By Adrian Banning
Adrian Banning charts the evolution of the stent from the original bare metal design to bioabsorbable scaffold and biodegradable polymers. He also looks at the challenges of further improving stent design
The recent development of fully bioabsorbable scaffolds has reminded interventional cardiologists of the desirable mechanical attributes of coronary stents and the evolutionary process that resulted in our current selection of drug-eluting stents.
Implantation of coronary stents, after initial dilation, restrains dissection flaps and creates a regular vessel lumen; thus, optimising acute lumen gain and prevents early vessel recoil and the constrictive effect of late adverse vessel remodelling.
The inevitable increase in neointima formation caused by the presence of the stent has been modified by use of polymer technology to bind antiproliferative drugs to stents, and drug-eluting stents have significantly improved the predictability stenting in the intermediate and longer term.
However, drug-eluting stent implantation necessarily attenuates the vessels healing processes, resulting in delayed and sometimes incomplete endothelialisation with an increased risk of catastrophic stent thrombosis—particularly if antiplatelet therapy is discontinued prematurely. Polymers that regulate drug delivery have been linked to hypersensitivity reactions and eosinophilic infiltration and the causative role of polymer in late stent thrombosis has been debated.
Profiles of the current metallic stents are low; they are highly flexible and track readily, giving ready access to tortuous anatomy. They have evolved to provide deliverability and sustained luminal area gain with minimal recoil with good wall coverage, minimising plaque prolapse. Thinner struts may enhance flexibility, but changes in strut architecture designed to improve stent tracking can have unforeseen consequences. This includes the potential for longitudinal shortening. Struts lying across the origins of branch vessels can be problematic and plaque embolization during stent deployments is not inconsequential, particularly in larger vessels. Stainless steel has been replaced by a variety of alloys, such as cobalt chromium and platinum chromium, and these stents can be dilated to a range of diameters without losing radial strength and low recoil. Notably, the increasing numbers of older patients with calcified stenoses, (sometimes with multiple bifurcations) still challenge interventional cardiologists to successfully implant these devices.
Evolution in drug-eluting technology has resulted in second and third generation drug-eluting stent featuring different less cytotoxic drugs delivered with less or no polymer. These devices have demonstrated lower rates of stent thrombosis and consequently, a better safety profile. Reducing durations of dual antiplatelet drugs are being trialled with the dual objective of reducing both the cost and potential haemorrhagic risk. The very long-term durability of the result of metallic stenting can now be examined. Neoatherosclerosis has been described occurring within contemporary metallic stents and this may be responsible for the continuing requirement for repeat revascularisation two to five years after device implantation.
The frequency of this problem and its prevention/management are uncertain, but the development of fully bioabsorbable vascular scaffolds may address this issue in particular. Providing a long lasting solution to obstructive coronary disease that involves a permanent implant after scaffold reabsorption could allow a biologically active atheromatous plaque to be “healed”, permitting truly normal vascular function to be restored. Scaffolds may permit future percutaneous and surgical revascularisation strategies without the hindrance of previous permanent metal prostheses. Additionally, they may have superior compatibility with non-invasive diagnostic imaging (eg. cardiac magnetic resonance imaging and computed tomography coronary angiography).
These developments have transformed the stents that we use on a daily basis. It is interesting to speculate about what further developments might be required. Changes in the mechanical properties of metallic stents are likely to be comparatively minor. Although we have concentrated on these properties for many years, there is little residual room for real improvement. Designated stents for the left main in particular will be welcome and the improvements in strategy and adjunctive technology for the treatment of chronic total occlusion provide new interventional opportunities. However, these challenges for our partners in industry are matched in my opinion, by our challenges to be more rigorous about lesion selection and improve our lesion preparation, accuracy of stent placement and stent expansion.
Adrian Banning, Oxford Heart Centre, John Radcliffe Hospital, Oxford, UK