Giuseppe Sangiorgi, Enrico Romagnoli and Antonio Colombo
Soon after the introduction of balloon-expandable bare metal stents (BMS) in common practice, the need for adequate expansion was recognised to avoid suboptimal stent deployment and reduce the incidence of target vessel revascularisation (TVR).1,2
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 a limited use of balloon postdilatation. Nevertheless, observational data continue to support the use of adjunctive balloon postdilatation following deployment of DES in the great majority of patients.3-8 Importantly, suboptimal or incomplete stent expansion, especially with DES, may not only be associated with increased restenosis and TVR but may also predispose to stent thrombosis.5,9-12
Aim of this article is to review the incidence and all the possible causes and consequences of a suboptimal stent expansion and to assess the possible role of non-compliant balloon postdilatation to minimise it.
Magnitude of suboptimal stent deployment
Since stent underexpansion is poorly recognised by angiography, the real incidence of suboptimal stent deployment is likely to be underestimated (Figure 1). Several trials, such as the STRUT, the CRUISE and the AVID trials, showed an incidence of post-procedural incomplete stent deployment raging from 4% to 22%, notwithstanding IVUS-guided postdilatation.13
A high rate of inadequate expansion, ranging between 24% (SES) and 28% (PES), has also been observed with DES. Both sirolimus-eluting stent (SES) and paclitaxel-eluting stents (PES), behave similarly to their bare-metal stent platforms, respectively achieving 75%-79% of predicted MSA.8 Also, a comparison study conducted by Javaid et al.14 showed that about 80% of patients with SES implantation and 63% of patients with PES implantation had a suboptimal stent expansion at conventionally used inflation pressures. Even at high initial deployment pressures of 20 atmospheres, underexpansion remained in 48% of implanted SES and 35% of implanted PES.
IVUS measurements have provided powerful predictive information regarding, stent thrombosis, restenosis and TVR. In particular, minimal stent area (MSA) and minimal stent diameter (MSD) measured by IVUS at end of the procedure are the strongest predictors of TVR after BMS implantation in numerous studies.15-19
The main consequences of stent underexpansion are thus represented by higher rate of restenosis and related necessity of repeated revascularisation at the long-term follow-up. Moreover, with DES, potential increase in the risk of stent thrombosis exists.20,21
In this context, the Multi-centre Ultrasound Stenting In Coronaries Study (MUSIC), was the first study that assessed the safety and feasibility of intravascular ultrasound-guidance to optimise stent deployment and its impact on immediate and six-month clinical and angiographic results.2 The intravascular ultrasound criteria introduced in the MUSIC study to define ’optimal’ stent expansion are listed in Table I. Subsequently, several studies with different stent types confirmed the clinical positive impact of optimisation of stent deployment on restenosis, TVR rates and thrombosis22-26 with impressive long-term results comparable to those from the latest DES trials.
In stent restenosis
In a retrospective analysis on 1090 in-stent restenosis lesions, Castagna et al.27 highlighted the role of suboptimal stent deployment in determining in-stent-restenosis. In this series, stent underexpansion, defined as CSA at the site of lesion <80% of the average reference lumen area and MSA <5.0mm2, was the most common mechanical problem accounting alone for 20% of ISR cases.
In the high-pressure stenting era, several other studies have demonstrated the importance of complete struts apposition, luminal cross-sectional area and concentricity of stent implanted.28 According to these data, IVUS measurement of MSA is the single most powerful predictor of long-term patency and clinical outcome.15,16,19,29,30 Indeed, an inverse relation between post-procedural MSA and angiographic restenosis15,19 and between MSA and TVR16 has been shown.
While the POSTIT trial31 has demonstrated the need of adjunctive balloon postdilatation to achieve optimal stent expansion, the effects of this IVUS-guided postdilatation on restenosis rate has been assessed in the Can Routine Ultrasound Influence Stent Expansion (CRUISE) study.32 The CRUISE trial clearly showed that postdilation can provide a larger minimal stent area, and how this may translate into better long-term outcome. Indeed, in this study, IVUS-guided postdilatation to optimise stent deployment led to an increase of 14% of the final MSA and this change resulted in a 44% composite relative reduction in TVR.32
With the advent of DES, determining a lower rate of angiographic restenosis and clinical TVR, the need for adjunctive balloon postdilation was considered less important. Moreover, after the results of the SIRIUS trial showing the risk of edge restenosis, vessel trauma at the stent margins due to predilation and postdilatation was indicated as the most important contributor to SES failure.33 For these reasons in subsequent trials,
eg E-SIRIUS, direct stenting was allowed and postdilatation was used only if strictly required by suboptimal stent placement.34
Thus, the message emerging from the first series of randomised trials on DES was that the beneficial effects of DES on subsequent neointimal proliferation may reduce the need for aggressive balloon dilatation.
