Inside The Exam Room™ 01/02/2007
INTRAVASCULAR STENTS: Part 3
By Mark Ombrellaro, MD
It is the formation of excessive scar tissue within and around the site of angioplasty or stenting which can cause recurrent narrowing of the treated blood vessel segment and reduce the longevity of these procedures. This phenomenon is what is referred to as restenosis. No matter which type of metal stent is used (balloon expandable or self expanding), all are associated with a small amount of fibrin and blood clot forming on its inside surface immediately after being implanted. Large diameter stents tend to remain open longer than small diameter ones because the larger surface area of the flow channel is better able to accommodate this early healing process. As a simple example, a 1mm buildup on the inside of a 3mm diameter stent is more significant than a 1mm build up on the inside of an 8mm diameter stent. Stents in larger blood vessels are also better able to withstand any development of scar tissue formation as compared to small diameter stents.
Understanding the biology of the healing process makes it easier to understand some of the medical treatments physicians use in order to maximize the functionality and longevity of stents in the circulation. The key goals are to try to minimize excessive arterial wall damage when performing angioplasty and stenting procedures, in addition to maximizing surface area contact between the arterial wall and the stent upon implantation. These factors (although somewhat difficult to control in the first instance) are aimed at trying to reduce the mechanical reasons that lead to excessive clot and scar formation within the vessel lumen. Clots develop in the circulation under these circumstances for several reasons. Clotting is a normal response to injury and one of the first steps in repairing a blood vessel after it has become damaged (and prevents us from bleeding to death after minor injuries). Endothelium is the normal cell lining found inside of blood vessels and is normally smooth and resistant to clot formation. Underneath the endothelial lining are tissue components (matrix and basement membrane) which are highly thrombogenic (clot promoting). When the endothelial lining of a blood vessel is damaged, exposure of the underlying structures promotes clot formation. As we also know, blood clots when it is not flowing and foreign materials within the blood stream also provoke a clotting response as well. Given these principles, it is easy to understand how procedures such as angioplasty and stenting can lead to clots within the arterial circulation and the development of scar tissue within the artery wall.
Fortunately, the vast majority of the time these procedures can be performed in such a way that even in the face of the normal healing response, the treatment zone remains open and an excellent clinical result can be achieved. It is how the body adapts to the initial treatment that explains how it “wears out” over time. Lesser amounts of trauma and injury require less scar formation and smaller amounts of residual blood clot also leads to smaller amounts of scar tissue formation. In order to reduce the amount of clot that forms in the treatment area, physicians often use antiplatelet agents such as aspirin or Plavix immediately following angioplasty or stenting. These medications are typically used until endothelial regrowth is completed and is why most physicians advocate their use for at least the first 1-2 months at a minimum. Quite often after any type of arterial intervention (minimally invasive or bypass) aspirin use is advocated indefinitely in order to reduce treatment complications as well as the future risk of heart attack and stroke. Antiplatelt therapy is especially important in small diameter stents: i.e. those less than 5mm in diameter.
Another potential strategy to improve the healing and performance of stents would be to try to turn off the process of scar tissue formation at the site of injury itself. This would mean that despite the trauma induced by the ballooning and stenting, we would be able to shut off this part of the healing process or fool the body into thinking that the treatment area is already healed when in fact it may not be. This is what drug eluting or “drug coated” stents are all about. The concept is that in addition to the normal support functions of the bare metal stent itself, the stent serves as a platform to deliver medication locally to the treatment area in order to influence the healing process. The 2 most common drug coated stents are Boston Scientific’s Taxus and Cordis’ (Johnson &Johnson) Cypher stents.