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Schiavone et al. Modelling of metallic and polymeric stents

For Elixir stent, it is confirmed that crimping-caused artery, the peak value of the maximum principal stress
residual stresses do affect stent expansion [Figure 11]. changed by ~5% (1.50 MPa to 1.43 MPa) when the
When stent expansion was simulated without residual stresses were considered.
considering residual stress, the stent expanded faster
and reached saturation earlier, as shown by the solid DISCUSSION
blue line in Figure 11A. Overall, residual stresses tend
to compromise stent expansion both at peak pressure Clinically, in-stent restenosis (ISR), i.e. re-narrowing
and after balloon deflation. At peak pressure, stent of stented artery, is one of the major drawbacks
[30]
outer diameter was found to be 2.61 mm and 2.75 mm associated with stent implantation. ISR is a direct
for simulations with and without considering residual consequence of the formation of neointima, largely
stresses, respectively, and settled as 2.10 mm caused by proliferating smooth muscle cells and
and 2.19 mm, respectively, after balloon deflation. accumulated extracellular matrix. According to
[30]
Although the recoiling had similar magnitude for both recent investigations, arterial wall biomechanics plays
cases (~20%; with and without residual stresses), a key role in ISR. The stenting-caused alteration of
less dogboning was found when residual stresses biomechanical environment controls the inflammatory
were excluded in simulations (i.e. 41%; versus 45% and remodelling processes of vessel walls. As reported
for simulations with residual stresses). As shown in by Timmins et al., stent designs that induced higher
[31]
Figure 12, residual stresses did not affect the pattern
of stress distribution in the stent and artery. On the
stent, stresses are still highly localised at the U-bend
regions, regardless of the residual stress state. Also,
the maximum stress on the stent changed only by
~0.4% (95.69 to 95.32 MPa) when residual stresses
were considered in the simulations. In the artery
[Figure 12B], stresses were again concentrated
towards the ends of the plaque as a result of stent
dogboning effect. The peak value of the maximum
principal stress changed by ~15% (0.64 to 0.54 MPa)
when the residual stresses were included.

For Xience stent, residual stresses affected the early
stage of expansion of the stent as shown in Figure 13.
The achieved final diameter (~2.40 mm) and the recoiling
effect (~11%) were similar for the two cases (with
and without residual stresses). Dogboning increased
when residual stresses were excluded, with a value
of 36% (compared to 24% for the case considering
residual stresses). As shown in Figure 14, the stresses
developed in the stent and the diseased artery were
similar, in terms of distribution, for the two cases (with
and without residual stresses). In terms of magnitude,
the maximum stress in the stent changed only by Figure 11: (A) Diameter change against pressure; and (B) recoiling
~0.7% (928.9 to 935.0 MPa) when residual stresses and dogboning effects obtained from simulations with and without
were included in the simulations. For the diseased considering crimping-caused post-crimping stresses on Elixir stent

Figure 12: (A) von Mises stress (MPa) on the Elixir stent; and (B) maximum principal stress (MPa) on the artery-plaque system for
simulations with (left) and without (right) considering residual stresses
18 Vessel Plus ¦ Volume 1 ¦ March 31, 2017