Among the possible reasons for suboptimal stent deployment, the first is certainly the undersizing of the stent delivery balloon related to the target vessel. It is worth noting that in case of undersizing of the stent delivery balloon, high-pressure stent deployment, especially with the current semi-compliant balloon, can compensate for the balloon undersizing only in part
Optimization of stent deployment during percutaneous coronary intervention (PCI) is a key element to obtain most favorable immediate and long-term results. Since the introduction of balloon-expandable bare-metal stents (BMS) in common practice, the need was recognized for adequate stent expansion to avoid suboptimal stent deployment and reduce the incidence of target vessel revascularization (TVR).
After the introduction of drug-eluting stents (DES) that dramatically improved restenosis and TVR, the importance of optimal stent deployment was initially underestimated, leading to less use of balloon post-dilation. In the major randomized clinical trials (RCTs) testing DES, post-dilation was not routinely performed. Nevertheless, despite the lack of evidence from RCTs, observational data continue to support the use of adjunctive balloon post-dilation after deployment of DES in the great majority of patients. Indeed, the current stent delivery systems of DES are similar to if not the same as in BMS, and the risk of suboptimal stent expansion is still high. Importantly, suboptimal or incomplete stent expansion, especially with DES with polymeric coating, not only might be associated with increased restenosis and TVR, but also might predispose to stent thrombosis.
Because stent underexpansion is poorly recognized by angiography, the real incidence of suboptimal stent deployment is likely to be underestimated. Indeed, it has been observed that discrepancies exist between angiographically-defined and IVUS-defined optimal stent deployment regardless of the stent implanted, with the IVUS success rate raging from 13% to 70% despite successful angiographic results. A comparison example between angiographic and IVUS results before and after high-pressure stent post-dilation is shown in Figure 1.
Conclusions & References
Another possible cause of stent underexpansion is strictly related to the compliance of balloons commonly used in delivery systems that are often not adequate to guarantee full stent expansion at nominal pressures (Several IVUS studies found that the real MSD after stent deployment was 20% to 26% less than the unconstrained stent size displayed in the compliance chart on the stent box). These differences were independent of stent manufacturer, length, diameter, and deployment pressure and related to the inherent resistance of dilating a stent within an atherosclerotic artery. Therefore, high-pressure stent deployment is still strongly recommended to obtain full expansion of both BMS and DES. In this context, IVUS analysis might have a role to check whether the pressures used have really fulfilled the job to optimally deploy the stent. .
Calcified vessels affect final stent lumen area, preventing complete expansion even when higher pressures or larger balloons are applied. In this situation, the use of high-pressure balloon inflations determines vessel overexpansion at noncalcified segments rather than compression of the calcific plaque. The net result is that a significant portion of stent remains underexpanded and asymmetric, which in turn probably explains the higher rate of restenosis found in this type of lesion. Therefore, in these situations, when ablative or atherectomy techniques are not feasible, the use of a noncompliant balloon post-dilation represents a good compromise to achieve good stent expansion and symmetry without increasing risk of dissection or rupture of the vessel
Bifurcation treatment is associated with a high incidence of nonuniform stent expansion in the side branch, resulting in a higher TVR rate. Indeed, the lateral opening of the stent in the main branch to gain access to the side branch causes strut deformation and malapposition. In this context, several studies showed that in bifurcation lesions, especially in the case of both branches stenting, final post-dilation with a kissing balloon is associated with more favorable long-term outcome, reducing the restenosis rate of the side branch and the need for TLR. Thus, kissing balloon dilation at the end of the procedure is mandatory for bifurcations, but balloon diameters and inflation pressures are yet to be defined to uniformly expand stent struts in both branches. In this context, the use of adequately sized noncompliant balloons at truly high pressures might represent a good compromise between safety and efficacy.
Treatment of long lesions, especially when more than 1 stent is required or when long stents are implanted, increases the risk of size mismatch between the proximal and distal portion of the target vessel. Indeed, in such cases, the stent is usually sized for distal reference diameter and results undersized for the proximal reference diameter. Furthermore, the presence of a double stent struts layer could reduce vessel compliance and can produce an incomplete stent apposition beneath the overlap. For these reasons, a systematic post-dilation of the proximal portion of a long stent and of the overlapping zone with high-pressure inflations or a larger balloon is strongly recommended.
In clinical practice, many interventional cardiologists generally avoid implantation of stents with a diameter size smaller than 2.5 mm. Thus, for small vessels it is common practice to deploy stents that are slightly larger but at lower pressures to avoid excessive vessel overstretch. .If the vessel’s small size were confirmed, post-dilation at higher pressures with a 2.5-mm noncompliant balloon would represent an indispensable solution to achieve the largest final lumen stent to reduce the risk of stent thrombosis and TVR.
Although DES use has also provided encouraging results in this setting, stent underexpansion is the main predictor of in-stent restenosis, ranging between 20% and 40% of the cases. With post-dilation with noncompliant balloons, significant stent re-expansion can be obtained during repeat angioplasty irrespective of the grade of initial stent underexpansion.
Figure 1. Comparison of angiographic and IVUS findings before and after high-pressure Stent post-dilation.