Optical Coherence Tomography and Histopathology Assessment After Implantation of First- and Second-Generation Drug-Eluting Stents in a Porcine Coronary Model

Abstract

Background: This study directly compared optical coherence tomography (OCT) and histopathology for the assessment of vascular response to first- and second-generation drug-eluting stents.

Methods and Results: Sirolimus-, everolimus-, and biolimus-eluting stents (SES, EES, and BES, respectively) were randomly implanted into the coronary arteries of 12 porcine. OCT was conducted after implantation: at 1, 3, and 6 months; histopathology was assessed at 3 and 6 months. At 1-month OCT, EES had the highest neointimal area (NA) and lowest neointimal unevenness score (NUS). At 6 months, NA and NUS were equivalent among the stent types. ∆NA from 1 to 6 months was lowest for EES, and ∆NA correlated with the histopathological inflammation score at 6 months, which was highest for SES (P<0.001). The mean signal intensity (MSI) and the attenuation were different for the stent types at 3 months, and were associated with inflammation score. Moderate diagnostic efficiency for measuring MSI was found, with an optimal cut-off of 6.88 predicting a high (≥grade 3) inflammation score.

Conclusions: EES had the greatest uniformity and the least neointimal proliferation and were associated with less persistent inflammation. OCT provides accurate morphometric data; furthermore, quantitative measurement of the optical properties may help assess histological inflammation, which was more predominantly associated with SES than with EES and BES.

Drug-eluting stents (DES) dramatically inhibit neointimal proliferation, significantly reducing the need for target-lesion revascularization.^1,2 Concerns have been raised, however, about the long-term safety of DES.^3,4 Human and animal pathological studies have shown that persistent inflammation, fibrin deposition, and poor re-endothelialization associated with first-generation DES may lead to vessel remodeling, strut malapposition, and occlusive thrombosis.^5–7

Editorial p ????

Optical coherence tomography (OCT), a catheter-based imaging modality, allows visualization of intra-coronary structures using infrared, back-reflected light and provides refined images of the neointimal structure in vivo,^8–14 and is useful for evaluating vascular response to coronary stent devices in preclinical animal models;¹⁵ in vivo OCT morphometric measurements, after stent implantation, have been shown to correlate well with histomorphometry.^16,17 OCT also allows identification of neointimal tissue morphologic patterns.^18–21

In humans, histological validation of coronary OCT for evaluating the coverage of stent struts with intimal tissue has not been systematically conducted due to the insufficient availability of autopsy samples. Only a few case reports have compared post-implantation histology and OCT patterns of restenosis.^22–24 Studies have not evaluated the OCT appearance of coronary arteries after the implantation of second-generation DES, conducted with an assessment of arterial wall histologic characteristics. Therefore, we evaluated 3- and 6-month OCT and histopathologic features associated with sirolimus-eluting stents (SES), everolimus-eluting stents (EES), and biolimus-eluting stents (BES), in a porcine coronary artery model. In addition, we conducted serial OCT observations at 1 and 6 months after stent implantation, to assess the time course of neointimal proliferation in association with histopathology.

Methods

Study Design

Three types of DES were implanted in the coronary arteries of 12 mini-swine: SES (Cypher Select, 3.0×18 mm; Cordis, Hialeah, FL, USA), EES (Xience V, 3.0×18 mm; Abbott Vascular, Temecula, CA, USA), and BES (Nobori, 3.0×18 mm; Terumo, Tokyo, Japan). Group A animals underwent OCT at 1 and 6 months after implantation, and were killed for histological analysis at 6 months. For the 1-month analysis, strut coverage and neointimal thickness (NIT) were measured only on OCT, without histologic assessment. Group B animals underwent OCT and were killed for histologic analysis 3 months after implantation.

Index Procedure

Animal handling and care procedures followed the recommendations of the National Institutes of Health (Bethesda, MD, USA), as published in the Guide for the Care and Use of Laboratory Animals (2012) and were consistent with the guidelines of the American Heart Association. All protocols were approved by the local animal care and use committee and followed the Association for Assessment and Accreditation of Laboratory Animal Care guidelines.

