Methods

The PES fracture group was derived from the HORIZONS-AMI study, a prospective, open-label, multicenter, dual-arm, 2 × 2 factorial randomized trial in patients with ST-segment elevation myocardial infarction presenting <12 hours after symptom onset undergoing primary percutaneous coronary intervention. The 2 randomization arms consisted of (1) the direct thrombin inhibitor bivalirudin (Angiomax, the Medicines Company, Parsippany, New Jersey) alone versus heparin plus a glycoprotein IIb/IIIa inhibitor (1:1 randomization) and (2) Taxus PESs versus otherwise equivalent Express bare metal stents (Boston Scientific Corporation, Maple Grove, Minnesota) (3:1 randomization). In patients undergoing stent randomization, 13-month angiographic follow-up was prespecified for 1,800 patients in whom acute stent implantation was successful (diameter stenosis <10% with Thrombolysis In Myocardial Infarction grade 3 flow with National Heart, Lung, and Blood Institute maximum type A peri-stent dissection) and in whom neither stent thrombosis occurred nor bypass surgery was performed within 30 days. A formal IVUS substudy enrolled 464 patients with baseline and 13-month follow-up imaging at 36 centers. Of these, paired baseline and follow-up IVUS images in 245 patients with 262 lesions were analyzable. This study was approved by the institutional review board or ethics committee at each participating hospital, and written informed consent was obtained from all patients.

The SES fracture group was derived from our institutional database (New York Presbyterian Hospital, New York, New York). We retrospectively analyzed high-quality IVUS images of 278 consecutive stented segments (128 Cypher, 53 Taxus, 27 bare metal, and 70 unknown types) in 261 patients who underwent follow-up IVUS from November 2005 to April 2008; there were 13 patients with 14 SES fractures that were not associated with a coronary aneurysm. Because stent fracture within a coronary aneurysm may have a different mechanism compared to stent fracture without a coronary aneurysm, we excluded aneurysm-related stent fractures from the present analysis. This study was approved by the institutional review board, and written informed consent was obtained from all patients. Patient demographics were confirmed by hospital chart review. Coronary risk factors included diabetes mellitus (diet-controlled, oral agent, or insulin-treated), hypertension (medication-treated only), and hypercholesterolemia (medication-treated or a measurement >240 mg/dl).

Stent fracture was defined as complete or partial separation of the stent at follow-up and was classified into types I (minor), II (V-form), III (complete separation without displacement), and IV (complete separation with displacement) by angiography.

IVUS was performed after successful, uncomplicated stent implantation. Allowable IVUS systems included iLab, Galaxy, or ClearView (all with Atlantis SR Pro, 40-MHz catheters; Boston Scientific, Fremont, California) or In Vision Gold with 20-MHz EagleEye catheters (Volcano Therapeutics, Rancho Cordova, California). IVUS imaging was performed using motorized pullback at 0.5 mm/s to include the stent and >5-mm segments proximal and distal to the stent. IVUS studies were archived onto s-VHS tape, CD-ROM, or DVD and sent to an independent, treatment allocation-blinded, IVUS core laboratory (Cardiovascular Research Foundation, New York, New York) for quantitative and qualitative analyses using validated planimetry software (EchoPlaque, INDEC Systems, Inc., Mountain View, California). Qualitative and quantitative analyses were performed according to criteria of the American College of Cardiology clinical expert consensus document on IVUS.

Complete stent fracture was defined as complete separation of the stent into ≥2 pieces separated by image slices with no visible stent struts; partial stent fracture was defined as the absence of struts ≥1/3 of stent circumference. In patients with multiple stents, (1) postprocedure IVUS and/or angiography confirmed that these stents were contiguous and overlapping at time of implantation and (2) distances from the fracture site to the stent edges were compared to distances between the overlap site and stent edges to ensure that the site of presumed stent fracture was distinct from and did not merely represent separation of overlapping stents.

We measured external elastic membrane (EEM) and lumen cross-sectional area (CSA) at the reference segments, at the minimum lumen site, and at stent fracture sites. We measured stent CSA at the minimum lumen site and at edges of the stent just proximal and distal to the stent fracture. Percent intimal hyperplasia was calculated as neointimal CSA divided by stent CSA. Lengths of stents and stent fracture sites were calculated from the known pullback speed and duration of the pullback.

Statistical analysis was performed using StatView 5.0 (SAS Institute, Cary, North Carolina). Categorical variables were summarized as frequencies and compared between groups using chi-square statistics or Fisher’s exact test. Continuous variables were presented as medians and interquartile ranges and compared between groups using Mann-Whitney U test or the Wilcoxon signed-ranks test. A p value <0.05 was considered statistically significant.

