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Thrombosis with bioresorbable vascular scaffolds beyond 1 year after implantation might be more of a problem than with everolimus-eluting stents (EES), according to a meta-analysis, but with big question marks remaining.

Studies that compared the Absorb bioresorbable vascular scaffold with EES turned up over the course of 2 years overall (2.5% versus 1.1%, OR 2.08, 95% CI 1.02-4.26), , of Japan's Kyoto University, and colleagues reported in the Jan. 9 issue of JACC: Cardiovascular Interventions.

Very late device thrombosis -- events between 1 and 2 years after implantation -- were numerically, but not statistically significantly, more common in bioresorbable vascular scaffolds (0.90% versus 0.37%, OR 2.03, 95% CI 0.62-6.71) in the analysis of trials and other comparative studies.

When single-arm registries were thrown into the mix, the odds of very late stent/scaffold thrombosis were similarly elevated for Absorb (rates 0.240%, 95% CI 0.022%-0.608% versus 0.003% for EES, 95% CI 0.000%-0.028%), which editorialists pointed to as a "significant" concern in this "first meta-analysis assessing the incidence of this dreadful complication at 2 years in a very large number of patients."

Very late target lesion failure was not significantly more frequent in either analysis.

The theoretical longer-term promise of disappearing scaffold stents remains just that, and must be balanced against the known thrombotic risk until complete scaffold resorption, Kimura's group suggested.

The investigators suggested that persistent strut malapposition of Absorb may be an important mechanism of very late scaffold thrombosis, and emphasized the importance of careful device sizing and meticulous implantation with optical coherence tomography guidance.

In an , , and , both of Hospital Universitario de La Princesa in Madrid, Spain also stressed the role of accurate sizing.

"Nevertheless, blaming suboptimal implantation techniques for some of the late adverse bioresorbable vascular scaffold results and, moreover, pretending that this problem could be readily overcome with a meticulous implantation technique seems like déjà vu in interventional cardiology," they argued.

They pointed out that optimal techniques for Absorb implantation were already highlighted when the device was first released.

"New-generation polymeric bioresorbable vascular scaffolds, with thinner struts, enhanced expansile capability, and improved resorption kinetics will be soon available. These iterations are expected to overcome most of the limitations encountered by first-generation bioresorbable vascular scaffolds," Alfonso and Cuesta wrote.

"However, we should learn our lessons and maintain close critical scrutiny on the clinical performance of these novel devices. Science and, more importantly, responsible patient care should be based on more than expectations."

Kimura and colleagues pooled 24 studies (a mix of single-arm registries, observational, and randomized studies) for their meta-analysis. They included 2,567 bioresorbable scaffold recipients and 19,806 of those who got EES.

After adding single-arm registries to the pool of comparative studies, event rates remained numerically elevated in the bioresorbable vascular scaffold group:

• Stent/scaffold thrombosis (1.43% versus 0.56% for EES)
• Target lesion failure over 2-year follow-up (7.90% versus 7.49%)
• Very late target lesion failure (1.88% versus 1.78%)

Yet the authors acknowledged several shortcomings of their analysis ranging from the insufficient number of patients with very late stent/scaffold thrombosis to the inclusion of observational studies that were prone to selection bias. Two years may be too early to evaluate the effects of the Absorb device, they added too.

Also relevant to the study's limitations was the lack of angiographic efficacy outcome measures, according to editorialists Alfonso and Cuesta. "This issue remains of major relevance, as some previous studies with mandated invasive surveillance suggested that classic angiographic surrogate endpoints, including acute lumen gain, late lumen loss, and other late angiographic parameters, may be slightly, but significantly, poorer with bioresorbable vascular scaffolds compared with EES."

"The disparity of findings between stent/scaffold thrombosis or very late stent/scaffold thrombosis and target lesion failure might be confusing from a clinical perspective," the editorialists noted.

"An explanation to reconcile these apparently discordant results could be that the adverse effects of bioresorbable vascular scaffold thrombosis would become diluted within mainly efficacy-driven composite device-oriented and patient-oriented endpoints. In this regard, specific information on the rates of MI, ideally dissecting the results to differentiate procedure-related versus spontaneous MI, would have been of major clinical value, especially considering concerns raised by previous studies in this regard."