Increased Efficiency in Spontaneously Passing Kidney Stones Using a Novel Ureteral Double-J Stent

Kidney stones are stiff deposit minerals formed in kidneys. Kidney stones affect a considerable number of the world popluation, and can be very painful during passing time through the ureter. Stone passing time in patients have been reported at an order of 20 days. In patients with kidney stones disease, double-J stents are used to increase openness of a ureter in case of kidney stones blockage. The stent surfaces normally are smooth and with symmetric friction along the longitudinal axis. This means that the friction coefficient between stone and stent along the distal end of the stent is considered to be the same. In this work, we show that a modified double-J stent with asymmetric friction surface facilitates expulsion of stone fragments after extracorporeal shock wave lithotripsy or trans ureteral lithoclast. Using Finite Element Method (FEM) and representation of fragmented ureteral stone motion and ureter, the stent was modeled and simulated with asymmetric friction inside the ureter in conjunction with random relative back and forth motion of ureter and kidney. The stent was modeled with linear elastic material with Young’s modulus of 0.4 MPa and Poisson ratio of 0.3 which is in contact with a linearly elastic ureter with Young’s modulus of 40 kPa and Poisson ratio of 0.49. Also, the kidney stone was assumed to be rigid, as it is significantly stiffer than the surrounding tissue, it also could help reduce the computational costs. The asymmetric friction was modeled by a Fourier series which periodically changed friction between 0.5 and 0.7. The periodicity in friction coefficient and vibration excitation effectively gave different friction coefficients in the axial direction, also, computationally reduces the modeling complexity. Simulation results reveals that at excitation vibration frequency of 20Hz fragmentation unidirectionally moves along the stent at an average speed of 5 mm/s. The speed at whitch fragmentations move in the modified stent is significantly increased compared to clinical results reporting average passing time of around 20 days. The proposed double-J stents, besides having their original function of opening the ureter larger kidney stones, with the asymmetric friction pushe the fragments towards bladder by the relative motions between ureter, kidney stone and stent. In reality patients undergo the vibrations induced by daily activities e.g. running, driving which can excite the kidney stone and move it unidirectionally towards the bladder. Practically, the difference of the friction coefficients along the stent could be made by laser micro-engraving techniques like CD writing techniques.