Bioresorbable scaffolds

What are the bioresorbable scaffolds?

Bioresorbable scaffolds are state-of-the-art stents made of materials that are reabsorbed over time. The principle behind this new technology is the possibility of obtaining transient mechanical support, avoiding the problems associated with the presence of a permanent implant made of metal. The presence of a permanent implant makes treatment of restenosis (narrowing inside a stent) more complex, may promote the formation of new plaques and, if needed, by-pass surgery becomes more difficult to be implemented.
Bioresorbable scaffolds:
\

support the vessel for the first six months like a permanent implant and they release a medication (sirolimus like) able to limit tissue growth.

\

following this initial “support period” start to dissolve losing mass with a complete disappearance after 3 years.

Without a permanent implant the blood vessel regains a normal contractile function. Intracoronary imaging studies confirmed the complete degradation of bioresorbable scaffolds 3-4 years after implantation, with recovery of normal vasomotion.

The unique features of bioresorbable scaffolds can be appreciated at a long-term follow-up (5-10 years), when we see that the morphology of the artery remained unchanged.

Below we see an angiogram a right coronary artery with diffuse disease, treated in 2012 with the implantation of 5 bioresorbable scaffolds. In 2021, nine years later, the vessel maintained excellent patency with the typical morphology of the right coronary artery and no foreign bodies inside.

The unique attribute, present only after implanting a bioresorbable scaffold or using a drug-coated balloon (a topic we will discuss below), is the possibility that the artery may naturally widen. This physiological process is called “positive remodeling”. The “positive remodeling” is a compensatory physiologic action counteracting the reduction of the lumen inside the artery caused by an atherosclerotic plaque. The presence of a permanent metallic structure prevents “positive remodeling” from taking place.

The image, present below, shows the lumen of a coronary artery after implantation of a bioresorbable scaffold with the lumen area of 8 mm2 (A). After 5 years, thanks to the possibility of “positive remodeling”, the lumen area increased to 10 mm2 and the scaffold completely disappeared (B).

Image A. At the time of scaffold implantation: lumen area 8 mm2
Image B. After 5 years: lumen area 10 mm2

Materials

The first bioresorbable scaffold was the Igaki-Tamai scaffold made of L-polylactic acid. Subsequently, scaffolds made by poly-L-lactic acid or similar bioresorbable materials became coated with a matrix capable to release an antiproliferative drug like sirolimus. Current bioresorbable scaffolds can guaranteed the same mechanical performance of metal stents despite a significant reduction of strut thickness compared to the scaffold of the first generation.

Studies

Contrary to expectations, the first studies conducted on bioresorbable scaffolds (Absorb), gave suboptimal results. Compared to drug-eluting metal stents, first generation bioresorbable scaffolds showed inferior safety.

However, these results must be correctly interpreted. It is important to consider that the bioresorbable scaffolds, evaluated in these early studies, had an excessive thickness and insufficient power to withstand the tension of the vessel wall. They were prototypes not yet ready for general clinical use

From the detailed analysis of the causes of this failure, two fundamental problems of the early scaffolds emerged:

\

The technological limitations of first-generation absorbable devices;

\

A sub-optimal implantation technique.

The present

Learning from the first studies, Scientists developed new bioresorbable scaffolds with a mesh thickness minimally higher than metal stents (100 microns compared to 70 microns) and with technical characteristics such as resistance to external pressure comparable to metal stents.

The implantation technique is now settled. Besides usage of angiography, the results are now evaluated with imaging techniques such as intravascular ultrasound or optical coherence tomography.

Which are the bioresorbable scaffolds we now utilize?

The Fantom Encore bioresorbable scaffold, manufactured by REVA Medical in San Diego (California, USA) is the scaffold we implant. The Fantom Encore is a latest generation scaffold built with a tyrosine polymer. The specific attributes of the Fantom Encore are:
\

Excellent radiological visibility

\

Good radial strength and flexibility: comparable to that of metal stents

\

Mesh thickness of 95 microns for the 2.5 mm scaffold, 105 microns for the 3.0 mm scaffold and 115 microns for the 3.5 mm diameter scaffold.

The figure below shows the good radiological visibility of the Fantom Encore similar to a metal stent.