A coronary medical stent is a small, mesh-like tube used to prop open a blocked or narrowed artery in the heart, a procedure known as angioplasty. This device restores proper blood flow to the heart muscle, preventing serious complications like a heart attack. Historically, these scaffolds were made of metal and designed to remain in the body indefinitely, providing permanent structural support to the vessel wall. Newer technologies, however, feature temporary structures designed to dissolve over time. The core difference between permanent and bioresorbable stents lies in whether the material persists or is absorbed by the body after its mechanical function is complete.
The Structure and Persistence of Permanent Stents
Traditional coronary stents, including Bare Metal Stents (BMS) and Drug-Eluting Stents (DES), are characterized by their permanent nature. These devices are typically constructed from medical-grade metal alloys, such as stainless steel, cobalt-chromium, or platinum-chromium, which are biologically inert. The metal lattice is pressed against the artery wall during implantation and remains embedded there for the remainder of the patient’s life. This persistence provides long-term mechanical strength, preventing the artery from collapsing or narrowing again, a problem known as restenosis.
Drug-Eluting Stents represent an evolution of the bare metal design, featuring an outer coating that contains a drug, often a polymer mixed with an anti-proliferative medication. This medication suppresses the excessive cell growth that can lead to re-narrowing of the artery around the stent struts. While the medication is gradually released and the polymer coating may be absorbed by the body, the underlying metal scaffold itself is permanent.
The main benefit of permanent metal stents is their long-term durability and structural integrity. They offer immediate and sustained radial strength, which is the force needed to hold the artery open against the pressure of the vessel wall. This robust mechanical support has made DES the standard of care for most percutaneous coronary interventions due to their high efficacy in preventing restenosis compared to BMS.
Bioresorbable Stents: Mechanism of Degradation and Timeline
Bioresorbable stents, also called bioresorbable scaffolds (BRS), provide temporary structural support before dissolving completely and being absorbed by the body. The materials used are typically medical-grade polymers, such as Poly-L-Lactide (PLLA), or sometimes metallic alloys like magnesium.
The degradation process occurs in two main phases, starting with the loss of mechanical function. For a polymer-based stent, the material begins to break down through hydrolysis, where water molecules split the long polymer chains into smaller fragments. This structural weakening leads to a loss of radial support, which typically occurs within six to twelve months after implantation. The goal is for the vessel to be healed and stable enough to maintain its own patency before the scaffold’s support disappears.
Following the loss of mechanical strength, the second phase involves the complete absorption of the material by the body. The smaller fragments of the polymer are eventually metabolized and eliminated, often as carbon dioxide and water. This full material absorption takes substantially longer than the initial structural breakdown, typically requiring one to three years for complete disappearance from the vessel wall. Magnesium-based scaffolds also undergo a similar process, dissolving into harmless ions that are cleared by the body, with a significant portion resorbed within one year.
Clinical Suitability and Comparative Outcomes
The choice between a permanent metal stent and a bioresorbable scaffold involves weighing the proven efficacy of permanent devices against the benefits of temporary ones. Permanent Drug-Eluting Stents (DES) are the current benchmark, largely due to extensive long-term safety data showing low rates of adverse events. They provide high mechanical support, which results in predictable and low rates of restenosis, the re-narrowing of the artery.
Bioresorbable scaffolds were developed with the advantage of restoring the natural flexibility and function of the treated vessel once the device is gone. A metal stent restricts the natural movement of the artery, but a dissolved scaffold allows for better long-term vessel remodeling and may reduce the risk of late-stage complications like chronic inflammation. This potential benefit is particularly appealing for younger patients or those with less complex blockages.
However, initial clinical trials for the first generation of bioresorbable scaffolds showed they were associated with a higher risk of adverse events, including target lesion failure and a higher rate of scaffold thrombosis, compared to modern permanent DES. This difference in early outcomes was partly attributed to the thicker struts and lower radial strength of the first BRS designs. While newer generations of bioresorbable technology are being developed to overcome these limitations, permanent metal DES currently maintain a superior safety and efficacy profile for a broader range of patients and lesion types.