In vitro simulation of stent fracture mechanisms in ureteric nitinol wire stents

The ZebraStent is a new-concept lumen-less teflon-coated nitinol double-J wire-stent, designed to facilitate the passage of residual fragments after extracorporeal shock wave lithotripsy. In clinical practice we observed a small number of stent fractures. Hence, an experimental model was designed to simulate the physical forces that may lead to material fatigue of the stent. Flexion force was simulated by "half circular kidney mimicking structures" (HCKMS) into which the upper part of the stent was placed. All experiments were done for a minimum of 5 million cycles representing a stent indwelling time of 9 months, or until stent fracture, and simulating respiratory kidney movement. It was demonstrated that as the diameter of the HCKMS decreases, thus leading to an increased bending of the stent, the likelihood of stent fracture increased proportionally and occurred earlier. From our results it appears that stent fractures can be avoided by observing a maximum indwelling time of 6-8 weeks (which should suffice for the duration of a average SWL treatment), by choosing the correct (and shortest possible) stent length, and perhaps by manufacturer's modifications decreasing the stent's resistance to flexion. The ZebraStent concept remains appealing if it is considered as a short-term stent for post-SWL residual fragments.