OCT imaging after carotid artery stenting reveals microdefects related to device design

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A study published in the European Journal of Vascular and Endovascular Surgery shows that in the carotid arteries, stent malapposition is more frequent with closed-cell stents while plaque prolapse is more common with open-cell stents. Gianmarco de Donato, Department of Surgery, Vascular and Endovascular Surgery Unit, University of Siena, Siena, Italy, explains the rationale behind the study, its implications and future directions.

Can you please explain the background for the study and what its objective was?

To date, limited information has been available regarding the complex interaction between carotid plaque and stent, considering that neither angiography nor intravascular ultrasound (IVUS) has micro-scale resolution able to identify such imperfections.

Optical coherence tomography (OCT) is a light-based imaging method that uses newly developed fibre optic technology. In the cardiovascular field, OCT is a catheter-based invasive intravascular imaging system that uses near-infrared light with orders of magnitude higher than those of medical ultrasound signals, providing unprecedented microstructural information on atherosclerotic plaques.

Regardless the use of an embolic protection device, percutaneous treatment of carotid pathologies has been correlated with a risk of cerebral ischaemic events related to distal embolisation. Many factors may cause an intraprocedural embolisation (including incorrect endoluminal manoeuvres, complex aortic arch, use of cerebral protection devices, stent deployment and ballooning, etc), while events occurring in the early postoperative period are contemplated as the consequences of remodelling of the atheroma, which is more or less contained behind the stent struts.

There is great interest in the possibility to recognise further details regarding the interaction between carotid plaque and stent by OCT, considering that plaque prolapse through the cell stent has been suggested as one of the major causes of postprocedural complications following carotid artery stenting and that available periprocedural imaging systems (angiography, IVUS and duplex ultrasound) may not be able to detect such microdefects.  

The objective of this study was to evaluate the rate of stent malapposition, plaque prolapse and fibrous cap rupture detected by OCT after carotid artery stenting according to stent design.

How was the study performed?

Forty consecutive patients undergoing protected carotid artery stenting were enrolled in the study. OCT images were acquired three times in each of the 40 patients: before stent deployment, immediately after stent placement, and following postdilatation of the stent.

The optical fibre of the Lightlab FD-OCT system used for the investigation is encapsulated within a rotating torque wire (0.014-inch compatible) built in a rapid exchange 2.6F catheter compatible with a 6F guiding catheter. It acquires 100 frames per second, scanning a 55mm artery segment in 2.7 seconds (pullback speeds up to 20mm/s).

Once the cerebral protection device is deployed in a straight portion of the internal carotid artery distally to the culprit lesion, the calibrated OCT catheter is advanced over the 0.014-inch guidewire of the filter, and completely passed over the lesion that needed to be scanned.

Pullbacks are started during a non-occlusive flush, mechanically injecting 24mL of 50% saline diluted contrast medium (Iodinoxanol: Visipaque 320mgl/mL; GE Healthcare) at 6mL per second, 750psi, using an automatic injection system (Mark V ProVis; Medrad) to completely replace blood from the artery. Injections are performed through an 8F guiding catheter with a minimum internal lumen of 2.3mm, placed just few centimetres proximally to the carotid bifurcation.

Of note, the pullback is started approximately 1.5–2 seconds after the injection of the diluted contrast medium, when the absence of blood scattering and signal attenuation are noted on the screen of the OCT system (real-time images).    

After stent deployment the same OCT manoeuvres are repeated, in particular two further scans are performed before and after stent dilatation.

OCT frames were then analysed offline, in a dedicated core laboratory by two independent physicians. Cross-sectional OCT images within the stented segment of the internal carotid artery were evaluated at 1mm intervals for the presence of strut malappossition, plaque prolapse and fibrous cap rupture according to stent design.

What are the results and how do you interpret them?

Closed-cell design stents (CC) were used in 17 patients (42.5%), open-cell design stents (OC) in 13 (32.5%), hybrid design stents (Hyb) in 10 (25%).

No procedural or postprocedural neurological complications occurred (stroke/death 0% at 30 days). Two transient ischemic attacks were recorded in the postoperative period (one in the open cell group and one in the hybrid group).

On OCT analysis the frequencies of malapposed struts were higher with closed cell compared to open-cell and hybrid stents (34.5% vs. 15% and 16.3%, respectively; p<0.01). Plaque prolapse was more frequent with open vs. closed cell design (68.6% vs. 23.3%; p<0.01) and vs. hybrid cell stents (30.8%; p<0.01). Significant differences were also noted in the rates of fibrous cap rupture between closed-cell and open-cell stents (24.2% vs. 43.8%; p<0.01), and between closed-cell and hybrid devices (24.2% vs. 39.6%; p<0.01), but not between open-cell vs. hybrid stents (p=0.4).

Our findings confirm that, when interacting with the plaque, all carotid stent designs are susceptible to those microdefects that are potentially responsible for cerebral embolisation.

Surprisingly, in our study less than 60% of stent struts analysed by OCT were considered “well apposed” to the arterial wall. Closed-cell stent struts were more likely to be malapposed, while open-cell stent struts were more likely to be embedded (p<0.01).  Moreover, plaque prolapse was more common with open-cell stents compared to closed-cell stents, but not to hybrid stents, while fibrous cap rupture occurred less frequently in the closed-cell stents compared to the open-cell or to the hybrid stents (p<0.01).

