Author + information
- Received September 29, 2014
- Revision received January 20, 2015
- Accepted January 29, 2015
- Published online June 1, 2015.
- Gabor G. Toth, MD∗,†,
- Stylianos Pyxaras, MD∗,
- Peter Mortier, MS, PhD‡,§,
- Frederic De Vroey, MD∗,
- Giuseppe Di Gioia, MD∗,‖,
- Julien Adjedj, MD∗,
- Mariano Pellicano, MD∗,
- Angela Ferrara, MD∗,
- Thomas De Schryver, MS¶,
- Luc Van Hoorebeke, MS¶,
- Benedict Verhegghe, MS, PhD‡,§,
- Emanuele Barbato, MD, PhD∗,‖,
- Bernard De Bruyne, MD, PhD∗,
- Matthieu De Beule, MS, PhD‡,§ and
- William Wijns, MD, PhD∗∗ ()
- ∗Cardiovascular Research Centre Aalst, OLV Clinic, Aalst, Belgium
- †University Heart Centre Graz, Medical University of Graz, Graz, Austria
- ‡FEops Besloten Vennootschap met Beperkte Aansprakelijkheid, Ghent, Belgium
- §IBiTech-bioMMeda, IMinds Medical IT, Ghent University, Ghent, Belgium
- ‖Division of Cardiology, Department of Advanced Biomedical Sciences, Federico II University, Naples, Italy
- ¶UGCT, Ghent University, Ghent, Belgium
- ↵∗Correspondence and reprint requests:
Dr. William Wijns, Cardiovascular Research Centre Aalst, OLV-Ziekenhuis, Moorselbaan 164, Aalst, B9300, Belgium.
Objectives The study aimed to evaluate the adequacy and feasibility of the single string bifurcation stenting technique.
Background Double-stent techniques may be required for complex bifurcations. Currently applied methods all have their morphological or structural limitations with respect to wall coverage, multiple strut layers, and apposition rate.
Methods Single string is a novel method in which, first, the side branch (SB) stent is deployed with a single stent cell protruding into the main branch (MB). Second, the MB stent is deployed across this protruding stent cell. The procedure is completed by final kissing balloon dilation. The single string technique was first tested in vitro (n = 20) and next applied in patients (n = 11) with complex bifurcation stenoses.
Results All procedures were performed successfully, crossing a single stent cell in 100%. Procedure duration was 23.0 ± 7.9 min, and the fluoroscopy time was 9.4 ± 3.5 min. The results were evaluated by optical coherence tomography, showing fully apposed struts in 83.0 ± 9.2% in the bifurcation area. Residual area obstruction in the MB was 6.4 ± 5.6% and 25.0 ± 16.9% in the SB, as evaluated by micro computed tomography. All the human cases were performed successfully with excellent angiographic results: the residual area stenosis was 27 ± 8% and 29 ± 10% in the MB and in the SB, respectively, by 3-dimensional quantitative coronary angiography. No relevant periprocedural enzyme increase was observed. During follow-up (6 ± 4 months), no adverse clinical events (death, myocardial infarction, target vessel revascularization) were noted.
Conclusions The single string technique for complex bifurcation dilation was shown to be adequate in vitro and feasible in humans, with favorable results in terms of stent overlap, malapposition rate, and low residual obstruction in both the MB and SB.
The basic concept of conventional stent systems relies on restoration of the native tubular geometry and expected luminal area of epicardial coronary arteries. Therefore, percutaneous coronary intervention (PCI) of complex bifurcation stenosis remains a challenge for interventional cardiologists. The evolution of bifurcation PCI resulted in several techniques, using either single- and double-stent techniques (1) or a number of dedicated devices (2–5). Yet the universally optimal solution is still lacking, mainly due to the highly complex morphology with large variation of bifurcations stenoses.
