Soluble stents
Despite the development and progression of metallic stents, they continue to have limitations such as stent thrombosis, which requires prolonged antiplatelet therapy, and mismatch of the stent to the vessel size, which often results in a smaller lumen after stent implantation. Further, metallic stents prevent the lumen expansion associated with late favorable remodeling.Permanent metallic stents impair the vessel geometry and often jail and obstruct side branches. Drug-eluting stents are a breakthrough in the development of stents, with their ability to significantly reduce restenosis rates and the need for repeat revascularization. Nevertheless, they are still associated with subacute and late thrombosis, and necessitate prolonged antiplatelet therapy for at least 12 months. Further, the polymer used as a vehicle for drug delivery may induce vessel irritation, endothelial dysfunction, vessel hypersensitivity and chronic inflammation at the stent site.Excessive use of stents in the coronary vasculature (full metal jacket) may interfere with traditional reinterventional techniques such as bypass graft surgery. Finally, metallic stents pose artifacts with modern imaging technologies such as magnetic resonance imaging (MRI) and multislice computerized tomography (MSCT), which eventually will become the default noninvasive imaging modality for the coronary anatomy.
In contrast, bioabsorbable stents, once they are bioabsorbed, leave behind only the healed natural vessel, allowing restoration of vasoreactivity with the potential of vessel remodeling. Late stent thrombosis is unlikely since the stent is gone, and prolonged antiplatelet therapy is not necessary in this instance. Bioabsorbable stents can also be suitable for complex anatomy where stents impede on vessel geometry and morphology and are prone to crushing and fractures, such as is seen in saphenous femoral and tibial arteries. Bioabsorbable implant stents can be used as a delivery device for agents such as drugs and genes, and will perhaps play a role in the treatment of vulnerable plaque. Transferring genes that code key regulatory pathways of cell proliferation inside the cells of the arterial wall using polymer stents as vehicles is feasible. Regardless of which agent (drug or gene) will finally conquer restenosis, a polymer stent remains an optional vehicle for such delivery. Finally, bioabsorbable stents are compatible with MRI and MSCT imaging.
Polymeric stents have the potential to act as local drug delivery systems. Polymeric material, especially biodegradable polymers, have been widely utilized for the controlled release of drugs, Therefore, it is possible to design a biodegradable polymer stent, not only offering a physical barrier to the vessel wall, but also presenting a pharmacological approach in the prevention of thrombus formation and intimal proliferation. These bioabsorbable polymers are currently loaded on the metallic stent for the purpose of drug or gene delivery, and completely erode by the time the drug has been released; yet the stent itself is still maintained in the vessel wall. The discussion of these bioabsorbable polymers is beyond the scope of this review, however.
There are several conditions to consider when selecting a polymer or alloy for the bioabsorbable stent. These include the strength of the polymer to avoid potential immediate recoil, the rate of degradation and corrosion, biocompatibility with the vessel wall and lack of toxicity. The change in the mechanical properties and the release profiles of drugs from bioabsorbable stents would directly depend on the rate of degradation of the stent, which can be controlled by selection of the stent alloy, passivation agents and the manufacturing process of the stent. Currently there are two types of materials used for bioabsorbable stents: polymeric-based and metallic-based.
Polymers have been widely used in cardiovascular devices and are currently primarily used as delivery vehicles for drug coatings.Among the polymers suggested for bioabsorbable stents are Poly-L-lactic acid (PLLA), polyglycolic acid (PGA), poly (D, L-lactide/glycolide) copolymer (PDLA), and polycaprolactone (PCL).Each of these polymers was designed as either self-expanding or balloon-expandable stents. Another proposed design is the hybrid stent, which combines polymeric absorbable stents with a metallic backbone to enable strength and prevent recoil.
Polymeric biodegradable stents are radiolucent, which may impair accurate positioning. Hence, this procedure requires extreme care and a highly experienced cardiologist to perfectly implant the stent. The polymer alone has a limited mechanical performance and a recoil rate of approximately 20%, which requires thick struts that impede their profile and delivery capabilities, especially in small vessels.Metal bioabsorbable stents are intuitively attractive since they have the potential to perform similarly to stainless steel metal stents. So far, two bioabsorbable metal alloys have been proposed for this application: iron and magnesium. The biocompatibility of these stents depends on their solubility and their released degradation products. Their local toxicity is related to the local concentration of the elements over time. The tissue tolerance for physiologically occurring metals depends on the change of their tissue concentrations induced by corrosion. Thus metals with high tissue concentrations are the ideal candidates for bioabsorption stents.
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