Subsequently, studies on SES restenosis showed that whereas a smaller MSA may be acceptable post-DES, underexpansion can result in restenosis.5 In particular the IVUS substudy of the SIRIUS trial showed MSA <5.0mm2 is the main predictor of restenosis. This finding was confirmed in other studies in which an incidence of 60-70% stent underexpansion determining a MSA <5.0mm2 was found in most of the cases of SES failures.35-37
In stent thrombosis
Acute stent thrombosis remains an infrequent yet devastating complication in patients undergoing PCI. In the current era of high-pressure stent implantation, the occurrence of stent thrombosis with BMS ranges from 1% to 2%.38,39 Conversely, the rate of DES thrombosis ranges between 0.4-0.6%33,40 of the randomised trial to 1.3-4.9% of ’real world’ registries.11,20 Despite this relatively low incidence, stent thrombosis has an important clinical impact, causing death in about 45% of the patients and nonfatal myocardial infarction in most of the survivors.11
The major post-procedural predictors of thrombosis with both BMS and DES are MSA and suboptimal stent expansion.10,13,41 In particular, stent underexpansion, resulting in abnormal shear stress, might explain as much as 80% of those events.9,42
Yet, the importance of complete stent expansion to prevent stent thrombosis might be more relevant with DES. Indeed, decreased endothelialisation associated with drug-related inhibition of neo-intimal proliferation might increase the risk of stent thrombosis in case of suboptimal stent expansion. Fujji et al. showed in a retrospective study that lesions leading to stent thrombosis after successful SES implantation more often have stent underexpansion (65±18% versus 85±14%, p<0.001), smaller MSA (4.3±1.6mm2 vs. 6.2±1.9mm2) and residual edge stenosis.10 This evidence has also been confirmed for the late-thrombosis cases of SES (>12 months) where stent expansion was less than in patients without stent thrombosis (58±25% versus 81±17%; p<0.001) and incomplete stent apposition was more common (55% versus 12%; p<0.0001).41
Causes of suboptimal stent deployment
Among the possible reasons of suboptimal stent deployment, the first is certainly the undersise of the stent delivery balloon for the target vessel. In patients with severe and diffuse target vessel disease, indeed, the choice of the adequate balloon size based only on angiographic evaluation, is often difficult and leads very often to a balloon-to-artery ratio less than 1.43,44 Our group demonstrated that a difference between intravascular ultrasound (IVUS) and angiography >1.0mm was found in 71% of cases with vessel size <2.75mm and in 49% of cases in patient with vessel size >2.75mm, respectively.43 Importantly, in case of undersizing of the stent delivery balloon, high pressure stent deployment, especially with the current semi-compliant balloon, can compensate only in part the balloon undersizing.14
Another possible cause of stent underexpansion is strictly related to the compliance of balloon commonly used in stent delivery systems that are often not adequate to guarantee full stent expansion at nominal pressures.45 Compliance charts are based on in vitro measurement (in air or in water), but several studies found that stent delivery balloon diameters during stent deployment were approximately 20% less than expected.45-48 These differences were independent of stent manufacturer, length, diameter, and deployment pressure and due to the inherent resistance of dilating a stent within an atherosclerotic artery.45,49 A critical IVUS analysis also showed that the minimal stent diameter usually achieved by DES is 25±10% less than the predicted one.8
Finally, heavy plaque burden and severe calcified vessel or fibrosis occasionally prevent optimal stent expansion by mean of an impaired distensibility, as demonstrated by histopathologic and clinical studies.46,49,50
Predictors of suboptimal stent deployment
Among all the different causes of stent underexpansion reported above, the most important is certainly the pressure level at which the stent is deployed. It is well recognised that most stents deployed at nominal “low” pressures are still underexpanded and stent struts are not completely apposed to the vessel wall.1,51 The importance of this phenomenon has not changed with the new stent delivery systems based on semi-compliant balloons, and drug-eluting stents showed the same properties as that of the corresponding BMS platforms.
In studies in which IVUS analysis after postdilatation was accomplished, including the POSTIT trial, none of the baseline clinical or angiographic variables seemed able to predict the final MSA or MSD after stenting.47 Similarly, neither quantitative IVUS lesion measurements nor qualitative IVUS assessment of plaque morphology can predict stent expansion.8 Finally, the response of the vessel wall to coronary stenting seems independently related to the technique used (e.g. direct stenting or predilation).52
Patients and lesion subsets in which postdilatation with non compliant balloons should be carefully considered are summarised in Table II.