The single-stent technique, namely, provisional T-stenting, is associated with excellent short- and long-term outcomes in the vast majority of cases (6–8) and is currently recommended as the default approach to bifurcation PCI with or without involvement of the side branch (SB) (9). For more complex pathologies, in which an important SB is involved or stenosed over a longer segment (more than 5 mm, starting at its ostium), the single stent strategy may not provide optimal results with adequate downstream flow in both territories. For such pathologies, double stent techniques or the use of dedicated devices may be recommended. A wide variety of techniques using 2 regular stents has been described and evaluated including T-, culotte, crush, minicrush, T- and protrusion stenting, but the best choice remains unclear. The main limitations of each of these techniques are due to superimposition of multiple metal layers and frequent stent strut malapposition. The introduction of novel dedicated SB stents aimed at eliminating these limitations (10). However, dedicated devices lack the anatomic conformability and flexibility of regular stents and may not be available as drug-eluting devices.
With the purpose of optimization of the pros and cons of conventional double-stent techniques, Kawasaki et al. (11) reported a modification of culotte technique that we propose to describe as the single string bifurcation stenting technique. The aim of the present study was the in vitro assessment of performance and feasibility of the single string technique by extensive technical and mechanical evaluation, using optical coherence tomography (OCT) and micro computed tomography (mCT). Additionally, we report here the experience in humans with a prospective pilot registry using the single string technique in 11 patients undergoing PCI for complex bifurcation stenosis involving a large SB.
In vitro testing
The single string technique was first extensively tested in vitro. Tests were performed in uniform silicone phantoms, manufactured using a rapid prototyping technique. Technical parameters are shown in Figure 1A.
Single string stenting procedure
Single string stenting procedures were performed with uniform materials using a standardized technique and predefined steps. Biomatrix and Biomatrix Flex drug-eluting stents (Biosensors Interventional Technologies Ltd, Singapore) were 2.5 mm and 3.0 mm in size. Both types of stents use the same platform in both sizes. Stents have a 120-μm strut thickness and are covered by 10 μm-thick bioresorbable abluminal polymer layer. Both platforms have 2 connectors between rings and a maximal cell size of 21.2 and 22.9 mm2, respectively.
Two hydrophilic coated, moderate support guidewires (GW) were used. For educational reasons, we call them the master GW (Figure 2) and the fellow GW (Figure 2). The Master GW was first advanced into the SB and the fellow GW into the distal MB (Figure 2A). A 2.5-mm stent was deployed in the SB (12 bars) with careful positioning of its proximal edge at the proximal rim of the SB ostium, resulting in no more than a single string of stent strut protruding into the MB (Figure 2B, Online Video 1). The stent was proximally optimized by reinflation of the stent delivery balloon at higher pressures (16 bars) after partial pullback (Figure 2C, Online Video 2). Next the master GW is slowly pulled back until it falls into the protruding cell, through which it was advanced into the distal MB (Figure 2D, Online Video 3). For achieving more support, the fellow GW can be advanced into the SB (Online Video 4). The protruding cell was opened up to reasonable size with a 2.5-mm balloon or a 1.5-mm balloon when needed (Figures 2E and 2F, Online Videos 5 and 6). Pre-dilation of the protruding stent cell (or the single string of the strut) allows the cell to be crossed easily with a 3.0-mm MB stent (Online Video 7). After positioning but before deployment of the MB stent, the fellow GW was removed to avoid its being confined under the MB stent and the string of the SB stent. The MB stent was deployed at 12 bars (Figure 2G, Online Video 8). The master GW was pulled back and positioned in the SB after crossing the most distal cell of the MB stent facing the SB ostium (Online Video 9). The fellow GW was positioned in the distal MB. The ostium of the SB was pre-dilated with the previously used 2.5-mm balloon or with smaller sized balloons when needed. Next, sequential final kissing balloon dilation was performed (12 to 12 bars) with the previously used 2.5-mm balloon in the SB and the 3.0-mm stent balloon in the MB (Figure 2H, Online Videos 10 and 11). Final optimization was performed in the proximal MB using a short 3.5-mm balloon (Online Video 12).
Optical coherence tomography
OCT imaging was done systematically at the end of each in vitro procedure for the evaluation of stent strut malapposition. Pullback runs were performed at 20 mm/s with a Dragonfly Duo OCT catheter (St. Jude Medical, St. Paul, Minnesota) and analyzed by a dedicated workstation (C7-XRTM OCT Intravascular Imaging System, St. Jude Medical). Images were recorded at 100 frames per second. Note that OCT pullback was only documentary and not meant to guide PCI.
For comparison purposes, analysis of the imaging data was performed by dividing each bifurcation into 8 areas, as shown in Figure 1B.
