Intra-coronary Stents - What are they ?

Dateline: 10/26/97

In last week's article, we discussed the catheter-based treatment modalities for coronary artery obstruction. The focus of this article will be Intra-coronary Stents, a topic which many of you have written to me about, and which consequently I hope will interest you all.

What is a stent ?
A stent is a "strut" or "scaffolding" inside the lumen of an artery, that supports the weakened artery wall from within.

Why are stents needed ?
As mentioned in last week's article, angioplasty involves the use of a special catheter called a balloon catheter. This is introduced into the arterial system and manipulated across the narrowed area of the coronary artery. The balloon is then inflated, widening the blocked artery and allowing blood flow to resume.

This effect of angioplasty is achieved by crushing or breaking an atheromatous plaque inside the coronary artery. This area is weak and when the balloon catheter is removed, the wall might collapse, causing a recurrent obstruction. Blood flow once again will stop.

Such a situation may be dangerous and life-threatening. Avoiding this complication has long been the bug-bear of interventional cardiologists who perform these angioplasty procedures.

How do stents work ?
A stent is a device that is also mounted onto a balloon catheter. In the usual manner the balloon catheter is placed across the area of obstruction in the coronary artery and inflated. The stent is also expanded by this maneuver. After the artery is opened up, the balloon catheter is deflated, and removed. The stent however retains its shape, remains inside the artery and does not collapse. It thus acts as a scaffold and supports the arterial wall.

How are stents designed ?
There are many different types of stents. One commonly used design is the Gianturco stent which appears like a metal coil (somewhat like the spring inside a ball-point pen !). The coil is loaded onto a balloon catheter, and when inflated, the spring uncoils partly and holds the artery open. Another model is the Wallstent, which appears like a cylindrical mesh (similar to a roll of wire fencing) which again is threaded onto a balloon catheter. Many more models of stents are available, and each has a specific advantage in particular situations.

What are the benefits of using stents ?
Stents are primarily used to prevent the acute (sudden) occlusion of arteries after dilatation by an angioplasty procedure. Stents are also useful in some other situations. Angioplasty can now be done in patients with severe and long-segment obstruction of coronary arteries. As soon as the initial part of the block is widened, a stent holds it open allowing further dilatation to proceed. Stents have also allowed angioplasty to be performed in patients with blocks of multiple vessels, and also in multiple blocks in a single artery.

Why are stents not used in all cases ?
Stents have some disadvantages as well. They are made of metal or polymers, and constitute a "foreign" and rough surface in contact with blood inside the artery. By its nature, blood is conditioned to harden (clot) when it encounters "foreign" material, and this may cause a blood clot (thrombus) to form inside the dilated artery. Rarely, the stent may dislodge and migrate in the blood stream to a more distant branch or to another part of the arterial tree. As it is made of metal, there is a small chance of the stent eroding the artery wall and causing a perforation.

What are the newer advances in coronary stents ?
Many new approaches have been tried using stents to improve the results of coronary angioplasty. One of the major shortcomings of angioplasty in the long term is the chance of recurrent narrowing of the dilated artery - a condition called "restenosis". Impregnating the stent with drugs and chemicals that emit radiation (gamma-rays) has been found to reduce the risk of restenosis. Also, using special polymers that coat the inside of the artery and then harden forming a smooth "pavement" inside it, the chance of restenosis has been reduced - "polymer paving". Self-expanding stents are the latest development. These are made of special alloys that can expand over time inside the artery.

I hope this article has cleared some of your doubts about coronary stents.