Inside The Exam Room™ 12/26/2006
INTRAVASCULAR STENTS: Part 2
By Mark Ombrellaro, MD
Currently, there are a wide variety of stents that differ with respect to several important characteristics. These include the radial strength (outward force or support) of the stent, its flexibility, crushability, as well as other technical considerations such as material construction, deployment method, radiopacity (visibility on x-ray), and predictability of final length after deployment. From the perspective of the interventionalist physician, optimal performance characteristics are the ability to provide the greatest amount of support but still be easy enough to track though the blood stream and be delivered to the site where it is needed. It must also be easy to see and very predictable with respect to its final length so that it can be accurately positioned. In a perfect word, the stent material would cause a minimal amount of inflammation (irritation) from the arterial wall: just enough for it to heal well without an over abundance of scar tissue formation. Radial strength and flexibility/deliverability are often related to the amount of metal in the stent. With the typical balloon expandable stent described previously, the slotted tube configuration is a fairly rigid structure which gives good support, but can be difficult to maneuver in the blood stream when manufactured in longer lengths. For these reasons, they tend to be made in short lengths so they have improved trackability. In order to balance these considerations, newer generations of stainless steel balloon expandable stents have been engineered to remove various segments of the metal arms. By reducing the amount of metal, some of the diamond shaped spaces become connected to other spaces. Eliminating the metal and opening up the diamond spaces improves flexibility and deliverability of the stent. The trick is to eliminate the metal in such a way as to retain as much of the radial strength of the stent as possible. Having the diamond spaces intact is what is referred to as a “closed cell” stent design (the cell being the diamond space) while connecting the diamond spaces is referred to as an “open cell” stent design.
Another method of obtaining flexibility, while retaining radial strength, is to change the type of metal used in stent construction. Most stents used in peripheral arteries are made of nickel titanium alloy or Nitinol. Nitinol is “memory metal” which can be made into a certain shape, then cooled down and packaged into a small, low profile delivery catheter, and unsheathed in the blood stream. When the stent is exposed to body temperature, it will assume its original shape. This type of stent is referred to as a self-expanding stent system. Nitinol stents have significant radial strength while being quite flexible. They allow for the manufacturing of very long stents which can be easily guided through the blood stream and positioned with great accuracy. Another advantage of Nitinol stents is that they are better at conforming to the arterial wall than balloon expandable stents. This characteristic is very desirable in circumstances where there are changing vessel diameters or when supporting long, complex areas of narrowing or obstruction. Nitinol, however, can be difficult to see in under x-ray, especially when used in the abdomen where there a lot of overlying structures that produce shadows. While any given stent of the proper size can be used in any appropriate vascular bed, some types of stents are better suited to specific applications or anatomic areas. Balloon expandable stents tend to be used most commonly in the coronary (heart), renal (kidney), mesenteric (gut), and iliac (pelvic) arteries, while self expanding stents in the legs, arms, chest, and iliac arteries.
The healing response to an intravascular stent is that immediately after implantation; fibrin and clots begin to form on the bare metal surface. This clot matrix typically accounts for a 1-2mm build up on the inner surface of the stent. Over the first week, immature endothelial cells begin to cover the metal stent arms as well as the open spaces in between. The third week after implantation, smooth muscle cells begin to invade the area and by week 8 fibrous tissue begins to develop. As a general rule, lesser degrees of angioplasty injury and wall damage, and better contact between the stent arms and the vessel wall surface, are associated with a smaller amount of scar tissue formation and a more complete healing response. The optimal result with either angioplasty or stenting is to have the blood vessel lumen restored to normal and the artery healed with an endothelial surface lining with the least amount of scar tissue formation.