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Schiavone et al. Modelling of metallic and polymeric stents

relatively non-compliant balloons, i.e. stiffer and larger between bioresorbable polymeric and metallic stents.
than the stent delivery system balloon, to ensure good
apposition and to ensure that the intended diameter On the other hand, the plaque can be classified as
is achieved. However, multiple cycles of inflation were hypocellular, cellular or calcified, depending on the
not simulated here which is a limitation of this study. composition. It is more difficult to treat calcified plaque
The simulation in this work was a single deployment by stenting due to its strong resistance to stretch when
[18]
of stent or direct stenting of a vascular lesion. In compared to hypocellular or cellular plaques. So far,
clinical practice, pre-dilatation of diseased artery the effect of plaque composition on stent expansion was
can also occur. Direct-stenting is common, but not only evaluated for isotropic tissue model, and further
with bioresorbable polymer stents. For bioresorbable work is required to study such effects by considering
polymer stents (or any new technology), the users vessel anisotropy. Finally, polymers generally possess
generally acknowledge the importance of careful lesion anisotropy and viscoplasticity effects which could
preparation by pre-dilatation as well as post-dilatation. also affect the simulation results and will need to be
The pre-dilatation balloon inflation step modifies the addressed in future studies.
plaque and results in less recoil after deployment
of a stent or scaffold. After a stent is deployed, an In conclusion, crimping and deployment of polymeric
additional post-dilatation generally occurs with a and metallic stents have been simulated using finite
second larger balloon. Sometimes this is to achieve element method to give a direct comparison of their
a better deployment through a tapered vessel, or to mechanical performances. Results demonstrated
“crack” a stubborn plaque. Sometimes, this is required that polymer stent has a lower rate of expansion than
when two stents are deployed with an overlap. This metallic stent. The overall expansion, reached at peak
type of balloon pre-dilatation and post-dilatation is inflating pressure and after balloon deflation, was lower
essential for polymer stents to be effective in difficult for polymer stent due to weaker material properties.
This is also associated with the higher recoiling effect
lesions. In fact, larger degrees of recoil and dogboning for polymer stent. Thus, it is a challenge to use polymer
predicted by the simulations indicate that adequate stent to treat patients with heavily calcified plaques or
pre-dilatation and post-dilatation are potentially critical stiffer vessels, without pre-dilation or post-dilation.
for polymeric stents to achieve optimal clinical results. Crimping generated severe residual stresses in the
Simulation of pre-dilation or post-dilation requires stent, which tend to affect stent expansion and increase
proper inelastic or damage models to describe dogboning for Elixir polymer stent. However, they did
unrecoverable deformation for the plaque and artery not alter the stress distribution during the deployment
wall, which is currently beyond the scope of the paper. process, and only imposed small changes to the stress
In this study, the artery and plaque were assumed to magnitude.
behave purely elastically (hyperelastic model), and the
efforts of pre-dilation or post-dilation will be nullified as Financial support and sponsorship
soon as the dilation pressure is removed. Modelling of Nil.
inelastic deformation and damage will be attempted in
our future work. Conflicts of interest
In addition, the stresses in the vessel layers at peak There are no conflicts of interest.
inflating pressure are generally beyond the ultimate
tensile strength of the tissue layer. This is also Patient consent
[25]
the case for the plaques. Therefore, tissue damage There is no patient involved.
will need to be modelled in the FE simulations at
high pressure levels. Tissue damage is associated Ethics approval
with unrecoverable deformation in the artery, which This article does not contain any studies with human
could reduce the stent recoiling considerably upon participants or animals.
balloon deflation. Consequently, stent is expected to
expand further if tissue damage is considered due to REFERENCES
irrecoverable mechanical deformation of the artery-
plaque system, thus resulting in a larger lumen diameter. 1. Hoffmann R, Mintz GS. Coronary in-stent restenosis - predictors,
Damage modeling of arterial layers is a limitation of this treatment and prevention. Eur Heart J 2000;21:1739-49.
study and will be studied in our future work. However, 2. Joner M, Finn AV, Farb A, Mont EK, Kolodgie FD, Ladich E, Kutys
R, Skorija K, Gold HK, Virmani R. Pathology of drug-eluting stents in
it does not affect the general conclusion of this paper, humans: delayed healing and late thrombotic risk. J Am Coll Cardiol
as we aim to make like-for-like comparison of the 2006;48:193-202.
scaffolding capability and mechanical performance 3. Yang TH, Kim DI, Park SG, Seo JS, Cho HJ, Seol SH, Kim SM, Kim
20 Vessel Plus ¦ Volume 1 ¦ March 31, 2017