Results

Among SES fractures, 12 patients had 1 fracture site and 1 patient had 2 fracture sites in the same stented vessel. In patients with serial (baseline and follow-up) IVUS analysis in the HORIZONS-AMI trial, 6 PES fractures were found in 6 patients (2.4%), all of which had 1 fracture site. Baseline clinical characteristics in individual patients with SES fractures or PES fractures are presented in Table 1 . There was no significant difference between the 2 groups except for systemic hypertension (92% in SES group vs 33% in PES group, p = 0.02) and stent implantation during ST-segment elevation myocardial infarction (7% in SES group vs 100% in PES group, p = 0.003). The mean duration between the index procedure and follow-up IVUS in the SES fracture group was not significantly different from the PES fracture group (200 [116, 422] vs 380 [321, 391] days, p = 0.25).

Of the 14 SES fractures 8 were located in the right coronary artery (57%), 2 in the left anterior descending coronary artery (14%), and 4 in the left circumflex artery (29%). In contrast, all PES fractures were located in the right coronary artery (p = 0.16). In the SES fracture group 71% (10 of 14) were detected by angiography and IVUS, and 4 were detected only by IVUS; 4 of the 10 (40%) angiographically visible SES stent fractures were type II, 5 (50%) were type III, and 1 (10%) was type IV. Conversely, all PES fractures were detected by angiography and IVUS; 5 were type III (83%), and 1 was type IV (17%).

Comparing PES fractures to SES fractures, qualitative and quantitative ( Table 2 ) IVUS analyses showed (1) similar frequency of complete stent fracture (17% in PES group vs 21% in SES group, p >0.99), (2) similar frequency of fracture adjacent to calcified plaque (83% in PES group vs 64% in SES group, p >0.99) or metal overlap (67% in PES group vs 57% in SES group, p >0.99), (3) more frequent complete malalignment of proximal and distal fragments in PES strut fractures compared to SES fractures (83% in PES group vs 7% in SES group, p = 0.002; best seen in longitudinal view, Figure 1 ), (4) similar stent lengths ( Figure 2 ), (5) similar fracture lengths, (6) larger EEM CSA and lumen CSA at PES fracture sites compared to SES fracture sites, (7) larger reference EEM CSA and lumen CSA in PES fracture group compared to SES fracture group, and (8) larger EEM CSA, lumen CSA, and stent CSA despite similar percent intimal hyperplasia adjacent to the PES fracture site compared to the SES peri-fracture site.

Table 2

Quantitative intravascular ultrasound findings

Variable	SES Fracture	PES Fracture	p Value
	(n = 14)	(n = 6)
Stent length (mm)	39.3 (22.6–73.4)	45.2 (23.8–50.7)	>0.99
Fracture length (mm)	0.7 (0.6–1.0)	0.5 (0.4–0.7)	0.14
Calcified plaque (number)	9	5
Arc of calcified plaque (°)	90 (75–100)	60 (48–88)	0.10
Distance from calcified plaque to fracture (mm)	3.6 (1.2–5.1)	2.4 (1.8–4.8)	0.95
Metal overlap (number)	8	4
Distance from metal overlap to fracture (mm)	4.1 (3.8–4.6)	6.2 (3.5–8.1)	0.45
Reference segment
External elastic membrane cross-sectional area (mm 2 )	10.4 (6.8–13.6)	15.0 (13.5–18.0)	0.01
Lumen cross-sectional area (mm 2 )	5.0 (4.5–5.4)	9.4 (7.9–12.0)	0.003
Plaque cross-sectional area (mm 2 )	5.3 (2.7–7.2)	5.8 (4.7–6.5)	0.46
Plaque burden (%)	50.4 (34.2–54.0)	33.8 (33.3–38.4)	0.16
Peri-fracture site
External elastic membrane cross-sectional area (mm 2 )	14.2 (11.4–15.3)	19.5 (16.1–23.1)	0.003
Lumen cross-sectional area (mm 2 )	4.7 (3.9–5.9)	8.4 (7.6–10.0)	0.002
Stent cross-sectional area (mm 2 )	5.8 (5.5–6.3)	9.9 (8.5–10.5)	0.003
Intimal hyperplasia cross-sectional area (mm 2 )	0.8 (0.2–1.5)	0.7 (0.3–1.1)	0.80
Intimal hyperplasia (%)	14.9 (3.2–25.9)	7.8 (4.6–11.1)	0.41
Fracture site
External elastic membrane cross-sectional area (mm 2 )	12.3 (10.2–15.4)	18.0 (15.7–20.0)	0.004
Lumen cross-sectional area (mm 2 )	2.8 (2.5–3.1)	7.0 (5.0–7.4)	0.0005

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