All animals were pre-medicated with oral clopidogrel (75 mg/day) and aspirin (100 mg/day) for 2 days before the procedure; thereafter, the drugs were given daily until the animals were killed. Anti-arrhythmics and broad-spectrum antibiotics were also given on the day of the procedure. Each pig was anesthetized with isoflurane, and surgical access was obtained via the femoral or carotid artery. Before catheterization, heparin (5,000–10,000 IU) was injected to maintain an activated clotting time of 250–300 s. Nitroglycerin was given intra-arterially to prevent or relieve vasospasms. A vascular introducer sheath of an appropriate size was placed into the access artery to advance the angioplasty guiding catheters. Vessel allocation to a particular experimental group was predetermined to equally distribute the different stent types in 3 different coronary arteries. The appropriate stent was delivered to the intended site over a guidewire using fluoroscopic guidance, and the stent was deployed using a 1:1.1–1.2 stent-to-artery diameter ratio. The animals were allowed to recover and were housed until their designated day of termination.

Follow-up OCT

One month after stent implantation (group A only) and before death at either 3 or 6 months (group B and A animals, respectively), each animal underwent follow-up OCT (M3 System, LightLab Imaging, Westford, MA, USA). OCT was performed using the balloon occlusion technique.

Tissue Harvest

Animals were killed under deep anesthesia, induced by i.v. injection of pentobarbital (100 mg/kg) and/or potassium (40 mEq). Hearts were excised and pressure-perfused with 0.9% saline until cleared of blood, followed by pressure-perfusion fixation in 10% neutral-buffered formalin until hardening of the heart muscle was clearly perceptible. Whole hearts were processed by a commercial facility (CVPath Institute, Gaithersburg, MD, USA).

Light Microscopy

The stented arterial segments were carefully dissected free from the heart (at CVPath), and embedded in methyl methacrylate resin. After polymerization, sections were cut every 2–3 mm along the stent (for a total of 3–5 sections). For each section, the distance from the proximal (and/or distal) edge of the stent was recorded to compare the histology and OCT results. Sections (4–6 μm thick) of the stents were cut on a rotary microtome, mounted, and stained with hematoxylin and eosin as well as with elastica van Gieson stain. Histomorphometry was used to quantify neointimal growth and to assess the extent of inflammation and fibrin deposition.

Histomorphometry

The cross-sectional areas (external elastic lamina, internal elastic lamina, and lumen area) of each section were measured using digital morphometry. NIT was measured as the distance from the inner surface of each stent strut to the luminal border; the neointimal area (NA=internal elastic lamina−lumen area) and %stenosis ([1−lumen area/internal elastic lamina area]×100) were also calculated.

Histopathology

An overall neointimal inflammation and fibrin score was determined for each section (range, 0–3). Vessel sections with ≥2 struts with granulomatous inflammation had an inflammation score of 4.²⁵

OCT

OCT was analyzed by 2 skilled, independent observers who were blinded to stent type and the histology results. The evaluations were conducted using proprietary software (LightLab Imaging). The assessments were performed every 1 mm through the stented segment; the assessed parameters included stent area, lumen area, %NA ([1−lumen area/stent area]×100), NIT, %uncovered struts (no. uncovered struts/covered struts per stent×100), and neoinitmal unevenness score (NUS=maximum NIT in the cross-section/average NIT of the same cross-section),²⁶ as measured and determined by experienced cardiologists.

According to the histology cutting record and the strut arrangement in the OCT and histological images, precise histology-OCT frame co-registration was possible. The OCT images were classified into the following 3 categories, based on the texture pattern: homogeneous with high signal (Homo), layered (Layered), and homogeneous with multiple low-signal spots (Hetero).¹⁸ Homo was characterized by a uniformly signal-rich appearance, containing largely signal-rich structures with low-signal regions only inside the stent strut. Layered had a signal-poor appearance, with a high-signal band adjacent to the luminal surface. Hetero was mainly signal poor, with islands of various signal intensities. In the case of mixed neointimal optical patterns, the classification was done according to the dominant neointimal pattern. This classification required the agreement of 2 independent and experienced observers (T.S. and H.O.). If there was disagreement, a final decision was made through discussion with a third independent observer (K.H).

