Most coating irregularities, or the potential to develop them, are inherent to the unexpanded DES. Important determinants of the formation of coating irregularities may be the stent geometry and the physical properties of the coating, while stent-balloon interaction plays no major role.

Background: Previous scanning electron microscopy (SEM) studies in expanded DES revealed differences in the frequency and size of coating irregularities between DES types and specific distribution patterns, however, the origin of these irregularities is unclear.The current study quantifies coating irregularities on unexpanded and expanded durable polymer-based drug-eluting stents (DES) to gain insights into the potential origin of coating irregularities. Methods: We assessed at bench side a total of 1200 SEM images obtained in 30 DES samples (15 expanded and 15 unexpanded) of Cypher Select, Taxus Liberté, Endeavor, Xience V, and Resolute. Results: For most types of coating irregularities seen on expanded DES (72%; 23/32), a matching irregularity (n=18/24) and/or its precursor (n=11/24) was observed in unexpanded corresponding DES. Only a few individual coating irregularities (13%; 4/32) could not be accessed in unexpanded samples, as these irregularities were located on the (invisible) luminal side. Unexpanded Cypher Select showed (small) crater lesions and cracks together with precursors of 'peeling of polymer'. On unexpanded Taxus Liberté, thinning of polymer, small bare metal areas, wrinkles and one type of precursor was found. Unexpanded phosphorylcholine-based Endeavor stents showed cracks, small bare metal areas, and crater lesions as well as precursors of the latter. Unexpanded Xience V and Endeavor Resolute mainly revealed crater lesions and their precursors.Cypher Stent:Precursor of peelin (left); peeling (right) Conclusion: SEM assessment demonstrated the presence of various coating irregularities and/or their precursors on unexpanded durable polymer-based DES. These findings provide evidence that coating irregularities arise during both DES manufacturing and stent expansion.