Pathology of First-generation Drug-eluting Stents in Humans

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Abstract

First-generation sirolimus-eluting stents (SES) and paclitaxel-eluting stents (PES) have dramatically reduced restenosis, although concern still exists about the long-term safety of this technology since observational studies have shown a steady increase in the rate of late stent thrombosis (LST), an infrequent but catastrophic complication. Although the mechanisms of LST are multifactorial, our laboratory has demonstrated that delayed arterial healing accompanied by poor endothelialisation is the primary pathogical substrate underlying this event. Delayed arterial healing is associated with penetration of necrotic core, long/overlapping stents and bifurcation stenting, especially in flow divider (high shear) regions. Grade V stent fracture is also associated with adverse pathogical findings including LST and restenosis. Moreover, localised hypersensitivity reaction is exclusive to SES as an underlying mechanism of LST, while malapposition secondary to excessive fibrin deposition is associated with PES. Uncovered struts are still identified in both SES and PES with implant duration beyond 12 months, particularly in stents placed for ‘off-label’ indications. In conclusion, the first generation of drug-eluting stents (DES) certainly reduce neointimal growth but this comes at the price of delayed healing.

Acknowledgement: CVPath Institute, Inc., Gaithersburg, Maryland, provided full support for this work.

Disclosure
Renu Virmani receives research support from Medtronic AVE, Abbott Vascular, Atrium Medical, OrbusNeich Medical, Terumo Corporation, Cordis Corporation, BioSensors International, Biotronik and Alchimedics, and is a consultant for Medtronic AVE, Abbott Vascular, WL Gore, Atrium Medical and Lutonix. The remaining authors have no conflicts of interest to declare.
Correspondence
Renu Virmani, CVPath Institute, Inc., 19 Firstfield Road, Gaithersburg, MD 20878, US. E: rvirmani@cvpath.org
Received date
29 January 2011
Accepted date
14 February 2011
Citation
ICR - Volume 6 Issue 1;2011:6(1):28-35
Correspondence
Renu Virmani, CVPath Institute, Inc., 19 Firstfield Road, Gaithersburg, MD 20878, US. E: rvirmani@cvpath.org
DOI
http://dx.doi.org/10.15420/icr.2011.6.1.28

Percutaneous coronary interventions (PCI) involving stenting are the most widely performed procedures for the treatment of symptomatic coronary disease.1 Although first-generation sirolimus-eluting stents (SES; Cypher™, Cordis Corp., Miami Lakes, FL) and paclitaxel-eluting stents (PES; Taxus™, Boston Scientific, Natick, MA) have radically reduced restenosis,2,3 complications of late (LST) and very late stent thrombosis (VLST) have emerged as an infrequent but important issue of this technology. While recent randomised clinical trials have reported similar rates of LST between first-generation drug-eluting stents (DES) and bare-metal stents (BMS),4 large registries have revealed a steady increase in the rate of LST with first-generation DES as compared with BMS.5–7 DES have been implanted in millions of patients worldwide; therefore understanding the histopathogical findings seen following deployment of such devices in patients in the global practice of medicine is of paramount importance. This article will focus on the pathological mechanisms of LST following first-generation DES implantation, and the different vascular healing responses observed between SES and PES from the pathological perspective will be highlighted.

Endothelial Coverage – The Most Important Morphometric Predictor for Late Stent Thrombosis

To determine the pathogical correlates of LST in DES, we investigated a total of 62 coronary lesions from 46 human autopsy cases with first-generation DES implanted for greater than 30 days.8 We identified stent thrombosis in 28 lesions (23 patients) and compared those with 34 lesions (23 patients) of similar duration without stent thrombosis (duration of implant: 254±235 days for lesions with LST versus 244±289 days for those without; p=not significant [NS]). In cases with LST, 13 occlusive and 15 non-occlusive thrombi were detected. In these lesions with thrombus formation, there were 16 SES (14 lesions) and 22 PES (14 lesions), while those without thrombi included 23 SES (18 lesions) and 17 PES (16 lesions).

