Biomedical Engineering Reference
In-Depth Information
anatomical structures, thus providing anchoring andmechanical stability to the whole
device. TAV stents can be classified in two main categories: balloon-expandable and
self-expandable. The first category refers to stents made of elasto-plastic alloys (e.g.
stainless steel): in order to be implanted, balloon-expandable TAVs are crimped onto
the delivering catheter which is equipped with an elastic deflated balloon; once in
the aortic position, TAV deployment is obtained by inflating the balloon. In this
phase, the stent is plastically deformed to an expanded configuration allowing TAV
positioning within the aortic root (AR). On the other hand, self-expandable stents
are made of super-elastic alloys (e.g. Nitinol) and do not need balloon inflation to
be deployed: in this case, TAV is crimped onto the delivering catheter by means of
a constraining sheath, that is simply retrieved to allow the expansion of the stent to
its undeformed configuration.
Since the first procedure, more than 50,000 TAV have been implanted worldwide
and TAV indications enlarged to lower risk patients: the randomized PARTNER I
trial revealed the non-inferiority of TAV with respect to standard surgery in high risk
patients and the superiority of TAV in inoperable patients if compared to alternative
therapies (e.g. valvuloplasty, drug therapies) [
2
,
3
]. Recently, a further randomized
trial, PARTNER II, has been designed to compare TAV with SAVR in low risk
patients.
Despite the promising results and the continuous improvements on the outcomes,
TAV application is still limited [
4
]: the cost-effectiveness of TAV with respect to
standard approaches is under an open debate and the evidence that TAV should be
considered a preferable procedure in moderate and high risk patients has not been
demonstrated. For this reason, many cardiovascular centers tend to prefer SAVR
when not contraindicated.
One reason of this trend resides on the fact that up to now clinical trials and registries
have been focused on elderly patients with comorbidities, while it is envisioned that
the potential benefits of TAV could stand out in less critical patients [
5
]; a second rea-
son could be related to the current limited knowledge on TAV biomechanics: a deeper
understanding of phenomenological aspects related to TAV implantation could sup-
port a better definition of clinical indications and help to optimize the procedure,
as well as the design of TAV devices [
6
]. Indeed, many complications affect TAV
performances: among them, vascular injuries, stroke events, complete heart blocks
requiring pacemaker implantation and prosthesis insufficiency are the most frequent
[
7
]. It should be noted that TAV implantation is performed in a complex anatomi-
cal site and without excising the pathological native valve, which is the biological
structure that mainly interacts with the prosthesis; it has been demonstrated that the
presence of the calcifications on the native AV leaflets affects TAV performances,
mostly in terms of TAV insufficiency and heart block events [
8
-
10
]: these clinical
observations suggest the importance to accounting for the degenerative condition of
the native AV in clinical investigations.
Recently, finite element (FE) models have been successfully employed as a tool
to investigate device implantation behaviour, giving complementary information to
clinicians and contributing to address device design and the planning of therapies
[
11
,
12
]. As regards TAV, FE analyses have been performed, mostly with the aim of
