Deformation Analysis of Balloon-Expandable Stents Considering Its Surface Roughness and Viscoplasticity

Stent implantation is a well-known non-surgical method to treat vascular diseases. As one of object implanted in the human body, it should have certain surface roughness. De Palma et.al ‎(1972) reported that surface roughness beyond the scale of a micrometer strongly hinders the process of surface passivation, thus it will lead to thrombus formation. Rough-ness of biomaterial implant turned out to have a significant effect to thrombogenicity. An observation on the outer surface of some intravascular catheters confirmed that surface roughness is one of the factors which can cause thrombo-genicity (Hecker J.F. and Scandrett L.A., 1985). Materials with a smooth surface and low coefficient of friction appear to be more biocompatible by reducing the mechanical skinning and the shear forces at the biomaterial-tissue interface. This phenomenon has been reported by Denstedt et.al (1998). The other study show that grooved surfaces promoted increased rate of migration of endothelial cells (Palmaz et.al., 1999). Furthermore, in vitro investigation into the effect of surface roughness of stainless steel confirmed that stainless steel with roughness of 671.8±27.8 nm causes endothelial cell activation and inflammation (E. McLucas et.al, 2006). These studies emphasize that the surface roughness of stainless steel is an indispensable surface property in the development of vascular stents. In the contrary, recently studies shows that the smoother surface of the stent may not guarantee to provide treatment effectively. Stent with a rough surface was as safe as a stent with a smooth surface. A rough stent surface does not increase late lumen loss after stent implantation as compared with a conventional smooth stent surface (Dibra et.al, 2005). Based on the facts, controlling surface properties of stent after deployment is extremely important. As is well known, stent should deform until reach plastic deformation. Many investigators confirmed that changes in the grain structure of metals will lead to changes in its surface roughness (T. Mizuno and H. Mulki, 1996, Heleen et.al, 1998, C. Lally et.al, 2004, O. Wouters et.al, 2005). Surface roughness actually has been changing during stent expansion. Conducting experiments either in vitro or in vivo on this subject in detail will encounter many obstacles and consume much time. By finite element simulation, changes in surface roughness during the deployment process can be observed easily. In reviewing the literature, there are numerous finite element simulations relating to the interaction between expanding stent, plaque, and arterial vessel wall. Several serious simulation and analyses regarding the behavior of stent in expansion and deflation process, with the presence of balloon, plaque, and artery, were reported (S.N. David Chua et.al, 2004, K. Takashima et.al, 2007, H. Zahedmanesh and C. Lally, 2009, F.S. Cui et.al, 2010). Another simulation compared the effect of two or more stent designs to stress within the vessel wall so that the design with the stent causing the least-injury can be determined (C. Lally et.al, 2005, Kiousis et.al, 2007). Otherwise, three different plaque compositions on underlying arterial wall were reported to greatly influence stress distribution within the vessel wall (I. Pericevic et.al, 2009). Thereupon, the effects of material on the deployment of coronary stents in order to establish an appropriate stent material in the treatment of vascular disease was also observed (S. Tammareddi and Q. Li, 2010). The most recent investigation regarding full computational simulation of stent deployment in a reasonable coronary artery was carried out on a very computationally expensive simulation (H. Zahedmanesh et al., 2010). However, no finite element simulation was found on the association between plastic deformations of the stent and its surface roughness. In this study, therefore, we attempted to observe the relationship between plastic strain and surface roughness of balloon expandable stent using finite element method, considering verious length of plaque and creep behavior, which could influence plastic strain distribution of stent.