Page 88 - Biomedical Engineering and Design Handbook Volume 2, Applications

OVERVIEW OF CARDIOVASCULAR DEVICES 67

3.3.2 Indications
Consensus documents developed through the collaboration of cardiovascular societies (American
College of Cardiology, American Heart Association, and the Society for Cardiovascular
Angiography and Interventions) detail coronary stenting indications currently supported by literature
findings (Smith et al., 2006). Data suggest that most stents (at least 95 percent) are placed with a
clear indication that follows the clinical standard of care (Anderson et al., 2002). Stents are most
often placed in nonemergent (or elective) settings to optimize outcome and improve patency of
vessels that have undergone balloon angioplasty (Al Suwaidi et al., 2000). An ideal PCI outcome has
various classifications, but a residual vessel diameter stenosis of less than 20 percent along with
TIMI 3 flow [defined as no visible reduction in the speed of contrast transit with good perfusion bed
runoff (TMI Study Group, 1985)] has broad acceptance as defining an ideal outcome in the poststent
era (Smith et al., 2006). Stents are also placed emergently in patients undergoing a myocardial
infarction due to coronary obstruction, a procedure termed primary PCI (Smith et al., 2006). Other
less well-established indications include stent placement for the treatment of chronic total vessel
occlusion, saphenous vein bypass graft lesions, gradual tissue overgrowth in an existing stent, and
lesions with difficult morphology, such as long, diffuse stenoses, stenoses in small vessels, and
lesions at a bifurcation or vessel opening (Holmes et al., 1998). Ongoing randomized trials continue
to delineate the optimal treatment of various coronary pathologies.
There has been considerable advancement in recognizing which patients and pathologies are
amenable to either traditional open revascularization or endovascular treatment (Hirsch et al., 2006;
Norgren et al., 2007). Reported indications for peripheral, noncoronary vascular stent placement
include immediate treatment of balloon angioplasty complications such as intimal dissection and flap
formation; the correction of unsatisfactory angioplasty outcomes such as residual stenosis, spasm,
recoil or occlusion; treatment of complicated, chronic lesions or occlusions; and as a routine, combi-
nation treatment with angioplasty (Mattos et al., 1999). The most common reason for placement of a
stent in the peripheral vasculature is an unsatisfactory result from angioplasty (Mattos et al., 1999).

3.3.3 Vascular Stent Design
The ideal stent would possess a number of characteristics designed to ease handling and permit
stable, long-term function. Desired handling characteristics include a simple, effective deployment
method, high radiopacity for visualization under fluoroscopy, flexibility to ease passage through
tortuous vessels, limited shortening during expansion so placement is optimal, high expansion ratio to
allow smaller profiles, and ease of retrievability or removal, if misplaced (Mattos et al., 1999; Henry
et al., 2000). Preferred functional characteristics include a high hoop strength to counteract arterial
spasm, biocompatibility that minimizes short-term and long-term complications, and durability in
the stressful, corrosive environment of the human body (Mattos et al., 1999; Henry et al., 2000).
Despite the large number of stent designs, with additional models under development, no one stent
possesses all these particular characteristics (Henry et al., 2000).
Stents can be divided into two main groups based on the method of expansion. Balloon-expandable
stents either arrive premounted on a balloon angioplasty catheter or are mounted by the doctor prior to
the procedure. A balloon catheter with inflation apparatus is shown in Fig. 3.5. While mounted, the
stent is moved into place and the balloon inflated to expand the stent to the desired diameter.
Figure 3.6 illustrates the placement and inflation procedure for balloon-expandable stents. In contrast,
self-expanding stents come premounted or sheathed. Once deployed to the treatment area, the sheath is
pulled back, allowing the stent to expand to its predetermined diameter. Balloon-expandable stents can
be further subdivided into slotted-tube and coil-based designs (Oesterle et al., 1998).
Each stent type possesses particular advantages and disadvantages. Self-expanding stents can
experience shortening during deployment which may complicate placement, although more recent
stent designs are able to mitigate this effect to a significant degree (Oesterle et al., 1998). Balloon-
expandable stents exhibit a greater stiffness than the self-expanding models, which can cause diffi-
culty navigating long or tortuous lesions and anatomy (Mattos et a1., 1999). In general, coil design