Role of the “Dogbone” Effect of Balloon-Expandable Stents: Quantitative Coronary Analysis of DUET and NIR Stent Implantation Int

Stent implantation has made its way into widespread clinical use since improvements in stent design and stent delivery systems facilitated deployment even in coronary lesions that are difficult to access. A variety of different stent designs are now available, which may, along with their delivery systems, account for different acute performance characteristics and clinical outcomes. Rieu et al. recently showed that the radial force of stenting determining elastic recoil of the coronary arteries varies considerably among different types of stent.1 The Multi-Link DUET stent (Guidant Corporation, Santa Clara, California) is the next generation of the Multi-Link stent. The DUET stent design features a wider strut-strut distance, more radio-opacity, better flexibility and better radial strength.2 The NIR on Ranger stent (Boston Scientific/Scimed, Inc., Maple Grove, Minnesota) was approved by the Food and Drug Administration for intracoronary use in 1998, and preliminary reports suggest similar or better acute results compared to older generation NIR stents.3–5 There are differences in stent design between the DUET and NIR stents. The DUET stent has a 3-2-3 linked corrugated ring structure, yielding an unsupported cell surface area of 4.0 mm2. The NIR stent has a truly “closed cell” structure, with only 2.2 mm2 of unsupported cell surface area. The other structural difference is the strut thickness (0.055´´ for the DUET and 0.040´´ for the NIR stent). Because the outcome of stent implantation may depend not only on the stent design but also on the deployment and expansion capacity of the stent delivery balloon, we compared the acute performance characteristics of these two second-generation, tubular, stainless-steel stents using quantitative coronary analysis (QCA). In this study, we introduce two new angiographic parameters of stent delivery performance: 1) the stent delivery balloon expansion ratio (SDBR) or “dogbone” ratio; and 2) the stent “scalloping” score to account for plaque protrusion through the stent struts. METHODS Inclusion and exclusion criteria. In this prospective study, the 3.0 and 3.5 mm diameter ACS Multi-Link DUET or NIR on Ranger stents were chosen for stent implantation by different operators; stent choice was left to the discretion of the operator. Inclusion criteria for the study were: 1) patients with coronary artery disease (CAD) who were treated with either the Multi-Link DUET or the NIR on Ranger stent delivery system at Borgess Medical Center; 2) patients with significant (> 50%) de novo lesions or restenosis in the coronary arteries; and 3) one stent in the coronary segment was analyzed per patient. Exclusion criteria for the study were: 1) stent deployment after recanalization of chronically occluded coronary arteries; 2) stent deployment in bypass grafts; 3) stent deployment in the setting of myocardial infarction; 4) stent deployment for treatment of in-stent restenosis; 5) multiple stents deployed in a single coronary artery; 6) coronary lesion length > 30 mm; 7) coronary lesion diameter Standard protocol for stent implantation. All operators were required to predilate the coronary lesion with one balloon inflation using a balloon diameter equal to or 0.5 mm less than the reference diameter of the coronary artery. The predilation had to be performed with the aim of obtaining an angiographic residual stenosis Inter- and intra-observer variability and reproducibility. In a cohort of 30 patients, intra-observer variability was low. A correlation coefficient (R2) of 0.996 was obtained for all QCA measurements by two observers. Intra-observer variability was low, with an R2 of 0.993 for the QCA measurements. Reproducibility of the QCA results using the MEDIS system was high, with an R2 of 0.995 against standard measure. Statistics. Comparisons of the data were performed by factorial ANOVA using Scheffe’s correction. The Chi-square test was used when appropriate. A multivariate regression analysis was used to evaluate the association of selected parameters with residual stenosis after stenting. A p-value Demographics and procedural data (Table 1). From September 1998 to September 1999, a total of 50 DUET stents and 50 NIR stents were implanted in 100 patients with CAD meeting the entry criteria (66 males, 34 females). The mean patient age was 66 ± 13 years. Forty-one left anterior descending coronary artery (LAD), thirteen left circumflex artery (LCX) and 45 right coronary artery (RCA) lesions were treated using either 3.0 or 3.5 mm diameter DUET or NIR stents and stent delivery systems. All coronary lesions were predilated with a conventional angioplasty balloon. Predilation was performed according to a standard protocol with a balloon size chosen to be 0.5 mm smaller than the visual estimate of the coronary artery reference diameter. Only 18 or 23 mm length DUET stents and 16 or 25 mm length NIR stents were included in the study. The mean lesion length was 14 ± 5 mm in the LAD, 15 ± 5 mm in the LCX and 15 ± 5 mm in the RCA (p = not significant). Mean deployment pressures during stent implantation were 14 ± 2 atm for the DUET stents and 13 ± 2 atm for the NIR stents (p = not significant). The calcium score was not different between the two groups (1.0 ± 0.7 prior to implantation of DUET stent and 0.9 ± 0.7 prior to implantation of NIR stent; p = not significant). There were no differences in calcium scores between the LAD, LCX or RCA lesion groups. Baseline quantitative coronary angiographic analysis (Table 1). Mean baseline reference diameters were 2.96 ± 0.65 mm for the LAD, 2.87 ± 0.36 mm for the LCX and 3.0 ± 0.4 mm for the RCA (p = not significant). The reference diameters were 3.0 ± 0.4 mm in patients receiving DUET stents and 3.0 ± 0.4 mm in patients receiving NIR stents (p = not significant). The coronary artery percent stenosis was 66 ± 13% at baseline prior to implantation of a