Malapposition of stent struts was calculated as described previously (12), and it was graded as 1) full apposition (no malapposition can be observed), 2) incomplete apposition (malapposition >0 μm), 3) marked malapposition (malapposition >200 μm), and 4) floating struts (malapposition >500 μm).
Micro computed tomography
Final stent deformations were visualized at a voxel resolution of 12 μm/voxel. From the scanned volumes, the stents were reconstructed by segmentation using Mimics (Materialise Inc., Leuven, Belgium).
The following parameters were quantified for all phantoms: ostial area stenosis in the distal MB, ostial area stenosis in the SB, wall coverage in the ostial SB, angulation between the MB and SB axes, and angulation between the MB plane and the plane of the protruding cell (string angle). Ostial area stenosis in the distal MB was quantified in a planar projection perpendicular to the MB axis according to the definition proposed by Ormiston et al. (13): (A1 − A2)/A1 × 100%, where A1 is the total area of the distal MB ostium and A2 is the largest area free of struts. Ostial area stenosis in the SB was quantified similarly (Figure 3). Loss of wall coverage in the ostial SB was quantified as (C2 − C1)/ C1 × 100%, where C1 is the area of the largest fitting circle in the most distal cell of the SB stent (regular deployment), and C2 is the area of the largest fitting circle in the most proximal cell at the SB (cell between the second and third rings of the SB stent, i.e., region of stent deformation).
Prospective human pilot registry
Patients with true bifurcation lesions, involving at least 1 of the MB segments plus the SB, with SB stenosis extending for ≥5 mm, were selected for single string procedure. The PCI procedure was performed according to the standardized protocol and procedural steps, as described for the in vitro study. Various stent brands were used: selection criterion was to have uniformly large cells of a minimum of 4.4-mm maximal cell expansion diameter. Procedural characteristics were recorded prospectively. Informed patient consent was obtained for the diagnostic and PCI procedures, data collection, and reporting. Results were evaluated by 3-dimensional quantitative coronary angiography (QCA). Angiographic images were acquired at 15 or 30 frames per second (Innova 4100, General Electric Inc., Fairfield, Ohio and Axiom Artis, Siemens Inc., Forchheim, Germany). Three-dimensional QCA was performed offline using QAngio XA 3D Research Edition 1.0 software package (Medis Specials BV, Leiden, the Netherlands) (14). This software allows the volumetric reconstruction of the luminal and reference diameters of the analyzed segments from 2 different projections at least 25° apart, preferably with the least foreshortening and yielding the best depiction of the stenotic coronary segments.
All analyses were performed with Prism GraphPad 5.0 (GraphPad Software Inc., La Jolla, California). Summary descriptive statistics are reported as mean ± SD, median (interquartile range), or n (%), as appropriate.
In vitro tests
All 20 in vitro procedures were successfully performed according to the protocol; OCT and mCT analyses were completed in each case. Procedure duration was 23.0 ± 7.9 min with a fluoroscopy time of 9.4 ± 3.5 min. It took in average 1:04 min (minimum, 0:40; maximum, 2:27) to position the master GW in the distal MB through the string cell. In 19 cases, the initially chosen workhorse guidewires were allowed to complete the procedure. On average, 2.1 ± 1.2 balloons were used per procedure, including 1 for the final proximal optimization. Of these dilation balloon catheters, 1.0 ± 1.2 were used for opening the string cell. Each procedure was completed with the 2 stents intended for use. Figure 4 shows examples from in vitro cases.
Optical coherence tomography
For the OCT runs obtained in 20 phantoms, 1402 OCT frames were analyzed, and 9,267 struts were identified for evaluation. Perfect apposition was seen in 8,040 struts (87%); 285 (3%) showed marked malapposition (>200 μm) and 96 (1%) were floating (>500 μm). Limiting the analysis to the bifurcation areas (area 2 to 6 as shown on Figure 1B), 6,328 struts were identified for evaluation. Of these, 4,368 struts (69%) were perfectly apposed, 276 (4%) showed marked malapposition (>200 μm) and 95 (2%) were floating (>500 μm). Within the bifurcation areas, marked malapposition was most frequently seen in the distal ostial area, namely, where a neocarina might have been created (16%), less frequently observed in the proximal ostial area (5%), and rarely found in other segments, namely, the SB ostial area (1%), the distal abostial (1%), and the proximal abostial (0.1%) areas of the main branch.
