Radial force and wall apposition of balloon-expandable vascular stents in eccentric stenoses: an in vitro evaluation in a curved vessel model.

Purpose: Important criteria for optimized stent implantation are high radial force and complete apposition of the stent itself. The aim of this study was to develop a simple method to assess these parameters under controlled experimental conditions and to compare vascular stents of different designs with regard to these parameters.

Materials And Methods: Five balloon-expandable stents of different designs (Jostent Flex, MAC stent, ML-Tristar, ML-Ultra, and S670) were tested. Fourteen stents of each type were implanted in a curved plastic vessel model (curve radius, 10 mm; lumen diameter, 3.5 mm) with use of a balloon inflation pressure of 12 atm. Part of the model was a nonflexible eccentric stenosis which was 2 mm or 8 mm in length. After stent implantation, a stenosis of 10%-70% was induced and radial force of the stent struts was measured at the site of the stenosis. The apposition of the stent to the vessel wall was imaged by high-resolution radiography.

Results: Analysis of variance showed significant differences of radial force between the tested stents (P <.001). The ML-Tristar and ML-Ultra stents had the highest radial force with maximum mean values of 687 cN and 846 cN at a stenosis of 70% in the model with the long stenosis. The radial force of the S670 stent was the lowest whereas the MAC stents showed an intermediate radial force. Radial force of the ML-Ultra stent was as much as 3.8 times higher than the force of the S670 stent (P <.001). Fifty percent of the ML-Tristar and ML-Ultra stents did not expand sufficiently to touch the vessel surface at the outer curvature. With an inflation pressure of 17 atm, complete apposition of these stents was achieved. The highest number of apposition irregularities was found in the S670 group (13 of 14 stents), whereas the MAC stents revealed the lowest number of irregular appositions (three of 14 stents). A significant correlation was found between the number of interconnecting struts and the number of irregular apposition events (P <.01).

Conclusions: This model allows an accurate in vitro evaluation of different stent parameters, such as apposition to the vessel wall and radial force. None of the investigated stents showed optimal results with respect to both parameters. The apposition behavior was significantly influenced by the architecture of the stents.