Clinical and Biomechanical Behavior of a Platinum-Chromium Stent Platform in a Large All-Comer Single-Center Population: Insights From the Novara-PROMETEUS Registry

Abstract: Aims. Longitudinal deformation has been described as a new complication affecting new-generation thin-strut coronary stents. Benchmark tests have suggested that the platinum-chromium (PtCr) Element coronary stent platform (Boston Scientific) might present increased susceptibility to this complication. Our study assessed the incidence of longitudinal stent deformation in a large single-center study population. Methods and Results. A total of 337 consecutive Promus Element PtCr stents deployed in an all-comer population underwent quantitative coronary angiography (QCA) analysis. Postdeployment QCA measured/nominal stent length ratio (SLR) was considered as a surrogate estimate of longitudinal stent deformation and averaged 0.95 ± 0.04 in the entire population. This small postdeployment reduction of stent length had no clinical relevance, leading to 3 cases (0.9%) of trivial geographical miss, which did not require further interventions. Plaque prolapse through the stent struts was observed in 19 cases (5.6%). Only 1 case of typical “concertina” effect (0.3%) complicated an ostial stenosis treatment requiring deployment of a second stent, while in 3 cases (0.9%), stent struts adapted to severely tortuous and calcified vessels mimicked longitudinal stent deformation, without further complications. Multiple regression analysis demonstrated a significant correlation between need for predilation and lower SLR values, while postdilation independently predicted higher SLR. Conclusions. Systematic QCA analysis of a large single-center all-comers PCI population treated with PtCr stents failed to detect any clinically relevant longitudinal stent deformation. Complex lesions needing predilation were associated with a reduced SLR; conversely, postdilation was associated with QCA stent measures close to nominal. Clinicaltrials.gov ID: NCT01759719.

J INVASIVE CARDIOL 2014;26(7):311-317

Key words: longitudinal stent deformation, stent, platinum/chromium, coronary angioplasty

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In the last years, the need to deal with increasingly complex coronary anatomy has polarized the effort of biomedical research toward improvement of stent crossing profile, conformability, and x-ray opacity.¹ In particular, longitudinal connectors between stent cells and strut thickness have been the main targets of stent engineering, as they profoundly affect stent longitudinal flexibility and deliverability, as well as vessel conformability and cell characteristics after deployment.² Stent materials have also changed, with the introduction of high-tech alloys like Cobalt/Chromium (CoCr) or Platinum/Chromium (PtCr),^3,4 allowing further miniaturization of stent struts without compromising radial strength.

Overall performance of stents, however, basically depends on a trade-off between crossing profile and flexibility and radial and longitudinal forces exerted after deployment.⁵ Newer-generation stents have been characterized by important improvements in strut thinning and flexibility while maintaining radial strength, and yet the very low metal density per mm³ of stent volume has eventually put the issue of longitudinal strength in the spotlight.^6,7

Among the latest developed stent platforms, the PtCr Element (Boston Scientific) has demonstrated good overall clinical results in the PERSEUS and PLATINUM programs.^8-11 However, several reports^12-26 recently suggested the susceptibility of this platform to an excessive longitudinal deformation during or following stent deployment, compared to other stent designs.²⁷

Published material about this phenomenon, however, is mainly limited to case reports/case series^17-25 or bench testing studies,^26-28 while only a few retrospective clinical registries have systematically explored a patient cohort^14-16 and only one has employed quantitative coronary angiography (QCA) to accurately test deployed stent dimensions.¹⁴

We prospectively designed the Novara-PROMETEUS (Novara Platinum CROMium Everolimus EluTing StEnt SpontaneoUs RegiStry), Clinicaltrials.gov ID: NCT01759719, with the purpose of analyzing both the incidence as well as factors associated with longitudinal stent deformation in an all-comer population treated by the new generation of PtCr Promus Element stent.

