It is not clear whether the thin struts and different alloy of a cobalt chromium stent will cause greater acute stent recoil compared to conventional stainless steel stents. We used postintervention intravascular ultrasound (IVUS) examinations to study 99 patients with 116 stented lesions: 61 Xience/Promus stents (cobalt chromium stent group) and 27 Taxus Liberté and 28 Cypher stents (stainless steel stent group). The IVUS images were obtained before and immediately after stent implantation with only the stent-delivery balloon. The ratio of the IVUS-measured to manufacturer-predicted stent diameter and area was the measure of acute stent recoil and expansion. The baseline patient characteristics, lesion morphology, and procedural details were comparable between the 2 groups. The ratio of the IVUS-measured to manufacturer-predicted stent diameter and area was 0.74 versus 0.73 (p = 0.57) and 0.63 versus 0.63 (p = 0.69), respectively, for the cobalt chromium and stainless steel stents. In conclusion, the acute performance of Xience/Promus was similar to that of previous stainless steel stents, and the thinner cobalt chromium metallic platform did not compromise the radial strength of the stent.
Bare metal and drug-eluting stent (DES) underexpansion has been related to stent thrombosis and restenosis. Although first-generation DESs were based on a stainless steel platform, some second-generation DESs have had a cobalt chromium stent backbone. Concerns have arisen that cobalt chromium stents will have greater acute recoil than stainless steel stents, leading to a greater incidence of underexpansion and a smaller minimum stent diameter (MSD) and minimum stent area (MSA). Manufacturers have provided a compliance chart reporting the implanted stent diameter as a function of the nominal stent size and deployment pressure. Although these charts were determined from in vitro measurements typically done in water, interventional cardiologists have routinely relied on these charts to optimize the stent dimensions according to stent size and inflation pressure during percutaneous procedures. We have used the ratio of the intravascular ultrasound (IVUS)-measured stent diameter and area to that predicted by the manufacturers’ compliance chart as a measure of acute stent expansion, including the effect of both expansion of the stent delivery balloon and acute stent recoil. On average, stainless steel bare metal stents and DESs achieved only 75 ± 10% of the predicted MSD and 66 ± 17% of the predicted MSA. This was similar for the sirolimus-eluting Cypher stent and the paclitaxel-eluting Taxus Express stent and almost identical to their bare-metal stent equivalents. From this previous work, we assessed the ratio of the IVUS-measured to manufacturer-predicted MSD and MSA, comparing the first-generation stainless steel and second-generation cobalt chromium DESs.
Methods
From January 2008 to June 2009, 99 patients with 116 stented lesions were enrolled in the present study. The inclusion criteria were a de novo native coronary artery lesion treated with a single 2.5- to 3.5-mm drug-eluting stent with IVUS performed immediately after stent implantation using only the stent delivery balloon. Patients with repeat stenosis lesions, lesions in bypass grafts, multiple/overlapping stents, or pretreatment with an atherectomy or plaque modification device were excluded. Our institutional review board approved the present study.
Stent implantation was performed according to standard techniques. All patients received heparin or bivalirudin during the procedure, with clopidogrel 300 mg as a loading dose before the procedure followed by clopidogrel 75 mg daily for ≥1 year, and daily aspirin. The final maximum deployment pressure was obtained from the physician’s report, and the predicted stent diameters were derived from the manufacturer’s compliance charts.
IVUS was performed before and after stent implantation using a commercially available IVUS imaging system (Boston Scientific, Fremont, California, or Volcano, Rancho Cordoba, California) with an automatic transducer or catheter pullback at 0.5 mm/s. Before the IVUS catheter was inserted, 100 to 200 μg of intracoronary nitroglycerin was routinely administrated. The IVUS images were recorded for offline quantitative analysis.
The images were analyzed according to the criteria of the American College of Cardiology’s Clinical Expert Consensus Document on IVUS using commercially available planimetry software (Echoplaque, Indec Systems, Mountain View, California). Lesion morphology was categorized according to plaque echogenicity. The preintervention lesion site and reference segment measurements included the external elastic membrane and lumen cross-sectional areas and diameters, plaque and media cross-sectional area, plaque burden (plaque and media divided by the external elastic membrane), remodeling index (lesion/average of the proximal and distal reference external elastic membrane cross-sectional area), and lesion length. The postintervention lesion site measurements included the MSD and MSA. Stent expansion was defined as the MSD or MSA divided by the predicted MSD or predicted MSA, obtained from the manufacturer’s compliance charts.
Statistical analysis was performed with Statistical Package for Social Sciences, for Windows, version 13.0 (SPSS, Chicago, Illinois). Continuous data are presented as the mean ± SD and categorical data as frequencies. The chi-square test or Fisher’s exact test was used for the comparisons of categorical variables. Student’s t test, analysis of variance, or linear regression analysis was used to compare the continuous variables. A p value <0.05 was considered statistically significant.