Quantifying Optical Properties of the Neointimal Tissue

Mean signal intensity (MSI), and attenuation (ATN) of the whole neointima inside the stent struts were quantified using the LightLab software. MSI represents the mean OCT signal reflected off the subject of interest, and ATN represents the degree of decrease of the OCT signal in the extraluminal direction, as previously reported.²⁷ NA was manually traced, as the lesion of interest, at exactly the same location on the cross-sectional image corresponding to the histopathology sections and the software automatically calculated the MSI and ATN of each cross-sectional image.

Statistical Analysis

Statistical analysis was done using StatView 5.0 (SAS Institute, Cary, NC, USA) and MedCalc (Acacialaan 22, version 12.4.0, B-8400, Ostend, Belgium). Qualitative data are presented as frequencies and quantitative data are shown as mean±SD. For continuous variables, comparison among the 3 stents was done using Bonferroni/Dunn test. P<0.05 was considered statistically significant.

Results

The OCT image of 1 BES in group A was of poor quality and that of 1 BES in group B was totally occluded, thus 17 stents in both groups A and B were ultimately analyzed. Figure 1A shows a representative OCT image and corresponding histology image, and Figure 1B shows comparable cross-sectional images of hematoxylin-and-eosin stain and macrophage immunostain using cluster of differentiation 68 antibody, with a grade 3 inflammation score.

Figure 1.

Optical coherence tomography (OCT) and histology. (A) Representative OCT images of the 3 stent types at 1, 3, and 6 months after implantation, and the corresponding cross-sectional histological images at 3 and 6 months; (B) immunostaining for macrophage identification; although lymphocytes with large, dark-staining nuclei and small cytoplasm are also observed, the inflammatory reaction shows mainly macrophages. BES, biolimus-eluting stent; EES, everolimus-eluting stent; SES, sirolimus-eluting stent.

OCT Morphometry

Stent length, average stent area per stent, and average minimum stent area per stent were similar among the 3 stent types at 1 and 6 months in group A and at 3 months in group B. In group A, at 1 month, NA, %NA, and NIT were the smallest and NUS was the highest for SES (0.9±0.5 mm², 14.1±7.0%, 102±63 μm, and 3.2±1.5, P<0.0001, respectively) followed by BES (1.7±0.9 mm², 24.0±12.2%, 198±103 μm, 2.5±1.3), and EES (2.2±0.6 mm², 31.5±11.5%, 269±98 μm, 1.4±0.1) . The percentage of the strut that remained uncovered was highest for SES (41.7±27.0%), followed by BES (24.5±23.8%) and EES (0.4±0.8%). At 6 months, those values were equivalent among the 3 stent types. The change in NA (∆NA) from 1 month to 6 months was highest for SES (3.70±1.09), followed by BES (3.06±1.32) and EES (2.24±1.07).

In group B, at 3 months, the minimum lumen area was the smallest for SES (1.0±0.7 mm², P=0.02), followed by BES (2.8±1.4 mm²) and EES (2.6±1.1 mm²). NA, %NA, and NIT were the highest in SES (5.7±1.0 mm², 78.4±8.3% and, 836±125 μm, respectively; P<0.001, respectively), followed by BES (4.1±1.0 mm², 52.3±15.7%, 527±171 μm) and EES (3.7±0.6 mm², 50.2±12.9%, 474±123 μm; Table 1). NUS and the percentage of the strut remaining uncovered were equivalent among the 3 stent types.