We found that neointimal thickness was lower in thrombosed DES lesions (median 0.074mm [interquartile range (IQR) 0.033, 0.129] versus 0.11mm [0.071, 0.19], p=0.05), and the percentage of endothelialisation was significantly less in thrombosed DES lesions as compared with patent DES lesions (40.5±29.8% versus 80.0±25.2%, p<0.0001). Total stent length was longer in thrombosed stents (25.9±11.5mm versus 20.3±9.6mm, p=0.04), and average stent length without neointimal coverage was significantly greater in thrombosed as compared with non-thrombosed lesions (20.1±11.5mm versus 9.9±10.1mm, p=0.0004). The mean number of uncovered struts per section was also significantly greater in DES lesions with thrombosis versus those without (5.0±2.7 versus 2.0±2.7, p<0.0001), and the ratio of uncovered to total struts per section was greater in thrombosed versus non-thrombosed lesions (0.50±0.23 versus 0.19±0.25, p<0.0001).

Moreover, the average distance between individual stent struts was significantly shorter in DES lesions with thrombus formation as compared with patent DES lesions (0.52±0.24mm versus 0.70±0.25mm, p=0.004). There was also a good correlation between the mean number of uncovered struts per section and the average distance between stent struts (R=-0.41, p=0.001), with the majority of uncovered stent struts showing shorter inter-strut distance than covered stent struts. On further examination, we found heterogeneity of coverage of stent struts, both within individual cross-sections as well as between sections from the same stent. Within the same DES, while some struts show healing, as demonstrated by neointimal growth, others remain bare and serve as a nidus for thrombus formation (see Figure 1). Within a DES, the middle section of the stent (versus the proximal and distal ends) was the most common location of stent struts lacking neointimal coverage and this was also the most common site of thrombus formation.

Multivariable logistic generalised estimating equation modelling demonstrated that endothelialisation was the best predictor of LST. The morphometric parameter that best correlated with endothelialisation was the ratio of uncovered to total stent struts per section. In a stent with greater than 30% uncovered struts, the odds ratio (OR) for thrombosis was 9.0 (95% confidence interval [CI]: 3.5–22.0) as compared with a stent with complete coverage.

The mechanisms by which the first-generation DES induces non-uniform incomplete healing are not fully understood; however, lesion characteristics, drug properties, total dose, release profile and drug distribution, and polymer bio-compatibility together all play an important role in inducing neointimal suppression. While sirolimus and paclitaxel reduce neointimal formation by impeding smooth muscle cell proliferation and migration, these drugs also impair the normal healing process of the injured arterial wall.9–11 Underlying plaque morphology may also affect the rate of healing when stent struts penetrate deeply into a necrotic core and are not in contact with cellular areas.12 Eccentric plaques may prevent uniform strut deployment, thereby increasing local toxicity due to higher concentrations of drug and polymer. Indeed, sections with evidence of thrombosis showed significantly lower inter-strut distances and this correlated with lower neointimal growth. Local concentration of drug is ultimately highly dependent on the spacing of stent struts, and the variance in distance between struts will amplify differences in concentration, leading to biological effects.13 Loaded dose of drug varies from strut to strut, with greater retention of lipophilic drugs in different regions of plaque affecting arterial drug concentration, resulting in non-uniform healing.14 Coating defects can explain some of these differences.15 The relationship between local drug concentration and cellular repair is further clarified by data from overlapping versus non-overlapping SES and PES that illustrate less coverage of stent struts in overlapping segments as compared with non-overlapping stent struts in the rabbit iliac model.16

In summary, the underlying pathology in cases of LST indicates incomplete stent strut coverage is the most important morphometric predictor of LST and is also the most powerful surrogate of endothelialisation. Both plaque- and device-related issues play a role in promoting uneven healing.

