Iron Bioresorbable Scaffolds Match Metallic Stents for Late Lumen Loss at Two Years

The Evolution of Coronary Scaffolding Beyond Permanent Implants

Percutaneous coronary intervention has advanced from balloon angioplasty to the current standard of second-generation drug-eluting stents [1, 2]. While effective at preventing restenosis, these permanent metallic implants can be associated with long-term complications, including chronic vascular inflammation and late stent thrombosis [3, 1]. Bioresorbable scaffolds were conceived to mitigate these risks by providing temporary vessel support before resorbing completely, theoretically restoring natural vasomotion [4, 5]. However, first-generation polymer-based scaffolds were limited by thick struts and higher rates of adverse events compared to metallic stents [6, 2]. Subsequent material science has produced biodegradable metallic alloys, such as iron, which permit thinner strut designs for improved mechanical strength and potentially better healing [3, 1]. A new randomized trial, IRONMAN-II, now provides a direct comparison of a thin-strut iron bioresorbable scaffold against a contemporary cobalt-chromium stent.

Trial Design and Patient Selection in IRONMAN-II

The IRONMAN-II study was a prospective, multicenter, single-blinded, noninferiority randomized trial conducted at 36 centers in China. Investigators enrolled patients with documented myocardial ischemia and one or two de novo coronary lesions suitable for intervention. Between March and December 2022, a total of 518 patients were randomized 1:1, with 259 assigned to receive the thin-strut sirolimus-eluting iron bioresorbable scaffold (IBS) and 259 assigned to a standard cobalt chromium everolimus-eluting stent (CoCr-EES). The single-blinded design masked the treatment allocation from patients to minimize potential bias in symptom reporting. A rigorous follow-up protocol was established, including clinical assessments at 1, 6, and 12 months, with annual evaluations planned out to five years. A key feature of the trial was the mandated angiographic and imaging follow-up at two years, a critical time point for assessing vascular response as the scaffold undergoes resorption. This included optical coherence tomography (OCT), an intravascular imaging modality that uses light to generate high-resolution, cross-sectional views of the vessel wall, allowing for precise measurement of healing and lumen dimensions.

Defining Success Through Vascular Patency and Flow Dynamics

To rigorously compare the two devices, the trial's primary endpoint was angiographic in-segment late lumen loss (LLL) at two years. This metric directly measures the amount of neointimal tissue growth that narrows the vessel after intervention, providing a surrogate for restenosis. The study also specified two powered secondary endpoints to assess physiological function. The first was the target vessel quantitative flow ratio (QFR), an angiography-based technique that computes the functional severity of a stenosis without requiring an invasive pressure wire. The second was the cross-sectional mean flow area as measured by optical coherence tomography, offering a precise anatomical assessment of the vessel's internal dimensions. These endpoints were selected to provide a comprehensive picture of the device's performance, combining a traditional angiographic measure of patency with a modern physiological assessment of blood flow and high-resolution imaging of the vessel lumen.

Noninferiority Confirmed for Primary and Physiological Endpoints

The sirolimus-eluting iron scaffold successfully met its primary and key secondary endpoints for noninferiority against the metallic stent. At the two-year follow-up, the primary endpoint of in-segment late lumen loss was 0.28 (0.52) mm in the IBS group versus 0.23 (0.43) mm in the CoCr-EES group. The small difference of 0.08 mm (95% CI: -0.02 to 0.18) fell within the prespecified margin, establishing noninferiority (P=0.03). This finding suggests that the bioresorbable scaffold controls neointimal hyperplasia to a degree comparable to a modern drug-eluting stent. The physiological and imaging endpoints reinforced this conclusion. The mean quantitative flow ratio was 0.90 (0.13) with IBS and 0.92 (0.09) with CoCr-EES (difference: -0.02; 95% CI: -0.04 to 0; P for noninferiority = 0.05). Furthermore, the mean flow area measured by optical coherence tomography was 6.92 (3.48) mm² with IBS and 6.64 (2.44) mm² with CoCr-EES (difference: 0.27; 95% CI: -0.09 to 0.63; P for noninferiority < 0.0001). For clinicians, these results indicate that the iron scaffold maintains vessel patency and hemodynamic performance on par with the current standard of care at two years.

Clinical Safety Profiles and Revascularization Trends

The trial also evaluated clinical outcomes to assess the safety of the iron scaffold. The rate of target lesion failure, a composite of cardiac death, target vessel myocardial infarction, or ischemia-driven target vessel revascularization, was not significantly different between the groups. Target lesion failure occurred in 7.4% of IBS patients and 5.4% of CoCr-EES patients at two years (HR: 1.37; 95% CI: 0.69-2.73; P = 0.37). Similarly, there were no significant differences in the patient-oriented composite endpoint (all-cause death, MI, or any revascularization) or its individual components. In a critical safety finding, no scaffold thromboses occurred in the IBS group, whereas one definite stent thrombosis was reported in the CoCr-EES group. While overall clinical event rates were comparable, the investigators noted that rates of binary restenosis and total revascularization were higher in the IBS arm. However, they specified that most of these repeat procedures were non-ischemia-driven, likely prompted by surveillance angiography findings rather than clinical symptoms. The ultimate clinical advantage of a bioresorbable device, if any, is expected to emerge after its complete resorption. Therefore, the planned five-year follow-up of the IRONMAN-II cohort is essential to determine if the temporary scaffold translates into long-term benefits such as restored vasomotion and reduced late adverse events.

References

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