Bioabsorbable Stents: Here Today, Gone Tomorrow?
Published date : 28 February 2013
Article date : 28 February 2013
Introduction by: Dr Phil Haslam, Director and Editor-in-Chief & Consultant Interventional Radiologist, Newcastle upon Tyne (UK)
Current stents used in the vascular system are permanent. They usually provide excellent radial force maintaining lumen diameter whilst eventually being endothelialised. They are however prone to intimal hyperplasia, hence the evolution of drug eluting stents which may themselves be prone to thrombosis. The Holy Grail of stenting would be a stent that is present for long enough to allow vessel wall healing and remodelling but is then dissolved when no longer needed. This is also true for other systems within the body. In the biliary tree and urinary tract we have plastic and even metallic stents that can be inserted for benign strictures and removed when no longer needed. This is not feasible in the vascular system. Drs Lee and Zaman present an excellent discussion of the theory and possible role of bioabsorbable stents in the coronary arteries.
By: Dr Kelvin Lee & Dr Azfar Zaman, Department of Cardiology, Freeman Hospital, Newcastle upon Tyne (UK)
Drug eluting metal stents are now the workhorses of percutaneous coronary intervention with efficacy data that justifies their use and higher cost. Whilst the drug/polymer composition has evolved from the pioneering cypher stent, the scaffold has remained metallic and as such, a permanent implant. All things being equal, the ideal coronary stent would be a device which was present when needed, gone when not and in the process of dissolving, disappear without trace or residual effects.
In order to gain a foothold in clinical practice, a bioabsorbable stent needs to maintain arterial patency by preventing acute recoil, providing mechanical support for the healing dissected artery and significantly reducing acute vessel closure by scaffolding intimal flaps and maximising vessel calibre. The ability to apply a drug to the bioabsorbable product to limit endothelial hyperplasia provides reassurance that long term restenosis can be successfully addressed. The singular long term advantage of the bioabsorbable product would be that subsequent resorption of the stent could allow for the potential recovery of physiological vascular activity with restoration of normal vasomotion and endothelial function. Any late increment in luminal size and positive remodelling of the artery would be unrestricted by a metal cage.
Other clinical advantages offered by bioabsorbable products would be freedom from stent strut obstruction of side-branches as well as the theoretical though yet unproven abolishment of late stent thrombosis. Long term vascular strategies remain possible as the vessel remains suitable for any future revascularisation strategy by percutaneous intervention or bypass surgery which is of particular importance to patients with chronic conditions such as diabetes and chronic renal disease. Immediate non-invasive imaging of the vessels by CT or MRI would also be possible with no residual metal shielding.
There are, as is inevitable with any first generation technology, also potential downsides. The bioabsorbable stents have thicker stent struts, are more bulky and therefore more challenging to deploy. There are limitations to its mechanical radial strength which may lead to luminal recoil and it has a narrow range in luminal diameter expansion, necessitating good lesion preparation and accurate sizing. It is less malleable than a metal stent which limits its use in anatomically complex PCI and calcified vessels. It is not yet clear what the ideal degradation profile is to provide a sufficient period of mechanical scaffolding to scaffold resorption without causing luminal recoil and restenosis. Too early a phase of resorption without sufficient endothelialisation may cause material embolisation and micro-infarctions. The material used must also be totally biocompatible such that it does not elicit a significant inflammatory response which itself may lead to stent thrombosis.
The pooled data for the metal based products have been tried and tested over the last 10 years and apart from a brief dalliance with unfounded safety concerns, have been shown to be safe and cost-effective. The bioabsorbable stent therefore, has a proven and trusted product to usurp if it is to become the stent of first choice. Long term safety and efficacy data need to be collected and only then will patient and physician confidence increase to a level where they are seen as a genuine alternative to their metallic counterparts. The issue of cost in the current economic climate is an obvious and essential obstacle that the product needs to overcome once the clinical data has confirmed equity or superiority to the metal stent.
In spite of the inevitable and obvious caveats, the bioabsorbable stent represents an undoubted paradigm shift in the world of coronary artery intervention. If we cast our minds back to the days of cypher and see the evolution in metal drug eluting stents over 10 years, it is possible to believe that 10 years from today bioabsorbable stents will be the new workhorses in percutaneous coronary interventions – a product trusted by physicians but with an important difference from metal stents – a product demanded by patients. Perhaps a more fitting title would be “bioabsorbable stents - here today and here to stay”.
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