Delayed Arterial Healing in First-generation Drug-eluting Stents for Acute Myocardial Infarction

Given that acute myocardial infarction (AMI) is one of the only clinical presentations in which PCI has been shown to decrease the risk of death as compared with medical therapy alone, the long-term outcome after DES for AMI is of immense clinical importance.17,18 Using our autopsy database of patients dying after DES implantation with duration of implant greater than 30 days, we compared the vascular pathogical responses to DES implantation in patients receiving first-generation DES for AMI (n=17) with those of patients receiving for stable angina (n=18).19 Histological sections were evaluated for the identification of culprit and non-culprit sites. Culprit sites in AMI were defined as stented segments with underling presence of a necrotic core and a thin cap with ruptured fibrous cap, whereas culprit sites in patients with stable angina were selected as the sections with the largest underlying necrotic core and overlying thick fibrous cap (>100mm). We compared culprit sites in patients with AMI with those in patients with stable angina as well as with non-culprit sites within each stent.

The incidence of LST was significantly higher in patients with AMI (seven of 17, 41%) as compared with those with stable angina (two of 18, 11%; p=0.04). VLST (more than one year) was observed in two patients with AMI (12%) and in no patients with stable angina. Morphometric analysis showed that culprit AMI sites versus stable plaque had significantly lower neointimal thickness (0.04mm [0.02, 0.09] versus 0.11mm [0.07, 0.21], p=0.008), greater fibrin deposition (63±28% versus 36±27%, p=0.008) and inflammation (35% [27, 49] versus 17% [7, 25], p=0.003), and higher prevalence of uncovered struts (49% [16, 96] versus 9% [0, 39], p=0.01). Representative images of culprit sites from patients with AMI and those with stable angina are illustrated in Figure 2.

In patients with AMI, neointimal thickness was significantly lower at culprit sites as compared with non-culprit sites (0.04mm [0.02, 0.09] versus 0.07mm [0.04, 0.20], p=0.008), whereas this difference was not observed in patients with stable angina (0.11mm [0.07, 0.21] versus 0.11mm [0.08, 0.19], p=0.56). Similarly, the percentage of struts with fibrin (63±28% versus 52±27%, p=0.04), struts with inflammation (35% [27, 49] versus 30% [13, 38], p=0.04) and uncovered struts (49% [16, 96] versus 19% [3, 34], p=0.02) was significantly greater at culprit sites as compared with non-culprit sites in patients with AMI, whereas there were no significant differences in these parameters in patients with stable angina.

We explored whether there was a significant correlation between fibrous cap thickness and the presence of uncovered struts and found a significant negative correlation (i.e. the thinner the fibrous cap the greater the presence of uncovered struts) (R=-0.60, p=0.0006). In addition, there was also a significant but weaker negative correlation between duration of stent implants and the presence of uncovered struts (R=-0.39, p=0.01). Similarly, fibrous cap thickness and neointimal thickness had a significant positive correlation (R=0.68, p=0.0001), whereas a weaker correlation was observed between neointimal thickness and duration of implant (R=0.38, p=0.03).

These findings reinforce our previous report demonstrating heterogeneity of healing within the same stent.8 Plaque rupture is the most frequent cause of AMI,20 and this underlying plaque morphology is the most reasonable explanation for the delayed healing at culprit sites in AMI as opposed to non-culprit sites as well as culprit sites from stable patients. Since sirolimus and paclitaxel are highly lipophilic,14 it is likely that these agents have high affinity for lipid-rich plaques (i.e. necrotic core) and dwell there for longer periods of time because of greater strut penetration as compared with when struts are exposed to adjacent inflamed regions of the plaque. In addition, the lipid-rich necrotic cores are avascular and have fewer smooth muscle cells within the fibrous cap. Therefore, these areas are less likely to be covered by migrating and proliferating cells from adjacent regions. Higher drug concentrations in these areas may also heavily influence healing by retarding smooth muscle cell proliferation as well as endothelial re-growth. In addition, thrombus burden may also play a role by increasing uptake of drug by the thrombus as shown by Hwang et al. with PES.21

