Page 87 - Biomedical Engineering and Design Handbook Volume 2, Applications

66 MEDICAL DEVICE DESIGN

3.3 STENTS AND STENT-GRAFTS: PERCUTANEOUS
VASCULAR THERAPIES

3.3.1 Market Size
According to Taber’s Cyclopedic Medical Dictionary, the term stent refers to “any material used to
hold tissue in place or provide a support for a graft or anastomosis while healing is taking place”
(Thomas, 1989). As this broad definition would suggest, stents are indicated for a wide range of dis-
ease states in the genitourinary, hepatobiliary, gastrointestinal, reconstructive, and vascular fields.
The focus of the current section is limited to those devices used in the vasculature, primarily the
peripheral and coronary arteries.
Many patients suffering from atherosclerotic vascular disease possess focal narrowings inside
their blood vessels. These narrowed regions, termed stenoses, are made up of fatty, and sometimes
calcified, plaques (atheromas) that can restrict the movement of blood to downstream tissue,
resulting in sequelae ranging from pain upon physical exertion to tissue breakdown. One danger-
ous scenario is thrombus formation at the lesion site with the potential for distal embolization.
Thrombosis and thromboembolization can directly lead to tissue ischemia (oxygen starvation) and
possible tissue death. Two categories of interventions to treat coronary stenoses and obstructions
(e.g., thrombus) have been defined in the literature, with similar definitions for interventions in the
peripheral vasculature. Percutaneous transluminal coronary angioplasty (PTCA), otherwise
known as balloon angioplasty, is a procedure wherein a catheter-mounted balloon is moved to the
lesion site and inflated, displacing the tissue and creating a wider lumen in the vessel.
Percutaneous coronary intervention (PCI) is a broad definition that encompasses PTCA and a
wide-ranging group of procedures, including atherectomy, thrombolysis, intravascular radiation,
and the placement of stents. Stents are designed to keep the PTCA-treated lesion open through
forceful opposition with the vessel wail; in essence, the stent braces the disrupted lesion in an
expanded position. Stent-grafts consist of a polymer fabric or sheet coating mounted on a stenting
device that excludes the vessel wall from the flowing blood, thus reducing its exposure to the pulse
pressure wave and minimizing blood contact with the damaged endothelium. At present, stent-
grafts have widespread application in the aorta and its major tributaries, with stents being applic-
able in almost all areas of the vasculature. Given the immense burden of CVD, stents and
stent-grafts are widely used to treat vascular lesions and are being employed in a growing number
of cases, expanding into anatomic areas once considered off-limits. In 2005, it was estimated that
more than 2 million PCIs were performed worldwide (Smith et al., 2006), with more than 1.2 million
in the United States (DeFrances et al., 2007). Stents are placed during most of these coronary
interventions either as a primary or adjunct therapy (Al Suwaidi et al., 2000), and registry
datasets report that less than 30 percent of interventions are now comprised of PTCA alone
(Anderson et al., 2002). Estimates for the number of interventional procedures in the peripheral
vasculature exceeded 200,000 per year in 1997, with the expectation that up to 50 percent of
traditional vascular procedure would be replaced with an endovascular analog in the near future
(Krajcer and Howell, 2000). One such endovascular intervention is the abdominal aortic stent-
graft and its use in abdominal aortic aneurysm (AAA) repair, which is traditionally a high-risk
surgical procedure with perioperative mortalities ranging from 5 to 60 percent, depending upon
patient characteristics (Krajcer and Howell, 2000). In 2000, it was estimated that 4000
abdominal aortic stent-grafts had been placed since their introduction (Krajcer and Howell, 2000).
However, recent data indicate that 21,000 stent-grafts were used in 2005 alone (DeFrances
et al., 2007), suggesting a dramatic rise in the use of these devices. Simple stenting is also a
major modality in the peripheral and central circulation. In 2005, it was estimated that almost
60,000 nondrug-eluting stents were placed in the peripheral vasculature, with an additional
10,000 carotid stent placements (DeFrances et a1., 2007). The clear shift toward endovascular
repairs is significant enough to have led to changes in the training of vascular surgeons
as their traditional caseload of open procedures diminishes (Sullivan et al., 2002; Diethrich,
2003).