Page 63 - Advances in Biomechanics and Tissue Regeneration

3.6 CUSTOMIZED PARAMETRIC DESIGN 57

FIG. 3.27 Surgical intervention: (A) introduction rectally of the catheter, guide wire, and transporter; (B) stent release and removal of guide and
transporter; (C) radiological monitoring of stent deployment.

The geometric typology used for this purpose corresponds to a tube-based stent with closed diamond-shaped cells
(Palmaz-Schatz type), which correspond to a basic geometry module used by the authors to design a colonic stent [53,
54]. In a previous comparative analysis of stents for the treatment of colorectal obstruction [48], it was shown that
stents based on closed cells with arms joined by rigid nodes develop higher radial force responses necessary for
reopening occlusive colonic strictures where greater strength is required.
The repose shape obtained after the subsequent shaping process, thermomechanical treatment (numerically: stress
field to zero), and electropolishing [53] (Fig. 3.20B) gives a bell-shaped profile similar to the commercial device
Medtronic-CoreValve used for the treatment of aortic valves.
By giving each stent a constant number of circumferential cells, it is possible to obtain areas with different stiff-
nesses, which vary the pattern cut along the longitudinal axis, and adjust the expansion ratio factor imposed on each
ring of cells until it achieves the desired shape.
Acting over these two design parameters, a shape profile with a longitudinally variable radial strength to facilitate
optimal adaptation to the duct walls and prevent migration risk can be set [54]. The pressure exercised against duct
walls by each annular cell may be adjusted independently of the profile adopted to avoid migration. Therefore it is
possible to obtain a stent with a shape adapted to each type of stenotic duct and variable radial stiffness according
to the needs of each patient and pathology.
As an example, a distribution of estimated required net radial forces is presented in Fig. 3.28 in three different areas
of the same acute stenosis, which by surgical prescription are considered suitable for restoring transit on the section of
affected colon. Those values might be estimated by balloon catheter via colonoscopy in subacute strictures; by per-
forming a minimal local stricture dilatation and recording the pressure to inflate the balloon, an estimation of radial
force needed in each area can be obtained.
Among the different possibilities (i.e., a single solution does not exist), by varying just length slots and keeping the
rest of the parameters constant, for example, the stent configuration can be obtained. The preliminary dimensioning of
geometry gives rise to a cutting pattern as illustrated in Fig. 3.29A, with zones 1–3 with slot length according to the
given specifications, and a transition zone 4. After the forming process simulation, the stent configuration shown in
Fig. 3.29B is obtained.

I. BIOMECHANICS