Recently published long-term (three to five years) follow-up results from randomised clinical trials in patients with ST-elevation AMI (STEMI) treated with either BMS or DES have demonstrated that the incidence of all-cause mortality, stent thrombosis and MI did not differ between BMS and DES.22–28 Spaulding et al.27 reported four-year follow-up results from the Trial to assess the use of the Cypher sirolimus-eluting coronary stent in acute myocardial infarction treated with balloon angioplasty (TYPHOON), where there was no significant difference in freedom from cardiac death (SES: 97.6% versus BMS: 95.9%, p=0.37), repeat MI (SES: 94.8% versus BMS: 95.6%, p=0.85) and definite/probable stent thrombosis (SES: 4.4% versus BMS: 4.8%, p=0.83) identified between the groups, although the complete data were available in only 70% of patients (n=501). All-cause mortality was also similar between the groups (SES: 5.8% versus BMS: 7.0%, p=0.61), whereas freedom from target lesion revascularisation (TLR) was significantly better in the SES group as compared with the BMS group (92.4% versus 85.1%, p=0.002). Vink et al.28 reported a five-year follow-up study of the Paclitaxel-eluting versus conventional stent in myocardial infarction with ST-segment elevation (PASSION) trial (n=619 patients), in which the occurrence of the composite of cardiac death, recurrent MI or TLR was comparable between PES and BMS (18.6% versus 21.8%, hazard ratio [HR]: 0.82, 95% CI: 0.58–1.18, p=0.28) and the rate of TLR was similar between PES and BMS (7.7% versus 10.5%, p=0.21). The incidence of definite or probable ST, which includes acute, subacute, late and very late stent thrombosis, was not different between PES and BMS (4.2% versus 3.4%, p=0.68); however, the PES group had higher incidence of (very) late definite or probable stent thrombosis as compared with the BMS group (3.5% versus 1.1%, p=0.06), and the incidence of (very) late definite stent thrombosis was significantly higher in PES as compared with BMS (3.3% versus 0.7%, p=0.04).

On the other hand, Brodie et al.29 showed the results from a large singlecentre registry with prospectively collected data on all patients undergoing primary PCI over more than a decade, which included consecutive patients receiving DES (SES, PES or second-generation DES; n=368) or BMS (n=1,095) for STEMI. The rate of definite/probable ST was similar between DES and BMS at one year (4.0% versus 5.1%) but increased more with DES after the first year (DES: 1.9%/year, BMS: 0.6%/year). Landmark analysis (more than one year) revealed that DES had a significantly higher incidence of VLST (p<0.001) and re-infarction (p=0.003), and DES was the only independent determinant of VLST (HR: 3.79, 95% CI: 1.64–8.79, p=0.002). These findings are reminiscent of previous long-term registries in which VLST after primary PCI was only noted with DES.30,31 Randomised trials generally recruit patients with less co-morbidity who are more likely to comply with study protocols, whereas registries include ‘all-comers’ who may have multiple comorbidities, are non-compliant or cannot afford close follow-up and long-term medical treatment. Given that the findings from large registries are inconsistent with those from randomised clinical trials, the long-term safety of DES for the treatment of AMI is still controversial; however, if in most cases there are no significant differences, the question arises as to why DES should be used in AMI when today cost containment is essential. Nevertheless, our data offer a pathophysiological underpinning for the possibility that the benefits of opening an infarct-related artery with DES might be outweighed by longterm risks of death and MI associated with DES-driven LST.

In summary, vessel healing at culprit sites in AMI patients treated with first-generation DES is substantially delayed as compared with non-culprit sites and culprit sites in patients receiving DES for stable angina, emphasising the importance of underlying plaque morphology in arterial healing and the risk of LST following DES implantation.

