The position of individual struts from the vessel wall can be assessed qualitatively (apposed or malapposed) or quantitatively (distance or area) with OCT. Immediately after stent implantation, individual stent struts are in touch with the vessel wall (apposed) or not (malapposed). Apposed struts are further classified into embedded or protruding. Struts are defined as embedded if more than half of the strut thickness is below the level of luminal surface [6] and as protruding if the adluminal strut surface is just above the vessel wall [6]. Considering the thickness of stent strut or abluminal polymer, struts that are detached from the vessel wall are defined as malapposed [7]. Accordingly, the distances for defining malapposed struts are different among types of drug-eluting stents (Table 16.1). Representative OCT images of apposed and malapposed struts are shown in Fig. 16.2.

The malapposed struts are commonly observed immediately after stent implantation. The incidence of acute stent malapposition was approximately 40–60% (Table 16.2), and about 70% of acute stent malapposition spontaneously disappeared at follow-up of 1 year [8, 10]. Determinants of the spontaneous resolution were malapposed distance or area, indicating that tiny malapposition can be resolved at follow-up [8, 10, 11]. The coverage of malapposed struts is delayed compared with apposed struts [12]. Recent two registries investigating mechanisms of stent thrombosis showed that stent malapposition was the most frequent finding (about one third of all cases) that caused stent thrombosis [13, 14]. In these studies, a significant or observed maximal distance of malapposition was >200–300 μm (Fig. 16.3) [13, 14]. Accordingly, it may be important to interpret malapposition as a quantitative, rather than binary phenomenon (present or absent), and to define the threshold of malapposition detachment that may benefit from optimization during stent implantation [15]. However, simple presence of acute stent malapposition >200 μm itself was not associated with worse outcomes [4], and results from prospective trials have not been addressed yet. On the contrary to malapposed struts, embedded struts at post-intervention were highly covered at follow-up of 6 months (median percentage of uncovered struts, 0% in embedded and 26.8% in malapposed, p < 0.001) [16]. Using contour plot analysis [17], the associations between strut apposition immediately after intervention and strut coverage at follow-up can be identified at a glance. On the contour plot, x-axis represented circumferential arc length of individual stent strut, and y-axis represented stent length. In (x, y) format, the locations of stent struts were delineated by their pixel coordinates [17]. In addition, individual stent struts could be marked regarding the status of apposition or coverage. Figure 16.4 is an example of serial contour plot analysis at post-intervention and follow-up.