Bioresorbable Vascular Scaffolds
Future Perspectives
Juan F. Granada, MD
The long-term presence of non-absorbable materials in the arterial wall (metal and polymer) following drug-eluting stent (DES) implantation has been linked to the development of neoatherosclerosis and delayed healing, factors thought to be responsible for late stent failure. Bioresorbable scaffolds (BRS) are emerging as an alternative approach to metallic DES due to their ability to mechanically support coronary arteries in the early phase post intervention, followed by complete absorption of the scaffold over time. Experimental studies have shown that BRS induce positive vascular remodeling and foster the restoration of the physiological vasomotor function. It has been hypothesized that this positive biological trait may result in lower long-term lumen loss, potentially leading to the improvement of clinical outcomes. Recent randomized, controlled trials support the safety and efficacy of this technological approach. However, its superiority in the long term compared to metallic DES remains to be proven.
In this session, we invited Dr. Antonio Colombo to share his thoughts about BRS, a technology that continues to gain traction amongst patients and physicians around the world.
What makes bioresorbable scaffolds (BRS) different from conventional metallic stents?
After the vessel is healed, which usually takes 6 months, the main advantage is that you no longer have a permanent cage (i.e., a metallic stent) present in the vessel. Absorption will take 1 to 2 years, depending on the type of scaffold. Not having a cage means the vessel is allowed to remodel, permitting: 1) the vessel to grow bigger if a plaque forms inside the artery; 2) dilation or constriction, which are physiologic actions of any blood vessel, especially arterial ones; and 3) treatment with bypass surgery, if this necessity comes about. The essential difference is that nothing occupies the wall of the vessel after scaffold absorption.
The ABSORB III trial, presented at the 2015 TCT meeting, demonstrated that the Abbott Absorb bioresorbable scaffold was non-inferior to the Abbott Xience drug-eluting stent (DES) after one year. However, there were issues with operator post dilatation and the scaffold technology affecting events such as thrombosis. What are your thoughts on the trial?
I will call it a neutral study with a little bit of negativity. The negativity is that all the events, even if not statistically different, were higher for the bioresorbable scaffold. My interpretation of what I will call a relatively inferior outcome is related to the implantation technique, which was not optimal. The scaffolds were not dilated sufficiently, especially when implanted in small vessels. Predilatation was done, but not in an optimal fashion. In addition, intravascular ultrasound was rarely utilized to check if the result was appropriate. The main blame has to be on the operators, which did not perform what I would call an optimal implantation technique.
What technique would you recommend?
I would advise, especially when implanting the Absorb in vessels smaller than 3 mm, that the vessel be fully predilated. Do not be afraid of a dissection, because having a stented dissection is no longer an issue. Following implantation of the scaffold, routinely post dilate the scaffold with a high-pressure, non-compliant balloon, at a minimum of 20 atmospheres. Then, I would say not routinely, but frequently, use intravascular ultrasound to check whether the result is acceptable, especially if you are treating a vessel of 2.5 mm in size.
Do scaffolds have a learning curve? Is it a matter of emphasizing certain aspects of implantation that operators now take for granted with the current generation of stents?
Yes, absolutely. Bioresorbable scaffolds do have a learning curve. For people like me, used to old-style implantation, it is a rejuvenating event, because it brings me back 10 years, so I feel younger. For other people, who have never been exposed to this period, it can be slightly disorienting.
Can you talk more about your specific technique?
First, use Rotablator (Boston Scientific) if you have a calcific lesion. Don’t be afraid to do that. Second, don’t be afraid of dissections. Many people call a dissection a complication. A dissection is part of the vessel preparation. Use a scoring balloon or AngioSculpt (Spectranetics), if the lesion is fibrotic. Basically, use so-called “preparation” tools in order to break the plaque and allow the stent to fully expand. Avoid direct stenting unless you are treating a thrombotic lesion or a lesion in the setting of acute myocardial infarction. Always do predilatation. Do not deploy the scaffold at high pressure, but post dilate at high pressure. This is the basic process; it is nothing complicated. If you are not sure of the result, use intravascular ultrasound or optical coherence tomography in a complex situation, and at the very beginning, in order to refine your technique.
What do BRS need to achieve in order to become competitive with metallic drug-eluting stents?
The first need for BRS is to get thinner. Currently, the core structure is 150 microns. I believe we need to go down to at least 120 microns, which will make the scaffolds more deliverable and maneuverable. The struts of bioresorbable scaffolds are not only thicker, but also wider, than those of metallic stents. The amount of foreign body that you are putting into the vessel, even temporarily, is greater than with metallic stents. Second, bioresorbable scaffolds need to be mounted on better balloons. The balloon currently utilized for the Absorb is too compliant. We need less compliant balloons. Third — and I am saying all of this in order of importance — the scaffolds should be better able to tolerate overexpansion. A 3 mm scaffold can only be expanded up to 3.5 mm. A tolerance permitting up to a 1 mm increase (i.e., up to 4 mm expansion for a 3 mm scaffold), would be an improvement. These three characteristics are the major drawbacks of the current generation of BRS. I expect the new generation will soon be ready to come into the field.
