Virtual Urinary Tract: Modeling the ureter and the urethra on a patient-specific basis

Many disorders of the urinary tract, such as urethral strictures and ureteral strictures, are problems of restricted urine flow. Strictures, if left untreated, can progress to infection, bladder dysfunction, and compromised renal function. Diagnostic tools for such urologic conditions have not evolved in decades. For example, the retrograde urethrogram (RUG), commonly used to evaluate for urethral stricture, has not changed since the 1920s. We propose a paradigm anchored on understanding the mechanics of voiding that will help identify strictures, quantify severity, and determine the surgical reconstruction plan. This will provide a new mechanics-based scheme for disease classification, diagnosis, treatment, and progression. In the long term, this will open new avenues to understand scar tissue formation by relating changes in elastic compliance to biochemical alterations in the tissue. A program that simulates flow through the ureter and urethra was developed, considering the specific mechanisms that underlie the transport of fluid. The properties of the tissue walls, such as the length, location, and stiffness of a stricture, can be specified to model strictures on a patient-specific basis. The system of hyperbolic conservation laws for fluid flow in a flexible tube are solved for the area and velocity as a function of space and time using the method of characteristics. The ureter and urethra can be individually modeled or can be connected with a simple bladder, which acts as a reservoir for holding fluid transported from the ureter. The pressure profile of the bladder, which controls the emptying of the fluid through the urethra, can be specified to account for different bladder functioning. A Virtual Disease Landscape (VDL) can be created to understand the different types of function that exist in a reduced parameter space. In particular, the impact of the length and location of the stricture on the maximum flow at the urethral meatus is explored.