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Chapter 31. Special Stents: Bioresorbable Coronary Scaffolds

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Which of the following statements regarding polymer-based scaffold bioresorption is true?

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A. Polymer-based scaffolds undergo bulk degradation following polymer hydrolysis.

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B. The end products of polymer-based degradation are elemental magnesium and inorganic salts.

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C. The average resorption time of poly-l-lactic acid is 1 year, as indicated by gel permeation chromatography. Regions previously occupied by polymeric struts are replaced by functional connective tissue at 2 years.

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D. Continuous cleavage of polymer’s amorphous tie chains leads to oligomeric molecules that enter the Cori cycle with end products carbon dioxide and water.

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E. Continuous cleavage of polymer’s amorphous tie chains increases the scaffold’s radial strength, enabling sufficient vessel support.

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The correct answer is A

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Polymer-based scaffolds undergo bulk degradation after polymer hydration. The first stage in the process of polymer bioresorption is hydration. Polylactides are relatively hydrophilic; thus water diffuses into the less dense amorphous regions of the implant and hydrolyzes the ester bonds. Random chain scissions occur at this stage, leading to reduction of the polymer’s molecular weight. The second stage is characterized by continuous cleavage of the amorphous tie chains, reducing the radial strength of the scaffold and leading to structural discontinuities. During the third stage, polymer chains that have been hydrolyzed to short lengths diffuse out of the implant (mass loss) as they are increasingly hydrophilic and soluble in aqueous solution. Following these sequential stages, oligomeric polylactic acid molecules hydrolyze to lactic acid monomers, which deprotonate (release of a proton [H+]) to lactate. Lactate is converted to pyruvate and enters the citric acid cycle (Krebs cycle), which is further metabolized in CO2 and H2O excreted through lungs and kidneys. The end products of magnesium-based scaffolds are elemental magnesium and inorganic salts.

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The ABSORB III trial randomized 2008 patients with stable or unstable angina to receive in a 2:1 ratio the everolimus-eluting bioresorbable vascular scaffold (BVS; Absorb) or the metallic Xience V stent. The findings of this trial include all of the following except:

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A. The primary end point was target-lesion failure (TLF), a composite of cardiac death, target vessel myocardial infarction (TV-MI), or ischemia-driven target lesion revascularization (TLR).

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B. TLF occurred in 7.8% of patients in the Absorb BVS arm versus 6.1% of patients in the Xience arm (difference, 1.7 percentage points; 95% confidence interval, −0.5 to 3.9; P = .007 for noninferiority and P = .16 for superiority).

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C. Device thrombosis among the groups reached 1.5% in the Absorb BVS arm versus 0.7% ...

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