Table 1. OCT Results

	Stent length (mm)	Stent area (mm2)	MSA (mm2)	MLA (mm2)	Neointimal area (mm2)	%Neointimal area (%)	Neointimal thickness (μm)	Neointimal unevenness score	%bare strut (%)
1 month
EES	16.5±0.9	7.2±0.8	6.5±0.6	3.5±1.4	2.2±0.6	31.5±11.5	269±98	1.4±0.1	0.4±0.8
BES	17.3±1.6	7.3±0.6	6.7±0.6	3.6±0.6	1.7±0.9	24.0±12.2	198±103	2.5±1.3	24.5±23.8
SES	16.3±1.9	6.8±1.3	5.6±1.9	3.5±1.1	0.9±0.5*	14.1±7.0*	102±63*	3.2±1.5*	41.7±27.0*
P-value	0.51	0.62	0.35	0.98	0.005	0.01	0.006	0.01	0.004
3 months
EES	17.5±1.6	7.5±0.7	6.4±0.7	2.6±1.1	3.7±0.6	50.2±12.9	474±123	1.3±0.0	0
BES	17.5±0.6	8.0±0.7	7.0±0.8	2.8±1.4	4.1±1.0	52.3±15.7	527±171	1.3±0.1	0
SES	16.1±2.1	7.3±1.2	6.4±1.0	1.0±0.7*	5.7±1.0*	78.4±8.3*	836±125*	1.3±0.1	0
P-value	0.24	0.42	0.44	0.02	0.002	0.002	0.0005	NS	NS
6 months
EES	18.4±0.9	6.8±0.9	6.2±0.8	2.2±1.6	3.9±1.2	59.6±20.9	572±219	1.4±0.2	0.2±0.6
BES	18.3±0.5	7.2±0.8	6.6±0.7	1.8±0.7	4.4±0.2	61.9±7.7	603±73	1.3±0.1	0
SES	17.8±1.7	6.5±1.3	5.8±1.1	1.7±1.0	4.2±0.3	66.5±9.1	616±168	1.3±0.1	0
P-value	0.64	0.50	0.42	0.78	0.58	0.7	0.86	0.3	NS

Data given as mean±SD. *Significantly different from EES.

BES, biolimus-eluting stent; EES, everolimus-eluting stent; MLA, minimum lumen area; MSA, minimum stent area; OCT, optical coherence tomography; SES, sirolimus-eluting stent.

Histomorphometry

In group B, at 3 months, the lumen area was the smallest for SES (1.7±0.7 mm², P=0.03), followed by BES (3.1±1.3 mm²) and EES (3.3±0.91 mm²). NA was highest in SES (5.1±1.5 mm², P=0.01), followed by BES (4.1±1.0 mm²) and EES (2.9±0.5 mm²). NIT and stenosis were the largest in SES (686±193 μm and 75.9±9.8%, respectively; both P<0.01), followed by BES (497±227 μm and 56.0±16.9%) and EES (393±89 μm and 47.2±10.8%). In group A, at 6 months, moderate neointimal formation was observed in all DES, and did not differ significantly with stent type (Table 2).

Table 2. Histology Results

	EEL area (mm2)	IEL area (mm2)	Lumen area (mm2)	Neointimal area (mm2)	Neointimal thickness (μm)	Stenosis (%)
3 months
EES	7.5±0.7	6.2±0.5	3.3±0.9	2.9±0.5	393±89	47.2±10.8
BES	8.2±0.9	6.8±0.8	3.1±1.3	3.8±0.9	497±227	56.0±16.9
SES	8.5±2.1	6.7±1.5	1.7±0.7	5.1±1.5**	686±193*	75.9±9.8*
P-value	0.46	0.52	0.03	0.01	0.0083	0.0047
6 months
EES	7.0±0.9	5.9±0.7	2.4±1.5	3.5±1.0	533±233	61.0±20.5
BES	8.0±1.1	6.8±0.8	2.9±1.4	3.9±0.7	483±143	59.0±14.9
SES	7.3±1.9	5.9±1.2	1.8±0.7	4.0±0.7	580±51	69.4±6.3
P-value	0.4549	0.2242	0.3681	0.5403	0.5928	0.4644

Data given as mean±SD. *Significantly different from EES and BES; **Significantly different from EES.

EEL, external elastic lamina; IEL, internal elastic lamina. Other abbreviations as in Table 1.

Histopathology

In group B, at 3 months, SES was associated with a significantly higher inflammation score than the others, and a similar pattern was also observed in group A animals at 6 months. The accelerated neointimal formation in SES animals was evidenced by an increased inflammatory reaction compared with EES or BES (Figure 2A). Fibrin deposition score did not differ with stent type in group B, at 3 months, and this pattern was also present in group A animals, at 6 months, with time-dependent regression (Figure 2B). The high inflammatory reaction at 6 months was related to the ∆NA from 1 to 6 months, as observed on OCT (r=0.523, P<0.0001; Figure 2C).

Figure 2.