What are the different scaffolds you have used?
I have a large experience with the Abbott Absorb bioresorbable scaffold, and a limited experience with the DESolve (Elixir Medical) and the Fortitude (Amaranth) bioresorbable scaffolds.
Do you see any differences?
I have been using the Elixir and Amaranth scaffolds in relatively simple lesions. In simple lesions, everything looks fine. I have experience with Absorb in very complex lesions, so until I can expand the utilization of the Amaranth and Elixir scaffolds to complex lesions, I will not be able to answer your question in any reliable way.
Can you describe your experience with Absorb in complex lesions?
For the past two years, I have been using Absorb almost in every lesion: bifurcations, total occlusions, calcific lesions, left mains, etc. The only restriction is when the vessel is greater than 4 mm or less than 2.5 mm. I basically use the Absorb scaffolds as a regular method of stenting. The only difference is that the post implantation technique is a little more meticulous, as I described previously.
How important is final lumen size and ensuring apposition of the scaffold?
Both apposition and final lumen cross-sectional area are important. If you have apposition, but the lumen is very small, this is not acceptable. The lumen has to be compatible with the size of the vessel. If the vessel is a 2.5 mm vessel, you need at least a 4 mm2 residual lumen. If the vessel is at least 3 mm, you need between 5 and 6.5 mm2. If you have apposition, but the residual lumen is 2 mm2, it is still unacceptable, even with full apposition. Lumen size is important.
What about the use of intravascular ultrasound (IVUS) and optical coherence tomography (OCT)?
IVUS, in my experience, is a little bit easier, because it is quicker. You don’t need extra contrast to be injected and IVUS gives you valuable information about the vessel size. Media to media is sometimes difficult to judge by OCT, while it is very easy to be judged by IVUS. OCT is more suitable in situations such as the evaluation of incomplete appositions, bifurcations, strut fractures, and strut reabsorption — so, more specific problems. IVUS is more for everyday use, as it is quicker and can be incorporated in the routine implanting technique.
Is it possible that metallic stents will become so good (bioresorbable polymers, etc.) that the choice of a metallic vs. bioresorbable stent could simply come down to patient preference and/or cost?
I don’t think so. I doubt metallic stents will ever be able to offer the features of bioresorbable scaffolds. Metallic stents will not permit remodeling or vasomotion — all of these attributes are negated by a permanent implant. That is my general perception. Of course, maybe things will become different, and perhaps we will have a metallic stent that is totally non-thrombogenic, and will not require any clopidogrel and aspirin. This may be a dream; at the present time, I don’t see it happening.
What questions would you like to see answered?
The questions are in the long-term follow-up. We need to extend the follow-up to 3 to 5 years before we will know if bioresorbable scaffolds are truly advantageous. In the short term, even going out to 2 years, I don’t expect to see any difference.
Do we know anything about coronary bypass patients?
Not yet, long-term data will show if surgeons have any difficulty putting a graft on a vessel that has been treated with a scaffold 3 or 4 years before.
Is any different antiplatelet therapy besides clopidogrel and aspirin required when you use BRS?
For short scaffolds in relatively large vessels, such as 3.5 mm or 3.0 mm, I would say no. When you implant very long scaffolds in smaller vessels, the initial amount of foreign body, i.e., strut thickness and width, is larger than with conventional metallic stents. In these cases, my personal opinion is that we should use stronger antiplatelet medication, such as prasugrel or ticagrelor, for at least the first 3 to 4 months, until the scaffold is partially endothelialized. We all know that the thicker the strut, the more risk of thrombosis, and if you have long stents in a small vessel, extra protection may be needed. There is a reason why, when we use a very long stent in a 2.5 mm vessel, we prescribe ticagrelor or prasugrel for 3 months. Our idea is not yet standard of care, but by following this approach, our thrombosis rate is below 1%.
Disclosure: Dr. Juan F. Granada is the executive director and chief innovation officer of the CRF-Skirball Center for Innovation. His research center has performed sponsored research for several BRS technologies under development including Abbott Vascular, Amaranth Medical, REVA Medical, Orbus Neich and Meril LifeSciences.
Dr. Antonio Colombo reports no conflicts of interest regarding the content herein.
Dr. Juan Granada can be contacted at jgranada@crf.org. Dr. Antonio Colombo can be contacted at colombo.antonio@hsr.it.
