## 4 Discussion  
To the best of our knowledge, this is the first study reporting the comparison of neointimal responses at long-term follow-up, assessed by OCT, according to the different doses of Vytorin after BP-BES implantation in patients with AMI. In this study, we did not find a significant difference between the Vytorin 10/10 mg and 10/40 mg groups in the percent of uncovered struts, despite marked reduction in uncovered struts at follow-up when compared with acute results. A sub-study of the LEADERS (Limus Eluted from A Durable vs. ERodable Stent coating) demonstrated better strut coverage of BP-BES as compared with a first-generation sirolimus-eluting stent (SES) at nine months (0.6% in the BP-SES group vs. 2.1% in SES group, \(p\) = 0.04) [22]. Another sequential OCT study showed that the proportion of uncovered struts tended to be lower in BP-BES than in SES at nine months (2.8% in the BP-BES group vs. 5.7% in the SES group, \(p\) = 0.31) [23]. In our study, the mean percentage of uncovered struts was 7.1% in the entire study population. The follow-up OCT results of our study showed higher uncovered struts when compared with two previous studies (7.1% vs. 0.6% and 2.8%, respectively). However, our study only enrolled patients with acute myocardial infarction (AMI). Other two studies enrolled patients with ischemic heart disease including angina, only 35-40% of enrolled patients with AMI. AMI is completely different pathophysiology from angina patients. This difference of enrolled patients might induce the proportion of uncovered struts between our study and other two studies. In one OCT study, a number of uncovered struts was observed in 12.8% of OCT-guided and 16.8% of image-guided primary PCI patients with STEMI at 9 months [24]. Regarding strut malapposition, our study showed that the percent of malapposed struts was similar in both groups. However, follow-up OCT of all individuals showed that the mean percentage of malapposed struts significantly decreased from \(5.6\pm 5.6\%\) after the index PCI to \(1.1\pm 2.2\%\) at follow-up (\(p\) = 0.004). Although the healing process of malapposition has not yet been fully understood, the improved apposition of BP-BES observed at follow-up can be explained by the reduced late-acquired malapposition due to the absence of a hypersensitivity reaction against the biodegradable polymer [20]. In the PESTO (Morphological Parameters Explaining Stent Thrombosis assessed by OCT) registry, malapposition was frequently observed in all stent thrombosis (ST) types, including acute/subacute and late/very late stent thrombosis [25]. The potential advantage of BP-BES might lead to improved long-term clinical outcomes, especially a lower risk of cardiac events associated with very late ST between one and four years [26]. The mean thickness of NIH was 47 \(\upmu\)m and 56 \(\upmu\)m in the 10/10 mg and 10/40 groups, respectively, and the mean was 51 \(\pm\) 74 \(\upmu\)m in all 18 patients. It was quite thin when compared to an estimation on the basis of two previous studies [27; 28]. A substudy from the RESOLUTE all-comers (Randomized Comparison of a Zotarolimus-Eluting Stent (ZES) With an Everolimus-Eluting Stent (EES) for Percutaneous Coronary Intervention) trials demonstrated that neointimal thickness was 116 \(\upmu\)m in ZES and 142 \(\upmu\)m in EES at 13 months after stent implantation [27]. In another sequential OCT study, neointimal thickness at 9 months' follow-up was 139 \(\upmu\)m and 124 \(\upmu\)m in the ZES and EES groups, respectively [28]. The presence of abluminal coating and the biodegradable polymer might be the reason that possibly explains the differences in neointimal thickness between the stents, although further studies should be conducted for confirmation. There were several limitations in this study. First, this is a single-centre study with the limitations of a small sample size, even though our study is a randomized trial. Second, inter-observer and intra-observer variability were not evaluated regarding the OCT analysis.