Methods

Device description. The PtCr Element bare-metal stent platform is laser cut from a tube of platinum chromium alloy with an offset peak-to-peak design and two links connecting adjacent rings,²⁸ with a stent strut thickness of 81-86 µm according to the nominal caliber. The PtCr Promus Element is an everolimus-eluting stent based on the Element stent platform in which the antiproliferative agent everolimus (100 µg/cm²) is applied in a biocompatible acrylic polymer and fluorinated copolymer.^29,30 Stent strut thickness is 81 to 86 µm of the stent struts plus 14 to 18 µm of the polymeric coverage.

Patient population. Novara-PROMETEUS is a single-center registry enrolling unselected patients undergoing PCI with a Promus Element.

Inclusion criteria were: (1) treatment with percutaneous coronary intervention and at least one Promus Element stent implantation; and (2) availability of high-quality coronary angiographic documentation in the same projection of the baseline lesion and the final result with the deployed stent.

The only exclusion criterion was an angiographic documentation deemed unsuitable for QCA analysis (lack of baseline and final angiography of the treated segment in the same projection, excessive foreshortening, insufficient contrast to detect stent edges, lack of guiding catheter for appropriate calibration).

Qualitative coronary angiography evaluation. Qualitative inspection of all the angiograms was performed by 2 independent expert reviewers (AL and AR). The presence of obvious stent shortening or stent elongation due to stent struts being pushed together or pulled apart (“concertina” effect), other minor stent deformations, intrastent plaque prolapse, and geographical miss were recorded. For this visual assessment, longitudinal compression/elongation was defined as inconsistency in the radiodensity pattern along the length of the stent, or other gross irregularities or deformities determining distortion or shortening of a stent in the longitudinal axis following successful stent deployment.¹³Geographic miss was defined when the deployed stent could not fully cover the entire length of the diseased or injured segment.³¹ Divergent opinions were resolved by consensus.

Quantitative coronary angiography (QCA). As routine in our center, standard image acquisition of the treated stenosis was performed using two or more angiographic projections, intracoronary nitroglycerin to provide maximum coronary dilation, and repetition of identical angiographic projections of the lesion at baseline and final angiography. Angiograms were analyzed on site by two independent reviewers (AL and DDV). Using the contrast-filled injection catheter as the calibration source, quantitative angiographic analysis was performed using a validated automated edge detection algorithm (QAngio XA; Medis).³² The best angiographic projection that minimized stent foreshortening and vessel overlap was used for analysis. Lesion length, defined as the distance from the proximal to the distal shoulder of the lesion, minimal lumen diameter (MLD), and percentage of diameter stenosis at the site of stent placement were evaluated for each lesion before and after the coronary intervention.

For each stent, final length measured by QCA was compared to the nominal stent length (stent length ratio; SLR). A ratio of 1.0 would indicate equivalent measured and nominal stent lengths. In cases of overlapping stents, only the first deployed stent was considered for the length analysis. The excellent radiopacity of the Promus Element stent allowed precise QCA sizing with clear identification of the overlapping zones.

Three-dimensional quantitative coronary angiography (3D-QCA). A 10% random sample from the study stent pool underwent 3D-QCA evaluation according to a validated protocol.³³ Available angiographic projections targeting the stent at the end of each procedure were analyzed by two interventional cardiologists (GLDM and IP) blinded to clinical data. Two electrocardiographically gated end-diastolic frames in two unmagnified views (one in each view) separated from each other by at least 30° (in either lateral or cranio-caudal planes) were selected. In each single biplane image set, two clearly identifiable reference points corresponding to proximal and distal stent edges were defined. Three-dimensional reconstruction of the stented coronary artery segment was obtained by CAAS QCA-3D system (Pie Medical Imaging BV), starting from the selected set of biplane angiographic images. After indicating a so-called “common image” point, which should be unambiguously visible in both projections, a path through the lumen covering the segment to be analyzed was designated. The luminal borders on both projections were found by automatic contour detection and manually corrected when required. The software then automatically generated a three-dimensional representation of the arterial lumen, giving a measure of the stented segment length.

Evaluation of outliers. Finally, a third expert interventional cardiologist (ASB) performed a blinded reexamination of the angiograms corresponding to the 20 lowest and highest ratios to identify any case of even mild stent deformation.

