in that patient. Urethra and surrounding tissue material properties were varied using Latin hypercube sampling to perform a sensitivity analysis. As in Part 1, urethral length, proximal and distal swinging, and shape parameters were measured at peak Valsalva for 50 simulations, and partial rank correlation coefficients were calculated between all model inputs and outputs. Cumulative influence factors determined which tissue properties were meaningfully influential (≥ 0.5).

The material properties of the urethra, perineal membrane, bladder, and paraurethral connective tissues meaningfully influenced urethral motion, deformation, and shape. Reduction of the urethral stiffness and/or the perineal membrane soft constraint resulted in simulated urethral motions and shapes associated with stress urinary incontinence in Part 1.

The data from Parts 1 and 2 suggest that connective tissues guide the controlled swinging motion and deformation of the urethra needed for passive closure during Valsalva. The swinging and kinking quantified in Part 1 and simulated in Part 2 are inconsistent with current continence theories.