Percutaneous Pulmonary Artery Biodegradable Stent: A New Armament to Fight Pulmonary Artery Stenosis?

Pulmonary artery stenosis presents a complex clinical challenge caused by congenital anomalies such as tetralogy of Fallot, pulmonary atresia, tricuspid atresia with pulmonary artery stenosis, and genetic syndromes like Williams and Alagille syndromes. These conditions often result in compromised blood flow through narrowed pulmonary arteries, requiring intervention to prevent progressive cardiopulmonary dysfunction. Traditional surgical approaches, including open repair with cardiopulmonary bypass, carry significant risks of trauma, restenosis, and complications associated with distal branch dissections. Over the past three decades, percutaneous stent implantation has emerged as a preferred therapeutic strategy, offering advantages in procedural minimalism, reduced recovery time, and efficacy in diverse vascular anatomies.

Evolution of Pulmonary Artery Stents

Conventional pulmonary artery stents utilize permanent metallic materials such as stainless steel, platinum-iridium alloys, nickel-titanium (Nitinol), and cobalt-chromium alloys. These stents are categorized into self-expanding and balloon-expandable types. Self-expanding Nitinol stents excel in navigating tortuous vessels and accommodating long-segment lesions but face limitations in mechanical strength for postoperative congenital heart disease cases. Balloon-expandable stents provide precise radial force but lack adaptability to vascular growth in pediatric patients.

Drug-eluting stents (DES) marked a paradigm shift by incorporating antiproliferative agents like rapamycin and paclitaxel to reduce neointimal hyperplasia. While DES reduced restenosis rates compared to bare-metal stents, they introduced new complications: delayed endothelialization, late thrombosis, stent fracture, and chronic inflammatory responses. These limitations highlighted the need for stents that provide transient mechanical support while enabling vessel remodeling without permanent implantation.

Biodegradable Stents: Materials and Mechanisms

Biodegradable stents address these challenges through temporary scaffolding and controlled degradation. These devices are classified into polymer-based and metal-based systems, each with distinct advantages and challenges.

Polymer-Based Stents

Poly-L-lactic acid (PLLA) is the predominant polymer, offering biocompatibility and gradual hydrolysis into lactic acid. However, polymer stents suffer from inadequate radial strength, requiring thicker struts that limit use in small-diameter vessels. Degradation byproducts like acidic monomers may induce local inflammation, while slow degradation kinetics (18–36 months) delay complete vascular healing.

Metal-Based Stents

Degradable metals—magnesium, iron, and zinc alloys—combine superior mechanical properties with rapid absorption. Magnesium alloys, composed of >99% Mg with rare-earth elements, exhibit density comparable to bone (1.74–2.0 g/cm³), elastic modulus matching vascular tissue (41–45 GPa), and complete degradation within 6–12 months. Early success was demonstrated in a 26-week preterm infant with left pulmonary artery stenosis, where a magnesium stent restored perfusion without long-term complications.

Magnesium’s degradation involves electrolytic corrosion:
Anodic reaction: Mg → Mg²⁺ + 2e⁻
Cathodic reaction: 2H₂O + 2e⁻ → H₂↑ + 2OH⁻
This process generates hydrogen gas and elevates local pH, potentially causing alkalosis and gas embolism. Surface modifications like micro-arc oxidation (MAO) create porous ceramic coatings to decelerate corrosion. Composite coatings combining MAO with polymers or bioactive molecules (e.g., rapamycin) further enhance corrosion resistance and enable localized drug delivery.

Clinical Applications and Outcomes

Right Ventricle-to-Pulmonary Artery Conduits

Biodegradable stents are used to palliate right ventricular outflow tract (RVOT) stenosis in congenital heart disease. A 10-year follow-up study demonstrated 82% freedom from reintervention in patients with RVOT conduits, outperforming traditional surgical patches prone to calcification and shrinkage.

Peripheral Pulmonary Artery Stenosis

Stent implantation for branch pulmonary artery stenosis, first reported in the 1980s, now achieves acute success rates >90% with transcatheter techniques. Case series show a 15–20% restenosis rate at 5 years for biodegradable stents versus 30–40% for permanent stents, attributed to reduced chronic inflammation and dynamic vessel expansion.

Hybrid Therapy

Combining intraoperative stent placement with surgical repair (e.g., tetralogy of Fallot correction) reduces cardiopulmonary bypass duration. A multicenter trial reported 94% procedural success in hybrid palliation for distal pulmonary artery stenosis, with 12-month patency rates of 88% for biodegradable devices.

Technical Innovations and Challenges

Drug-Eluting Capabilities

Biodegradable stents offer larger surface areas for drug loading compared to metallic counterparts. Multilayer coatings allow sequential release of antirestenotic agents (e.g., rapamycin), anti-inflammatory drugs (dexamethasone), and endothelialization promoters (vascular endothelial growth factor). Experimental models show 60% reduction in neointimal thickness with dual-drug stents versus single-drug systems.

Degradation Rate Optimization

Iron-based stents degrade too slowly (24–36 months), risking late restenosis, while zinc alloys exhibit ideal absorption profiles (12–18 months) but require alloying to enhance ductility. Current research focuses on ternary alloys (Mg-Zn-Ca) and nanostructured coatings to balance degradation kinetics with mechanical performance.

Imaging Compatibility

Magnesium stents cause minimal artifact on magnetic resonance imaging (MRI), enabling noninvasive surveillance. Computed tomography (CT) quantification of stent degradation correlates with intravascular ultrasound (IVUS) measurements (r=0.89, p<0.01), supporting multimodal imaging follow-up.

Future Directions

Fourth-generation stents integrate shape-memory polymers for thermal deployment and bioresorbable electronic sensors to monitor hemodynamics. Preclinical trials of endothelial progenitor cell-seeded stents demonstrate 50% faster endothelialization compared to uncoated devices. Large-animal studies using 3D-printed, patient-specific stents show 100% procedural success in complex anatomies, heralding personalized solutions for pulmonary artery stenosis.

doi.org/10.1097/CM9.0000000000001061

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