Pathological Findings in Bifurcation Stenting

Atherosclerotic lesions tend to form at specific regions of the coronary vasculature where flow is disturbed, in particular in areas of low shear.32–34 Because dramatic haemodynamic alternations occur at branch points within the arterial tree, coronary bifurcations are extraordinarily susceptible to atherosclerosis. Indeed, our human pathological data in non-stented coronary bifurcation lesions showed that low-shear areas (the lateral wall) had significantly greater intimal thickness and necrotic core size as compared with high-shear areas (the flow divider).35 The use of DES at bifurcation lesions has reduced restenosis rates compared with BMS; however, long-term outcomes are tempered by an increased risk of thrombosis,36 which raises the possibility that the delayed healing seen after DES implantation might be exacerbated at bifurcation sites. Given the difference in atherosclerotic plaque burden between low- and high-shear regions, neointimal growth following stent implantation may be different between these lesions. To investigate this hypothesis, we evaluated the pathological arterial response to bifurcation stenting with DES and BMS.35

From our stent registry, a total of 40 stented bifurcation lesions (DES=19 and BMS=21) from 40 patients were reviewed. Duration of implant was similar between the DES and BMS groups (330 days (188, 680) versus 150 days (54, 540), p=0.14). To assess the impact of flow disturbance on arterial healing in stented lesions, the differences between high-shear (flow divider) and low-shear (lateral wall) regions were compared. Neointimal thickness was significantly lower at the high-shear (flow divider) site as compared with the low-shear (lateral wall) site in DES (0.07mm (0.03, 0.15) versus 0.17mm (0.09, 0.23), p=0.001), whereas this difference did not reach statistical significance for BMS cases (0.26mm (0.16, 0.73) versus 0.44mm (0.17, 0.67), p=0.25). Similarly, the percentage of uncovered struts was significantly greater at high shear as compared with low shear in DES (40% (16, 76) versus 0% (0, 15), p=0.001), while there was no significant difference in BMS (0% (0, 21) versus 0% (0, 0), p=0.09). Fibrin deposition was also frequently higher at sites of high-shear blood flow as compared with low-shear and was only observed in DES (60% [21, 67] versus 17% [0, 55], p=0.01). Although the difference remained of borderline significance because of a limited sample size, a greater incidence of LST was documented in the DES group as compared with the BMS group at bifurcation sites (main vessel: 75% versus 36%, p=0.06; side branch: 42% versus 14%, p=0.19). Interestingly, most of the thrombi originated at the flow divider sites where uncovered struts were frequently observed (see Figure 3).

Previous experimental study has shown that greater neointimal formation occurs in the lateral as compared with the flow divider region following stent implantation in a porcine ilio-femoral bifurcation model,37 which is consistent with our findings. On the other hand, these differences were not significant in BMS, which may be due to more rapid healing and more uniform neointimal formation after BMS implantation. Thus, a combination of drug effect and blood flow disturbance both are likely to accelerate the delayed healing in bifurcation lesions.

In summary, arterial healing at bifurcation lesions with first-generation DES was impaired, with greater delay at the flow divider (high shear) as compared with the lateral wall sites (low shear), which may be caused by a combination of drug effects and difference in flow conditions.

Impact of Stent Fracture on Adverse Pathological Findings

Stent fracture has emerged as a complication following DES implantation and is recognised as one of the contributors of in-stent restenosis38–40 and possibly stent thrombosis.41,42 Clinically the incidence of stent fracture is reported in 1–2% of patients at eight- to 10-month follow-up angiography,38,43 although the sensitivity of angiography to detect stent fracture is limited. We sought to assess the incidence of stent fracture at autopsy using high-contrast film-based radiography and investigate the impact of stent fracture on the pathogical findings and clinical outcomes.44

High-contrast film-based radiographs of 177 consecutive lesions (SES=77 and PES=101, one lesion had both SES and PES) from the CVPath DES autopsy registry were reviewed. Stent fracture was graded as I (single strut fracture), II (two or more strut fractures without deformation), III (two or more strut fractures with deformation), IV (multiple strut fractures with acquired transection but without gap) or V (multiple strut fractures with acquired transection with gap in the stent body). The incidence of adverse pathological findings (thrombosis and restenosis) was assessed histologically.

