Stent Undersizing and Malapposition: Mechanical Causes of Acute and Late Stent Failure

Stent undersizing and malapposition are critical mechanical failures in contemporary percutaneous coronary intervention (PCI). These deficiencies create a substrate for both immediate and delayed complications, including acute stent thrombosis, delayed healing, and late stent restenosis. Despite technological advances, angiography often fails to detect clinically relevant malapposition, leading to a systematic underestimation of its prevalence and clinical impact. This review evaluates the mechanisms underlying stent undersizing and malapposition, the associated pathophysiological consequences, and the pivotal role of intracoronary imaging (ICI) in optimizing stent deployment and improving clinical outcomes. Key definitions include stent underexpansion, defined as an expansion index <0.8; malapposition refers to a physical gap between stent struts and the vessel wall. Malapposition triggers a cascade of disturbed flow, low shear stress, and endothelial dysfunction, in part through inhibition of the MER proto-oncogene tyrosine kinase receptor (MerTK), thereby promoting a prothrombotic and proinflammatory environment. Inadequate deployment is established as the primary mechanism for early thrombotic events, with underexpansion observed in 44.4% of stent thrombosis cases. ICI-guided PCI consistently demonstrates improved stent expansion and better clinical outcomes compared with angiography-alone strategies. Optical coherence tomography (OCT) offers superior resolution for detecting acute malapposition, whereas intravascular ultrasound (IVUS) provides more accurate vessel sizing in large or heavily calcified vessels. Consequently, a fundamental shift toward a "mechanism-based PCI strategy" is required, emphasizing that optimal stent sizing and deployment should be guided by ICI, rather than by angiographic endpoints alone. Wider adoption of imaging, specialized training in ICI for interventionalists, and the implementation of standardized definitions of malapposition are essential to minimize stent failure and achieve mechanistic excellence in coronary intervention.