DUET stent and 65 ± 15% prior to implantation of a NIR stent (p = not significant). SDBR and final percent diameter (residual) stenosis (Figures 1–3). The extent of SDBR (“dogbone” effect) after stenting using both types of stent delivery system significantly correlated with the magnitude of residual stenosis after stenting. SDBR and residual stenosis after stenting were higher with the DUET stent versus the NIR stent (15 ± 5% versus 12 ± 5% and 14 ± 5% versus 11 ± 6%, respectively; p Multivariate analysis. Several demographic and procedural variables were analyzed to predict the extent of residual stenosis after stenting. The multivariate regression analysis included: 1) patient age; 2) baseline percent stenosis; 3) lesion length; 4) calcium score; 5) stent recoil; 6) scalloping score; and 7) SDBR. SDBR and elastic recoil were predictive of final residual percent diameter stenosis after stenting (r = 0.45 and p Comparison with other stent studies. In this study, the acute outcome of stent implantation was evaluated for a community-based interventional cardiology department. Implantation of DUET and NIR stents was performed by several interventional cardiologists with no predetermined implant guidelines except those given by the protocol, as one may encounter in a sponsored investigational trial. The films were evaluated consecutively by the order of the procedural data. The data of this study therefore represent the “real world” of stenting, which may explain why this study failed to show acute results of stenting as excellent as those reported in previous controlled investigational trials. Because the overall number of coronary lesions evaluated by QCA was small, a lesion classification for each individual coronary artery was not reported. Although a recent study suggests that the American College of Cardiology/American Heart Association stenosis morphology classification may influence the long-term angiographic and clinical outcomes after stenting,7 the value of this lesion subtyping when comparing different stents will require further investigation. Preclinical animal studies clearly suggest that stent design influences acute and late outcomes after stent implantation;8 however, the moderate-sized stent equivalency trials have failed to show significant differences in clinical endpoints. Equivalency studies, however, may be an insensitive means for detecting minor but important differences in stent design, as was pointed out by Edelman et al. in a recent editorial.9 In equivalency trials, new stents are compared to a “standard” stent, but not in a challenging subset of patients with complex lesion morphology. These studies are also not powered to detect small but potentially significant differences in clinical outcomes. The first studies with the Multi-Link stent reported excellent deployability.10 Nakano et al. reported a high procedural success rate and a low rate of late angiographic restenosis in a series of patients treated with the Multi-Link stent.11 Others also reported a low percentage of residual stenosis after deploying the Multi-Link stent, although the majority of implanted stents did not meet intravascular-ultrasound defined criteria for “optimal stenting”.12 The ASCENT trial, a multicenter randomized study that compared the Multi-Link stent to an older-generation stent (the Palmaz-Schatz stent), reported a lower in-stent residual stenosis of 8 ± 11% after Multi-Link stent deployment in 520 patients.2 The Multi-Link DUET stent is the next generation of the Multi-Link stent. A prospective, non-randomized multicenter registry conducted by 17 United States centers in 269 patients using the Multi-Link DUET stent in patients with de novo coronary lesions suggested an improvement in post-procedural percent residual stenosis to 6.2 ± 10% with the DUET stent.2 In the NIRVANA equivalency trial, which compared the NIR Primo (n = 420 patients) to the Palmaz-Schatz (n = 429 patients), the final diameter stenosis after implantation of the NIR stent was 7 ± 10%. The 6-month angiographic restenosis rate of the NIR was comparable to the Palmaz-Schatz stent in the NIRVANA trial.13 This study shows that acute performance of the NIR stent delivery system is superior to the DUET stent delivery system, even though both utilize low-profile, semi-compliant balloons. It is possible that some or all of the stent deployment problems with the DUET system will be improved with the Tristar™ stent delivery system. Further randomized studies are currently underway to better evaluate the role of a stent and its respective stent delivery balloon in acute and late clinical outcomes. As direct stenting becomes more routine, differences in the acute performance of stents and their delivery systems may become even more relevant to clinical outcomes. Study limitations. Larger series of patients will be required to compare clinical endpoints associated with the implantation of the DUET and NIR stents. Thus, other studies may find that differences in performance characteristics of stents will not result in different clinical outcomes. In addition, new indexes such as those introduced here require further validation in larger trials. Intravascular ultrasound was not used in this study to differentiate the acute performance characteristics of the DUET and NIR stents and their delivery systems. Stent malapposition or plaque protrusion through stents can be more precisely evaluated with intravascular ultrasound, which is considered to be the “gold standard” for the evaluation of stent performance. However, the purpose of this study was to introduce novel criteria for differentiating stent performance from angiographic films in catheterization laboratories, where intravascular ultrasound is not available. Acknowledgments. Supported in part by grants from the Borgess Heart Institute, Kalamazoo, Michigan; this work was conducted during Dr. Hehrlein’s tenure as a visiting professor at the Institute from 1998–1999.

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