Rationale and Feasibility Study of a Mechanical Model for the Testing of Material Fatigue in Metal Ureteral Stents

Abstract

Stents are used abundantly to maintain ureteral patency. The majority are plastic tubes that adjust easily to upper urinary-tract motion. Recently, a coiled-wire lumenless stent was introduced (ZebraStent, Neo Medical, Munich, Germany) to facilitate expulsion of stone fragments after lithotripsy. Its metal core is composed of Nitinol, with the soft J ends being of titanium. The thin shape considerably increases the extraluminal space. The ZebraStent stretches the ureter and also provides a surface for the fragments to glide along. In our 18-month experience with the ZebraStent, two of them fractured along the shaft. We sought to learn whether this complication resulted from a defect in stent design or from material fatigue secondary to constant movement. Our model is powered by an electric motor that produces a constant displacement similar to stent movements in vivo. The whole ZebraStent is embedded in a 37 degrees C waterbath to simulate physiological conditions within the ureter. We used an average displacement of 16 mm. The average frequency of ventilatory-cycle simulation was 20 times that in vivo, allowing us to collect data in a shorter time. All 10 stents broke within the proximal Nitinol shaft at the equivalent of 4 to 6 months (125-179 days). Our preliminary results show that all stents break after the equivalent of 4 or more months. The fact that this occurs in the homogenous proximal Nitinol shaft rather than at the welding point between the shaft and the titanium curl implies that breakage is secondary to material fatigue and not design error. Extensive testing is under way to confirm material fatigue as the cause of breakage. We hope to determine a safe dwelling time for these stents, which at the moment should not exceed 3 months.