Histopathological score and the relationship between change in neointimal area and inflammation score. (A) At both time points, the sirolimus-eluting stents (SES) had the highest inflammation grade among the 3 types of stents; (B) at both time points there were no significant differences in fibrin score among the 3 types of stents. (C) High inflammation score at 6 months correlated with larger change in neointimal area between 1 and 6 months after implantation. Chronic inflammation causes neointimal growth. BES, biolimus-eluting stent; EES, everolimus-eluting stent.

There were no significant differences between EES and BES in the various parameters, including inflammation and fibrin deposition score (Figure 2).

Co-Registration of Histopathology and OCT Cross-Section

A total of 202 histopathology sections were co-registered with their corresponding OCT cross-sections. On independent OCT categorization the 202 OCT sections were classified as follows: 123 Homo sections (60.9%), 31 Hetero sections (15.4%), and 48 Layered sections (23.7%; Table 3A). Agreement between the 2 independent observers at the initial evaluation was 98% (118/120) for Homo, 78% for Hetero (31/40), and 93% for Layered (55/44). The interobserver reproducibility for classification of OCT tissue was modest (κ=0.65).

Table 3. OCT and Histology Results of Neointima

A. OCT texture classification and histopathology score
Inflammation	0	1	2	3	4	Total
Homo	96 (47.5)	9 (4.5)	3 (1.5)	0 (0)	15 (7.4)	123 (60.9)
Hetero	9 (4.5)	6 (3.0)	3 (1.5)	1 (0.4)	12 (5.9)	31 (15.4)
Layered	21 (10.4)	12 (5.9)	4 (2.0)	0 (0)	11 (5.4)	48 (23.7)
Total	126 (62.4)	27 (13.4)	10 (5.0)	1 (0.4)	38 (18.8)	202 (100)
B. OCT morphological classification
	Attenuation			Mean signal intensity
Homo	7.28±0.28			1.22±1.17
Hetero	6.67±0.25			2.75±0.60
Layered	6.72±0.25			2.83±0.54
P-value	<0.0001			<0.0001
C. OCT tissue characterization
	Attenuation			Mean signal intensity
3 months
EES	1.70±0.80			7.19±0.24
BES	1.52±1.12			7.0±0.40
SES	2.88±0.33			6.66±0.15
P-value	0.0227			0.0143
6 months
EES	1.81±1.05			7.40±0.27
BES	1.61±1.64			7.33±0.26
SES	1.58±1.64			7.41±0.10
P-value	0.9578			0.784

Data given as mean±SD. Abbreviations as in Table 1.

When the histopathology scores were compared between the corresponding histology sections and the 3 OCT texture classifications, Homo had the lowest grade of inflammation (P<0.0001, Homo vs. Hetero). There were no differences in the fibrin deposition scores among the Homo, Hetero, and Layered classifications (Figure 3). MSI was higher in the Homo (7.28±0.28) classification than in either the Hetero (6.67±0.25) or Layered (6.72±0.25) groups (P<0.0001). ATN was lower in the Homo (1.22±1.17) group than in either the Hetero (2.75±0.60) or Layered (2.83±0.54) groups (P<0.0001; Table 3B). The diagnostic accuracy of the optical density measurements was assessed on receiver operating characteristics curve analysis for the MSI of the neointima vs. a high inflammation score (>2), showing moderate diagnostic accuracy for the measurement of MSI (area under the curve=0.702; Figure 4).

Figure 3.

Optical coherence tomography (OCT) texture classification and histopathology score in corresponding cross-sections. The stent was classified on OCT morphology classification and compared to the histopathology score. There were no differences in fibrin score, but the homogeneous (Homo) type was associated with a significantly lower grade of inflammation, compared with the heterogeneous (Hetero) type.

Figure 4.

On receiver operating characteristic (ROC) curve analysis, optical density measurement of mean signal intensity of the neointima vs. high inflammation (>2), was found to have moderate diagnostic accuracy. AUC, area under the curve.

Texture and Quantitative OCT Analysis of Neointima

At 3 months, the predominant pattern of the EES neointima was homogeneous (78% of lesions were classified as Homo, 6% as Hetero, and 16% as Layered), in comparison to BES (60% of lesions were classified as Homo, 17% as Hetero, and 23% as Layered) or SES (28% of lesions were classified as Homo, 34% as Hetero, and 38% as Layered). At 6 months, however, there was no difference with stent type: SES, 61% of lesions classified as Homo, 25% Hetero, and 14% Layered; EES, Homo 64%, Hetero 8%, Layered 28%; and BES, Homo 67%, Hetero 8%, Layered 25% (Figure 5).