What has the study concluded?

The main message from our investigation is that an unexpected high number of micro-imperfections after carotid artery stenting are noticeable with all carotid stent designs by OCT images acquisition. Stent malapposition is more frequent with closed-cell stents, while plaque prolapse is more common with open-cell stents. It remains, however, unknown whether these figures now detected with OCT are of any clinical and prognostic significance.

An additional conclusion is that the ideal carotid stent did not exist at time of this investigation.

What information can OCT provide that other modalities are not able to offer?

OCT is a progressively accepted intravascular modality to study coronary arteries, stent implantation and vessel injury, as it permits accurate measurements of luminal architecture and provides insights into plaque coverage, plaque prolapse, stent apposition, overlap, and neointimal thickening. The superiority of OCT as a modality in the setting of coronary stent apposition has been demonstrated; it is due to its ability to resolve small gaps between stent strut and the vessel wall, which are often missed by IVUS.

As demonstrated by our investigation, OCT can also clearly define and provide precise details of the conformability of actual carotid stents to lesion contours and vessel tortuosity after deployment (defined as stent apposition/malapposition), or of the ability of the stent to effectively cover the plaque.

OCT with its high-resolution capability of 10μm and its ability to carotid plaque definition has to be considered the innovative tool to define new indications to treatment, new suggestions in the carotid artery stenting vs. endarterectomy debate, and above all new criteria for the development of new carotid stent and testing their performance.

 

What does your study add to the literature and how can the results of the study impact clinical practice?

The present study is the first in the literature focusing on the complex interaction between carotid plaques and stents by analysing OCT findings (stent malapposition, plaque prolapse and cap rupture) according to stent design. The results of this investigation offer some original and unexpected information, which is available for the first time at such a high definition, and that may influence our future clinical policies in patients with carotid artery disease. Moreover, these results clearly show that the carotid stent designs available at the time of investigation were tremendously suboptimal with regards to plaque coverage and wall apposition as depicted by OCT.

 

What are the next steps in this investigation?

The next step in our investigation is to study the new carotid stents that have been recently launched on the market. The interest in testing the performance of any new endovascular product according to the OCT high-resolution capability is at the top of my list.

The so called “mesh-stents” are a new class of carotid stent that promise to offer a higher scaffolding of carotid plaque in comparison to previous carotid stent design, avoiding, or at least limiting plaque prolapse through the cell struts.

We are now collecting the first OCT data after carotid artery stenting with the Terumo Roadsaver (a double layer micromesh design nitinol stent) and with Inspire C-Guard (a nitinol stent wrapped with an expandable, MicroNet—PET mesh). Up to now we have studied only a small number of cases where these products have been used and the interest in analysing the functioning of these new mesh stents is increasing. In particular the question is whether this new technology is really able to reduce the unexpected high rate of plaque prolapse that we observed while performing OCT with both closed- and open-cell stents.

Although we still do not have enough data to compare this information with the results from our previous OCT investigation, the first impression is particularly promising. The rate of plaque microprolapse seems much lower. We are now also setting up a 3D volume reconstruction from OCT images that allow us to better identify the profile of the micromesh, which is only 20μm for the C-Guard stent. This kind of volume reconstruction at very high-resolution makes it possible to recognise how some microprolapses that occur through the cell struts are in fact well contained by the micromesh.

Finally we hope to collect more OCT records in mesh-stent usage and to publish new data that can influence our medical decision and help tailor our clinical strategies for carotid artery disease.

At present, three designs of carotid stents with distinctive features are available: closed-cell, open-cell and hybrid cell design, and different rates of neurological events after carotid artery stenting have been previously reported according to the stent design.

A previous research by Drs Marc Bosiers, Alberto Cremonesi and our group at University of Siena reported different neurological rates after stenting with different carotid stent designs. The study focused on postprocedural events (when the struts of the stent and its scaffolding properties are the only protection against plaque embolisation) and found that the symptomatic population had a higher event rate when an open-cell model was implanted. Unfortunately, when the study was performed, OCT was not available and a correlation with plaque microprolapse and neurological complications could only be suggested.

Coronary stent-strut coverage and apposition have been linked to the risk of stent thrombosis, and recent randomised clinical trials have selected these variables detected by OCT as their primary end point, revealing new and unexpected data.

OCT with its high-resolution capability of 10μm can detect them, and our study has shown that in a relevant number of cases stent strut malappostion, plaque prolapse or rupture of the fibrous cap persist despite technical success detected by angiography.

Safety and feasibility of OCT applied to carotid arteries have been recently reported, both with an occlusive and a non-occlusive technique.

Unfortunately existing intraprocedural imaging systems have not been able to clearly define and provide precise details of the conformability of actual carotid stents to lesion contours and vessel tortuosity after deployment (defined as stent apposition/malapposition), or of the ability of the stent to effectively cover the plaque. High-resolution OCT images might provide more satisfactory results, and this prospective study is the first to investigate the distribution of such micro-defects after stenting according to stent design.

The aim of this study is to evaluate the rate of stent malapposition, plaque prolapse and fibrous cap rupture detected by OCT after stenting according to stent design.