Stent fracture was documented in 51 lesions (29%; grade I=9, II=14, III=11, IV=6 and V=9). There was no significant difference in age, gender and cause of death between patients with fracture and those without. Lesions with stent fracture had longer duration of implant as compared with those without fracture (172 days [31–630] versus 44 days [7–270], p=0.004), whereas no statistical difference in duration of implant was identified between each grade of stent fracture (grade I: 31 days [5, 616], II: 105 days [27, 1,095], III: 376 days [72, 570], IV: 331 days [31, 833] and V: 172 days [44, 450], p=0.70). Furthermore, lesions with stent fracture showed a higher rate of SES usage (63% versus 36%, p=0.001), longer stent length (30.0mm [22.0–40.0] versus 20.0mm [14.0–27.3], p<0.0001), and a higher rate of overlapping stents (45% versus 22%, p=0.003) as compared with lesions without stent fracture. A forward stepwise logistic regression analysis demonstrated that longer stent length (OR: 1.07, 95% CI: 1.036–1.100, p<0.0001), use of SES (OR: 3.40, 95% CI: 1.57–7.33, p=0.002) and longer duration of implant (OR: 1.002, 95% CI: 1.001–1.003, p=0.002) were independent determinants of stent fracture.

Histological evaluation showed that neointimal thickness was similar between lesions with stent fracture and those without (0.11mm [0.06, 0.19] versus 0.11mm [0.03, 0.19], p=0.62). There was no significant difference in fibrin deposition (fibrin score, fracture [+]: 1.0 [0.1, 1.5] versus fracture [-]: 1.4 [0.4, 2.0]) and inflammation (inflammatory score, fracture [+]: 1.0 [0.5, 1.6] versus fracture [-]: 1.4 [0.4, 2.0]), including a similar degree of giant cell and eosinophil infiltration. Furthermore, there were no differences in these parameters among the various fracture grades (i.e. grade I–V). Six adverse events (five thrombosis and one restenosis) were associated with grade V fracture (67%), while there were no fracture-site-related adverse pathological findings in stents with grades I–IV except for one stented lesion with grade IV that had a long overlapping stent (grade I–IV versus grade V, p<0.0001). A representative case of grade V fracture of DES is shown in Figure 4.

Although it is not fully understood why stent fractures cause adverse events, lack of stent integrity such as distortion or acquired underexpansion may play an important role in the occurrence of adverse events. Previous clinical studies have reported that the main risk factors for stent fracture are longer stent length, right coronary artery (RCA) or saphenous vein graft lesion location, lesion with high motion, overlapping stent and SES use.38,40,45 Our findings showed SES were associated with higher incidence of stent fracture; the flexible ‘N’-shaped undulating longitudinal inter-sinusoidal ring linker segment was the most frequent location of the fractures, which are smaller in width than the sinusoidal ring portion. The relationship between longer implant duration and higher incidence of stent fracture suggests that stent fracture may result from continuous stress over time on the implant that leads to greater metal fatigue with eventual fracture. However, it should also be noted that stent fracture was seen even in the patients who died shortly after stent implantation, which is probably procedure related (high pressure and/or oversized balloon, overlapping stent, etc.).

In summary, the incidence of DES fracture is 29% of the stented lesions at autopsy, which is much higher than clinically reported. A high rate of adverse pathogical findings was observed in lesions with grade V fracture, while fracture with grades I–IV did not have a significant impact on the clinical outcome. Longer stent length, SES usage and longer duration of stent implant were identified as independent predictors of stent fracture.