Rationale of non compliant balloon postdilation
For safety and deliverability reasons, most of stent delivery systems are currently based on semi-compliant balloon device. Compliant nature of these balloons causes significant deformation of profile and volume with increases in pressures, resulting in stretching of the balloon itself. Thus, although current semi-compliant balloons are precisely matched with stent length with very short balloon shoulders, and assure a uniform diameter expansion along the balloon length, the risk of vessel stretch and edges injury is important at high pressures.
Conversely, non compliant balloons have little change in volume even at high pressures concentrating dilating force at the lesion site.53 Bench tests and clinical studies, indeed, have shown that non compliant balloons exert more dilating force against a lesion or a stent than compliant balloons for a given balloon size and inflation pressure. Thus, postdilatation with non compliant balloons results in a significant improvement of MSA compared to the current semi-compliant stent deployment balloons.54
These features can be important to prevent unwanted and potentially severe complications caused by over-dilation or un-controlled fast stretch of the vessel wall. Using semi-compliant balloon in calcified or very stiff lesions requiring higher pressure to open the stent, for example, might cause dissection at stent edges. High injury scores due to break of the intima or media might also cause a long-term inflammatory response resulting restenosis.50 Moreover, the SIRIUS IVUS substudy suggested that more injury to the contiguous vasoelastic normal wall, coupled with a drug that delay the healing process, could contribute to late stent malapposition due to focal positive vessel remodelling.55 Finally, in small vessels the oversizing of balloon delivery systems might facilitate coronary rupture.
These considerations have led to common use of non compliant balloons for postdilatation at high pressure.
High pressure initial delivery vs. high pressure non-compliant balloon postdilation technique
The POSTIT trial31 was the first trial to assess the impact of higher delivery pressures and adjunctive non-compliant, IVUS-guided, balloon postdilation to optimise stent deployment. This multi-center study revealed that in only 29% of patients an optimum stent deployment is achieved with the current stent delivery systems. Higher deployment pressures (=12 atmospheres) were associated with a lower frequency of suboptimal stent expansion, but only 36% of patients achieved optimal stent deployment according to IVUS criteria. The MSD achieved with the stent delivery systems was about 20% less than expected according the manufactures’ chart.
Conversely, with postdilation using non-compliant balloons, there was a significant increase in MSA and MSD, and the frequency of achieving optimal stent deployment was double.
In conclusion, IVUS studies showed the real minimum stent diameter following stent deployment is about 20% less than the predicted stent diameter based on the manufacturer’s balloon compliance charts, regardless of stent brand.
Observation data suggest stent underexpansion and incomplete stent apposition may be one of the most important causes of DES failure and thrombosis, advocating that, while neo-intimal hyperplasia is suppressed, the optimum stent deployment is still fundamental.
Further to this, several randomised and prospective studies have demonstrated optimal stent expansion and MSA are the most powerful predictors of long-term stent patency and clinical outcome showing an inverse relation with TVR.15,16,19,29 Data from literature suggest that achieving adequate stent expansion during PCI is thus important to reduce restenosis and the need for TVR, but can also minimise the risk of stent thrombosis in DES era.
Thus, postdilatation with non compliant balloons at high pressures, improving final MSA and MSD, may greatly increase the frequency of optimum DES deployment and actually lead to reduction of restenosis and TVR rates. The POSTIT trial showed indeed that, even at high pressure (=14 atmospheres), an optimum stent deployment is achieved in only 36% of patients31, and that using a non-compliant balloon to post-dilate the stent, this percentage can double.
In clinical practice, according to the rate of suboptimal stent deployment reported in different studies, a considerable number of patients may then benefit from repeat inflations with non compliant balloon at higher pressures and/or with larger diameter size.
Unfortunately, it is not practical nor cost-effective to perform postdilatation in all patients undergoing stent implantation. A more realistic approach is therefore to selectively postdilate at high pressure with non compliant balloons in situations where the risk of suboptimal stent delivery is higher (Table II (page8)).
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55 Ako J, Morino Y, Honda Y, Hassan A, Sonoda S, Yock PG, Leon MB, Moses JW, Bonneau HN, Fitzgerald PJ. Late incomplete stent apposition after sirolimus-eluting stent implantation: a serial intravascular ultrasound analysis. J Am Coll Cardiol. 2005 Sep 20;46(6):1002-5.