Ureteral stents: coil strength and durometer.

OBJECTIVES: To evaluate the coil strength before and after urine exposure and the stiffness of commercially available double-J ureteral stents because both properties may affect stent performance and patient comfort. METHODS: Twelve commercially available 6F ureteral stents were tested for coil strength before and after 30 days of urine exposure. The proximal end of each stent was inserted through a 2-mm hole in bologna, allowed to recoil, and then pulled using a handheld force gauge. Ten different commercially available ureteral stent models were tested for tensile strength using an MTS MicroBionix Testing System and Testworks II software and a 5 N load cell. RESULTS: The Cook Black Silicone and Cook C-Flex stents had the strongest coil strengths before urine exposure at 0.480 +/- 0.0 lb (P < or = 0.0006) and were also the stents that had the greatest decrease in coil strength after urine exposure. After urine exposure, the weakest stent was the Applied Vertex stent at 0.088 +/- 0.008 lb (P < or = 0.02) and the strongest was the Cook Endo-Sof AQ at 0.223 +/- 0.014 lb (P < or = 0.03). Calculating the Young's modulus, E, the Cook C-Flex stent was the stiffest (E = 1472 +/- 196 KPa) and the Cook Black Silicone was the least stiff (E = 122 +/- 18 KPa). The stent models that demonstrated consistent E values across different lot numbers were the Circon Double J stent and Bard InLay. CONCLUSIONS: Ureteral stents can be differentiated according to their coil strength and stiffness. The impact of these properties on stent performance and patient comfort deserve additional evaluation. The significant variability found in stent stiffness among stents from different lot numbers suggests poor quality assurance in biomaterials or stent processing and increases the complexity of cross-stent comparisons.

Radial dilation of ureteral balloons: comparative in vitro analysis.

BACKGROUND AND PURPOSE: The dynamics of ureteral balloon expansion may differ with increasing extrinsic compressive forces and inflation pressures. This study compared the ability of ureteral balloons to expand under different conditions. MATERIALS AND METHODS: The balloons tested were the Cook Accent, Ascend, Ascend AQ, and Pursuit; the Bard 195503 and UroForce; and the Boston Scientific Microvasive UroMax Ultra. When available, multiple balloon diameters and lengths were tested. With a guidewire in place, the balloon tip was secured by elevated vise grips on either side of the balloon. A string was wrapped around the balloon center once, and incremental increases in load were added (2 g, 42 g, 82 g, 122 g) to represent increasing extrinsic compression. The balloon was inflated with contrast medium, and circumference changes were measured at increments of 2 atm up to burst pressure. Balloons were tested in triplicate for each weight. RESULTS: The majority of the balloons were unable to reach 90% of their expected diameter with larger constrictive loads (122 g) at low inflation pressure (4 atm). The only balloons that achieved a diameter at 4 atm that was at least 90% of the expected diameter with a coefficient of variance (CV) of <10% at all radial loads were the Pursuit 6 mm x 4 cm (98.2 +/- 2.2%; CV 7.88%), UroMax Ultra 7 mm x 4 cm (97.5 +/- 1.4%; CV 5.94%), and the UroMax Ultra 7 mm x 6 cm (101 x 1.2%; CV 7.67%). At inflation burst pressure, the balloons able to maintain a diameter at or above 100% of expected with a CV of <5% at burst pressure were the Ascend AQ 4 mm x 4 cm (116 +/- 1.0%; CV 3.34%) and the Pursuit 6 mm x 4 cm (108 +/- 2.0%; CV 4.53%). CONCLUSION: Reaching maximum inflation diameter at low pressures in the face of increasing extrinsic compression may help minimize the risk of ureteral injury. Reliable expansion to maximum diameter even with higher extrinsic compressive forces is another important characteristic of ureteral balloons. Balloon material, configuration, and dimensions may contribute to differences in dilation properties.