In the bare metal stent (BMS) era, several intravascular ultrasound(IVUS) studies have validated the strategy of reducing in-stent restenosis rates by a strict adherence to an optimal stent implantation technique, predominantly by avoiding stent underexpansion.1 In a first phase after the introduction of drug-eluting stents(2003), the spectacular reduction in restenosis rates, based on pharmacological inhibition of neointimal growth, distracted to someextent the focus on optimal implantation technique. However,concerns of an increased risk of stent thrombosis and the association,in human pathology studies of stent thrombosis, of delayed healingand malapposition of stent struts,2 fostered a renewed interest instent implantation optimisation techniques. Around the same time, abreakthrough was seen in the use of a new high-resolutionintracoronary imaging technique, optical coherence tomography(OCT). The spatial resolution of intravascular OCT in biological tissue is<20 μm, which is 10-fold higher when compared with the current goldstandard, IVUS. With the newest type of the technology, frequencydomain OCT (FD-OCT), a detailed, fast, safe and reproducibleassessment of intracoronary stents, just after implantation as well asin follow-up examinations, is possible. This manuscript describes therationale of the use of OCT in the guidance of percutaneous coronaryintervention (PCI) with respect to the correction of stentunderexpansion and malapposition in drug-eluting stents anddescribes the results in a first small set of patients.
In the BMS era, studies using intracoronary imaging for guidance of PCI constituted predominantly the use of IVUS for the assessment of stent expansion. Inadequate stent expansion is usually not detectable by angiographic assessment3,4 and in studies using IVUS adequate stent expansion is usually achieved only in a minority of patients. In the Postdilation ClinicalComparative Study (POSTIT1), minimal stent diameter (MSD) >90 %of the average reference lumen diameter was reached in only29 % of patients and the mean MSD was 20 % lower than the nominal stent diameter.5 Different studies have pointed to the beneficial effects of IVUS-guided DES implantation.6 Because of their more straightforward implementation in daily routine,criteria using lumen dimensions (e.g. Multicenter UltrasoundStenting in Coronaries study (MUSIC) criteria) are more often used than those relying on vessel dimensions.
Although aggressive stent expansion might be less important with DES for the prevention of restenosis, it might be important with respect to stent healing. IVUS findings at the time of DES implantation inpatients who subsequently developed stent thrombosis showed that a smaller minimum stent area7 and stent underexpansion at baseline (assessed with IVUS) were associated with incomplete neointimal coverage (on angioscopy) after serolimus-eluting stent(SES) implantation in another study.8
The question arises whether criteria validated for IVUS can be transposed without modification to data acquired with OCT. To date,no adequately powered validation study of these criteria has been published and analysis of larger OCT data sets will be needed to prove the validity of OCT in the assessment of stent underexpansion and its implications. Some comparative studies between IVUS andFD-OCT reported slightly smaller lumen dimensions measured withOCT as compared with IVUS, while other studies and assessment in a phantom model reported good correlations.9–12
Incomplete Stent Apposition
Usually, a discrimination is made between acute (assessed immediately after stent implantation) incomplete stent apposition(ISA) and late ISA as detected during a follow-up invasive intracoronary examination. Acute stent malapposition is commonly caused by sub-optimal stent implantation, but it is reasonable to say that in some lesion subsets (e.g. heavily calcified lesions), it can be impossible to achieve full apposition of all stent struts, even with very high implantation pressures or aggressive postdilation with noncompliant balloons.
In late incomplete stent malapposition, persistent early malapposition is to be discriminated from late acquired malapposition (LAISA). The latter can be caused by positive remodelling, chronic stent recoil(which is rare with currently implanted devices) or by thrombus dissolution, a phenomenon occurring when stents are implanted in thrombus-rich lesions as in ST-segment elevation myocardial infarction (STEMI) or non-STEMI.
Our current knowledge on the incidence, causes and possible implications of ISA relies predominantly on studies with IVUS. LAISA,more specifically when occurring to a severe degree, has been associated with adverse events,13–15 while for acute ISA no association with adverse events has been demonstrated thus far.
OCT offers a dramatic increase in the sensitivity of detecting malapposition when compared with IVUS.16 This is predominantly due to the higher resolution of OCT, but also to the fact that inIVUS images stents tend to produce reverberation artefacts and that IVUS is a very poor discriminator between thrombus and soft plaque. Kubo et al. made a comparative assessment of acute ISA withOCT and IVUS, detecting a much higher incidence of malapposition(47 % versus 18 %, p<0.001) when assessed with the much more sensitive OCT technique.17
Interestingly, the vast majority of late stent malapposition (4.67 %) was due to persistent acute malapposition. LAISA, due to vessel or lesion remodelling, accounted for only a small percentage of late stent malapposition (0.37 %). This finding completely contradicts what we knew previously from IVUS studies and further research will certainly shed more light on this issue. Another striking finding from this report was that late stent malapposition was associated with a 10-fold higher(20.8 % versus 2.0 %) incidence of thrombus detected on the struts at the time of the follow-up study as compared with well-apposed struts.
Percutaneous Coronary Intervention Guided by Intravascular Ultrasound and/or Optical Coherence Tomography
The Can Routine Ultrasound Influence Stent Expansion (CRUISE) study18 proved that an IVUS-guided PCI procedure can achieve larger MSAs and that this is associated with less target vessel revascularisation(TVR). It is commonly believed that a semi compliant (SC) balloon is unable to achieve full stent expansion in many cases involving lesions with heavy plaque burden and increased resistance and that non-compliant (NC) balloons are needed to apply a bigger local force to the underdeployed stent. In one study, postdilation with NC balloons failed to improve MSA probably because the NC balloon was relatively undersized.19 Therefore, in our series of patients, we used oversizedNC balloons to achieve sufficient gain in minimal stent CSA.
Study Aims and Methodology
The aim of this small pilot study was to determine and quantify the acute effects of high-pressure NC balloon postdilation on parameters of underexpansion and malapposition immediately following implantation of DES, as assessed with OCT. In 12 patients with OCT characteristics of frank malapposition immediately after DES implantation, high-pressure postdilation with an NC balloon was performed (see Figure 1).
The sizeof this balloon was chosen according to the reference vessel diameter on the OCT images. Mean postdilation balloon diameter was 3.5 ± 0.6mm. The final result after postdilation was re-evaluated with OCT at the end of the procedure (see Figure 2). MSD and minimum CSA were assessed with OCT before (baseline) and after postdilation.
Results and Conclusion
A total of 3,843 of stent struts were analysed using a previously described fully automated algorithm20 followed by an accurate visual inspection of the results. The mean MSD reached a diameter equal to79.9 ± 1.8 % of the diameter of the final balloon postdilation (as reported by company-provided theoretical compliance charts). In this small series of patients, there were no procedural complications related to OCT use or high-pressure postdilation (see Table 1).
We concluded that high-pressure NC balloon postdilation for underexpansion and malapposition in DES significantly improves MSD and minimum CSA, as assessed with OCT. There was a highly significant decrease in the percentage of malapposed struts.
Obviously, the limited sample size is a major limitation of the current study. However, it clearly shows that OCT allows for a very precise detection and quantification of malapposition after stent implantation and that clearly statistically significant differences can be demonstrated despite low patient numbers. Larger series, correlated with OCT and clinical follow-up, will shed light on the clinical implications of postdilation in case of severe acute ISA.