By per-case analysis, the rate of full apposition was 87.0 ± 7.1%. Limiting the analysis to the bifurcation areas, the rate of full apposition was 83.0 ± 9.2%, the rate of marked malapposition was 4.4 ± 5.3%, whereas the rate of floating struts was 1.6 ± 3.3%. In bifurcation areas, full apposition was the least frequent in the distal ostial area with 59.0 ± 26.0%, namely, where the neocarina might have been created. Results of detailed analysis are shown in Table 1 and Online Table 1.
Micro computed tomography
The mCT analysis confirmed that the single most proximal cell of the SB stent was crossed successfully in all the 20 cases. Single string procedures resulted in residual distal MB obstruction of 6.4 ± 5.6% area stenosis. In the ostial SB, post-stenting obstruction was 25.0 ± 16.9% area stenosis. In the ostium of the SB, the loss of wall coverage was found to be 16.0 ± 9.3%. Post-procedural angulation showed a negligible deviation (3.3 ± 2.2°; p = 0.34) from the manufactured 60° MB-SB angulation. The string angle was found to be 5.9 ± 6.5° with a maximum of 16.0° (Figure 5). The results of a detailed analysis are shown in Table 1 and Online Table 1.
Prospective human pilot registry
Of 11 patients, 8 were male, with mean age of 73 ± 11 years. Body mass index was 27 ± 5 kg/m2. The indication for performing a PCI was stable angina in 9 cases and acute coronary syndrome in 2 cases. Target bifurcation was located at left anterior descending artery and the diagonal branch in 10 cases and at left main stem in 1 case. Stenosis significance and indication for revascularization were justified by morphological and functional evaluation.
On 3-dimensional QCA, the area stenosis in the MB and in the SB was 65 ± 16% and 62 ± 16%, respectively. All the procedures were performed successfully according to the standardized protocol. Procedure duration was 110 ± 21 min with a mean fluoroscopy time of 25 ± 8 min. It took an average 3:24 min (minimum, 1:16; maximum, 5:57) to position the master GW in the distal MB through the string cell post-PCI 3-dimensional QCA measurements showed that the residual area stenosis in the MB and the SB was 27 ± 8% and 29 ± 10%, respectively. As shown on 3-dimensional QCA, native angulation of the bifurcation was 38 ± 19°, which was modified by 0 ± 16° (p = 0.99) after stenting (Online Table 2).
Equipment use consisted of 3 guidewires (IQR: 2 to 3) and 4 balloon catheters (IQR: 3 to 5). Contrast media use was 340 ± 118 ml. Figure 6 shows case examples.
Pre- and post-procedural cardiac enzyme levels showed no relevant periprocedural enzyme increase. Clinical follow-up was available for all patients at 6 ± 4 months post-PCI. During this period, no adverse clinical event occurred, including death, myocardial infarction, and target vessel revascularization. No patients reported any anginal symptoms during the follow-up period.
According to different calculations and observations, nearly 10% to 15% of all PCI procedures involve a clinically relevant bifurcation stenosis. Although provisional T-stenting represents a good solution for simpler anatomies, the proper treatment of more complex coronary bifurcation lesions remains a challenge. This is mainly due to their anatomic variation in terms of diameters, angulations, and the involvement by atherosclerotic disease. Various approaches have been developed for bifurcation stenting using conventional stent platforms. When transforming simple tubular shapes to a much more complex Y-shape, undesirable or suboptimal geometries appear such as stent distortion, overlap, uncovered spots, or malapposition. Numerous dedicated devices have been developed and tested to fill this therapeutic gap. However, due to the wide variations in bifurcation anatomy and pathologies, universally suitable dedicated bifurcation stents have not yet been developed, if they ever will be.
In this study, we evaluated in detail the single string bifurcation stenting technique, first reported by Kawasaki et al. (11). In brief, the first stent is implanted in the ostial SB, after lesion preparation and being positioned exactly at the proximal rim of the SB ostium, resulting in a single cell protruding to the MB. Then the GW can cross this single protruding cell. After predilation of this cell to optimal size, the MB stent can be positioned and deployed across it. After rewiring the SB, the procedure finishes with final kissing balloon dilation. Accordingly, we proposed the name single string because the single protruding cell results in only a fine string of strut, coming from the SB stent that is overlapping with the MB stent.