Results
Of the 116 stents used to treat the 116 de novo lesions, 61 were Xience/Promus (Abbot Park Laboratories, Abbot Park, Illinois; cobalt chromium stent group) and 27 were Taxus Liberté (Boston Scientific) and 28 Cypher (Cordis, Bridgewater, New Jersey; stainless steel stent group, n = 55). The baseline characteristics were comparable between the cobalt chromium and stainless steel stent groups ( Table 1 ). No difference was found between the 2 groups in terms of lesion location and complexity or stent size, except that (1) direct stenting was more frequently performed in the stainless steel stent group (54.5% vs 40%, p = 0.04); (2) the stents were longer in the stainless steel stent group (20.0 vs 17.4 mm, p = 0.04); and (3) the maximum stent deployment pressure was greater before IVUS evaluation in the stainless steel stent group (13.7 vs 11.9 atm, p <0.001; Table 2 ).
| Variable | Cobalt Chromium Stent (Xience/Promus; n = 52) | Stainless Steel Stent (Taxus/Cypher; n = 47) | p Value |
|---|---|---|---|
| Age (years) | 65.5 ± 10.1 | 65.0 ± 10.4 | 0.66 |
| Men | 27 (52%) | 33 (70%) | 0.06 |
| Diabetes mellitus | 19 (37%) | 23 (49%) | 0.21 |
| Hypertension ⁎ | 44 (85%) | 35 (75%) | 0.21 |
| Hyperlipidemia † | 38 (73%) | 38 (81%) | 0.36 |
| Current smoker | 22 (42%) | 25 (53%) | 0.28 |
| Previous myocardial infarction | 6 (12%) | 4 (9%) | 0.74 |
| Previous percutaneous coronary intervention | 24 (46%) | 22 (47%) | 0.95 |
| Previous bypass surgery | 5 (10%) | 3 (6%) | 0.72 |
| Clinical presentation | |||
| Stable angina pectoris | 32 (62%) | 34 (72%) | 0.27 |
| Unstable angina pectoris | 15 (29%) | 12 (26%) | |
| Acute myocardial infarction | 5 (10%) | 1 (2%) |
⁎ Systolic blood pressure ≥140 mm Hg, diastolic blood pressure ≥90 mm Hg, or medical treatment of hypertension.
† Total cholesterol ≥240 mg/dl or medical treatment of hyperlipidemia.
| Variable | Cobalt Chromium Stent (Xience/Promus; n = 61) | Stainless Stent (Taxus/Cypher; n = 55) | p Value |
|---|---|---|---|
| Target coronary vessel | 0.58 | ||
| Left anterior descending | 26 (43%) | 26 (47%) | |
| Left circumflex | 16 (26%) | 10 (18%) | |
| Right | 19 (31%) | 19 (35%) | |
| American Heart Association/American College cardiology lesion morphology | 0.07 | ||
| A | 26 (43%) | 20 (36%) | |
| B1 | 23 (38%) | 17 (31%) | |
| B2 | 10 (16%) | 8 (15%) | |
| C | 2 (3%) | 10 (18%) | |
| Stent size (mm) | 0.85 | ||
| 2.5/2.75 | 17 (28%) | 13 (24%) | |
| 3.0 | 22 (36%) | 22 (40%) | |
| 3.5 | 22 (36%) | 20 (36%) | |
| Stent length (mm) | 17.4 ± 5.8 | 20.0 ± 7.4 | 0.04 |
| Direct stenting (%) | 40.0 | 54.5 | 0.04 |
| Maximum pressure (atm) | 11.9 ± 2.1 | 13.7 ± 3.3 | 0.00 |
No differences were present in the quantitative or qualitative IVUS analyses between the 2 groups ( Table 3 ). The postintervention MSD, MSA, and persistent plaque burden were also similar, as were the IVUS-measured/manufacturer-predicted MSD ratios (0.74 vs 0.73, p = 0.57) and MSA ratios (0.63 vs 0.63, p = 0.69; Figure 1 ). In addition, if stent expansion were defined as the MSA/postintervention distal reference lumen area, the ratio would be 0.82 ± 0.15 for the cobalt chromium stents and 0.82 ± 0.20 for the stainless steel stents (p = 0.94).
| Variable | Cobalt Chromium Stent (Xience/Promus; n = 61) | Stainless Steel Stent (Taxus/Cypher; n = 55) | p Value |
|---|---|---|---|
| Reference segment | |||
| EEM area (mm 2 ) | 11.0 ± 3.3 | 11.4 ± 3.1 | 0.52 |
| Lumen area (mm 2 ) | 6.4 ± 1.7 | 6.6 ± 1.7 | 0.63 |
| Lesion site preintervention | |||
| EEM area (mm 2 ) | 9.5 ± 2.9 | 10.6 ± 3.5 | 0.10 |
| MLA (mm 2 ) | 2.8 ± 0.7 | 3.0 ± 0.8 | 0.28 |
| MLD (mm) | 1.7 ± 0.2 | 1.8 ± 0.3 | 0.15 |
| Plaque burden (%) | 68.2 ± 10.8 | 70.1 ± 8.2 | 0.35 |
| Remodeling index | 0.88 ± 0.16 | 0.94 ± 0.22 | 0.11 |
| Lesion length (mm) | 14.1 ± 6.6 | 15.5 ± 8.2 | 0.35 |
| Calcification length (mm) | 5.6 ± 4.7 | 6.9 ± 7.2 | 0.34 |
| Maximum calcification arc (°) | 99.3 ± 87.7 | 94.6 ± 86.9 | 0.80 |
| Plaque morphology | |||
| Hyper/isoechoic | 28 (46%) | 21 (38%) | 0.64 |
| Hypoechoic | 5 (8%) | 5 (9%) | |
| Calcified | 11 (18%) | 6 (11%) | |
| Mixed | 5 (8%) | 7 (13%) | |
| After intervention | |||
| MSD (mm) | 2.4 ± 0.4 | 2.4 ± 0.4 | 0.76 |
| MSA (mm 2 ) | 5.3 ± 1.4 | 5.3 ± 1.7 | 0.98 |
| Peri-stent plaque burden (%) | 52.9 ± 10.7 | 55.9 ± 10.8 | 0.13 |
| IVUS MSD/predicted MSD ratio | 0.74 ± 0.09 | 0.73 ± 0.08 | 0.57 |
| IVUS MSA/predicted MSA ratio | 0.63 ± 0.11 | 0.63 ± 0.12 | 0.69 |
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