Stent Function, Material, and Types
So, what is a vascular stent? A stent is a short hollow expandable tube placed inside the lumen of an artery or vein to allow unobstructed flow of the blood. The procedure in which stents are implanted in the lumen of the blood vessel using a catheter will be discussed in detail later. Stents are made of metal mesh that helps it expand when placed inside the vessel. The idea of the stent is to restore the adequate blood supply of a certain organ by maintaining the patency of the supplying blood vessel (scaffolding) with fewer risks of restenosis. The procedure in which the stent is implanted is called stent angioplasty.
There are different materials from which stents are made. stents have metal mesh wire used to be made of stainless steel. However, Platinum-chromium, cobalt-chromium, nickel-titanium alloys, and tantalum have recently substituted stainless steel as they offer better structural durability and flexibility with thinner struts. Since the 80s, stents were adopted widely in clinical trials which eventually resulted in shaping their current form. Nowadays, stents have mainly two types which we will discuss separately:
1) Bare Metal Stents (BMS): are the traditional stents that were approved by the medical community in 1986 after proving their superiority over balloon angioplasty. The design and materials used in BMS are developed to overcome its poor flexibility and thick struts. Bare metal stents could be self-expanding or balloon expanding. Although BMS was an excellent solution for vascular occlusion and offered great results, acute and subacute restenosis of the unblocked portion was a little common complication. Doctors found that blood clots get stuck in the stents causing partial obstruction of the bloodstream and 25% of patients may need another procedure within six months after the procedure. Therefore, doctors had to give double anti-platelet therapy (DAPT) to their patients who underwent stenting to prevent the recurrence of stenosis.
2) Drug-Eluting Stent (DES): was introduced to the medical community in the late 90s when a Brazilian interventional cardiologist called Eduardo Sousa succeeded in performing the first-in-human sirolimus Drug-Eluting Stent implant in Sao Paolo, Brazil. Doubtlessly, Dr. Sousa had taken a step forward in revascularization and encouraged cardiologists to develop second and third generations of DES. Drug-eluting stents (DES) were created to particularly overcome the restenosis issues associated with BMS. They are made up of a BMS covered with a polymer that slowly releases medication to suppress the cell growth that may lead to restenosis. The potential risk of restenosis has been reduced significantly by the action the drug has been released by the polymer. DES releases the pharmacological agents for six to nine months.
In 2003, johnson & johnson and cordis corp. had developed the first generation of DES approved by the FDA and called it CYPHER. The active agent released from the CYPHER stent was macrocyclic lactone, sirolimus which is a class of drugs that have a broad spectrum of action (antibiotic, antiproliferative and immunosuppressive). The first generation of DES was coated with a durable non-degradable polymer such as polyethylene-co-vinyl acetate (PEVA) and poly n-butyl methacrylate (PnBMA). This generation of DES was believed to have a controversial correlation with the incidence of MI (myocardial infarction)
The second generation of DES came out after a few years of the first generation. In this generation, doctors applied functional and structural modifications as Polymers were composed of newer, more biocompatible materials including zotarolimus, everolimus, and novolimus, resulting in a quicker onset of drug elution and better anti-proliferative action. Furthermore, pharmacological companies developed DES with biodegradable polymers which simply break down once it finishes releasing the drug leaving a bare-metal stent.
Since FDA approved the second generation of DES in 2008, it has become the option of choice, especially for patients with CAD (coronary artery disease). By 2017, eight companies were providing second-generation DES in the market. Currently, DES is used widely as it provides a non-invasive solution instead of the bypassing procedure. Cardiologists usually favor DES over BMS, they predict that DES will undergo more adjustments to fit almost every case scenario of arterial narrowing.