Statistical methods and ethical issues. Categorical data were presented as counts and percentages and compared by χ² test. Continuous data with normal distribution were expressed as mean ± standard deviation (SD) and compared by t-test. Non-normally distributed data were expressed as median and interquartile range and compared by Wilcoxon signed rank sum test. To test whether a significant change occurred in repeated measurements of QCA data before and after intervention, ANOVA followed by Newman Keuls test for repeated measurements was used. When data did not fit normal distribution, they were compared by Kruskal-Wallis test. Correlation between QCA and 3D-QCA was assessed with the Pearson’s correlation coefficient.

Multiple regression analysis was performed to determine independent predictors of Pt/Cr QCA SLR. The covariates tested in this model were male sex, age, hypertension, diabetes, dyslipidemia, body mass index (BMI) >30 kg/m² of body surface area (BSA), estimated glomerular filtration rate (eGFR) <30 mL/min, cigarette smoking (active or quit <2 years prior), previous myocardial infarction (MI) and/or PCI, previous coronary artery bypass graft, peripheral artery disease, multivessel CAD, left main, type B2/C lesion, multivessel PCI, need for predilation, high-pressure postdilation, number of implanted stents, total stent length (mm), presence of thrombus, ostial lesions, tortuous lesions, lesions longer than 20 mm, bifurcations, severely calcified lesions, chronic total occlusion (CTO) lesions, kissing balloon dilations, maximum inflation pressure (atm), deep intubation with guiding catheter, use of intravascular ultrasound (IVUS), use of thrombectomy catheters, use of Rotablator (Boston Scientific), and use of distal filters.

Univariate predictors that achieved a P-value <.10 were included en bloc in the multivariate model. All statistical tests were 2-tailed and a P-value <.05 was considered statistically significant. The investigation conforms to the principles outlined in the Declaration of Helsinki.³⁴ All patients provided written informed consent. The study protocol was approved by the institutional review board of the “Maggiore della Carità” University Hospital and is registered at https://www.clinicaltrials.gov (NCT01759719).

Results

This analysis included 337 PtCr stents deployed in 253 consecutive all-comer patients treated in our laboratory from January 2011 to August 2012 (Figure 1). Clinical and interventional characteristics are summarized in Table 1. A moderate-to-high general profile of risk could be observed, with a significant prevalence of diabetes, peripheral artery disease, multivessel atheroma, and left main disease. Most interventions were performed transradially, with an average of 1.33 Promus Element stent deployed per patient.

Lesion characteristics are summarized in Table 2. Most of the 286 treated lesions showed a high degree of complexity. About 15% of the lesions were ostial, and two-thirds were longer than 20 mm. True bifurcational disease was also well represented. As a result of such complexity, there was a relatively frequent use of IVUS and thrombectomy catheters (about 1 in 10).

Measurement of QCA stent length: stent length ratio. Stent length ratio averaged 0.95 ± 0.04 in the whole population, with maximum and minimum values of 1.07 and 0.73, respectively (Figure 2). The cumulative distribution curves of SLR showed similar ratios according to various stent diameters and lengths (Figure 3).

The small postdeployment reduction of stent length had no clinical relevance, with only 3 cases (0.9%) of small geographical miss, which did not require further intervention. Only 1 clear case of concertina effect was observed (0.3%) (Figure 4), requiring a second stent implantation, while in 3 cases (0.9%) close fitting of the stent struts to very tortuous and calcified lesions mimicked longitudinal stent deformation (“pseudoconcertina” effect).

Blinded reexamination of the angiograms corresponding to the 20 lowest and highest SLRs did not identify any cases of stent deformation. These extreme measured-to-nominal ratios appeared to result from inherent QCA variability, out-of-plane magnification from the calibration source, and foreshortened projections.

Measurement of 3D-QCA stent length: stent length ratio. Of the 337 stents evaluated with QCA, a total of 34 stents (10.0%) underwent 3D-QCA analysis. The clinical and angiographic characteristics of this sample were similar to the whole population of the study. Measured nominal SLRs obtained through the 3D-QCA analysis averaged 0.95 ± 0.09, with maximum and minimum values of 1.11 and 0.74, respectively. 3D-QCA measurements significantly correlated to standard QCA measures (r=0.63; P<.001).

