Case Report: The "metal ring" phenomenon: a rare cause of recurrent in-stent restenosis.

Background: Recurrent in-stent restenosis (ISR) remains a challenging issue in coronary intervention. Localized intractable ISR caused by a non-expandable metal ring is extremely rare. This report describes a rare case of recurrent in-stent restenosis attributable to a non-dilatable metal ring and demonstrates a viable interventional strategy for its management. Case presentation: A 64-year-old female patient was admitted with complaints of "intermittent chest pain for over one year, worsening over the past 10 days". In June 2022, the patient underwent drug-eluting stent implantation in the proximal-to-mid right coronary artery (RCA) for acute coronary syndrome. She experienced recurrent angina at 4 and 9 months post-procedure, with coronary angiography on both occasions revealing 95% ISR in the RCA. During the third intervention on January 31, 2023, optical coherence tomography (OCT) identified a non-dilatable metal ring at the site of severe stenosis. Attempts to disrupt the ring using intravascular lithotripsy and high-pressure non-compliant balloon inflation were unsuccessful. Ultimately, a modified Side Flap Technique was employed: a guidewire was advanced through the lesion lateral to the metal ring, enabling successful stent-in-stent implantation at the original site of the metal ring in the mid-RCA with adequate expansion. Post-procedural symptoms resolved substantially, and the patient remained free of major adverse cardiovascular events during three years of follow-up. Conclusion: Non-dilatable intracoronary metal rings represent an exceedingly rare cause of recurrent ISR following stent implantation. OCT plays an essential role in identifying such structural lesions. When conventional dilation techniques fail, a modified Side Flap Technique combined with stent-in-stent strategy may represent a feasible bail-out approach in this clinical setting.