Coronary Responses and Differential Mechanisms of Late Stent Thrombosis Attributed to Sirolimus-eluting and Paclitaxel-eluting Stents

Previous clinical trials have reported differences in angiographic late lumen loss in patients receiving SES or PES,46 whereas it remains unclear whether the long-term histological responses to each stent type are different and how this relates to the time course of arterial healing and mechanism(s) of LST. Therefore, we evaluated vascular healing response and the mechanism(s) of LST in patients with first-generation DES.47 The overall analysis included 174 cases (230 DES lesions) from the CVPath autopsy registry, and histomorphometry was performed on coronary stents from 127 cases (171 lesions) who died ≥30 days after receiving stent implants. Analysis of individual lesions with duration of implant <30 days (SES=25 and PES=34) revealed that the incidence of early stent thrombosis was equivalent for lesions with SES and PES (44% versus 38%, p=0.79). Histologically, no differences in the extent of inflammation and fibrin deposition were noted between SES and PES implants <30 days.

Lesions with duration of implant ≥30 days comprised of 77 SES and 94 PES lesions: 40 SES (52%) and 53 PES (56%) lesions were treated for off-label indications, which included stents deployed for AMI or bifurcation lesions, left main artery, bypass graft, restenosis, chronic total occlusion or lesion lengths >30mm.48 There was no significant difference in the incidence of LST between SES and PES (21% versus 27%, p=0.47). Neointimal thickness was significantly greater in PES as compared with SES (0.13mm [0.03, 0.20] versus 0.10mm [0.04, 0.15], p=0.04). Similarly, PES had greater maximum neointimal thickness than SES (0.23mm [0.13, 0.37] versus 0.17mm [0.06, 0.28], p=0.04). The heterogeneity in neointimal thickness between sections was also significantly greater for PES versus SES (0.14mm [0.08, 0.31] versus 0.10mm [0.03, 0.22], p=0.02). On the other hand, the percentage of uncovered struts was similar between PES and SES (20% versus 21%, p=0.72). There was a progressive and significant increase in neointimal thickness beyond nine- and 18-months’ duration in lesions with PES without evidence of LST (p=0.009); although similar trends were observed for SES, findings were of borderline significance (p=0.08).

Accumulated fibrin as assessed by fibrin score was significantly greater in PES as compared with SES (1.8 [1.0, 2.5] versus 0.8 [0.0, 2.0], p=0.001). On the contrary, SES implants were associated with a significantly greater inflammatory score as compared with PES (1.3 [0.5, 2.0] versus 1.0 [0.5, 1.5], p=0.007). The contributing cells resulting in greater inflammation observed with SES were predominantly eosinophils and giant cells. The incidence of malapposition was comparable between SES and PES (14% versus 19%, p=0.40), although the mechanism of this phenomenon was different (see below). Further analysis revealed near-complete healing in stents placed for ‘on-label’ indications with implant durations of >12 months, while the majority of DES with off-label usage remained unhealed beyond this similar time-point.

Underlying pathological causes of LST were determined as penetration of necrotic core, bifurcation stenting, long/overlapping stents, stent underexpansion, isolated uncovered struts, localised hypersensitivity reaction and malapposition from excessive fibrin deposition. Significant difference in underlying cause of LST was identified between SES and PES; for SES, there were localised hypersensitivity reactions consisting of eosinophils, lymphocytes and giant cells throughout the stented segment (see Figure 5A), while LST in PES was attributed to malapposition secondary to excessive fibrin deposition on the abluminal surface (see Figure 5B). Localised hypersensitivity was documented in five cases involving seven lesions treated by SES that developed LST, whereas no PES lesion showed this reaction. The majority of patients with hypersensitivity reaction following SES implantation died in the very late phase (>1 year) where the mean duration of implant was 649 days. Malapposition was observed in five lesions (71%) with a mean section of struts from the vessel wall of 944μm. In most SES with severe inflammation there was positive remodelling of the vessel resulting in malapposition. In contrast, malapposition secondary to excessive strut fibrin as the primary contributor towards LST was observed only in PES (seven lesions from six patients) with a mean implant duration of 611 days. The mean distance separating the struts from the vessel wall was 404μm. The luminal surface generally lacked endothelial cell coverage as well as evidence of granulation tissue consisting of macrophages, smooth muscle cells or proteoglycan matrix (see Figure 5B).

