COMMENTARY: Optimal Stent Expansion: Is the Eye of the Beholder as Good as Intravascular Ultrasound?

The introduction of coronary stenting marked a major milestone in percutaneous coronary interventions. Acute or threatened closure of the artery was once the most serious complication of conventional coronary balloon angioplasty. It was mostly due to the unpredictable nature of arterial wall disruption (dissection) due to barotrauma, arterial recoil and inadequate antiplatelet therapy, making it the “Sword of Damocles”. Stents, with their scaffolding design, together with improved antiplatelet therapy, have resulted in a marked reduction in acute and subacute abrupt occlusion of the artery which is now mostly due to stent thrombosis.^1,2 Numerous variables have been implicated for this continued potential threat, including the length of the stented segment, final minimal luminal diameter, inadequate antiplatelet therapy, low ejection fraction, more stents per lesion, smaller balloon size, persistent slow flow and dissection and, most importantly, stent underexpansion.³ Restenosis of the treated segment was reduced by stenting compared to balloon angioplasty but continued to plague coronary interventions until drug-eluting coronary stents were introduced. Late loss is a major predictor of restenosis and is defined as the minimal luminal diameter (MLD) of a treated segment at the completion of intervention minus the MLD at 6 to 9 months after the procedure. Larger vessel sizes are associated with lower re-stenosis rates. These factors prompted emphasis on achieving greater MLDs at the time of intervention. In an analysis of 22 clinical trials examining late loss after deployment of bare-metal (BMS) or drug-eluting stents (DES), Mauri et al found that late loss with BMS ranged from 0.65 to 1.21 mm, while late loss for DES was -0.01 to 0.81 mm, and that late loss correlated strongly with binary restenosis.⁴ Hong et al demonstrated in an intravascular ultrasound (IVUS)-guided study on patients undergoing DES deployment that the angiographic restenosis rate was highest in lesions with stent area <5.5 mm² and stent length >40 mm.⁵ Similarly, Mintz et al pointed out the correlation between stent underexpansion and both thrombosis and restenosis.⁶
Optimal stent implantation has been very difficult to evaluate angiographically. Angiographic projections do not reveal the three-dimensional geometry assumed after stent expansion. What may seem like an angiographically perfect result may in fact, on an intracoronary ultrasound study, show areas of stent malapposition, irregularity of the stented segment and incomplete stent expansion.^7–11 Contrast angiography may completely miss narrowed segments within the stented vessel if they are <1 mm, unless the beam of X-ray is absolutely perpendicular to the artery.¹¹ Colombo and associates have shown that 70% of the stents that were reported as apparent success angiographically were suboptimally placed in the artery.³ Intravascular ultrasound imaging (IVUS) has the distinct advantage of providing a detailed cross-sectional view of the stent and the vessels from within the lumen. IVUS has now become the gold standard and has been widely used to ascertain complete expansion of intracoronary stents.¹² IVUS guidance of stent implantation is also clinically relevant, as its use has resulted in significantly lowering not only the thrombotic complications, but also long-term target vessel revascularization as demonstrated in the CRUISE and RESIST trials.^12,13 Although IVUS has played an integral role in understanding and guiding coronary stent placement, its routine use is not supported in contemporary clinical practice. The interpretation skills, time constraints and the cost of IVUS studies have been prohibitive.¹⁴
The foremost variable in preventing acute and subacute stent thrombosis is placement of the stent with adequate inflow and outflow. Several techniques have been developed as surrogate markers for optimal stent expansion and to guide stent implantation without having to use IVUS. The most commonly used techniques in day-to-day clinical practice are the use of a balloon-to-artery ratio of 1.1, high-pressure stent deployment, postdilatation with a 0.25 mm oversized balloon, visualization of angiographic step-up and step-down and luminal regularity.^7,15 Other techniques to guide optimal stent deployment include the use of fractional flow reserve (FFR) and StentBoost (Philips Medical Systems, Amsterdam, The Netherlands). Studies analyzing the impact of high- versus low-pressure stent implantation techniques have concluded that high-pressure stent implantation results in greater stent expansion and larger lumen dimensions without any increase in complications.¹⁶ Numerous studies have now shown that high-pressure deployment of stents without IVUS guidance and dual antiplatelet therapy not only increases the minimal lumen diameter, but also decreases the rates of stent thrombosis to an acceptable level (3.1%).^9,17 Although high-pressure expansion improves stent apposition to the luminal wall, it does not guarantee homogeneous stent geometry and optimal stent expansion in all patients.^11,18,19 The era of high-pressure stent expansion had coincided with dual antiplatelet therapy, and thus the reduction in stent thrombosis is due in major part to a combination of dual antiplatelet therapy and high-pressure balloon inflation, and not just the latter.^17,20
The inability to achieve optimal stent deployment despite high-pressure inflation is not due to undersizing but rather to an inability of the stent balloon to fully expand the stent to nominal size and acute recoil of the stented vessel.^{10,18,21,22,23} Thus postdilatation of stents with a 0.25 mm oversized balloon has become a common practice. In patients who undergo postdilatation, the frequency of achieving optimal stent expansion doubles, minimal stent area increases by approximately 1 mm and minimal stent diameter increases by 0.2 mm.²¹ Postdilatation studies have also shown a decrease in target vessel revascularization rates without increasing the risk of dissection or perforation.²⁴
The use of FFR after stent deployment can indeed give information regarding adequacy of stent deployment by providing functional information but by no means can provide any information about stent expansion or apposition to the vessel wall.²⁵ This inadequacy poses the continued threat of stent thrombosis and has been largely abandoned.²⁶
A novel and new fluoroscopic image processing technique has recently emerged with a fairly good correlation with IVUS in demonstrating stent expansion.²¹ It is known as StentBoost and uses motion-corrected fluoroscopic images with balloon markers as reference points to enhance coronary stent visualization. Although the technique is inferior to IVUS, it is superior to conventional contrast angiography.²⁷ It does have some limitations in lesions with heavy intimal calcification, but may be very advantageous in bifurcation stenting or overlapping stents. It is a relatively new technique and awaits critical appraisal.
The step-up and step-down technique described by Haldis et al in this issue of the journal, although not unique and has been previously well described, reiterates the simplicity of surrogate markers of good stent expansion.²⁸ It was a small study with 25 patients where 12 patients were randomized to standard sizing and high-pressure stent deployment and 13 patients to angiographic step-up and step-down, both followed by IVUS to evaluate stent expansion and strut apposition. The stent expansion index was calculated using the MUSIC (Multicenter Ultrasound Stenting in Coronaries) criteria. The results were impressive with 33% in the standard arm and 92% in the step-up arm, achieving the primary endpoint of optimal stent expansion. A similar study was performed by Colombo and his team with 62 patients, all of whom underwent stent deployment with angiographic step-up and step-down followed by IVUS.⁷ In contrast to the present study, Colombo’s study showed a high frequency of suboptimal stent expansion by IVUS criteria (80% of the lesions requiring further balloon dilatation). It is difficult to speculate on the reason(s) for the significant differences in the findings of the two studies. It would be important to bear in mind that the Colombo study was performed using Palmaz Schatz coronary and biliary stents with older balloon technology.
The step-up and step-down technique, although very attractive, can lead to edge dissection and edge restenosis.^{15,23,29–33} There were no edge dissections noted in this study and there was no angiographic follow up to look for edge restenosis. It was a relatively small study and as mentioned by the authors, was not powered to analyze adverse outcomes. Overall, this study helps provide the interventionalist with the reassurance that the step-up and step-down effect is a useful surrogate marker for optimal stent expansion.

References

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