Multiple regression analysis. Multiple regression analysis was performed to identify independent predictors of SLR in the registry population. Need for predilation was associated with lower SLR values, while postdilation predicted higher final SLR (Table 3) in both univariate and multivariate analyses.

Discussion

The main result of our all-comer registry of patients treated with a Pt/Cr Promus Element stent is that severe longitudinal stent deformation is an extremely rare phenomenon. In the sample evaluated in our study, SLR, a surrogate marker for longitudinal stent deformation, was substantially similar to those previously reported in literature.¹⁴ Of note, the anatomical complexity of the population of our study far exceeded that of the currently available randomized controlled trials,^8,10,11 increasing the practical value of our observation. Interestingly, the small differences recorded in our study between nominal and postdeployment longitudinal stent dimensions did not translate into a significant increase in geographical miss requiring unplanned interventions.

These results, obtained with standard QCA, were corroborated by 3D-QCA analysis, confirming measured to nominal SLRs similar to the main analysis. Of note, 3D-QCA is considered the best approximation for measuring stent length in vivo.³⁵

Finally, we observed that the need for predilation was an independent predictor of a lower SLR, possibly as a proxy for lesion complexity. Furthermore, postdilation was demonstrated to be independently associated with higher final SLR. 

Longitudinal deformation: a “necessary” defect of modern stents? Modern coronary stents have made it possible to treat extreme cases of coronary obstructive disease without resorting to surgery. However, the task to face complex coronary anatomy has prompted biomechanical researchers to maximally improve stent properties such as profile, conformability, cell design, and x-ray opacity.¹ Longitudinal connectors and strut thickness have been the preferred target of stent engineering,⁷ as they profoundly condition stent longitudinal flexibility and deliverability before deployment, as well as stent conformability to vessel and cell size and shape after deployment. Stent materials have also significantly changed with the introduction of high-tech alloys like Co/Cr or Pt/Cr, with further reduction of stent strut thickness.³ Performance of stents, however, ultimately depends on a trade-off between low crossing profile and flexibility on one side, and the radial and longitudinal forces exerted by the expanded stent on the other.⁵ Thus, while new-generation stents have reached excellent deliverability, concerns about the radial strength of the scaffolding offered to the vessel wall have developed.

Manufacturers have meticulously addressed the problem of radial strength, which is generally maintained despite the reduction of metal density,⁶ and yet data about stent longitudinal strength (eg, resistance to compressing or elongating forces) is seldom found in the technical documentation of novel stents.

Longitudinal stent deformation: real-world occurrence versus selection bias. Reports of longitudinal stent deformation, consisting generally of stent shortening and elongation,^13,15 were thus somewhat expected, as it is conceivable that the combination of fewer connectors and thinner struts may have adversely affected stent longitudinal integrity. Stent distortion has usually occurred after deployment, but also during positioning, high-pressure postdilation, or withdrawal in the guiding catheter.^{13,17,18,36,37} Recently, benchmark evaluation comparing the four most diffuse stent design families associated stent susceptibility to longitudinal compression with stent design, particularly to the two-link offset peak-to-peak design.²⁸

In contrast with these data, a pooled analysis from the PERSEUS and PLATINUM randomized controlled studies involving more than 2400 stents has demonstrated only a modest length reduction after deployment.¹⁴ The analysis by Kereiakes et al, however, suffers from the limitations of most randomized trials, comprising mainly moderate-risk patients and low-to-medium lesion complexity. In particular, their sample included 25% diabetics, 21% American College of Cardiology/American Heart Association (ACC/AHA) type-C lesions, 25% calcified lesions, 3% tortuous lesions, and mean lesion length of 14 mm, which are characteristics not representative of a real-world population. Moreover, in this selected population, postdilation was used in only 50% of lesions with a mean pressure of 16 atm, ST-elevation MI was excluded, while no data about IVUS and filters were reported.