Figure 5.

Distribution of optical coherence tomography (OCT) morphological classification. At 3 months, the percentage of homogeneous neointima (blue) was high, and the percentage of heterogeneous neointima (red) was significantly lower for everolimus-eluting stents (EES), compared with biolimus-eluting stents (BES) and sirolimus-eluting stents (SES). By 6 months, these differences had disappeared.

On quantification of the optical properties of the neointima, a difference was seen in MSI and ATN that varied according to stent type in group B, at 3 months. MSI was higher for EES (7.19±0.24; P=0.0143) than for SES (6.66±0.15). ATN was higher for SES (2.88±0.33; P=0.0227) than either EES (1.70±0.80) or BES (1.52±1.12). In group A, at 6 months, neither MSI nor ATN differed with stent type (Table 3C).

Discussion

This study quantitatively and qualitatively analyzed the OCT findings in concert with histological findings after SES, BES, and EES implantation in pig coronary arteries: 1 month in swine is equivalent to 3–6 months in humans, 3 months in swine is equivalent to 1 year, and 6 months in swine is equivalent to ≥1 year in humans. Hence, this study provided a direct comparison of OCT with histopathological findings for assessing long-term vascular response to different generations of DES.

OCT

On OCT, SES had the greatest extent of uncovered struts at 1 month after implantation, confirming that first-generation stents induce severely delayed arterial healing compared with the second-generation DES, EES. At 3 months, however, SES had thicker neointimal growth than EES, with the difference disappearing by 6 months.

The gradual increase in NIT for first-generation platforms (SES) reinforces the notion that the arterial response to DES is a dynamic process lasting well beyond the 6–8-month time frame that was widely applied during clinical trials.²⁸ Stone et al reported that EES were associated with significant reductions in the 1-year incidence of adverse cardiac events, compared with paclitaxel-eluting stents. Therefore, the long-term clinical efficacy and safety of the second-generation DES (EES, BES) might be better than those for first-generation DES (SES).²⁹ Moreover, clinical DES studies also reported a gradual increase in target lesion revascularization and neointimal volume, as determined on intravascular ultrasound, occurring over a 4-year follow-up.^30–33

Histology

On histology a consistent increase in inflammatory reactions associated with implanted SES was seen at both 3 and 6 months, whereas EES and BES had minimal inflammation at those time points. A previous study of human coronary stents showed that extensive intimal inflammation was an independent predictor of restenosis. Nakazawa et al showed that although a dramatic late catch-up in restenosis has not been reported for first-generation DES, target vessel revascularization rate increases over time, consistent with the observed progressive neointimal growth.³⁴ Sheehy et al reported that fibrin accumulation and the accompanying inflammation is directly related to the early phases of thrombus healing;³⁵ fibrin is often detected 1 month after stenting in animal studies. Moreover, deposition of fibrin decreases, such that there is no difference between the first-generation DES 3 months after stenting. In the present study, we analyzed the pathology of healing after implantation of second-generation DES, at 3 and 6 months after stenting; there was no difference in amount of fibrin deposition. The presence of fibrin following stenting with EES was not different from bare-metal stent (BMS) 90 days following stenting in a porcine model. The data suggest that the EES-implanted vessels continue to heal despite chronic (90 days) hyperglycemia in a swine model. Fibrin was observed to be minimal at 90 days, with inflammation almost completely resolved.³⁵

OCT and Histology

Preclinical studies have suggested that intimal inflammation is a key determinant of in-stent neointimal growth, which is accompanied by excessive neointimal thickening.⁷ Histologic data documents a progressive increase in injury and inflammation scores from the early phase through the chronic phase for SES as compared with BMS in porcine arteries. This observed progression of injury and inflammation represents a chronic vascular response to the drug or polymer. The vascular response to ongoing injury and inflammation, induced by the stent with residual polymer, may simply overwhelm the biological effects of the drug during the late formation of neointima. Arterial inflammation, characterized by the presence of giant cells, gradually progressed with a corresponding increase in neointima formation for SES in porcine coronary arteries.³⁶ Conversely, multiple studies have shown that treatments that inhibit inflammation and inflammatory cell adhesion molecules also reduce intimal growth.³⁷ With regard to BMS, the inflammation around the stent struts typically consists of macrophages and T lymphocytes, with a few B lymphocytes and giant cells in the short term. In the mid-term, however, BMS had minimal fibrin deposition and nearly complete endothelialization. In contrast, DES have shown evidence of significantly greater delays in arterial healing, as manifested by persistent peri-strut fibrin deposition and poor endothelialization. The present study showed that high inflammation score was related to greater ∆NA, supporting the thesis that neointimal growth is associated with chronic inflammation.

