Late positive remodeling and late lumen gain contribute to vascular restoration by a non-drug eluting bioresorbable scaffold: a four-year intravascular ultrasound study in normal porcine coronary arteries.

BACKGROUND The interplay between mechanical dilatation, resorption, and arterial response following implantation of bioresorbable scaffolds is still poorly understood. METHODS AND RESULTS Long-term geometric changes in porcine coronary arteries in relation to gradual degradation of bioresorbable scaffolds were assessed in comparison with bare metal stents (BMS). Intravascular ultrasound (IVUS)-derived lumen, outer stent/scaffold, and reference vessel areas were evaluated in 94 polymer scaffolds and 46 BMS at 5 days and 3, 6, 12, 18, 24, and 55 months, in addition to polymer scaffold radial crush strength and molecular weight (M(W)) at 3, 6, and 12 months. BMS outer stent area and lumen area remained constant through 55 months (P=0.05, but within 1 standard deviation of 100%, and P=0.58, respectively), while significant increases were exhibited by polymer-scaffolded vessels with the maximum late lumen gain at 24 months, paralleled by the outer scaffold area increase, and then remaining at that increased level at 55 months (P<0.01). By 12 months polymer scaffolds experienced significant reductions in radial strength and M(W), while the animals underwent the largest weight gain. At 3 months and beyond, the patency ratio (lumen area/reference vessel area) of BMS remained constant (0.71 to 0.85, P=0.49). In contrast, that of polymer scaffolds increased and approached 1 (P=0.13). CONCLUSIONS Bioresorbable polymer scaffolds allow restoration of the treated segment's ability to remodel outward to achieve level lumen transition between reference vessel and scaffold-treated regions, a process mediated by animal growth and scaffold degradation. This also introduces a challenge to standard analyses of IVUS outcomes relying on constant stent diameters over time.