Regional remodeling as the cause of late stent malapposition

Background - Late stent malapposition (LSM) is only detected if intravascular ultrasound (IVUS) is performed at implantation and follow-up. We used a novel regional IVUS analysis to assess the mechanism of LSM. Methods and Results - Corresponding image slices on postimplantation and follow-up IVUS studies of 11 malapposed stents were identified and electronically rotated until they were aligned. The geometric center of the stent was identified, and the angle of late malapposition measured. Radii were drawn from this center through the transition points between complete apposition and LSM. These two circumferences were divided into equal arcs, and radii were drawn to the external elastic membrane (EEM). Measurements included EEM radius and circumference, plaque and media (P& M = EEM minus stent radius) thickness and area, and stent-intima separation. Mean baseline EEM radius and P& M thickness were similar in apposed and malapposed circumferences. At follow-up, mean EEM radius increase within the malapposed circumference (0.57 +/- 0.34 mm) was larger than within the apposed circumference (0.16 +/- 0.18 mm; P = 0.0004). DeltaEEM for each malapposed radius was greater than for each apposed radius (P < 0.05 for all comparisons). Stent-intima separation correlated with EEM radius increase within the malapposed circumference (r = 0.83, P < 0.0013). At follow-up, the mean P& M thickness decreased in the malapposed circumference (-0.31 +/- 0.22 mm; P < 0.0001). However, the decrease in P& M thickness in the malapposed circumference occurred because the same P& M area was distributed over a larger circumference (4.1 +/- 1.6 mm to 5.4 +/- 3.0 mm; P = 0.05), the result of positive remodeling. Conclusion - The main cause of LSM is a regional increase in EEM ( regional positive remodeling).