Conceptually, but also supported by data derived from detailed in vitro analysis, the single string technique may be superior to many, if not all, double-stent approaches because of the following reasons:
1. Overlap of stents is minimized to 1 single ring. However, it is important to realize that in case of extremely shallow angulations, the protrusion might be up to 2 cells, resulting in double-strut overlap. Compared with other conventionally applied techniques (culotte, crush, mini-crush), this level of overlap is still minimal.
2. Because the SB stent is literally pulled toward the MB while inflating the balloon in the single string, this portends optimal and full coverage of the SB ostium and the carina (unlike T-stenting and provisional T-stenting).
3. Malapposition and neocarina formation is minimal (unlike T and Protrusion stenting or kissing stent).
4. The single string technique is applicable for any size vessel, as well as any extent of atherosclerotic plaque (unlike dedicated devices).
5. The single string technique can theoretically be applied with any commercially available conventional stent, with sufficiently large cell designs, even though specific design features may offer better solutions than others.
At this point, data are not sufficient to decide which currently available stent platform might be better suited for application of the single string technique. In vitro comparisons with different commercially available stent platforms are ongoing. However, analysis of structural stent behavior suggests that the single string technique can be performed with most of the current platforms having a large open cell design. String angle data suggest that any stent, where the maximal expanded cell diameter reaches 4.4 mm or more, should be suitable for implantation in the SB stent, up to at least a 4-mm MB diameter.
From a practical point of view, testing showed that positioning of the SB stent must be performed with highest possible accuracy. For that reason, a projection has to be chosen where the bifurcation is maximally opened with possibly no overlapping of the distal branches. It is crucial to know where exactly the stent is folded on the delivery balloon. Note that the distance between the markers of the balloon and the edge of the stent shows relevant variation between different vendors. Using an extra-support hydrophilic master GW facilitates the crossing of the string cell. Conceptually, the latter step could be well controlled by OCT.
Recent studies suggesting that the physiological integrity of the vessel lumen plays a key role in the maintenance of normal flow patterns, shear stresses, and hemodynamics. Accordingly, the presence of any disturbance (i.e., oval shape, changed angulation, malapposed struts, localized stent deformation) might be associated with turbulent flow patterns and loss in driving pressure, impaired shear-stress pattern, and even abnormal platelet activation, leading to potential risk of adverse events (15–17). Increased risk of stent thrombosis or restenosis might be expected in case of suboptimal post-PCI results (18). In vitro tests with the selected stent platform demonstrated both high technical feasibility and excellent procedural results, i.e., minimal strut distortion, preserved vessel-wall coverage, optimal MB and SB openings at the ostium, favorable malapposition rates, and intact SB angulation. These findings suggest that post-procedural anatomy after a single string procedure can be ideally close to the native anatomic structure. Although detailed physiological evaluation was beyond the scope of this project and thus remains to be investigated, it can be anticipated that near-normal geometry will translate into hemodynamic and pathophysiological benefits.
The numbers in the presented first-in-humans pilot registry are limited, which must be taken into consideration while interpreting the derived data, but clinical cases remain informative: all the cases were performed successfully, within a reasonable length of time and using the usual amount of contrast and radiation exposure. Furthermore, post-procedural laboratory tests did not show clinically relevant periprocedural myocardial necrosis. It is important to emphasize that the single string technique is safe to perform because, in case of difficulties, the procedure can be immediately converted to the culotte technique (if the protrusion into the MB was unnecessarily long), the mini-crush technique (if the protrusion into the MB was too small), or T-stenting (if no protrusion into the MB was obtained at all).
Our in vitro results cannot be compared directly with other bifurcation techniques or dedicated devices, mainly due to the fact that such standardized and detailed evaluation with the same imaging protocol has not been universally performed. In vitro and in vivo studies with limited sample sizes investigating various bifurcation techniques reported malapposition rates in the region of bifurcation ranging between 30% and 45% (19–21), suggesting a potential advantage of the single string technique compared with those results. Of note, in vitro phantoms can never represent the variety of in vivo coronary anatomies and pathologies in terms of angulation, disease distribution, and calibers.