Novara-PROMETEUS adds to and extends Kereiakes’s data. In our cohort, 41.7% had  diabetes was, 51.7% had ACC/AHA type-C lesions, 37% had calcified lesion, mean lesion length was 21 mm, and ST-elevation MIs were included. Of note, a significant proportion of patients were treated with bulky devices, such as IVUS (11.6%), aspiration thrombectomy (11.0%), and filters (4.5%), which have been previously addressed as potential causes of stent deformation or fracture.¹³ Also, postdilation was very frequent (two-thirds of patients), and performed with very high pressures (>20 atm on average). A significant fraction of patients also required deep intubation, with the guiding catheter passing through the deployed Pt/Cr stent, to increase back-up for distal stent passage. That longitudinal stent deformation remains a rare phenomenon in this all-comer real-world population, without a clear clinical effect, is quite reassuring.

Risk factors for longitudinal deformation. It is noteworthy that of the cited potential risk factors for stent deformation, none except lesion length correlated to SLR when considered singularly. Multiple regression analysis identified only the need for predilation as an independent predictor of low SLR, while postdilation was significantly and independently correlated with a reduced longitudinal deformation. The most reasonable explanation for our findings is that both variables acted as proxies for lesion complexity, linked to higher risk of incomplete stent expansion and consequently to stent deformation. A causative effect for the absence of postdilatation, however, cannot be fully excluded, supporting our practice of an (almost) systematic postdilatation of implanted stents with non-compliant balloons.

Study limitations. First, we are aware that using angiography has limitations in the evaluation of stent deformation. Indeed, the longitudinal stent deformation definition is based on a pattern of dense or overlapping stent struts, and the ratio of measured/nominal stent length can only be considered a surrogate marker of inappropriate stent expansion over the longitudinal plane. Longitudinal stent deformation may occur without significant change in stent length and may be detectable only by IVUS.¹⁸ It must be noted, however, that in our study both visual evaluation of stent strut deformity and QCA analysis were performed, with evidence of a very low rate of longitudinal stent deformation, similar to previously published data.¹⁴ Moreover, the subanalysis using 3D-QCA is in full agreement with the main results. 

Furthermore, QCA was performed by trained personnel (at least a decade of experience in QCA) at the core lab of our institution, and not by an independent core lab. 

Finally, our analyses could have been affected by the typical biases affecting QCA studies,³² and mainly depending on angiographic angulations, leading to foreshortening and resulting in low SLRs. However, to minimize bias, only patients who had at least 3 cine loops (baseline, after stent deployment and after postdilation) in the same angiographic projection were included in the analysis. Moreover, the independent confirmation by 3D-QCA, known to significantly reduce two-dimensional biases,³⁸ is reassuring.

Conclusion

Our study demonstrates that longitudinal stent deformation, a recently recognized complication of coronary stent deployment, is rare in an all-comer population treated with PtCr stents at a large tertiary center. Multivariate analysis confirmed the importance of lesion complexity for longitudinal deformation. 

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From the ¹Cardiology Department, Maggiore della Carità Hospital, Novara, Italy; ²Cardiology Department, San Donato Hospital, Arezzo, Italy; ³Division of Interventional Cardiology, Ospedali Riuniti Marche Nord, Pesaro, Italy; ⁴Cardiovascular Medicine Department, Catholic Unversity of the Sacred Heart, Rome, Italy; ⁵Interventional Cardiology, Division of Cardiology, University of Turin, Turin, Italy; ⁶University Medical Center Utrecht, Utrecht, The Netherlands; and ⁷Department of Clinical and Experimental Medicine, University of Eastern Piedmont, “Maggiore della Carità” Hospital, Novara, Italy.

Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. The authors report no conflicts of interest regarding the content herein.

Manuscript submitted October 7, 2013, provisional acceptance given January 9, 2014, final version accepted January 29, 2014.

Address for correspondence: Dr Alessandro Lupi, Cardiologia ospedaliera, Ospedale Maggiore della Carità, Cso Mazzini 18, 28100 Novara, Italy. Email: lupialessandro1@tin.it