Successful Reproduction of In-Vivo Fracture of an Endovascular Stent in Superficial Femoral Artery Utilizing a Novel Multi-loading Durability Test System

Aim: High rates of stent fracture have been reported in the treatment of diseased superficial femoral artery (SFA). Understanding of in-vivo mechanical-loading environments, therefore, draws serious attention. We have developed a novel multi-loading durability tester which can apply cyclic twist and stretch on stents. Here, in-vitro stent fracture patterns were compared with stent fracture observed in vivo. Methods: The angle of cyclic twist as well as cyclic stretch exerted to the stents was regulated to physiologic conditions by utilizing referenced data of MR angiography of SFAs obtained in the spine and fetal position. Stiffness of the silicone vessel models were regulated to that of SFAs. As the SFAs are intrinsically stretched in vivo, stents were tested not only in the nonstretched vessel model but also in the 50% pre-stretched vessel model, respectively. The simultaneous cyclic multi-loads were exerted to the stents under the mean luminal pressure of 100mmHg at 60 cycles per minute. Considering the data that average Japanese walk 7378 steps per day, the cyclic numbers of the in-vitro durability tests were converted to the equivalent durability in vivo. Results: Only minor fracture was observed at the stent strut after one-year-equivalent duration when the stents were tested using the non-stretched vessel models. However, employment of the novel pre-stretched vessel model resulted in complete stent fractures in the middle of longitudinal direction in 2-months-equivalent duration. Moreover, the fracture pattern was clearly coincident with the stent fracture in vivo. Conclusions: This is the first demonstration that the pre-stretch of vessel model is a requisite condition in durability tests of stents for SFA. Moreover, these results indicated that reproduction of multi-loading environments including cyclic twist and stretch is of great significance in order to obtain reliable durability data of endovascular stents for SFA.