Stents are known to fracture in tortuous vessels which have severe angulation, such as at bifurcations, where the angle between the main artery and the side branch can vary from 15° to 120° (Chaichana et al. 2011). Furthermore, the cardiac cycle causes the arteries to undergo a significant amount of repetitive hinge-type motion, with the degree of movement depending on the intensity of blood flow and location of the vessels. The angle of this hinge-type motion has been measured as 22.8±4.9° in the left anterior descending while in the right coronary artery it increases to 31.0±13.1° (Ino et al. 2009). The aim of this bench testing study was to observe and better understand the mechanism of stent fracture under this type of loading for a number of different stent designs.
Methods A test rig has been designed and built to enable the deployment of 36 stents at a predetermined angle into silastic mock arteries. The stents were deployed at an initial artery angle of 90°, and then subjected to a 20° continuous repetitive hinge-type movement, at a rate of approximately 1100 rpm. To clarify the loading, a vessel initially angulated at 90° would have that angle further exacerbated to 110° during testing.
9 each of four different stent types have been tested :BioMatrix Flex (Biosensors Europe SA); MULTI-LINK Vision (Abbott Vascular); Promus PREMIER (Boston Scientific); Integrity Prokinetic (Biotronik AG) for up to 166 million cycles. All stents were deployed at nominal pressure and maintained at that pressure for 30 s, with 3 mm ID, with a length of 28 mm for all except Prokinetic which were 30 mm length as 28 mm is not available in that design. Stents were periodically examined in a micro-CT scanner every 7 million cycles to identify strut fractures.
Results Fractures have been identified in 7 stents in total so far, and are limited to only the BioMatrix Flex samples. Fractures were first seen to occur at 13.5 million cycles (which corresponds to 133 days of average cardiac cycles assuming 70bpm). These initial fractures were observed in 2 stents, (one having a single strut fracture, and the other exhibiting 3 strut fractures) as seen in figure 2. All fractures were seen to occur at the ring linker parts of the stent, and in the mid-portion of the stent as is observed clinically (Aoki et al. 2007), in areas of high tensile stress. By 75 million cycles (corresponding to approximately 763 days), 7 of the BioMatrix stents had fractured, exhibiting between one and four strut fractures in each stent. Testing is ongoing, however clinically it has been observed that the mean time to stent fracture is 450 days ± 330 days, with little likelihood of new fractures being observed after this time.
Conclusions This is an observational study, to increase understanding of the way that stent fracture occurs in these types of conditions. The results show that the test-rig can replicate in vivo stent loading conditions and will be a useful tool for further studies.