This is a very interesting article from Wiley Online Library http://onlinelibrary.wiley.com/doi/10.1002/cce2.54/full on Stent Selection For Percutaneous Coronary Intervention (PCI) by P.D Williams M Awan Its a tad long but covers all the angles with regard to the selection.
Endovascular treatment of coronary artery disease was initially performed using balloon angioplasty alone but this technique was limited by a high rate of acute vessel closure and late restenosis. The introduction of coronary stents more than 20 years ago revolutionized practice and percutaneous coronary intervention (PCI) with stent deployment is now the dominant form of myocardial revascularization, with millions of procedures performed worldwide each year.
Stent design has evolved substantially over time and there are now a wide range of options available to the interventional cardiologist. This review will cover the development of intracoronary stents and the patient and vessel factors which are important in stent selection.
Early Developments in Stent selection for Percutaneous Coronary Intervention
The first human percutaneous coronary intervention (PCI) procedure was performed by Andreas Gruentzig in 1977. Although a huge step forward in the treatment of obstructive coronary artery disease, the original procedure of balloon angioplasty alone had important limitations. In the short-term there was a risk of acute vessel closure due to vessel wall dissection or thrombus formation, which would often require emergency bypass surgery. In the longer term, there was a high rate of vessel restenosis which occurred in 20–50% patients within 12 months, often necessitating a repeat procedure. The major contributors to restenosis were identified as acute vessel recoil (within 6–24 h of angioplasty), the contraction of the external elastic lamina of the vessel (constrictive remodelling) and neointimal hyperplasia secondary to local trauma inflicted by balloon inflation.
The bare metal stent era
The introduction of coronary stents, facilitated by improvements in antiplatelet therapy, was the next major evolution in the percutaneous management of coronary disease. In 1994, two randomized control trials demonstrated the superiority of uncoated stainless steel balloon-expandable stents for the treatment of de novo coronary artery lesions in native coronary vessels. The use of these bare metal stents (BMS) reduced angiographic restenosis, and hence the need for repeat procedures, by about one-third compared with balloon angioplasty only . As a result, stent use rapidly proliferated and stent deployment became routine practice during PCI procedures.
Coronary stents solved many of the problems of balloon angioplasty. By scaffolding the vessel and tacking down vessel dissections they reduced the risk of acute vessel closure. They also reduced the risk of restenosis by combating vessel recoil, thereby improving acute lumen gain and constrictive remodelling.
However, early stents had significant limitations. Restenosis due to neointimal hyperplasia within the stented segment remained an important clinical problem, and repeat revascularization was required to treat in-stent restenosis (ISR) in approximately 15% of patients treated with a BMS . Initial stent designs such as the Palmaz-Schatz stent were also stiff and bulky and were difficult to deliver in tortuous, calcified anatomy.
First generation drug eluting stents
Drug-eluting stents (DES) were developed to specifically address the issue of ISR encountered with BMS. They typically consist of a metallic stent coated with a layer of polymer which gradually releases an anti-mitotic drug to inhibit the cell proliferation that causes restenosis.
The first two DES to reach market were Taxus (Boston Scientific) and Cypher (Cordis). Cypher consisted of a 140 mcm stainless steel stent coated with a permanent polymer containing the drug sirolimus. The Taxus Express DES consisted of a 132 mcm stainless steel stent coated with a permanent polymer containing paclitaxel. The later iteration Taxus Liberte stent possessed the same polymer and drug as the Taxus Express but with a thinner strut platform (97 mcm).
Clinical trials with these DES platforms demonstrated dramatic reductions in ISR compared to BMS and a consequent reduction in the need for repeat revascularization. However, first generation DES did not prove to be a panacea for coronary artery disease and were associated with an increased risk of late stent thrombosis related to delayed endothelialization of the stented segment . Later generations of drug eluting stents were designed to improve on these late outcomes.
Components of a Drug Eluting Stent Platform
A modern drug eluting stent platform typically has three elements: the stent backbone, the antiproliferative drug, and a mechanism for bonding the drug to the backbone to allow controlled elution, which is usually a biocompatible polymer coating. Modern DES have seen significant developments in all three of these elements compared to the first generation devices.
The stent backbone
Arguably the most important component of a coronary stent platform is the stent backbone itself. An ideal coronary stent has several desirable characteristics: it should have a low profile, low strut thickness, a good conformational profile and flexibility, acceptable radial and longitudinal strength, allow easy side branch access and have adequate radio-opacity. Stent design is a delicate balancing act, however, and improving some of these characteristics may have a deleterious effect on others.
Thin strut stents reduce the profile and make the stent more flexible and hence improve deliverability. Thin strut BMS have also been shown to result in a reduction in restenosis, possibly related to reduced flow disturbance within the vessel. Development of thin strut stents has therefore been a major design focus for stent manufacturers.
Early stents were mostly manufactured from 316L stainless steel with relatively thick struts. Alloys such as cobalt chromium and platinum chromium are increasingly used which have allowed the construction of thinner strut backbones while maintaining strength and radio-opacity. Strut thickness has fallen dramatically over the last 15 years. For instance the original Cypher stent (Cordis) had a strut thickness of 140 mcm, while the Orsiro stent (Biotronik) has a strut thickness of 61 mcm. Innovative stent designs have been required to maintain radial and longitudinal strength.
The Cypher stent had a closed cell configuration and was relatively inflexible and prone to stent fracture . Most modern DES are constructed with an open cell configuration which results in increased flexibility, as well as improving ease of access to side branches.
Paclitaxel has a narrow therapeutic index with an increased risk of arterial toxicity compared to sirolimus . Evidence from multiple comparative trials of the first generation DES showed that sirolimus-coated stents were superior to those coated with paclitaxel, with reduced rates of both restenosis and late stent thrombosis . Subsequently, all mainstream contemporary DES are coated with a limus analogue, although paclitaxel is still utilized on many drug-eluting balloons because of its tissue retention characteristics.
Bonding of drug to stent
Most DES are coated with a layer of polymer to allow controlled release of the antimitotic drug, typically over the period of 3–9 months in which vessel healing occurs. Polymers have been shown to cause persistent local inflammation and delayed endothelialization in animal models, which represents one possible mechanism for late stent thrombosis. Therefore modern DES have been designed with thin layer polymer coatings. Stents coated with bioabsorbable polymer and/or polymer solely on the abluminal surface have been developed in an attempt to further improve long term safety, although this theoretical benefit has not been shown to improve clinical outcomes compared to modern permanent polymer DES in studies to date. The novel polymer-free BioFreedom DES has been roughened on the abluminal surface to increase the surface area allowing direct application of the drug.
Newer Generation DES
There are now several commercially available newer generation DES (see Table 1). The increased risk of late events with first generation DES compared with BMS has not been shown with these later generation DES and numerous studies have shown the superiority of modern DES over first generation DES for both efficacy and safety, including at longterm follow-up. As a result, the 2014 ESC guidelines on myocardial revascularization recommend unrestricted use of new-generation DES in all patients undergoing PCI including patients with diabetes, multivessel and left main disease, STEMI, vein grafts, restenosis, and chronic total occlusions.Comparison of selected commercially available current generation DES with first generation DES