Page 41 - Advances in Biomechanics and Tissue Regeneration

3.1 INTRODUCTION 35

Despite the plethora of stent designs, diameters, and materials from which they are fabricated, the technology is
imperfect. Uncovered SEMSs allow tissue ingrowth and fully covered SEMSs migrate. New designs and materials
to minimize these risks as well as those associated with occlusion and device-associated perforation are needed [22].
Stent design principally has two different manufacturing strategies: braided or knitted wire-based stents, and tube-
based stent designs [31] to which laser cuts are made to shape their final repose configuration. They can also be clas-
sified according to whether they are covered or uncovered. Although the tendency is to design stents covered to pre-
vent tumor growth and thus reduce the risk of restenosis, in practice these stents have not shown clear advantages over
other types [32]. Tumor ingrowth occurs more frequently in the uncovered group, while late migration is more com-
mon in covered stents. In [33], an increase in late migration (40%, P¼.005) and greater loss of stent function (60%,
P¼.0018) with covered stents are reported.
Different designs of colorectal stents are available for clinical use. The characteristics of some of the most extended
and used such as Ultraflex, Wallstent, and colonic Wallflex are included in Table 3.1.
In the literature, little quantitative data are found to characterize the behavior of the different types of colonic stents.
A limited number of studies have investigated the influence of design parameters for helical coils [34] and braided
stents [23, 35] on mechanical behavior. Experimental bench testing to understand the properties of self-expandable
metallic stents for different biliary and airway endoprostheses have been reported [36, 37]. Finite element analysis
is commonly used to contrast the main mechanical parameters (radial and axial strength, flexibility, etc.) of different
stents [38–42].
The combination of different geometries and materials provides stents with a wide range of mechanical properties
related to radial strength, trackability, elastic recoil, longitudinal flexibility, etc. For example, an expanded balloon
together with a slotted tube provides great radial strength, while a self-expanding coil stent is highly flexible with good
trackability. In the next section, some of the most relevant mechanical and functional characteristics are defined.
Since the first case of colorectal stent placement reported by Dohmoto et al. [25] in 1991, different stent designs have
been widely used to relieve various forms of malignant gastrointestinal obstructions. In fact, the first colonic stent was
primarily designed for vascular or esophageal use. Nowadays, different configurations (braided mesh, knitted, cell
wire, etc.) and materials (stainless steel, Nitinol, polymers) have been developed and marketed with clear differences
between them in their clinical performance, such as those observed in comparative studies of clinical trials [44–47].
The following configurations strive to fulfill the criteria of the ideal stent for colonic obstructions:

• Mechanical properties such as flexibility, high expansion rate, mechanical stability, and radial compression
resistance.
• Properties related to the stent delivery system, such as tractability, crossability, and pushability, needed to advance
the stent to the point of the obstruction.
• Clinical features such as the prevention of the migration of propulsive forces of colonic mobility, perforation, good
radiopacity, and tumor- related ingrowth or overgrowth.

TABLE 3.1 Main Available Commercial Colorectal Stents [43]
Diameter flare Diameter body
Stent name Material Length (mm) (mm) (mm) Manufacturer

Ultraflex Precision Colonic Nitinol 57, 87, 117 30 25 Boston Scientific
Stent
Wallflex Colonic TTS Nitinol 60, 90, 120 25 22 or 25 Boston Scientific
Wallstent Enteral TTS Elgiloy 60, 90 NA 20 or 22 Boston Scientific
Colonic Z stent Stainless 40, 60, 80, 100, 120 35 25 Wilson-Cook
steel
Niti-S Colorectal Stent Nitinol 60, 80, 100 28–30 20, 22, 24 Taewoon-Medical Co.,
covered, uncovered Ltd.
TTS Niti-S Colorectal Stent Nitinol 60, 80, 100 28 20 Taewoon-Medical Co.,
Ltd.
Hanarostent colorectal Nitinol Covered: 60, 90, 110, NA 18, 22 MI Tech Co. Ltd.
(Choo stent) 130
Uncovered: 80, 110, NA 18, 22
140
NA, not available.

I. BIOMECHANICS