Image credit: Artur Tumasjan on Unsplash
Upper urinary tract obstruction is a condition where the normal passage of urine from the kidneys to bladder is impeded or blocked. It can happen to newborns and children with congenital anomalies and anyone with kidney stones or external compressions caused by tumor or pregnancy.
When obstruction happens, a thin polymer tube known as ureteral stent can be inserted to restore the urinary flow. The indwelling stent can be either short-term, as a perioperative procedure to assist stone removal, or long-term as means of relieving external compressions.
Complications of long- term indwelling stents are particularly of concern, including biofilm formation and infection, growing encrustations, and eventual stent blockage.
In case the patient fails to follow up their appointment to remove the stent, the indwelling time can be prolonged indefinitely and the stent will become stuck. As such, its removal will require more invasive approaches or even open surgery. “It feels like pulling a barbed wire or sandpaper through the ureter and can cause severe damage,” says Professor Fiona Burkhard, Chairwoman for Functional Urology at the Inselspital, Bern University Hospital.
These microscale crystals move and aggregate in the urine flow and eventually grow into larger crystals that fuel blockage. To elucidate the interplay between urinary flow and the deposition/growth of crystals, various benchtop and computer models have been implemented.
Dr. Dario Carugo and his team at the School of Pharmacy, University College London and the University of Southampton, designed microfluidic chips simulating different geometries of stent side holes, which are small circular openings on the stent wall connecting the intraluminal and extraluminal fluids. By tuning the vertex angle of the side hole edges and optimizing the stent wall thickness, they achieved more than 80% reduction of deposits.
The application of fluid mechanics to urology extends also to ureteroscopy, a minimally invasive procedure to remove kidney stones. In ureteroscopy, a long thin ureteroscope, which contains a hollow lumen called the working channel, is inserted through the urethra and into the kidney. Using a laser, the stone is broken into small fragments or dust that can then be washed out by injecting saline solution through the working channel.
Professor Sarah Waters and colleagues at the Mathematical Institute, University of Oxford, recently investigated the washout efficacy by means of mathematical modelling. Looking at the region near the exit of the working channel, they proposed a method to optimize the channel geometry by minimizing the recirculation of injected flow. The washout time, defined as the time needed for 90% of the stone fragments to be washed out of the region, could be reduced by up to 10%.
Continuing discussions and collaborations between researchers and clinicians allow exploration of potential translatable engineering solutions to the daily challenges in urological practice. In this context, researchers are developing an in vitro platform to mimic the physiological pressure and flow during bladder filling and emptying. The platform is being used to test current ureteral stents and investigate novel designs to minimize problems.
“Clinicians and researchers are learning so much from each other,” says Dr. Francesco Clavica, head of the Urogenital Engineering Group. “Together we will keep exploring new ideas for stent design with fewer side effects to reduce the negative impact on patients’ quality of life.”
References: Shoakai Zheng, et al., Fluid mechanical modeling of the upper urinary tract, WIREs Mechanisms of Disease (2021). DOI: 10.1002/wsbm.1523; J. Williams, et al., Shape optimisation for faster washout in recirculating flows, J. Fluid Mech. (2021) DOI:10.1017/jfm.2020.1119