STENT Types Compared For Efficacy

By Dr Harold Gunatillake Health Writer

No doubt about it, innovation of stents has extended the lives of older people in the atherogenic age all over the world. People, who would have died of coronary artery disease, are living today enjoying life just the way they were before the episodes. What is required is to minimise risk factors for further episodes, by maintaining your body weight within normal range, exercising daily and eating a Mediterranean diet, taking medication regularly, along with the blood thinning medication.

Stents make you feel better overnight. One would say, "Life has come back to me, I could breathe better and the chest pains are gone."

Most of them, especially Sri Lankans and those abroad, tend to relax on risk factors and invariably end up in hospital with a recurrence, for further revision stenting or open heart surgery in a few years. Attending weekend parties, it is most shocking the way such individuals indulge in food and sweets as though it were nobody's business – especially those having diabetes, enjoy the ice-creams and puddings without limits.

There are three generations of stents that you should know about: earliest stents were bare metal stents, and then came the durable polymer drug eluting stents. The latest generation is the biodegradable polymer drug eluting stents for coronary artery blockage.

What are drug eluting stents?
A drug-eluting stent (DES) is a peripheral or coronary stent (a scaffold) placed into narrowed, diseased peripheral or coronary arteries that slowly release a drug to block cell proliferation. This prevents fibrosis that, together with clots (thrombus), could otherwise block the stented artery, a process called restenosis. The stent is usually placed within the peripheral or coronary artery by an Interventional Cardiologist or Interventional Radiologist during an angioplasty procedure.

Drug-eluting stents in current clinical use were approved by the FDA after clinical trials showed they were statistically superior to bare-metal stents(BMS) for the treatment of native coronary artery narrowings, having lower rates of major adverse cardiac events (MACE) (usually defined as a composite clinical endpoint of death + myocardial infarction + repeat intervention because of restenosis).

Efficacy of the biodegradable stents was compared with the previous generations, bare metal and durable polymer drug eluting stents.

Biodegradable polymer drug eluting stents were found to be more superior to first generation durable polymer drug eluting stents but not to newer generation durable polymer stents in reducing target vessel revascularization. Newer generation durable polymer stents, and especially

cobalt chromium everolimus eluting stents, have the best combination of efficacy and safety. The utility of biodegradable polymer stents in the context of excellent clinical outcomes with newer generation durable polymer stents needs to be proven.

On a study of quarter million patient years of follow-up, showed that biodegradable polymer drug eluting stents are not superior to newer generation durable polymer drug eluting stents for either efficacy or safety outcomes. Newer generation durable polymer stents (cobalt chromium everolimus eluting stents, platinum chromium everolimus eluting stents, and zotarolimus eluting stents were the most effective stents in having the lowest rate of repeat revascularization, no increase in very late stent thrombosis, and a significant decrease in the risk of myocardial infarction. Furthermore, a decrease in definite stent thrombosis, myocardial infarction, and death were seen when cobalt chromium everolimus eluting stents were compared with bare metal stents.

Biodegradable stents are promising because traditional metallic stents, with or without drug laden polymer coatings, require the patient to take blood thinners such a Plavix for 6 months or more after the procedure. This is because the coatings, while quite good at serving as a reservoir for the various therapeutic agents that prevent or retard the healing cascade, are often thrombogenic in and of themselves (i.e. the coating can make your blood clot).

A properly designed biodegradable stent could, in theory, provide initial structural rigidity sufficient to hold the clogged artery open and possibly deliver a therapeutic agent, but then dissolve over time so you don't have the long term risk of thrombosis and thus come off the Plavix.

There are a few problems or "cons" with this strategy. One is that the commonly used biodegradable materials such as PLA and PLGA are not stiff enough to support the artery wall when produced in standard stent geometry. The stiffness can be increased by playing with the geometry (width, thickness, strut length, etc.) but the end result is a stent that is too big to be easily delivered to the diseased vessel and would be bulky in the deployed state compared the small diameter of a diseased coronary vessel. The other problem lies with controlling the rate of the degradation and the size of the degradation products. As the stent degrades over time, certain portions of the stent may erode preferentially based on non-uniform shear stress from flowing blood, non-homogeneous vessel wall morphology, etc. If, for instance, the middle of the stent degrades preferentially, a large chunk of the stent may break off and float downstream raising embolism concerns.

In lieu of novel, biodegradable materials, a more practical approach is to have a thin, biodegradable coating over a traditional metallic stent. In this case, therapeutic agents may be delivered from the coating as it dissolves, eventually leaving a bare metal stent with a low risk of blood clotting. This type of technology has already been put into practice.

This article aims to help those people with stents to understand the workings and take measures to prevent risk factors. Maybe the future candidates may be luckier to have the newest generation stents with less problems.

Leading a health conscious life and avoiding risk factors is the alternative to prevent the experience in having stents.

The writer has worked as a senior registrar in a cardio-thoracic surgery unit in Singapore in the 1970s.

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