As previously reported, mechanism(s) of LST are likely multifactorial.49 Although there is a certain commonality in the mechanism of LST for both SES and PES in that all cases demonstrated poor endothelialisation, our findings indicate the final stimulus for thrombus development may be different based upon DES type. The disparities in vascular responses regarding the stent milieu are undoubtedly attributable to differences in the loading drug, polymer coating and unique elution profile for each device. The hypersensitivity reaction observed in SES is likely attributed to the polymer rather than drug,50 which is presumably completely eluted by three months. Pre-clinical DES implants in porcine coronary arteries showed escalating amounts of inflammation over time,51 which is consistent with our findings showing that the majority of hypersensitivity cases were documented in devices >1 year old. Greater fibrin accumulation around struts in PES also remains consistent with prior pre-clinical findings that showed a dose-dependent increase in fibrin deposition and medial necrosis following deployment of PES in rabbit iliac arteries,52 as well as similar dose escalatory findings in a porcine model.53 Therefore, we believe that paclitaxel itself is responsible for excessive fibrin deposition.

Since histological sections of PES demonstrating the thinnest neointima are typically accompanied by persistent fibrin, the heterogeneity of arterial healing may result from an uneven distribution of drug and polymer. Further support for variations of available paclitaxel within a single stent are from scanning electron microscopy (SEM) studies demonstrating webbing and delamination of polymer, which is a frequent finding in PES.15 Progressive neointimal growth, although slow to develop, is likely related to persistent fibrin and inflammation. In both DES platforms, biological signs of a drug effect such as fibrin remain beyond the reported durations of drug release. Fibrin degradation products, in particular fibrin fragment E, are a known initiator of smooth muscle cell migration and proliferation,54,55 a phase that generally occurs early after BMS placement. In addition to fibrin, persistent inflammation is yet another plausible explanation for the late increases in neointimal formation associated with DES. Non-erodible polymers used in firstgeneration DES are associated with chronic inflammation and, in particular for SES, eosinophils,16,49,50,56 lymphocytes and giant cells, especially in the presence of hypersensitivity vasculitis. However, hypersensitivity cases failed to show an increase in neointimal thickness. Clinical studies have shown that target vessel revascularisation rates do increase with time57–59 and the gradual growth of neointima seen in our study may partly account for this phenomenon. Moreover, the greater prevalence of unhealed stents in DES placed for off-label as compared with on-label indications may have implications for the duration of dual antiplatelet therapy after first-generation DES placement.

In summary, first-generation DES exhibit divergent mechanisms of LST, where hypersensitivity likely plays a significant role in SES while for PES the aetiology appears to be association with excessive fibrin deposition on abluminal surface with malapposition. Another important finding was near-complete healing in DES placed for greater than 12 months with confirmed on-label usage, while off-label indications of both stents resulted in incomplete healing even in DES beyond 12 months.

Conclusion

Stent thrombosis is an infrequent but catastrophic complication following stent implantation. First-generation DES have dramatically reduced restenosis; however, the steady increase in the rate of LST still raises questions regarding the long-term safety of this technology. Pathological studies have identified incomplete neointimal coverage of stent struts as the most important predictor of LST, and have shown that delayed arterial healing is associated with penetration of necrotic core, long/overlapping stents and bifurcation stenting, especially in flow divider (high-shear) regions. Grade V stent fracture is also associated with adverse pathogical findings including LST and restenosis. Localised hypersensitivity reaction is exclusively attributed to SES as an underlying mechanism of LST, whereas malapposition secondary to excessive peri-strut fibrin deposition is associated with PES implants. In addition, uncovered struts are still identified in both SES and PES with implant duration beyond 12 months, particularly in stents placed for off-label indications. These concerns about first-generation DES have driven efforts to develop newer DES that incorporate differences in drug loading, including release kinetics, non-erodible and erodible polymers, non-polymeric drug delivery and fully bioresorbable scaffolds; however, DES have been implanted worldwide and careful long-term follow-up is necessary to determine the risk and benefit of this technology.

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