MECHANICAL PERFORMANCE OF POLYURETHANE URETERAL STENTS IN-VITRO AND EX-VIVO

A serious problem associated with the use of ureteral stents is fractu re in situ. Following clinical observations of fracture of polyurethan e stents in vivo, this study examined the mechanical properties of 17 polyurethane stents (double-J containing drainage holes) retrieved fro m patients over a 24-week period of insertion. In addition, stents wer e immersed in human and artificial urine in an in vitro model at 37 de grees C to determine their general propensity to fracture. Mechanical properties of ureteral stents were examined using the standard ASTM D- 412 tensile test and by the novel application of dynamic mechanical an alysis (DMA). The ultimate tensile strength and elongation at break (b ut not the Young's modulus) of unused polyurethane stent sections cont aining side-drainage holes were greater than stent sections devoid of side-drainage holes. No correlations were observed between increased o r decreased Young's modulus, ultimate tensile strength or elongation a t break of polyurethane stents and their time of immersion in either h uman urine or artificial urine in simulated upper urinary tract condit ions of 37 degrees C and 5% CO2. Similarly, no correlations were obser ved between Young's modulus, ultimate tensile strength or elongation o f polyurethane stents and stent dwell time in situ. DMA of retrieved s tents revealed that their tan delta value and storage modulus did not differ significantly from unused stents following dwell times in situ of up to 24 weeks. No changes in the glass transition temperatures wer e observed in retrieved stents. Although patient variation was observe d, the results indicate that the polyurethane stents examined in vitro and following removal from patients did not exhibit any greater prope nsity to fracture than their unused counterparts. Fracture of retrieve d polyurethane stents, arising in vivo and also during subsequent tens ile testing, was observed to occur along the drainage holes, suggestin g that elimination of these holes will reduce the incidence of polyure thane ureteral stent fracture in use. (C) 1997 Elsevier Science Limite d. All rights reserved.