Coronary stenting: Difference between revisions

=Bare Metal Stents=

Bare-metal stents are made of different types of metals, for example stainless steel, nitinol, cobalt-chromium or platinum-chromium. Stents can be either self-expanding, or balloon-expandable, and exert a scaffolding force to prevent acute recoil of the treated coronary artery stenosis. Bare-metal stents were introduced in the 1980s in an effort to prevent acute vessel occlusion and restenosis, two common complications after POBA. An early report on the use of BMS in coronary arteries was cause for optimism, in 19 patients who received stainless-steel BMS in the mid-1980s (before the era of dual antiplatelet therapy, DAPT) two acute stent occlusions occurred, and one patient died (without suspicion for a thrombotic occlusion), and no further restenosis or occlusions were observed up to nine-month follow-up.(2) <br />

However, a study published in 1991 in the New England Journal of Medicine reported sobering outcomes in a series of 105 patients undergoing BMS implantation.(5) Complete occlusion of the coronary artery stent was observed in 24% of patients at follow-up coronary  angiography after one month. Out of the 27 occlusions, 21 occurred within the first 14 days after stent implantation. Moreover, restenosis (defined as >50% diameter stenosis at follow-up) was observed in 14% of patients with patent stents. An excerpt from the accompanying editorial reflects the disappointment that ensued “I believe that the introduction of most these new devices will be of passing interest only. The development of mechanical interventions (such as stents) that attempt to overcome the response to injury caused by intravascular therapeutic interventions (such as balloon angioplasty) is probably futile.”(6) <br />

During the 1990s, important developments in pharmacology, stent implantation technique, and BMS technology resulted in markedly improved outcomes after PCI with BMS compared to the aforementioned early experiences. <br />

- Pharmacology:  Stenting with dual antiplatelet therapy (DAPT) with aspirin and a P2Y12 inhibitor (at that time, ticlopidine) instead of a large variety of combinations of warfarin with varying antiplatelet agents was introduced by Antiono Colombo et al.(7)  Angiographically documented stent thrombosis occurred in 1.6% of patients after 6 months. <br />

However, there was an important safety issue with the first generation DES; the occurrence of very late stent thrombosis (>1 year after stent implantation).(14) As stent thrombosis is a relatively rare complication (typically occurring at a rate of <1% per year), these issues were only discovered after data from several randomized controlled trials were pooled. A meta-analysis of >5000 patients showed significantly increased rates of very late stent thrombosis with first-generation DES compared with BMS.(14) The pathophysiological mechanisms for late stent thrombosis with DES included prothrombotic effects of the non-biocompatible polymers. Moreover, the large strut thickness of first-gerenation DES was problematic; on top of the thick-strut stent platforms was a layer of polymer on both the luminal- and abluminal stent surfaces. Furthermore, particularly with the TAXUS stent there were issues with stent coverage with neointima. Because of the aggressive action of the cyotoxic paclitaxel, many struts remained uncovered with neointima in lesions treated with TAXUS stents, which may be a trigger for very late stent thrombosis.<br />

''Current-Generation Drug-Eluting Stents:'' Stent technology has evolved significantly since the first DES were introduced. Important improvements have been made in the stent platform, the polymer, and the antirestenotic drugs.<br />

[[Improvements in stent plaforms:]] Stent platforms are the metal backbones of the DES. First-generation DES were designed as a standard BMS coated with a polymer and drug. The stent designs of current-generation DES are mainly different from their first-generation predecessors because of a thinner strut thickness. An overview of strut thicknesses of first-generation and current-generation DES is shown in table 1. Another advancement in stent platform design is the use of novel metallic alloys such as cobalt-chromium and platinum-chromium rather than stainless steel. These alloys allow for thinner struts with equal radiopacity and radial support. <br />

[[Improvements in polymer design:]] Improvements in polymer design were again aimed at minimizing the thickness of the polymer. This can be achieved by decreasing the thickness of the polymer itself, or by only coating the abluminal side of the stent with the polymer. Moreover, as the polymers of first-generation DES were thought to be responsible for the increased rates of very late stent thrombosis, an effort was made to create bio-inert and biocompatible polymers e.g. the fluoropolymer (not unlike a Teflon layer in an anti-sticking pan) used in the XIENCE everolimus-eluting stent. Other stent types employ a bioresorbable polymer which is broken down over a period of several months, essentially leaving a BMS behind. However, the benefits of a bioresorbable polymer seem to be limited as shown by a large network meta-analysis involving 63,242 patients showing that durable polymer everolimus-eluting stents (such as XIENCE and PROMUS) and zotarolimus-eluting stents (such as RESOLUTE) had a superior safety profile compared with biolimus-eluting bioresorbable DES.(15) Yet another type of stent (BIOFREEDOM) uses no polymer at all, but elutes a drug from small reservoirs on its abluminal surface. This stent has shown superior outcomes in terms of repeat revascularization when compared to a similar BMS, but has not been compared against other DES types in a randomized controlled trial.(16)<br />

[[Improvements in antirestenotic drugs:]] Improvements in antirestenotic drugs have been relatively modest compared with the aforementioned improvements in stent platforms and polymers. Experiences with first-generation DES led the interventional cardiology community to conclude that sirolimus and its analogues were superior to paclitaxel for use on a DES. The cytotoxic properties of paclitaxel are probably too aggressive for the purpose of preventing formation of neointima, the more subtle anti-inflammatory and cytostatic effects of sirolimus seem to be better suited for this purpose. Several analogues of sirolimus have been developed, these are chemically altered forms of the drug with improved lipophilicity such as biolimus A9 or zotarolimus.  <br />