A New SWL Titanium Stent (Zebra Stent®): Resistance to Shockwave Exposure

Urinary Stent Development and Evaluation Models: In Vitro, Ex Vivo and In Vivo-A European Network of Multidisciplinary Research to Improve Urinary Stents (ENIUS) Initiative

Background: When trying to modify urinary stents, certain pre-clinical steps have to be followed before clinical evaluation in humans. Usually, the process starts as an in silico assessment. The urinary tract is a highly complex, dynamic and variable environment, which makes a computer simulation closely reflecting physiological conditions extremely challenging. Therefore, the pre-clinical evaluation needs to go through further steps of in vitro, ex vivo and in vivo assessments. Methods and materials: Within the European Network of Multidisciplinary Research to Improve Urinary Stents (ENIUS), the authors summarized and evaluated stent assessment models in silico, in vitro, ex vivo and in vivo. The topic and relevant sub-topics were researched in a systematic literature search in Embase, Scope, Web of Science and PubMed. Clinicaltrials.gov was consulted for ongoing trials. Articles were selected systematically according to guidelines with non-relevant, non-complete, and non-English or Spanish language articles excluded. Results: In the first part of this paper, we critically evaluate in vitro stent assessment models used over the last five decades, outlining briefly their strengths and weaknesses. In the second part, we provide a step-by-step guide on what to consider when setting up an ex vivo model for stent evaluation on the example of a biodegradable stent. Lastly, the third part lists and discusses the pros and cons of available animal models for urinary stent evaluation, this being the final step before human trials. Conclusions: We hope that this overview can provide a practical guide and a critical discussion of the experimental pre-clinical evaluation steps needed, which will help interested readers in choosing the right methodology from the start of a stent evaluation process once an in silico assessment has been completed. Only a transparent multidisciplinary approach using the correct methodology will lead to a successful clinical implementation of any new or modified stent.

New developments in ureteral stent design, materials and coatings

Ureteral stents are used in a variety of urological diseases and procedures. The majority of patients with indwelling ureteral stents experience bothersome symptoms and are at increased risk for urinary tract infection. Stent encrustation and the associated complications can also result in significant patient morbidity. The development of new stent designs using novel biomaterials, stent coatings and drug-eluting technologies are being applied to reduce the disadvantageous features of ureteral stents.

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.