Background-Late stent malapposition has been reported to be an abnormal finding after vascular brachytherapy and, possibly, implantation of drug-eluting stents. It can only be detected if intravascular ultrasound (IVUS) is performed at follow-up. However, the "background" frequency of late stent malapposition after bare-metal stent implantation is not known. Methods and Results-We studied 206 patients with native artery lesions who had tubular-slotted bare-metal stent implantation and who had IVUS performed at index and after 6Ϯ3 months of follow-up. There were 9 patients (4.4%) with late malapposition, which is separation of at least 1 stent strut from the arterial wall intima that does not overlap a side-branch, with evidence of blood flow (speckling) behind the strut, and where the immediate postimplantation IVUS revealed complete apposition of the stent to the vessel wall. The location of late malapposition was the stent edge in 8 of 9 patients. The maximum area, length, volume, and arc of late malapposition measured 3.1Ϯ2.4 mm 2 , 3.3Ϯ2.2 mm, 21Ϯ27 mm 3 , and 110Ϯ61 degrees, respectively. There was an increase in external elastic membrane (EEM) area (20.7Ϯ4.9 to 26.9Ϯ4.2 mm, Pϭ0.0021) and plaque area (10.1Ϯ3.7 to 14.8Ϯ3.6 mm, Pϭ0.0022); however, the increase in EEM was greater than the increase in plaque. The area of late malapposition correlated directly with the increase in EEM area (rϭ0.75, Pϭ0.0205). Conclusion-Late

Background-Late stent malapposition (LSM) is only detected if intravascular ultrasound (IVUS) is performed atimplantation 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&MϭEEM 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 (0.57Ϯ0.34 mm) was larger than within the apposed circumference (0.16Ϯ0.18 mm; Pϭ0.0004). ⌬EEM for each malapposed radius was greater than for each apposed radius (PϽ0.05 for all comparisons). Stent-intima separation correlated with EEM radius increase within the malapposed circumference (rϭ0.83, Pϭ0.0013).At follow-up, the mean P&M thickness decreased in the malapposed circumference (Ϫ0.31Ϯ0.22 mm; PϽ0.0001). However, the decrease in P&M thickness in the malapposed circumference occurred because the same P&M area was distributed over a larger circumference (4.1Ϯ1.6 mm to 5.4Ϯ3.0 mm; Pϭ0.05), the result of positive remodeling. Conclusion-The main cause of LSM is a regional increase in EEM (regional positive remodeling).

Diagnostic study, Level III.