Helically Ridged Ureteric Stent Facilitates the Passage of Stone Fragments in an Experimental Porcine Model

We have developed a ureteric stent with features designed to facilitate active expulsion of ureteric stone fragments. Its unique characteristics include a helix-like ridge along a thin central core; the superior edge of the ridge is bevelled caudally and the inferior is concave. The ridge's outer edge is smooth and hydrophilic. As the stent is moved in relation to the ureteric wall (owing to respiratory motion and/or peristalsis), stone fragments will slip over the superior edge of the helical ridge. Once past this edge, they cannot migrate in a cephalad direction; they will pass down the helical trough or be ratcheted by the repetitive bevel and emerge through the ureteric orifice. To evaluate the effectiveness of the design it was compared with a conventional, smooth-surfaced double-pigtail stent in an ex vivo porcine model. A single artificial stone fragment (3 x 2 x 2 mm, 4 x 3 x 2 or 5.5 x 3 x 2 mm) was placed in the proximal ureter and the stent was moved up and down (56 studies with each type of stent). The helically ridged stent required a mean of 79 excursions (standard deviation 60, range 8-291). In contrast, the conventional stent was unable to move any stone fragments--even after more than 1000 excursions.