Stents were designed because balloon angioplasty (PTCA) was fraught with two major problems:
- The area that was opened by balloon angioplasty abruptly closes up.
- The ballooned area is incomplete opening and if the balloon is expanded too vigorously, the arterial wall is torn causing a dissection.
In the early 90s, stents were introduced. Initial deployment of stents were plaque by excessive clotting and bleeding. The agent initially used to keep stents open was Coumadin and Aspirin. This turned out to cause excessive bleeding at puncture sites but the combination didn’t prevent the stents from clotting off. Finally around 1995, Ticlopidine, an new antiplatelet drug, was introduced. Ticlopidine and Aspirin turned out to be the correct combination that didn’t lead to excess bleeding and allowed the stent to not clot off.
Around the same time, Dr. Antonio Colombo made a very interesting observation that would change the way we implant stents forever leading to durable excellent stent results. Using intracoronary unltrasound or IVUS (a catheter that can be advanced within a coronary artery that has an ultrasound probe at the tip allowing for ultrasound pictures of the internal part of the coronary artery), he noticed that most of the stents he implanted were incompletely expanded and failed to touch the internal wall of the coronary artery. He then went back and reintroduced balloons at high-pressures to achieve proper expansion of the recently implanted stent; this lead to little to no stent blockage or clotting off of the stents.
It is these two developments that made stent implantation a practical solution to abrupt closure issue of the PTCA as well as sealing any dissection or tear of the artery. Stents implantation then took off and quickly supplanted PTCA as the primary method of opening blocked coronary arteries.
Ticlopidine was replaced in 1998 by Clopidogrel, an agent that did not have the side effects of Ticlopidine. Clopidogrel was much better tolerated and also essentially supplanted Ticlopidine as the adjuvant antiplatelet drug necessary after stents were implanted. There are newer drugs now on the market that can be used in replacement of Clopidogrel: Ticagrelor and Prasiguel. Both these drugs are more potent than Clopidogrel. These antiplatelet drugs have come to be known as P2Y12 inhibitors. These P2Y12 inhibitors must be taken daily as prescribed or else the patient has a risk of the very disastrous complication of sudden and complete closure of the stent with clot.
As stent implantation became commonplace, problems with stent implantation was noticed. Principally, metal stents tended to cause an over-zealous response of surrounding smooth muscle cells which if continuing to multiple, would in itself block the flow. Ideally, a single layer of cells were supposed to cover (endothelialize) the metallic surface of the stent so as to allow for stopping the antiplatelet drugs. It turns out that the process of endothelialization is complete after 4 weeks. However, in many patients, smooth muscle cell proliferation was so vigorous that the cells themselves created a blockage where the stent had been placed. There was no good solution to this process of excessive smooth cell proliferation, until new stents were developed. These new Drug Eluting Stents(DES) had drugs which combatted the smooth muscle cell growth impregnated within a collagen scaffold that surrounded the metal of the stent struts. The scaffold allowed the drug to slowly diffuse into the surrounding smooth muscle in contact with the stent so that the smooth muscles wouldn’t excessively react to the stent scaffold. While this did solve the problem of over vigorous cell growth and subsequent blockage, it also caused incomplete coverage of the metal in the long term (beyond one year). This lead to the metal being exposed to blood thus resulting in blood clots (the same problem that plagued the early stent implanters). Blood when it comes into contact with metal or any foreign object will clot.
This lead to the development of two new stent designs:
- Bio-absorbable stent platforms that are not made of metal but instead are made of a different substance that dissolves over time. So the stent struts actually dissolve and disappear.
- Metal stents impregnated in drug that prevent smooth muscle cells from proliferating.
Both stent designs are currently under research evaluation. While stent designs are constantly evolving, the actual procedure has remained relatively the same. (see How Stents are deployed). It turns out that devices used to image the inside of the artery (IVUS and OCT) have found the newly developed implanted stents were still underexpanded (sometimes even floating in the coronary lumen). Until the we continue to refine the technique of stent implantation, no matter what the design advancement, we may still have problems with stents coming back blocked or clotted off.
Despite these continuing issues, stent implantation has become the standard way Cardiologists deal with coronary artery blockages in one or two vessels. They have become a life-saving procedure in people having an acute heart attack.