Stent deployment in coronary artery produces a series of physiological responses, which sequentially lead to platelet and fibrin deposition, inflammatory cell recruitment, smooth muscle cell hyperplasia, deposition of cellular matrix, and re-endothelialization in the segment treated by stent [12]. Unfortunately, the persistence of metal and/or durable polymers in the vessel induces chronic inflammation and hypersensitivity reaction, which can cause complications including neoatherosclerosis and late or very late stent thrombosis [13–17]. BVS can offer potential benefits over metallic stents for these problems with the process of “bioresorption ” of scaffold. Intracoronary imaging techniques such as intravascular ultrasound (IVUS) and virtual histology intravascular ultrasound (VH-IVUS) have been used to analyze the process of bioresorption of BVS [10]. Polymeric strut is recognized as hyperechogenic tissue in IVUS and as areas of apparent dense calcium surrounded by necrotic core due to the strong backscattering properties of the polymer in VH-IVUS, and resorption process can be assessed by the reduction in the percentage hyperechogenicity and by change in quantitative analyses of these areas, respectively.

OCT also provided crucial information for the BVS resorption process. Thorax center investigators have proposed the terminology to describe OCT findings associated with various stages of BVS strut resorption in the vessel wall (Fig. 18.4) [18]. An intact scaffold strut footprint is denominated as a “preserved box ,” which is defined as a box appearance with sharply defined borders with bright reflection, and the strut body shows low reflection. The first OCT changes in the strut footprint are named as “open box” which is characterized by luminal and abluminal “long-axis” borders thickened with bright reflection and short-axis borders that are no longer visible at follow-up. The last change on OCT in the process of resorption is “black” and “bright” “dissolved boxes,” which are defined as black spot with poorly defined contours, often confluent but with no box-shaped appearance and partially visible bright spot with poorly defined contours and no box-shaped appearance, respectively [18]. This serial change of OCT findings reflecting resorption process of BVS was firstly evaluated with histology in porcine coronary artery model [19]. In this study, BVS was serially assessed immediately, at 1 month and 2, 3, and 4 years after implantation. The proportion and sequential changes of OCT findings over time are summarized in Table 18.1. Immediately after implantation, all struts had a preserved box appearance. However, the proportion of box appearance decreased over time and only dissolved boxes were seen at 4 years. The preserved box in OCT corresponded well (86.4%) with 2-year histology in which the struts were first covered by a thin, fibromuscular neointima and then replaced by proteoglycan-rich matrix gradually over time, whereas the dissolved bright and black boxes corresponded well (88.0 and 90.7%, respectively) to 3-year histology showing inspissations of the provisional matrix and connective tissue infiltration in the region of the preexisting struts. Struts indiscernible by OCT corresponded to the integrated strut footprints seen at 4 years (100%) [19].

OCT also demonstrated that BVS implantation led to the formation of a symmetrical neointima with a mean thickness of 220 μm during 6–12 months [18], which nearly completed the healing process without further increase of neointima over time [20, 21]. This formation of a circumferential neointimal layer, with resorption of polymeric struts, creates a “de novo” cap , which may help to seal a thin-cap fibroatheroma [20].

OCT is the gold standard for the evaluation of metallic stent strut tissue coverage with its high resolution [22, 23]. It is important to assess the tissue coverage of strut after stent implantation because this coverage is generally considered a marker of endothelialization [24]. BVS has translucent polymeric struts which enable OCT to image the abluminal surface of scaffold. Gutiérrez-Chico et al. demonstrated that most of the malapposed and side-branch struts were covered by neointimal tissues on both the abluminal and adluminal side 6 months after BVS implantation, with thicker neointimal coverage on the abluminal side (101 vs. 71 μm; 95% confidence interval [CI] of the difference: 20–40 μm) (Fig. 18.5) [8]. This OCT finding for BVS strut coverage may provide the understanding of the mechanism by which acute stent malapposition could be spontaneously corrected over time. Long-term follow-up data of BVS showed that all incomplete appositions (incomplete, persistent, and late-acquired incomplete stent apposition) were resolved over 2 years [10]. ABSORB JAPAN trial using OCT also demonstrated that the incidence of malapposed struts decreased from 4.9% immediately after BVS implantation to 0.12% at 2-year follow-up with 0.6% of uncovered struts [25].

OCT was able to reveal the advantage of BVS for early vascular healing with optimal strut coverage [26]. In this study, overall 99% of BVS struts were covered at mean 47.6 ± 6.3 days, in the setting of acute coronary syndrome and stable angina. ABSORB-STEMI TROFI II, which enrolled the ST-segment elevation myocardial infarction patients undergoing primary PCI with BVS or everolimus-eluting metal stent (EES) , evaluated the 6-month OCT healing score (HS) based on the presence of uncovered and/or malapposed stent struts and intraluminal filling defects. BVS showed a nearly complete arterial healing with lower HS when compared with EES arm [1.74 (2.39) vs. 2.80 (4.44); difference (90% CI) −1.06 (−1.96, −0.16); P _{non-inferiority} < 0.001] [27]. Accordingly, OCT is considered the gold standard for strut coverage evaluation of BVS at follow-up. Indeed, OCT made it possible to clearly identify the fibrotic de novo cap (a neointimal layer covering the scaffold struts) manifested by signal-rich low-attenuating tissue layer, even when struts are no more identifiable (Fig. 18.6) [11].

BVS has the potential for greater scaffold underexpansion and malapposition due to its intrinsic differences in recoil characteristics and its less distensibility as compared with metallic stents [28]. Therefore, it is very important during BVS implantation to get the accurate measurement of vascular lumen, to select the appropriate size and length of BVS, and to achieve optimal apposition after deployment. OCT can allow more accurate detection of luminal border at both lesion and reference segments, which enables to select the optimal size of BVS, and quantification of scaffold malapposition and underexpansion with its high resolution, as compared with conventional intravascular imaging modalities (Fig. 18.7) [29]. However, the limitation of clinical data and the lack of standardized criteria for OCT measurements are still problematic in its clinical use, although a comprehensive consensus document has been issued from international working group for OCT standardization and validation [24]. Also several studies demonstrated that lumen dimensions measured by OCT were smaller than those measured by IVUS [30, 31]. Despite these limitations, OCT is now considered a useful intravascular imaging modality for the evaluation of BVS, thanks to the characteristics of BVS to allow the assessment of the vessel wall behind the struts without any metal shadowing [8]. Recently, one study revealed that further optimization after BVS implantation was required in over a quarter of lesions on the basis of OCT findings, despite angiographic success [32].

Another important advantage of OCT in the evaluation of BVS treatment is that it can prove the potential benefit of BVS to get late lumen gain [11], which is a common phenomenon between 6 months and up to 5 years after successful balloon angioplasty with myointimal regression at the lesion site. Two-year follow-up data of ABSORB with OCT demonstrated that there was an increase in minimal and mean luminal area with a significant decrease in plaque volume without change in vessel size between 6 months and 2 years [10]. On the other hand, there was a decrease in lumen area between the immediate post-procedural and 6-month follow-up measurements. There was no significant vascular remodeling over 2 years. Long-term follow-up study using OCT showed that both minimum and mean luminal area increased from 2 to 5 years (Figs. 18.8 and 18.9) [11]. Therefore, OCT can provide the information for the late lumen enlargement and vascular remodeling during follow-up after BVS implantation, although late lumen enlargement is a phenomenon that needs to be confirmed.