Due to geometric incoherence (i.e., the angulation between the plane of the OCT image and the true cross-sectional plane of the SB ostium), the accuracy of OCT in evaluating malapposition is hampered by marked overestimation, especially in the area of the carina. In those cases, reality might be better than measured.
Although feasibility was thoroughly tested in vitro and in humans, the number of clinical cases remains limited, and no long-term follow-up is available. Accordingly, medium- and long-term clinical safety, as well as more detailed procedural data such as procedure duration, radiation exposure and use of equipment, and intravascular imaging, will be evaluated within the context of an ongoing prospective registry. In the first-in-humans pilot registry, the protocol did not include standard use of intravascular imaging methods, such as OCT. Therefore, despite optimal crossing of the SB stent on fluoroscopy, it cannot be confirmed that the most proximal cell was taken in all cases. No randomized comparison with any other bifurcation technique or dedicated device is available yet either. Being a proof-of-concept study, comparison with any other bifurcation technique or dedicated device either in vitro or in human cases was beyond the scope of this work.
Bifurcation PCI remains challenging because any technique needs to find the optimal balance between wall coverage, stent overlap, and malapposition, whereas adapting to variable anatomy. In this proof-of-concept study, single string bifurcation technique was shown to be feasible in vitro and in humans as well. Data indicate minimal overlap, maximal wall coverage, and a favorable malapposition rate. Clinical effectiveness remains to be confirmed in a larger study population.
WHAT IS KNOWN? Percutaneous coronary intervention of complex bifurcation stenosis is still a challenge for interventional cardiologists. Evolution of bifurcation interventions resulted in several approaches, using single-stent techniques, double-stent techniques, or dedicated devices, but the best choice remains unclear. The main limitations of each of these techniques are due to superimposition of multiple metal layers and frequent stent strut malapposition. The introduction of novel, dedicated stents aimed at eliminating these limitations; however, dedicated devices lack the anatomic conformability and flexibility of regular stents and may not be available as drug-eluting devices.
WHAT IS NEW? In this study, we evaluated in detail the single string bifurcation stenting technique. In brief, the first stent is implanted into the ostial side branch, resulting in a single cell protruding to the main branch. Then the main branch stent is deployed across this single cell, and the procedure is completed by final kissing dilation. The concept ensures full coverage of the whole bifurcation area in both branches although having a minimal double layer as a single string of a strut. In this proof-of-concept study, the single string bifurcation technique was shown to be feasible in vitro and in humans as well. Data indicate minimal overlap, maximal wall coverage, and a favorable malapposition rate.
WHAT IS NEXT? Although in vitro and the first-in-humans data are encouraging, further more detailed in human evaluation of the technique is essential. Systematic intravascular imaging will be applied within the context of an ongoing prospective registry. Clinical effectiveness and comparison with other bifurcation techniques remain to be confirmed in a larger study population.
The authors thank Medis Inc. (Leiden, the Netherlands) for providing the QCA software.
Testing equipment, including guidewires, stents, and balloon catheters were provided by Abbott Laboratories Inc. (Abbott Park, Illinois), Biosensors Interventional Technologies Ltd. (Singapore), Biotronik SE & Co. (Berlin, Germany), Boston Scientific Inc. (Natick, Massachusetts), and Medtronic Inc. (Minneapolis, Minnesota) without financial involvement or intellectual restriction. Drs. Verhegghe and Mortier are cofounders of FEops. Dr. Adjedj is supported by a grant from Federation Française de Cardiologie. Dr. De Bruyne has received institutional grant support and consulting fees from St. Jude Medical. Dr. De Beule is a shareholder in FEops. Dr. Wijns has received institutional grants from Abbott Vascular, Biosensors Interventional Technology, Biotronik, Boston Scientific, and Medtronic. All other authors have reported that they have no relationships relevant to the contents of this paper to disclose.
- Abbreviations and Acronyms
- main branch
- micro computed tomography
- optical coherence tomography
- percutaneous coronary intervention
- quantitative coronary angiography
- side branch
- Received September 29, 2014.
- Revision received January 20, 2015.
- Accepted January 29, 2015.
- American College of Cardiology Foundation
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