Page 20 - Read Online

Schiavone et al. Modelling of metallic and polymeric stents

incorporated with anti-proliferative drugs, to overcome have been many devices available in preclinical and
the notorious in-stent restenosis (ISR) that represented clinical evaluations. Elixir DESolve (Elixir Medical
an undesired drawback of bare metal stents. Corporation, USA) stent, made of PLLA, is one of
[1]
Although ISR rates can be substantially reduced by the major biodegradable stents currently available
DESs (lowered by 74% in high-risk patients), the life- on the market. The polymer stents showed sufficient
long presence of metallic alloy stents, under loads and radial strength within 6 months of implantation, with
corrosive conditions, could increase the risk of strut the occurrence of bioresorption between one and two
fracture and failure with time. This may lead to possible years. [13]
complications such as delayed healing, lethal migration
and unstable angina. [2,3] Bioresorbable stents provide Finite element (FE) analyses have been largely used
the advance of overcoming these possible long- in modelling of stent expansion and deformation
term complications of metallic stents. Biodegradable during the deployment process. [14-16] However, majority
polymers are the most promising materials investigated FE analyses were performed on metallic stents. For
[17]
to date. [4-6] Fully expandable polymeric biodegradable example, Imani et al. modelled the effects of design
stents initially support the vessel wall as scaffolds with on vessel wall stresses for Palmaz-Schatz, Xience
sufficient radial strength to prevent mechanic recoil after V and NIR stents. The comparative study confirmed
implantation. In about 6 months, arterial remodelling that Palmaz-Schatz stent generated 15.6% and 7.6%
enters a stable phase, and no substantial scaffolding higher stresses in the arterial wall than Xience V stent
is required. Consequently, it is desirable for stents to and NIR stent, respectively. The results suggested a
dissolve after 6 months in the body, leaving behind an direct correlation between vessel wall stresses and
intact vessel with no pro-inflammatory substances or in-stent restenosis rate, as Palmaz-Schatz stent gave
obstacles for future treatments. During the process, the highest restenosis rate in clinical trials. Recent FE
[7]
the polymers gradually soften, which allows for a simulations on metallic stents by Schiavone et al.
[18]
smooth disappearance of high stresses, imposed also confirmed that stent design is a major factor that
by the permanent stents, in the recovering artery. It controls the expansion of the device and also the
should be noted that recent reports have suggested an stresses generated in the artery. In addition, shape
independent relationship between bioresorbable stents optimization of metallic stents, in terms of improved
and stent thrombosis. Instead, the higher incidence fatigue resistance and radial flexibility, can also be
[8]
rates of stent thrombosis were associated with the 2.5 achieved by FE modelling. On the contrary, there is
[19]
mm platforms. [9] very limited work dedicated to modelling the deformation
of bioresorbable polymer stents, particularly during the
One of the major concerns for polymeric BRSs process of deployment in diseased arteries.
is their mechanical performance, especially their
interaction with blood vessels during and after In this paper, FE modelling was carried out to simulate
deployment. Agrawal et al. carried out the earliest the deployment of Elixir polymer stent in a diseased
[10]
work in assessing the in vitro performance of the artery, with head-to-head comparison against Xience
Duke biodegradable stents made of Poly-L lactide metallic stent. Particularly, the simulations considered
(PLLA) fibres. Results showed that a successful the crimping step, which is necessary to fix the as-
biodegradable stent could be achieved by carefully produced stent to the catheter. The mechanical
balancing the mechanical properties of PLLA fibres performance of the two stents has been compared
and geometrical design of stents. Nuutinen et al. directly, in terms of radial expansion and stresses in
[11]
conducted in vitro tests of a woven fibre polymeric the stent-vessel system. In addition, results were also
braided stent subjected to radial compression. The compared with those obtained without considering the
performance of such stents was not as good as metal residual stresses generated from the crimping process.
ones, and the collapse pressure was found lower, even
for thicker PLLA fibres. Bünger et al. compared the METHODS
[12]
in vivo performance of stents made of biodegradable
PLLA and stainless steel, by monitoring the stents after Models for stents, balloon and artery
the implantation in carotid arteries of living pigs. Both Geometries for Xience and Elixir stents were built using
stents achieved initial technical success. However, the NX design software. Both stents were created in
PLLA was reported to be significantly softer than expanded shape, with a diameter of 3 mm and a length
metallic alloys, which affected the radial stiffness of the of 10 mm. The geometries of both stents were based
implanted stents and needed to be further investigated. on dimensions found in open resource. The strut
Over the last two decades, great effort has been made thickness is 80 µm and 150 µm for Xience stent and
for the development of bioresorbable stents, and there Elixir stent, respectively. The FE mesh contains eight-
Vessel Plus ¦ Volume 1 ¦ March 31, 2017 13