Restenosis in bare metal and drug-eluting stents: distinct mechanistic insights from histopathology and optical intravascular imaging.

An increasing body of evidence points to the existence of important differences in the processes of restenosis following drug-eluting stent (DES) as compared to bare metal stent implantation. Preclinical investigation and human autopsy studies have shown that the high efficacy of DES in comparison with bare metal stents in preventing restenosis is achieved at the collateral cost of a delay in healing of the stented arterial segment. Moreover bare metal stent restenosis is typically characterised by a homogeneous tissue rich in smooth muscle cells; whereas DES restenosis is more often hypocellular and proteoglycan-rich. In addition, in-stent neoatherosclerosis appears to have an accelerated course in DES. Angiographic surveillance studies show that while neointimal formation peaks six months after bare metal stenting, neointimal formation after DES therapy is temporally right shifted and remains a dynamic ongoing process (late luminal loss creep) even out to five years. The widespread availability of high resolution optical coherence tomography (OCT) is affording better understanding of the pathophysiology of in-stent restenosis. While bare metal stent restenosis is characterized by predominantly homogenous high-signal tissue echogenicity, layered pattern or heterogeneous tissue composition is more common in DES restenosis. Moreover, preliminary data suggests that tissue attenuation may increase in a time-dependent manner. Nevertheless, the paucity of direct histopathological correlation studies means that the tissue composition of these lesions remains speculative. Data from specifically designed imaging-pathology correlation studies in suitable preclinical models of restenosis and in autopsy specimens is eagerly awaited. Furthermore, although long-term longitudinal clinical follow-up is necessary to define the clinical relevance of optical imaging findings, the nature of restenosis as a disease entity means that its natural history is often altered by a mandate for repeat intervention directly following data acquisition.