In the current study the proportions of the OCT texture patterns and histological “inflammation scores” were different among the stent types at 3 months. MSI and ATN, at 3 months, also differed among the 3 stent types. Because MSI and ATN quantified the OCT texture patterns, we believe that MSI and ATN are associated with inflammation. Therefore, ATN and MSI, on OCT images, are indicators of immaturity and the degree of neointimal inflammation, and are consistent with the pathology findings. We can estimate that high MSI causes proliferation of the neointima and a high inflammatory response.

The pathological assessment by Joner et al suggested that more significant, incomplete endothelialization, persistent fibrin deposition, and inflammation were observed after DES implantation, compared with BMS implantation, and reflected so-called delayed healing.⁵ Gonzalo et al evaluated the morphological characteristics of the intimal tissue in restenosis lesions, using OCT.¹⁸ Furthermore, according to Tanaka et al, low-signal pattern neointima, as assessed on OCT, was observed with high frequency in patients with diabetes mellitus.³⁸ There are few histological data, however, to validate the OCT findings in these studies.²⁴

The OCT characteristics of intimal tissue have not been compared with histology specimens, but the assessment of OCT, compared with histology, has emerged as a potential avenue for evaluating intimal conditions after DES implantation, based on OCT findings. The present qualitative OCT analysis, at 1 month after implantation, has suggested that the intima was observed as a low-signal area in the Layered type. We could gather from OCT and histopathological analysis at 3 and 6 months that the neointima, at 1 month after implantation, comprises immature intima with a high degree of inflammation. Similarly, the intima observed as a low-signal area in the Hetero type may be composed of intramural thrombi and inflammation. These findings, caused by delayed re-endothelialization, suggest a potential clinical role for OCT in the detection of high-risk, thrombosis-prone DES patients. Therefore, the OCT texture pattern was associated with differences in the histological inflammation scores, and the measurement of MSI and ATN can help quantify the OCT texture pattern.

One of the critical limitations of this study is that DES were implanted and evaluated in healthy pig coronary arteries, not in the diseased vessels where stents should be deployed in humans. As Schwartz et al described the rationale for using healthy animal to assess the efficacy and safety profiles of stent devices, this study successfully presented the various vessel reactions to first and new generations of DES.³⁹ Although frequency domain-OCT provides better resolution than time domain-OCT, dedicated software for the quantitative measurement of OCT signal intensity was available only for time domain-OCT at the time of this study. Because of the low penetration ability of OCT, once neointima becomes thickened, it may be difficult to distinguish natural signal ATN from low backscattering tissue character within neointima on OCT. From the present observations, a layered OCT pattern can be produced from tissue with both low and high inflammation, while a heterogeneous OCT pattern may be closely associated with higher histological inflammation.

Conclusions

SES had uneven tissue coverage, with peculiar light properties at 1 month, leading to progressive neointimal proliferation and persistent inflammation at 6 months after implantation. In contrast, EES and BES had minimal inflammation, even at the long-term follow-up point in this animal model. Therefore, long-term clinical efficacy and safety might not be different between the 2 types of second-generation DES, but may be better than the first-generation DES. Moreover, the OCT texture patterns and quantitative measurements may help to assess the histological inflammation more commonly found associated with SES than with EES or BES.

Disclosures

This study was supported, in part, by Abbott Vascular Japan. Gaku Nakazawa, MD, is a consultant for Abbott Vascular Japan, Terumo Corp, and Japan Stent Technology. Toshiro Shinke, MD, Hiromasa Otake, MD, and Junya Shite, MD, are consultants for St. Jude Medical Japan and Terumo Corp. None of the other authors have any conflicts of interest.

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