Author – Dr. Alan Koay Choon Chern, Consultant Cardiologist, Columbia Asia Hospital – Taiping
An overview of coronary stents evolution, its current status and future directions
The heart has a network of coronary arteries which circulates blood within the organ. From the aorta, vessels branch off to form the right and the left coronary arteries for coronary circulation. Coronary artery disease is the obstruction of any of these vessels, resulting in heart attacks or acute coronary syndrome.
Coronary artery disease (CAD), either narrowing (stenosis) or blockage (occlusion), arises from plaque deposition in the arterial wall, also known as atherosclerosis. This results in restriction of blood flow and hence oxygen to the heart muscle, which is called ischemia. Atherosclerosis is beyond just the accumulation of lipids, as it also a series of responses characteristic to inflammatory disease. CAD could be chronic (arteries narrowing over time) or acute (sudden rupture of plaque), both of which require stent-based coronary intervention, to relieve the arteries of the obstruction and regulate normal blood flow to the affected cardiac muscle.
Percutaneous coronary intervention (PCI) was introduced in the late 1970s by Dr. Andreas Gruentzig and that has revolutionized the management of stable and unstable coronary artery disease. Before that, coronary artery bypass grafting surgery was the only option of treatment besides medications. PCI offers an effective, safe and readily available method for coronary revascularization for many patients. Over the past four decades, this specialty has witnessed rapid development which eventually led to the introduction of a number of new technologies, including coronary stents that have resulted in improved efficacy and long term safety. These technologies including new generations of drug-eluting stents, non-polymeric stents, bioresorbable polymer-coated stents, and fully bioresorbable scaffolds.
Initially, balloon angioplasty was first introduced as a less invasive alternative to coronary artery bypass grafting surgery. However, the major setback of balloon angioplasty was the high rate of acute arterial closure due to recoil and dissection, which resulted in myocardial infarction. The introduction of coronary stents in the 1990s has drastically reduced the incidence of acute arterial closure and ever since stent implantation became the standard of care percutaneously.
A stent is a tiny mesh tube that is designed to remain permanently implanted at the intended vessel. Briefly, a guide-wire is advanced across the diseased segment of the artery. Then, a balloon catheter is positioned to compress the plaque against the arterial wall, and then a stent is deployed to provide permanent mechanical support, thus preventing acute arterial closure.
First generation stents were made of bare metal (stents without medication), which had an in-stent restenosis rate of 20-30%. Arterial re-narrowing within the area of stent implantation (in-stent restenosis) occurs due to a process called neo- intimal proliferation. As a solution to this problem, drug-eluting stents (DES) was designed in 1999. DES are coated with medication to disrupt the process of restenosis and hence effectively reduced the rate of restenosis to less than 10%. However, DES also unexpectedly increased the risk of stent thrombosis, a life- threatening complication. There are a number of reasons why stent thrombosis can occur, namely delayed endothelialisation of stent struts, hypersensitivity to drugs or polymer coating of the stent or stent mal-apposition. To prevent this from happening, prolonged dual- antiplatelet therapy is mandatory following DES implantation, but at a slightly increased risk of bleeding. The various problems encountered have been the constant impetus for innovation in coronary stent designs and technology which ultimate aim was to improve clinical outcome.
Early coronary stents were made of stainless steel that has relatively large stent struts. As such, their flexibility is limited and resulted in higher restenosis rate. Newer generation stents have largely replaced stainless steel with cobalt alloys. This enabled longer stents with thinner struts to be developed and these have provided more flexibility to achieve better vessel conformability, without compromising its radial strength.
Additionally, new DES technologies comprise novel polymeric materials, for controlled release of medication at the implanted site. Since polymer coatings are redundant after drug release, newer generation DES are therefore made up of biocompatible or biodegradable polymer coatings that dissolve after drug release is completed. Further development has explored an approach that removes the polymer coating entirely, hence forming a polymer-free DES which releases medication directly from the stent surface. All these have made possible a shorter duration dual- antiplatelet therapy, which eventually translates to lower bleeding risk.
The current trend is seeking to replace a permanent metallic prosthesis with something which will completely disintegrate or disappear once its supporting purpose is fulfilled. This results in what is known as a bioresorbable scaffold. The potential advantages of having the stent disappear include reduced risk of late stent thrombosis, future vessel assessment with computed tomography and restoration of normal arterial motion. However, the setbacks include bulky stent struts that are less deliverable, stent fracture due to aggressive balloon dilatation and lack of radio-opacity. Hence, a lot of research and development in this area is ongoing as the existing bioresorbable scaffolds are still far from perfect.
The field of PCI is evolving to facilitate safe and optimized treatment for patients with coronary artery disease. Stent technologies will continue to focus on the above issues to create even better stents. Although the ideal stent is still nowhere in sight, most contemporary stents are reasonably safe and effective options, providing low incidence of stent complications