From permanent to resorbable: self-dissolving stents as heart implants

Cardiology is increasingly transitioning from traditional metal stents to self-dissolving stents for the treatment of coronary heart disease. These new magnesium stents have the potential to improve the treatment and outcomes for a large patient population.

In an interview with MEDICA-tradefair.com, Dr. Christian Napp explains the criteria for patient selection as well as the requirements and future trends of this technology.

Dr. Napp, what criteria are used to select patients for the implantation of self-dissolving stents?

Dr. Christian Napp: The standard therapy to date has been the implantation of a non-absorbable metal stent, which achieves good results but requires follow-up treatment in the event of re-narrowing. These follow-up treatments can be more difficult due to the remaining metal.

We are therefore particularly reluctant to implant metal stents in younger patients so as not to leave metal in their bodies for another 40 or 50 years. Self-dissolving stents are therefore particularly suitable for younger patients and those with less pronounced calcification.

However, in cases of severe atherosclerosis, which requires a high radial force, the use of self-dissolving stents may not be sufficient. We therefore take into account both the age and the quality of the narrowing in the coronary arteries.

What are the requirements for implanting self-dissolving stents compared to conventional metal stents?

Napp: When implanting metal stents, adequate preparation of the vessel is important. However, this is even more important with self-dissolving stents. This includes thorough pre-dilatation with balloons to ensure that the vessel is fully dilated and that there are no more constrictions before the product is implanted. When the stent dissolves, the vessel should ideally be completely free of constrictions.

Another important requirement when implanting self-dissolving stents is the use of intravascular imaging. We don't just rely on imaging with contrast medium to see that the vessel is open again. We also use methods such as ultrasound and, above all, optical coherence tomography. Using this light-based imaging of the vessel from the inside, we can prove with an accuracy of up to 20 micrometers that the self-dissolving stents are fully attached to the vessel wall.

How is the resorption of the stent in the body monitored?

Napp: We do not regularly monitor the healing or release of the drug, as this cannot be checked using a single method. However, for patients with constrictions in all three coronary arteries, treatment is carried out step by step. These constrictions should not be treated all at once, not even with metal stents. If the patient had to undergo surgery in this condition due to an accident, the antiplatelet drugs would have to be discontinued. This could lead to stent thrombosis, which can occur in all three vessels at the same time and is life-threatening.

This is why we treat the vessels one after the other in cases of severe coronary heart disease. When treating the second vessel, which takes place around two months after treatment of the first vessel, we can use the fiber optic camera to check how the stent has dissolved in the vessel that has already been treated.

Are there any risks or complications that can arise from the degradation process of the stents?

Napp: The idea of producing self-dissolving stents has been around for more than 30 years. The magnesium stents that we are now using have also been around for a while and have been continuously developed further. There are other materials from which self-dissolving stents are made, such as lactic acid or special plastics.

However, lactic acid products, also known as polylactides, have led to an increased number of complications in studies, which has had a negative impact on the overall perception of self-dissolving stents. Even magnesium stents, which did not have these problems at all, were also perceived negatively due to press reports about complications with self-dissolving stents. This was unjustified, as this is a completely different design.

Although there are risks and complications that can occur with self-dissolving stents, this depends heavily on the material. This does not typically apply to magnesium. Magnesium is an endogenous substance that is broken down into calcium acetate. This calcium acetate is an amorphous mass that remains in the vessel wall, where it does not cause any problems.

What technology is used for the controlled release of medication from the magnesium stent?

Napp: The release of medication itself cannot be controlled, as the active levels of the medication cannot be detected in the blood. They are too low if they are only released locally. However, intensive research is being carried out into the release of the drug locally in the vessel. For this purpose, a polymer is applied to the struts of the magnesium stent, creating a two-part product. This polymer has the special property of being able to release the drug in the vascular pathway in a controlled manner over a certain period of time.

What trends are currently emerging in stent technology, particularly with regard to bioresorbable materials?

Napp: Many companies are currently researching self-dissolving stents made of various materials in order to revitalize the field. In an ideal world, we would no longer be implanting metal stents in ten to 15 years' time. But that is still a long way off.

An important future technology is not to implant a stent at all, but only to use a balloon to expand the narrowing and deliver a drug. The same limitations that apply to self-dissolving stents also apply to this therapy. Nevertheless, there are many constrictions that can be treated with a drug-releasing balloon alone. Some centers already treat ten to 20 percent of all dilatations without a stent.

Another trend is stents that can “grow” with the patient. There are stents with small joints or predetermined breaking points that are particularly suitable for children. If children need stents, the question arises as to what happens when they grow. So far there are no stents that grow with the child, but there are stents that can be expanded further in the future to keep pace with the growth of the vessel.