Stents, by Ryan Colombo

Background

Precutaneous coronary intervention, pioneered by Dr. Gruntzig in 1977, is the most frequently performed medical procedure [1]. In the past, this was performed using balloon angioplasty. However, ballon angioplasty alone was subject to detrimental vessel closure due to dissection and restentosis [1]. The stent was developed to maintain lumen integrity.

Angioplasty and Implantation of Stent

Figure 1. Once the catheter has been maneuvered to the site of blockage [A], the balloon is inflated [B], which deploys the stent. The stent remains open upon removal of the catheter [C] Ref. [3]

In an angioplasty, imaging techniques are used to direct a balloon-tipped catheter into an artery or vein [2]. Once the site of blockage is reached, the balloon is inflated to open the vessel, deflated, and removed. A stent may be placed around this balloon, which is deployed upon balloon inflation and will maintain the opened artery [2]. This balloon can be inflated to different

Angiplasty with stenting is initiated when a nick is made at the site of insertion. A sheath is inserted into the artery, into which the catheter is inserted and guided using fluoroscopy to the blocked site. Fluroscopy converts x-rays into video images that the doctors can see [2]. A thin guide wire is used to place the balloon-tipped catheter at the blocked site. Next, the balloon is inflated, which pushes the stent against the arterial wall. The stent remains in place once the balloon is deflated [2]. Following successful deployment, the catheter is removed and bleeding is stopped using pressure. No sutures are needed, although a closure device may be needed to seal the small hole in the artery [2]. Additionally, anesthetics are not administered during this procedure.

History

1977 - First Percutaneous coronary intervention was performed Gruntzig

1986 - Puel and Sigwart deployed the first coronary stent to act as a scaffold to prevent vessel closure during PTCA and vessel closure

1994 - 250000 PTCA's performed each year

1999 - Stenting associated with 84.2% of all PCI's

2012 - Over 500,000 patients implanted with stents each year

Bare Metal Stents

One of the major problems with bare metal stents is the occurrence of neointimal hyperplasia following impantation [1]. This leads to restentosis, or a re-narrowing of the blood veseel, and requires revascularization. This has occurred in up to a third of patients treated with bare metal stents [1].

Drug Eluting Stents

Drug eluting stents are characterized by the controlled release of immunosupressive and antiproliferative agents, which act to inhibit the accumulation of smooth muscle cells [1]. These stents are superior to bare metal stents with respect to prevention of restenosis.

Traditionally, stents have been constructed from stainless steel [1]. New generation stents are now being fabricated from cobalt-chrome alloys, which exhibit improved radial strength with thinner struts. Radial strength is being further improved upon via the introduction of platinum-chrome stents [1]. Thinner struts are thought to result in less arterial injury, thus reducing thrombosis and restenosis.

The polymer coating on the stent enables controlled drug release. These coatings must be biocompatible in order to decrease local inflammatory reactions and thrombosis [1]. Polymers that biodegrade are currently undergoing clinical trials [1].

Drug eluting stents have been deemed more cost effective than bare metal stents, as the higher cost of these stents is offset by the reduced need for revascularization procedures.

First Generation Drug Eluting Stents

First generation DES's are characterized by the timed release of sirolimus or paclitaxel. These antiproliferative drugs reduce revascularization as a result of neointimal hyperplasia. Similar risks of death and myocardial infraction exist as compared to BMS's [1].

In 2006, the European Society of Cardiology Congress cited stents releasing paclitaxel or sirolimus as slightly increasing the risk of thrombosis [1]. As of 2013, drug eluting stents are implanted in more than 500,000 patients per year [1]. Clinical studies of paclitaxel-eluting stents have ceased, and sirolimus-eluting stents are no longer manufactured.

Too much antiproliferation can retard or inhibit arterial wall healing, which leads to chronic inflammation [1].

Second Generation Drug Eluting Stents and the Future

New generation stents release everolimus and zotarolimus [1]. Clinical trials have demonstrated that everolimus reduces the risk of repeat revascularization, heart attack, and stent thrombosis compared to paclitaxel and sirolimus eluting stents [1]. Compared with paclitaxel, zotarolimus resuces the risk of myocardial infraction [1].

Uses

Stents are used to treat stable coronary artery disease, in which a drug-eluting stent is used in conjunction with an antiplatelett therapy [1]. Atherosclerosis, or the hardening of arteries, occurs when fat or cholesterol accumulates on the arterial walls, forming plaques. This is commonly associated with diabetes. Multivessel disease and Left Main Coronary Artery disease are also treated using stents [1].

References

[1] Stefanini G., Holmes D. Drug eluting coronary-artery stents. New England Journal of Medicine, 2013, 368:254-65.

[2] http://www.radiologyinfo.org/en/info.cfm?pg=angioplasty